Amsats and Hamsats Amateur Radio and Other Small Satellites [1 ed.]

Table of contents :
Amsats and Hamsats
Key factors for working satellites
Working the ‘Easy Sats’
Working satellites with linear transponders
Working digital satellite transponders
Working the big satellite
“Mistakes, I’ve made a few”
Quick quiz questions
Visible satellites
Listening to satellite transmissions
New satellite proposals
Maidenhead grids
IARU satellite coordination WRC-15
Satellite bands
Satellite transponders and repeaters
Satellite deployment
Miscellaneous facts
The FUNcube satellites
Weather satellites
The International Space Station
Link budgets
Coaxial feeder cables
Masthead preamplifiers
Antenna systems
Quick quiz questions
Doppler shift
Satellite prediction and tracking software
CAT (CI-V) control of your radio
Automated rotator control
Types of orbit
Space and orbits
Quick quiz questions
Understanding TLE files
Satellite pass prediction
AMSAT satellite designations
Satellite history
Amateur radio on manned space missions
Vector mathematics
Glossary of abbreviations and acronyms
Drawings and images
Online resources and videos
About the author

Citation preview


By Andrew Barron ZL3DW

Copyright © 2018, 2019 Andrew Barron All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the copyright holders. U.S. Kindle edition 1.2 July 2019. First U.S. Kindle edition 1.0 March 2018

It is a breach of copyright to scan or copy any part of this book or to post its content on any Internet site. Legal copies of this book are only available as printed books or as a Kindle eBook. If the document you are reading is a PDF file or you downloaded it from the Internet, it is an illegal copy! I hope you didn’t pay anything for it! The cover image was created by Dr. Fabian Neyer. From ASAT, the Aristotle Space & Aeronautics Team. It is used with permission.

The information contained in this book is my own analysis of the satellites created and used by amateur radio operators and by non-commercial or government interests worldwide. It intends no criticism of any association, group, manufacturer, model of radio equipment, or software application. The author has no association with any equipment manufacturer or software developer. Research material for the creation of this document has been sourced from a variety of public domain Internet sites. The author accepts no responsibility for the accuracy of any information presented herein. It is to the best of my knowledge accurate, but no guarantee is given or implied. Use the information contained in this document at your own risk.


The Radio Today guide to the Icom IC-7300 The Radio Today guide to the Icom IC-7610 Software Defined Radio for Amateur Radio operators and Shortwave Listeners

An introduction to HF software defined radio (Out of print)

AC KNO WL EDG E MENT S Thanks to my wife Carol for her love and support and to my sons James and Alexander for their support and their insight into this modern world. Thanks also to everyone who has ever helped with the design, production, or ground control of amateur satellites and amateur radio satellites. Finally, many thanks to you, for taking a chance and buying my book.

ACRONYMS Amateur radio, commercial radio, and the satellite operator’s world are chock full of commonly used acronyms and TLAs (three letter abbreviations). They can be very confusing and frustrating for newcomers. I have tried to expand out acronyms and explain abbreviations the first time that they are used, but some like TV, FM, and SSB are in common use, so I didn’t bother. Near the end of the book, I have included a comprehensive glossary, which explains what the terms mean. My apologies if I have missed any.

DEDICAT IO N I dedicate this book to Elon Musk, a visionary who dreams of impossible things and turns them into reality. At a time when many Americans believe that the world is flat and that satellites are a hoax perpetrated by their own government instead of celebrating the achievements of their country, Musk put a car into space and sent it out past Mars to the Asteroid Belt! With Bowie on the stereo and “Don’t Panic” on the SatNav, Elon’s SpaceX team capped a truly remarkable achievement with a touch of humor. I can’t wait to see what he does next.

PREFA CE When I was four my mother bought me a book titled, ‘You will go to the Moon.’ I have given up waiting for the call from NASA, but I have remained interested in space and science fiction ever since. I guess it was natural for me to combine those interests with my passion for amateur radio and have a go at satellite operation. I started a quite a few years ago by beginning to set-up a station for operation on the AO-40 satellite. I decided to concentrate on getting the receive side going first so I purchased an 800 mm dish, made a patch feed and got a, ‘modified for Sband,’ California Microwave down converter. Unfortunately, by the time that I got all this together, the AO-40 control station was having problems with the satellite and they turned off the S-band passband. I could hear the beacon but there were no QSOs (conversations). Then, early in 2004, disaster struck and the satellite failed completely, so it was back into the garage for all of the satellite gear. Years later, my interest was rekindled when I downloaded some APRS (automatic packet reporting system) software from the Internet. I had fun scaling maps and plotting station beacons from all around New Zealand. Then my thoughts turned to the International Space Station (ISS). I tried sending APRS beacons through the ISS, and then I tried the packet store & forward system. It all worked fine, but once it was achieved, it quickly lost its appeal. I could see my APRS beacon plotted on the map with a few others from New Zealand and occasionally Australia. I could retrieve messages that I had stored on the ISS bulletin board on the previous orbit, but I was not actually communicating with anyone. So, I moved on to the ‘analog’ satellites, working AO-07 and RS 11/12, which I found much more interesting. These days there are many small satellites in orbit providing a great range of activities for people interested in talking to other amateur radio operators via satellite or participating in the scientific and educational programs by sending the reception reports or information received from the satellite. You can chat with an astronaut or receive pictures from the International Space Station. Working satellites does not need to be expensive; it can be achieved with a hand-held radio and a small homemade handheld Yagi antenna. It’s a new challenge and a load of fun!

CONTENTS Introduction Amsats and Hamsats Key factors for working satellites Working the ‘Easy Sats’ Working satellites with linear transponders Working digital satellite transponders Working the big satellite “Mistakes, I’ve made a few” Quick quiz questions Visible satellites Listening to satellite transmissions New satellite proposals Maidenhead grids IARU satellite coordination WRC-15 Satellite bands Satellite transponders and repeaters Satellite deployment Miscellaneous facts The FUNcube satellites Weather satellites The International Space Station Link budgets Coaxial feeder cables Masthead preamplifiers Antenna systems Quick quiz questions Doppler shift Satellite prediction and tracking software CAT (CI-V) control of your radio Automated rotator control Types of orbit Space and orbits Quick quiz questions Understanding TLE files Satellite pass prediction AMSAT satellite designations Satellite history Amateur radio on manned space missions Formulas Vector mathematics Glossary of abbreviations and acronyms Drawings and images Online resources and videos

Index About the author

INTRODUCTION This book provides a broad overview of how to communicate through amateur radio satellites and how to receive signals from other small satellites and ‘weather’ satellites. It also answers questions like, “how do satellites stay in orbit” and “why are they so expensive to launch?” There are sections about the history of amateur radio satellites, the mathematics governing orbits, different types of satellite and their orbits. I cover the equipment you need, to track and use the amateur satellites and some of the satellite tracking software that is available. A lot of readers will want to get straight into receiving or transmitting, so I have put the operating sections near the beginning and followed up with the technical and general interest chapters. ‘Working’ the low orbit satellites can be done relatively cheaply. You can talk through some of the satellites with as little as a dual-band handheld FM (frequency modulation) radio and a homemade handheld Yagi antenna. Interest in amateur satellites is increasing because of the large numbers of ‘Cube,’ ‘Micro,’ and ‘Pico’ satellites being launched into space. Listening to signals from satellites or sending signals through them is a challenging and rewarding part of the radio hobby. Unfortunately, the prohibitive cost of getting satellites into high orbits means that we don’t currently have the capability to make transcontinental calls via amateur satellite. But after a long wait of twelve years, this is about to change. There are exciting proposals for amateur radio payloads to be deployed into a geostationary or geosynchronous orbit as hitchhikers on large commercial satellites and two separate proposals for amateur radio to be included on satellites destined for orbits around the Moon. There is a long history of amateur radio being used to communicate with manned space missions. The MirEX1 / SAFEX1 program controlled amateur radio communications with the Mir1 space station and the SAREX1 program scheduled amateur radio communications with the crew of the Space Shuttles. If you are lucky, you might have the opportunity to talk to an Astronaut or Cosmonaut on the International Space Station. The program is called ARISS (Amateur Radio on the International Space Station). There are transponders on the space station for APRS and occasionally voice traffic. They sometimes transmit photographs using the SSTV modes. The amateur radio equipment aboard ISS is primarily used for educational outreach. ‘ARISS lets students worldwide experience the

excitement of talking directly with crewmembers of the International Space Station, inspiring them to pursue interests in careers in science, technology, engineering and math, and engaging them with radio science technology through amateur radio.’ (Note 1: see glossary).

AMSATS AND HAMSATS Some small satellites are built by amateur radio operators, but the majority are built by non-commercial teams at universities, technical colleges, clubs, or schools. Their projects usually have educational and scientific goals, although some art projects have been flown. The satellites they build usually carry at least one science experiment or they facilitate a research project. Some satellites have cameras to relay images from space and they normally have radio equipment that operates on VHF or UHF frequencies. The radio transceivers on the satellite are used to receive control signals from the ground ‘control’ station and to send flight data about the health of the spacecraft and mission data from the experiments back to Earth. Amateur radio operators often contribute by building the radio communications, computer, and power systems. I call these satellites Amsats because they are constructed by amateur teams. Even though these satellites may carry radios that operate on amateur radio frequencies, they are not primarily designed for use by the wider amateur radio community. A few of the amateur built satellites carry transponders or repeaters that operate on amateur radio frequencies. These satellites can be used by amateur radio operators to communicate with other ’hams.’ I call them Hamsats. The key difference is that these satellites have uplinks that are open for any licensed amateur radio operator to send a signal up to the satellite and have it re-transmitted back to Earth on the downlink frequency. Many of the Hamsats also carry science experiments and they are often built in conjunction with university teams. These days it is very expensive to get a satellite launched and free rides into space are rare. Universities and similar organizations can justify the expense but it is beyond the reach of most amateur radio organizations. To get funding for a launch, or backing from NASA (National Aeronautics and Space Administration) to have a satellite deployed from the International Space Station for free, you have to demonstrate a solid educational or scientific proposal. We are lucky to be able to get amateur radio transponders onto a few of the CubeSats and there is some scope for ride-shares on commercial satellites or the experimental programs of commercial operators. Not every amateur satellite enthusiast is an amateur radio operator intending to communicate with other ‘radio hams’ through the Hamsats. Many hobbyists are interested in receiving signals from satellites. You can

receive pictures from weather and commercial TV satellites, listen to ‘radio hams’ using the Hamsats, receive files from the Outernet, or decode the telemetry data from the Amsats. Some people like to receive signals from the big commercial satellites, mostly TV from the geostationary TV satellites and weather maps from the NOAA polar orbiting weather satellites. A few snoop on signals from commercial, navigation, and military satellites, but this is quite difficult and it is outside the scope of this book. Another large group of satellite enthusiasts is interested in the visible observation of satellites. There are websites devoted to ‘visible satellites,’ which will assist you with locating satellites in the sky and the best time to look. The International Space Station is very big and it can be seen quite easily. Using binoculars, you can even see it has a roughly square shape. You can see sunlight reflected from satellites that are in low or medium Earth orbits. They look like tiny stars traveling in oddly straight lines across the sky. Their motion looks quite different to observing a meteoroid or ‘shooting star,’ which is an object (rocks, dust, or possibly a dead satellite or space junk) that is burning up as it falls from space into the Earth’s atmosphere. Amateur astronomers are sometimes interested in using their small telescopes for satellite spotting. Others hate satellites because sunlight reflected from them causes annoying streaks across the images of star clusters and galaxies that they are trying to photograph. Photographers use long exposure times to capture the faint light from stars. During the exposure time, the satellite moves creating a line across the image.

The best place for satellite spotting is out in the countryside away from city light pollution. The best time is just after sunset or just before dawn when you are in darkness or low light, but the satellites still in full sunshine. Some satellite users are not even interested in satellites! Timing signals from the GPS (global positioning system) satellites are often used as accurate clock references for amateur radio transceivers and microwave transverters. APRS enthusiasts use the GPS satellites to provide the location of their vehicle, which is included as a latitude and longitude or Maidenhead grid reference into their APRS ‘beacon’ transmissions. It is quite difficult to pin down how many Amsats and Hamsats are up in space at any given time. The recent launch of ‘PSLV-C37’ a ‘Polar Satellite Launch Vehicle,’ by the Indian Space Research Organization deployed a record 104 new CubeSats. Mineo Wakita’s list of amateur and amateur radio satellites, for 7th March 2017, shows 98 active satellites and 182 inactive but still in orbit. The number of active satellites changes all the time. Some of the satellites that are launched don’t work, some only last a few days or months and some don’t

transmit signals on the amateur radio bands. Most missions have a team website and that is often the best place to get the latest information. One of the big problems that you will encounter is that many of the satellite websites are out of date and have not been updated for years. The satellite fails, reaches the end of its mission, or re-enters the atmosphere, but the website lives on. The Google Group at has a wealth of up to the minute satellite related news including the weekly AMSAT news bulletin. AMSAT (Amateur Satellite) is a group of amateur radio satellite organizations worldwide. AMSAT groups launch some of the amateur radio satellites and assist with coordination of radio frequencies and equipment for many of the satellites built by radio clubs, universities, and other groups. Amateur radio operators use the transponders, repeaters, and digipeaters on Hamsats to communicate with other operators. Different satellites support different voice or data modes. At the moment, there are four FM repeater satellites, twelve satellites with linear transponders and four satellites with digipeaters. The AMSAT-NA Satellite Status page shows up to date status reports from amateur radio operators who are using the Hamsats. See In this book an Amsat is an amateur built satellite, which may have radios using amateur radio frequencies for control and telemetry purposes. It does not have uplinks and downlinks for general amateur radio use. A Hamsat is an amateur built satellite that carries at least one radio transponder or repeater that allows amateur radio operators to communicate with other radio ‘hams’ via satellite. It must have at least one uplink and downlink channel for general amateur radio use. AMSAT (capitalized) refers to any or all of the AMSAT groups worldwide. AMSAT’s goal is to foster Amateur Radio’s participation in space research and communication.

KEY FACTORS FOR WORKING SATELLITES OK, I know that many readers are itching to get into the practical part, so let’s get started. In the next few chapters, I cover using the amateur radio Hamsats to make amateur radio contacts. Then there are chapters on visible satellites and receiving signals from commercial satellites. In order to successfully receive signals from or send signals through a satellite, you need to know some important information. Low orbit satellite passes only last ten to fifteen minutes. You will be busy so you need to plan ahead. Before heading outside with your radio and an antenna, you need to do some research to find out when the satellite will be available and you need to find a practical and safe location to operate from. Until you set up a permanent home station, you will probably be wandering around waving a small handheld antenna in front of you. It is difficult to do that and work the radio while still remaining aware of your surroundings. Make sure that there is no risk of stumbling into obstacles, traffic, people, animals, water, or tripping hazards. It is a good idea to have a helper on hand. You can work as a team with one person managing the antenna while the other manages the radio and makes the calls. Once you have, your gear prepared and tested and you have found a good location. There are seven key factors that need to be considered. 1. You need to know the time when the satellite will rise above your local horizon. Satellites receive and transmit on VHF, UHF, or SHF frequencies, which limits satellite operation to ‘line of sight’ communication. To receive a signal from the satellite, or to send a signal to it, it must be above your horizon otherwise, the radio signal will be blocked by the ground. This is colloquially referred to as being able to ‘see’ or ‘hear’ the satellite even though most satellites are much too small to see and they make no noise other than through your radio’s speaker or headphones. The time when the satellite will rise above the horizon is called the AOS (acquisition of satellite) time. The period when the satellite is above the horizon and you can communicate through it is called a ‘pass.’ 2. You also need to know when the satellite will set below the horizon at the end of the pass. There is no point in continuing to listen for signals from a satellite when it is no longer in range. The time when the

satellite will set below your local horizon is called LOS (loss of satellite). 3. Assuming that you are using some kind of directional antenna, you need to know where to point it. You have to know the point on the horizon where the satellite will rise and the path it will take across the sky to the point on the horizon where it will set. Tracking the satellite’s path across the sky involves two numbers that change as the satellite passes over. They are the ‘azimuth,’ which is the horizontal bearing with respect to ‘grid’ or ‘true’ north, and the ‘elevation.’ The elevation is how high above the horizon the satellite is. Azimuth and elevation are both measured in degrees. Azimuth bearings range from 0 to 360 degrees. As you, turn your antenna towards the right, ‘east of north,’ the azimuth increases. North is at 0O, East is at 90O, South is at 180O, and West is at 270O. Grid north is the direction marked as ‘N’ on a map or displayed on your GPS navigation unit. It is different to magnetic north as indicated by a magnetic compass or the compass app on your smartphone. The elevation bearing to the satellite is a number between 0 and 90 degrees. When your antenna is pointing towards the horizon, it is at 0O elevation. When it is pointing straight up, vertical, it is at 90O elevation. Many rotators are able to continue the elevation drive past vertical and back down to horizontal with the antenna facing 180 degrees away from the starting azimuth. In that case, the elevation is stated as being between 90 and 180 degrees. A negative elevation reading displayed on your satellite prediction program indicates that the satellite is below your horizon. You won’t be able to send or receive signals to or from it. If the negative number is decreasing, the satellite is heading towards AOS and will rise above the horizon when the elevation changes from a negative to a positive number. If the negative number is increasing, the satellite has already passed over. Many people manage antenna pointing by hand, holding the antenna, or mounting it on a small tripod. More advanced satellite ground stations use a computer controlled satellite tracking system. If you are holding the antenna, you are using the ‘Armstrong’ method. You work out where the satellite will rise and where it will set. Point the antenna to the point on the horizon where the satellite will rise, until you can hear a signal on the satellite downlink frequency. During the pass, follow the satellite’s path by listening to the signal. If you get too far

ahead or too high, the signal will fade or disappear. Most satellite passes are not directly overhead so usually, you will face in the same general direction for the complete pass. It is called the ‘Armstrong’ method because you need strong arms. 4. You should use a phone application, a computer program, or an online resource to find the AOS time, pass duration, azimuth heading at AOS and at LOS, and the peak elevation, for the next few satellite passes. I have included a chapter about satellite prediction and tracking programs later on. To get accurate satellite prediction information you need to know the latitude and longitude or the Maidenhead grid locator code for your operating location. You will also need an accurate clock, preferably displaying UTC (coordinated universal time). A mobile phone is ideal because the time is set by the cellular network provider. If you decide to use local time, make sure that you have allowed for daylight saving. There is no point being an hour late or early. Note that satellite prediction programs always use ‘grid’ or ‘true’ north. That is the ‘north’ that is represented on maps. Magnetic north is the direction that magnetic compass point, this includes the ‘Compass’ application on your smartphone. The direction of magnetic north is affected by variations in the Earth’s magnetic field. It is not used by satellite prediction programs because the relationship to ‘true north’ varies across the Earth’s surface. Along the length of New Zealand, magnetic north ranges from 18 to 26 degrees offset from true north. True north is the same everywhere. 5. If you want to communicate with a specific station via a satellite, you will need to check that you can both ‘see’ the satellite at the same time. The easiest way to do that is to look at the satellite footprint displayed on an image of a map by a satellite tracking program. For example, the SatPC32 tracking program can be set to ‘manual mode’ to show you the footprint at any time. You can step advance the time and discover the time in UTC or local time when the satellite will be available at both locations. A satellite footprint is an area on the surface of the Earth that can be reached by the satellite’s antennas. The diameter of the satellite’s footprint tells you the maximum distance that you can communicate via the satellite. The footprint is smaller for satellites that are closer to Earth. If you want to work a station that is a long way away, you need a high satellite. To

demonstrate this effect, shine the light from an electric torch directly at a basketball. Hold the torch a couple of inches from the ball. The torch bulb is the satellite. The small circle of light on the ball is the footprint. Move the torch away from the ball and the footprint circle gets bigger. As you continue to move the torch further away, eventually the light will cover an entire side ‘hemisphere’ of the ball. The International Space Station is in a low Earth orbit (LEO) around 400 km above the ground. At that height, it has a footprint about 4,400 km across. Stations within the footprint can communicate via the satellite. Typical LEO satellite orbits range from 400 to 1500 km in altitude, so their footprints can be anywhere from 4,400 km, to 8,000 km. Satellites in very high orbits can cover an entire hemisphere (side of the Earth). 6. Next, you need to pick the frequencies and mode of operation. All Hamsats and most Amsats use frequencies in the amateur radio bands. Nearly all of the VHF, UHF and microwave amateur radio bands have internationally agreed on ranges of frequencies called ‘satellite sub-bands’ that have been specifically allocated for satellite operation. Weather, TV, telephony, and GPS satellites use commercial frequencies outside of the amateur radio allocations. Most amateur radio satellite operators start with satellites that have a repeater for FM voice. These work much like your local ham radio hilltop repeater except instead of having a 600 kHz or 1 MHz split between the receive and transmit frequencies they operate ‘cross band.’ Transmitting on one amateur radio band and receiving on another. Other Hamsats are set up for SSB (single sideband), SSTV (slow scan television), APRS (automatic packet reporting system), or digital data modes. There is even a Hamsat with a D-Star (digital voice mode) repeater. 7. You need to know how much Doppler shift will affect the signals that you are receiving or transmitting. You can pre-program your radio channels, use a lookup table, or read the information from a satellite tracking program. Some tracking programs use CAT (computer-aided transceiver) computer control to change your radio’s receiver and transmitter frequencies automatically. Icom calls the CAT control on their radios ‘CI-V’ (communication interface version five). Satellites in low orbits travel faster than satellites in higher orbits. The higher relative speed between the satellite and your location creates more Doppler shift on the radio signals. Satellites in high orbits travel

more slowly so there is less Doppler shift. Higher frequencies are more affected by Doppler shift than lower frequencies are. Because of these two factors, most LEO (low Earth orbit) satellites use frequencies in the VHF and UHF bands. High orbit satellites travel slowly enough to allow the use of microwave frequencies. Adjusting for Doppler shift is much easier when you use FM modulation. FM receivers are naturally tolerant of frequency errors. With SSB modulation, the receiver must accurately track the downlink signal, because any frequency errors affect the audio frequency of the resulting audio, making it sound squeaky or distorted. When you use an SSB transponder, your radio frequencies must be continuously corrected for Doppler shift. These seven factors can be summarized to, “when where and how.” The when and where information can be provided by a phone app, a web page, or a satellite prediction program. To use the prediction information successfully you need to know the time reasonably accurately. The time displayed on your cell phone is good because cell phones are synchronized to the ‘network time.’ Many people use UTC time to avoid being confused by different time zones and daylight saving. Getting the current time correct is important. An error of one minute can cause a 30 degrees error in the direction that you point your antenna. That could make the difference between having a successful contact and receiving nothing. Satellite web pages such as the AMSAT websites can provide information about transponder frequencies and availability of the satellite. But the information is fixed in time. Tracking programs display the changing Doppler shift, range, azimuth, and elevation throughout the pass, while it is happening. If you have configured CAT control, the tracking program can also adjust your radio(s) to the right frequencies. Tracking the satellite’s motion and correcting your receiver and transmitter frequency for Doppler shift can be done manually and many people do it that way, especially if they are operating the FM voice satellites using a hand-held antenna. However, talking on SSB through a satellite with a linear transponder, while correcting the uplink and downlink Doppler shift, and steering the antenna, at the same time can be quite difficult. I never seem to have enough hands. Manual tracking certainly adds an element of difficulty for beginners struggling to manage a conversation (QSO) in a short time frame with an unfamiliar setup. Luckily, there are many satellite tracking programs available. They can manage all of these factors so that

you can get on with making calls or listening to the data from the satellites. If you are using a home based station with antenna rotators and a desktop radio, computer control is a great investment. Most FM voice operation is done using pre-programmed channels and handheld antennas. SSB and digital mode operation are more commonly done using home stations or portable setups with tripod mounted antennas and computer control of the radio frequencies. Most LEO satellites use frequencies in the 2m and 20cm bands. UV mode satellites have an uplink on the 70cm amateur band and a downlink on the 2m amateur band. VU mode satellites have an uplink on the 2m amateur band and a downlink on the 70cm amateur band. The next three chapters are a guide to getting started with the amateur radio Hamsats. The first includes sections on picking a suitable FM voice mode satellite, working out what equipment you will need, and how to set it up for satellite operation. The subsequent two chapters build on that experience to include operating through satellites with linear transponders and digipeaters.

WORKING THE ‘EASY SATS’ This chapter is about how to use your amateur radio transceiver to send signals through amateur radio ‘Hamsats’ to communicate with other amateur radio operators. You must be a licensed amateur radio operator to transmit on the satellite uplink frequencies. You don’t need a license to receive the downlink signals from the satellite. The easiest satellites, or at least the ones that require the least investment in equipment, are the ‘FM’ satellites. To work them you need a dual band FM 2m / 70cm handheld radio, or separate radios for the 2m and 70cm bands. You can use a dualband mobile or a VHF/UHF base station radio instead of a handheld. You may need a diplexer to combine two antennas onto a single coax feeder cable for the radio. Or possibly to combine two radios on to a single antenna. Finally, you will need an antenna for each band or a dual-band antenna. The minimum practical requirement for an antenna is a handheld ‘Crossed Yagi’ or ‘Log Periodic’ antenna with three elements on the 2m band and five elements on the 70cm band. These are easy to make on a boom made of plastic or wood. Alternatively, there are commercial models available. The most favored commercial antennas are the Arrow II LEO satellite antenna, the Alaskan Arrow, and the Elk 2M/440L5 log periodic antenna. The Arrow antennas are named after the lightweight aluminum arrow shafts they use as antenna elements. The ‘Arrow II’ antenna has three elements on 2m and seven on 70cm. The ‘Alaskan Arrow’ has four elements on 2m and ten elements on 70cm. See Arrow antennas are available from the manufacturer, amateur radio dealers, and the AMSATNA or AMSAT-UK web store. The Elk 2M/440L5 antenna is a five element log periodic antenna. You get dual-band operation with a single antenna connector without the need for a diplexer. The antenna is flat meaning that you may not have to disassemble it to get it into your car. Elk antennas are available from the manufacturer, amateur radio dealers, and the AMSAT-UK web store. To make your own antenna search on ‘building an Arrow antenna’ or ‘build your own Elk antenna’ etc. There are dozens of websites with suitable designs. is a good place to start.

You can even make antennas by cutting up a metal tape measure for the elements. The elements can be rolled up for storage making this type of antenna very easy to pack up and carry. TIP: The SMA antenna connector on most handheld radios is small and not very robust. To avoid the possibility of damaging the antenna connector, some form of strain relief such as a loop of coax taped to the radio is recommended.

ALTERNATIVE ANTENNAS You might be able to hear the satellite, when it is at high elevation, i.e. relatively close to you, using the ‘rubber duck’ aerial that came with your handheld radio. But that kind of antenna really does not have enough gain, so reception will be difficult at best. However, it is likely that your transmitted signal would reach the satellite. Please do not transmit if you haven’t already proved that you can hear the downlink signal. An FM satellite can only be used for one conversation or ‘QSO’ at a time. Transmitting when you cannot hear the downlink may interfere with other people’s ability to use the satellite and doing that can make you rather unpopular. More advanced stations might use a small Yagi or log periodic antennas mounted on a tripod or a mast. Perhaps with an azimuth and elevation rotator. Home stations typically use longer (higher gain) antennas, masthead amplifiers, computer controlled azimuth and elevation rotators and crossed, or circular polarized antennas. Fixed antennas like J-pole, discone or collinear antennas don’t perform very well because they have deep signal nulls at high elevations. But, if that is all you have, they can be used. Fixed antennas should preferably be mounted in a location where they are able to cover the whole of the satellite pass. Better fixed omnidirectional antennas, with circular polarization, include the Turnstile, Lindenblad, and ‘Eggbeater’ antennas. The ‘Turnstile antenna’ is a pair of crossed dipoles mounted horizontally, usually with a reflector below the two driven elements. The Eggbeater II and the Texas Potato Masher (TPM) are variants of the eggbeater design. The Eggbeater II is an improved omnidirectional antenna. The Texas Potato Masher is somewhat directional but it can be mounted pointing upwards as an omnidirectional antenna. The Quadrifilar helix is a circular polarized antenna favored by weather satellite enthusiasts. They are quite tricky to build and tune, but they work well. Helix, patch, and Yagi variants like the Loop Yagi and the Quagi can be used for satellite work. For frequencies above 1000 MHz, most operators use patch, dish, horn, or helix antennas. For LEO satellites, most home stations use separate 2m and 70cm Yagi antennas mounted on a non-metallic ‘cross beam.’ They use an azimuth rotator or better still, both azimuth and elevation rotators.

Linear polarized Yagis may be mounted in either a horizontally or vertically polarized configuration. It is common for the 2m antenna to be mounted with horizontal polarization and the 70cm antenna to be mounted with vertical polarization. This ‘cross polarization’ minimizes RF (radio frequency) coupling between the antennas. Some people mount their antennas at opposing 45-degree polarizations. Serious satellite stations employ crossed Yagis on both bands, with switched polarization, or circular polarization. Kent Britain WA5VJB has a homebrew antenna design for a low cost dualband Yagi on his excellent website at The file is at, If you are looking for a commercial solution, AMSAT-NA is selling the M2 Antenna Systems ‘LEO pack’ at a discount to AMSAT members. It includes a 16 element (8x8) circular polarized 70cm Yagi, an 8 element (4x4) circular polarized 2 m Yagi, and a cross beam. It is also available from M2. To ‘work’ satellites that carry L band (1260 MHz) or S-band (2400 MHz) transponders, you will need a dish, patch, or helix antenna. However, this kind of operation does not fall into the category of ‘working the Easy Sats.’

MINIMUM EQUIPMENT LIST FM radio(s) for 2m and 70cm bands (preferably radios that can be programmed with 5 kHz channel spacing or less) Handheld or tripod mounted antenna, a few meters of coax cable, and a diplexer (if required) A headset or earphones can be handy to avoid distractions An external microphone is very handy, or a ‘speaker-mic’ A portable folding table if you are using a mobile or base station radio A DC power supply if you are using a mobile or base station radio Spare battery for handheld radios Protractor, preferably a full circle, rather than a semi-circle. As big as you can find in your local school supply stationery shop. It should only cost a couple of dollars A magnetic compass, cell phone application, or map of the area Patience. Don’t expect perfect results the first time. Satellite operation takes practice.

PREPARATION FOR ACTION OK, you have assembled the necessary gear. A radio (or two), a diplexer (if required), some coax and a dual-band antenna. Check that it all works. There will not be time to make changes during the 10 - 15 minute satellite pass. Now you have to choose a satellite and set up your radio(s) with the right frequencies. You can use an online prediction program to find the time and pass information. See the section on ‘satellite prediction and tracking software’ for some useful website links, or Date (UTC)



AOS Maximum Max El LOS Azimuth Elevation Azimuth Azimuth


26 Sep 16








26 Sep 16








26 Sep 16








26 Sep 16








You can see from the satellite prediction table above that there will be a pass with a good maximum elevation, starting at 05:12 UTC. Later the same day there is another pass with a much lower maximum elevation and two passes that are unusable because the satellite will only rise to two degrees above the horizon for a few minutes. The table has the azimuth heading to point your antenna at when the satellite rises above the horizon at AOS time. It also lists the time and azimuth heading where you will lose the satellite. UTC = universal coordinated time. AOS = acquisition of satellite. It is the time the satellite will rise above your local horizon. Duration is the number of minutes that the satellite will be available (i.e. above the horizon). AOS Azimuth is the horizontal bearing towards the satellite at the time that it is expected to rise above the horizon, i.e. the direction you should point your antenna when you are waiting for the satellite to come ‘up.’ Azimuth is always expressed in degrees east of true north, (not magnetic north). The range is 0 to 360 degrees.

Maximum elevation is the highest above horizontal that the satellite will get in the sky during the satellite ‘pass.’ (Range 0 to 90 degrees). Maximum El Azimuth is the bearing (east of true north) to the satellite when it is ‘mid path’ and at its highest point. It is the direction you should be pointing your antenna at the middle of the satellite pass. At that point, the antenna should also be pointing upwards at the maximum elevation. (Range 0 to 360 degrees). LOS Azimuth is the bearing towards the satellite in degrees east of true north that the satellite will be at when it sets below the horizon at the end of the pass. It is the direction you should end up facing when the satellite pass has finished. (Range 0 to 360 degrees). LOS (UTC) ‘loss of satellite,’ is the time that the satellite will set below your local horizon. In this case, it is UTC time, not ‘local’ time. Find out the Maidenhead grid square for your operating location. Your ‘grid’ is one of the pieces of information that is commonly swapped during a satellite conversation or ‘QSO.’ There is more about Maidenhead grids later in the book. Look it up at

CHOOSING A SATELLITE The AMSAT-NA website lists amateur radio satellites that have transponders, under the heading of ‘Communications Satellites.’ The site currently lists three FM satellites, fourteen satellites with linear transponders, and five digipeaters. Some of the data may be out of date. Other information websites listing satellites include; The AMSAT-NA ‘Current Status’ page shows activity reports supplied by amateur radio operators. Visiting the website is a very good way to find out which satellites are active. SO-50 (SaudiSat 1-C) is a VU mode 1Unit CubeSat with a 250mW transmitter. It requires a 67.0 Hz pilot tone on your transmissions. It also has a 10 minute timer to avoid being hung up by interference for long periods. To wake the satellite up for ten minutes you, or someone else must transmit a 74.4 Hz pilot tone for about 2 seconds. SO-50 uplink 145.850 MHz - downlink 436.795 MHz. AO-85 (Fox-1A) is a UV mode 1U CubeSat. It has an 800mW transmitter. Its uplink is different to the figures published before launch. It is about 8 kHz lower than planned on about 435.172 MHz due to an unforeseen temperature in the satellite. The satellite requires a 67 Hz pilot tone to activate a one-minute timer. AO-85 (Fox-1A) UV mode uplink 435.172 MHz - downlink 145.979 MHz. LilacSat-2 (CAS-3H) is a VU mode satellite. The FM repeater is only active three days a week. Its uplink is out of the 2m satellite sub-band, but it is legal to use the frequency on a non-interference basis to other 2m users. Please listen near the uplink frequency before using the channel. Remember that you will be using the uplink frequency ± about 3.5 kHz. LilacSat-2 also has an APRS digipeater, a linear transponder, and a CW beacon. CAS-3H uplink 144.350 MHz - downlink 437.200 MHz. IO-86 (Lapan-A2 / Orari) is a UV mode CubeSat. It requires an 88.5 Hz pilot tone. Unlike almost all CubeSats, IO-86 is in an equatorial orbit with an inclination of 6O. To use it you will have to be located at a latitude between 30 degrees north and 30 degrees south. IO-86 uplink 435.880 MHz - downlink 145.880 MHz.

AO-91 (Fox-1B) UV mode uplink 435.250 MHz - downlink 145.960 MHz. AO-92 (Fox-1D) UV mode uplink 435.350 MHz - downlink 145.880 MHz.

PROGRAMMING THE FREQUENCIES We expect there to be up to ± 10 kHz of Doppler shift on the 70cm frequency and up to ± 3.5 kHz on the 2m frequency. Ideally, we would have the radio change frequencies continuously to account for Doppler shift during the satellite pass. Unfortunately, most amateur radio handhelds can only be programmed with fixed channels. Fortunately, since we are using FM this is not a ‘deal breaker’ because FM is tolerant of quite large frequency errors. The pre-launch published frequencies for some satellites, e.g. AO-85, sometimes prove to be inaccurate once the satellite reaches orbit. This is because the temperature in the satellite is often different to what was planned. Use the Internet to check the frequencies for the satellite that you wish to use. If your radio supports 12.5 kHz channel spacing, you should program three channels. The 2m frequency does not require Doppler correction because the expected Doppler shift is less than 5 kHz. Set the first channel to the nominated 70cm frequency + 12.5 kHz, the second channel to the nominated frequency, and the third channel to the frequency - 12.5 kHz. As the pass proceeds, switch from channel one at the beginning of the pass, to channel two in the middle of the pass and then to channel three late in the pass. Table 1: programming chart for LilacSat-2 with 12.5 kHz spacing Channel Transmit Receive 1 144.350 MHz 437.2125 MHz 2 144.350 MHz 437.2000 MHz 3 144.350 MHz 437.1875 MHz Some handheld radios support 5 kHz channel spacing, in which case you can program five channels with the 70cm frequency incrementing in 5 kHz steps. Again, all of the channels are set to the same 2m frequency. Table 2: programming chart for LilacSat-2 with 5 kHz spacing Channel Transmit Receive 1 144.350 MHz 437.210 MHz 2 144.350 MHz 437.205 MHz 3 144.350 MHz 437.200 MHz 4 144.350 MHz 437.195 MHz


144.350 MHz

437.190 MHz

In the event that you can program channels in 1 kHz steps. I would still use five channels, but I would add Doppler correction to the 2m frequencies. You could also set the 70cm frequencies to increment in 4 kHz steps. Table 3: programming chart for LilacSat-2 with 1 kHz spacing Channel Transmit Receive 1 144.348 MHz 437.208 MHz 2 144.349 MHz 437.204 MHz 3 144.350 MHz 437.200 MHz 4 144.351 MHz 437.196 MHz 5 144.352 MHz 437.192 MHz However, I expect that most radios with a channel spacing of 1 kHz or less would also have CAT control, in which case you can use computer control rather than having to pre-program the channels. Pilot tones: if the satellite requires a pilot tone to keep the squelch open during your transmission, then you have to program the pilot tone onto each of the transmit channels. It is not usual to program the pilot tone on the receive channels because it could desensitize the receiver squelch on your handheld radio. In fact, most people don’t use receiver squelch at all, preferring to leave the receiver ‘open’ so that you can hear even the faintest of signals from the satellite. Some satellites, currently SO-50, need a different tone to activate a timer before you start using the satellite. In that case, program an additional channel with the same frequencies as channel one, but with the ‘timer start’ 74.4 Hz pilot tone. Squelch is the system that turns off the receiver’s audio output when there is no input signal being received. It stops you having to listen to noise when there are no signals.

PICK A GOOD OPERATING SITE You need a site where you can ‘see’ (follow) the satellite throughout its pass across the sky. Close obstacles such as buildings will block the radio path and your conversation will fail. Trees in the foreground can also be a problem. Mountains and distant objects are less important because they will not affect the radio path once the satellite is above a few degrees of elevation. Basically, if you don’t have a clear line of sight path, you will not be able to work the satellite. Find out where ‘true’ or ‘grid’ north is. You can use an application like Google Earth, a mapping program, a phone app, or even the ‘Sat Nav’ in your car. It is best to identify a landmark or place a marker on the ground. You can use a magnetic compass to find magnetic north and add or subtract the ‘magnetic declination’ between magnetic north and grid north. There is likely to be around 20 degrees, of difference between true north and magnetic north. The north point does not have to be super accurate. The small antennas that you will be using have a wide beam angle so a few degrees of error will not be noticeable. Besides, if you are hand holding the antenna you will be waving it all over the place. But it is nice to be starting with the antenna pointing in the right direction. Use the satellite prediction table and the protractor to work out where the satellite will rise at the AOS time and where the satellite will set at the LOS time. Work out how high to point the antenna in the middle of the pass when the satellite is at peak elevation. This practice will make it much easier to find the satellite and to track it by hand. Remember that the satellite prediction program is probably using UTC ‘Universal Coordinated Time.’ You will need to convert the UTC time to your local time. Your phone will probably have a ‘world time’ application that can show UTC and local time. Make sure that you allow for daylight saving time if it is in effect. You won’t be able to work the satellite if you are an hour late, or an hour early! Make sure that your watch is accurately set, or use the clock on your mobile phone. Be ready and listening, with the antenna pointing the right way a minute before the AOS time. Listen to make sure the satellite is not already in use. Listen again and if the satellite is free, you can make your CQ call. Keep it short. “A2AA CQ A2AA” is enough. If nobody comes back and nobody calls another station, you can call again. Alpha codes are a distraction and too long for an FM satellite, just use your call letters.

Or you may choose to respond to another station’s CQ call or an invitation such as “listening for any callers.” The format is “K2DW this is A2AA.”

SAFETY TIP: To avoid exposure to possibly dangerous levels of RF (radio frequency) radiation, it is strongly recommended that you do not exceed 10 W of transmitter power when using any handheld antenna.

FIRST SATELLITE CONVERSATION “QSO” You should listen to a couple of complete satellite passes before attempting to transmit. It will give you a feeling for tracking the satellite across the sky when to switch channels and the way that satellite conversations are carried out. The polarization of the signal from the satellite is unpredictable and it will change as the satellite passes over. You can twist the antenna to change the polarization. Listen for the ‘quietest’ least noisy signal. Your ear is remarkably good at determining when the wanted signal sounds best and you will soon get the hang of twisting the antenna as well as moving it to follow the satellite. After you have had some practice following the satellite and you know what to expect to hear, you can make a CQ (“seek you”) call, or answer a call from another station. Don’t worry about making mistakes. We have all done it. The vast majority of satellite operators are tolerant and willing to help a ‘newbie.’ Just remember that many hams may be hoping to use the satellite channel during the pass. Conversations should be brief and to the point. This is not the local repeater. Remember that other stations will be able to ‘see’ the satellite before you can and they may lose the satellite before you do. It is pretty common to lose a contact, mid-conversation, for this reason. Get the formalities such as signal reports and names out of the way early. Don’t leave pauses, in your overs. While you have the satellite, keep up the pace, then drop off and let others have their turn. Other operators may have only just acquired the satellite and they may not have heard your previous ‘over.’ For FM satellites, I recommend using your call letters rather than the long alpha codes used on the HF bands. Satellite etiquette is more like using a repeater. Make your CQ calls short. I like the ‘two call signs one CQ’ method. For example “A2AA CQ A2AA.” It gives the other station all the information they need. They can tell you are looking for anyone rather than a specific station and they get two chances to catch your call sign. If you are calling another station or responding to their CQ call, “K2DW this is A2AA,” is enough to establish the contact. Listen to the pattern that the locals are using in your area. Satellite operators like to know the Maidenhead grid of the place that you are transmitting from so that they can locate your position relative to theirs. If you don’t know your ‘grid,’ look it up online before you start.

For your first attempt at transmitting, I recommend picking a pass with a maximum elevation of at least 50 degrees. High passes usually have stronger downlink signals and you will have more time to make a contact. Remember to change channels as the pass progresses. If you have three channels programmed, you need to change to channel two when the satellite is halfway to full elevation. Leave it on channel two as the satellite passes through peak elevation and then change to channel three when the satellite is halfway down to the horizon. If you have five channels programmed, change to channel two when the satellite reaches one-third of the maximum elevation, switch to channel three at two-thirds. Stay on channel three until the satellite passes through peak elevation and has descended to two-thirds of maximum elevation, then switch to channel four. When the satellite has reached one-third of maximum elevation select channel five for the remainder of the pass. ‘Full duplex operation’ means you can receive on one band at the same time as transmitting on the other. You can hear your own transmission via the satellite. ‘Half duplex operation’ is more usual for FM satellites. With half duplex, you can receive on one band and transmit on the other, but you can’t do both at the same time.

STEP BY STEP 1. Get all your gear together and check that it all works. There is no time during a satellite pass to fix anything. See ‘minimum equipment list’ above. Be prepared. Check out your location. Make sure that it is a safe place to operate. 2. Look at the direction where the satellite will rise at AOS time. And note a landmark such as “just to the left of the tree.” Note the direction where the satellite will reach maximum elevation and practice pointing the antenna at the right angle and direction. Finally, determine a landmark that will define where the satellite will set below the horizon at the end of the pass. 3. A minute before the satellite pass starts (AOS), point your antenna in the direction where the satellite will rise. Set your radio for channel 1 and listen. When the satellite comes into range, sometimes you can hear a very slight change in the noise even if there is nobody currently talking. 4. If you don’t hear anything by the time the satellite is up about 10 degrees, you can make a CQ call. If you do hear an ongoing conversation wait until it is finished, or a request is made for other callers. If you hear a CQ call, go back with a quick response. This should be only your call sign or their call sign followed by your call sign. No more than that! Just use your call letters, not phonetics. If the other station is having trouble getting your call, you can use phonetics for the second over. 5. If you make or respond to a CQ, make very sure that the other station is responding to your call and not someone else. It is very common to get a “double” where two stations call or respond at the same time. 6. Every minute or so, turn a little to follow the satellite path and raise the angle of your antenna a little. By mid pass, probably between five and eight minutes after AOS, you should be facing the mid-point azimuth and pointing your antenna at the max elevation. You should also have moved your radio to the middle channel. 7. You may need to twist the antenna to change its polarization. Listen to the signal. If it gets noisy, you are losing signal strength. You may need to adjust the direction that you are pointing the antenna, the elevation, or the polarization (twist). Or the low signal may be due to a foreground obstruction.

8. Keep talking, listening, or calling CQ. At mid-point, you should have a solid signal. The satellite is at its closest and high enough that there should be no objects blocking the path. 9. Keep overs short. Long overs are a recipe for losing track of the conversation. Remember that the satellite may be just coming up, or just about to set, at the other operator’s location. 10. Swap essential information first. Your name, grid reference, and QTH. I don’t bother with a signal report because it is pretty meaningless. The third over, if there is one, can be more conversational. Allow others into the conversation. Make it a three or four-way contact. Then get off the satellite and give others a go. With luck, someone might call you or the other station that you just worked. If no one jumps on within 15 seconds, feel free to call CQ. 11. Keep tracking the satellite down towards the LOS direction. Remember to change channels on the radio. It is probably not a good idea to start a new contact if you have less than two minutes left before you lose the satellite. If you are in a QSO try to warn the other station that you are close to losing the signal. That way they won’t be surprised when you drop out. 12. After LOS, breathe. You probably forgot to in all the excitement. If you managed to make a contact, congratulate yourself on a job well done. If you didn’t, don’t be discouraged. It will probably take two or three tries, to get everything working together. Check out the problem solving section then get set up for the next pass, or satellite. 13. Check out the videos listed in the Online Resources and Videos section.

PROBLEM SOLVING The most common problem is ‘losing the satellite’ or the contact by not following the satellite’s path with your antenna. It is a quite a handful operating the radio, adjusting the channels and maintaining a conversation all at the same time. It is not a bad idea to have another person do the antenna tracking for you, while you make the calls. Both operators need to be able to hear the downlink so you would need to arrange dual headsets or a speaker. As well as pointing the antenna at the satellite, you may need to twist the antenna to adjust the polarization. You should be able to hear the difference as the signal starts to move out of the antenna beamwidth and also if the polarization is causing a null in the signal strength. The signal will become noisier (hissy) as the signal strength reduces. Unfortunately, it is not possible to pre-determine the difference in polarization between your antenna and the satellite while you are transmitting, so it is very likely that a null will occur mid-transmission, and that could cause your uplink signal to drop out. Another typical problem is ‘operator error.’ It is quite common to get a bit flustered the first few times you transmit through a satellite. There is a lot to remember and the ten to fifteen minutes you have available passes amazingly quickly. Don’t worry! We have all made most of the mistakes that are possible. I have even forgotten my own call sign in all the excitement. Transmitting on the downlink frequency instead of the uplink is a good one, and mistaking your own signal on the downlink for another station is another. On linear transponders, you could transmit on the wrong sideband. At least you can’t fall into that trap when using the FM satellites. “I forgot to change channels.” This is very easy to do if the downlink is in the 2m band because you don’t hear the signal degrading the way you do if the downlink is in the 70cm band. If you forget to change channels, it is pretty likely that it will be the 70cm part of the radio path that will fail. If the satellite has a UV repeater, the 70cm uplink will eventually fall outside of the passband of the satellite and you will disappear off the satellite downlink, or your uplink channel might be hijacked by another uplink signal that is closer to the correct frequency. If the satellite has a VU mode repeater, the 70cm downlink will eventually fall outside of the passband of your radio unless you change channels to adjust for the Doppler shift. “I’m not hearing anything.” Wait for an hour or so for the next pass, or try a different satellite. Not all satellite passes are used. Down here in

New Zealand at the end of the world, any passes that don’t cover at least the east coast of Australia are unlikely to have any traffic. It can be quite good for testing but remember there could be someone listening. In more populated areas such as Europe and the USA, I expect that there would be traffic on most passes. There may be exceptions if it is late at night or the middle of a workday. If you still don’t hear anything you probably got the time of the pass wrong or the satellite may be inactive. Try to check. Find out if you can hear a local repeater to check your receiving system. You will almost certainly need some kind of ‘gain’ antenna. The rubber ducky antenna on your handheld probably won’t be good enough. If you are using full duplex, you should be able to hear yourself on the downlink while you are talking on the uplink. Power loss in the feeder cable can be a major problem for any weak signal reception. Use good quality coaxial cable and keep it short. A long cable run may work fine for receiving signals from a local 50 watt repeater. But the satellite could be 4000 km away and running 250 mW into a short piece of steel tape measure as an antenna. “Nobody hears me!” So long as you have the correct pilot tone and some sort of directional antenna, five watts of transmitter power should be enough to reach any of the FM satellites. Remember that you have to wake up SO-50 with a 74.4 Hz pilot tone. It is sometimes a bit flakey if the satellite is at low elevation. Check your gear and ensure that the frequencies are correct. Try again! Try one of the other FM satellites. “If it was easy it wouldn’t be fun!” “I lost him (or her).” This happens all the time on satellites. It is one of the reasons that QSOs are kept short and informative. It is almost always because either you or the other station has reached LOS and the satellite has gone down below the horizon. If you are using tracking software and you know where the other station is located, you can keep an eye on the footprint drawing so that you get an idea of when the other station is going to lose coverage. Other reasons include nulls in the antenna pattern or a misdirected antenna at either end. Inadequate Doppler correction such as forgetting to change to the next channel can cause your signal to slide off the satellite’s receiver bandwidth. ‘Hi-jacking’ is a common problem. Radios using FM modulation inherently exhibit ‘capture effect.’ This helps you when your signal is a little off frequency due to Doppler shift but it also means that a stronger signal, particularly if it is closer to the satellite’s receive frequency will

capture the receiver, blocking your transmission in favor of the bigger signal. Your signal will disappear and the interloper will take over. Don’t get angry. It is probably unintentional on the part of the new station. Your signal will have the same effect when the satellite is at high elevation at your place and you become the ‘big gun.’ That is why you must listen and make sure that the repeater is not in use before you transmit. Finally, “I don’t really like standing outside on a freezing cold night pointing an aerial at the satellite and mumbling into the microphone. I want a warm cup of cocoa.” Yep, I fall into that camp myself. Time to build a home station and have a go at the linear transponders!

WORKING SATELLITES WITH LINEAR TRANSPONDERS Many people try the FM satellites first and then move on to using the linear transponder satellites. You can still do it with a portable setup, preferably using a tripod for your Yagi antennas. The antenna and transmitter power requirements are the same for any type of LEO satellite. You will probably need a different radio because you can’t use FM modulation. FM handheld or mobile radios are not suitable. I guess that it is possible to work satellites with linear transponders using a handheld antenna, provided Doppler control is being managed by a computer. Doing both by hand would be interesting to watch. Realistically a higher level of automation is required. With a fully computer automated station; I actually find using the ‘linear transponder’ satellites easier than using the FM ones. For linear transponder operation, you need a transceiver or a separate receiver and transmitter that is capable of single sideband (SSB) operation. Most linear transponders require you to transmit to the satellite on lower sideband (LSB) and receive the downlink signal from the satellite on upper sideband (USB). To help counter the effects of Doppler shift, many linear transponders are ‘reverse tracking.’ As the uplink frequency increases, the downlink frequency decreases. Like the FM satellites, the most common transponder arrangements for LEO satellites are UV mode (UHF uplink and VHF downlink) or VU mode (VHF uplink and UHF downlink). Some satellites have linear transponders that use the 10m (A band), 23cm (L band), or 12cm (S-band) amateur radio bands. A ten element Yagi antenna for 70cm and a five element Yagi antenna for 2m is typical for a permanent home station. Bigger antennas or arrays designed for EME or meteor scatter operation can be used. But being much larger, they are more difficult to point at the satellite. Also, larger antennas have narrower beamwidth so you have more accurate rotator control. Crossed Yagis configured for circular polarization are very handy but not essential. If you have long coax feed lines, masthead amplifiers to overcome the feeder loss are a definite advantage, and they may be a necessity. When you use single sideband, any uncorrected Doppler shift of the received frequency causes a corresponding change in the audio frequencies coming from the receiver. It is essential to constantly correct for the Doppler shift. You cannot use fixed channels as you do for the FM

satellites and you must be able to tune the receiver frequency independently of the transmitter because UHF frequencies are more affected by Doppler shift than VHF frequencies. Some operators do control the adjustment for Doppler shift by hand, but I strongly recommend software control of the uplink and downlink frequencies.

MINIMUM EQUIPMENT LIST SSB radio(s) for 2m and 70cm. Full duplex is just about mandatory and is certainly strongly recommended. Tripod mounted antenna, coax cable, and a diplexer (if required). Using a handheld antenna for SSB satellite operation is pretty tricky because operating the radio is more difficult due to the need for very regular Doppler shift correction. You can still use a homebrew antenna or an Arrow / Elk antenna but it really needs to be mounted it on a tripod or a mast. The M2 LEO pack, available from M2 Antenna Systems Inc. or AMSAT-NA is an excellent commercial antenna system. You can’t use a handheld radio due to the requirement for SSB modulation, so your station becomes much less portable. You will need a power supply or a battery and a folding table is useful. A preamplifier for the receive band (if the coax feeder cable is long). A PC, a satellite tracking program, and preferably CAT (CI-V) control of the radio frequencies for Doppler shift correction Patience. Don’t expect perfect results the first time. Satellite operation takes practice.

PREPARATION Have a plan. Use a prediction program to find where to point the antennas for AOS and during the pass. Or better still use a tracking program. Have the transceiver set to the right frequencies plus the expected amount of Doppler shift so that you are ready for AOS. If you are not using computer control of the radios but you do have a ‘satellite capable radio,’ set the radio for reverse or forward tracking depending on the transponder configuration of the satellite. This moves the transmit frequency in the right direction as you tune across the transponders downlink band. It won’t correct for Doppler shift, but it will track correctly, as you move the VFO control. If you are using a radio without CAT control, you can manually adjust the frequencies to the numbers indicated by the satellite tracking program. Select a satellite and find out its transponder details. You need to know the uplink and downlink frequencies; and whether the transponder is reverse or forward tracking. There is a chapter about satellite transponders later. If the transponder is reverse tracking, your uplink should be lower sideband and the downlink will be upper sideband. This is important. You won’t make contacts if you transmit on the wrong sideband. If the transponder is forward tracking, both the uplink and the downlink will be upper sideband.

CHOOSING A SATELLITE OSCAR 7 (AO-07) and JAS-2 ‘Fuji OSCAR 29’ (FO-29) are probably the easiest satellites for SSB or CW operation. AO-07 OSCAR 7 has a strong downlink, which makes it easy to hear but there are some special considerations. The satellite is very old. It was launched in 1974. Its batteries are dead but the transponder works while the solar panels are in sunlight. The satellite can only be used when it is not eclipsed. Just because you are in the dark, does not necessarily mean that the satellite is. Your tracking program will indicate whether the satellite is eclipsed or not. Another problem is that OSCAR 7 is no longer under, ground station control. The satellite has two operational modes. ‘Mode A’ (now called VA mode) has a 2m uplink and 10m band downlink. ‘Mode B’ (now called UV mode) has a 70cm uplink and a 2m downlink. The trick is that you never know which mode the satellite will choose when it powers up after being eclipsed. Because the satellite is running off the solar panels with no batteries, the power budget is compromised. As the transponder is a linear amplifier, strong incoming signals result in strong downlink signals. But, if you send too much power to the satellite, the high current drawn by the transponder power amplifier has the effect of pulling down the voltage rail during your voice peaks. This effectively modulates the entire downlink bandwidth with your transmission. If you use too much power on the uplink, your transmission will affect every other conversation across the transponder. Doing that will make you very unpopular so only use the minimum power that you need to hear your signal on the downlink. You should reduce your transmit power as the satellite reaches higher elevation because you won’t need as much uplink signal at the closer range. It is actually quite hard to tune to a signal that is sending too much uplink power into AO-07. It can be heard as distorted choppy speech peaks spread right across the transponder making it difficult to find the actual downlink frequency. So using too much uplink power actually makes it more difficult to get a contact on AO-07. OSCAR 7 has a relatively high orbit so it has a larger footprint and longer passes than most LEO satellites. Providing you are careful not to transmit too much power, AO-07 in UV mode, ‘mode B’ is a great satellite to start with.

AO-07 VA mode (A) non-inverting forward tracking (100 kHz wide) Uplink USB 145.850 to 145.950 MHz Downlink USB 29.400 to 29.500 MHz Beacon 29.502 MHz constant carrier AO-07 UV mode (B) inverting reverse tracking (50 kHz wide). Uplink LSB 432.125 to 432.175 MHz Downlink USB 145.975 to 145.925 MHz (8 watts) Beacon 145.975 MHz CW FO-29 (JAS-2) was launched in 1996. It is much bigger than a CubeSat and has more area for solar panels. This allows the satellite to run a relatively powerful 1 watt power amplifier on the 70cm downlink. FO-29 VU mode inverting reverse tracking (100 kHz wide) Uplink LSB 145.900 to 146.000 MHz Downlink USB 435.900 to 435.800 MHz Beacon 435.795 MHz CW AO-73 (FUNcube-1) is a 1U CubeSat launched in 2013. It is primarily intended for educational outreach to schools in the UK. AO-73 has a 300 mW transponder, which is usually only active while the satellite is in eclipse. AO-73 UV mode inverting reverse tracking (20 kHz wide) Uplink LSB 435.130 to 435.150 MHz Downlink USB 145.970 to 145.950 MHz Beacon 145.935 MHz BPSK telemetry XW-2 (CAS-3). There are six XW-2 satellites. They were all launched at the same time along with three other small satellites. They all have essentially the same amateur radio payloads but not all of the satellites have their transponders activated. They each have a CW beacon and an AX-25 19k2 and 9k6 baud GMSK telemetry downlink. They all use the same type of antennas, quarter wavelength monopole whip antennas made of steel tape. XW-2A (CAS-3A) UV mode inverting reverse tracking (20 kHz wide) Uplink LSB 435.030 MHz to 435.050 MHz Downlink USB 145.685 MHz to 145.665 MHz

CW Beacon 145.660 MHz 22 wpm CW Digital Telemetry 145.640 MHz 9.6/19.2kbps, GMSK XW-2B (CAS-3B) – Not Activated Uplink LSB 435.090 MHz to 435.110 MHz Downlink USB 145.750 MHz to 145.730 MHz CW Beacon 145.725 MHz 22 wpm CW Digital Telemetry 145.705 MHz 9.6/19.2kbps, GMSK XW-2C (CAS-3C) UV mode inverting reverse tracking (20 kHz wide) Uplink LSB 435.150 MHz to 435.170 MHz Downlink USB 145.815 MHz to 145.795 MHz CW Beacon 145.790 MHz 22 wpm CW Digital Telemetry 145.145.770 MHz 9.6/19.2kbps, GMSK XW-2D (CAS-3D) – Not Activated Uplink LSB 435.210 MHz to 435.230 MHz Downlink USB 145.880 MHz to 145.860 MHz CW Beacon 145.855 MHz 22 wpm CW Digital Telemetry 145.835 MHz 9.6/19.2kbps, GMSK XW-2E (CAS-3E) – the satellite is inactive XW-2F (CAS-3F) – UV mode inverting reverse tracking (20 kHz wide) Uplink LSB 435.330 MHz to 435.350 MHz Downlink USB 146.000 MHz to 145.980 MHz CW Beacon 145.975 MHz 22 wpm CW Digital Telemetry 145.955 MHz 9.6kbps, GMSK EO-79 (QB50p1 / FUNcube-3) is a 2U CubeSat launched in 2014 into a sun-synchronous orbit. It has a 400 mW transponder, which is only active while the satellite is in sunlight. UV mode inverting reverse tracking (30 kHz wide) Uplink LSB 435.035 MHz to 435.065 MHz Downlink USB 145.965 MHz to 145.935 MHz

CW beacon / 1.2kbps BPSK telemetry 145.815 MHz. UKube-1 (FUNcube-2) is a 3U CubeSat launched in 2014 into a sunsynchronous orbit. It is primarily intended for educational outreach to schools in the UK. Check the project website for an activity schedule. UV mode inverting reverse tracking (20 kHz wide) Uplink LSB 435.060 MHz to 435.080 MHz Downlink USB 145.950 MHz to 145.930 MHz Primary beacon145.840 MHz CW / 1.2 – 9.6 kbps telemetry FUNcube telemetry 145.915 MHz 1.2kbps BPSK telemetry LO-87 (LUSEX-OSCAR 87) is a hitchhiker on NuSAT-1 an Argentinian Earth observation satellite. It carries a 250 mW linear transponder and a 70 mW beacon. The transponder is only active while the satellite is above Latin America and Europe. UV mode inverting reverse tracking (30 kHz wide) Uplink LSB 435.935 MHz to 435.965 MHz Downlink USB 145.965 MHz to 145.935 MHz Beacon 145.900 MHz CW LilacSat-2 (CAS-3H) has an APRS digipeater, an FM voice repeater, and a SSB/CW linear transponder. It was launched in 2015 on the same rocket as the six XW-2 satellites. The linear transponder may not be active, check the project website for the activity schedule. Note that it is not reverse tracking so the round trip Doppler shift will be higher than most other satellites. VU mode non-inverting forward tracking (30 kHz wide) Uplink USB 144.3425 MHz to 144.3825 MHz Downlink USB 437.180 MHz to 437.220 MHz Beacon 437.225 MHz GPSK telemetry

DIGITAL MODES ON ANALOG SATELLITES Some people operate digital voice, PSK-31, SSTV, or even RTTY over linear transponders. It is not very common and unless you have prearranged a ‘sked,’ it is unlikely that you will get a two-way contact. If you call CQ on several passes, you might get lucky. The only absolute rule is that the mode should be narrow band, less than 3 kHz. The mode must be transmitted via your SSB transmitter. You should never use FM or FM packet on a linear transponder. If you are intending to use digital modes, I recommend staying away from the busy central part of the satellite transponder and conduct experiments nearer the lower edge of the uplink band. But not right at the bottom of the transponder. Try not to squash any CW signals on that part of the band.

PSK TRANSPONDERS Some satellites specifically support BPSK-31 operation. For example, PSAT (NO-84) has an AU mode PSK transponder. PSK is a good digital mode for satellite operation because it is narrowband. But it is very sensitive to the frequency errors caused by Doppler shift. PSK can be used on any satellite with a linear transponder, but the PSK specific transponder on NO-84 has an SSB uplink and an FM downlink. Using FM for the downlink reduces the problems caused by Doppler shift of the UHF downlink frequency. A PSK signal received without any Doppler correction will appear as a slanted line on your digital mode program waterfall. Because the frequency is changing over time. Using the AFC mode on the digital mode software will help the software to track the signal as it drifts across the waterfall, but it may also move the transmit signal in the wrong direction. CAT control of the radio frequency is a very big help. Because PSK is a narrow bandwidth mode, many PSK users can operate through a linear transponder simultaneously. Full duplex conversations can be held using WinPSK or running two PSK programs on your PC. DopplerPSK by Andrew Flowers K0SM can transmit PSK with automatic Doppler correction while you use another application for decoding the received PSK signal. PSAT ‘ParkinsonSat’ (NO-84) has a 350 mW APRS digipeater on 145.825 1200 baud and an AU mode PSK digipeater. The APRS digipeater is harder to work than the one on ISS because it is further away (higher orbit) and it has less transmit power. The PSK repeater has an SSB uplink and an FM downlink. APRS Digipeater on 145.825 MHz 1200 baud FM PSK31 Downlink 435.350 MHz +/- 5 kHz FM (300 mW) PSK31 Uplink 28.120 MHz +2 kHz PSK31 SSB (up to 75 watts of uplink power into an omnidirectional vertical antenna is authorized).

SELECTING A FREQUENCY Linear transponders can support several users at the same time. You may hear SSB conversations on either side of the transponder center frequency. CW, digital modes, and SSTV tend to be more towards the lower side of the uplink range. Some people send a string of ‘dits’ while netting their uplink and downlink frequencies. So, you might hear CW ‘beeps’ anywhere on the transponder. Where I live, we have the luxury and the curse of wide open spaces on the linear transponders. It is very unusual to encounter more than two conversations on the satellite. The rule over here is that the first QSO will use the center of the transponder and any other conversations will operate a few kHz higher or lower. The satellite will probably be busier where you live. Pick a spot at least 3 kHz above or below another conversation. A wider spacing is even better because it will give more room for mistakes in the Doppler correction. Listen to a pass before transmitting to check what spacing between transmissions the operators in your area are using.

STEP BY STEP The ‘step by step’ approach differs a little depending on whether you have a minimal portable station operating, with your radio on a camping table and a small Yagi on a tripod, or a fully integrated home station with computer control of the radio and the rotator. For this example let’s assume that you are somewhere in the middle of the range. You are operating portable, with manual adjustment of your antenna tripod. You have SatPC32 or a similar satellite tracking program running and it is managing the radio frequencies using CAT control. We will also assume that you have a fancy satellite capable radio or two separate radios, so you can employ full duplex operation. It is so much easier that way! 1. The same basic rules apply as for the FM satellites. Get all your gear together and check that it all works. There is no time during a satellite pass to fix anything. Be prepared. Check out your location. Make sure that it is a safe place to operate. 2. Turn ‘tracking’ off on the radio so that it is not fighting against the CAT control. If the ‘tracking’ button is set to off, the ‘forward/reverse’ setting does not matter. 3. Look at the direction where the satellite will rise at AOS time. Make a mental note of a landmark such as “just to the right of the tree.” Also, note the direction where the satellite will reach maximum elevation and practice pointing the antenna at the right elevation and direction. Finally, determine a landmark that will define where the satellite will set below the horizon at the end of the pass. 4. A minute before the satellite pass starts (AOS), point your antenna in the direction where the satellite will rise. SatPC32 (or similar) will show you when the satellite is about to appear. Make sure that you have selected the correct transponder and satellite. Select the ‘both’ mode of Doppler correction. Turn on CAT control on the radio and SatPC32. The radio should tune to the current frequencies and both the receiver and transmitter frequencies should change every few seconds. The frequencies will be below the transponder center frequency because of the expected Doppler shift. Open the receiver squelch so that you hear band noise. Wait for AOS and listen. 5. If you don’t hear anything by the time the satellite is up about 10 degrees, you can make a CQ call. If you do hear an ongoing

conversation wait until it is finished, or a request is made for other callers. If you hear a CQ call, go back with a quick response. This should be only your call sign or their call sign followed by your call sign. No more than that! Just use your call letters, not phonetics. If the other station is having trouble getting your call, you can use phonetics during the second over. 6. If you make or respond to a CQ, make very sure that the other station is responding to your call and not to someone else. It is very common to get a “double” where two stations call or respond at the same time. 7. If the transponder center frequency is in use and you want to make a CQ call or to listen for other stations, select VFO mode on SatPC32 and use your radio VFO knob to tune up or down the transponder. Make sure that you don’t tune too far and end up off the end of the transponder. When you have found a signal or a spot to call CQ, turn off the VFO mode on the SatPC32. It will save you if you accidentally bump the big knob on the radio. The CAT control will track the new frequency at the correct offset. 8. While you are in a conversation, don’t touch the VFO knob on the radio. Use RIT on the radio or the offset controls on the SatPC32 software to fine tune the receiver for best audio. If you use the software offset controls, save the setting when it is correct and it should remain correct next time you use the same transponder. If the SatPC32 VFO button is on, adjusting the VFO knob on the radio will make the other station sound better, but it will also shift your transmit frequency. If the SatPC32 VFO button is off, the CAT control will pull the radio back and your tuning adjustment will be lost. If the other station is not correcting ‘both’ uplink and downlink frequencies, their downlink will not be on the same frequency as your own. It should be within a couple of kHz. Don’t touch the VFO knob on the radio. 9. Every minute or so, turn the antenna a little to follow the satellite path and raise the angle of your antenna a little. By mid pass, probably between five and eight minutes after AOS, it should be facing the midpoint azimuth and pointing at the max elevation. 10. You may need to twist the antenna to change its polarization. Listen to the signal. If it gets noisy, you are losing signal strength. You may need to adjust the direction that you are pointing the antenna, the elevation, or the polarization (twist).

11. Keep talking, listening, or calling CQ. At mid-point, you should have a solid signal. The satellite is at its closest and there should be no objects blocking the path. 12. Keep overs short. Long overs are a recipe for losing track of the conversation. Remember that the satellite may be just coming up, or just about to set, at the other operator’s location. 13. Swap essential information first. Your name, grid reference, and QTH. I don’t bother with a signal report because it is pretty meaningless. The third over, if there is one, can be more conversational. Because there is room on the transponder for multiple users and modes, it is more common to have a QSO that lasts for the entire pass. Unlike the single channel FM satellites, there is no need to cut things short to give others an opportunity. It is also more common to call people that you have worked before and build on previous contacts. With luck, after your conversation has finished, someone might call you or the other station that you just worked. Or if you ‘own’ the frequency, you can call CQ. Normally the station that called CQ initially retains a right to the frequency, but they may let you have it if they are nearing the end of their available time. If no one jumps on within 15 seconds, feel free to call CQ. It is basically the same etiquette as operating on the HF bands. 14. Keep tracking the satellite down towards the LOS azimuth direction. It is probably not a good idea to start a new contact if you have less than two minutes left before you lose the satellite. Keep an eye on the antenna position and the SatPC32 footprint. If you are in a QSO try to warn the other station that you are close to losing the signal. That way they won’t be surprised when you drop out. 15. After LOS, breathe. You probably forgot to in all the excitement. If you managed to make a contact, congratulate yourself on a job well done. If you didn’t, don’t be discouraged. It will probably take two or three tries, to get everything working together. Check out the ‘problemsolving’ section then get set up for the next pass, or satellite.

CARRYING OUT A QSO The big additional challenge with operating through a linear transponder is managing the Doppler shift. Of course, you still have to steer the antennas so that they point at the satellite, manage the QSO, and log the call. If everything is right, you should be able to hear your own signal on the downlink and other operators as you tune across the transponder. Full duplex operation Being able to hear your own signal on the satellite downlink is a really good indication that everything is going well. It means that the Doppler correction is effective, your transmit power is sufficient, and that your antennas are pointing in the right direction. If hearing yourself is distracting once a QSO has been established, you can turn down the volume while you are transmitting. Most operators leave the radio set for full duplex all of the time. If you can hear your own downlink, and the other station transmits on the same frequency at the transponder, you will be able to hear their downlink signal. Sideband inversion Nearly all linear transponders are ‘inverting’ meaning that a lower sideband signal on the uplink results in an upper sideband signal on the downlink. If the transponder is ‘reverse tracking,’ in other words, an increase in the uplink frequency creates a decrease in the downlink frequency, it will also be inverting. Remember to transmit on LSB and receive on USB. The same goes for CW and digital modes. Sideband inversion will not affect a PSK or CW transmission but it will move the audio offset to the other side of your nominal displayed frequency. Transmitter power You do not need a huge amount of power for LEO satellite operation. Five to ten watts into a Yagi antenna is plenty. At high elevations, one watt can be enough. The more power you put into the transponder the stronger your downlink signal will be. But creating a big signal means that you are taking more than your share of the available transponder power and reducing the available power for everyone else using the satellite. Some of the early satellites had an ‘alligator’ switch called LEILA, which dumped your downlink if your uplink power was too high. If you can hear your downlink so can everyone else. You don’t need any more than the minimum power that gives you a signal that you can hear. Received signal strength

Don’t expect a noiseless received signal. LEO satellites usually have very low power transmitters and pretty poor antennas (due to size considerations). They are also a long way from you, so there is a lot of path loss. See the section on link budgets. Polarization skew (your antenna polarization not matching that of the satellite's antenna) adds more loss and it changes during the pass because of the satellites changing orientation towards your station. Signal polarization is also affected by Faraday rotation. The best way to reduce polarization errors is to use circular polarized antennas. It is normal for the received signal to be weak and for it to fade up and down as the satellite passes over. Adding a masthead preamplifier can help enormously. At my place, it makes the difference between the signal fading out on every pass, and full copy throughout the whole pass. I could hardly believe the improvement. Adding a preamplifier inside the shack is probably pretty pointless. See the section on masthead amplifiers for an explanation. Using long Yagi antennas does improve the received signal strength, but they introduce other issues. They are more difficult to mount and steer accurately. Doppler correction If you are not correcting your uplink signal for Doppler shift, your transmission will move across the transponder as the pass progresses. There is a high risk that your transmission will drift across the frequency being used by another operator. If you are using tracking software and CAT control, always select the option to correct both the uplink and the downlink frequencies. The only exception is when you are working a station that is not correcting their uplink frequency. If you are manually adjusting your transmitter and receiver frequencies rather than using computer control, you should use full duplex so that you can hear your downlink signal and correct for Doppler using the ‘one rule.’ The ‘one rule’ states that you should correct whichever frequency (uplink or downlink) is the highest because it suffers from more Doppler shift. You should not attempt to manually change both the uplink and the downlink frequencies because you will almost certainly get hopelessly lost. My recommendation is to do yourself and other satellite operators a favor and use CAT control to automatically adjust both frequencies. It gives you

one less thing to do during a busy satellite pass and it ensures that your signal stays on the same frequency at the satellite. If your PC software is correcting both the uplink and downlink frequencies, you should be able to hear your transmitted signal returning on the downlink frequency. You may have to fine-tune your receive frequency to make yourself sound ‘normal.’ In SatPC32, you can adjust the offset setting on the CAT menu. When your downlink sounds good, you can save the CAT offset and it should never need to be adjusted again (for that satellite). You can use the RIT (receive incremental tuning) control on the radio. Do not adjust the VFO knob on the radio because it will cause a corresponding shift in your transmit frequency. You can tell if this is happening because tuning the receiver VFO will fail to make your voice sound normal on the downlink. Moving frequencies Now that you have adjusted the fixed offset between your uplink and your downlink, the satellite tracking program will keep the relationship constant while allowing for more Doppler shift on the higher frequency. Switch to the ‘VFO’ mode on the tracking software. It will allow you to tune across the transponder using your radio’s VFO knob. As you tune the receiver the software will recalculate the appropriate transmit frequency. You may be able to hear the beacon and you should hear any conversations that are taking place. With luck, you will hear a station calling CQ and you can give it a call. Netting to another station Another station will experience completely different amounts of Doppler shift. The uplink and downlink frequencies may be decreasing at their place while they are increasing at your place. But that does not matter! So long as you are both on the same frequency at the satellite transponder input, if the other station can hear their own downlink signal, they will also be able to hear your downlink signal. The receiver frequency displayed on your radio will be different to the frequency displayed on the other station's radio because the amount of Doppler correction being applied is different at their location. If you can hear your downlink, you should be able to hear the other station when they transmit. Many operators get confused about this so let’s run through an example. There is more on Doppler shift later and the formulas are in the ‘Formulas’ chapter.

As the satellite comes up over the horizon, it is getting closer as it rises. But it will not be coming straight at you unless it is destined to pass directly over your location (i.e. a max elevation of 90 degrees). The satellite’s path as it rises can be expressed as a vector sum of the motion towards your location and motion at right angles. Only the motion directly towards you adds Doppler shift to your uplink and downlink frequencies. To be pedantic, the Doppler shift is due to the difference in velocity between the satellite and your location. The satellite is moving and your location is moving as well due to Earth’s rotation. Here is an example of the effect of Doppler shift at two locations. Say you want to talk to Sally via the FO-29 satellite and you plan to use the center of the transponder. You set the tracking software for an uplink of 145.950 MHz, which corresponds to a satellite downlink at 435.850 MHz. The range to the satellite is constantly changing, decreasing as the satellite approaches, and then increasing after the satellite has reached peak elevation. The relative velocity (speed) between the satellite and your location is called range rate. It is negative as the satellite approaches, zero at the highest point of the pass, and positive as it moves away again. For this example, we will say that the satellite is approaching us and currently has a range rate of -2.5 km per second. Using the formulas later in the book, we find that a range rate of -2.5 km per second at 146 MHz creates a Doppler shift of 1.2 kHz. To correct for the Doppler shift on the uplink we need to transmit 1.2 kHz low, on a frequency of 145.9488 MHz. This will make the signal arrive at the satellite on the transponder center frequency of 145.950 MHz. You transmit a little low and the Doppler shift brings the frequency up to the right frequency. The transponder translates the uplink signal to a downlink frequency of 435.850 MHz. By the time the downlink signal reaches your location, it will be at 435.8536 MHz, about 3.6 kHz higher than the transponder center frequency due to Doppler shift. The satellite tracking software can calculate the Doppler shift in both directions and it sets the radio frequencies using CAT control. When you transmit a lower sideband (LSB) signal at 145.9488, you hear your own signal as an upper sideband (USB) signal at 435.8536 MHz. At the same time, the satellite is moving away from Sally’s location. The range rate at her location is currently +4 km/s. That range rate will create a Doppler shift of -1.95 kHz at 146 MHz. Sally has to transmit 1.95 kHz

high, on 145.9519 MHz to hit the center of the transponder. She will hear her downlink signal (and yours) 5.82 kHz low, at 435.8422 MHz. As long as she gets her signal up to the satellite on the same frequency as you do, she will hear her downlink at the same 435.8422 MHz frequency as yours. You will probably have to use your RIT control to fine tune a bit. But don’t touch the main VFO or you will change your transmit frequency and you will end up chasing each other all over the transponder.

TRACKING SOFTWARE VFO MODE The SatPC32 software sets its VFO mode to ‘on’ if the mode is SSB, indicating that you are using a linear transponder. You can move the radio VFO knob to tune across the transponder. When the SatPC32 VFO indicator is showing ‘off,’ the software tracks the frequencies that were entered in the ‘Doppler.sqf’ file. This is usually the uplink and downlink transponder center frequencies. With VFO set to ‘on,’ you can tune across the transponder using the buttons on the PC screen, the mapped keyboard buttons, or the VFO knob on the radio. As you tune the receiver, the Doppler shift will be calculated and the required transmit frequency will be adjusted automatically. With the frequencies changing all the time, it can be rather confusing and difficult to tell where you are on the transponder. To make things less confusing, you can set an option on SatPC32 that displays the frequencies at the satellite rather than on your radio. If you do get lost, temporarily setting the VFO back to ‘off,’ pulls the radio back to the transponder’s center frequency ± Doppler.

TRACKING SOFTWARE ROTATOR CONTROL Toggling the SatPC32 ‘R’ control turns rotator control on or off. If you have computer-controlled rotators, you will probably want to turn rotator control off while the satellite is below the horizon. There is no point in attempting to track satellites that are out of range.

PROBLEM SOLVING All of the problems discussed in the ‘working the Easy Sats’ chapter also apply when you are working linear transponders so I won’t go over them again. The most common problem experienced while using linear transponders remaining netted to another station. It is reasonably easy if both stations are correcting both the uplink and the downlink. But it is much more difficult if the other station (or you) are only correcting the uplink or the downlink. Netting gets easier with practice. See the tuning tips below. The next most common error is caused by moving the main VFO when you are working another station. You can end up chasing each other around the transponder. If you are both correcting both the uplink and downlink frequencies, you should never have to touch the main tuning knob during a conversation. But if either station is not correcting both uplink and downlink, you will have to move the VFO to track the other station. A game of chase is usually unavoidable. Try to match the type of Doppler correction that the other station is using. If in doubt, leave your correction set to ‘both,’ it is almost always the best method. The most likely technical issue that you can encounter is your transmitter overloading your receiver. This is a pretty common problem if you are trying to use full-duplex mode, receiving on one band while transmitting on the other. It is especially prevalent when working VU mode satellites because the third harmonic of your 2m transmission will fall into the 70cm band. That means that some RF power will pass through any front-end filters in your receiver and it might overload the 70cm receiver. If you are using a handheld antenna to work a VU mode satellite, you are very likely to encounter this problem because the two antennas are very close to each other. The only complete solution is to use half-duplex mode so that the receiver is not working while you are transmitting. Using separate antennas spaced at least a meter apart helps a lot because less energy is coupled from the ‘transmit’ antenna into the ‘receive’ antenna. Using lower transmit power will help as well, but of course, that could limit your success in accessing the satellite. You should definitely use as little power as you can. Full duplex means that you are able to transmit and receive at the same time, like talking on a phone. Half duplex means transmitting and then receiving. Not doing both at the same time. Half duplex is the

normal amateur radio style of operating, such when you are talking to someone via a local repeater. A harmonic is an integer multiple of a frequency. Two times a frequency is its second harmonic. Three times the frequency is its third harmonic. VU mode means transmitting on the 2m amateur radio band and receiving the satellite downlink on the 70cm amateur band. UV mode means transmitting on the 70cm amateur radio band and receiving the satellite downlink on the 2m amateur band. The use of UV mode eliminates the harmonic problem, but the larger 2m antenna is often quite good at picking up signals on 70cm. If you are using two Yagis, orient them so that they have 90 degrees difference in polarization. (2m vertical and 70cm horizontal or vice versa). If you are using circular polarized ‘crossed’ Yagis to work into LEO satellites, which are not transmitting circular polarization. You can use alternate polarization to limit the amount of coupling. For example, RHCP for 2m and LHCP for 70cm. RHCP is the acronym for ‘right hand circular polarization,’ I will let you work out what LHCP stands for.

TRACKING BUTTONS ON THE RADIO Radios that were designed for full duplex SSB satellite operation have a ‘tracking’ button and a ‘forward/reverse’ button. These controls lock the transmitter and receiver VFOs together so that as you tune the main VFO knob the transmitter frequency moves as well. The ‘reverse’ setting causes the transmitter frequency to decrease as the receiver frequency is increased. These controls are useful if you don’t have computer CAT control of the frequencies. If you are using CAT control, the ‘tracking’ button on the radio should always be turned off to avoid the CAT control fighting the tracking control on the radio. If you are not using CAT frequency control. Before the start of the pass. Turn the ‘tracking’ button off, so that the transmitter and receiver VFOs are not linked. Choose an uplink frequency that will be near the transponder center (or choose the FM uplink channel frequency). Add the amount of Doppler shift expected at the start of the satellite pass and set the transmitter VFO to the calculated frequency. Calculate the downlink frequency corresponding to the chosen uplink frequency. Set your receiver VFO to the appropriate downlink frequency plus the amount of Doppler shift expected at the start of the satellite pass. Make sure that you have the ‘normal’ / ‘reverse’ setting set correctly for the transponder that you are using. Most linear transponders use reverse tracking. FM satellites use fixed frequencies so set the switch to ‘normal’ if you are using one with a satellite capable radio. When the satellite becomes available, transmit CW ‘dits’ (or talk) and tune the receiver VFO until you hear the CW dits (or your voice) coming down on the downlink at about the right audio tone. Once netted, turn the ‘tracking’ button on, to gang the two VFOs together. Now your transmitter will follow the receiver VFO as you tune across the transponder looking for signals. However, using the ‘tracking’ control on the radio will not adjust for Doppler shift as the pass progresses. So you will have to turn tracking off periodically and re-net the radio to the sound of your voice on the downlink while you are transmitting. My advice is to turn the radio’s tracking button off and use PC CAT control instead. It makes life so much easier.

TUNING TIPS WHEN BOTH STATIONS ARE USING ‘BOTH’ MODE Firstly, you must have set the SatPC32 (or similar) ‘frequency offset’ so that you can hear your downlink signal. This only has to be done once for each transponder. Make sure that you save the setting. After that, you should be able to tune anywhere across the transponder and hear your own downlink. If you can hear another station, they should be able to hear you when you transmit. You should never have to touch the radio VFO knob during the conversation. Use the RIT control to make minor adjustments to your receiver frequency without upsetting the netted VFOs. If you fail to resist the temptation, you will tune the radio so that it sounds good when receiving the other station, but when you go to transmit you will have moved slightly on the transponder and the other station will have to tune to find your signal. Of course that, in turn, moves his or her transmitter frequency and you will have to tune again to hear him (or her) during the next over. You end up chasing each other up and down the transponder’s band. The only way to break the cycle is, to never move your VFO knob during the QSO.

TUNING TIPS WHEN EITHER STATION IS USING THE ‘ONE RULE’ Remember that the ‘one rule’ states that you should correct whichever frequency (uplink or downlink) is the highest because it suffers from more Doppler shift. If you are only adjusting the higher frequency it needs to be moved enough to counteract the combined effect of Doppler on the uplink and the downlink. With a VU mode reverse tracking transponder, this can require you to adjust the frequency opposite to the way you might expect. There is a lot more about this in the chapter about Doppler shift chapter. Table 4: Doppler correction types for a typical UV mode transponder Correction type Both Uplink (one rule) Downlink

Frequency change for uplink ± 11.3 kHz ± 7.5 kHz

Frequency change at transponder 0 kHz ± 3.8 kHz

Frequency change for downlink ± 7.5 kHz 0 kHz

0 kHz

± 11.3 kHz

± 7.5 kHz

Table 5: Doppler correction types for a typical VU mode transponder Correction type Both Downlink (one rule) Uplink

Frequency change for uplink ± 3.8 kHz 0 kHz ± 7.5 (inv)

Frequency change at transponder 0 kHz ± 3.8 kHz

Frequency change for downlink ± 11.3 kHz ± 7.5 kHz

kHz ± 11.3 kHz 0 kHz (inv)

Using ‘both’ uplink and downlink Doppler correction: If you use ‘both’ correction, the tracking software changes both the receiver and the transmitter tuning with the result that your signal stays in the same place on the transponder throughout the conversation. This is the preferred Doppler correction mode because it eliminates the risk of your transmission interfering with another user. Also, it has the big advantage that the other station will appear at the same receiver frequency. You should not have to adjust the radio tuning at all. In practice, Doppler correction is never perfect so you may need to use the RIT control. Using the one rule: If you are not using ‘both’ mode you should use the ‘one’ rule.’ This will minimize the possibility of interference to other users

because the frequency change at the transponder is a moderate ± 3.8 kHz (in this example). This is not as good as using ‘both’ correction but it is significantly better than ignoring the rule. If you fail to use the ‘one rule’ and correct the lower frequency, instead the change at the transponder is ± 11.3 kHz (in this example). There is a much greater chance of causing interference to another user and you are quite likely to slide off the transponder completely. One rule - Uplink correction: To perform uplink Doppler correction, leave your receiver tuned to the same frequency and adjust your transmitter frequency so that you can hear yourself on the downlink. Uplink correction would be used on a UV mode transponder because the uplink is the higher frequency. The combination of Doppler shift and your adjustment of the transmitter frequency will cause the conversation to slide across the transponder bandwidth as the satellite pass progresses. If we look at the ‘Uplink (one rule)’ line on table 4, you can see that you would adjust your transmitter from the transponder frequency -7.5 kHz at the start of the pass to the transponder frequency +7.5 kHz at the end of the pass. At the satellite, this ± 7.5 kHz change in the transmitter frequency will cause the signal to move from the transponder frequency -3.8 kHz at the start of the pass to the transponder frequency +3.8 kHz at the end of the pass. By the time the downlink signal reaches you, the Doppler will have changed the downlink frequency so that your receiver can remain tuned to the nominal transponder downlink frequency for the duration of the satellite pass. When you use the one rule for Doppler correction, your downlink signal will stay at the same frequency on your receiver throughout the pass, but the signal received from the other station will be on a different frequency and worse still, it will change the pass proceeds. This is because the overall Doppler shift is different at the other location. The other station is applying a different amount of uplink correction so the other station’s transmission will be on a different part of the transponder. Using uplink correction uses two spots on the transponder whereas using ‘both’ correction mode, both sides of the conversation use the same transponder frequencies. So ‘both’ mode is not only easier to manages, it is also more efficient in terms of the amount of satellite transponder bandwidth that your conversation is using. The offset is reciprocal. For example, if you are receiving the other station 3 kHz above the frequency that you are receiving your downlink, the other

station will receive you 3 kHz below the frequency that they are receiving their downlink. One rule - Downlink correction: With downlink correction, you leave your transmitter tuned to the same frequency all the time and tune your receiver until you hear your downlink signal. The Doppler shift on your uplink signal will cause the conversation to slide across the transponder bandwidth as the satellite pass progresses. Because the other station’s signal appears on a different transponder frequency, there will be a difference between the frequency that you receive your downlink and the frequency that you receive the other station’s downlink. If you receive the other station higher than your own signal, the other station will receive your signal lower than their own. Any time that Doppler causes an uncorrected shift on the transponder there will be a difference in frequency between your downlink and the other station’s downlink. In summary, If possible, use software CAT control and select the ‘both’ option. If you have to use manual control, always follow the ‘one rule.’ If the other station is using the ‘one rule,’ you will have to tune your receiver to hear it. If the other station is using the ‘one rule’ on a UV transponder use ‘both’ mode, or alternatively uplink correction. If the other station is using the ‘one rule’ on a VU transponder use ‘both,’ mode or alternatively downlink correction.

CAT CONTROL If you are building a serious satellite ground station and you plan to use satellites with linear transponders, having CAT (CI-V on Icom) control of your transceiver or receiver is a definite advantage. It is easy to achieve and should not cost more than the price of a connecting cable between your radio and your computer. For more on CAT, see the CAT section later in the book.

ROTATORS AND COMPUTER CONTROL When you first start with linear transponder satellites, you can use a portable setup much like the handheld arrangement that many people use for the FM satellites. I do recommend mounting the antenna(s) on a small tripod because you have to contend with Doppler correction and less portable radios. You can take things to the next level by adding an azimuth (horizontal) rotator or an azimuth and elevation (vertical) rotator. This will allow you to steer the antenna without having to leave your operating position to adjust the tripod. It also means that you can move inside and set up a home satellite station. If you are building a ‘home’ station, you will probably mount the antennas on a pole or tower in the backyard. Some people mount VHF and UHF Yagis above or below their HF beam, but you can’t usually employ elevation control with this arrangement. The 2m and 70cm antennas don’t need to be particularly high provided there is room for them to rotate and elevate without hitting anything. When you are planning a location for your antenna pole, think about foreground obstructions. It is best if there is an unobstructed path to the satellite in all directions when the elevation is above about ten degrees. If you place the antenna system in a spot where you can achieve that, you are very lucky. Normally compromises are necessary. But plan where the pole is placed with these points in mind. Computer control of the rotators is the final luxury. Being a bit of a ‘computer nut’ I designed and built my own rotator controller, but there are commercial options available.

WORKING DIGITAL SATELLITE TRANSPONDERS Digital satellites carry a completely different type of transponder. The signal on the uplink is received and demodulated. When the signal is retransmitted on the downlink channel, the digital information has been regenerated and it should be completely error free. This is different to using a digital mode like PSK or SSTV on a linear transponder satellite. Linear transponders do not regenerate the signal so any data errors caused by noise on the uplink path will be faithfully repeated onto the downlink transmission. The early digital satellites were set up for ‘packet store and forward.’ This is a system where written messages can be sent to the satellite and downloaded later by another operator. Similar packet radio bulletin boards were popular on the VHF bands before the Internet, but they have now fallen into disuse. To use the packet mode you had to use a TNC (terminal node controller) but these days that can be achieved using a software application running on your PC, so no special hardware is required. OSCAR 6, launched in 1972, was the first amateur radio satellite to have a store and forward ‘packet’ radio system. Messages uploaded to the satellite could be retrieved later in the same orbit or during a subsequent orbit. The system was used to relay messages between the two ground control stations in Canada and Australia. The format for satellite packet radio is AX.25 at either 1200 bauds or 9600 bauds. AX.25 is a ‘link layer protocol.’ A data frame structure specifically designed for amateur radio. It can work as a ‘connected’ link between two stations or you can send UI (Un-numbered Information) frames, which don’t require a reply from the other station. For 1200 baud transmission, the signal is transmitted using AFSK (audio frequency shift keying). Digital ones are represented by a 1200 Hz tone and digital zeros are represented by a 2200 Hz tone. It is rather like RTTY except that it uses 8 bit ASCII data instead of the 5-bit Baudot data format. The two tones are FM modulated by your transceiver. For 9600 baud transmission, direct FSK (frequency shift keying) is used. The radio frequency is changed according to the 9600 baud digital signal appearing on the data port on the radio. This port is often an RS232 serial port, which is also used for CAT control. There are a couple of software applications that can replace the hardware TNC to produce the AX.25 frame structure. The most commonly used is

‘AGWPE’ written and supported by George Rossopoulos SV2AGW. AGWPE is available at and at An alternative is to use the ‘software packet radio TNC’ written by MixW and some other digital mode programs can also act as a TNC. These days the use of satellite bulletin boards is discouraged because it requires you to ‘connect’ with the satellite. It takes several minutes to upload and download your messages blocking other users from access. If you lose, coverage before logging off the board, the satellite will continue to listen for your signal and this will continue to block other users even when your satellite pass is over. Most digital transponders are now only used for APRS.

APRS DIGIPEATERS The digital satellites are used to relay APRS (automatic packet reporting system) data. You send a short ‘APRS beacon,’ to the satellite and it will be repeated as the satellite orbits. Several ground stations run by amateur radio operators receive the beacons from the satellite and send them to an APRS Internet server. Your call sign and a short message will be displayed at your location on an online map. These Internet connected receiving sites are called iGates. There is a shortage of iGates in some locations, causing gaps in the worldwide APRS coverage. Please consider adding your station to the worldwide community of iGates, especially if you live in Hawaii, India, China, or anywhere the Southern Hemisphere. APRS was invented by Bob Bruninga, WB4APR. It is commonly used to track and display the position of amateur radio mobiles and to convey data from home weather stations. It can be used to send short messages and even emails although these are not private. APRS is also used to track amateur radio balloon flights. Some balloons with APRS beacons fly right around the world. The digital repeater uplink requires AX.25 data sent at either 1200 bauds or 9600 bauds. It uses FM modulation. Most of the satellite digipeaters, like terrestrial APRS repeaters, use the same frequency for receiving and transmitting. The internationally agreed frequencies for APRS on satellites are 145.825 MHz FM 1200 BPS and 437.550 MHz FM 1200 BPS. Some satellites have multiple uplink frequencies and a single downlink frequency. Unlike the connected AX.25 bulletin board mode, which hogs the satellite, the APRS data is sent as a short burst using the AX.25 UI frame structure. Your APRS software and radio will send ‘APRS beacons’ up to the satellite. If the signal is successfully received at the satellite, it will be repeated several times around the orbit. Like the terrestrial APRS systems, there is a possibility of two stations sending a beacon at the same time. It is normal practice to send your beacon every five minutes or so during the satellite pass. The data only has to be received at the satellite one time. If you set the resend time to something other than exactly five minutes, you will have less chance of repeatedly clashing with another station. Doppler correction for both the uplink and downlink is advisable for the VHF frequency and very advisable for the UHF frequency. Particularly for ARISS because it travels so fast.

APRS MAP DISPLAY PROGRAMS Using Win APRS, X-APRS, YAAC, UI- View32 or other software, you can plot your signal and others received from the satellite on a map displayed on your PC. You don’t even necessarily have to receive signals from the satellite. Because you will be able to see your data and pin displayed on a map when you visit the APRS website. Note: UI-view32 will not run on 64 bit operating systems like Windows 10. YAAC “Yet another APRS client,” can operate on Windows platforms, Mac, Linux, and RaspberryPi. It can also interface with hardware TNCs including Outernet receivers.

APRS ONLINE MAPPING WEBSITES displays APRS packets received from satellites other than the ISS. Currently, the traffic seems to be coming from PCsat and PSAT. displays APRS packets received from the ISS. displays every APRS packet from anywhere on the planet, that makes it to an iGate. It defaults to a map of your local area. is a link to the UI-View APRS software page. links to an excellent tutorial called, ‘Add ISS Packet Operation to Your Satellite Activity.’ As well as repeating APRS beacons received from ground stations, PSAT uses an onboard GPS receiver to work out its own location and reports it via an APRS beacon.

CHOOSING A SATELLITE ISS (ARISS) The APRS digipeater on the International Space Station is usually configured to use the VHF frequency. APRS digipeater: 145.825 FM 1200 bps APRS digipeater: 437.550 FM 1200 bps Aliases ARISS or RS0ISS-4 ISS is in a very low orbit so there is significant Doppler shift on the UHF frequency. Remember that you should correct your transmit frequency for the uplink Doppler shift as well as correcting for Doppler shift on the downlink. There is a tutorial at PCsat-1 (NO-44) was launched 30 September 2001. The batteries are dead, but the satellite is functional while in sunlight. After periods of eclipse, the satellite usually boots up into a failsafe, ‘standby downlink transmitter mode.’ It should not be used as a digipeater while it is in this mode. The control station will restore it to normal mode as soon as possible. APRS digipeater: 145.825 1200 baud and 9600 baud FM Aliases ARISS or RS0ISS-4 PSAT ‘ParkinsonSat’ (NO-84) has a 350 mW APRS digipeater on 145.825 MHz at 1200 baud, and an AU mode PSK digipeater. The APRS digipeater is harder to work than the one on ISS because it is further away (higher orbit) and because PSAT has less transmitter power. APRS Digipeater on 145.825 MHz 1200 baud FM PSK31 Downlink 435.350 MHz +/- 5 kHz FM (300 mW) PSK31 Uplink 28.120 MHz +2 kHz PSK31 SSB (up to 75 watts into an omnidirectional vertical antenna is authorized) LilacSat ‐2 (CAS‐3H) has the call sign BJ1SI. It carries a VU mode linear transponder, an APRS digipeater, and a VU mode FM voice repeater. The FM voice channel and the APRS digipeater share the same uplink and downlink frequencies. Transmitter power is +23 to +27 dBm (200-500 mW). APRS Uplink

144.350 MHz AX.25 1200 bps AFSK FM

Downlink 437.200 MHz 1200 bps AFSK (not 144.390 MHz) FM voice transponder Uplink 144.350 MHz FM Downlink 437.200 MHz 1200 FM Telemetry downlink 437.225 MHz 1200/4800 bps GFSK Linear transponder (non-inverting normal tracking) Uplink 144.3425 - 144.3825 MHz SSB/CW Downlink 437.180 - 437.220 MHz SSB/CW Outernet The Outernet system has been mentioned in the section about ‘listening to satellite transmissions.’ It uses three geostationary satellites to relay educational and general interest information in the form of text files. The three satellites are placed so that they provide coverage to almost the whole planet. All APRS beacons that are received by any iGate, anywhere, end up at, which displays them on one of the three web pages listed above. There is a proposal for the Outernet to relay the APRS data from findU so that all APRS traffic worldwide can be received without access to the Internet anywhere within the footprint of the three Outernet satellites. Proposed digital satellites QIKcom-1 is on the ISS ready to throw out the window, but there is a problem with FCC approval. QIKcom-2, PSAT-2, and BRICSAT-2 are scheduled for launches in 2017 or 2018.

SATELLITE TELEMETRY The AX.25 1200 baud and 9600 baud ‘packet’ protocol is often used for telemetry downlinks. There is no uplink or APRS capability but you can use the same radio equipment and ‘packet’ modem to receive and download the information that the satellite transmits on the downlink frequency.

SENDING APRS BEACONS Sending APRS beacons to a digital satellite is much like working FM voice. You can use a handheld antenna or even a fixed antenna because you only need to send one successful data burst to the satellite. Then you can sit back and monitor the downlink for other APRS bursts or wait for the spot to be displayed on the Internet page. The setup for sending and receiving packet data is similar to sending APRS over a terrestrial APRS repeater. You need an FM transceiver (preferably a dual bander), with a data port to accept the APRS data signal. These days many mobiles can do that, some even have APRS capability built in. Before transmitting to a satellite, it is best to test your setup by sending beacons to a local APRS digipeater. When that is successful, you can change over to the Satellite frequency, generally 145.825 MHz or 437.550 MHz Simplex. Set your path to ARISS,SGATE,WIDE2-1 and when the satellite is in range, send your beacon. Low power can be used. The satellite will hear you. At reasonably high satellite elevations, even a handheld will get in. Although you will most likely not be able to hear the satellite downlink and will have to view your data on the Internet. Please be considerate to others wanting to try APRS via the satellite. Set your beacon rate so that your radio only transmits the APRS data two or three times during a typical ten-minute pass. Path settings: ARISS is a common alias used by all of the digipeater Satellites. It means that you don’t have to change any settings when you move to another satellite. SGATE tells the satellite to pass your position report on to the terrestrial network, and WIDE2-1 is the path used once the beacon is in the terrestrial network. If you already have a mobile in your car that can send position information to a local APRS gateway, you should be able to send to the satellite simply by changing the APRS path and the radio frequency. Wait until the satellite is up at 30 degrees and send a beacon. Note that higher satellite elevations may not work as well because the antenna on your vehicle will have a vertical null.

PROBLEM SOLVING There are really no additional problems when you work the digipeater satellites. In many ways, digipeater operation is simpler than the voice modes. Because the digipeater uses FM, you can program three or five channels into the radio memories, just the same as you do for the FM voice satellites. If you are already set up to operate APRS on terrestrial VHF digipeaters, you should have little difficulty with APRS on the satellites. If the digipeater is using the UHF APRS frequency. You will need to account for Doppler shift on the uplink and the downlink if you are bothering to receive it. Table 6: programming chart for 2m band APRS with 5 kHz spacing Channel Transmit Receive 1 145.820 MHz 145.830 MHz 2 145.825 MHz 145.825 MHz 3 145.830 MHz 145.820 MHz Table 7: programming chart for 70cm band APRS with 5 kHz spacing Channel Transmit Receive 1 437.540 MHz 437.560 MHz 2 437.545 MHz 437.555 MHz 3 437.550 MHz 437.550 MHz 4 437.555 MHz 437.545 MHz 5 437.560 MHz 437.540 MHz Make sure that the satellite is operational. Check when the last APRS beacons were received by visiting the website for ARISS or the for the other digital satellites. The forum on has up to date information about ARISS.

APRS INFORMATION WEBSITES has a wealth of great information about APRS. Wiki

WORKING THE BIG SATELLITE The most challenging satellite for amateur radio operators to use is the largest by a wide margin and it also has the highest orbit. Yes, of course, I am talking about the Moon. The mode is called ‘Moon Bounce’ or more commonly ‘EME’ (Earth – Moon – Earth). I am definitely not an expert on the subject, having to date a single confirmed EME QSO, on the 2m band with RX1AS a ‘big gun’ EME station in St Petersburg, Russia. The Moon is a satellite, even though it’s not an artificial one, so it is worthy of a mention. The equipment for EME is about the same as a ‘big’ satellite station, but you only need equipment for one band. At VHF and UHF frequencies, EME enthusiasts generally use high gain Yagis and sometimes switched cross-polarized Yagis, but not circular polarization. Because the Moon is a lot further away than the artificial satellites and the signal is not being amplified before it returns to Earth, you need a lot more transmitted power and a high gain antenna for receiving the very weak returned signals.

EME HISTORY During January 1944, German technicians were surprised to see echoes with a 2.5 second delay while they were testing an experimental 120 kW Radar station operating on 564 MHz. The station, codenamed ‘Würzmann,’ was located on Rügen Island (Göhren) on the shore of Baltic Sea, (locator JO64UI). They had inadvertently beamed the collinear radar antenna array towards the rising Moon. When the Moon passed out of the antenna lobe, the mystery signal disappeared. The technicians repeated the experience the following evening. They were seeing the first known radio echoes from the Moon. In the years following World War II, ‘Moonbounce’ techniques were developed for military and commercial use by the United States Military. Their first successful reception of echoes reflected off the Moon was carried out at Fort Monmouth, New Jersey on January 10, 1946, by John H. DeWitt as part of ‘Project Diana.’ This was followed by the ‘Communication Moon Relay’ project, which led to more-practical uses for EME, including a teletype link between the naval base at Pearl Harbour Hawaii and the United States Navy headquarters in Washington, DC. The first amateur radio echoes from the Moon were sent and received by Ross Bateman W4AO and Bill Smith W3GKP, on the 2m band, on July 15, 1950. But it was not until July 17th, 1960 that the first Trans-USA EME ham QSO was made. W6HB and members of the Eimac radio club San Carlos California sent a signal one way via the Moon on 1296 MHz to members of the W1BU (Rhododendron Swamp V.H.F. Society) club station in Boston Massachusetts. Both teams used a 1 kW transmitter and an 18-foot microwave dish. Four days later signal reports were exchanged both ways marking the first two-way amateur radio EME QSO.

THE MOON AS A REFLECTOR The Moon is not a particularly good radio reflector because its surface is rocky and irregular and because it is roughly spherical which makes radio signals reflect off at an angle rather than straight back towards Earth. The Moon is only about 7% as good at reflecting radio waves back to Earth as a flat reflector of the same radius would be. The signals received back from the Moon over the 770,000 km average return distance, are very weak indeed. To make matters worse, the received signals become fluttery and distorted because of the multiple reflections from the Moon’s curved and irregular surface, plus path loss, heat noise, ducting, attenuation in the Earth’s atmosphere, and Faraday rotation, which alters the polarization of the radio signal as it passes through the atmosphere. The round trip path loss for an EME transmission is approximately 375 dB for the VHF 2m band and 395 dB for the UHF 70cm band. At microwave frequencies, the path loss is higher, approximately 448 dB at 10 GHz. The higher path loss at microwave frequencies is partly offset by the ability to use high gain dish antennas. Four 10-element Yagis on the 2m band could have a gain of 20 dBi. A 2m diameter dish working at 10 GHz has a gain of around 44 dBi. But even 24 dB of additional antenna gain is not enough to offset the 36 dB of additional path loss in each direction. The Moon’s orbit is almost circular so the path loss is always about the same. The return path loss is 2.04 dB higher when the Moon is at apogee than it is when the Moon is at perigee. To overcome the radio path loss, you need high gain antennas and reasonably high power levels. You also need to radiate a signal that is not going to be bent back to Earth by the Ionosphere. For this reason, EME operation is restricted to the VHF, UHF, and microwave bands. Most activity is on the 2m band with significant activity on 6m and 70cm, and some activity on the higher amateur radio bands up to 10 GHz. The next best extra-terrestrial object for ‘echo’ purposes is Venus. It has a five-minute round-trip delay. It is roughly a million times more difficult to detect a reflected signal from Venus than from the Moon. Amazingly, this feat has been achieved by amateurs using the 20 m dish antenna at Bochum in Germany. They transmitted 5 kW signal on the 2.4 GHz band into the 20-meter diameter dish. Not your average ham station!

JT65 MODE Because of the very weak signals that you can receive from the Moon, almost all EME QSOs are conducted using the JT65 weak signal modes developed by Joe Taylor, K1JT. JT65 is one of the digital modes included in the WSJT software package that Joe makes available for free, from CW can be used as well, but SSB is not often used because of the weak signals, flutter, noise, and distortion on the received signals. SSB conversations are usually only possible between ‘big gun’ stations running high power into very large antenna arrays, or big dishes if you are using microwave frequencies. But sometimes, short SSB contacts can be made between modest stations. The WSJT software is extremely effective for very weak signal conditions. It has opened up EME and other weak signal modes like meteor scatter to hundreds of hams who have modest stations and aerial systems. I know from personal experience that it is possible but not easy to have a successful JT65 mode EME QSO using a single 12 element 2m Yagi and 100 watts of transmitted power. Especially if the station at the other end has more power and a four or six-antenna array. To quote the WSJT operating manual, “WSJT is a computer program for amateur VHF/UHF communication using state of the art digital techniques. It can help you to make contacts using fraction-of-a-second signals reflected from meteor trails, as well as steady signals more than 10 dB weaker than those needed for conventional CW. The software includes several operating modes; FSK441, designed for high-speed meteor scatter, JT6M, optimized for meteor and ionospheric scatter on 6meters, JT65A, B and C for Earth-Moon-Earth (EME) and weak troposcatter and CW for EME using timed computer-generated transmissions.” “JT65A is generally used on 50 MHz and for HF band non-EME contacts. JT65B is used on 144 and 432 MHz, and JT65C on the 1296 MHz band and above. The B and C JT65 modes are slightly less sensitive than mode A, but progressively more tolerant of frequency drifts and rapid libration fading.” When you use the JT65B mode, your station transmits continuously for 60 seconds and then receives for the following 60 seconds. The periods are different for other JT65 modes. During each period, around 20 characters of text information can be sent, so the information you send in an EME QSO is kept to an absolute minimum. A typical initial transmission might

contain ‘CQ’, your call sign and Maidenhead grid (CQ ZL3DW RE66HM). The response would be your call sign, the answering station’s call sign, and their grid (ZL3DW RX1AS KO59XW). In the following over you would send the signal report which is always ‘O’ (normally sent three times as OOO), the other station will respond by sending ‘RO’ which means I have seen your O report and respond with an O report for you. You normally go back with ‘RRR’ meaning I have received your report thanks, and they finish the QSO by sending ‘73’. For EME QSOs, the signal report on your QSL card is always ‘O’, rather than the 59 / 599 type of report used for other types of ham contacts.

ELEVATION AND GROUND GAIN Unlike satellite operation, you don’t really need elevation control on your Yagi antennas for 2m EME. The longest distance amateur radio ‘contacts’ occur when the Moon is at low elevation. Most EME activity takes place at local ‘Moonrise’ and ‘Moonset.’ Another important factor is ‘ground gain’. This is the effect when the signal from the Moon is reflected from the ground onto your antenna in phase with the direct signal from the Moon. Ground gain can add a theoretical maximum signal improvement of 6 dB. For any given antenna height above ground, the phase of the signals from the reflected and direct path will add together when the Moon is at a particular elevation, or more usually at several elevations with deep nulls between. The height of the antenna above ground affects the radiation pattern from the antenna. This can be modeled accurately with antenna modeling software such as NEC4. The best scenario is to place the antenna at a height above ground where the highest gain of the main antenna lobe is at the same elevation as the maximum amount of ground gain. If you do use elevation control on your antenna, you will be able to detect signals reflected from the Moon when it is higher in the sky and extend the time that EME operation can occur, but you don’t get the added ground gain once your antenna is pointing upwards. Ground gain gives a muchneeded boost to the received signal on the 6m and 2m bands, but it is less useful on the higher bands and counterproductive at microwave frequencies. The earth is a relatively hot body compared to space. At high UHF and SHF frequencies, this heat adds noise to the reflected signal from the ground, which impairs the amount of signal improvement from ground gain. You get more signal but also more noise, so the received signal to noise ratio is not necessarily improved by the ground gain. For this reason and because the dish antennas used at microwave frequencies have a narrow beamwidth, EME stations using microwave frequencies usually track both the azimuth and elevation of the Moon. Even on 2m and 70cm, you may not be able to get any ground gain advantage if you live in a built-up area because the foreground may be too cluttered to reflect very much signal onto your antenna. So if you are working EME in the suburbs, you might as well use elevation control. When the Moon is at higher elevations, the coverage footprint is smaller but you may be able to work stations that are closer.

You can try EME if you have a medium size satellite station. For EME on the 2m band, 100 watts of transmitter power into a 10 or 12 element Yagi will let you work ‘big gun’ stations via the Moon. Load up WSJT point your antenna at the Moon and see if you can decode any signals. But to avoid hours of frustration I recommend that you seek help from a local EME expert before trying to transmit. 2m activity is at the low end of the band, just above 144.1 MHz. You can find out who is transmitting, their operating frequency, their transmitting period (1st or 2nd), and an idea of their setup, by looking at the N0UK logger For example the text; ‘K6MYC/4XP32/KW’ means that amateur radio station K6MYC is using four cross-polarized 32 element Yagis and 1kW of RF power.

ONE YAGI OR TWO? Serious EME stations use crossed Yagis because the signal that is received from the Moon may be at any angle of polarization. The difference between the polarization of the incoming signal and that of your antenna causes attenuation. If the polarization error is near 90 degrees, there will be significant attenuation and you will not be able to communicate with the other station. Crossed Yagis allow an operator to switch to whichever antenna (horizontal or vertical1) has the best signal. Use of an SDR (software defined radio) with two synchronous receivers allows an operator to combine signals from both polarizations to get the best overall signal. Note 1: you can use 45O / 45O angled crossed Yagis. They work just as well. There are two primary reasons for polarization error. The first is that the world is (roughly) a sphere so a signal sent from another station may not be in the same plane as the signals that are sent from your station. This error is predictable and can be calculated. To work a station sending a horizontally polarized signal you may receive more signal if you orient your antenna to vertical polarization. The other reason is that the polarization of the signal will be changed by Faraday rotation as it passes both ways through the atmosphere. The amount of Faraday rotation is unpredictable. The amount of Faraday rotation is proportional to the square of the wavelength, so the effect is worse at lower frequencies. The net result is you can’t predict the polarization of signals received from the Moon.

ANTENNA POLARIZATION At VHF and UHF frequencies, the Moon acts as a fairly poor flat reflector. Circular polarization can’t be used because the reflected signal has the opposite polarization to what you transmit. A Left Hand Circular Polarized (LHCP) signal will be reflected back as Right Hand Circular Polarized (RHCP). To avoid the resulting cross polarization attenuation, you would need a different antenna for receiving. Linear polarized signals are OK because the reflected signal maintains linear polarization at the point of reflection. A 180 degree, change in polarization is the same as no change as far as a linear polarized antenna is concerned, so the same antenna can be used for receiving and for transmitting. At microwave frequencies, the irregular surface of the Moon causes multiple reflections at different phases and in differing directions. The net result is that the polarization breaks down into chaos. The reflected signal has components in all different polarization planes. The net result is that the reflected signal will be around 3 dB lower than it would be if the Moon were dead flat. There is no advantage in using either vertical or horizontal polarization, or in being able to switch between polarizations. It also means that if you transmit a circular polarized transmission, you won’t receive the opposite circular polarization back. So at microwave frequencies, there is no real advantage or disadvantage in using circular polarization. Faraday rotation does not affect microwave frequencies very much so that is not really a factor either. For VHF or UHF use a linear polarized antenna or better still switch between vertical and horizontal antennas depending on which is receiving the best signal. For the microwave bands, linear or circular polarization is acceptable. There is no advantage in being able to switch from vertical to horizontal polarization and no advantage or disadvantage in using circular polarization.

FARADAY ROTATION The ionosphere consists of a plasma containing free electrons, which in conjunction with the Earth's magnetic field can cause rotation of the polarization of radio waves and also light. It was from observing this effect the Faraday was able to prove the relationship between light and electromagnetism. This twisting of the polarization of radio waves as the signal passes through the ionosphere is known as Faraday rotation. The density of electrons in the ionosphere varies greatly on a daily basis, as well as more slowly over the seven-year sunspot cycle. Accordingly the amount of Faraday rotation that your signal experiences is affected by the time of day, solar activity, and the sunspot cycle. Faraday rotation is greater in the daytime. The electron density is also affected by the Earth’s magnetic field so the further from the equator you are, the worse the problem becomes. What actually happens? The Faraday Effect causes left and right circularly polarized waves to propagate at slightly different speeds. This does not matter for circularly polarized transmissions because it just causes them to take very slightly less or more time to pass through the ionosphere layer. However, linear polarized radio waves are made up of two equal-amplitude circularly polarized components of opposite handedness and a different phase. The speed differential introduced by the Faraday Effect slows one side and speeds the other, causing a twist (rotation) in the polarization. The propagated electromagnetic wave stays linear but it changes relative to the antenna from which it was transmitted. The Faraday rotation of the signal returning from the Moon will be in the same direction as for the outgoing signal. When the Moon is at a low elevation the signal passes through more of the atmosphere so there is more Faraday rotation when the Moon is low in the sky. The amount of Faraday rotation is proportional to the wavelength of the radio wave, which makes it inversely proportional to the frequency. At 144 MHz, Faraday rotation can cause as many as seven complete 360 degrees rotations of the signal plane. The polarization of the received signal can change rapidly. At 432 MHz, a single 360-degree rotation of the signal plane is more typical and at microwave frequencies, the amount of rotation is quite small. At 1.4 GHz, the Faraday rotation will typically be less than 11 degrees and at 10 GHz, it is typically less than 0.2 degrees. A polarization error of 11 degrees only represents a 0.13 dB attenuation in the received signal so it is not a big deal. Attenuation due to Faraday

rotation only really becomes a problem at VHF and UHF frequencies. The polarization error could end up between 45 degrees and 90 degrees. At 45 degrees, you would experience 3 dB of signal loss and at 90 degrees; you could experience more than 30 dB of loss. In this situation, the ability to switch to an antenna on the alternate polarization can make the difference between having a successful contact and “nothing heard.” Being able to see your own signal reflected back from the Moon is a big plus, but it is no guarantee that another station will be able to see your signal on the waterfall. Faraday rotation means that it is common for propagation between stations to be fine in one direction but not the other. The effects of Faraday rotation can’t be predicted. You just have to take what you get. It is just another part of the challenge of EME operation.

DOPPLER SHIFT The Moon is a satellite that orbits the Earth so there is Doppler shift on the signals that are sent to it and on the reflected signals that come back. There isn’t a huge amount of Doppler shift because relative speed between the Earth and the Moon is quite low. The Moon takes 27 days 7 hours and 43.19 minutes to complete an orbit of the Earth. But the Earth is rotating as well and that actually has a greater effect on the relative velocities. The low relative speed means that the amount of Doppler shift is small for VHF and UHF frequencies. More importantly, the Moon is a long way away so the rate of change in the Doppler shift is quite small. Frequencies do not need to be continuously adjusted. The Doppler shift can be treated more like an offset between the frequency that you transmit and the frequency that you receive. The higher the band that you are operating on the more offset due to Doppler shift you will see on the waterfall display. As with all Doppler shift, the received signal will be higher than the transmitted signal when the Moon is rising and lower than the transmitted signal when the Moon is setting. The maximum amount of Doppler shift for a typical EME path up to the moon and back to a station on Earth is around 440 Hz for transmissions at 144 MHz, 4 kHz at 1296 MHz, and 30 kHz at 10 GHz. There is also a delay because of the distance. The returned signal will be delayed by about 2.4 seconds when the Moon is at perigee and 2.7 seconds when it is at apogee.

LIBRATION FADING The signals you receive back from the Moon have been reflected from many points on the surface. Because of its spherical shape, there is a shorter return path from the center of the Moon than from the visible edges. The many different path lengths cause multiple reflected signals with different phase delays. As well as phase delay, the difference in relative speed caused by the rotation of the Earth and the orbit of the Moon causes differential Doppler shift between the reflections from different parts of the Moon’s surface. This causes frequency spread on the reflected signals. Combine this with the effects of ionosphere scintillation distortion (the effect that makes stars appear to twinkle) as the signal passes through the atmosphere and there are a lot of random phase relationships. All of the signals may add roughly in phase at your antenna or events may conspire against you and they may cancel. The effect is called Libration fading. Stations nearer to the equator observe more rapid libration fading and wider signal spreading.

The fading rate varies over time and is proportional to the frequency that you are using. The frequency spread causes tone errors on CW signals making medium speed CW hard to copy on EME. Higher transmit frequencies have more Doppler shift and therefore a greater frequency smear on the received signal. The frequency smear is usually accompanied by rapid fading. At 144 MHz, you may hear several Morse code characters on each of the signal peaks, but at higher frequencies, the rapid fading can break up the reception of individual characters. At 144 MHz, the differential Doppler can cause a 4 Hz spread of the CW tone. Not a problem. At 10 GHz, it is 295 Hz, which would make CW reception a challenge. I hope this chapter gives you a taste of the EME world. It is not for the faint-hearted. You need a great deal of skill and patience. Some days the signals are weak or there is too much distortion or fading and it just does not work, other days it is really exciting.

“MISTAKES, I’VE MADE A FEW” Initially, you are going to make mistakes. Working satellites is a complex business and there is a lot to manage over a short period of time. The key to success is to plan ahead and to try not to get flustered. Most satellite operators are pretty tolerant. They have all made the same mistakes themselves. Don’t get frustrated. It is normal to have a few busted calls. In fact, even if you don’t make any mistakes, many calls are lost before the contact is completed. This is usually because the satellite goes out of range at your location or at the other operator’s location.

TIME ERRORS You need to know when the satellite will rise above your horizon and how long it will be available. If you are using a satellite-tracking program, make sure that the clock on your PC is set accurately. An error of one minute could mean an antenna pointing error of up to 20 degrees. If you are not using a satellite-tracking program, your watch needs to be accurate. The clock on your cell phone is always correct, so that is a good time reference. Check that the time you are predicting for the satellite is your local time and that daylight saving has been taken into account. There is no point looking for a satellite an hour after it has passed over. If you use UTC time, make sure that you calculate the offset correctly. You may laugh but it has happened. It is easy to lose track of time in all the excitement and you may end up calling on a satellite that has already passed over or is just about to go out of range. The latter is very annoying to anyone who comes back to your call and then receives silence. It is also annoying to anyone who is patiently waiting for his or her turn to use the satellite.

BEING TOO LONG WINDED Satellite conversations, especially on the FM satellites where there is only one voice channel, should be brief and to the point. Other people may be waiting to “have a turn.” Brevity is not such an issue on linear transponders because there is room for several conversations. But generally, I advise making a series of short ‘overs’ rather than a few long ones. If you talk for a long time, the satellite may have passed out of range at the other station while you are talking. This leads to confusion. LISTEN to what the other station is saying! If they say, “I am going to lose the satellite soon,” it means that they will only have coverage for a minute or two longer. This is your cue to sign off with a cheery, “Thanks for the QSO, 73.” Your first ‘over’ should include your call sign, the other station’s call sign, your name, your QTH, and your Maidenhead grid, followed by “over to you.” There is no need to repeat your call sign again. Most people trade signal reports. But I believe that this is pretty redundant. The signal that you receive is mostly dependent on your reception of the downlink from the satellite. It has very little to do with the other stations uplink transmission. It has more to do with your station's configuration than the configuration and power at the other end. It is similar to when you use a terrestrial repeater, signal reports don’t mean very much. Your second ‘over’ can be more conversational, but on an FM satellite, it should end with a gap or a comment inviting others to join in. If no other stations respond or you are using a linear transponder, by all means, carry on conversing throughout the pass. The rules for digital modes like SSTV, PSK or RTTY are similar. Digital satellite operation involves sending bursts of APRS or packet data so being ‘long-winded’ is not really a problem. I guess if you sent a long email or a big file you could be accused of “hogging the satellite,” but I don’t think this is a common issue. Most of the digital satellites don’t get much bulletin board or packet traffic they mostly just handle APRS. You don’t need to send an APRS beacon more than two or three times in a pass.

FREQUENCY PROBLEMS Frequency issues are a common source of aggravation. Firstly, the satellite may not be transmitting or receiving on the published frequencies. The temperature inside the satellite may be different to what was expected and it may change dramatically as the satellite moves from the intense cold of an eclipse to the intense heat of solar illumination. Heat affects the stability of crystal oscillators so a change in temperature can cause the transmitter and/or receiver frequencies diverge several kilohertz from the published frequencies. The amount of illumination of the solar panels affects the voltage on the batteries and this also can change the oscillator frequencies. Check the satellite operator’s website and satellite forums for up to date information. Or you might make a mistake with the frequencies. In the heat of the moment, you may forget to change channels on your FM radio as the satellite passes over. Or you could inadvertently select the frequencies for a different satellite. If you are using a linear transponder, you could fail to adjust for the Doppler shift or tune outside the bandwidth of the transponder. It is very easy to lose the downlink signal from another station between overs. You transmit and when you have finished the ‘over,’ you find that you can’t hear the other station. This is very easy to do if you are using different Doppler correction to the other station. The best remedy for frequency related operating errors is experience. The more contacts you make the smoother your operating procedure will become.

CROSS BAND INTERFERENCE One classic mistake, which has caught me out several times is listening to a harmonic of your transmit frequency and believing that you are hearing the satellite downlink. If the satellite pass has finished and you can still hear yourself on your receiver when you transmit, that is almost certainly, what is happening. The issue arises when you are transmitting on the 2m band and receiving on the 70cm band (VU mode). The third harmonic of your 2m band transmission falls into the 70cm band. It will not be an exact match for the wanted downlink frequency because satellite builders aren’t that dumb, but it can overload the 70cm receiver causing you to hear distorted audio. You can improve the situation by using wide spacing between your antennas and cross polarization to minimize the coupling between your antennas, for example, vertical for 70cm and horizontal for 2m. If the problem is very bad, you may have to abandon the use of full duplex operation when you are operating in the VU mode. UV mode transmissions are not usually a problem, but it is still a good idea to use cross-polarization isolation.

SHARING THE TRANSPONDER In my part of the world, the convention when using satellites with linear transponders is that the first person to call when the satellite comes into range will use the center frequency of the transponder. If that slot is in use, the next operator will choose a frequency above or below the center frequency. If you are operating CW, you will probably call near the lower end of the uplink frequency range and if you are transmitting SSTV or a digital mode, you will probably operate nearer the high end, or between the CW and SSB operators. This convention may be different in your part of the world. Please listen to what is happening and ask other operators what the ‘unofficial’ rules are. Traditionally the lower 1/3 of the transponder is for CW, but you don’t often hear CW on satellites these days, (not round here anyway), so that rule can be relaxed a bit.

USING THE WRONG SIDEBAND Using the wrong sideband. Most linear satellites use ‘Inverting Transponders’ to reduce the overall Doppler shift. You transmit lower sideband (LSB) on the uplink and it appears as upper sideband (USB) on the downlink. Transmitting USB by mistake results in LSB on the downlink. This is not actually illegal but it will limit your chances of success and it marks you as a ‘rookie.’ It is rather like going on the 20m HF band and calling CQ on lower sideband. You are pretty likely to get growled at. It is quite easy to make this mistake as you normally use USB when transmitting on the UHF or VHF bands. I have had a satellite QSO with a ham who called on the wrong sideband. I didn’t make a fuss about it because I was sure that he would get it right the next time.

USING THE WRONG TRANSPONDER Using the wrong transponder. Some satellites have several linear transponders. Normally only one of them will be active at a time, so you need to know which one is currently in use. Some satellites only turn on the transponder or repeater at certain times, when the satellite is over a particular location, or when the satellite is eclipsed, or not eclipsed. Check the satellite operator’s website for the operating schedule.

NOT ALLOWING FOR DOPPLER SHIFT Doppler never sleeps. You always need to take Doppler shift into account. Doppler can cause a frequency shift of as much as ±10 kHz on the 70cm band. There is no point listening for a satellite if you are several kHz off frequency. If you are using an FM handheld transceiver, program three or five channels and switch to each as the satellite pass progresses. If you are using a linear transponder, you must regularly adjust the receiver and transmitter frequencies to compensate for the Doppler shift, preferably using computer CAT control. It is best if the ‘both’ mode is used to adjust both the uplink and downlink frequencies (i.e. your transmitter and receiver frequencies). Correcting Doppler shift is so important it even has its own chapter later on.

NETTING Netting is the process of adjusting your receiver frequency so that you can hear your own transmission or the other station on the downlink. Once this has been achieved, your satellite control program will adjust for Doppler shift and you should not have to touch the radio’s VFO knob again. If both stations are using ‘both’ mode and adjusting both their uplink and downlink for Doppler shift you should not have to tune anything throughout the pass. Minor frequency errors should be adjusted using the RIT control rather than using ‘the big knob.’ Unfortunately, this often doesn’t happen and you end up tuning your receiver in order to hear the other station. Then you can’t hear your own downlink when it is your turn to transmit, so you make another adjustment. By the time, you finish talking, the other station has moved even further. All this makes operating a challenge. One big netting mistake is having radio tracking set to ‘on,’ allowing the radio to change the transmitter frequency in response to any changes you make to the receiver frequency. Another is ignoring advice and adjusting the ‘big VFO knob’ while you are receiving, which can cause the tracking software to move the transmit frequency. Changing your transmitter frequency, other than for Doppler correction moves your transmission across the transponder and of course, that makes it difficult for the other station to track your downlink. He or she will typically respond by changing frequency as well and you end up chasing each other all over the transponder. There is a risk of crashing into another user’s frequency or disappearing off the transponder bandwidth. During a conversation, you should only adjust your receiver frequency. Do not switch on the tracking button on your radio and allow the transmitter to ‘track’ the receiver. Do not adjust the transmitter frequency yourself. Allow the PC software to do the work.

ACCESS TONES Some FM satellites use a CTCSS (continuous tone coded squelch system), to stop the repeater being held on by noise. Similar tones are used by most terrestrial FM repeaters for the same reason. When you program your radio, set ‘tone’ to ‘on’ for each of the transmitter frequencies. Most of the satellites that use tone squelch require you to send a tone of 67.0 Hz. Do not program your receive channels for tone operated squelch. Doing so may result in you missing weak signals from the satellite. In fact, it is often better to have the squelch on your receiver open so that you can hear any faint difference in the noise level as the satellite comes into range. To complicate things further, to use SO-50, (SaudiSat-1c) you or another operator must send a 74.4 Hz tone for two seconds to turn on a 10-minute timer and then you must send the 67.0 Hz tone during each transmission. You can achieve this by programming an additional first channel pair on your FM radio. One with the 74.4 Hz wakeup tone and the other with the 67 Hz access tone. If someone else has already woken the satellite up it will continue to operate for 10 minutes before it needs another burst of 74.4 Hz tone to reset the timer.

ANTENNA POINTING PROBLEMS Successful operation requires you to point directional antenna(s) at the satellite throughout the pass. The two most common problems are not getting the azimuth angle right and losing the satellite by pointing the antenna(s) in the wrong direction. The satellite prediction program will provide the azimuth bearing at the AOS and LOS times. These bearings are in degrees east of ‘true’ north. If you use a magnetic compass, you need to factor in the amount of magnetic declination because a magnetic compass points towards magnetic north, not true north. There is likely to be around 20 degrees, of difference between true north and magnetic north. If you are using a handheld antenna, the bearing does not have to be super accurate. Small antennas have a wide beam angle so a few degrees of error will not be noticeable. Besides, if you are hand holding the antenna you will probably be waving it all over the place. If you have a big station with large Yagi antennas, you should make more effort to make sure that the azimuth and elevation calibration is correct. If you are using a handheld antenna, it is a good idea to practice tracking the path that the satellite will take before the pass starts. You will know the point on the horizon where the satellite will rise, how high it will get in the sky and where it will set. Run through it a few times, so that you know where the antenna should be pointing as the pass progresses. While you are receiving, it is pretty easy to keep the antenna pointing correctly, because you can hear the signal change. The problem is that it is instinctive to stop moving the antenna while you are transmitting. You need to keep tracking. Keeping your transmissions short really helps, because the satellite will probably stay within the antenna beam width even if you stop moving it. There is a temptation to equip your station with really big antennas such as 14 elements on the 2m band, or 4x4 antenna arrays. But this is much too big for operating the LEO satellites. Big antennas are difficult to manage. You need high towers and expensive rotators. Yes, you do get a strong signal from the satellites but you are also likely to send too much transmit signal. It is OK to use big antenna systems for satellite operation if your station is primarily, configured for EME, DX operation or Meteor scatter etc. but watch your transmit power levels. If your station is being designed specifically for satellite tracking, big antennas are more of a challenge than a blessing. They have very narrow beam width so they

need to be pointed more accurately. That means that you need accurate computer control and rotators capable of at least one-degree precision. And the rotators need to move fast enough to track the satellite across the sky in a ten-minute satellite pass. That puts a strain on the whole antenna system. Frankly, you just don’t need that much gain for LEO satellites. Big linear polarized Yagis still suffer from cross polarization loss as the satellite moves across the sky, so in spite of the higher antenna gain, you may still have deep nulls in the received signal strength. You are much better off with shorter cross-polarized Yagis. They are easier to move, have a wider beamwidth, and suffer much less cross polarization loss.

TOO MUCH TRANSMITTER POWER You don’t need a lot of transmitter power to work satellites. A three-watt handheld radio working into a modest three-element handheld antenna is perfectly adequate for the FM satellites and ten watts into a small beam antenna is fine for linear transponders. Always use the minimum amount of power possible. If you can hear your signal on the downlink, so can everybody else. More power does not change that and it can make matters worse. If you use too much power into a linear transponder, you will get more than your share of the transponder downlink power. This may affect other users, making you unpopular. Transmitting too much power is a particular issue with AO-07. Too much uplink signal increases the load on the DC system and the whole transponder becomes modulated with your voice. This can be heard right across the transponder and it will make you unpopular. It also makes it very difficult for other stations to determine the frequency of your downlink signal. You become equally poor at all frequencies. Generally, you need to transmit slightly more power when a satellite is at low elevations because that is when the range and hence the path loss is the greatest. There is also more signal lost due to foreground obstructions such as trees and buildings. At higher elevations, you can reduce your transmitter power. You may not need any more than a watt or two. You definitely should never use a linear amplifier such as would be used for EME or DX operation.

BEING AN ALLIGATOR An Alligator has a large mouth and tiny ears. In the ham radio world, the term refers to a station that runs a lot of transmitter power but has a poor receiving setup. They tend to call endlessly but can’t hear the people who respond. It takes surprisingly little power to reach a satellite, but it is difficult for your station to receive the very small amount of RF energy that the satellite repeater or transponder is radiating. An Alligator is a person who transmits to the satellite but can’t hear the downlink. This is exceptionally annoying to people who are already using the satellite and have to tolerate the Alligator calling over the top of their conversation, or to people who respond to your call but get no reply. For FM satellites, you should not transmit unless you can hear signals from the satellite. OK, sometimes there is nobody using the channel and you have to be the first caller, but please leave time for other users before you call again. For digital satellites set your system up to beacon once every four or five minutes. You should not send lots of beacons on the same pass. Stations using linear transponders normally monitor their own downlink signal while they transmit. This gives you confidence that the call is going smoothly and it ensures that you can hear the downlink. It also allows you to back off your power to the point where the downlink just starts to sound a little noisy.

DEAD SATELLITES Unfortunately, there is a huge amount of obsolete information on the Internet. This can lead you to try to work a satellite that is no longer operational. Your best defense is to keep an eye on the operating schedule and other information on the satellite operator’s website. Be warned, many satellite websites are enthusiastically updated during the design and build phase of the project, until just after the satellite is launched. But often they are not updated when after a few days, months, or years the satellite fails. The AMSAT-NA web page lists satellites with recent activity.

QUICK QUIZ QUESTIONS 1. The most important thing to remember when operating with a portable setup and a handheld antenna is; A. to work as many stations as possible while the satellite is available B. to remember to change channels as the satellite passes over C. to be aware of your surroundings and make sure that you are operating in a safe environment 2. When you are operating through a LEO FM voice satellite; A. you don’t have to worry about Doppler shift B. you should talk for a long time to maximize the time you have available C. you must remember to change channels as the satellite passes over 3. Full duplex operation is virtually essential for; A. operating through an FM voice satellite. B. operating through an SSB linear transponder voice satellite C. operating through a digipeater satellite 4. Preparation is important. Before attempting satellite communication you should; A. check that your gear is all connected up correctly and working properly B. find out when and where the satellite will rise and set, the frequency and mode of operation, duration of the pass and maximum elevation C. make sure that you packed some snacks and a drink 5. If you have setup computer software control of your radio frequencies; A. you should use the displayed frequencies as a guide. It is much easier to adjust the frequencies manually B. it is always best to allow the computer to adjust both the uplink and the downlink frequency

C. the computer should be set to follow the ‘one rule’ and adjust the higher frequency.

QUIZ ANSWERS 1. The correct answer is C. While options A and B are worthy goals, it is absolutely essential that you ensure you’re own and everyone else’s safety while you operate. You will be distracted pointing the antenna and adjusting the radio as well as thinking what to say, so you have to make sure that you won’t walk into someone, trip over an obstacle or fall down a bank. This is even more important when operating at night or in extremely cold conditions. 2. The answer is C. You always have to worry about correcting for Doppler shift. It is not as difficult for FM operation because FM is more tolerant of frequency errors than SSB. When working through FM satellites you can manage Doppler by pre-programming your radio with a series of channels and stepping through them during the satellite pass. You should be courteous and share the 10-15 minute pass with other stations and you need to be aware that the station that you are talking to may lose the satellite before you. Keep your conversations brief with short ‘overs.’ Leave time for others join in or make their own calls. 3. The answer is B. You can use full duplex for FM voice satellites, but because the satellite uses single fixed frequencies for the uplink receiver and the downlink transmitter, it is not essential. Full duplex (being able to hear your downlink signal while you are transmitting on the uplink) is very useful, if not mandatory, with SSB transponders. It helps you keep your frequencies correct. Any frequency error results in a similar change in the received audio, making the SSB signal sound squeaky and hard to copy. Digipeaters receive your signal and later re-transmit it, usually on the same frequency as the uplink channel. So full duplex operation adds no benefit. 4. A and B are both correct and C is probably a good idea. You need to know that all of your equipment is OK. There is no time to make adjustments or repairs during the short satellite pass. If you are using a handheld antenna or manually adjusting a rotator, you need to know the timing and shape of the pass. Where to point the antenna at the start of the pass, the mid pass elevation and direction, and where the antenna should be pointing at the end of the pass.

5. The answer is B. If you have computer frequency control adjusting the radio frequencies for Doppler shift. It is always better to use the mode that adjusts both the uplink and the downlink. If you are doing manual frequency control you should follow the one rule and use the frequencies displayed on your satellite tracking program as a guide.

VISIBLE SATELLITES There is a whole separate hobby related to visually observing satellites. Out in the country away from the city lights, it is pretty easy to see satellites streaking across the sky. The best time is just after sunset when you are in the dark but the low orbit satellites are still in sunshine. Early in the morning works just as well if you are an early riser. If you stare at the sky for ten minutes or so, you are likely to see a satellite. They look like tiny stars moving in what looks like an unnaturally straight line from horizon to horizon. You are not actually seeing the satellite. The bright light that you can see is sunlight reflected from the satellite’s chassis, antennas, and solar panels. After observing for a while, you will see that there are satellites moving in all sorts of directions. The polar-orbiting satellites travel pretty much north to south or south to north, but others travel more west to east, or even east to west. The ‘Heavens Above’ website is dedicated to visible satellites, It has an online calculator to show you when and where to look. You can’t see geostationary satellites with the naked eye. They are 35,786 km away. But you can see sunlight reflected from satellites in LEO and MEO orbits and from large bits of space junk in low orbits such as rocket bodies. Some satellites appear very bright for a brief period and then they disappear. This happens if the satellite has a particularly reflective surface on one side, most often the extended arrays of solar panels. It is like flashing light across a room using a mirror. ‘Satellite flare, also known as satellite glint, is the phenomenon caused by the reflective surfaces on satellites (such as antennas, SAR or solar panels) reflecting sunlight directly onto the Earth below and appearing as a brief, bright "flare.” The Iridium constellation of 66 active telecommunication satellites in low Earth orbit is known to cause the brightest flares of all orbiting satellites.’ One amateur satellite was fitted with high-intensity LEDs (light emitting diodes). It was configured to flash Morse code characters and other sequences that were visible from Earth when the satellite was eclipsed, (not illuminated by the Sun). The International Space Station (ISS) is by far the biggest and brightest “human-made” object orbiting the Earth. The station’s huge solar panel arrays are highly reflective making the ISS very bright when it is in full sunlight. With the naked eye, the ISS looks like a fairly bright star. With

binoculars, you may see a tiny bright square. A telescope or a camera with a big zoom lens can reveal the ‘H’ shape of the solar arrays. This may seem disappointing but the ISS is visible at a range of 400 to 2,320 km. Remember how small an airplane at cruising altitude of 36,000 feet (10km) looks at a distance of only about 40 km and you can understand why you can’t see more. Go to the to check the times for visible passes of the International Space Station. The International Space Station is huge! It weighs 419,725 kg. If you include the solar panel arrays, it is about the size of a football field. The station is 109 meters long and 73 meters across the span of the solar panels. Other large satellites in low orbits include China's Tiangong-1 space laboratory, the Tiangong-2 space station, and the Hubble Space Telescope. Tiangong-1 is orbiting at an altitude of 370 km, but it is no longer being controlled and without a re-boost to a higher orbit, the satellite is likely to re-enter the atmosphere during 2017.

BRIGHTNESS OF VISIBLE SATELLITES In astronomy, the brightness of stars is measured using a magnitude scale originally by devised by ancient Greek astronomers. They classified the stars from ‘first magnitude’ (the brightest star in the sky) to ‘sixth magnitude’ (the dimmest star that can be observed with the naked eye). A first magnitude star is 100 times brighter than a sixth magnitude star. Since the invention of the telescope, the brightness scale has been extended at both ends. It now extends down to +24 to include the very faintest stars that can be observed using giant telescopes and upwards to include very bright objects like the Moon at about -12.5 and the Sun at -27. Satellite spotters use the same brightness scale. The light that you see as a satellite passes over, is sunlight reflected from the surface of the satellite and its solar panels. If the satellite is eclipsed, (not illuminated by the sun), you will not be able to see it. Satellites are easiest to see when the satellite is in the sunshine and you are not. The best times are just after sunset or before sunrise. It helps a lot if you are out of the city away from light pollution from vehicles, buildings, and streetlights. Out in the countryside, you will be able to see a lot more stars as well. In the far north, you might even see the Aurora Borealis (northern lights) or in southern latitudes the Aurora Australis (southern lights). On the astronomical brightness scale, +6 is the dimmest satellite that you can see with the naked eye. A camera with a zoom lens, binoculars, or a small telescope will allow you to see fainter objects, improving the limit to about +10. However, this magnification comes at the price of a much reduced field of view. The International Space Station is very bright and it can usually be seen from the suburbs in spite of the city lights. Visible passes range from magnitude 0 to -3.5, so it is easy to see with the naked eye. Sometimes the light from a satellite winks out halfway across the sky. This happens when the satellite moves into Earth’s shadow and it is no longer illuminated by the sun. Or it can happen when the angles change and you no longer see light reflected from the solar panel arrays. While staring into space, you might be lucky enough to see a ‘shooting star.’ These are usually meteoroids burning up as they streak through the atmosphere, but very occasionally, you could be watching the death of a satellite or piece of space junk burning up on re-entry.

PHOTOGRAPHIC IMAGES OF SATELLITES There are two main types of satellite images. The first are images where the stars rotate in the background creating ‘star trails’ and the satellite is fixed in the center of the frame. This is what you get when you use a long, five to ten minute, exposure time with the camera pointed towards a geostationary satellite, or when you use rotators to track a satellite as it moves across the sky. You might use this technique to get an image of the International Space Station. The second are images where the stars do not move (much) and the satellite shows up as a bright streak across the picture. You place the camera on a tripod and start the exposure as the satellite comes into view. Stop the exposure when the satellite passes out of the field of view. A wider lens with an exposure time of 30 seconds or less will yield an image with little streaking or blurring of the stars. Astronomical telescopes, which track the stars, adjusting for Earth’s rotation, will allow you to use longer exposure times and take images with no blurring or streaking of the background field of stars.

PHOTOGRAPHING GEOSTATIONARY SATELLITES Most geostationary satellites are quite large, often bigger than a small car, (think Volkswagen Beetle). But they are in the geostationary orbit a very long way from Earth. None of them reflect enough sunlight to make them visible to the naked eye. Generally, they reflect enough sunlight to reach an astronomical brightness magnitude of about +12. About 100 times fainter than the faintest star that the unaided human eye can see. You can see geostationary satellites with the small telescopes favored by amateur astronomers but they are very hard to distinguish from faint stars of a similar magnitude. If you use a very long exposure time you can capture images of geostationary satellites with a camera, or a camera connected to a telescope. You place the camera on a tripod and use an exposure time of five to 10 minutes. The Earth’s rotation will make all of the stars, galaxies, and other space stuff appear as streaks across the photograph. But because the geostationary satellite is keeping pace with the Earth’s rotation, it alone will be a small dot of light. Where do I point my camera or telescope? All of the geostationary satellites are in the same circular orbit 35,786 km above the equator. So it is fairly easy to work out where they are in the sky. Because they orbit at the same rate as the Earth’s rotation, you only need a fixed azimuth bearing and elevation. There are many online resources for finding the position of geostationary satellites. The stationary position makes it fairly easy to photograph the small amount of reflected sunlight from them. Set the camera on a tripod, point it at the right elevation and direction, and use a long exposure time.

PHOTOGRAPHING ORBITING SATELLITES Satellites in low Earth orbit are traveling fast. They cross from horizon to horizon in ten to fifteen minutes. You can use a tripod or rotator mounted camera to photograph the light reflected from a satellite. Note that the intention is to capture a streak of reflected light as the satellite passes over, not a photo of an astronaut waving at you through the space station window. Using an exposure time from thirty seconds to a minute, the stars and galaxies look normal and the reflected sunlight from the fast-moving satellite will create a streak or line across the image. As with any visual observation of a satellite, you need to be in darkness or semi-darkness, and the satellite needs to be in sunlight (not eclipsed). The best times are up to about 45 minutes after dusk and the 45 minutes as the sky begins to brighten before dawn. The choice of location is important. A dark site is preferable because it allows you to photograph faint satellites and minimize image fogging from manmade light pollution. Try to avoid locations with air traffic. Airplanes carry bright lights, which will wreck your picture. Light from the Moon will also affect your pictures, so nights with a new Moon are preferable. The Heavens Above website will tell you the time and direction of the visible satellite pass. It can provide predictions for several days ahead. You need to pick a pass where the satellite will be visible for a reasonable time, will reach a fairly high elevation, and will be not be blocked by buildings, trees etc. I recommend trying for a picture of an ISS pass for your first attempt since it is the brightest and therefore easiest satellite to photograph. The Iridium series of satellites have really big solar panels. When the angles are right, a bright flare of reflected sunlight can be visible for a few seconds. The flare can make the satellite glow up to -8 in magnitude. That is much brighter than the ISS, which tends to range between 0 and -3.5 and it is also brighter than Venus at around -4, or the moon at -13. The flares only last a few seconds so they are much more difficult to photograph than the ISS. You need luck and perfect conditions. You can get predictions for Iridium flares from the Heavens Above or CalSky website Set your camera to manual focus and the lens to infinity. Your camera will not be able to autofocus to a tiny bright light in a dark sky. Setting the camera to automatic may cause it to hunt, shifting the focus when you don’t want it to. You will only be able to capture about 30 – 60 seconds of

the satellites ten to fifteen-minute pass due to the capture area of the camera lens. The more magnification the lens has, the less wide angle it will be. For this type of photography, a wide-angle lens is better. Get the satellite data from an online source. Use google earth or a GPS unit to determine the exact latitude and longitude for your location. Pick a safe location with a good view of the satellite’s path. Place the camera on a tripod set the direction and elevation according to the data provided. Make sure you use grid north references, not magnetic north. Use an accurate time source so that you know exactly when to begin the exposure. Your cell phone should be good. Set the camera to a wide-angle setting, 30-second exposure, manual setting, and focus set to infinity. When the satellite comes into the camera’s field of view start the exposure. Good luck! The results that you get depend a lot on the time of the satellite pass (how dark the sky is), light pollution, sky conditions, and the brightness of the satellite. You will get a better photo on cold crisp frosty nights when the air is clear than warm nights with higher humidity. Use a pass with a fairly high elevation. At least 30 degrees. At low elevations, the light has to pass through more atmosphere so it will be more affected by water vapor refraction, which can smear or distort the image. Satellite spotting and photography is a fun activity that with careful planning can produce some exciting results. It is magic to see the space station with your own eyes for the first time and realize that there are people living up there who are traveling right around the world every ninety-three minutes.

LISTENING TO SATELLITE TRANSMISSIONS Many people are interested in ‘listening’ to the signals transmitted by satellites. This can include decoding telemetry data from satellites in low Earth orbits, watching TV, receiving images from weather satellites, receiving data from the Outernet, or even listening to unscrambled military traffic. Many ‘non-radio’ people don’t realize that they are using satellite receivers all the time. Your smartphone receives location signals from the GPS satellites and so does the navigation system in your car. Many people have microwave dishes mounted on their houses for the reception of TV from geostationary satellites and of course, we are very familiar with seeing images taken by weather satellites. You can see these types of images on TV and the Internet. But where’s the fun in that? The challenge for radio hobbyists is to receive the signals directly from the satellite. With a suitable antenna and receiver, you can receive transmissions from satellites in geostationary orbit, medium Earth orbit, or low Earth orbit. Each option has its own challenges and rewards. Geostationary satellites use microwave frequencies, so a dish or patch antenna, a low noise preamplifier, and a suitable antenna feed arrangement are required. Usually, each of these only cover a single band, so setting up the required equipment can be quite a big investment of time and money. The upside is that for receiving from a geostationary satellite the dish can be fixed, you don’t have to track the satellite. You do need to be able to re-point the dish if you want to receive transmissions from a different satellite. Sometimes if you are using a small dish, the antenna beamwidth will be wide enough to receive signals from two satellites. The easiest signals to receive from geostationary satellites are TV broadcasts and studio feeds. There is also the Outernet. You can assemble the equipment to receive signals from the Outernet satellites very cheaply, so it is a good way to get started. Medium Earth orbit satellites are usually involved with mapping the Earth’s surface or navigation services. The most likely candidate for amateur reception is obtaining time and location signals from the GPS satellites. This is now very cheap and easy to do. GPS receivers suitable for Arduino microcontrollers only cost a few dollars. GPS receivers for making GPS disciplined clocks and oscillators are available for a few hundred dollars. The most popular low orbit satellites are the NOAA weather satellites. Receiving and decoding weather images appeals to many people, who

are interested in weather forecasting or viewing images of the Earth taken from space. Yes, you can find similar pictures on the Internet, but there is something special about receiving and decoding them yourself. Receiving and decoding weather satellite images is quite easy to do especially with the recent availability of cheap SDR ‘dongle’ and ‘small box’ receivers. The NOAA satellites transmit on 137 MHz but the signal is quite wideband. The filters in most conventional FM receivers are too narrow for good image reception. Special wideband receivers preset for the six NOAA channels are available, but I recommend using a software defined radio. SDR receivers are cheap and they have adjustable bandwidth so they are ideal for NOAA satellite reception and they can be used for receiving other bands. Most people use a fixed antenna for weather satellite reception. Suitable antennas are not too difficult to build or you can buy a commercial one. Other low orbit satellites include the multitude of small CubeSats and the International Space Station (ISS). You might hear a conversation with an Astronaut or Cosmonaut, receive APRS beacons from amateur radio stations, or listen to conversations between radio hams. Periodically the ISS transmits images, which can be decoded and displayed with free SSTV software. Many of the CubeSats transmit telemetry data with information about the health of the satellite and data relating to onboard experiments. Some CubeSats send images or video from onboard cameras. The project teams rely on help from radio hobbyists receiving and downloading the telemetry from their satellites. By sending them your received data files, you can become a part of that global support team and contribute to the scientific goals of the space research projects carried on the satellites. To receive signals from the ISS or CubeSats, you need to track their movement across the sky. A handheld, tripod mounted, or rotator mounted antenna is best, although may be able to get a taste of the action using a fixed antenna.

TECHNIQUES FOR RECEIVING The techniques for receiving signals from CubeSats are much the same as for amateur radio operation covered in the ‘working satellite’ chapters. So it is probably worth browsing those sections. Just ignore the bits about transmitting. You don’t need tracking antennas for Geostationary, GPS, or weather satellites. You do need tracking antennas for low orbit satellites like the CubeSats and the International Space Station. The proliferation of SDR receivers and cheap wideband low noise amplifiers has made the reception of microwave band signals cheaper and easier, but operation above 6 GHz still requires a level of home building and experimentation. The way that you proceed and the equipment that you will need depends on the type of satellite that you want to use. Receiving TV from a geostationary satellite is a completely different proposition to getting images from the NOAA weather satellites, or listening to the ham radio operators on LEO Hamsats. There is a very wide range of possibilities. I will cover the easiest and most popular options. Listening to encrypted or military traffic from satellites is much more difficult and outside the scope of this book.

GEOSTATIONARY SATELLITES Geostationary satellites are used for TV and radio broadcasting, weather forecasting, scientific Earth observation, navigation (not GPS), Internet services, telephony and commercial data links. All of these services use digital transmissions and most of them are scrambled and encrypted. It is unlikely you will be able to hack international telephone calls or commercial data. You may be able to receive and download weather information and images, although it is much more difficult than receiving images from the NOAA satellites. That leaves free to air TV and radio broadcasts, covered in the next section and the ‘Outernet,’ also covered later in this chapter. Inmarsat Inmarsat is a telecommunications service provider with several geostationary satellites in orbit. They provide satellite phone communications, broadband internet, and short text and data messaging services. Inmarsat is primarily used for telephone and data services to ships, aircraft, and field teams that are outside of cellular coverage or in countries without cell phone services. The network also handles beacons from ships that identify the vessel, it's heading, speed, and cargo details. Inmarsat is also responsible for GMDSS, the global maritime distress, and safety system, which receives signals from ships in distress, lifeboats, or personal emergency beacons and passes them on to an emergency response center. Inmarsat satellites transmit in the L-band at around 1.5 GHz. With an SDR dongle receiver and a patch, dish, or helix antenna, you can receive data from an Inmarsat and decode the STD-C NCS channel. This channel is mainly used by vessels at sea. It contains EGC (enhanced group call) messages, which are like short text messages containing information such as search and rescue and coast guard messages as well as news, weather, and incident reports. The Outernet patch antenna is an affordable antenna for L band reception. The current model includes a low noise amplifier but also a filter that may have to be removed for reception of some L band frequencies. Some people have modified GPS antennas for the job. Look on the Internet for details. For more gain, you could build a circular polarized (RHCP) helix antenna or use a dish with a 1500 MHz feed. If the antenna is any distance from the receiver, an LNA (low noise antenna) is a good

idea. You will need the Inmarsat decoder from I based this section on the excellent tutorial on receiving signals from Inmarsat using the RTL Dongle. See

TV SATELLITES You can use a dish antenna, LNB (low noise block), and a standard digital satellite receiver to receive ‘free to air’ TV and radio signals from some of the TV satellites that cover your region. Satellite TV and radio is broadcast from are big geostationary satellites with one or two large general coverage dishes and several smaller dish antennas which are pointed towards high population areas or specific countries or regions. The smaller antennas create enhanced coverage areas called ’spot beams.’ TV satellites transmit a lot of power so even though they are a very long way away, they are easy to receive with a modestly sized dish, usually 600 – 900mm for the Ku band or 1.5 to 3 meters in diameter for the C band. The smaller Ku band receiving dishes use an offset feed so that the signal is not blocked by the LNB (low noise block) and the feeder mounting bar. The photo shows larger ‘C band’ dish antennas with a ‘prime’ feed arrangement rather than the smaller offset fed Ku band antennas. Can you guess why they are pointing straight up? The answer is at the end of this section.

TV satellites are very big. For example, Optus D1, which serves Australia and New Zealand, is 3.9 meters high and 17 meters across, including the solar panel array. It weighs 1,006 kg. The solar panels can generate 4.8 kilowatts of electrical power. Optus D1 carries twenty-four Ku band transponders with eight backup channels. It has a total transmitter power of more than 2800 Watts, divided into 16 x 150 Watt transponders and 8 x 44 Watt transponders. LNB (low noise block) An LNB is a combination of a low noise preamplifier and a frequency down converter. The LNB receives the Ku or C band frequencies and translates them down to a block of IF (intermediate frequencies) between 950 MHz and 2150 MHz. This lower band of frequencies can be carried over the coaxial cable to the satellite receiver inside or next to your TV set with far less loss than there would be at microwave frequencies. LNBs typically have a gain (amplification) of about 65 dB, boosting the signal to compensate for the coaxial cable loss. They are powered by a DC voltage sent up the coaxial cable from the satellite TV receiver. It is very important

that the preamplifier section adds the minimum possible amount of noise to the received signal. So the best LNBs have a very low Noise Figure. The LNB uses a fixed local oscillator frequency to translate the microwave frequency to an IF output between 950 MHz and 2150 MHz. The satellite TV receiver tunes across the IF band to enable you to select any of the TV channels that were carried on the satellite transponder. LNB output frequencies for various satellite downlink bands Satellite downlink Local oscillator Band LNB output MHz GHz GHz 2.5-2.7 S 3.65 1150-950 3.4-4.2 C lower 5.15 1750-950 C 4.5-4.8 5.95 1450-1150 upper 10.7-11.8 Ku 9.75 950-2050 10.95-11.7 Ku 10.00 950-1700 11.7-12.5 Ku 10.60 1100-1900 11.7-12.5 Ku 10.75 950-1750 12.25-12.75 Ku 10.60 1650-2150 12.25-12.75 Ku 10.75 1500-2000 12.25-12.75 Ku 11.30 950-1450 12.5-12.75 Ku 11.475 1025-1275

Ku band These days most satellite TV is broadcast using the digital DVB-S modulation standard. The Ku band extends from 10.7 GHz to 13.25 GHz with most regions receiving downlinks at 11-12 GHz with uplinks around 14 GHz. The ‘fixed satellite service’ (FSS) band downlinks are from 11.7 to 12.2 GHz. ‘Broadcast satellite service’ (BSS) downlinks are from 12.2 to 12.7 GHz. In Europe and Africa (ITU region 1), the Ku downlinks are from 11.45 to 11.7 GHz and 12.5 to 12.75 GHz. In Europe 10.7 to 12.75 GHz is used for direct broadcast satellite services. The 11.7 to 12.5 GHz sub-segment is allocated to the BSS (broadcasting satellite service). In the Americas (ITU region 2), the Ku satellite downlink band is from 11.7 to 12.2 GHz. In the Pacific, Australia and New Zealand (ITU region 3), the Ku satellite downlink band is from 11.70 GHz to 12.75 GHz.

Using the Ku band has advantages and disadvantages over the previously used C band. The major advantage is that smaller dishes can be used due to the higher frequencies, making the dishes cheaper and easier to mount. The disadvantage is that frequencies above 10 GHz are more affected by rain. This causes dropouts of the satellite service in severe weather. If you have satellite TV, you may have seen rain fading cause the picture to disintegrate into pixelated squares. C band The old C band satellites were common in the days when TV was broadcast as an analog signal. Many of you will remember seeing two or three-meter dishes in front or backyards. You could get a variety of paid subscription channels and enthusiasts could tune to free channels and often studio channels. Studio channels were used to link the TV studio to terrestrial TV broadcast sites. They were also used to send programs and news feeds between studios and sometimes from outside broadcast locations. Usually, studio feed channels did not include any advertising. I remember receiving AFRTS broadcasts in the early 1990s. AFRTS was the ‘American Forces Radio and Television Service.’ A network beamed at US Service men and women stationed worldwide. It was even received by US Navy ships. We could watch an endless series of American NFL football games punctuated with adverts suggesting that it would be a great idea to re-enlist. AFRTS broadcast radio and TV programs, 24 hours a day from its base in Sun Valley California. C band was used for TVRO satellite free to air broadcasts in the USA from the 1970s until the mid-1990s. Receiving dishes were commonly 12 to 16 feet in diameter. The broadcast consisted of a ‘baseband’ of analog TV channels modulated onto the microwave transponder as an FM signal. You could rotate the antenna azimuth to pick up different satellites. The system transmitted one group of channels on vertical polarization and another group on vertical polarization. Often one Pay-TV company sold the channels on one polarization and a second company sold a similar set of channels on the other. A motorized horn feed could be used to easily change your dish polarization so that you could receive the alternate set of TV channels. There are a few C band satellites still operational and some of the new satellites broadcast on both the C and the Ku band. But there is no more analog TV, they all broadcast using the DVB-S2 digital standard. Websites:

LyngSat seems to be the best information website for information about ‘free to air’ TV and radio channels carried on TV broadcast satellites. Free to air TV and radio services ‘Free to air’ means that you don’t have to pay to view the channels so there are no encryption or conditional access codes. Many TV satellites carry ‘Freeview’ or other non-encrypted radio and TV signals. Use the LyngSat website to find satellites and transponders that cover your region. Encrypted (conditional access) services Pay-TV channels such as ‘Sky TV’ or ‘HBO’ are also broadcast from TV satellites. You need to enter a special ‘encryption key’ code or plug a small smart card into your satellite receiver to access the service. The digital TV signals are scrambled using an encryption code. You can receive the channels on a standard satellite receiver but you won’t be able to decode and view the individual channels. The most common encryption systems used by TV broadcasters are: DigiCipher II VideoCipher II RS Nagravision VideoGuard Irdeto Mediaguard Viaccess, and Conex As far as I am aware, there is no way to reliably break the encryption. Some people have hacked Pay TV set-top boxes, but that is not usually a long-term solution because the encryption coding is changed by the broadcaster periodically. My advice is to stick to the free to air channels. Why are the dishes pointing straight up? No, it’s not to collect rainwater! They must be at a location very near to the equator. At the equator, geostationary satellites are straight above you. As you travel north or south, the elevation reduces. You can tell the latitude of a satellite dish pointed at a geostationary satellite by the angle of elevation. 90O at the equator, 45O at 45 ON and 45 OS, and 0 O near the poles.

OUTERNET The Outernet system uses three Inmarsat geostationary satellites to relay educational and general interest information in the form of text files. The three satellites are placed so that they provide coverage to almost the whole planet. The company’s aim is to provide a sort of library of information to people, especially in developing countries, who don’t have access to the Internet. The downlink is very narrow bandwidth so it takes a long time to download files. The system only transmits 20 Mb of data per day, not per second… per day! Outernet is often touted as the Internet for the four billion people who can’t access the Internet, but it is not like the Internet. There is no uplink. You cannot select the content that you receive. The idea is to leave the receiver and computer running for an extended period of time. When you come back after a few hours, there are downloaded files about all kinds of things. It could be weather, encyclopedia items, teaching resources, news, or even ham radio. Outernet uses an L band Inmarsat transponder on 1542 MHz. You use a cheap SDR receiver to receive the signal and decode it using a program called Skylark running on a small computer called a Dreamcatcher board. The Skylark software creates a web style interface and organizes the received files into different categories. Skylark is available as an ‘ISO image’ for the Dreamcatcher board. The SDR software and data management system are integrated into its Linux based operating system. You need to write the ISO ‘image’ to an SD card, in the same way as you would create an operating system for a Raspberry Pi. It is not difficult, but you need to learn a lot of Linux stuff to get everything working. Dreamcatcher is a single board computer that includes an RTL based receiver (like an RTL dongle). You don’t need a separate radio. Outernet also sells an L band patch antenna that incorporates a low noise amplifier. The two are $59 and $25 respectively, so this is a very cheap way to get into satellites. All prices are in USD current in August 2017. You used to be able to receive and decode Outernet with a separate SDR dongle receiver such as a FUNcube, an RTL dongle, or the Outernet SDRx. The decoding software ran on a CHIP or a Raspberry Pi single board computer. But apparently, the latest release of Skylark does not support either the Raspberry Pi or the CHIP. Note that the Outernet L band patch antenna is designed for experimentation on a good day. It is not weatherproof and it can’t be left

outside. Refer to There used to be a project called Outernet Lantern and another called Outernet Lighthouse. The latter seems to be a modified satellite TV box used to receive Outernet data from Ku band satellites. However, it seems that Outernet no longer broadcasts data from any of the Ku band satellites. I must say that the Outernet website is terrible. It has no information about the Outernet service, content, or goals, and very little documentation about the products that are for sale. There is no information on the website about the satellites they are using, or the Skylark software. There is a link to a user forum, but nothing from the Outernet Company or organization. There is little about Outernet on the Internet either and a lot of the web pages and reviews that you can find are out of date.

WEATHER SATS ‘Weather satellites’ are used by climatologists to study the weather and the effects of climate change. Data from weather satellites are used to measure things like; ground temperature, humidity, wind speed and direction, sea temperature, drift rate, sea currents, erosion, river deltas, melting of the polar ice sheet, glaciers, air pollution, snow, storms, cyclones, hurricanes, rainfall, and of course what the weather will be like at your place tomorrow. They relay radar images of the Earth’s surface, which can be decoded using the freely downloadable software. Some of the images are enhanced with false colors to make them easier to interpret. Some of the weather satellites are in low Earth orbits, some are in geosynchronous (sun-synchronous) polar orbits so that they appear above the same place every day, and some are in the geostationary orbit. The pictures from the low Earth orbit NOAA satellites are the easiest to receive. They are transmitted on 137 MHz. You get better images using a software defined radio (SDR) receiver because they can be adjusted to match the wide bandwidth of the satellite transmission. This topic is so interesting and popular I have included a special chapter about weather satellites later.

GPS SATELLITES The GPS satellites transmit two kinds of information useful to amateur satellite hobbyists. You can use the navigation information or the extremely accurate timing signals. Receiving GPS navigation signals is cheap and easy. A simple Arduino board with a GPS module costs less than $100. The GPS antenna is incorporated into the GPS module. Alternatively, some of the new single board computers like the ‘Dragon Board’ incorporate a GPS receiver or you can connect a GPS ‘shield’ to a Raspberry Pi. Simple software can display the location position received and decoded by the chip. This includes the number of GPS satellites being received, current UTC time and date, and your speed, latitude, longitude, and altitude. The altitude data seems to be less reliable. You could build a ‘world clock’ or your own satellite tracking system. These cheap GPS modules are amazingly sensitive, I found that they even work encased in a plastic box, sitting inside the house on my computer desk. The other popular use for GPS satellites is the creation of a GPS disciplined oscillator (GPSDO). A GPSDO is an oscillator or clock generator that is being kept accurate by regular signals from a GPS receiver. There are several control techniques used such as digital division or phase locked loops. The result is a super accurate square wave clock or sine wave oscillator that can be used to keep your radio equipment or digital network precisely on frequency. There is more on this subject under the heading ‘Satellite disciplined clocks.’ You can use an Arduino to make a GPSDO, but it is cheaper and easier to buy a commercially made GPSDO that is already designed to output clock signals. A GPS disciplined oscillator (GPSDO), is a combination of a GPS receiver and a high-quality, stable oscillator such as a quartz or rubidium oscillator. GPSDO boards are available online for less than $100. Good ones cost more like $300. They usually output a 10 MHz signal and often a 1 PPS (pulse per second) signal. They should be stable to at least one part in 1011 (0.000001 ppm), which is far better than an OCXO or TCXO can manage.

CUBESAT TELEMETRY Most CubeSats, whether they carry amateur radio transponders or not, have some sort of control station uplink and a downlink to send status information back to the ground station. It is common for the command teams for small amateur built satellites to enlist the help of volunteers to receive and decode the downlink telemetry and to send it to the control station by email. You can contribute to the success of the satellite mission in this way. See the following websites for the latest in telemetry satellites and CubeSat news. The AX.25 1200 baud and 9600 baud ‘packet’ protocol used for APRS is often used for the telemetry downlinks. You can use the same radio equipment and ‘packet’ modem to receive and download the information that the satellite transmits on the downlink frequency. See the section on working digital satellites. The telemetry data contains information about the health of the satellite systems, temperature, battery capacity, solar panel voltage and current etc. It may also contain data from onboard experiments or cameras. Many satellite project teams rely on enthusiastic amateurs sending them downloaded data files or decoded data. Their websites often have small programs designed to take the data and display it in a meaningful way. Unfortunately, there is currently no standard data structure, so each of these programs only works for an individual satellite or series of satellites. The FUNcube satellite website has an application that can take audio from your receiver or the digital output from a FUNcube dongle receiver and display the data on your PC screen. The downlink telemetry frequencies for LEO satellites are usually in the VHF or UHF bands. Historically they have used frequencies in the 2m or 70cm amateur radio bands, however since the ITU has now adopted ‘resolution 659,’ this may change. New non-amateur radio satellites may be required to use other VHF or UHF ‘space segment’ frequencies. If you are interested in receiving CubeSat telemetry this probably won’t matter much. The frequencies will be published and it is likely that your receiver will be able to tune to the frequency of interest even if it is outside the ham

bands. It is more of a concern that your antennas may not be optimized for the new frequency.

HAMSATS You don’t have to transmit through the Hamsats to enjoy them. As well as receiving and decoding the satellite telemetry, you can tune to the downlink and listen to the ham radio operators calling through the satellites. I imagine this is often quite amusing. Working through the satellites is a tricky business and mistakes are commonplace. Receiving the downlinks from the Hamsats is best covered by reading the ‘working the… satellite chapters.’ Just ignore everything that relates to transmitting. There are three classes of amateur radio satellite (Hamsats). Satellites with FM voice repeaters Satellites with linear transponders, and Satellites with digipeaters The FM satellites are the easiest. FM is much more forgiving of Doppler shift so you can use three or five fixed channels on your radio. You may be able to hear several upper sideband conversations as you tune across the downlink band of a satellite with a linear transponder. There may be CW, PSK, JT-65 or even RTTY signals as well. Any narrowband transmission capable of modulating an SSB transceiver can be used. You will need a receiver with SSB mode and VFO tuning or an SDR. Digipeaters relay APRS beacons that have been received from amateur radio stations. They will be displayed on the APRS Internet sites but you can also receive them directly from the satellite and use the software on your PC to plot them on your own maps.

EME You could listen to EME transmissions reflected off the Moon. But I don’t know why you would bother. If you are not an active EME participant, the conversations are pretty slow and boring. Most EME contacts only consist of call signs, signal reports, and received signal strength. You need very big antennas and low noise amplifiers to receive the extremely weak signals reflected from the Moon, so the financial investment is quite high. If you go to the trouble and expense of setting up an EME station, you really should include transmitting capability. See ‘working the big satellite’ for more about EME history, challenges, and operation. To receive EME on the 2m band you need at least a 10 element Yagi. For the 70cm band a 32 element Yagi. You will also need a low noise preamplifier. If you already have a big Yagi, you can download WSJT and have a go.

INTERNATIONAL SPACE STATION (ISS) The ISS currently has two amateur radio stations. The newer station is in the Columbus module. It includes Ericsson VHF and UHF radios and a specially constructed ‘HamTV’ DATV (digital amateur television) video transmitter used to transmit video or photographs. The older station in the Service module has three Kenwood VHF/UHF dual band radios. The ham radio stations are used for ham radio contacts between ISS crew members and licensed amateur radio operators, sending still SSTV (slow scan television) pictures and postcards, 1200 baud packet radio for APRS beacons, and digital DVB-S TV transmissions on the S-band at 2.395 GHz. For up to date information check out: The frequencies, modes, and call signs for ISS are included in the next chapter, ‘International Space Station.’ It is possible to hear an Astronaut or Cosmonaut making amateur radio contacts with amateur radio stations, but this depends on the crew schedule. The crew can only make ham radio calls during leisure hours. Some ISS crew members are interested in making amateur radio contacts but the majority do not use the station. There used to be an FM repeater for amateur radio operators to use for ‘ground to ground’ contacts but this is currently deactivated. Most of the current ISS amateur radio activity is the digipeater relaying APRS beacons. The SSTV system is activated sporadically, usually on special occasions such as the anniversary of notable space-related events like the first manned space flight.

BALLOONS OK, “you got me,” balloons are not satellites, but they can reach the edge of space and they carry similar payloads to some CubeSats. They can be tracked using your satellite station while they are within range, although their path is less predictable. Balloons are much cheaper to launch than CubeSats. They offer a similar challenge to building a satellite, at an affordable price, and the results are often spectacular. There are some amazing videos and photographs online. Some balloons stay airborne for several weeks and at least one has circumnavigated the Earth twice. Balloons often carry cameras relaying video and they always carry an APRS transmitter so that their location can be tracked. You need the same sort of station, as you would use for receiving from a CubeSat. Preferably, a Yagi antenna with a PC controlled rotator. You do not need an elevation rotator because the balloon is unlikely to fly close to your location. A hand-held antenna would be OK if you know roughly which way to point it. See Amateur Radio High Altitude Ballooning.

NEW SATELLITE PROPOSALS A SATELLITE PROPOSED FOR GEOSTATIONARY ORBIT Es’hailSat, the Qatar Satellite Company is currently building a commercial communications satellite called, Es'hail-2. It will include an amateur radio payload, making it a Hamsat and a commercial satellite. The launch of Es’hail-2 was planned for December 2016 but has now been put back to sometime in 2018. The satellite will be placed into geostationary at 25.5 degrees East. Coverage from the two amateur radio transponders should extend from Brazil to Thailand. Es’hail-2 will carry two ‘Phase 4’ amateur radio non-inverting transponders operating in the 2,400 MHz and 10,450 MHz bands. A 250 kHz bandwidth linear transponder is intended for conventional analog amateur radio operations and an 8 MHz wide transponder will be used for experimental digital modulation schemes and DVB amateur television. Narrowband Linear transponder 2,400.050 – 2,400.300 MHz Uplink 10,489.550 – 10,489.800 MHz Downlink 250 kHz bandwidth Up to 90 simultaneous voice transmissions for 180 users. Wideband digital transponder 2,401.500 – 2,409.500 MHz Uplink 10,491.000 – 10,499.000 MHz Downlink 8 MHz bandwidth Video transmission for two or three video channels S-Band 2.4 GHz Narrowband Uplink: narrowband modes like SSB, CW 5W nominal Uplink power (22.5 dBi antenna gain, 75cm dish) RHCP polarization S-Band 2.4 GHz Wideband Uplink (DATV): wideband modes, DVBS2 peak radiated power of 53 dBW required (2.4m dish and 100W) RHCP polarization Source: CQ-DATV 39 - September 2016

A SATELLITE PROPOSED FOR GEOSYNCHRONOUS ORBIT AMSAT-NA has accepted the opportunity to be a “hosted payload” on a spacecraft that Millennium Space Systems (MSS) of El Segundo, California, is under contract to design, launch, and operate for the US government. The satellite’s potential footprint would extend over the US from the Mid-Pacific to Africa. The amateur radio payload will comprise a Software Defined Transponder capable of supporting many different modes, including analog SSB. The uplink will be in the 5830-5840 MHz band and the downlink in 10450-10500 MHz. A satellite in a geosynchronous orbit stays roughly over the same longitude because it has an orbital period of one sidereal day (23 hours 56 minutes and 4 seconds). It is orbiting the Earth at the same rate as the Earth spins. But it has an inclination greater than zero, in other words, it is not orbiting in the equatorial plane. Looking from Earth it moves north and south, tracing out a ‘figure eight’ pattern over the course of the 24 hour period.

TWO SATELLITES PROPOSED FOR LUNAR ORBIT The possibility of placing an amateur radio satellite into orbit around the Moon has been a long-held dream and now there is a real plan to make it a reality. “The Cube Quest competition, sponsored by NASA’s Space Technology Mission Directorate Centennial Challenge Program, offers a total of $5.5 million to teams that meet the challenge objectives of designing, building and delivering flight-qualified, small satellites capable of advanced operations near and beyond the Moon. Cube Quest teams will have the opportunity to compete for a secondary payload spot on the first mission of NASA’s Orion spacecraft, which will launch atop the agency’s Space Launch System (SLS) rocket.” uest/about.html The Ragnarok Industries Nano-Satellite Company team, comprised of former NASA Goddard Space Flight Center Ph.D. engineering interns, is designing the 6-unit (6U) Heimdallr CubeSat to test advanced propulsion and communication technologies for lunar and deep-space missions. AMSAT would develop the 5 GHz uplink/10 GHz downlink — the so-called “five and dime” paradigm — Phase 5 Amateur Radio transponder for the spacecraft, and AMSAT’s Ground Terminal initiative is supporting the effort. The five teams announced on September 9 [2016] scored highest in the first of four “ground tournaments” making up the initial phase of the $5 million CQC. The three teams with the highest total cumulative scores will be offered rides as secondary payloads on the first Space Launch System (SLS) mission, Exploration Mission 1 (EM-1) in 2018. Heimdallr would be a 3-axis stabilized 6U CubeSat weighing approximately 8 kg. It would have a cold-gas thruster and a star tracker for navigation. Deployable, gimbaled solar panels would produce up to 100 W of dc power. Electric propulsion will be used to achieve lunar orbit. The onboard communication gear would use a combination of omnidirectional and directional patch antennas on one side of the spacecraft. The first part of Heimdallr mission is to provide telemetry, tracking, and command to obtain lunar orbit, the second is the data downlink experiment, and the final component is to provide a two-way regenerative repeater and analog repeater in lunar orbit for the lifetime of the satellite. The omnidirectional, directional, and analog transponders would downlink in the 10 GHz amateur band, while the uplinks would be in the vicinity of 5.6 GHz.” The Heimdallr-1 CubeSat AMSAT-NA is planning to place three 5 GHz to 10 GHz linear transponders on the Heimdallr-1 CubeSat. An omnidirectional dipole and several directional patch antennas will be mounted on one side of the spacecraft. The first part of the mission will be to establish Telemetry Tracking and Command in order to place the satellite into a lunar orbit. The second part is to perform the ‘data downlink experiment’ and the final part is to provide a two-way regenerating digital repeater and an analog transponder in lunar orbit for the remaining lifetime of the satellite. The proposed 500 kHz wide ‘Omni’ CX mode transponder will have an uplink frequency centered at 5.651 GHz with a downlink at 10.451 GHz. The 3.5 MHz wide ‘Directional’ CX mode transponder connected to the patch antennas will have an uplink frequency centered at 5.6575 GHz and a downlink at 10.4575 GHz. The 2 MHz wide ‘Analog’ CX mode transponder will have an uplink centered at 5.665 GHz and a downlink centered at 10.4665 GHz. It is anticipated that you will need a 1.2 to 2 meter diameter dish to successfully work Heimdallr-1, assuming that you are using the AMSAT design for your ground station radio equipment. The transponder information above was sourced from,

Using any satellite in a lunar orbit will be quite challenging, but it would enable you to work any station on Earth that can see the Moon at the same time as you can. The available ‘pass’ time that you will be able to use the satellite will be limited by the satellites orbital period around the Moon. It will be determined by the altitude of the satellite above the Moon and the shape of the orbit. The Apollo Command Modules orbited in about two hours, which allowed communication with the ground stations for about an hour per orbit. A highly elliptical orbit would allow longer access times, but probably wouldn’t be as good for photography of the lunar surface and other onboard experiments. The path loss on the 5.6 GHz uplink to the Moon will be around 219 dB, so a significant amount of transmitter power and a dish antenna will be required. The path loss on the 10.5 GHz downlink is around 224 dB, so a dish antenna and a low

noise amplifier (LNA) or low noise down-converter block (LNB) will be required. Stations set up for 5 GHz or 10 GHz microwave EME operation will have a head start. The satellite should be much easier to work than ‘Moonbounce’ operation at the same frequencies. The DSLWP project Students at the Harbin Institute of Technology are developing a pair of satellites to be placed into a 200 x 9000 km orbit around the Moon. The project is scheduled for June 2018. The satellites are DSLWP-A1 and DSLWP-A2. The amateur radio payload onboard DSLWP-A1 will provide a telecommand uplink and telemetry plus a digital image downlink. An open telecommand structure is proposed which will allow amateurs to send commands to request the satellite to take an image and transmit it on the downlink. The two satellites will be 50x50x40 cm with a mass of about 45 kg and are to be 3-axis stabilized. Two linear polarized antennas will be mounted along and normal to the flight direction. The team proposes downlinks for DSLWP-A1 on 435.425 MHz and 436.425 MHz. Downlinks for DSLWP-A2 will be on 435.400 MHz and 436.400 MHz using either 250 bps GMSK with concatenated codes or JT65B. Source

HFSAT The US Naval Academy has proposed that HFsat will carry a 30 kHz wide reverse tracking HA mode transponder. The 15m uplink will be at about 21.4 MHz and the 10m downlink will be about 29.42 MHz. The satellite is intended for use by HF mobile, portable, and home stations with simple vertical antennas. The transponder will support most HF modes but the emphasis seems to be on SSB and PSK operation. With the low frequency of operation and a reverse tracking transponder, the amount of Doppler shift will be minimal. The US Naval Academy has already achieved several very successful satellite missions. PCsat was launched in 2001, PSAT-1 in 2015, and PSAT-2 in 2016.

GOLF In October 2017, AMSAT-NA announced a new AMSAT program called GOLF, ‘Greater Orbit - Larger Footprint.’ The project is dedicated to achieving the deployment of amateur radio CubeSats into higher orbits so that they cover more of the Earth’s surface and allow longer distance and more transcontinental ham radio contacts. The aim is to develop a series of three unit CubeSats that can be held ready for rapid deployment into MEO, HEO, or geosynchronous orbits, as launch opportunities become available. The initial ‘GOLF-T’ satellites will be test satellites launched into low Earth Orbits. They will be designed to test the technologies that will be required for future GOLF satellites destined for higher orbits. The GOLF-T satellites will be used to investigate the use of commercial off the shelf components to build satellites capable of withstanding the higher level of radiation that satellites are exposed to in MEO or HEO orbits. They will also test new IHU management computers, new attitude determination and control systems, CX mode 5 GHz uplink to 10 GHz downlink SDR transponders, and deployable solar panels. I assume that they will also carry transponders or repeaters for amateur radio use. IHU = Integrated Housekeeping Unit. MEO = Medium Earth Orbit. HEO = Highly Elliptical Orbit. SDR = Software Defined Radio.

MAIDENHEAD GRIDS It is usual to swap Maidenhead grid references during a satellite conversation and some satellite tracking programs use your ‘grid’ to determine your operating location. The Maidenhead grid locator system divides the Earth’s surface up into ‘squares.’ Normally a six-digit code is used, but the code can be extended to ten digits for very fine location definitions. The system is very popular because it is much easier to state a six digit ‘grid’ reference than a pair of eight or nine-digit latitude and longitude numbers. Maidenhead grids are derived directly from latitude and longitude. My Maidenhead grid is RE66hm, which equates to a latitude of -43.479167 and a longitude of 172.625000 degrees. The first two letters are written as upper case characters. They denote one of 324 large areas called ‘fields’. The world is divided into an 18 x 18 array of fields using the letters AA to RR. The first letter relates to the longitude and the second to the latitude. Each field covers 10O degrees of latitude and 20O degrees of longitude. Refer to the drawing. Most of the UK is in Field IO.

As the inset drawing on the previous page illustrates, each field is divided into 100 ‘grid squares’ ranging from 00 in the bottom left corner of the field up to 99 in the top right. Each grid square covers 1° of latitude by 2° of longitude. The first number relates to the longitude and the second to the latitude. The ‘IO’ field covers Scotland, Ireland, and most of England. It is broken into 100 ‘Grid squares,’ IO00 to IO99. This four digit code is accurate enough to pinpoint your location down to about county, district, or regional level. It is often used by HF contesters and DX stations. HF digital mode enthusiasts and people using frequencies above the HF range generally use the more accurate six digit version. For example ‘IO92ab.’ Each of the 100 grid squares is broken into 576 ‘sub-squares.’ They measure 2.5 minutes of latitude by 5 minutes of longitude. Subsquares are usually written in lowercase letters ranging from aa to xx. In Morse code, grid squares are usually prefixed with “LOC” to indicate that the next four or six characters are a Maidenhead grid. The actual area inside each field, square, or sub-square depends on its latitude. Grid squares near the equator are bigger than grid squares near

the poles. They are actually trapezoid sections, not squares. The top and bottom (latitude lines) are parallel, but the sides (longitude lines) are not. For the IO92ab Maidenhead grid in England, the latitude is 52.06 degrees. A field is approximately 1109 x 1369 km (689 x 851 miles). A grid square is about 111 x 137 km (69 x 85 miles) and a sub-square is about 4.6 x 5.7 km (2.9 x 3.5 miles). However, for EM17ip, a grid in the continental USA (Wichita Kansas), the latitude is 37.65 degrees so the fields, squares, and sub-squares are bigger. A field is approximately 1109 x 1763 km (689 x 1095 miles). A grid square is about 111 x 176 km (69 x 110 miles) and a sub-square is about 4.6 x 7.3 km (2.9 x 4.6 miles). The website at can display a map of the location around a supplied Maidenhead grid, amateur radio call sign, or address. The website at can determine the latitude, longitude, and Maidenhead grid of any spot on the world map. Just zoom into the location and click your mouse on it. The “Ham Square” phone app (for Apple iOS) displays your current Maidenhead grid, altitude, latitude, and longitude. The “Maidenhead” phone app (for Apple iOS) displays your current Maidenhead grid, places a pin on the map, and your latitude, and longitude. Google Earth displays the latitude, and longitude of the current mouse position.

IARU SATELLITE COORDINATION WRC-15 You are probably wondering whether you need to read this chapter and why I put it in the book! I am not usually very bothered by the activities of the IARU and the ITU but this notification creates an important change. It separates the frequency bands that amateur built, educational and other small ‘Amsat’ satellites are allowed to use, from the frequency bands that amateur radio ‘Hamsats’ are allowed to use. The International Amateur Radio Union (IARU) is a global federation of national amateur radio associations from more than 150 countries. For many years, the IARU has provided a free frequency coordination service for amateur radio and amateur built satellites that include radios that operate on amateur radio frequencies. Frequency coordination is essential to limit interference between satellites or ground stations and to ensure that satellite uplinks and downlinks are placed on sensible frequency allocations within the amateur radio ‘satellite sub-bands.’ Traditionally, nearly all amateur built low Earth orbit satellites, whether Amsats or Hamsats have used amateur radio frequencies in the 2m and 70cm amateur radio bands. However, since 2014, the large number of amateur built satellites being launched has made it impossible for IARU to coordinate interference-free frequencies in the fairly busy 144 – 146 MHz band. The International Telecommunications Union (ITU) is the international group that coordinates radio spectrum usage worldwide. It makes recommendations about what frequencies should be allocated to various radio services. National regulators usually adopt the ITU recommendations. In this way, television, broadcast radio, amateur radio, satellite, maritime, emergency services, land mobile, aeronautical, and many other bands are standardized worldwide. Nearly every year the ITU holds a World Radio Conference (WRC), which votes on any changes to the ITU recommendations. In 2012 (WRC-12) the ITU noted the sudden proliferation of “nanosatellites and picosatellites” and suggested that this should be discussed at a future conference. At WRC-15 (2015), the ITU adopted ‘resolution 659,’ which stated that the use of 144-146 MHz and 435-438 MHz by non-amateur radio satellites is not in accordance with the international radio regulations. [Previously a blind eye had been turned to this]. The resolution goes on to say that, non-amateur radio satellites should operate in the licensed part of the radio spectrum allocated to

“Space Operation.” There are bands at 137-138 MHz, 148-149.9 MHz, 174-184 MHz, 267-273 MHz, 400.15- 402 MHz and 1427-1429 MHz. [above and below the 2m and 70cm amateur radio bands]. There are also a few other ‘space operation’ bands that are restricted as to their use or only available in certain countries. Forcing future amateur built, non-amateur radio satellites to use these licensed bands instead of the amateur radio bands could cause problems such as satellites not being licensed to operate while they are over particular countries. Control stations wishing to transmit on the satellite uplink frequencies may be required to pay a license fee. For us, it means that we might need different antennas and possibly different radios to receive signals from the amateur built ‘Amsats’ than we do for the amateur radio ‘Hamsats,’ which will continue to use amateur radio frequencies. On the 30th of June 2017, the IARU issued a news release stating that it will be issuing revised guidelines for satellite frequency coordination. The IARU believes that the definitions in article 25 of the Radio Regulations, which governs the ‘amateur services,’ are broad enough to encompass nearly all educational satellite projects that include hands-on experience with radio communication and are conducted under an amateur radio license. Their strong preference is for all satellites using amateur radio frequencies to be operated under amateur licenses. This means that the operation of a satellite control station would have to be under the direct control of a licensed amateur radio operator and satellite constructors who propose to use amateur radio frequencies for their satellites would be required to use the IARU frequency coordination process. IARU will not coordinate frequencies for satellites operating in the ‘space operation’ bands. It is unclear what organization will take over that responsibility. However, it will continue to coordinate satellites with combined amateur radio and non-amateur radio missions. As I write, this information is “hot off the press,” and the situation will probably have changed by the time you read the book. As it stands, the change could create a division between amateur radio satellites and experimental satellites. At present, most CubeSats only have one transmitter. It is used to downlink satellite control and information telemetry, send data from onboard cameras or experiments, and possibly for amateur radio. If the telemetry and the educational or experimental downlink have to be on ‘space operation’ frequencies, it is unlikely that the

satellite will be able to carry an amateur radio repeater or transponder as well. On the other side of the coin, satellites built by amateur radio groups to carry amateur radio transponders or repeaters often carry an educational experiment as well, as this is a good way to get funding towards the launch cost. I believe that there will be no problem operating this type of satellite on amateur radio frequencies provided any uplink traffic is managed by a licensed amateur radio operator. So it is possible that in the future more CubeSats will be designed to carry an amateur radio payload so that the constructors can justify using amateur radio frequencies.

SATELLITE BANDS AMATEUR RADIO SATELLITE BANDS The amateur radio bands contain agreed ranges of frequencies that have been set aside for satellite operations. Uplink and downlink pairs of frequencies within these band allocations are usually called modes, which can be a bit confusing since we also use the term ‘mode’ to describe the type of transmission, CW, FM, SSB, SSTV, AX.25, digital voice etc. The satellite frequencies within the amateur radio bands are allocated by the ITU and they are slightly different in each of the three ITU regions. The legal frequencies for your zone will be marked on your local amateur radio band plan. There are satellite allocations in most of the amateur bands from the 7 MHz (40 m) band, up to the 248 - 250 GHz band (1 mm). Some allocations are restricted to uplinks only or downlinks only. Because of the effects of Doppler shift, most LEO satellites use the VHF 2m band and the UHF 70cm band allocations. Amateur radio satellites in higher orbits are more likely to use the L, S, C, or X bands. So far, all Hamsats and the majority of Amsats have used frequencies in the following amateur radio satellite bands. Range Band HF









23 cm


12 cm


9 cm

Letter Preferred Allocation H 21.000 21.450 MHz A 29.300 29.510 MHz V 145.800 146.000 MHz U 435.000 438.000 MHz L 1.260 1.270 GHz S 2.400 2.450 GHz S2 3.400 3.410




Entire band


Entire band


Sat subband


Sat subband


Uplinks only


Sat subband


Not ITU zone 1


5 cm



5 cm



3 cm



1.2 cm





GHz 5.650 5.670 GHz 5.830 5.850 GHz 10.450 – 10.500 GHz 24.000 24.050 GHz 47.000 47.200 GHz


Uplinks only

Secondary Downlinks only Secondary Sat subband Primary

Sat subband


Entire band

*NIB = satellite operation is allowed on a non-interference basis Bands that allow both uplinks and downlinks have sub-bands allocated for each purpose. The convention for labeling amateur radio satellite transponders or repeaters is to use the letters relating to the uplink and downlink bands. For example, a transponder with an uplink on the 2m band and a downlink on 70cm is called a VU mode transponder. A transponder with an uplink on the 70cm band and a downlink on 2m is called a UV transponder. An LS transponder has an L-band uplink and an S-band downlink. ISS crew contacts use the 2m band for both the uplink and downlink. This is called ‘V V’ mode, (usually written as V-V so it isn’t mistaken for a W). The method of operation is half-duplex like a repeater contact on Earth. The early ‘OSCAR’ and ‘RS’ satellites used allocations in the amateur 15m, 10m, 2m, and 70cm bands. Some of the frequencies they used are now outside of the allocated satellite bands. OSCAR 7 is the only early OSCAR that is still operational. Use of the 432.125 MHz to 432.175 MHz uplink frequencies used by OSCAR 7, (AO-07), is granted by special dispensation. Some ‘old timers’ still refer to ‘mode A,’ ‘mode B,’ or ‘mode J,’ especially when referring to the “good ol’ days” or operating through OSCAR 7. Mode A was a 2m uplink and 10m band downlink. Now VA mode. Mode B was a 70cm uplink and a 2m downlink. Now UV mode. Mode J was a 2m uplink and a 70cm downlink. Now VU mode.

When AO-07 was launched into space in 1974, the UV, two-letter convention had not yet been adopted. The two-letter format was first used for OSCAR AO-40 (16 November 2000 - 25 January 2004) SK RIP.

TV AND COMMERCIAL SATELLITE BANDS Commercial satellite bands vary from region to region and within individual countries. The table below shows the general structure. The table on the next page adds more detail about the downlink frequencies used by various satellite services. Band L S



K Ka V

Commercial Satellite Bands Nominal Typical Satellites Frequencies 1 – 2 GHz GPS, Sat phones, Inmarsat, Iridium 2 - 4 GHz Weather radar, communications, NASA, Inmarsat and Solaris mobile 4 - 8 GHz Communications & TV 8 – 12 GHz Meteorological, military and law enforcement. (USA 812.5). 12 -18 GHz TV 10.7-12.75 GHz is called Ku band even though most of it is technically in the X band. 18 - 27 GHz NASA Communications and short27 - 40 GHz range radar. (USA 26.5-40). 40 – 75 GHz Research

The K band was split into three sections because it contains 22.24 GHz which is the frequency most absorbed by water vapor in the atmosphere, which makes it unsuitable for long distance communications.





Commercial Satellite Band Segments Typical Satellite Frequencies Satellite Service downlink segments GHz GPS 1176.45 MHz GPS 1227.60 MHz GPS 1381.05 MHz GPS 1227.60 MHz GPS 1575.42 MHz Iridium sat-phones 1616 - 1626.5 800 – 1525 MHz MHz Inmarsat 1525 – 1645.5 MHz Thuraya sat1525 – 1661.0 phones MHz DAB digital audio 1400 MHz broadcast band Amateur 23cm sat 1240 – 1300 band MHz DARS digital audio 2.31 – 2.36 radio GHz DAB digital audio 2.3 GHz band broadcast (USA) Inmarsat and 2.17 – 2.2 GHz 2 – 4 GHz Solaris Direct to home TV 2.5 – 2.7 GHz Amateur S band 2.4 – 2.45 GHz 13cm Amateur S2 band 3.4 – 3.41 GHz 9cm C band Downlinks 3.7 – 4.2 GHz C band Uplinks 5.925 – 6.425 Do not transmit! GHz Amateur 5cm band 5.83 -5.85 4 – 8 GHz downlinks GHz Amateur 5cm band 5.65 – 5.67 uplinks GHz Fixed satellite 7.25 – 7.75 service FSS GHz



7.9 – 8.4 GHz

10.7 – 12.55 GHz


18 - 27 GHz


26.5 - 40 GHz


40 – 75 GHz

Military / NATO Military / NATO (ITU region 1 Europe) TV downlinks (ITU region 2 Americas) TV downlinks (ITU region 3 Pacific ) TV downlinks NASA Kepler spacecraft. Not used for broadcasting due to severe rain attenuation Inmarsat, Iridium Next, James Webb Space Telescope. Very wide bandwidth data. Research and ‘Space to Space’ Communications.

7.75 – 7.90 GHz 7.9 – 8.4 GHz 10.7 - 12.75 GHz 11.7 - 12.20 GHz 11.7 - 12.55 GHz

18 - 27 GHz

18 - 20 GHz

40 – 75 GHz

SATELLITE TRANSPONDERS AND REPEATERS Most of the amateur satellites that are intended for amateur radio communication rather than research or educational roles, in other words, Hamsats rather than Amsats, are equipped with transponders or repeaters operating on frequencies within the designated satellite subbands of the amateur radio bands. Other amateur built satellites may have beacon transmitters and control uplink and downlink frequencies on amateur radio frequencies, but these are not intended for use by the wider amateur radio community. A transponder on a satellite receives a band of frequencies and transmits the same bandwidth on a different range of frequencies. It is this frequency transposition that makes it a transponder. Technically a single channel FM satellite is a narrow bandwidth transponder, but to avoid confusion I prefer to refer to FM satellites as cross-band repeaters.

LINEAR TRANSPONDERS A linear transponder is a mixer followed by a wideband linear amplifier. It accepts a range of frequencies at the input and retransmits the entire range to the output. The signal is not usually processed in any way other than frequency translation and amplification. Weak noisy signals will be retransmitted as stronger noisy signals. The technology is simple. The incoming band is mixed with a fixed frequency oscillator to shift the frequencies up or down to the required downlink band. Typically, this causes an inversion so that a lower sideband signal on the uplink becomes an upper sideband signal on the downlink. CW signals transmitted at an audio offset from the nominal carrier frequency are shifted. A CW signal transmitted at an offset lower than the nominal frequency will be shifted to a frequency higher than the nominal frequency on the downlink. RTTY mark and space signals also become ‘inverted.’ Reverse tracking transponder – The downlink frequency decreases as the uplink frequency increases. Generally, reverse tracking transponders are also ‘inverted.’ Forward or ‘normal’ tracking transponder – The downlink frequency increases as the uplink frequency increases. Forward tracking transponders are not inverted. Inverted transponder – for SSB transmission you transmit to the satellite on LSB and receive the downlink signals on USB. Non-inverted transponder – for SSB transmission you transmit to the satellite on USB and receive the downlink signals on USB.

The reverse tracking satellite transponder above receives a lower sideband signal at 145.920 MHz. It translates it to an upper sideband signal at 435.880 MHz. Signals received at higher 2m frequencies will be translated to lower 70cm signals. The frequency reversal occurs because the transponder’s local oscillator is on the ‘high side.’ Reverse tracking reduces the effect of Doppler shift. Positive Doppler shift moves the uplink higher causing the downlink frequency to be a little lower, reducing the effect of the positive Doppler shift on the downlink. Conversely, negative Doppler shift on the uplink causes the downlink frequency to be a little higher, reducing the effect of negative Doppler shift on the downlink.

The forward or ‘normal’ tracking satellite transponder above receives an upper sideband signal at 145.980 MHz. It translates it to an upper sideband signal at 435.880 MHz. Signals received at higher 2m frequencies are translated to higher 70cm signals. The transponder’s local oscillator is on the ‘low side.’ Doppler shift is not partially canceled. It is the sum of the uplink and the downlink Doppler shift. Linear transponders will faithfully reproduce any signal that appears at the input. You can use any narrowband mode that is permissible for the bands concerned. You would not normally transmit FM to a linear transponder because it would take too much of the available bandwidth, but CW, SSB, narrowband SSTV, and narrowband digital modes are permitted. The transponder has a wide bandwidth, so several amateur radio conversations can take place simultaneously. The available downlink power is spread across the active transmissions. Larger input signals create larger output signals. It is very important not to be an ‘Alligator’ sending a very strong signal to the satellite. A strong transmission will leave less power for other users. You can lose friends quickly if you take more than your share of the downlink power. A linear transponder includes: an input band-pass filter connected to the receive antenna a low-noise amplifier (LNA), designed to amplify the weak signals received from the Earth stations a frequency translator, normally an oscillator and a mixer, which is used to convert the frequency of the received signal to the frequency required for the downlink an RF (radio frequency) power amplifier an output band-pass or low pass filter

FM REPEATERS The FM radio systems on satellites work in essentially the same way as your local terrestrial repeater. The main difference is that they are crossband repeaters, receiving on one amateur band and transmitting on another. Most ‘FM satellites’ use the 2m and 70cm bands. They usually only have a single FM channel. Unlike a linear transponder, which is little more than bandpass filters, an oscillator, a mixer, and an amplifier. The FM repeater has a full receiver and transmitter. The audio output signal from the receiver is connected to the audio input of the transmitter. Some recent satellites use SDR (software defined radio) technology and/or DSP (digital signal processing). Because the repeater frequencies are fixed, you must adjust your transmitter frequency for Doppler shift in order to make sure that the right frequency arrives at the satellite. Luckily, FM is naturally tolerant of frequency errors so the Doppler shift correction does not need to be exact. You must also adjust your receiver frequency or channel to adjust for Doppler shift on the downlink. The usual method is to pre-program three or five channels on your radio(s) and switch through them during the pass.

PILOT TONE Many local repeaters use a pilot tone to prevent the receiver squelch from opening on noise bursts or other interference. Some FM satellites use the same technique. CTCSS is the ‘Continuous Tone-Coded Squelch System.’ It uses a low-level pilot tone to control the FM receiver’s squelch. The tones are below 100 Hz so they are usually filtered out of the 300 Hz to 3 KHz receiver audio output and are not noticeable on the on the received signal. To use the satellite, you have to transmit a CTCSS tone, usually 67.0 Hz, to open the satellite receiver squelch. The pilot tone is sent continuously while you are transmitting to the satellite. For SO-50, (SaudiSat-1c) you must send a 74.4 Hz tone for two seconds to turn on a 10-minute timer and then send 67.0 Hz during each subsequent transmission. You can achieve this by programming an additional first channel on your FM radio to transmit the 74.4 Hz tone. Program the other three or five channels to send the 67 Hz tone. Many terrestrial repeaters also transmit a CTCSS tone to control the squelch on your receiver. But, for satellite operation, the wanted signals are often weak so programming pilot tone operated squelch onto your receiver frequencies is not recommended. DCS is digital coded squelch. It uses FSK to send a one, two, or three digit code, which is decoded by the receiver at the far end. Most VHF and UHF radios can generate and decode either CTCSS tones or DCS codes. I’m not aware of any satellites that use DCS coded squelch.

FULL DUPLEX Because the satellite FM repeater or linear transponder is operating cross-band, you can, and should, listen to the downlink at the same time as transmitting to the satellite on the uplink. This type of operation is called full duplex. You need two radios or a radio especially designed for satellite full duplex operation to do this, but it is worth it. It allows you to ensure that your signal is sounding good through the satellite. Remember that you have antenna tracking and Doppler shift to manage while you are transmitting. Full duplex can be used on FM satellites, but it is not mandatory. It is almost essential for SSB or CW operation through linear transponders. In SSB or CW modes, the speech or audio tone changes as you drift off frequency due to the Doppler shift on the downlink signal. Speech may become unintelligible and it is easy to lose a conversation completely. Being able to hear your own signal helps you to keep track of things. FM operation is easier because your FM receiver will hold onto the signal even if it drifts a few kHz off frequency and the audio note is not affected by frequency error.

DIGIPEATERS A digipeater is a repeater, which receives a digital signal, regenerates it, and then transmits it on the same or another frequency. This is different to an FM repeater, which does not regenerate the signal. In a digipeater, the digital signal is received and demodulated. Then the resulting data is used to modulate the downlink transmitter. The advantage is that any noise or distortion occurring on the uplink path is completely eliminated from the downlink signal. Noise received on analog systems such as FM repeaters or linear transponders is retransmitted along with the wanted signals. This is true even if you are using a digital mode through an analog satellite.

DIGITAL SIMPLEX Most digipeaters on satellites do not work cross-band. They receive the signal from your ground station, demodulate the digital data stream and then later re-transmit it on the same frequency as the uplink. This is known as ‘packet store and forward.’ The mode was originally used for bulletin board systems where you sent a message to the satellite and another operator could request that it be transmitted back to them. Since the advent of the Internet, BBS systems have fallen out of popularity and packet radio is not often used these days. However, the digipeater satellites are great for APRS. You can send a ‘beacon’ to the satellite and the information will be re-transmitted several times as the satellite orbits. There are several ‘digital’ satellites and the ISS also has a digital simplex packet system for APRS.

SIMPLEX V-V MODE V-V mode is used for FM voice contacts with the crew on the International Space Station. Both the uplink and downlink are on the 2m band. The uplink is 145.2 MHz for ITU region 1 and 144.490 MHz for ITU regions 2 and 3. The downlink is 145.8 MHz ± Doppler. The V-V mode is halfduplex and your signal is not re-transmitted. Set up your radio with a fixed transmit to receive offset, like a standard repeater channel. It is like working a station in the ‘split’ mode used on the HF bands. The ARISS call sign depends on the nationality of the ISS operator; USA: NA1SS, Russian: RS0ISS, European: DP0ISS, OR4ISS, or IR0ISS. Other call signs may be allocated for other nationalities.

BENT PIPES AND MIRRORS Linear transponders and FM repeaters are sometimes referred to as ‘bent pipe’ repeaters, probably because of the frequency translation, or possibly because the signal travels up to the satellite and is then ‘bent’ back down to Earth. It is completely wrong to refer to a satellite as a mirror, because that implies that, the signal is being reflected rather than being amplified and re-transmitted. The inverting sideband nature of reverse tracking linear transponders could be called a mirror effect. But, the only satellite that could correctly be referred to as a mirror is the Moon because there is no repeater or transponder there. EME transmissions rely on the signal being reflected from the Moon’s surface.

COMMERCIAL COMMUNICATIONS SATELLITES Most communication satellites are radio relay stations in orbit and carry dozens of transponders, each with a bandwidth of tens of megahertz. Most transponders operate on a bent pipe principle, sending back to Earth what goes into the conduit with only amplification and a shift from uplink to downlink frequency. However, some modern satellites use onboard processing, where the signal is demodulated, decoded, re-encoded and modulated aboard the satellite. This type, called a "regenerative" transponder, has many advantages but is much more complex. With data compression and multiplexing, several videos (including digital video) and audio channels may travel through a single transponder on a single wideband carrier. http:\\

Geostationary satellites are mostly used for national and international telephone and data communications and for television distribution. They are also used for providing Internet service to remote locations and for television linking such as sending programs, sports coverage, and news feeds between broadcasters or studios. The broadcaster will edit the feed and then transmit their ‘program feed’ via the same or a different satellite, or from terrestrial television transmission sites. These days the TV signals are digital DVB-S transmissions and they may be encoded so that they can only be viewed via a subscription service. However, some satellites still carry un-encoded TV signals that you can view if you have suitable station equipment. The commercial Internet, telephony, and data services are private and encrypted. Most digital TV (DVB-S) signals from geostationary satellites are transmitted on the Ku band (10.7 GHz to 12.7 GHz). The old analog TV satellites which required a two or three-meter mesh dish in your yard, transmitted on the C band (4 GHz to 8 GHz). The other popular commercial satellites are the ‘weather satellites’ used to study the weather and climate change. They transmit radar images of the Earth’s surface, which can be decoded using freely downloadable software. Some of the images are enhanced with color to make them easier to interpret. Some weather satellites are in low Earth orbits, some are in geosynchronous (sun-synchronous) polar orbits so that they appear above the same place every day, and some are in geostationary orbits. The pictures from NOAA satellites are easy to receive. They are transmitted on 137 MHz. You get better images using a software defined

radio (SDR) receiver because they can be adjusted to the wide transmission bandwidth.

SATELLITE BEACONS Nearly all Hamsats and many Amsats transmit a beacon signal. A satellite beacon is a transmission that indicates the satellites identity. Receiving the beacon tells you that the satellite is alive and you can hear signals from it. Satellite beacons used to send a Morse code identification signal, but these days it is more common for the beacon transmitter to send telemetry data relating to the health and operation of the satellite down to the satellite control station. You may be able to help by receiving and decoding the information and sending it to the control station via the Internet. Small CubeSats typically only have one transmitter that is used for telemetry and sometimes as an FM repeater or a digipeater. The beacon transmitter may also be used to transmit data relating to the science or educational experiments or payload on the satellite. This sometimes includes pictures from onboard cameras. Even the beacon on OSCAR 1, the very first amateur radio satellite, had a dual amateur radio and scientific function. The satellite transmitted the ) as a beacon. The speed that the Morse code letters H I ( CW signal was sent indicated the temperature inside the satellite. This was important scientific research data in those early days. The first artificial satellite, Sputnik-1, launched into orbit on the 4th of October 1957, carried a radio beacon transmitting on the frequency of 20.005 MHz. Sputnik-2 carried a live dog ‘Laika’ into space. Not only was a beacon needed to track the spacecraft and relay information about its health, but it was also needed to send information about the dog's condition. Some satellites transmit a CW beacon signal; others send telemetry and mission data using AX-25 ‘packet’ data, AFSK, FSK, or PSK. The beacon is usually in the same RF band as the main downlink. If the satellite has several transponders, it may have a beacon on more than one band. On amateur radio satellites with linear transponders, the beacon is usually just below the transponder downlink band of frequencies. As you know the beacon frequency being transmitted, you can use the signal to work out how much Doppler shift there is on the downlink. This was more useful in the early days before everyone had a PC running satellite tracking software.

Some commercial satellites including geostationary satellites also have beacons. They are usually in the same band as the downlink, often in the 4 GHz or 11 GHz range. The beacon is used to align the feed arrangement in your satellite dish. Most of the geostationary satellite beacons use horizontal polarization, referenced to their position above the equator. If you are at a substantially different longitude and latitude, the polarization may be skewed but it will remain constant once you rotate your feed for best signal.

RESEARCH BEACONS Signals from satellite beacons are used for research into the effects of the ionosphere and troposphere layers on RF signals passing through them. Beacons operating in the VHF and UHF range are used for ionosphere research. The ionosphere is a region of the upper atmosphere, which lies at altitudes between 80 and 1000 km. Air is ionized to form a weak plasma by UV and X-ray radiation from the Sun, forming the ionization layers that we rely on for long distance HF communication. LEO beacon satellites combined with chains of ground receivers have been used for ionospheric imaging based on computerized ionospheric tomography (CIT). Troposphere research is based on beacons operating in the microwave range. The signals from satellite beacons in the X-band (8-11 GHz) are little affected by the Ionosphere, but they are affected by the troposphere, which is in the first 10 km – 20 km of the Earth's atmosphere. The troposphere is the lowest layer of Earth's atmosphere, where nearly all weather conditions take place. The layer contains 99% of the water vapor held in the atmosphere and most of the trapped dust, smoke particles, and greenhouse gasses. The top of the troposphere is highest over the equator and lowest over the poles. It is also lower over the hemisphere experiencing winter. Beacons can also be used to determine the amount of Faraday rotation that is occurring on the path from a particular satellite. Ionization in the upper atmosphere makes the polarization of radio wave rotate as the signal passes through the atmosphere. Faraday rotation affects both uplink and downlink signals. has more information on satellite beacons including a description of techniques for measurement of Faraday rotation, electron content, ionosphere scintillation, and ionosphere research. Ionospheric scintillation is a rapid fluctuation of radio-frequency signal phase and amplitude as the signal passes through the ionosphere. The effect of scintillation of the radio wave upsetting the phase and frequency of the signal is the radio equivalent of the twinkling of stars. The scientific name for the twinkling of stars is ‘stellar’, or ‘astronomical’ scintillation. Stars twinkle because the light from them is refracted by water vapor, air turbulence, and temperature layers in the atmosphere. Temperature layers cause density changes that the light passes through. Higher air

density refracts light more than low air density. The charged particles in the ionosphere cause a similar fast fluttering on radio signals. In both cases, starlight and radio, signals viewed at low elevations pass through more atmosphere and therefore suffer more scintillation twinkle, than starlight or radio signals from sources at high elevations.

SATELLITE DEPLOYMENT Building a satellite even a small CubeSat is a major undertaking, which can take several years, but building the satellite is the easy part. Getting the satellite launched into orbit is the real problem. A good example is the AMSAT Fox program. The project was proposed in July 2011. After construction, the launch was delayed twice from the scheduled date in November 2014 until the satellite was eventually launched in October 2015. Another example is KiwiSAT, a 20 cm CubeSat first proposed in 2002. It was initially scheduled for a launch in 2005 and then in 2012. The satellite is complete, tested, and ready to go, but it is still waiting for a launch opportunity. The team is now hoping for a launch early in 2018. Amateur built satellites have been constructed in many different sizes and shapes. Some of the amateur radio satellites were quite large and heavy. For example, AO-40 weighed 400 kilograms. The early AMSAT OSCAR satellites were designed and constructed so that they could be fitted into the launch vehicle as weight balancing ballast. Since about 1990, the number of amateur built satellites being constructed and launched has risen dramatically. It is now common for ten or more amateur built satellites to be launched at a time. Dozens of universities, clubs, and amateur radio groups began to build satellites. The variation in designs was becoming a big problem for the organizations that were responsible for launching them into orbit. Each satellite had a different set of requirements. There were different shapes, sizes, weights, cradles, and deployment methods. Some sort of standard was required. In 1999, California Polytechnic State University (Cal Poly) and Stanford University jointly developed a set of standard specifications for CubeSats. They are available at These days most satellites launched into low Earth orbit are built following the CubeSat guidelines. They carry one or two scientific instruments as their primary mission payload and some carry amateur radio transponders. CubeSats are based around 10cm x 10cm x 10cm units weighing no more than 1.33 kg. They can be constructed in configurations from one to twelve units. Inside, the CubeSat will have several circuit boards. The outside surfaces are usually covered with solar panels and antennas for the uplink and downlink radios. Sometimes they carry items such as bigger antennas, fold-out solar panels, or sails that can deploy once the satellite is in space.

The launch cost for a CubeSat is generally directly proportional to their weight. It is quite expensive, so many institutions now build even smaller ‘pico’ satellites typically based on collections of 10 x 10 x 10 mm modules. It is a big challenge to fit in radios, a power supply, solar panels, and antennas.

Building a CubeSat has never been easier. You can buy ‘off the shelf’ CubeSat frames and a range of commonly used technology cards and panels. Unfortunately launching a CubeSat remains much more difficult than building one. If you are a well-funded University or organization, you can pay full price and get your satellite into space relatively quickly. Many countries offer commercial satellite launching services, including USA, Russia, China, India, and even New Zealand. The alternative is to persuade NASA to launch your satellite from the International Space Station for free or at a discounted rate. There is a huge demand for these cheaper opportunities and a limited NASA education budget. To qualify, your satellite must have a valid educational and/or scientific purpose, such as an experiment that meets NASA’s educational objectives. Even if your

project is approved, you will have to wait for about two years for your project to reach the top of the heap.

DEPLOYMENT PLATFORMS P-PODs were designed to provide a common deployment platform for CubeSats. The P-POD, pre-packed with up to three 1U CubeSats, is mounted onto a launch vehicle or the outside of the International Space Station. When the launch vehicle has reached orbit and the deployment command is triggered, the front hatch of the P-POD pops open and a spring is released to push the CubeSats out into space. The latest P-POD Mk III model holds three 1U CubeSats, or 0.5U, 1U, 1.5U, 2U, or 3U combinations up to a maximum volume of three CubeSat units. The NanoRacks CubeSat Deployer (NRCSD) is also used on International Space Station. It can launch up to a 10×10×68.1 cm (1x1x6 units) 6U CubeSat. ISIPOD (Innovative Solutions In Space BV) can launch a different form of 6U CubeSat measuring 10×22.63×34.05 cm, (1x2x3 units). Other CubeSat deployment systems include SPL, X-POD, T-POD, and J-SSOD.

CUBESAT DESIGN The actual design of a CubeSat is up to the design team as long as they stay within the specification, but most CubeSats have the same basic structure. They all need as much power as possible, so the external surfaces are covered with solar panels. The inside consists of several printed circuit boards arranged in layers. One board will be some sort of management computer, traditionally called an IHU (internal housekeeping unit). There will be a power supply board to manage the charging of batteries or supercapacitors and to provide all of the voltages required. One board will carry the experimental payload, possibly including cameras or sensors. Finally, there will be a radio board, which will include at least a receiver for receiving commands from the control station and a transmitter to send satellite status data back to Earth. The transmitter and receiver may also be used to interface with the experiment payload. Amateur radio satellites may include a beacon transmitter, one or more amateur radio transponders, an FM repeater, or a digipeater. If the satellite is a one unit CubeSat or a picosatellite, several functions may be performed on the same circuit board.

The satellite might include other features such as a stabilizing mechanism to stop it tumbling, a GPS receiver, or extendible solar panels. Some CubeSats have included solar sails or ‘electric’ booster motors used to extend orbital lifetime by boosting the CubeSat into a higher orbit. VHF and UHF antennas are typically ‘tape measure’ antennas, which can be

stored, folded up. They spring out after the satellite leaves the pod. Patch or dish antennas can be used but they take up space that would otherwise be used for solar panels.

MISCELLANEOUS FACTS This chapter contains some of the odd or interesting satellite facts that don’t fit into any of the other chapters.

OVER THE HORIZON Generally, we accept that the radio path between your location and the satellite is a straight, “line of sight” path. But you might actually be able to hear signals from the satellite and even transmit signals through a satellite when your satellite prediction software reports that the satellite is below your local horizon! This is because at low antenna elevations the radio path from the satellite through the Earth's atmosphere is not perfectly straight. It is refracted a little by the air. The refraction tilts the signal downwards allowing you to hear and send signals through the satellite when it is a degree or two below the horizon. Most of this refraction occurs in the lowest parts of the troposphere where the refractive index changes most rapidly with the change of height. The actual amount of refraction of the radio wave varies. It depends on factors such as humidity, air pressure, and air temperature. At elevations higher than 10 degrees, the effect of refraction becomes minimal. Some satellite prediction programs correct the elevation calculation to compensate for the refraction effect. The C++ version of Plan-13 applies a correction of about half a degree when the real elevation is zero degrees.

In the drawing, refraction causes the satellite to appear as if it is at the position marked by the dot at the end of the upper line when in fact it is below the horizon at the position indicated by the dot at the end of the lower line. Also, operating from a high site increases the distance to the horizon. Which is why for accurate readings, the satellite prediction program needs the Observer’s height above sea level as well as the latitude and longitude.

Refraction in the atmosphere affects the visible spectrum as well as radio waves, causing stars and the Moon to be visible when calculations show that they are actually below the horizon.

As this time-lapse series of photos shows, refraction makes the Moon look fatter as it sets. The refraction is allowing you to see the wider part of the Moon that has already set below the horizon. The effect compresses the vertical dimension making the Moon appear oval. The refraction effect reduces at higher elevations, so the Moon looks normal when it is higher than 10-15 degrees above the horizon.

FLYING IN FORMATION There have been some amateur satellite experiments seeking to discover the behavior of satellites that are in orbit and close together. Some of these are experiments in distributed communications such as increasing the effective capture area of the receiver antennas by combining signals received by several satellites. Other experiments are to do with tethering two or more satellites together or investigating the way that orbital drag and micro-gravity affects satellites. To quote the novel ‘Seveneves’ by Neal Stephenson, “in space there is no such thing as formation flying.” In the air, or on the surface, objects can travel beside each other fairly easily. The position of an object can be defined using three numbers, height, latitude, and longitude or ‘x, y, z.’ but to define the position of an object that is in orbit, you need the six Kepler numbers. Objects in orbit behave differently. Rather than staying in the same spatial relationship unless deliberately moved, they will always tend to drift apart. In almost every case, each object in space is in its own unique orbit. Let’s study the example of a supply craft in an orbit near the International Space Station. If the supply craft is at a higher altitude than ISS, it will drift backward away from the ISS. This is because objects in higher orbits travel more slowly. If it is at a lower altitude, it will drift ahead of the ISS. If it is to the left or the right, is orbit will be inclined differently and it will drift away to the side and then return close to ISS each time they both complete an orbit. A swarm of small satellites will not naturally stay together. The group will always spread apart. Satellites that are slightly lower will drift ahead of the others. Satellites that are slightly higher will drift behind. Satellites that are at exactly the same orbital height will drift away to each side but will return to the initial spacing at the same part of every orbit. The only exception is when the supply craft is directly ahead or behind the ISS in exactly the same orbit. In that case, the two spacecraft will maintain the same distance apart because they are traveling at exactly the same speed. They could be thousands of kilometers ahead or behind and they will stay separated by the same distance. This is why the hundreds of geostationary satellites stay separated. Of course, the second that any force is applied in the form of a thruster ‘burn’ in any direction, the spacecraft’s orbit will change and the satellites will move relative to each other. This makes docking two spacecraft together a very tricky process.

BIRDS Some people like to call satellites 'birds.' It is quite common to hear about radio hams “working the birds.” This might be because satellites fly and they have a “bird’s eye view.” Or it might be because the name Early Bird was used for INTELSAT 1, the first commercial communications satellite to be placed in geosynchronous orbit, (April 6, 1965). However, I believe that the most likely derivation is military slang. Apparently, US military personnel traditionally refer to pretty much anything that flies as being a “bird.” Since the 1950s, the term has been selectively used to describe, helicopters, aircraft, rockets, missiles, and satellites. The first American artificial satellites were launched by the military from Cape Canaveral Air Force Station in Florida. I am sure that is why it was common in those days to refer to the new satellites as ‘birds.’

SATELLITE DISCIPLINED CLOCKS The GPS satellites transmit extremely accurate time signals. GPS navigation devices such as your smartphone or the satellite navigation in your car use the difference in the time that the signals are received from three or more GPS satellites to calculate (triangulate) the position of your car or phone. These super-accurate time references can also be used as very accurate and stable clock references for any digital system or radio. This is especially useful if you are employing diversity receiver systems where two receivers must operate in exact phase or large networks where you want a common clock source. GPS clocks are routinely used as a reference clock to control (discipline) the internal clock oscillators in fiber optic transmission systems, digital microwave radio links, and cell sites. Amateur radio operators use time signals from GPS receivers to lock the local oscillator of their transceiver or receiver. This makes sure that the radio is exactly on the wanted frequency and ensures that it won't drift as the radio warms up. The technique is particularly useful when you are using an HF or VHF radio with a transverter to generate microwave frequencies. Multiplying a frequency multiplies any frequency error on the original signal, and mixing a signal with the output of a reference oscillator adds any frequency error on the original signal to that of the reference oscillator. So very accurate reference frequencies result in more accurate, transmit frequencies on the high bands. A GPS disciplined oscillator (GPSDO), is a combination of a GPS receiver and a high-quality, stable oscillator such as a quartz or rubidium oscillator. GPSDO boards are available online for less than $100. Good ones cost more like $300. They usually output a 10 MHz signal and often a 1 PPS (one pulse per second) signal. They should be stable to at least one part in 1011 (0.000001 parts per million), which is far better than an OCXO or TCXO can manage. In older HF transceivers the local oscillator may have a drift of 5 - 20 ppm, (parts per million). The oscillators in modern HF transceivers are typically stable to 1 or 2 ppm. Most VHF and UHF band radios now use a TCXO or an OCXO, which will improve the oscillator stability to between 0.5 and 0.1 ppm. An ‘OCXO’ is an "Oven Controlled Crystal Oscillator." A tiny oven heats the crystal in the oscillator and then keeps it within a narrow temperature

range. This minimizes frequency drift and keeps your operating frequency stable. A ‘TCXO’ is a Temperature Compensated Crystal Oscillator. A circuit adjusts the oscillator frequency as the temperature increases to compensate for the natural frequency drift of the crystal oscillator. A GPS signal can be used to make very small adjustments to the TCXO in your radio, usually every second or so. This can improve the stability to one part in 1012 (0.0000001 ppm). An oscillator or clock generator that is governed using GPS time signals is called a GPS disciplined oscillator or ‘GPSDO.’ PPM stands for ‘parts per million.’ Say your transmitter or receiver is working at 30 MHz (30 million Hertz) and the local oscillator is stable to ±1 ppm, (1 Hertz in a million). That means that your frequency could be anywhere from 30 Hz high to 30 Hz low. If the radio has a TCXO with ±0.1 ppm stability, your frequency will always be in the range between 3 Hz high and 3 Hz low. If your transmitter or receiver is working at 450 MHz with a similar ±0.1 ppm TCXO, your frequency could be anywhere from 45 Hz high in frequency to 45 Hz low. In most cases, this level of error is absolutely fine. But using a GPS reference can bring the possible frequency error down to essentially nil. PPM is usually measured over a long period, for example, 48 hours. Of course, your measuring equipment needs to have better frequency stability than the oscillator under test! This usually means it either will use an atomic frequency standard or will have its own GPS disciplined frequency reference.

WHY ARE SATELLITES SO EXPENSIVE TO LAUNCH? Space is pretty close. The LEO satellites are in orbits that are only about 350 km above ground. You probably travel further than that for your summer holidays. So why does it cost so much to get a satellite into orbit? The answer, of course, is that the rocket carrying the satellite has to fight the force of gravity. It’s a bit like going on vacation with a huge tree dragging behind your car. That would really hurt your fuel economy. As well as having to fight gravity, there is also air resistance, or ‘atmospheric drag,’ which is quite a big factor when the spacecraft is traveling at several thousand kilometers, or miles, per hour. Overcoming air resistance accounts for about 10% of the energy that is required to launch a rocket into space. Overcoming gravity and atmospheric drag is only a part of the problem. It takes about a million joules of energy to lift one kilogram of weight up to an altitude of 100km. But, that is a lot less than the energy that is required to keep it there. Hold on! Didn’t I say that satellites stay in orbit without any additional energy being required? Yes, I did! I will explain. If you launch a rocket straight up, to an altitude of 350 km, it will fall straight back down again. To make the satellite stay in space it has to be ‘inserted’ into an orbit. The rocket has to achieve enough speed, parallel to the Earth’s surface, to allow the released satellite to stay in the wanted orbit, typically around 7.8 km/s or 17,400 mph. It takes thirty times as much energy to get the satellite up to orbital velocity than it does to achieve the orbital height. If you watch a video of a rocket or space shuttle launch, you will see that the rocket does not go straight up. It follows a curved path, eventually becoming horizontal near the final altitude. The flight path gradually transfers from gaining altitude to gaining velocity. Now you know why! To get a satellite established into a 350 km orbit takes a total energy of about 32 megajoules per kilogram of weight (32MJ/kg). To get a kilogram into a stable geostationary orbit requires around 54 megajoules (54MJ/kg) of energy per kilogram. A typical car engine with a power output of 200 horsepower can create enough energy to lift about 16kg into a 350km orbit. But since the car engine weighs considerably more than 16kg, an engine that feeble would have no chance of getting itself into orbit, let alone lifting the car and your family. So I’m afraid that it is unlikely that you will be going to space for your summer vacation.

Rocket launches typically cost between $100M and $260M per launch. An American Atlas V rocket can lift its own weight plus 4.75 metric tons of payload into orbit at a cost of about $ 164 million. That is around $34,500 per kg. Ariane launches cost $200 million to carry a maximum payload of 10 metric tons to geostationary transfer orbit (GTO). About $20,000 per kg. The SpaceX Falcon 9 rocket can lift a payload of 5.5 metric tons to GTO for a cost of around $62,000,000, which is a bargain at only $11,270 per kg. A ‘payload’ is the items that you want to get into orbit. It could be people, supplies for the International Space Station, a big satellite, or a collection of CubeSats. To lift the rocket’s payload, a large amount of energy is required and that requires a lot of high-energy fuel. The fuel must be stored in the rocket, which makes the rocket bigger and heavier. The bigger rocket and the weight of the fuel must also be accelerated up to orbital height and speed, although the weight of fuel decreases as it is used up. That requires, even more, fuel and bigger engines, which of course weighs more. Big rockets like the Saturn V have multiple stages that can be dropped to shed weight as the fuel they contain is used up. The Space Shuttle used solid fuel boosters, which fell away after they were depleted. Dropping weight during the flight allows the rocket to accelerate more at higher altitudes where atmospheric drag is less. As the weight of the payload and the intended orbital height increases, the size and cost of the rocket needed to get it there increase exponentially. Rockets capable of lifting big, five-ton, TV or telecommunications satellites into geostationary transfer orbit have to be very large. Delta V. Even when the rocket is on the launch pad, it has some velocity due to the rotation of the Earth. The difference between the velocity at the launch pad and the final orbital velocity is called ‘delta V’ (change in velocity). The initial velocity is not very relevant for rockets launching satellites into low Earth polar orbits, but it does give a slight boost to rockets launched from near the Equator into geostationary orbit. Conversely, satellites being boosted into retrograde orbits that travel in the opposite direction to the direction of Earth’s rotation, have to attain orbital velocity plus the rotational velocity. In other words, the delta V that must be achieved is greater. So, it costs a little more to launch a satellite into a retrograde orbit.

SPACE JUNK AND DEBRIS IN LEO The LEO environment is becoming congested with space debris due to the frequency of launches. As well as satellites, there are protective fairings, booster stages, ice, foam packaging, insulation materials, and even tools dropped by astronauts. This is causing growing concern since collisions at orbital velocities can be deadly or at least expensive. Any collisions that do occur produce even more space debris, creating a domino effect known as Kessler Syndrome. The ‘Joint Space Operations Center,’ part of the United States Strategic Command currently tracks more than 8,500 objects in low Earth orbit that are larger than 10 cm. However, a limited Arecibo Observatory study suggested there could be a million objects larger than 2 millimeters, which are too small to be visible from Earth-based observatories.

THE FUNCUBE SATELLITES In October 2009, a team of volunteer experts from AMSAT-UK, in collaboration with their colleagues in the Netherlands at AMSAT-NL started work on the FUNcube satellite concept. The initial plan was to design, build and launch a single spacecraft but further flight opportunities became available and the project now comprises five satellite missions. The FUNcube satellites have a dual role, functioning as amateur radio transponders during night time and weekend passes over the UK and as educational outreach satellites for school programs during daylight passes. The Educational telemetry transponder runs higher power during the day when the amateur radio transponder is turned off. Information from

FUNCUBE DONGLE The FUNcube project needed a cost-effective ground station that could be used by teachers to capture the telemetry data directly from the satellite. Team member Howard Long, G6LVB, designed the FUNcube dongle SDR receiver as the ground station companion to the FUNcube satellite. The original FCD had a typical frequency range of 51.5 MHz to 2000 MHz with a gap between about 1100 MHz and 1270 MHz. The newer FCD Pro+ has a typical frequency range of 150 kHz to 260 MHz and 410 MHz to 2.05 GHz. The FUNcube dongle receivers are a bit more expensive than the ubiquitous RTL Dongles, but they have dedicated front-end filters with excellent SAW filters on the 2m and 70cm ham bands. The frequency coverage is amazing and they were specifically designed to be a receiver for amateur operators and for schools involved with the FUNcube satellite project.

THE FUNCUBE SATELLITES FUNcube-1 (AO73) is a complete educational 1U CubeSat with the goal of enthusing and educating young people about radio, space, physics and electronics. It was launched from Russia on a DNEPR rocket on Nov 21st, 2013. FUNcube-1 is in a Sun-Synchronous Low Earth Orbit approximately 630km above the earth. In such an orbit a satellite passes over most locations approximately three times in the morning and three times in the evening, every day. This schedule allows the morning passes to be used for educational purposes and the evening passes to be used for amateur radio communications. Primary mission: During the sixty-five minute sunlit part of its orbit, FUNcube-1 operates in ‘educational’ mode. The amateur radio transponder is turned off and the telemetry beacon runs in ‘high power’ 300mW mode. Students can use the telemetry data to study; orbital mechanics, the time in sunshine and time eclipsed, Doppler shift, satellite temperature, data from three sun sensors to determine the satellites spin rate, battery condition, and solar power charge rates. The satellite also carries a Materials Science Experiment, from which the school students can receive telemetry data regarding heat absorbed by different materials. They can compare the results they obtained from similar reference experiments in the classroom. An internet-based data warehouse records the telemetry collected from more than 450 users around the world and makes the data available for download. Secondary mission: During the 32 minutes of each orbit that the satellite is in eclipse, FUNcube-1 operates in amateur radio transponder mode. Telemetry is still transmitted but at a lower power of 30mW. FUNcube-2 on UKube-1 was a follow-on project comprising of a set of FUNcube boards mounted on the UKube-1 triple CubeSat. It was launched on a Soyuz rocket from Baikonur on July 8th, 2014. FUNcube-3 on QB50p1 (EO-79) is a 2U CubeSat. It was launched June 19, 2014, into a 621 x 604 km sun-synchronous orbit, inclined at 97.98 degrees. Due to power budget constraints, the transponder cannot be operational 24/7 and an orbit specific schedule has been developed. The transponder

will commence operation 27 minutes after the spacecraft enters sunlight and will stay on for a period of 25 minutes. This schedule may be modified over the forthcoming weeks as a result of experience. FUNcube-5 (EO88) on Nayif-1 is a 1U CubeSat project developed by the Emirates Institution for Advanced Science and Technology (EIAST) in partnership with students at the American University of Sharjah (AUS). It carries an enhanced, FUNcube communications package to provide educational outreach telemetry and an amateur transponder. Nayif-1 was successfully launched from India on Feb 15th, 2017 together with 103 other spacecraft. FUNcube-4 is to be a payload on the ESEO satellite mission. ESEO is a 20kg class microsat which incorporates experimental payloads from a number of universities around Europe. The FUNcube payload will provide similar telemetry to its predecessors but will have a more powerful transmitter. The amateur radio payload will be an FM voice LV mode repeater.

WEATHER SATELLITES Weather satellites are primarily used to monitor the weather and climate of the Earth. Some are in geosynchronous (sun-synchronous) polar orbits so that they appear above the same place every day and some are geostationary. The geostationary satellites stay in a fixed place in the sky and can be received all the time. However, they are much further away than the polar orbiters so their signals are weak. They transmit at microwave frequencies. You need a dish or patch antenna and specialized radio equipment to receive images from them. The geostationary satellites provide images taken in the visible spectrum and in the infrared spectrum. Some also provide radar images used mostly to indicate rainfall. Receiving weather pictures from geostationary satellites is relatively difficult compared to getting pictures from the NOAA satellites. Most people use an SDR receiver, a low noise amplifier or LNB, and a 1.2m dish antenna. The polar-orbiting weather satellites are in sun-synchronous orbits at an altitude of about 850 km. The polar orbit means they can observe any place on Earth and being sun-synchronous means that they can view every location twice a day with the same general lighting conditions because they pass over at the same time every day. Polar orbiting weather satellites offer a much better image resolution than the geostationary satellites because they are so much closer to the Earth. The United States government, ‘National Oceanic and Atmospheric Administration,’ operates the NOAA series of polar-orbiting meteorological satellites. Presently NOAA 15, NOAA 18 and NOAA 19 are active. Europe has the Metop-B satellite operated by EUMETSAT. Russia has the Meteor-M N2 satellite. China has Feng Yun 3A, 3B, and 3C and India have polar-orbiting weather satellites as well. The NOAA satellites are by far the easiest way to receive weather images. The pictures are transmitted in APT (automatic picture transmission) format using FM modulation on one of six channels near 137 MHz. You will get much better images using an SDR receiver or a receiver especially designed for weather satellite reception because the signals are quite wideband at about 50 kHz. A ‘FUNcube Pro plus’ SDR is excellent. Even a cheap RTL dongle will do. SDR receivers have fully adjustable bandwidth whereas most amateur radio FM receivers are narrowband (5 kHz). If they do have a wideband mode, it is usually 150

kHz for FM stereo broadcast reception, which is too wide, resulting in unwanted noise being received. The effect of using a narrowband receiver is that the loudest bits of the signal representing the brightest parts of the image are filtered out, resulting in an image with poor contrast. In most cases, the received signal is saved as a file on your computer and then analyzed later using a weather satellite image, processing program. However, some weather image software can manage Doppler correction as well as downloading and decoding the weather pictures while they are being received. Most, probably all, satellite tracking and prediction programs can be set to display weather satellites. The choice of antenna is critical for good reception. Discones, dipoles and collinear antennas have deep radiation pattern nulls, which cause noise bands or synch loss on the received images. You can track the satellite with a Yagi or similar directional antenna but most, weather satellite enthusiasts use fixed circular polarized antennas, particularly the Quadrifilar helix.

WEATHER SATELLITE SOFTWARE WXtoImg is shareware for Windows, Linux, or Mac OS X. Additional features can be purchased but they are not necessary for amateur use. WXtoImg can process APT (Automatic Picture Transmission) and Wefax (weatherfax) images. The program supports computer control of several receiver models. It is more sensitive than some other programs, has additional options for displaying data from nighttime passes, and it can overlay country borders onto the received image, making it much easier to see what you are looking at. WXtoImg was developed by Craig Anderson from Auckland, New Zealand. WXSAT was developed by Christian Bock. It produces excellent APT images and it can produce images while the signal is being received, or from a saved audio file. WXSAT is provided free for amateur use. APTDecoder developed by Patrik Tast Västervik in Finland is freeware. It supports computer control of several receiver models and it produces very nice images. SatSignal by David Taylor has a free trial version with the ability to purchase additional capability. It was specifically designed to process APT images from the NOAA satellites. Check out Les Hamilton’s guides to these programs and his extensive website., and also the NOAA Satellite and Information Service The Automatic Picture Transmission (APT) system is an analog image transmission system developed in the 1960s specifically for weather satellites. WEFAX ‘weatherfax’ is a fax format used to transmit weather images over HF radio. There are still some WEFAX broadcasts on the HF bands but they are being replaced by digital images provided via satellite Internet.

DEDICATED WEATHER SATELLITE RECEIVERS The main reason to buy a dedicated 137 MHz weather satellite receiver is their ability to receive wideband FM. They are specifically designed to receive the six NOAA channels with suitable noise and bandwidth filters. Some have two antenna inputs, enabling diversity reception. These days there are many ‘software defined radios’ which cover the same frequencies with similar wideband FM reception. However, you may prefer to buy a radio that was designed for the task. Popular options include: The R2FU APT receiver is designed for the NOAA series satellites. It is pre-set for the six APT channels and it has a USB interface to connect to the PC. The radio is also powered by the USB connection. Timestep sells the PROscan APT receiver, a compatible Quadrifilar antenna, and a low noise masthead preamplifier. The receiver covers the six APT channels and has extensive filtering. The Wrasse WeSaCom APT-06 Weather Satellite Receiver is available in several formats, including a model with antenna diversity. Like the RF2U receiver, it is UFB connected and powered. Using two antennas and the diversity option provides excellent protection from fades causing noise on the pictures. The company offers antennas and preamplifiers including an active antenna, which incorporates the preamp.

IMAGE / TRANSMISSION STANDARDS Each NOAA satellite transmits APT pictures, using frequency modulation, on one of six channels around 137 MHz. The Automatic Picture Transmission system is an analog image transmission system developed for use on weather satellites in the 1960s. The signals can be decoded using the applications listed above. APT pictures have a resolution of about 4 km. The NOAA satellites also transmit high-resolution pictures using the HRPT format on channels in the 1.7 GHz band. HRPT stands for highresolution picture transmission. The mode uses L-band downlink frequencies around 1.7 GHz. You can use a down converter or an SDR that can reach 1.7 GHz such as the FUNcube dongle or a HackRF SDR. Reports online suggest that RTL dongles are not good enough for this job. HRPT pictures have a resolution of 1.1 km. Much better than the APT images. The European Metop-B satellite and the Russian Meteor M N2 satellite also transmit pictures on channels around 137 MHz but they use the LRPT image format. LRPT (low-rate picture transmission) is a digital format sent using QPSK modulation at 72 or 80 kbits. LRPT transmissions are not compatible with the APT software. Also, there is some difference between the Russian and the European images, which causes sync bars on the picture from the EU satellite if you are using image-processing software developed for the Russian satellite. The decoding software requires a recording of the IQ signal from a software defined radio. See; The European and Russian polar orbiting satellites also transmit a highresolution version of the LRPT images using a format called AHRPT. It is a QPSK signal transmitted at a 665.4 kbit data rate, on L-band downlink frequencies around 1.7 GHz. AHRPT stands for advanced high-resolution picture transmission. The geostationary satellites use the LRIT and HRIT digital transmission standards, with most downlink frequencies between 1680 and 1691 MHz. Although some of the satellites use downlink frequencies as high as 18 GHz. LRIT is ‘low rate information transmission.’ According to the NOAA website, the LRIT broadcast is operational on both GOES-13 (GOES-

East) and GOES-15 (GOES-West) spacecraft. The GOES satellites offer images of the visible spectrum, infrared spectrum, and water vapor. Each satellite also sends the infrared images captured by the other GEOS satellite. The transmission is a digital signal at about 128 kbits on the 1691 MHz downlink. HRIT is ‘high rate information transmission.’ The data rate for HRIT is 400 kbits. The current GEOS satellites transmit both LRIT and HRIT coded pictures. As new weather satellites are launched, HRIT will replace the older LRIT standard. Both formats include weather images and data used for other services. Some amateurs have built stations capable of receiving these signals, but this is still very experimental and the decoding software is not easily available. There is an Apple iOS app called ‘GOES’ that allows you to view satellite images from the geostationary GOES satellites that have been stored online. But of course, using a phone application doesn’t present the same challenge as building your own setup.

WEATHER SATELLITE WEBSITES GEO magazine website Weather satellite Yahoo group. Meteosat Yahoo group Polar APT / LRPT low Earth orbit satellites Satellite Frequency NOAA 15 137.6200 MHz NOAA 18 137.9125 MHz NOAA 19 137.1000 MHz Meteor M N2 137.1000 MHz Polar HRPT / AHRPT low Earth orbit satellites Satellite Frequency Mode and Notes NOAA 15 1702.5 HRPT Omni weak MHz signal NOAA 18 1707.0 HRPT RHCP good MHz signal NOAA 19 1698.0 HRPT RHCP good MHz signal Feng Yun 3A 1704.5 AHRPT non-standard MHz format Feng Yun 3B 1704.5 AHRPT non-standard MHz format Feng Yun 3C 1704.5 AHRPT non-standard MHz format Metop A 1701.3 AHRPT RHCP good MHz signal Metop B 1701.3 AHRPT RHCP good MHz signal Meteor M N2 1700.0 AHRPT RHCP good MHz signal Geostationary weather satellites

Satellite Mode Frequency Position Meteosat HRIT / 1691.0 / 57.5OE 7 WEFAX 1691.0 Meteosat HRIT 3.5OE 8 Meteosat HRIT / 9.5OE 9 LRIT Meteosat HRIT / 0OW 10 LRIT GEOS GVAR / 1685.7 / 75OW 13 LRIT 1691.0 GEOS GVAR / 1685.7 / 105OW 14 LRIT 1691.0 GEOS GVAR / 1685.7 / 135OW 15 LRIT 1691.0 MTSAT- HRIT / 1687.1 / 140OE 1R LRIT 1691.0 MTSAT- HRIT / 1687.1 / 145OE 2 LRIT 1691.0 Feng SVISSR 104OE Yun 2E / LRIT Feng SVISSR 112OE Yun 2F / LRIT Feng SVISSR 86.5OE Yun 2G / LRIT Satellite data as at September 2017.

Status working standby working working working standby working standby working working working working

THE INTERNATIONAL SPACE STATION The International Space Station (ISS) is a special case because it has people living in it. It is also a Hamsat so it deserves inclusion. The ISS has two amateur radio stations. The newer station is in the Columbus module. It includes Ericsson VHF and UHF radios and a specially constructed ‘HamTV’ video transmitter used to transmit video or photographs. The older station in the Service module has three Kenwood VHF/UHF radios. The ham radio stations are used for; ham radio contacts between ISS crew members and licensed amateur radio operators, sending still SSTV pictures and postcards that anyone can receive, 1200 baud packet radio and APRS beacons, and digital DVB-S TV transmissions on the S-band at 2.395 GHz. There used to be an FM voice transponder but this is currently deactivated. For up to date information check out:

ISS moves fast because it is in a relatively low orbit. A typical pass only lasts about ten minutes. You need the same equipment as receiving from CubeSats. The primary use of both the FM radios and the DATV transmitter is educational ‘contacts’ between ISS crew members and school groups. These have to be pre-arranged. The schedule for educational contacts is published months ahead. If the ISS happens to be passing over your place while the contact is in progress, you can listen in to the downlink. The audio downlink is 145.800 MHz and the video feed is on 2.395 GHz. Note that almost all ‘School contacts’ use the Telebridge facility. This means that the school making the contact may not be inside the ISS footprint while the contact is proceeding.

It is possible to hear an Astronaut or Cosmonaut making amateur radio contacts with amateur radio stations, but this depends on the crew schedule because the crew can only make ham radio calls during leisure hours. Some ISS crewmembers are interested in making calls but the majority do not use the station. There used to be an FM repeater for amateur radio operators to use for ‘ground to ground’ contacts but this is currently deactivated. Most of the current ISS activity is relaying APRS beacons. The digipeater is configured the same as the digipeater on LEO satellites like PCSat. Amateur radio operators send a data packet containing their call sign location and optionally other information such as weather data. The data packet, known as a ‘beacon,’ is relayed to ground stations via ISS or another satellite and then forwarded to an Internet site which displays all of the received APRS beacons worldwide on a map. There is a lot more on APRS in the ‘working digital satellites’ section.

SSTV TRANSMISSIONS FROM ISS On special occasions such as the anniversary of notable space-related events like the first manned space flight, SSTV images are transmitted from the International Space Station. The system utilizes Kenwood D700 and D710 radios and the ARISS antennas mounted on the ISS Service Module. The pictures are usually postcards or photographs, but sometimes there are live video images of the inside of the Service Module or views taken through a window. Sometimes a Kenwood VC-H1 ‘Visual Communicator’ is used to provide near real-time automatically transmitted images, usually Earth views, once every 3 minutes.

John Brier, KG4AKV has provided an excellent online tutorial explaining in detail how to configure a simple system to acquire and view the SSTV images.

See and SSTV is transmitted on the standard ISS downlink frequency of 145.800 MHz so other activities such as APRS are suspended while ISS is transmitting SSTV pictures. To decode SSTV images you need one of the packages available as a free download. MMSSTV is the most popular SSTV application. It can automatically sense the mode that is being transmitted. RXSSTV is designed for receiving only. Digital Master 780 is an SSTV add on to Ham Radio Deluxe QSSTV is for Linux MultiScan 3B is for Mac MultiPSK is a digital mode program that supports some SSTV modes MixW is a digital mode program that supports some SSTV modes

THE AMATEUR RADIO PROGRAM ABOARD THE SPACE STATION Everything that happens on the space station has a program name. The program governing all amateur radio activities in the ISS is called ARISS (Amateur Radio on the International Space Station). The following frequencies are used but may not be active. Voice and SSTV downlink 145.800 MHz (Worldwide) Voice uplink: 144.490 MHz for ITU Regions 2 and 3 (The Americas, and the Pacific and Southern Asia) Voice uplink: 145.200 MHz for ITU Region 1 (Europe, Russia, and Africa) VHF packet (1200 baud AX.25 AFSK APRS) 145.825 MHz UHF packet (1200 baud AX.25 AFSK APRS) 437.550 MHz UV mode FM voice repeater uplink 437.80 MHz - downlink 145.80 MHz (currently inactive) VU mode FM voice repeater uplink 145.990 MHz (67 Hz pilot tone) downlink 437.800 MHz LV mode FM voice repeater uplink 1269.0650 MHz - downlink 145.800 MHz SSTV downlink 145.800 MHz DVB-S TV downlink 2.395 GHz Check and The following call signs are available for use on the ISS: • Russian: RS0ISS • USA: NA1SS • European: DP0ISS, OR4ISS, IR0ISS Packet Station Mailbox: RS0ISS-11 and RS0ISS-1 Other call signs may come into use as the station and crew change. Many of the ISS crew are licensed amateur radio operators. If you are patient and very very lucky, you may be able to chat with an astronaut or Cosmonaut on board the space station. You can confirm a contact with a QSL card. ‘Some ISS crew members make random, unscheduled, amateur radio voice contacts with earth-bound radio amateurs, often called "hams.” They can make radio contacts during their breaks, pre-sleep time, and before and after mealtime. Astronauts have contacted thousands of hams around

the world. The work schedules of the ISS crew dictate when they are able to operate the radios. The crew's usual waking period is 0730 - 1930 UTC. The most common times to find a crew member making casual periods are about one hour after waking and before sleeping when they have personal time. They're usually free most of the weekend, as well.’ Source:


According to the ISS Fan Club, these frequencies are used for spacesuit to ISS and ISS to ground communications. Do NOT transmit on these frequencies! RS EVA from Orlan spacesuit 121.125 MHz FM [Credit N5VHO] Downlink from Soyuz-TM (voice). RS EVA from Orlan spacesuit. Soyuz VHF-2. Progress Telemetry. 121.75 MHz FM [Credit N5VHO] VHF-2 FM downlink from Zarya (Service Module). RS EVA to Orlan suits. 130.167 MHz [Credit N5VHO] VHF-1 FM downlink. 143.625 MHz. Main Russian communications channel. Often active over Moscow. You can hear air to ground conversations in Russian. Sometimes English when US crews talk to their NASA representative in Star City. [Credit IZ6BYY] US Space to Space voice 414.200 MHz Soyuz / Progress telemetry 166.000 MHz AM, Soyuz / Progress beacon 922.763 MHz CW


ISS PARAMETERS ISS crew: fully crewed: 6, currently aboard: 6 Launched: 20 November 1998 (more than 19 years in orbit) Mass: 419,455 kg (924,740 lb) Length: 72.8 m (239 ft) Width: 108.5 m (356 ft) Height: approx. 20 m Typical apogee: 410 km Typical perigee: 408 km Inclination: 51.65 degrees Orbital speed: approx. 7700 m/s The ISS is in a low circular orbit normally between 330 and 430 km above Earth. It has an inclination of 51.6 degrees, which means that it passes over all latitudes between 51.6 degrees north to 51.6 degrees south. At a typical altitude of 409 km, the ISS has a velocity of 7667 m/s. That is 7.7 km per second, 27602 km/h, or 17151 mph. The speed of a satellite as it orbits the Earth is always called its ‘velocity.’ The ISS travels completely around the Earth every 92.6 minutes, completing around 15 orbits per day. The low orbit means that you can only communicate with the satellite for about 10 minutes each ‘pass.’ The maximum distance that two ground stations can communicate through the ISS is about 4,450 km. The ISS is so big that it is easily visible through a pair of binoculars or a small telescope. To the naked eye, it looks like a fairly bright star moving in a straight line from horizon to horizon. The best time to see it is when the sun has just set at your location but the satellite is still in sunlight (not eclipsed by the Earth’s shadow). See for a list of visible passes over your location. also has a lot of information about ISS and other satellites. The low orbital height of the ISS means that it suffers quite a lot of orbital drag. The huge solar arrays mean that the ISS has a very big surface area, which makes the orbital decay problem worse. The rate of orbital decay is the amount of altitude the satellite loses per day due to orbital drag. The space station can lose 60 to 90 meters of altitude per day. To overcome this problem the space station is periodically boosted into a

higher orbit, by either using the station’s two main engines on the Zvezda service module or getting a push from Russian or European spacecraft docked to Zvezda's aft port. Visiting spacecraft often have spare fuel that is not required for re-entry maneuvers, so using it in this way is cost effective. “Orbits do not decay without some friction-like mechanism, which robs energy from the orbital motion. This can be any of a number of mechanical, gravitational, or electromagnetic effects. For bodies in a low Earth orbit, the most significant effect is the atmospheric drag. Atmospheric drag at orbital altitude is caused by frequent collisions of gas molecules with the satellite. It is the major cause of orbital decay for satellites in low Earth orbit. It results in the reduction in the altitude of a satellite's orbit.” You might wonder why the space station was not placed into a higher orbit where less fuel would be required to compensate for orbital drag. There are two main reasons. One is that the low orbit makes it cheaper to launch supply rockets that will dock with the ISS. The more important reason is that a low orbit is safer. There is a lot of space junk, such as rocket casings, old boosters, and bits that have fallen off spacecraft and satellites, in low Earth orbit. The big items are reasonably easy to locate and plot well in advance, so the ISS can be moved to avoid them, but there are thousands of tiny items such as dust, paint chips, micrometeorites, chunks of ice, and even dropped screws, that can’t be seen and avoided. That is not to say that the ISS is flying through a soup of objects, but at very high relative speeds, any collision could be deadly. The orbit down at 400km is relatively clear of space junk and risk from micrometeorites because the high orbital drag pulls them out of orbit and they burn up in the atmosphere. This effect also reduces the risk of the space station hitting items ejected or dropped from itself or the supply craft that visit.

LINK BUDGETS A link budget is a calculation of how much transmit power you need to reach the satellite and how much signal you can expect to receive from it. With the inclusion of some additional factors, relating to the poor reflectivity of the Moon’s surface you can even calculate link budgets for EME operation. Working LEO satellites is comparatively easy and there is plenty of signal in both directions so you don’t really need to calculate the link budget. But if you are using a minimal system such as a hand-held transceiver with a rubber duck antenna it is still a useful exercise. If you want to receive signals from a spacecraft in deep space or from satellites in HEO highly elliptical orbits, the received signals are weak and the link budget calculation becomes much more relevant. If we are lucky enough to ever get an amateur radio satellite on or in orbit around the Moon, or Mars, then link budget calculations will be absolutely essential. To make life easier, all of the calculations are done using decibels. All of the losses and gains can simply be added together.

DECIBELS AND DBM. Decibel measurements follow a logarithmic scale and they are always a ratio. Components that have attenuation or ‘loss’ absorb some of the energy of your signals. The loss can be calculated from either the ratio of the voltage at the output and the voltage at the input of the device, or from the ratio of the power at the output and the power at the input of the device. Objects that have loss include your coaxial cables, any fixed attenuators that you have in the circuit, antenna tuners, and the space between the transmitting station antenna and the receiver antenna. Fixed attenuators generally have pretty much the same amount of loss across all of the frequencies that you are using. The loss of coax cables and the ‘free space path loss’ is proportional to the frequency of operation. Components that have amplification or ‘gain’ include amplifiers and antennas. Amplifiers use electronic devices to increase the signal voltage or power. They get the energy from some sort of power supply. Antennas are not powered. Antenna ‘gain’ comes from concentrating the power being radiated into a wanted direction. Antennas with no gain in any direction, radiate equal amounts of power in all directions. Because they are ratios, gain and loss have no unit of measurement i.e. they not measured as a particular voltage or power. We express gain or loss in dB, never as dBm. When we use a dB scale to measure the actual amount of power or voltage at a point in your radio system, the measurement must be expressed as a ratio in comparison with a known a reference level. For example, we might measure the power coming out of your transmitter as decibels referenced to one Watt, or the received signal level in decibels referenced to one microvolt. For measurements of the amount of RF power we often use the dBm scale. The dBm scale is used to indicate RF Power referenced to one milliwatt dissipated in a 50-ohm termination. 0 dBm = 1mW into a 50 Ohm load. For power comparisons, the ratio of two power measurements (P1 and P2) expressed in decibels is 10 log (P1/P2). When we reference the measurement to one milliwatt, P1 is the measured power and P2 becomes our reference power of 1 mW (0.001 watts). If P1 is expressed in Watts and P2 is the 1mW reference level. Power in dBm = 10 log (P1/0.001)

Power in dBm = 10 log (P1) +30 Two times the amount of power is an increase of 3 dB. Ten times the amount of power is an increase of 10 dB. For example, ten times a power of 1 watt is 10 watts. A 10 dB increase in power from +30 dBm to = +40 dBm. Power 1 mW 10 mW 100 mW 1W 10 W 100 W 1000 W

dBm 0 dBm +10 dBm +20 dBm +30 dBm +40 dBm +50 dBm +60 dBm

Power 2 mW 20 mW 200 mW 2W 20 W 200 W 1500 W

dBm +3 dBm +13 dBm +23 dBm +33 dBm +43 dBm +53 dBm +61.8 dBm

The amount of attenuation (loss) or amplification (gain) between two points in the radio path is expressed in dB because it is the difference between the power going in and the power coming out (i.e. a ratio). You can say that an amplifier has a gain of 10 dB if the power coming out of it is ten times the power going into it. For example, the power at the amplifier input might be +23 dBm and at the output, it would be +33 dBm. Decibels can be added and subtracted. For example: Transmitter power = +23 dBm (power is measured in dBm) Amplifier gain = 10 dB (gain is always in dB) Amplifier output power = 23 + 10 = 33 dBm (power in dBm) Feeder cable loss 3 dB (loss is always in dB) Power at the antenna is 23 + 10 - 3 = 30 dBm (power in dBm)

TRANSMIT PATH The transmit path calculation starts with the amount of power from your transmitter or linear amplifier described in dBm. Some of the power is lost in the coaxial feeder cable between the transmitter and the antenna. Attenuation or ‘loss’ is expressed in decibels (dB). The antenna radiates the signal and the radio wave travels through the atmosphere and then empty space to the satellite or perhaps all the way to the Moon. On the way, more power is lost. This is called the free space path loss.

RECEIVE PATH The path back from the satellite is calculated separately because the downlink signal is usually on a different frequency to the uplink. In situations where the receive frequency is the same as the uplink frequency, such as satellite digipeaters or EME operation, the free space path loss will be the same in both directions. The amount of Doppler shift on the signals is very small relative to the frequency so is not normally included in the calculations.

FREE SPACE PATH LOSS As a transmitted signal traverses the atmosphere its power level decreases at a rate inversely proportional to the distance traveled and proportional to the wavelength of the signal. Free space does not attenuate the signal. The loss of signal between the transmit antenna and the receiving site is mostly due to the signal spreading out as it travels according to the ‘inverse square law,’ and partly due to the formula being based on using 0 dB gain ‘isotropic’ radiators to transmit and receive the signals. Free space path loss = 20*log(f) + 20*log(d) + 32.45 d = path distance in km f = frequency in MHz

RECEIVED SIGNAL STRENGTH IN DBM AND MICROVOLTS To get the received signal level power in dBm subtract the free space path loss from the transmit power in dBm and add the dBi gain of the transmit antenna and the receive antenna. Receive signal in dBm PRX = PTX – (32.44 + 20*log(f) + 20*log(d)) + Gtx + Grx Receive signal µV PD =SQRT( 10^( PRX/10)*1000000000*50) The antenna gain Gtx and Grx is in dBi (dB referenced to an isotropic antenna). PRX and PTX are in dBm. µV PD is microvolts across a 50 Ohm termination. F is frequency in MHz. d is path distance in km. An isotropic antenna or radiator is a theoretical ‘point source’ antenna with no physical length. It radiates equally in all directions. dBi is the gain of an antenna compared to the gain of an isotropic antenna. A dipole antenna has a gain of 2.14 dBi. A 2m band 6 element Yagi has a gain of about 11 dBi meaning that the antenna works 11 dB better than an isotropic antenna, but only in the direction of its main beam. dBd is the gain of an antenna compared to the gain of a dipole antenna. A dipole antenna has a gain of 0 dBd which is equivalent to 2.14 dBi. An antenna with a gain of 11 dBi has a gain of 9.14 dBd in the direction of its main beam i.e. it is 9.14 dB better than a dipole antenna and 11 dB better than an isotropic antenna. Because an isotropic antenna radiates equally in all direction, it is ideal for path loss calculations, which depend on the inverse square law. The inverse square law shows how power spreads out from the transmit antenna. In addition to being proportional to the distance between the transmitter and receiver antenna, the loss at the receive site is also proportional to the frequency of operation. That is because although the theoretical isotropic antenna is a point source, it has an antenna aperture and that affects how efficiently it can receive different frequencies. The aperture is derived from the gain of the antenna which is 1 (0 dBi) for an isotropic radiator, and the wavelength of the radio wave.

Antenna aperture is the reason that the lower the frequency, i.e. the longer the wavelength, the bigger an antenna needs to be to

achieve a given antenna gain. In the real world, there is no such thing as an isotropic antenna, excluding things like atomic and sub-atomic particles. All real antennas should have at least the 2.14 dBi gain of a dipole. But poorly designed, mismatched, or non-resonant antennas could have less than 2.14 dBi of gain. They would radiate inefficiently resulting in an antenna working more like a radiating attenuator with a net loss rather than gain.

UPLINK LINK BUDGET EXAMPLE Let’s take the example of a 4-watt transmitter being used for a 2m band uplink to a LEO satellite. The satellite has an average altitude of 500 km. That means that the range when the satellite rises over the horizon would be about 4668 km. If the satellite passes directly over your house, the minimum range would be 500 km, but a more likely scenario would be a minimum range of about 1000 km. If you are using a small handheld, three element Yagi you will have plenty of transmitted power during the middle of the satellites pass. But will you be sending enough power at the beginning and end of the pass when the satellite is at maximum range? Let’s find out. 4 watts of transmit power = +36 dBm Cable loss to the antenna = 0.7 dB (3 m of RG58 coax cable) Diplexer loss at 145 MHz = 0.9 dB Gain of 3 element 2m band Yagi antenna = 9 dBi (typical) Frequency = 145.850 (SaudiSat-1c SO-50) Free space path loss over 4668 km = 149.1 dB Free space path loss over 1000 km = 135.7 dB Free space path loss over 500 km = 129.7 dB Satellite receiver antenna gain (quarter wave tape measure) = 2 dBi. We will assume that the satellite has a maximum sensitivity of -124 dBm. The signal arriving at the satellite receiver is approximately, PRX = PTX – cable loss – diplexer loss + TX ant gain - FSPL + Sat RX ant gain PRX = 36 - 0.7 - 0.9 +9 -149.1 +2 = -103.7 dBm at AOS and LOS PRX = 36 - 0.7 - 0.9 +9 -135.7 +2 = -90.3 dBm at 1000 km range PRX = 36 - 0.7 - 0.9 +9 -129.7 +2 = -84.3 dBm at 500 km range Even at maximum range, there is plenty of your transmitted signal arriving at the satellite. The received signal is more than 20 dB above the minimum signal of -124 dBm. Dropping the transmitter power to 1 watt would reduce the signal received at the satellite by 6 dB making it marginal but still OK. Keeping the power level at 4 watts but using a rubber duck antenna on your handheld would reduce the transmit antenna gain down to 2 dBi (at best). That would reduce the signal at the satellite by 7 dB. Your signal could still reach the satellite, even at the maximum range, but it is

probable that you would not be able to hear the satellite downlink at that range with a Rubber Duck antenna. Consider a using a similar setup to send a signal to a satellite that is in orbit around the Moon. The path loss over the 382,500 km path to the Moon is 187.4 dB so the signal received at the satellite would be -142 dBm. That is not enough to meet the minimum signal requirement of -124 dBm. We would need an additional 18 dB to even be heard at the satellite plus another few dB to ensure a reliable signal to noise ratio above the minimum signal level. We would need at least 24 dB more gain to ensure reliable communications to a satellite in lunar orbit. Increasing our transmitter power from 4 watts to 1000 watts would get us the 24 dB of additional signal that we need. Of course, if you plan to use a big Yagi, radiating 1000 watts of RF energy you are committed to a more permanent antenna mounting arrangement. Most handheld antenna manufacturer’s state that is not safe to use a handheld antenna at power levels exceeding 10 watts. An antenna radiating 1000 watts could cause serious RF burns. You would not touch the element of a working 1000 watt radiant heater and RF burns are much worse than heat burns!

DOWNLINK LINK BUDGET EXAMPLE OK, we have enough signal arriving at the satellite. What about the signal that we receive back from the satellite on the 70cm band downlink? We will assume that you are using a seven element handheld Yagi on the 70cm band, with a diplexer to combine the two antennas on to a single coax feeder cable. 250 mW satellite transmit power = +24 dBm Transmitter antenna (quarter wave tape measure) 2dBi. Frequency = 436.795 (SaudiSat-1c SO-50) Free space path loss over 4668 km = 158.6 dB Free space path loss over 1000 km = 145.3 dB Free space path loss over 500 km = 139.2 dB Gain of 7 element 70cm band Yagi antenna = 12 dBi (typical) Cable loss to the antenna = 1.3 dB (3 m of RG58) Diplexer loss at 437 MHz = 1.4 dB Your receiver will have a maximum sensitivity of about -130 dBm. The signal arriving at your receiver is approximately: PRX = Sat PTX + Sat TX ant - FSPL + RX ant gain – cable loss – diplexer loss PRX = 24 +2 -158.6 +12 -1.3 -1.4 = -123.3 dBm at AOS or LOS PRX = 24 +2 -145.3 +12 -1.3 -1.4 = -110.0 dBm at 1000 km range PRX = 24 +2 -139.2 +12 -1.3 -1.4 = -103.9 dBm at 500 km range Even at maximum range, there is plenty of signal arriving from the satellite when you are using a seven element handheld antenna. However, if you attempted to use a rubber duck antenna on your handheld. The antenna gain would reduce from 12 dBi to between 2 and 4 dBi. Using a 4 dBi antenna would reduce the signal from the satellite to -131 dBm when the satellite is at low elevations and -118 dBm at a range of 1000 km. That would make reception of the satellite very difficult, probably impossible when the satellite is at low elevation but still possible when the satellite is at higher elevations. Receiving a signal from our imaginary satellite in orbit around the Moon is a real challenge. The received signal using a 7 element Yagi would be around - 162 dBm. To even hear the satellite we would need at least 30 dB more gain. A 20 dB masthead preamplifier would help, but the best

solution is to move up to microwave frequencies where you get more antenna gain for the same sized antenna.

COAXIAL FEEDER CABLES FEEDER CABLE LOSS Having a long cable between your radio and the antenna can break your whole satellite station. The importance of keeping the antenna feeder loss as low as possible cannot be overstated. Always use the best coax cable that you can. Stacking two antennas gives you a theoretical maximum gain in the signal of 3 dB. In practice, you will get less than that, probably about 2.7 to 2.8 dB of enhancement. Reducing the loss of your feeder system by 3 dB is equivalent to having added that stacked antenna system. It is much cheaper and easier to improve the coax feeder cable than to add a second antenna. Portable satellite operation using a handheld antenna completely eliminates the problem of long feeder runs between the radio and the antenna. But as soon as you decide that, it would be convenient to put the radio on a table and run a cable to the antenna on a tripod you may begin to run into problems. Overcoming feeder cable loss while transmitting is not too difficult. You just transmit more power. But when you are receiving signals from the satellites, the cable loss may attenuate the signal so much that operation becomes marginal or even impossible. Signals reflected back from the Moon when you are doing EME operation are very weak indeed and it is essential that the effect of coaxial feeder loss is minimized. The best way to do that is using a masthead mounted preamplifier. There are some important tips regarding the use of preamplifiers in the next section. Cable loss increases with the frequency of operation, so a cable that is OK for HF may be unsuitable for VHF and UHF frequencies and it may be completely unusable at microwave frequencies. A major problem with low loss coaxial cable is that it usually has a larger diameter and it is often considerably less flexible. If you have an antenna that is being turned by a rotator, the feeder needs to be flexible so that it doesn’t kink or break as the antenna turns. Also, you don’t want to place too much strain on the antenna connectors or the rotator. The most common solution is to run low loss ‘hardline’ cable from the shack to the antenna pole and then a short flexible coax tail from the end of the hard line to the antenna. The loss in the short tail is negligible and the long run back to the shack has low loss. Normally you would also use a flexible tail at the shack to connect from the hard-line coax to your receiver, transceiver, amplifier, or tuner. Low loss coaxial cable costs more, but for satellite and EME stations, it is the best investment that you can make.

Most receivers and transceivers are designed to match 50-ohm coaxial cable. 75-ohm coaxial cable is used for runs to TV aerials, satellite TV receivers, and for video and data cables. It is not suitable for your satellite station antennas. Other coax cable impedances are sometimes used for impedance matching sections, but not for the main feed line. Hardline coaxial cable has a ribbed metal outer conductor and a solid copper inner conductor. It is much less flexible than ‘normal’ coaxial cable and it is more expensive. The connectors cost a lot more too. The most common hardline cable types for amateur and hobby stations are LDF450, FSJ4-50, and LDF5-50. These cables are also known as Heliax cables. I prefer not to use FSJ4-50 because the protective outer plastic sheath is thin and easily damaged. LDF4-50 is a ½ inch diameter cable. It is the most suitable ‘hardline’ cable for use at home. LDF5-50 is 7/8 inch diameter cable. It is extremely stiff with a fairly wide minimum bend radius. I believe that using LDF5-50 is overkill for most satellite stations. I would only recommend it for very long coax runs. The larger coax types like LDF6-50 and LFD7-50 are used for high power TV and radio broadcast transmitters, and for cellular radio systems. The high cost of these cables and the matching connectors makes them uneconomic for home use unless you can source them on the surplus market. It is perfectly acceptable to use LMR-400 or RG213 coax cable for VHF and UHF satellite antennas. These types are more flexible and the loss is not excessive. I use RG213 for the tails at both ends of my LDF4-50 runs. RG58 cable is Ok for very short lengths but it has too much loss for long runs. You could use FSJ2-50 or LDF1-50 but there is little benefit. The loss per hundred feet is about the same as LMR-400 and the cable and connectors are considerably more expensive.

TYPICAL LOSS FOR 100 FEET OF COAX CABLE Cable loss is directly scalable. Choose the frequency of operation and read the loss per 100 feet. For a length of 10 feet, simply divide the loss figure by 10. Different manufacturer’s products may have significantly different attenuation for the same cable type. Refer to the manufacturer’s product specification sheets. They should be available online. Loss per 100 feet (dB) Cable

Dia. Type

LDF5-50A LDF4-50A FSJ4-50B FSJ2-50 LMR-400

7/8" 1/2" 1/2" 3/8" 10.3 mm 1/4" 10.3 mm 1/4" 10.3 mm 8.4 mm 6.2 mm 5 mm 5 mm 2.8 mm

LDF1-50 RG-8/U (Belden 9913) FSJ1-50A RG213 RG-6A (Belden 8215) RG-8X (Belden 9258) RG58 (Radioshack) RG58C RG174 (Belden 8216)

Heliax Heliax Heliax Heliax Coax

30 144 450 1000 MHz MHz MHz MHz 0.195 0.439 0.808 1.25 0.357 0.799 1.45 2.22 0.557 1.26 2.31 3.6 0.649 1.45 2.64 4.06 0.667 1.232 2.688 4.109

Heliax 0.667 Coax 0.68

1.49 1.541

2.71 2.853

4.16 4.471

Heliax 0.973 Coax 1.105

2.16 2.59

3.91 5.000

5.96 8.166 9.905

75ohm Coax






8.281 14.228




9.672 15.861



6.642 13.376 22.692


4.459 10.516 20.456 33.638

TYPICAL LOSS FOR 100 METERS OF COAX CABLE Cable loss is directly scalable. Choose the frequency of operation and read the loss per 100 meters. For a length of 10 meters, simply divide the loss by 10. Different manufacturer’s products may have significantly different attenuation. Refer to the manufacturer’s product specification sheet. Loss per 100 meters (dB) Cable

Dia. Type

LDF5-50A LDF4-50A FSJ4-50B FSJ2-50 LMR-400

7/8" 1/2" 1/2" 3/8" 10.3 mm 1/4" 10.3 mm 1/4" 10.3 mm 8.4 mm 6.2 mm 5 mm 5 mm 2.8 mm

LDF1-50 RG-8/U (Belden 9913) FSJ1-50A RG213 RG-6A (Belden 8215) RG-8X (Belden 9258) RG58 (Radioshack) RG58C RG174 (Belden 8216)

144 MHz 1.442 2.622 4.13 4.76 4.043

450 MHz 2.65 4.75 7.59 8.66 8.818

1000 MHz 4.12 7.28 11.8 13.3 13.48

Heliax 2.19 Coax 2.23

4.89 5.056

8.88 9.361

13.6 14.667

Heliax 3.19 Coax 3.626

7.10 8.498

12.8 19.6 16.408 26.792

75ohm Coax


11.103 20.647 32.495


13.294 27.169 46.679



16.363 31.734 52.036



21.793 43.885 74.449


14.268 34.501 67.113 110.36

Heliax Heliax Heliax Heliax Coax

30 MHz 0.641 1.17 1.83 2.13 2.189

RF POWER OUTPUT TO THE ANTENNA. Every 3 dB of cable loss results in a halving of the power that you are sending to the antenna. For example; at 450 MHz on the 70cm band with a transmitted power of 100 watts and a perfect 1:1 match to the antenna, compare the 4.6 watts being sent to the antenna after a 100-foot length of RG58C coax, with the 71.6 watts you would get using LDF4-50A Heliax.

Power output in watts, for 100 feet of coax. Assuming 100 watts input power and an antenna load with a perfect 1:1 VSWR. Cable

Dia. Type

LDF5-50A LDF4-50A FSJ4-50B FSJ2-50 LMR-400

7/8" 1/2" 1/2" 3/8" 10.3 mm 1/4" 10.3 mm 1/4" 10.3 mm 8.4 mm 6.2 mm 5 mm 5 mm 2.8 mm

LDF1-50 RG-8/U (Belden 9913) FSJ1-50A RG213 RG-6A (Belden 8215) RG-8X (Belden 9258) RG58 (Radioshack) RG58C RG174 (Belden 8216)

Heliax Heliax Heliax Heliax Coax

30 MHz 95.6 92.1 88.0 86.1 85.8

144 MHz 90.4 83.2 74.8 71.6 75.3

450 MHz 83.0 71.6 58.7 54.5 53.9

1000 MHz 75.0 60.0 43.7 39.3 38.8

Heliax 85.8 71.0 53.6 38.4 Coax 85.5 70.1 51.8 35.7 Heliax 79.9 60.8 40.6 25.4 Coax 77.5 55.1 31.6 15.3 75ohm Coax

71.0 45.9 23.5 10.2


61.4 31.7 10.8 2.6


53.3 21.7 4.6



35.8 8.9


68.5 39.3 14.9 3.8


Since the table uses an input power of 100 watts, you can use the numbers as a percentage. For example, using a 100-foot of RG213 cable on the 2m band, you can expect to radiate 55% of the power that you are transmitting. Using a similar length of RG58C coax, you will only be radiating about 22% of your transmitter power. More than 70% of your power would be wasted as heat. Power at end of feeder cable in dBm = Transmit power dBm – Cable loss dB Output power P = +50 dBm – 2.59 dB = +47.41 dBm = 55.08 watts Also, Output power P = 10^((50-Cable loss)/ 10) / 1000 = 55.08 watts

SWR / VSWR SWR stands for standing wave ratio. VSWR stands for voltage standing wave ratio. In most cases, they are synonymous and VSWR is very often shortened to SWR. SWR occurs when there is a mismatch between the impedance of your transmitter and the impedance of your antenna or coaxial feeder cable. High SWR is bad because it reduces the amount of power you are transmitting. Very high SWR can cause high voltages or currents that could damage your transmitter. You could write a complete book about coaxial cables, impedance matching, and standing waves, but this is a book about satellites, not antenna theory, so I will keep it brief. Assuming that the coaxial cable that you use has a 50-ohm impedance and matches the output impedance of your transmitter, the RF power that is generated by the transmitter will travel down the coaxial cable towards your antenna. Some of your transmitter power will be lost as heat due to cable loss in the coaxial cable. But, all going well, your antenna will have a good 50-ohm impedance match and nearly all of the power that does arrive at the antenna will be radiated from it. However, antennas are not perfect. Your antenna will not present a perfect 50-ohm match right across the band. If you send RF power to an antenna that is not presenting a perfect 50-ohm resistive match at the frequency you are using, the impedance mismatch causes some of the RF power to be reflected back down the coaxial cable towards the transmitter. The reflected signal is the same frequency as the transmitted signal but the phase is different. There is a 180-degree phase change due to the reflection and an additional phase change due to the capacitive or inductive reactance of the antenna load. In some places along the coaxial cable, the forward and reflected signals add together, creating a series of higher voltage points. At other points along the cable, the two waves cancel creating voltage nulls. The places that have a high voltage have a reduced current flow, and the places with low voltage have a higher current. Unless you change the frequency of the transmitted signal or the length of the cable, these voltage peaks and troughs along the cable occur in fixed positions. The pattern of voltage peaks and troughs is called a standing wave because they stay in the same place. The ratio of the voltage across the coax cable at the voltage peak, to the voltage across the coax cable at the voltage trough, is the voltage standing wave ratio (VSWR). You would get the same ratio if you measured the current at each position. It is not practical to find and

measure the position of the voltage peaks along the cable, so we measure the amount of ‘forward’ power being sent towards the antenna and the amount of ‘reflected’ power being reflected back to the transmitter. We actually measure the voltage of a small sample of the forward power across a known resistance and the voltage of a small sample of the reflected power across another load resistor of the same resistance. The ratio of these two voltages is the reflection coefficient. The VSWR can be calculated from the reflection coefficient. Return Loss is the reflection coefficient converted to decibels. The reflection coefficient is covered in more detail under ‘SWR meters’ below and in the formulas section under Antenna Reflection Coefficient.

FEEDER CABLE LOSS The maximum acceptable feeder loss depends on your circumstances but as a rough rule of thumb, I would say that it is good if your whole feed line from the transmitter (or power amplifier), to the antenna, has a total loss of less than 3 dB on the highest band of operation. At that point, you can still radiate 50% of your transmit power and a reduction of 3 dB on the received signal is probably acceptable. Less than 1 dB of cable loss is ideal, but not always achievable. When receiving you can compensate by adding a low noise masthead preamplifier, or by installing a higher gain antenna. A better antenna will help with both your transmitted and received signal so it is the preferred option. However higher gain antennas are larger and physically more difficult to manage. You will probably need a rotator and you may need to mount the antenna higher to avoid hitting things when it is elevated or rotated. Also, higher gain antennas are more directional. They have a narrower beam angle, (angle of radiation), so you have to aim the antenna at the satellite more accurately. You don’t actually need much transmit power for the LEO satellites. Ten watts of power going into a small Yagi antenna is plenty. Most FM handheld operation is done with three to five watts into a three element Yagi on the 2m band and a five to seven element Yagi on the 70cm band. Later in this chapter, I have written about various ways that you can find out the actual amount of feeder cable loss you have.

ALWAYS BUY GOOD QUALITY COAXIAL CABLE AND CONNECTORS Saving a few dollars by buying cheap coax cable or connectors is the worst investment decision that you can possibly make. It is much better to buy high-quality coax cable and avoid the need to spend mega-bucks on a power amplifier, preamplifier, or bigger antenna just to make up for the additional losses. The easiest way to spot good coax cable is to look at the braid layer. If there are a lot of wires and possibly a foil layer, the coax is probably good quality. Cables with very little braid, very thin wires in the braid, or thin plastic sheathing, is cheap and nasty. Clear labeling of the coax cable type and its impedance on the outside of the cable is a good sign. Unlabeled cables or cables with smudgy labeling are likely to be poor quality. Make sure that the coax you buy is actually 50-ohm cable. A lot of cheap coaxial cable is made for TV or data cabling. It is quite likely to be 75-ohm cable and not suitable for transmitting. Most good RF connectors have gold center pins and silver or nickel plating on the other surfaces. An exception is PL259 connectors, which don’t usually have gold plated center pins. They have a larger center pin and the additional surface area ensures a good connection. Cheap connectors usually look cheap. They are not as well machined and often the screw threads don’t work well. They may bind or cross-thread when you connect them to other connectors. Cheap connectors often have inferior plating and poor dielectric insulation. In most cases, shiny plated connectors should be avoided. Chrome is a poor conductor, so you don’t get a particularly good connection between connectors. It is also impossible to solder to a chrome plated surface. To terminate a non-crimp connector you may have to burn or file the chrome off and solder the coax braid to the underlying brass. Silver plated connectors are easy to solder. Nickel plated connectors are OK too. Cheap connectors may have high loss and they don’t last as long outside. They have a tendency to corrode and if left exposed, the chrome finish can weather or flake off. In bad cases, wet or corroded connectors can be a source of intermodulation distortion, which will cause interference problems to others when you transmit. If there are other transmitters operating nearby, you may also get ‘passive intermodulation’ (PIM) interference while you are receiving. As a rule of thumb, if you don’t know the manufacturer or the connectors are unbranded. Don’t buy them!

COMMONLY USED RADIO FREQUENCY COAX CONNECTORS Most HF and VHF amateur radio transceivers and shortwave receivers are fitted with ‘UHF’ connectors. ‘UHF’ connectors were designed in the 1930s for the US Navy Signal Corps for use with RG213 coax. They can also be fitted to LMR-400 and to RG58 coax using an adapter sleeve. They are commonly known by their original Navy part numbers, PL259 for the plug and SO239 for the socket. ‘UHF’ connectors are not actually usable at UHF frequencies, i.e. above 300 MHz. They are only suitable for use up to about 160 MHz. In the 1930s, when they were designed, anything over 30 MHz was considered to be an ‘ultra-high’ frequency. They are not often used for commercial ‘land mobile’ band radios, but they are used for marine band radios. ‘UHF’ connectors are not weatherproof and must be protected when used outside. It is quite difficult to solder the outer shield of the coax to the PL259 connector body. Some folks say that you don’t have to solder the braid at all. But the Amphenol installation guide says that you do. Provided you have the correct crimping tool, the crimp versions are easier to terminate. The male (PL259) connector is usually used for cables. It has a barrel-shaped sleeve with an internal thread and a center pin. Because it does not have a hex nut, tight connections can be difficult to unscrew. Some versions have a gold plated center pin, but the large center pin ensures a good connection either way. Female (SO239) connectors for cable are available, but much less common. Female to female barrel adapters can be used to join two cables together. The connector on the radio is always female. Avoid the chrome plated shiny solder type PL259s. You will have to file the solder holes down to the brass to get them to solder correctly. Nickel or Silver plated connectors are better quality and much easier to solder. For frequencies above 150 MHz, the most common antenna connector is the ‘Type N’. It was designed in the 1940s, for LMR-400 and RG213 coax. Versions are available for the smaller RG58 / RG8 coax and even the very small RG174 coax. As a personal preference, I use Type N connectors for all outside tails and antenna connections. They are reliable and can handle any power or frequency that I am likely to use. Inside the shack, my hard line LDF4-50 coax is terminated with Type N connectors and the far end of the connecting tails are terminated with whatever the radio equipment needs. The Type N male connector has a captive hex nut and

a gold plated center pin. The socket is threaded with a gold plated center socket. Radio equipment usually has the female connector. Cables may be fitted with the male or the female connector. The Type N was one of the first connectors capable of carrying microwave-frequency signals. They were originally rated for use up to 1 GHz, but these days good quality Type N connectors can be used up to 11 GHz. Type N connectors can handle more power than the other types listed in this section. They can be used for up to 5 kW on the HF bands reducing to about 500 W at 2 GHz. Like the BNC connector, the Type N comes in 50 and 75-ohm variants. You must not mix them because the 75-ohm connector has a smaller center pin. You are fairly unlikely to come across the 75-ohm variety because they are mostly used for TV broadcasting and medical imaging equipment. If you do find some, throw them away! Avoid the cheap shiny chrome connectors and use the Silver or Nickel plated connectors. Type N connectors should always have gold plated center pins. Type N connectors are not waterproof. If they are not properly protected you will probably get water into your coax feeder cable which will increase the cable loss and wreck your station’s performance. BNC (Bayonet Neill Concelman) connectors are OK for test leads and connecting up the equipment in your shack. They are fine for handheld portable antennas and you might see them on outdoor switches or preamplifiers. The BNC connector was designed for the smaller RG58 sized coax. It has a ‘bayonet’ action for quick connect and disconnect making them ideal for test cords. Some BNC connectors have gold-plated center pins but the ones with silver center pins seem to work just as well. BNC connectors can be used up to 4 GHz. BNC connectors are available for either 75-ohm or 50-ohm cables. You cannot mix them. The 75-ohm male connector (plug) has a narrower center pin, which will not mate reliably with a 50-ohm female connector. Inserting a 50-ohm BNC plug into a 75-ohm BNC socket will spread the center socket, making it unreliable for later 75-ohm connections. Cheap BNC cables bought online or from computer or TV stores are very likely to be 75-ohm cables intended for video cabling and the coaxial cable used is often poor quality as well. Throw them out immediately, before you cause yourself problems!

Many handheld radios use SMA connectors. SMA connectors have a 50ohm impedance and are suitable for use up to 18 GHz. Confusingly they come in two flavors, the standard SMA plug is a male connector with a captive hex nut outer and a gold center pin. The matching SMA female has a threaded outer and a gold center socket. Some SMA connectors are gold plated all over, but the silver ‘shiny’ ones are fine as well. Apart from chassis connectors, the braid connection on SMA connectors is always crimped, never soldered. Many handheld radios use ‘reverse polarity SMA’ (RPSMA) connectors. The ‘plug’ has a captive hex nut outer and a center socket. The matching RPSMA connector, mounted on the radio, has a threaded outer and a gold plated center pin. You need to be careful to buy the correct connector if you are making up a cable to connect to your handheld radio or when buying an antenna for one. The SMB connector is like an SMA without the screw thread it simply pushes onto the matching connector and latches with a click. They are more often used for connections inside a radio than as a general connector. There are other RF (radio frequency) connectors such as the TNC (Threaded Neill-Concelman) connector, which is a BNC with a screw thread rather than the bayonet action. They are not commonly used in amateur radio applications. The mini-UHF connector is a scaled down PL259 / SO239. It is used on at least one SDR transceiver and on some cellular radio equipment. Commercial operators terminate the larger diameter hard line Heliax cables with either a 7/16 DIN or a C Type connector. These are very good connectors but they are expensive. It is also possible to buy Type N connectors for those cables. The 7/16 DIN connector has a 16mm outer connection for the shield and a 7 mm center pin. It is widely used for high power radio systems and cell sites. The C connector was invented by Amphenol engineer Carl Concelman, who also helped to design the BNC connector. It is weatherproof without being overly bulky. The mating arrangement uses a bayonet action similar to that of the BNC connector. Both types have very good passive intermodulation (PIM) performance. You may see cables terminated with F-connectors. The F connector is standard for satellite TV systems, so you often see them on TVs, set-top boxes, splitters, and amplifiers. F connectors were specifically designed for use with RG6 coax cable. The male connector has no center pin, simply using the solid center conductor of the coax to mate with the F socket. These cables are low loss and they work well at UHF frequencies,

but they are always 75-ohms. The outer braid is not soldered to the connector. It is held either by the connector being screwed over it or by a crimp ferrule. There is a fairly rare waterproof variant with an internal Oring. Female F connectors are fitted to equipment but not usually to cables. You can get female barrel adapters if you need to join two cables together. The male F connector can only be installed on RG6 cable. I have seen, right angle male to female F adapters for connecting to equipment in confined spaces. The F connector is not designed for use with devices that transmit very much RF power, but you could use them for a ‘receive only’ system. The RG6 cable is cheap with low loss and the 75 to 50-ohm mismatch loss is not very significant if you are only receiving.

WATERPROOFING YOUR CONNECTIONS Water kills coax! By far the most common problem with permanent antenna installations is a failure of the coax cable due to water getting in through the connectors. Any nicks in the coax sheath that are through to the braid should be taped up in the same way as the connectors. Heliax cable has a solid ribbed copper outer conductor and most of the connectors are weatherproof, so you are much less likely to have problems. On the other hand, if you do get water damage in a Heliax cable, the cost of replacement is significantly higher. I recommend taping up those connectors as well. If water gets into a connector and then freezes, ice will cause gaps to open up, possibly damaging the connectors, and definitely making more water ingress likely in the future. Everyone has their own favorite method for waterproofing RF connectors. I will share mine. The choices are to use the tape method or heat shrink tubing. If the connection is to be long-term, or permanent, heat shrink sealing is a very good option. But you have to use the right product. Standard heat shrink tubing that comes in a range of colors is no good at all. It is not designed for use outside and there is no barrier to moisture creeping up underneath the tube. Standard heat shrink tube has no UV protection. It will quickly degrade if left outside in the weather and it is very thin and subject to damage. The right heat shrink to use is black and UV stabilized. It has a greater wall thickness making it more rugged and most importantly it includes an adhesive layer. When you heat the tubing with a hot air gun, it shrinks over the connectors and the adhesive melts, rather like the product used in hot glue guns, forming a waterproof seal under the tubing. If you think that ‘one day’ you might like to open up the joint again you can put a layer of PVC tape over the connectors before placing the heat shrink tube. That will keep the metal parts clean from the glue. But ensure the heat shrink tube extends at least 25mm or ‘an inch’ wider than the taped area, on both sides. The sealant must be able to make a seal between the coax sheath and the heat shrink tube. This type of seal is very effective and you really only need about 10mm (half an inch) each side of the connectors, but a little more won’t do any harm. If you want to open the joint, you will have to cut the heat shrink tube off with a craft knife and then clean off the adhesive. Be careful not to

damage the coax sheath. The other disadvantage is that you need a heat gun to shrink the tube. If you are using tape, the key is to use layers of different materials. A single layer of PVC tape, no matter what quality, is not enough. It will eventually fail and water will almost certainly get in. The adhesive on PVC tape may seem sticky, but it is not waterproof. Try licking it and see how sticky it stays! If you put a layer of tape over the joint, it will remain sticky where tape covers tape, but water will be able to penetrate underneath the tape layers. Also, most PVC tape is not UV stabilized so after a year or so in the weather, it will break down and become useless. Some PVC tape is specifically designed for use outside and it is definitely better to use it. PVC tape does have its uses and if the connection is only temporary, a single layer will keep the rain off the connectors for a few days. PIB (Polyisobutylene) rubber self-amalgamating tape is a ‘must have’ product for anyone building or maintaining an antenna system. The tape is usually about an inch (25mm) wide and it is not sticky because it does not have an adhesive layer. You must stretch the tape as you wind it in layers over the connector. Overlay by about half the width each time. As the tension leaves the rubber, the tape amalgamates into a seamless rubber coating that is waterproof. But it does not stick to the coax, so it is possible for water to get under it. When you cut off PIB tape it will come away completely clean so you do not need to put PVC tape underneath it. PIB tape is silicone rubber so it is not particularly tough. You can apply a layer of PVC tape over the top for added protection. PIB tape is very handy stuff. It can be used for a lot of other things such as sealing the end of a duct or tube or even making temporary repairs to low-pressure water hoses. It is good for sealing jars as well, but it has to be cut off. It cannot be unwrapped. Both PVC tape and PIB tape will allow water to creep up between the tape and the coax cable sheath. It will take a while, but eventually, it will probably leak. So now for the final ingredient. Black Mastic tape or goo. The rubber tape is waterproof except that water can get under it at the ends where the rubber layer stops. Apply a little mastic on the coax at each end of the connectors. Do not put mastic on the connectors. Rub the tape or goo so that it covers right around the coax with no gaps. You don’t need very much. A finger width is fine. Then apply the PIB tape and the PVC tape.

I use ‘Andrew’ brand mastic. It comes in a sort of tape with greaseproof paper both sides and it works well. Before taping all your outside connections. It is worth trying out a sample of the mastic with the PIB and the PVC just to make sure that the chemicals in the mastic don’t melt the PIB or the PVC. In “the good ol’ days,” we used to use Denso tape for waterproofing RF connectors. It is horrible stuff but it does work. Denso tape is a cloth tape impregnated with petrochemical grease. You wrap it around the joint and massage it until the grease makes a waterproof seal. Make sure you have mineral Turpentine and a rag to clean your hands. The grease is sticky and very messy. When the joint dries out after a few days, it will be dry to the touch. Removing Denso tape is messy and difficult. It has to be cut off and the grease cleaned away. It is a very good idea to wrap the connectors with PVC tape to keep them clean before applying the Denso tape. Of course, the Denso must extend past the tape by a few millimeters either side. You should not put Denso tape over PIB tape. The petrochemicals melt the PIB tape and turn it into sticky black goo. The joint will stay waterproof, but the mess when you cut it off is terrible. Denso is weatherproof and parrot proof and lasts for years. You can use RTV or other silicone sealants on your antenna system. I put it on stainless steel screws to keep them looking good. And it is great to protect the ends of antenna elements, exposed solder joints etc. However, I don’t think it works well on coaxial cable. It does not seem to bond with the coax sheath plastic and after it dries, I think you will find that it can be peeled off. If that is the case, I am sure that water could get under it. Mastic sticks and will not easily peel off so it blocks the water.

TESTING COAXIAL CABLE FOR WATER DAMAGE. Water in your coax cable can be very difficult to detect. The SWR (standing wave ratio) on transmit may look good and you may still hear other stations. Your setup may seem normal but you struggle to make contacts and your station seems a bit deaf. The water in the coax changes the properties of the dielectric (the layer between the inner and outer conductors). It increases the loss in the coax and may also change its impedance. You might not notice the problem on transmit because the reflected signal from the poor match is attenuated by the additional loss. In that situation, your SWR meter sees a big signal going out and a very small amount of reflected power. It displays a low SWR indicating that all is well. But in fact, there is almost no RF power being radiated. If the effect is getting gradually worse over a long period of time, you may be completely unaware that you have a major problem. If you have a ‘receive only’ station, the best way to test the antenna cable is with an antenna analyzer. Or you could compare the normal feeder cable with a temporary feeder cable run out to the antenna. An antenna analyzer can perform tests from the shack into the antenna, from the shack into a 50-ohm load substituted for the antenna at the antenna end of your main feeder, and directly into the antenna via the short coax tail. Many analyzers have a ‘distance to fault’ mode, which can be used to compare the reflected signal from a 50-ohm load with the reflected signal from an open or short-circuited feed line. This will identify whether you have a problem and where it is in the system. While you have the antenna tail disconnected, check for moisture inside both connectors and for any signs of corrosion. If it is wet inside but passes the tests, dry it out with a hair dryer or similar and give it a spray of CRC 2.26 or WD40 to dispel any water then reconnect and tape everything properly this time. If either connector is corroded, replace both connectors. If you have a transmitting station, there are several ways to test the coax run. You could use an antenna analyzer as discussed above or you can use your transmitter as a test signal source. One method is to disconnect the tail at the antenna end of the main feed line and place a power meter and 50-ohm load at that point. When someone in the shack keys the transmitter, you will be able to see you how much power is going to the antenna. You may be shocked at how little there is! Of course, you must

transmit a carrier at the correct frequency for the antenna. Switch your all mode rig to CW or FM. It is easy to work out the cable loss in dB. Cable loss = 10 log x (power at the shack / power at the antenna) dB. If you have an SWR meter, and you should, you can compare the SWR with the antenna connected to that with a 50-ohm load connected. Record the SWR with the antenna connected. Note that antennas are affected by anything close to them such as a building, tower, or you. Make sure that the antenna is in its normal operating condition when you are testing. The SWR when transmitting into the antenna via the feeder cable should be better than 1.2:1 although this may depend on the design. If it is high at around 1.5 to 1, you may have water in the feeder cable. If it is really high, more than 6 to 1, then you have a bad feeder or a bad antenna. Disconnect the antenna tail at the antenna end and terminate the cable that leads back to the shack with a 50-ohm termination. You can buy 20 watt, 50-ohm, loads that screw directly onto a Type N female connector. Or you can use a 20 dB or 30 dB attenuator if the power rating is sufficient. Use a barrel adapter if required. When you transmit into the cable with the load connected at the far end, you should see an almost perfect 1:1 SWR. If you still see a higher SWR, you definitely have a problem with the feeder cable. The next stage is to check the actual amount of feeder cable loss and compare it with what you would expect for the cable type and length of the feeder. A ‘quick and dirty’ method of finding out if the cable is faulty is to; reduce your transmitter power to 1/20 normal power, say 5 watts, to avoid possibly damaging your transmitter. Recalibrate your SWR meter and write down the SWR figure when you transmit into the antenna, for example, 1.2:1. Disconnect the antenna tail at the antenna end. Then transmit briefly into the now open circuit coax cable. If the cable is good, the SWR should be bad, at least 10:1. If the SWR stays “good” then the meter is not seeing power reflected from the open circuit at the far end, which means that your coax has a very high loss. You can calculate the cable loss at the frequency that you are transmitting from the SWR into the open circuit cable. See the formula for calculating the Return Loss from the SWR below. The cable loss is equal to half the return loss when transmitting into the cable when it has either a short or an open circuit at the far end. See the ‘testing coaxial feeder cable with an SWR meter’ section below. If the cable tests OK but you still suspect that the antenna system, perhaps the antenna is faulty. This is much less likely than the cable being

faulty but it is still a possibility. Move your transmitter and SWR meter outside so that you can transmit, at a frequency near the center of the band, directly into the short tail connected to the antenna. This eliminates the long feeder cable from the equation. The antenna SWR should be better than 1.2:1 (depends on the antenna design). If the antenna SWR is high, you have a problem with the antenna, probably in the matching arrangement or balun.

MEGGER CABLE LEAKAGE TEST A ‘Megger’ is a test meter designed to test telephone cable leakage and other insulation tests. It can be used to check your antenna feeder cable but you must be very careful. Disconnect the antenna tail and your radio equipment and then use the Megger to test only the open circuit feed line. Megger testers generate a high voltage that might damage your antenna or balun. A Megger is very likely to kill your masthead preamplifier. If you are testing from the antenna end of your feeder cable, a Megger is very likely to kill your receiver or transceiver. Always disconnect the antenna and radio equipment! Test the cable only. A Megger only performs a DC test. It cannot determine how well the cable will work at radio frequencies. However, a wet cable will probably fail the insulation test.

DIGITAL VOLTMETER CABLE LEAKAGE TEST Testing the antenna coax with the resistance setting on your digital voltmeter only checks the DC resistance of the center conductor and the braid. It does not test the RF performance of the cable. It is not a very good way to test your antenna cable, but it may be the only test instrument that you have. You can’t test the feeder cable for DC resistance with the antenna connected. Disconnect the antenna tail and place a short across the connector on the cable facing the shack. Back in the shack use the DVM to measure the resistance between the center conductor and the outer conductor. It should only be a few ohms. You could check the cable specification sheet. The specification will include DC resistance for the braid and the inner conductor. Your test reading should be close to those two numbers, adjusted for the feeder cable length, added together. Remove the short circuit and in the shack test the leakage between the center conductor and the outer conductor. It should now be infinite, or very high. Compare your results against a similar length of ‘known good,’ coax cable. Failing either of these tests would indicate a really bad issue with the coax. Passing the tests does not guarantee that the coax is OK. Just that it is not terrible. Note that a DC resistance test of the antenna, measured without including the feeder cable, should definitely not read 50-ohms. It will probably read a very high resistance or a very low resistance, near zero ohms. If it reads something in the middle such as 100 ohms, the antenna is probably faulty. 50-ohms is the ‘complex impedance’ at the frequency of operation, not the DC resistance. Complex impedance is a combination of the capacitive or inductive reactance and the resistance presented to the radio frequency. At resonance, the reactance is zero and the antenna “looks” resistive.

SWR METER SWR (standing wave ratio) is important because it affects the amount of your transmitter power that you are actually transmitting towards the satellites. A high standing wave ratio indicates that some of your transmit power is not being transmitted, but is instead being reflected back towards your transmitter. This will happen if the impedance of the antenna or the coaxial feeder cable does not match the impedance of your transmitter. Your transmitter has a nominal output impedance of 50-ohms and the coaxial cable and the antenna should be chosen to match that impedance. A fault in either the cable or the antenna such as water damage or a break in the cable will create a change in the impedance match, resulting in a high SWR meter reading. Any loss of transmitted power due to high SWR is added to the cable loss of your coaxial cables, potentially absorbing most of your radio frequency power. Cable loss affects both the forward power traveling towards your antenna and the reflected power traveling back towards your transmitter. SWR is expressed as a ratio. It is not related to the power level that you are transmitting. A standing wave ratio of 2 to 1 (often written as 2:1) or less is acceptable for an antenna system. SWR meters measure the power that is reflected back from the ‘termination’ at the end of the cable under test. Usually, the termination is your antenna. You might change the termination to a 50-ohm load, an open circuit, or a short circuit while you are testing the feeder cable. If the antenna is a good match for the cable and the transmitter, there will be very little signal reflected back down the feeder cable towards the transmitter. But if the antenna is faulty or the cable is damaged, the match will be poor and a lot of your transmitted power will be reflected back along the feeder cable towards your transmitter. The SWR meter uses a small transformer configured as a directional coupler to take a tiny part of the power that is on the cable between your transmitter and the antenna. The meter displays the voltage of the ‘sniffed’ signal measured across a load resistance. The meter scale is marked to indicate the amount of ‘forward’ power being transmitted. A small percentage of the power that is reflected back from the antenna end of the cable emerges from a different port on the directional coupler. It

is measured as a voltage across another load resistor. The meter displays the voltage as an indication of the amount of reflected power.

Some SWR meters have a single meter movement and a switch so that you can select the forward or reverse power reading. Some have two meters so that you can see both forward and reflected power at the same time and some have a single meter with two movements. These meters show forward and reflected power at the same time and the point where the two pointers cross indicates the SWR. Cross meter SWR, meters don’t have to be calibrated prior to performing an SWR test.

In most cases, the SWR is indicated on a nonlinear scale. The cross meter type of SWR meter indicates forward power, reflected power and SWR simultaneously. Photo:

The ratio of forward power being sent towards the antenna to the reflected power returning from the antenna end of the cable is mathematically related to the VSWR (voltage standing wave ratio). VSWR is usually shortened to SWR (standing wave ratio). You will never have to do this because your SWR meter already has an SWR scale, but I will include the formulas for the sake of completeness. To calculate the VSWR from the two voltages, you first calculate the reflection coefficient. It is the ratio of the reflected voltage measurement to the forward voltage measurement. Reflection coefficient, Γ = VREV / VFWD Also reflection coefficient, Γ = (PREV / PFWD) Then calculate the VSWR VSWR = (1 + Γ) / (1 – Γ)

The ratio of power being reflected is the square of the reflection coefficient. % of reflected power = Γ x Γ If you are using a non-cross meter, you will need to calibrate the instrument before reading the SWR. Calibration is done by switching the meter to ‘Cal’ and then transmitting a fixed carrier signal. Adjust the ‘calibrate’ control to set the meter to full scale. Stop transmitting and switch to the ‘SWR’ meter position. Then transmit again and read the SWR will be indicated on the relevant scale of the meter. The meter shows the SWR as seen at the transmitter end of the feeder cable. SWR meters don’t actually read the voltage standing wave on the feeder cable. They read the forward and reflected power at the point where they are inserted into the circuit, and display that ratio as a standing wave ratio.

ANTENNA ANALYZER An antenna analyzer works in much the same way as an SWR meter. Instead of using your transmitter to provide a test signal, the analyzer transmits a low-level output signal, moving across the band of interest in small frequency steps. It uses a directional coupler to measure the reflected signal at each frequency step. The instrument can ‘sweep’ across the band of interest, for example, 144 to 148 MHz for an antenna on the 2m band. Or across much wider bandwidths if you are testing a multiband antenna or a cable. An SWR meter shows you the antenna and feeder cable’s performance at the single frequency that you are transmitting. An antenna analyzer can show you its performance across the whole radio band. There are cheap antenna analyzers that have manual tuning rather than an automatic sweep. Some of these instruments are not much more than an SWR meter with a built-in test oscillator. Most of them don’t cover the VHF and UHF frequencies that you need for testing satellite tracking antennas. The best analyzers have automatic sweep and can display plots of your antennas performance on an internal screen or better still on a PC. The better antenna analyzers are fairly expensive but they are wonderful if you are building your own antennas, or if you have a problem with your setup. They can display the performance of your antenna and its feeder cable, test matching harnesses, and tuning stubs, find the loss, length, and impedance of your feeder cable or the distance to a fault. They display the complex impedance, SWR, and Return Loss, so you can determine if the antenna is tuned too low or high in frequency or if the elements are too short or too long. The vector network analyzer (VNA), a close cousin of the antenna analyzer, has two test ports rather than a single one for connection to the antenna or feeder cable. It has an output port and a separate input port, so it can be used to test through devices such as filters, cables, or antenna traps. With the addition of a directional coupler, a vector network analyzer can also carry out one port tests on antenna systems.

RETURN LOSS The difference between the amount of power being sent into the antenna feeder cable and the amount of power being reflected is called Return Loss (RL). It is the reflection coefficient converted to decibels. Telecommunications professionals use return loss measurements rather than SWR because it is easier to perform calculations using decibels than voltage ratios. Return loss (dB) = Power in (dBm) – Reflected power (dBm) A high return loss means that little power is being reflected back. It indicates a good impedance match between the antenna, the feeder cable, and the transmitter. High return loss is equivalent to low SWR. A low return loss means that a lot of the transmitted power is being reflected back. Low return loss indicates a ‘bad’ impedance match. It is equivalent to a high SWR and it usually means that you have a fault. A simple calculation allows an antenna analyzer to display a plot of the standing wave ratio or the return loss (RL). It can also display a plot of the phase variation that is caused by the capacitive or inductive reactance of the antenna load. The phase of the reflected signal changes as the frequency increases, because the antenna moves from having an inductive reactance at frequencies below the antenna’s resonant frequency to a capacitive reactance above the frequency of resonance. The antenna only presents a purely resistive load at the frequency where the antenna is resonant. The resonant frequency is usually the point where the SWR is lowest and the return loss is highest, near the center of the band that the antenna is designed to operate on. A perfect match at the end of the cable absorbs or radiates all of the power sent from the transmitter that has not already been lost as heat due to cable loss. There is no power reflected back down the cable to the shack, so your SWR meter reports a 1:1 match and the antenna analyzer reports a high return loss. A typical antenna will not have a perfect match, particularly at frequencies near the band edges. If your antenna plot looks like a perfect 1:1 match over the whole band, you probably have a high antenna feeder loss, masking the true state of affairs. Temporarily move your SWR meter out to the antenna pole and try the same measurement with a short test lead instead of the long feeder cable. Cables that are water damaged may test

OK with your bench top SWR meter or an antenna analyzer in the shack, because the high loss in the cable absorbs most of the reflected power, making the antenna system appear better than it actually is.

Return loss = -20 x log( Γ ) and In Excel format, return loss in dB = 20*LOG10((VSWR+1)/(VSWR-1))

If you know the return loss, you can reverse the formula to calculate the SWR.

In Excel format, VSWR = (10^(RL/20)+1)/(10^(RL/20)-1) A really bad load such as an open or short circuit on the far end of the antenna cable will have a high SWR, usually more than 10:1 and a low return loss around 2 dB. You can use this known relationship to measure the actual loss of the cable.

TESTING COAXIAL FEEDER CABLE WITH AN SWR METER Amateur radio operators should have a standing wave ratio (SWR) meter in the shack. If you don’t have one, you may be able to borrow one from another ham. An SWR meter shows you the amount of power that you are transmitting and the amount of power that is being reflected back from the antenna or ‘termination.’ In some cases, you can read the return loss directly off the dB scale on the meter. If there is no dB scale, you can record the SWR measurements and then use the formula above to convert SWR to return loss. If you have a ‘good’ 50-ohm termination capable of withstanding the output power of your transmitter, you can connect the load to the output port of the SWR meter and use the device as a power meter, (see the next section). You can check that the load is a true 50-ohm load by connecting it to the SWR meter in place of the antenna feeder cable. You should get a perfect 1:1 match when you transmit. For this test, you do not need full power. The power that you transmit into the cable is unimportant so long as there is enough to calibrate your SWR meter. To check the feeder cable. Disconnect the antenna tail at the far end of the coax feed line. Terminate the cable that leads back to the shack with a 50-ohm RF load that can withstand the power that you are transmitting. Connect your transmitter to the ‘transmitter’ or ‘TX’ port on the SWR meter with a short jumper cable and connect the ‘antenna’ or ‘ANT’ port to the antenna feeder cable. Calibrate the meter according to the maker’s instructions and then read the return loss or SWR. Calibration is usually a matter of setting a switch to the ‘Cal’ position and adjusting a knob for full scale, then setting the meter to the SWR or RL position. ‘Cross meter’ SWR meters do not need any calibration. After the meter has been calibrated, switch it to the SWR or RL position, transmit briefly, and record the return loss or the SWR. The return loss should be higher than 20 dB. The SWR should be low, very close to 1:1. As soon as you have recorded the measurement, stop transmitting. If the readings are different to what is expected you probably have a bad feeder cable. If the readings are good when transmitting into the 50-ohm load, but not good when you are transmitting into the antenna, the antenna may not be resonant on the band, or it may be faulty. The problem could be water or corrosion in the connectors, the antenna’s coax tail, a matching harness problem, or least likely of all, the antenna itself.

If you want to determine the actual loss of the feeder cable, you can go on to the next step. Do not transmit full power or you may damage your transmitter. I suggest using 1/10th of your normal operating power. If you can’t turn down the power on the transmitter or use a ‘low power’ setting, use a fixed 10 dB attenuator between the transmitter and the SWR meter. The attenuator must be able to handle the power that you are transmitting. This is not a 100% accurate method but it is better than nothing. Go out and remove the 50-ohm load leaving the antenna cable disconnected. Back in the shack briefly transmit into the open circuit feeder and record the return loss. Then place a shorted connector on the cable and test again. Because the signal travels to the termination and then back again before it is measured, the cable loss is approximately one-half of the return loss into an open circuit or a short circuit termination. The short circuit reading may be different to the open circuit reading because of the high reactance that each type of load presents to your transmitter. So the best approach is to average the readings. Cable Loss (dB) = (RL(short) + RL(open)) / 4 Cable Loss = 5*(LOG10((SWRS+1)/(SWRS1))+LOG10((SWRO+1)/(SWRO-1))) The SWR reading while transmitting into the open feeder should be high. At least 10:1. The return loss should be low, not more than 2. If the open circuit, or short circuit, feeder performs in much the same as when the antenna or the 50-ohm load is connected. For example, the SWR is between 1.3 and 5 then you almost certainly have water in the coax. It will have to be replaced.

TESTING COAXIAL CABLE LOSS WITH A POWER METER If you don’t have an antenna analyzer or an SWR meter, you can measure your coax cable feed line loss using a power meter. Use the power meter to measure the power output of your transceiver on the band of interest (use CW or FM mode). Then move the power meter to the antenna end of the feed line and measure the power again. You will need long arms to reach the PTT switch on the microphone. Enlist a helper. The ratio of input power to output power indicates the cable loss in dB. Cable loss = 10 log x (power at the shack / power at the antenna) dB. Note that to turn power ratios like this measurement into dB, we use 10 log(ratio). For voltage ratios like the SWR measurement, we use 20 log(ratio).

ANTENNA ANALYZER TYPES There are several different sorts of antenna analyzer available, with differing capabilities. The cheapest models cost a couple of hundred dollars for an analyzer with manual tuning that is unable to determine between the capacitive and inductive reactance. The best value analyzers cost $500 - $1000 for a box that has a small internal display or connects to a PC for displaying the results. They automatically ‘sweep’ or step through the band of frequencies that you are interested in and store the reflected values. The PC can display the results as an SWR or Return Loss plot and you often get phase and reactance displays as well. Some units can display a Smith chart or the distance to a fault. You can use a spectrum analyzer with a tracking generator and a directional coupler as an antenna analyzer, but that is a bit ‘old school’ these days. Or you can use a vector network analyzer with a directional coupler. These devices cost more but they can also be used for testing filters and combiners. Antenna analyzers are very useful if you are building or tuning an antenna because you can immediately see the effect of moving or shortening the antenna elements. If you have one of the better antenna analyzers, you can use it to find the length of coax cables. The distance to fault plot will show any discontinuities or cable sections that have a different impedance. You can see the effect of a shorted or open stub transmission line or find the distance to a faulty connector. Even a kink in the cable can cause a poor SWR match. Antenna analyzers are not cheap, but they are pretty cool.

SWR AND RETURN LOSS TABLE The table shows the amount of radiated and reflected power for various degrees of antenna mismatch.

VSWR RL Γ Reflected Forward Mismatch dB :1 Power Power loss dB 1.1 26.4 0.05 0.2% 99.8% 0.01 1.2 20.8 0.09 0.8% 99.2% 0.04 1.3 17.7 0.13 1.7% 98.3% 0.07 1.4 15.6 0.17 2.8% 97.2% 0.12 1.5 14.0 0.20 4.0% 96.0% 0.18 2.0 11.1% 88.9% 0.51 9.5 0.33 2.5 18.4% 81.6% 0.88 7.4 0.43 3.0 25.0% 75.0% 1.25 6.0 0.50 4.0 36.0% 64.0% 1.94 4.4 0.60





4.5 5.0 6.0 7.0 8.0 9.0 10.0 15.0 20.0 30.0 40.0

3.9 3.5 2.9 2.5 2.2 1.9 1.7 1.2 0.9 0.6 0.4


0.64 0.67 0.71 0.75 0.78 0.80 0.82 0.88 0.90 0.94 0.95

Reflected Power 40.5% 44.4% 51.0% 56.3% 60.5% 64.0% 66.9% 76.6% 81.9% 87.5% 90.5%

Forward Mismatch Power loss dB 59.5% 2.25 55.6% 2.55 49.0% 3.10 43.8% 3.59 39.5% 4.03 36.0% 4.44 33.1% 4.81 23.4% 6.30 18.1% 7.41 12.5% 9.04 9.5% 10.21

VSWR = voltage standing wave ratio. RL = return loss (dB). Γ = reflection coefficient. Reflected power is the percentage of RF (radio frequency) power that is being reflected back from the mismatched termination. Forward power is the percentage of RF (radio frequency) power that is not being reflected back from the mismatched termination. Mismatch loss is the amount of loss in dB that the impedance mismatch is causing to the forward power. At a VSWR of 1.1 to 1, a healthy 99.8% of the RF power arriving at the antenna end of the feeder cable is available to be radiated by the antenna. The actual amount of power that is radiated depends on the radiation efficiency of the antenna. It is important to note that this table only shows what happens at the antenna end of the feeder coax. It does not take into account the amount of power that is being lost due to cable loss.

At a VSWR of 2.1 to 1, the antenna is still acceptable. 88.9% of the RF power arriving at the antenna end of the feeder cable is available to be radiated by the antenna. By the time the VSWR rises to 6 to 1, the mismatch loss is 3.1 dB and only 49% of the power arriving at the antenna end of the feeder cable will be radiated. An impedance mismatch that causes a 6 to 1 SWR has the same effect as having 3 dB cable loss in your feedline. If you happened to have a coax feeder cable with 3 dB of cable loss and also an SWR of 6:1 the cumulative loss is would be 6 dB and you would only radiate 25% of your transmitted power.

DOES POOR SWR AFFECT RECEIVED SIGNALS? Yes, it does! Feed line loss is the main problem when receiving but mismatch loss does affect your received signals as well as the signals that you are transmitting. The best power transfer between the radio and the antenna through the antenna feeder cable occurs at that frequency where the antenna is resonant. This is true for transmitting and for receiving. As the antenna reactance increases near the band edges, the SWR degrades and mismatch loss increases. See reflection coefficient and mismatch calculations in ‘formulas.’ For a good antenna system with better than 2.1 to 1 SWR at the edges of the band, the worst case mismatch loss is only 0.5 dB. It can be ignored. The SWR needs to be quite bad before there is any significant effect on your received signal levels. At an SWR of 6:1, the mismatch loss is 3.1 dB. But if your SWR gets that bad, you definitely have a fault.

WHAT HAPPENS WHEN THE REFLECTED POWER GETS BACK TO THE TRANSCEIVER? What happens when the reflected power gets back to the transceiver? It gets reflected back towards the antenna again, further complicating the standing wave. After being attenuated for a third time by any coaxial cable loss, any remaining power arriving at the antenna is once again partly radiated and partly reflected. Will the reflected power harm my transceiver as everybody says? Reflected power will not damage your transceiver. After all, by the time the power has been reduced by coax cable loss as it travels to the antenna and then reduced again by the same amount as it travels back from the antenna to the transceiver, there is little left to cause any problems. The reflected power can never be more than the power that was being generated in the first place. No, it is not the reflected power that can cause a problem. It is the standing wave. The standing wave causes high voltage points and high current points along the coax feeder cable. If the high voltage happens to appear right at the transceiver end of the coax, components in the transceiver could be damaged by excessive voltage. Similarly, if the high current happens to appear right at the transceiver end of the coax, the final transistors could be damaged by the excessive current flow. The capacitive or inductive reactance transformed back to the transceiver may cause detuning of the final power stage causing overheating of filter components and reduced transmitted power.

MASTHEAD PREAMPLIFIERS Masthead preamplifiers are used to improve the signals that you receive from the satellites. It is critical that they are placed close to the antenna and not on the bench next to your transceiver or receiver. I have seen online comments from hams who say that preamplifiers are useless and that they cause more problems than they solve. This is not true. Their preconception might be due to poor placement in the overall antenna system or it may be attributable to using a preamplifier with too much amplification or bandwidth. This chapter discusses when it is appropriate to use a masthead amplifier, why we should mount it close to the antenna, what noise figure is, and why it matters. There is a common misconception that masthead preamplifiers are employed boost the strength of received signals. That assumption is wrong. The main reason for using a preamplifier is to improve the quality, not the level, of the received signal. A boost to the signal level may occur, but it is not the reason that we use a preamp. Signal quality is measured in terms of ‘signal to noise ratio.’ To measure the signal to noise ratio (SNR), you compare the audio output level of the receiver when it has an incoming signal and the normal background noise ‘S+N,’ with the level of received background noise on its own with no test signal ‘N.’ The test measures the ‘signal plus noise, to noise’ ratio (S+N / N). For FM receivers, the quality measurement is called SINAD (Signal to Noise and Distortion ratio). It is measured with a special test instrument but is effectively the same measurement as a signal to noise ratio. Amplifying the signal level and the noise level together does not improve the quality of the signal. It just makes it louder. You can do that by turning up the volume on your radio! We need to boost the wanted signal without adding more noise so that the signal to noise ratio improves. The key to this the preamplifier’s noise figure, not the amount of amplification (gain) that it has.

WHY USE A MASTHEAD PREAMP? You don’t need a preamp if you are using a handheld antenna but for home stations with longer coaxial cables, a preamplifier can dramatically improve the quality and intelligibility of the signals that you receive from the satellite. The preamplifier overcomes the losses incurred in your coaxial feeder cable and improves the overall noise factor performance of your receiving setup. The extra signal helps a lot when the signal fades due to cross polarization, antenna radiation pattern, or tracking problems. The trick is to use a preamplifier that improves the signal quality and which has enough gain to overcome the loss of your feeder cables…. plus a bit! Even very good coaxial cables have a lot of loss at microwave frequencies. A masthead amplifier can compensate for the cable loss but it is more common to use a ’Low Noise Block’ (LNB) mounted right at the dish antenna feed point. A low noise block combines a low noise preamplifier with a frequency down converter. Since the amount of feeder cable loss is proportional to the frequency, converting the received frequency to a lower frequency eliminates the need for expensive low loss coax or waveguide between the radio and the antenna. A good example is your Satellite TV dish and receiver. The LNB is the little white block mounted right at the focal point of the dish. The output can be carried to the TV set-top box over relatively cheap RG6 coaxial cable.

BUYING A PREAMPLIFIER When you are buying a masthead preamplifier the two most important specifications are the gain and the noise figure. Other important considerations are whether it can be bypassed so that you can transmit on the same antenna, its bandwidth, how it is powered, and weatherproofing. Too much gain can cause overload and intermodulation problems. The gain should be enough to overcome the loss in your feeder cable, ‘plus a bit.’ (Preferably not more than your feeder loss +10dB). Wideband amplifiers should only be used for wideband antennas. Some preamplifiers will work up to a GHz or higher. Wideband amplifiers are subjected to a wider range of interference signals which makes them more prone to overload and intermodulation. The amplifier must be mounted be as close to the antenna as practical. Overall noise figure is dominated by the first device in the feeder chain, so that is where your low noise preamplifier needs to be. Lower noise figure is better but more expensive. For EME or VHF DX, it needs to be as low as possible. For Satellite operation the noise figure is less important, and for your local repeater, you probably don’t need a preamplifier at all. Power feeding over the coax means that you do not need to run a separate cable to power the preamp. Some transceivers and receivers can feed a masthead amp without the need for an external DC power injector.

TRANSMITTING WHEN YOU HAVE A PREAMPLIFIER Masthead preamplifiers are very sensitive devices. The active device is designed to amplify very, very, small signals. They have no tolerance at all for the relatively huge amount of RF power generated by your transmitter. If you want to transmit to an antenna with a preamplifier in the circuit, your preamplifier MUST have bypass relays. The relays switch the amplifier into circuit while you are receiving and out of circuit while you transmit. In ‘transmit’ mode, the power supply to the preamplifier is disconnected and usually relay contacts short the input to ground for extra protection. Depending on the preamplifier design, the relay and power switching may be controlled by sensing RF energy coming from your transmitter, or by applying and removing the DC power supply. Sometimes DC power for the preamplifier is provided via a separate cable, but the most commonly used method is to send DC current over the coaxial feeder cable. Power is injected into the shack end of the coax cable while the radio is receiving. At the preamplifier, the voltage switches the bypass relays and supplies power to the preamplifier. When the radio switches to transmit, the DC is removed, turning the amplifier off and causing the bypass relays to revert to the bypass mode, shorting out the preamplifier input and routing the transmitted signal safely around the amplifier and out to the antenna. Some transceivers and a few receivers like the FUNcube SDR Dongle have a built-in power injector to provide a 5-volt signal to power and switch a masthead preamplifier. Commercial power injectors are available, or they can be easily constructed using a small capacitor and an inductor.

PREAMPLIFIER AMPLIFICATION A masthead preamplifier usually has a single stage of amplification, provided by one transistor or more commonly a FET (field effect transistor) or a MMIC (Monolithic Microwave Integrated Circuit). FETs provide a large amount of amplification and add very little noise to the signal. The amount that the amplifier increases the level of the incoming signal is called the ‘amplification factor’ or ‘signal gain.’ The amplification factor is the ratio of the voltage at the output divided by the voltage at the input. The amplifier may be said to have an amplification of ‘times two’ or ‘times ten.’ When the voltage ratio is converted into the logarithmic decibel scale the amplification is called ‘gain’ and is measured in dB. A voltage amplifier with an amplification factor of ‘times two’ has 6 dB of gain. A voltage amplifier with an amplification factor of ‘times ten’ has 20 dB of gain. Gain (dB) = 20 * log(VOUTPUT / VINPUT) The gain of your preamplifier should be greater than the loss in your antenna cable, but not much more than the loss of your antenna cable +10 dB. Typical commercial models have between 10 and 20 dB of gain. More than that is likely to cause overload problems in your receiver. Too much gain will limit the dynamic range of your receiver. The S (signal strength) meter on your receiver should not rise to more than S3 (30%) when the amplifier is on and connected to your antenna, with no other signal present. If the S meter is higher than S3 when the receiver is just receiving noise, you are limiting the amount of wanted signal that the receiver will be able to manage without overloading.

WHY ARE WEAK SIGNALS NOISY? As a radio signal travels through space or the atmosphere it gets weaker because it is spreading outwards. The longer the radio path, the less signal reaches your antenna. This loss of signal level is proportional to the distance between the stations and to the frequency of operation. It is called ‘free space path loss.’ The loss per km while traveling through the Earth’s atmosphere is not significantly different to the loss while the signal is traveling through space. The signal also picks up ‘atmospheric noise’ as it travels. This is caused by natural and man-made noise sources, such as lightning, static build up in clouds, rain, vehicles, power lines and a million other things. The signal that reaches your receiver is much weaker and noisier than the signal that was transmitted from the distant station. We have all heard strong signals with little noise and weak stations which sound noisy. FM receivers are much more immune to the noise that is added along the radio path because noise affects the amplitude (level) of the received signal more than it affects the phase or frequency of it. The weak signal arrives at your antenna and is converted by it to a very small voltage, normally only a few microvolts. After that, the noise problem gets worse! Every item in your receive chain from the antenna to the receiver including coax feeder cables, attenuators, filters, and amplifiers, adds noise to the received signal. This noise is due to the thermal noise characteristic of each device. I am not going to discuss ‘noise temperature’ in this chapter because it is complex and I don’t feel that it is necessary when discussing the merits of using a masthead preamplifier. It only really becomes important when you are designing a microwave satellite system with dish antennas. Just take it as a fact that each item whether it is an active component like an amplifier or a mixer, or a passive component like a coax cable, adds some thermal noise to the signal that you are receiving from the satellite. The thermal noise degrades the signal to noise ratio making weaker signals harder to listen to and understand. On a digital signal, noise impairs the software’s ability to decode the modulation without errors.

PREAMPLIFIER AND RECEIVER NOISE FIGURE / NOISE FACTOR Preamplifiers are often advertised as having a low ‘noise figure’ or ‘noise factor.’ This is a much more important number than the gain in dB that the amplifier provides. The noise factor is the amount of noise that is added to the wanted signal as it passes through a device. It is defined as the signal to noise ratio at the input divided by the signal to noise ratio at the output. If the device does not add any noise, F=1 because the SNR at the output of the device is the same as the SNR at the input. F = SNRInput / SNROutput. The device could be anything in the receive chain, such as an amplifier, a filter, a coaxial cable, or an attenuator. Noise factor (F) is used in some calculations but for most purposes, it is converted to the logarithmic decibel scale and called noise figure. NF=10*log(F). For satellite antennas, you want a preamplifier with a low noise figure, say 1 or 2 dB. For EME operation you need all the help you can get because the signals are so weak. It is best to use a preamplifier with a very low noise figure, less than 0.5 dB and preferably better than 0.2 dB. Amplifiers with these extremely low noise figure are a lot more expensive and they are not necessary for the signal levels that you expect from satellites. Your receiver has a noise figure but most manufacturers don’t quote it. The ARRL does test it and it can be found in the QST reviews. For HF radios the noise figure is not a particularly important factor because the amount of atmospheric background noise that is picked up when you connect an antenna is much higher than the internal noise created inside the receiver. It is much more relevant for VHF and UHF receivers and quite important for microwave frequency receivers, particularly if they are receiving signals from space which is cold and therefore electrically quiet. The noise figure of a receiver is the difference in decibels (dB) between the noise output of the actual receiver to the noise output of an “ideal” receiver with the same overall gain and bandwidth, when the receivers are connected to matched sources at the standard noise temperature (T0), usually 290 K. Source Wikipedia. If a receiver has a noise figure that is 2 dB better than another receiver, it will produce a signal to noise ratio that is 2 dB better than the other receiver with the same input signal.

Noise figure is related to temperature. If the antenna is pointing out towards outer space, it is pointing to a place that is much colder than the reference temperature of 270 degrees Kelvin. In that case, the receiver with a 2 dB better noise figure may produce slightly more than 2 dB improvement in signal to noise ratio. This is not really a big deal at the frequencies we use for LEO satellites, but it becomes important for microwave systems. All amplifiers add some thermal noise. The noise figure of a preamplifier is equal to the amount of additional noise that it is adding. NF = SNR in dB at the input – SNR in dB at the output. A good preamplifier adds very little noise and has a low noise figure.

WHY NOISE FIGURE MATTERS The “thing” about noise figure is that it is dominated by the first device in the chain from the antenna to the receiver. If the first thing after the antenna is a lossy feeder cable, the overall noise figure will be high and the resulting signal quality as measured by the signal to noise ratio will be poor. If on the other hand, the first device is a low noise preamplifier with a low noise figure, the overall noise figure will be low and the resulting signal to noise ratio will be much higher. This is how a preamplifier improves the quality of your received signal. But it only works if the preamplifier is early in the receive chain close to the antenna. It seems strange! Why does it matter where the preamplifier is located? It matters and I can prove it! ‘Passive’ devices with no gain such as attenuators, filters or the coax feeder cable to your antenna have a noise figure that is equal to their attenuation, (loss). So if your feeder cable has 3 dB of loss at the frequency of operation, the NF of the feeder cable will be 3 dB and it will degrade your signal to noise ratio by 3 dB. Let’s think about that because it seems a bit odd at first. Assume that your UHF receiver has a maximum sensitivity of –140 dBm on SSB mode. In that case –140 dBm is the weakest signal at which you can faintly hear a wanted signal. It is called the MDS (minimum discernible signal) level. An input level of –130 dBm, 10 dB above the MDS level, will give a SNR (signal to noise ratio) of 10 dB, which is about the minimum for acceptable reception of a voice transmission. If you have loss of 3 dB in the feeder coax cable from your antenna, the input level at the receiver would reduce to –133 dBm and therefore the signal to noise ratio will reduce to 7 dB, which would make the received signal difficult to copy. A loss of 6 dB in the feeder coax cable would reduce the input level to –136 dBm making the signal to noise ratio 4 dB and the signal almost impossible to copy. In the example above we found that the signal to noise ratio is degraded by exactly the amount of feeder cable loss. It is the same effect as leaving the signal level the same and increasing the noise level. In other words, the cable loss degrades the noise figure. Noise Figure (NF) = SNR (input) – SNR (output). This is why it is important to use good quality coax feeder cable. The higher the frequency the more loss the coax will have and the better

quality it needs to be. For example, at 435 MHz 20 m of RG213 coax cable will add about 3.2 dB of loss, but 20 m of RG58 would add 6 dB of loss. Active devices such as amplifiers have a noise figure related to their bandwidth, input impedance, gain, and the thermal noise created by semiconductors and other components. A really good low noise preamplifier might have a noise figure between 0.5 dB and 0.8 dB. Most FET preamplifiers can manage a noise figure less than 3 dB. Your VHF or UHF receiver will probably have a noise figure between 6 dB and 20 dB.

WHY THE PREAMPLIFIER MUST BE NEAR THE ANTENNA All of the things between the receiver and the antenna have a noise factor. This includes preamplifiers, coax cables, filters, tuners, and your receiver. The formula for overall noise factor demonstrates that the noise factor of the overall chain, from the antenna -> coax tail -> masthead preamplifier > feeder coax -> the receiver, is dominated by the performance of the first device. The gain or loss of each stage affects the noise factor of the following stage to produce an overall noise factor at the receiver. Note that this formula uses noise factor, which can be calculated from the noise figure.

The formula was developed by Harald Friis in 1942. F = Noise Factor of device = 10^(NF of device in dB/10)) G = Gain ratio of the device = (10^(Gain in dB/10)) NF (noise figure in dB) If the ‘gain ratio’ of a device is greater than one, the device is acting as an amplifier. A ‘gain ratio’ of less than one but greater than zero indicates that the device is acting as an attenuator, for example, it may be a loss-making component such as a coaxial cable. Each stage has less effect on the overall noise factor than the preceding stage. F1 is the noise factor of the first device. Add to that the noise factor of the second device -1, divided by the gain ratio of the first device. For the third device, add the noise factor of the third device -1, divided by the gain ratio of the first device multiplied by the gain or loss of the second device. Here is an example of a noise figure calculation with no preamplifier.

The coax feeder has a noise figure of 3 dB, so its noise factor F1 = 10(3/10) =2 The receiver has a noise figure of 6.2 dB, so its noise factor F2 = 10(6.2/10) = 4.17 The coax feeder has a gain of - 3 dB (i.e. a loss of 3 dB). G1 = 10(-3/10) = 0.5 Overall noise factor F = F1 + (F2-1)/G1 = 2.0 + 3.17 / 0.5 = 8.34 and Overall noise figure NF = 10 log (F) = 9.2 dB This means that the overall degradation of the signal quality over the whole chain between the antenna and the receiver (SNR in – SNR out) is 9.2 dB.

Here is an example of a noise figure calculation with a preamplifier

In the second example, we have the same radio and feeder cable with the addition of a masthead preamplifier and a short coax cable tail from the amplifier to the antenna. (As per the picture). The coax tail has a noise figure of 0.5 dB, so its noise factor F1 = 1.12 The preamplifier has a noise figure of 1 dB, so its noise factor F2 = 1.26 The coax feeder has a noise figure of 3 dB, so its noise factor F3 = 2.00 The receiver has a noise figure of 6.2 dB, so its noise figure F4 = 4.17 The coax tail has a gain of – 0.5 dB (i.e. a loss of 0.5 dB). G1= 0.89 The preamplifier has a gain of +20 dB, G2= 100 The coax feeder has a gain of - 3 dB (i.e. a loss of 3 dB). G1= 0.5 Overall noise factor F = F1 + (F2-1)/G1 + (F3-1)/G1*G2 + (F41)/G1*G2*G3 F = 1.49 Overall noise figure NF = 10 log (F) = 1.74 dB This means that the overall degradation of the signal quality over the whole chain between the antenna and the receiver (SNR in – SNR out) is

1.74 dB. Using the preamplifier has added a 16.5 dB (20 -0.5 -3) boost to the signal strength, but much more importantly it has improved the quality of the signal. The signal to noise ratio using the preamplifier is 7.46 dB better than it would be without the preamplifier, (9.2 - 1.74 dB). Placing the preamplifier in the shack instead of the antenna. What would happen if the preamplifier and the short coax cable was at the ‘shack’ end of the feeder? F1 (feeder cable) = 2.00, G1 = 0.5 F2 (preamp) = 1.26, G2 = 100 F3 (coax tail) = 1.12, G3 = 0.89 Overall noise factor F = F1 + (F2-1)/G1 + (F3-1)/G1*G2 +(F41)/G1*G2*G3 F = 2.58 Overall noise figure NF = 10 log (F) = 4.12 dB When we place the preamplifier near the receiver rather than near the antenna, we only get a 5 dB signal to noise improvement (9.2 - 4.12 dB). It is better than not using the preamplifier at all but not as good as placing it at the masthead, before the feeder cable. The boost in signal strength is exactly the same no matter where the preamplifier is placed. In the example above, adding a good 20 dB preamplifier with a 1 dB noise figure improves the overall noise figure from 9.2 dB to 1.74 dB and the signal to noise ratio by 7.46 dB. What was a 3 dB signal to noise ratio degradation due to the feeder loss is turned into a 4.45 dB signal to noise ratio enhancement due to the improved overall noise figure.

PREAMPLIFIER GAIN Don’t use too much preamplifier gain. The masthead preamplifier gain only needs to be enough to make up for the coax loss, plus around 10 dB. Adding more gain has little effect on the Signal to Noise ratio but it increases the risk of intermodulation and overload on strong signals. A rule of thumb is that the preamplifier gain should be set so that the receiver is showing S0 – S1 with the antenna and preamplifier connected but with no received signal. Any more gain than that will result in you listening to background noise and will limit the receiver’s dynamic range on strong signals. Some operators switch in the receiver’s attenuator or add external attenuators at the receiver input, to reduce the background noise level to less than S2 on the signal strength meter. This will reduce the receiver input level, but because the overall noise figure is mostly related to the noise figure of the preamplifier at the masthead, the SNR (signal to noise ratio) will not be badly affected.

PREAMPLIFIER PERFORMANCE CHARTS After a certain point, the signal to noise improvement becomes largely independent of the gain of the preamplifier. But it is always significantly better to place the preamplifier at the masthead, rather than at the other end of the feeder cable near the receiver.

If the preamplifier or the overall noise figure is poor, no amount of gain will result in an improved signal to noise ratio. When the overall noise figure is similar to the noise figure of the receiver on its own there is no improvement in signal to noise ratio no matter how much gain the preamplifier has.

Coax loss after the preamplifier can be made up with preamplifier gain. You need more preamp gain to compensate for a larger value of feeder cable loss. Increasing preamplifier gain past a certain point has little effect on the overall signal to noise improvement but will reduce the receiver's dynamic range and it can add intermodulation distortion in the presence of strong signals.

Coax cable loss before the preamplifier adds directly to the overall noise figure and should be kept as low as possible. Too much loss before the preamplifier can result in your amplifier producing no net improvement in the signal to noise ratio or even a degraded signal to noise ratio, no matter how much gain the amplifier has. Note the radical difference between cable loss before the preamplifier (this diagram) and cable loss after the preamplifier (previous diagram). With cable loss after the preamplifier, you can add more amplifier gain to overcome the cable loss and end up with a signal to noise improvement. With cable loss before the preamplifier, adding more amplifier gain may not result in a signal to noise improvement. You get a stronger received signal but the signal to noise ratio is not improved.

FREE SPACE PATH LOSS Free space path loss is not really a loss at all. As a radio wave travels through space the signal does not lose energy by being converted to heat the way it does in a cable or an attenuator. However, the energy density reduces as the wavefront spreads out due to the according to the ‘inverse square law.’ (See link budgets and formulas sections). The further away from the transmitter, you are, the less signal you will receive. The reduction in signal level due to the distance between the transmitter and the receiver is equivalent to a loss due to an attenuator between the transmitter antenna and the receiver antenna. In other words, a 140 dB free space path loss results in the same received level as having a receiver connected to a transmitter via a 140 dB attenuator.

GOLDEN RULES FOR MASTHEAD PREAMPLIFIERS: 1. If the overall noise figure is lower than the receiver’s noise figure, the signal to noise ratio will be improved. Achieving a better signal to noise ratio is the primary goal. Not getting more received signal strength. 2. If possible use a preamplifier with a low noise figure. But almost any masthead mounted preamplifier will improve your overall noise figure. 3. Coax cable loss after the preamplifier has little effect on performance provided the amplifier gain is higher than the coax loss. 4. Coax cable loss before the preamplifier has a large effect on performance no matter how much gain the preamplifier has. 5. Mount the preamplifier as close as possible to the antenna and use low loss (but flexible) coax and high-quality connectors to connect it to the antenna. The preamplifier does not literally have to be at the top of the mast. A good place is at the end of your main feed line at the point where the main feeder connects to the flexible tail going up to the antenna. 6. Preamplifier gain should be kept as low as possible to avoid overloading on strong signals and unnecessary receiver background noise. If you have too much gain, use the attenuator inside your receiver (if it has one). You can use a fixed attenuator at the antenna port on your receiver but it would have to be removed before you transmit on that antenna port. 7. Don’t use a wideband preamplifier unless you need a wideband preamplifier. For LEO satellite stations, buy separate amplifiers for the 2m band and the 70cm band. Wideband amplifiers could be overloaded by strong signals that are nowhere near your band of operation. 8. The FET devices in masthead preamplifiers are very easily damaged. Even if you are not intending to transmit on the feeder coax, buy a preamplifier with bypass relays that bypass the amp when the DC supply is off. That way the amp will have some protection from static discharge during lightning storms and signals coupled from your transmit antenna. 9. If you want to transmit and receive on the same antenna, the preamplifier must have bypass relays. These can be switched by sensing the RF from your transmitter, or via a DC control voltage usually over the coax cable.

ANTENNA SYSTEMS Loads of antennas are suitable for satellite operation and reception, but some are better than others. Of course, your antennas must fit the band you are operating. For most LEO satellites that means, you need an antenna for the 2m amateur radio band and another for the 70cm amateur band. For reception of pictures from weather satellites, you need an antenna for 137 MHz. For TV or other high orbit satellites, you need a dish or patch antenna. But, you can receive signals from the GPS satellites with tiny patch antennas less than the size of a postage stamp. This chapter discusses some of the antennas commonly used for satellite ground station, reception and transmission. Note that I say antennas, not antennae. Antennae are the things that wave about on the top of the heads of insects, crustaceans, and Martians on 1960s science fiction shows. They are not good for receiving signals from satellites. OK, maybe the Martian ones are… but not generally.

POOR SATELLITE GROUND STATION ANTENNAS A ‘rubber duck’ is the type of antenna that you got with your handheld FM transceiver. They are usually a helical design. Rubber duck antennas are a compromise solution. They are physically short and they have to provide a safe RF load for the transmitter over a very wide range of frequencies. They usually have less gain than a dipole antenna. Not enough for reliable satellite work. A quarter wave vertical is a dipole antenna with one-half of the dipole being provided by the ground plane beneath the antenna. They are the type of antenna that you used to see mounted in the middle of a car roof. A quarter wave antenna can be used for satellite reception, but by definition, they only have the same 2.14 dBi gain as a dipole antenna. Still not enough for reliable satellite work. The same applies to vertical half wave dipoles and vertical folded dipole antennas. All vertical antennas have a null in their radiation pattern, directly overhead, end on to the dipole, which dramatically reduces the level of the received signal when the satellite is at very high elevations. You may still be able to get adequate reception because the signal from the satellite is much stronger when it is at minimum range. A ‘J-pole’ antenna is dual band vertical dipole. Because they are simple to make and you can access two bands using one feeder coax and antenna, they are very popular as a fixed antenna for accessing local repeaters. They work as well as a vertical quarter wave antenna but have the same lack of performance for satellite operation. A 5/8th wavelength vertical antenna has a little bit more gain by virtue of its longer length. Its radiation pattern is a little squashed at the top making the antenna work better than a quarter-wave antenna at low elevations and worse than a quarter-wave antenna at high elevations. It is not a very suitable antenna for satellite reception. The same is true of the collinear antenna. A collinear can be made for a single or dual band and they are great for working repeaters and even DX on the 2m and 70cm bands. They are made up of several vertical dipoles arranged as a phased array. They have more gain than a dipole antenna or the 5/8th whip. The amount of gain depends on how many dipoles are stacked inside. But the increased gain at low elevations is offset by the loss of gain at high elevations.

Discone antennas are commonly used for ‘scanners.’ They have an extremely wide (10:1) frequency bandwidth with a reasonable SWR over the whole range. The antenna is vertically polarized and it has about the same gain as a dipole. It has an omnidirectional radiation pattern with most of the pattern at low elevation. All of the above antennas are ‘omnidirectional’ designs. If they are mounted vertically, they radiate signals to and receive from all horizontal directions equally. They all have a null at the end of the antenna, which causes a pronounced dip in signal strength when the satellite is at high elevations. The more gain an omnidirectional antenna has at low elevations, the less gain it has at high elevations and the worse it is for satellite work. The last type of unsuitable antenna is large antenna arrays designed for EME operation. Unlike the others, the problem with EME arrays is too much gain. There is a good chance of overloading the satellite with your transmitter power, especially if you are using a linear transponder. High gain antennas have narrow beam angles and that makes it difficult to track fast-moving LEO satellites. You need good rotators because you are moving a large mass relatively quickly and the pointing accuracy needs to be high.

WAVELENGTH AND FREQUENCY We usually talk about the frequency of the radio waves in MHz, or GHz. But some calculations use the wavelength of the signal. Antenna lengths and widths are usually multiples or sub-multiples of the wavelength that they are designed for and the bands that we use for satellite operation are commonly referred to in terms of their nominal wavelength. Wavelength is represented by the Greek symbol Lambda ( λ ) All radio signals are based on a sine wave at the frequency of operation. We modify the phase, frequency, and/or the amplitude of the ‘carrier’ sine wave to add modulation (intelligence) to the signal. Without modulation, we can’t transmit any information over the radio path. The radio wave travels out from the transmit antenna at the speed of light. The wavelength is the distance between the peaks (or troughs) of the radio frequency sine wave while it is traveling at the speed of light (in a vacuum). The formula to convert a frequency into a wavelength and vice versa is easy. Frequency F = C / λ Wavelength λ = C / F λ = Wavelength in meters F = Frequency in MHz

C= 300 = approximate speed of light in millions of meters per second

High frequencies have short wavelengths. Lower frequencies have longer wavelengths. That is why an antenna for the 80-meter ham band is so much bigger than a UHF aerial on your car. The wavelength for a frequency of 145 MHz is about 2 meters. 145 MHz is in the 2-meter amateur radio band. A ‘half wave’ dipole antenna for use on 145 MHz is about 1 meter long, which is half of the wavelength of the radio signal. The wavelength for a frequency of 10 GHz is about 3 centimeters. A dipole antenna used to feed a dish at 10 GHz has a length of about 1.5 centimeters, which is half of the wavelength of the 10 GHz radio signal. The amateur radio band at 10 GHz is called the ‘3-centimeter’ band.

ANTENNA GAIN Antenna gain is a bit like free space path loss. It is not really a gain at all. It is more like a perception that the signal is being amplified. Normally when we talk about gain, we mean the increase in level that we get from an amplifier. But an antenna has no transistors and it is not connected to a power supply. An antenna does not amplify the signal. It just concentrates the energy that is being radiated, into a particular direction. The effect is reciprocal. An antenna that transmits more power in a given direction will also receive signals more strongly from that direction. Do you remember the mythical isotropic reference antenna we used for the link budget calculations? The isotropic antenna radiates (or receives) signals equally well in all directions. It has no gain in any direction. We can’t build a real isotropic antenna. The most basic antenna that we can build is a dipole antenna. The length of a dipole antenna is ‘electrically’ half of the wavelength that the antenna is made for. A dipole has 2.14dBi gain in directions broadside to the dipole. Almost no signal is radiated off the ends. Consider a small electric torch bulb running off a three-volt battery. It is like the isotropic antenna casting light in all directions. Place the same bulb into a torch with a dish-shaped reflector behind it and a much stronger beam of light is created in the wanted direction with almost no light being radiated in any other direction. The bulb is emitting the same amount of light but the reflector concentrates it into a single direction. The effect is the same as adding a dish to reflect the signal from a microwave antenna. Or adding a reflector and director elements to a dipole to create a Yagi antenna. The gain of an antenna is the increase in signal in the direction getting most of the radiated power. But, you don’t get “something for nothing.” The more gain an antenna has in any given direction, the less energy is radiated in the other directions. Sometimes you want the antenna to radiate all around and in other situations, you want to send your signal to or receive signals from, a particular direction. An FM broadcast station wants to reach a lot of people over a wide area, so the engineers use omnidirectional collinear antennas, which send most of the transmitter power out at low elevations, where people live, and don’t waste energy sending signals into the sky. The omnidirectional factor means that the antenna radiates equally in all horizontal directions.

For satellite work, we want to send as much signal as possible to the satellite and to receive signals only from the satellite. We want an antenna that concentrates the signal into a narrow beam that we can point directly at the satellite. High gain antennas are larger than low gain antennas made for the same frequency. Unfortunately, big antennas are more difficult to manage, if you need to track the satellite across the sky. High gain antennas have a smaller beam angle. They concentrate the radiated power into a narrower beam. The downside is that they need to be pointed more accurately. Antenna gain is reciprocal. All antennas have the same gain and directivity when receiving signals as they do when used for transmitting signals, assuming the same operating frequency.

RADIATION PATTERN A dipole does not radiate (or receive) signals equally in all directions. It has a radiation pattern that looks like a round doughnut with the antenna sticking up through the hole. This means that there will be more signal radiated (or received) broadside to the dipole antenna than at the ends of the dipole element (where the doughnut hole is). The radiated energy has been concentrated in the broadside direction at the expense of energy radiated from the top or bottom of the dipole element. If you happen to be in the broadside direction, you will receive more signal that you would from an isotropic antenna, because the dipole has gain in that direction. If you are ‘end on’ to the dipole, you get less signal than you would from

an isotropic antenna because the dipole has no gain in that direction.

When the vertical dipole antenna is close to the ground, reflection off the ground causes the radiation pattern to change. The maximum radiation is at a higher elevation. If you mount the dipole antenna horizontally, the radiation pattern rotates through 90 degrees. Looking down from above, the radiation pattern looks like a figure eight. It is like cutting the doughnut in half and placing it cut side down on the ground.

As shown in the drawing above, a Yagi antenna concentrates most of the energy into a single direction. Typically, there are small ‘side lobes’ but unlike a dipole antenna which sends energy equally in the forward direction, (towards the station of interest) and backward, (away from the station of interest), most of the transmitted power is in the ‘forward’ direction. The radiation pattern (but not the polarization) is usually pretty similar no matter whether the Yagi is mounted vertically or horizontally. The radiation pattern from a dish antenna with more gain than the Yagi would show a narrower forward beam and smaller side lobes.

FRONT TO BACK RATIO The ratio of power radiated in the forward direction to the power radiated in the reverse direction is called ‘front to back ratio.’ Having a high front to back ratio is better because it means that you are wasting less power radiating signals in the wrong direction. Antennas with a high front to back ratio receive more signal from the wanted direction as well, so there is less chance of picking up interference signals from other directions.

HANDHELD ANTENNAS FOR LEO SATELLITE OPERATION Handheld antennas give just enough extra gain to make working FM satellites easy and reliable. They need to be very light and small enough to be pointed by hand in the general direction of the satellite as it passes over. Larger antennas are too unwieldy and heavy to hold for the duration of a ten to fifteen-minute satellite pass and the narrower beamwidth makes keeping the antenna pointed at the satellite more difficult. You can use a handheld antenna for working FM or digipeater satellites. It is more difficult to work a satellite with a linear transponder because you need accurate Doppler control as well as pointing the antenna. It can be done if you have PC control of the receiver and transmitter frequencies. It is amazing how you quickly adapt to tracking the satellite’s motion by listening to the background noise on the downlink signal. If you want to transmit as well as receive the LEO satellites, you need an antenna system that works on both the 2m and 70cm amateur bands. The most commonly used handheld antennas for working LEO satellites are the Elk and the Arrow. The Elk antenna is a 5-element log periodic optimized for dual band operation. It has the advantage of not needing a diplexer to combine the two bands onto a single coax feeder cable. Receiving signals on the 70cm band while transmitting on the 2m band (full duplex operation) is probably not possible because the antenna provides almost no isolation between the two bands. This is not a big problem because you don’t really need full duplex for FM or digipeater operation and handheld operation through linear transponders is pretty tricky anyway. The Arrow antenna is a three or four element 2m Yagi with a six to ten element 70cm Yagi mounted on the same beam. When the 70cm antenna is held with vertical polarization, the 2m Yagi is horizontal. The two Yagis are combined into a single coax feeder cable using a diplexer. The 90degree difference in polarization should provide enough isolation between the bands to use full duplex mode. Arrow and Elk are brand names for commercially available antennas. They both work very well. Alternatively, building a handheld antenna is a cheap and easy ‘homebrew’ project. You can build them using cheap wire tacked onto a broomstick or even make the elements by cutting up a steel tape measure. There is plenty of project information on the Internet.

DIPLEXER OR DUPLEXER? A diplexer is a coupler that combines or separates signals of different frequencies. A single antenna could be split to serve 2m and 70cm antenna ports on the radio, for example on a Yaesu FT-847 transceiver. Or it could be used to connect an antenna to two radios operating on different frequencies. Alternatively, a diplexer can be used to combine antennas on different bands onto a single coax feeder cable. A duplexer is a device that couples a transmitter and receiver on the same frequency or band to the antenna while producing isolation between the transmitter and the receiver. They can also be used to couple a single antenna to multiple receivers, or multiple transmitters to a single antenna. A diplexer is a coupler for different bands. A duplexer is a coupler for the same band.

ANTENNA POLARIZATION The way that your antennas are oriented while you are using them is quite important. It should be a major design consideration for your satellite ground station. A dipole or Yagi antenna, which is oriented so that the elements are horizontal, transmits a radio wave with horizontal linear polarization. If you change the antenna so that the elements are vertical, the antenna transmits a radio wave with vertical linear polarization. Fixed antennas like the collinear, Discone, ¼ wave and 5/8th wave dipoles are mounted vertical and have vertical polarization. An antenna is a device that converts radio frequency energy into electromagnetic waves that are radiated into space. These ‘radio waves’ have an electric field known as the ‘E’ plane and a magnetic field known as the ‘H’ plane. The ‘H’ plane is always aligned at 90 degrees to the ‘E’ plane. The ‘E’ plane determines the polarization of the transmitted radio wave. If the antenna is horizontally polarized, the ‘E’ plane is horizontal and the ‘H’ plane is vertical. If the antenna is vertically polarized, the ‘E’ plane is vertical and the ‘H’ plane is horizontal. In the case of circular polarization, the ‘E’ and ‘H’ planes stay at 90 degrees to each other and the combined wave rotates as the wave travels. The polarization rotates 360 degrees in the time that it takes the radio wave to move away from the antenna a distance of one wavelength. Let’s pretend that your hand is an antenna. Hold out your arm straight in front of you, palm downwards. That is horizontal linear polarization. Palm up is the same. It is still horizontal polarization. Rotate your wrist so that your thumb is up, and your hand is vertical. Now your hand antenna has vertical polarization. If your hand could spin continuously clockwise, “don’t try this at home,” your antenna would be circularly polarized with right hand circular polarization. Spin your hand in the opposite direction and the radio wave will be circularly polarized with left-hand circular polarization. A linear polarized antenna radiates RF energy in one plane containing the direction of propagation. Unless it is twisted by Faraday rotation or reflected off something like the Ionosphere for HF signals or perhaps the Moon, the signal will stay in the same plane as it travels through space to the receive antenna. If the receive antenna is oriented the same way it will pick up the signal. If the receive antenna is ‘cross-polarized,’ i.e. not

oriented the same as the transmit antenna, the received signal will be significantly reduced. Mathematically, cross polarization loss is infinite. But in the real world, the signal reduction is about 25 dB. As the receive antenna is rotated closer to the angle of the transmit antenna, the loss due to cross polarization decreases quite rapidly, reducing to 3 dB when the receive antenna is at a 45-degree angle compared to the transmit antenna and to 0 dB when both antennas are aligned to the same polarization. Some FM broadcast stations exploit this property and arrange their antennas to a 45-degree polarization. That way, antennas fixed to cars, which are nominally vertical and antennas on portable radios and houses, which are nominally horizontal can both pick up the signal from the FM station. Satellite operators such as yourself can use cross polarization loss to advantage as well. Most LEO satellites use either UV mode or VU mode. The VU mode poses a problem if you want to use full duplex operation because the 3rd harmonic of the 145 MHz uplink falls into the 70cm downlink band and this can overload your receiver while you transmit. By aligning your 2m antenna to one polarization and your 70cm antenna to the opposite polarization, the cross polarization loss helps to protect your receiver from receiving too much of the 2m band transmission. It does not matter if your transmitting antenna is vertical, horizontal, or at 45 degrees, you will get maximum isolation by arranging the receive antenna with a 90-degree difference in polarization.

With circular polarized antennas, the plane of polarization rotates as the radio wave travels through space, making one complete revolution during each period of the wave. If the rotation is clockwise looking in the direction of propagation, the antenna is using right hand circular polarization (RHCP). If the rotation is counterclockwise, the polarization is called lefthand circular polarization (LHCP). Unless it is reflected off something, a

signal transmitted with circular polarization will arrive at the receiver with the same polarization. The receive antenna needs to be either linear or circular polarized. An antenna with opposite circular polarization will suffer significant cross polarization loss. A linear polarized receive antenna, either vertical or horizontal, will receive 3 dB less signal than it would from a linear antenna with matching polarization at the same distance.

Faraday rotation speeds up or slows down the rate that the polarization rotates, so unlike linear polarization, it has no noticeable effect on the reception of the circular polarized signal. Circular polarization is created by the design of the antenna. It is usually achieved by feeding one driven element 90 degrees out of phase with another. It is not caused by the satellite spinning or tumbling in space. The rate that the circular polarization rotates is very fast. It is directly related to the wavelength of the transmission. For example, 145 MHz is a frequency of 145,000,000 cycles per second. During each cycle, the wave travels one wavelength (about 2 meters) away from the antenna and the polarization completes a complete rotation. At a frequency of 145,000,000 Hz, the polarization rotates at a rate of 145,000,000 revolutions per second. It takes the radio wave 6.89655 nanoseconds to travel a distance of 2.067 meters (one wavelength) and for the polarization to complete one rotation.

WHAT ANTENNA POLARIZATION SHOULD I USE? There are two types of antenna polarization, linear and circular. Linear polarization can be vertical or horizontal, or something in between. The radiated signal stays fixed in a linear plane. Circular polarization can be right handed or left handed, RCHP or LHCP. Which should we use? Most TV satellites use both horizontal and vertical linear polarization. They transmit different TV channels on each, relying on the cross polarization loss to enable the receiving station to separate the signals. The LNB on your dish at home will be mounted for either vertical or horizontal polarization depending on the TV service that you subscribe to. A house down the street may pay a different provider and their TV dish may be looking at the same satellite, but getting a different TV service. Or you may have a dual polarization LNB on your satellite dish so you can receive twice as many channels from the same satellite. Some satellites transmit on both RHCP and LHCP, right hand circular polarization and left-hand circular polarization. The theory is exactly the same except that the LNB must be either RHCP or LHCP. You can’t turn a circular polarized LNB or antenna ninety degrees to change its polarization, the way you can with a linear polarized device. You can’t use a linear polarized LNB to receive signals from a satellite that is sending circularly polarized transmissions because it would receive a mixture of both signals. The standard for amateur radio satellites in HEO orbits is to use RHCP (right hand circular polarization). But there are no Hamsats in HEO at present. GPS satellites transmit RHCP (right hand circular polarization). You can use either a linear polarized antenna or a RHCP antenna to receive GPS signals. A linear polarized antenna can be in any orientation and the signal that you receive will be 3 dB less than if you use a RHCP antenna. You can’t use an antenna with LHCP (left-hand circular polarization) because the cross polarization loss would be 25 to 30 dB. Small satellites in low Earth orbits tend to have rudimentary antennas. Often quarter wave linear polarized dipoles made from steel tape measure or spring wire. These can be rolled up or wound around the satellite until the satellite is released from the launch vehicle. They spring out straight after the satellite is released from the pod. Some LEO satellites are stabilized so that the antennas point down to Earth but

others spin and tumble as they orbit. In either case, even if by chance the polarization of the satellite transmission was the same as your receive antenna when the satellite rises above your horizon it will change as the satellite rises to peak elevation and then descends back to the horizon. If you are using a handheld antenna you may be able to compensate by turning the antenna to change its polarization as the satellite passes over, but if you are using larger or fixed antennas there will be times when the polarization matches and times when it does not. When the polarization mismatch is close to 90 degrees, you will experience a deep fade in the signal strength. It will only last a few minutes because the polarization and the satellite’s location and orientation change continuously. These fades in the signal strength are very common and a great annoyance. The situation can be improved by the use of a low noise masthead preamplifier or by using a circular polarized or crossed Yagi antenna. A circular polarized antenna will give you 3 dB less signal than a correctly aligned linear polarized antenna, but it will never get 90 degrees out of phase with the linear antenna on the satellite. You won’t experience the deep nulls and signal dropouts. A cross-polarized antenna is really two separate antennas, one vertical and the other horizontal. You can switch to whichever antenna has the best signal. When you experience a null due to cross polarization loss, you simply switch to the other antenna. To add even more confusion, the radio signals traveling through the atmosphere towards the satellite and the downlink signals traveling back through the atmosphere from the satellite experience Faraday rotation. Faraday rotation is not a big issue at microwave frequencies but, at VHF and UHF, it can cause the polarization to change unpredictably. Combined with the satellite’s random orientation, it is impossible to predict the received signal’s polarization, or whether you will experience fading. All of these factors affect your uplink signal to the satellite as well as the downlink signal. It can cause the signal to drop and you may lose a conversation. It is all a part of the fun! If you are using a handheld antenna, you can compensate for cross polarization fading by twisting the antenna boom to improve the signal. Stations using fixed linear polarized antennas or linear polarized antennas on a rotator may experience signal fades during satellite passes. The fading won’t be a problem if the received signal is strong. For example, you may have a high gain antenna or a

masthead preamplifier, or the satellite might be at a high elevation when the cross polarization happens. The fading will be a problem if the received signal is weak. The best solution is to install either, a circular polarized or switchable cross-polarized antenna. A masthead preamplifier can also help. Many people just put up with the fading rather than go to the trouble and expense of installing circular polarized antennas. Cross-polarized antennas are better for EME operation than circular polarized antennas. This is partly because most operators transmit linear polarized signals towards the Moon. A circular polarized antenna would impose a 3 dB signal strength penalty that you can’t afford with extremely weak signal operation. Another problem is that if you do transmit a circularly polarized signal to the Moon, the reflected signal will have the opposite circularity. If you transmit RHCP, you will receive LHCP. That means that you would need a separate antenna or feed arrangement for receiving even though the frequency is the same. In the real world, things are a bit different. At VHF and UHF frequencies, the reversal of circular polarization does happen and it is the reason that EME operators do not use circular polarization on the 6 meter, 2 meter, or 70cm bands. At microwave frequencies above 1 GHz, the Moon ceases to look like a flat reflector. Surface curvature and details such as mountains and craters scatter the reflected signal, breaking up the polarization of the returned signal. At these frequencies, it makes no difference whether you transmit RHCP, LHCP, or linear polarization, the returned signal is the same. It can be received equally well with a linear polarized or circularly polarized receive antenna. There is no longer a 3 dB penalty due to the difference between circular and linear polarization. It is replaced with an approximately 3 dB penalty in signal compared to what you would get if the Moon was a flat reflector.

FIXED ANTENNAS I have already mentioned that some popular fixed antennas that are not very good for satellite operation. The features that make them good for reaching distant terrestrial signals make them less suitable for satellites. For satellite operation, you want a fixed antenna that is high enough to have an unimpeded view of a typical satellite pass from horizon to horizon. It is better if it does not have a large null in the radiation pattern at high altitudes and even better if it has circular polarization. Fixed antennas are usually omnidirectional in the horizontal plane. They need to cover the complete satellite pass without being moved. A fixed Yagi antenna would not be any good because the satellite would move out of its coverage beam. Unfortunately, omnidirectional antennas don’t have very much gain. They usually have about the same gain as a dipole antenna. This means that although you may be able to work some of the LEO satellites, you will be struggling. A masthead preamplifier would be a very good investment. You can use fixed antennas for the LEO satellites. But most amateur radio operators who intend to transmit to them prefer to use rotatable or handheld directional antennas with more gain. However, fixed circular polarized antennas are very popular for reception of pictures on 137 MHz, from the polar weather satellites. The weather satellites transmit strong signals and you probably will not need a masthead preamplifier. You will probably get better pictures from a fixed circular polarized antenna than you will from a tracking linear polarized Yagi antenna. With a linear polarized antenna, the signal dropouts experienced during cross polarization nulls causes noise bars across the pictures. Circular polarized fixed antennas that are optimized for receiving signals from satellites include the Turnstile, Moxon Pair, Lindenblad, and Eggbeater. The ‘Turnstile antenna’ is a pair of crossed dipoles mounted horizontally, usually with reflectors or a larger ground plane, below the two driven elements. The reflector or ground plane makes each antenna a two element Yagi giving the turnstile a more upwardly focused radiation pattern and a small amount of gain. A two element Yagi has a pretty wide beam angle and the ground underneath it pushes the radiation pattern up a bit. In the vertical plane, it has a null at a very low elevation and is then

pretty consistent from about 20 degrees through 90 degrees elevation. In the horizontal plane, each of the driven elements exhibits a typical dipole figure eight pattern and the combination creates a cloverleaf pattern with deep nulls between the nodes. This means that the performance of the Turnstile antenna is largely unaffected by the elevation of the satellite, but it is affected by the azimuth. One driven element is fed from the feeder coax cable with the other fed via ¼ wavelength of coax. The ¼ wavelength-phasing line delays the signal to the antenna that is physically at 90 degrees, by 90 degrees so that the radiation is circular polarized. Turnstile antennas are very easy to build, but they don’t perform as well as some of the other fixed antenna options. The nulls in the horizontal radiation pattern affect the performance even though the antenna has circular polarization. ‘L.B.’ Cebik W4RNL (SK)1 tested and wrote about hundreds of antenna designs. His huge legacy of reports and more than 1000 antenna modeling data files are now maintained by who have made them available on CD. Note 1: SK stands for ‘silent key’. Amateur radio shorthand for a deceased amateur radio operator. These reports are a ‘must read’ for anyone interested in comparing the merits of alternative antenna designs. The documents include a lot of research about how the radiation patterns change when you add more elements or mount the antenna at different heights. The CDs are available from LB’s old website at The ARRL Satellite Handbook includes an article by L.B. Cebik about an improvement to the Turnstile antenna design, which substitutes the two element Yagis with crossed Moxon Rectangle antennas. This design is much better looking than the Turnstile antenna, it works better and it is still easy to build. A Moxon Rectangle is a two element Yagi with the tips of the driven element bent towards the reflector and the tips of the reflector are bent towards the driven element to form a rectangle. The antenna works as well as a standard two-element Yagi but it is smaller. Arranging two upward pointing Moxons at 90 degrees to each other and feeding them with a phasing line the same as the Turnstile, creates a circular polarized antenna. The Moxon Pair has less gain than the Turnstile, which gives it a radiation pattern with fewer deep nulls and a wider beam angle, which is preferable for a fixed satellite antenna. I have not tried either the

Turnstile or the dual Moxon versions but it seems that the Moxon Pair is a better option.

An eggbeater antenna has is a crossed loop with a reflector below to push the radiation pattern a little higher. There are eggbeater variants such as the Eggbeater II and the Texas Potato Masher. The Eggbeater II is an improved omnidirectional antenna. The Texas Potato Masher is slightly directional. It is more commonly used with an azimuth, but no elevation, rotator. It has square or rectangular loops rather than the round loops on the eggbeater antenna.

The Lindenblad antenna is a phased array of four dipole antennas. The standard Lindenblad is quite difficult to construct as it has open wire feeders made from tubing connecting each of the four driven elements. The ARRL Satellite Handbook contains an article by Anthony Monteiro AA2TX (SK), which describes how to build an easy version called the EZLindenblad. His version uses 75-ohm coax for the phasing lines and deliberately cuts the four dipoles short in order to force their impedance up to 200 ohms. Combining the four antennas together in parallel, you get a 50-ohm feed point impedance. The dipoles are mounted at 45 degrees around a square and this creates the circular polarization. Use of four dipoles rather than two creates a more omnidirectional horizontal radiation pattern. The vertical radiation pattern is like a dipole with a null at zero degrees elevation and pretty much the same power from ten degrees up to 90 degrees and back down to ten again. A Lindenblad basically has the same amount of gain as a dipole but with the advantage of circular polarization. They work better than the Turnstile antenna but not as well as a Quadrifilar Helix.

HELICAL /HELIX ANTENNA The Helix antenna is also known as a Helical antenna. The wire element is evenly wound in a spiral. They look a bit like the spring inside a ballpoint pen. Helix antennas can operate in two different ways depending on the relationship between the wavelength of the operating frequency and the diameter of the spiral. If the wavelength is much larger than the diameter of the helix, the antenna works in ‘normal’ mode. It radiates broadside to the antenna with a similar pattern to a ¼ wave dipole. The ‘rubber duck’ antenna on your handheld radio is a helical working in ‘normal’ mode. For satellite operation, we use the Helix in ‘axial mode.’ The diameter of the helix is about the same as the wavelength. In the axial mode, the antenna functions as a directional antenna radiating a circularly polarized beam off the ends of the helix along the antenna's axis. The helix is normally constructed with a ground plane at the feeder end to concentrate the signal into the wanted direction. The longer the Helix is the more gain it has.

THE QUADRIFILAR HELIX ANTENNA. The Quadrifilar Helix is a circular polarized antenna favored by weather satellite enthusiasts. They are quite tricky to build but they outperform the Turnstile and eggbeater antennas. You can nest a Quadrifilar Helix antenna for a higher band inside one for a lower band. For example, a 145 MHz antenna inside a 137 MHz weather satellite antenna. The antenna is four half turn axial mode helix antennas fed with one pair in phase and one pair 90 degrees out of phase. Each of the four helices is about half a wavelength long. The antenna is usually constructed in the form of two twisted loops fed 90 degrees out of phase to create circular polarization. There are two methods of phasing the loops. One method involves using a coax cable to form one of the four helices and using it to feed one of the loops from the top of the antenna. The other method makes the connections to both loops at the bottom. One loop is made shorter to resonate slightly above the design frequency and the other slightly longer so that it resonates below the design frequency. The different lengths make one helix loop inductive and the other loop capacitive at the operating frequency. This introduces +45 and –45 degree phase changes so that one loop is fed 90 degrees out of phase with the other creating circular polarization. Quadrifilar Helix antennas are usually mounted as a fixed antenna pointing upwards, but they have some gain so they can also be used as a directional antenna. A half turn, half wavelength Quadrifilar Helix has a gain of about 5 dBi and a beam angle of 115 degrees. So they are nowhere near as directional as a Yagi antenna. Because the helices are working in the axial mode, the antenna radiates off the end of the helix not broadside to it.

STACKING YAGI ANTENNAS A single Yagi antenna has more than enough gain for listening to or transmitting through LEO satellites. The more distant high orbit satellites usually use frequencies in the microwave range, where dish antennas are the norm. However, stacked Yagi antennas are often used for EME operation. Using two identical antennas spaced apart by the correct amount doubles the ‘aperture’ or capture area of the antenna system. So it is not a surprise that adding a second identical antenna gives you a theoretical increase in gain of 3 dB. In practice, you lose some efficiency due to the proximity of the second antenna and a little additional loss in the coax cable coupling harness. The real gain from using a stacked array is usually closer to 2.8 dB. A stacked pair of Yagi antennas has a narrower beamwidth than a single antenna, but only in the same plane as the stack. If you place two antennas side by side, the horizontal beamwidth is reduced. If you the place two antennas one above the other, the vertical beamwidth is reduced. If you stack four antennas into a square, you compress both the horizontal and vertical beamwidth. You could use a longer single Yagi to achieve the same 2.8 dB increase in gain, but it would need to be double the length of the original Yagi. In that case, you don’t need a matching harness, but the coax feed point of the antenna is further away from the mast, so the saving in power is minimal. As well as narrower beamwidth in the same plane as the stack, a stacked pair of antennas has a different radiation pattern. If the stack is vertical, one antenna mounted above the other, the elevation of the major lobe will be higher from the bottom antenna than from the higher antenna. The stacked combination is likely to have several major lobes with deep nulls in between. This can be a benefit or a curse for the EME operator. It means that as the Moon rises the signals will peak several times as they pass through the antenna lobes. You may be able to work EME stations that you could not with a single Yagi. But because the lobes are narrow, you could lose a contact halfway through. Some operators add a switch so that they can select the bottom antenna, the top one, or the stacked pair. As the Moon rises, it gives them the choice of the higher antenna with a low elevation main lobe, the stacked pair, or the lower antenna with a higher main lobe. The antennas should not be mounted too close together or they will affect each other. Conversely, placing the antennas too far apart makes them

even more difficult to rotate and manage. It also tends to increase the power going into side lobes, which decreases the gain in the wanted direction. The rule for short Yagi antennas is that the stacking distance should be 2/3 of the boom length. For long Yagi antennas, it is more usual to use the formula below. DL6WU created a formula for an appropriate stacking separation for the sort of antennas usually used for EME i.e. 10 elements or longer with a boom length of two wavelengths or longer. D = W / (2 * sin(B/2)) In Excel format D =W/(2*SIN(RADIANS(B)/2)) where D = the stacking distance, vertical or horizontal W = wavelength, in same units as D (usually meters) B = beamwidth between -3dB points. Use the vertical beamwidth for vertical stacking and the horizontal beam width for horizontal stacking. The formula can also be used for four antennas arranged in a square. For a pair of 10 element Yagi antennas on 145 MHz, the stacking distance is about 2.95 meters.

STACKING ANTENNAS FOR DIFFERENT BANDS Satellite operators often mount two or more antennas for different frequencies on the same boom so that a single rotator can point both (or all) of them at the satellite. Ideally, you would place the antennas far enough apart that they would be outside of the capture area of the other antenna. But that is neither practical nor really necessary. If you have an azimuth rotator but no elevation rotator, it is most likely that you will place your higher band antenna, for example, the 70 cm band, above a horizontal antenna for a lower band, say for example the 2m band. The longer wavelength lower band antenna will not be particularly affected by the higher frequency antenna. But the high-frequency antenna will “see” a reflector below it. This will have the effect of pushing the main transmission lobe up to a higher elevation, the same as mounting an antenna close to the ground does. Also, there is a strong possibility when transmitting on the 2m band that some of your transmitted power will be coupled from the 2m antenna into the 70 cm antenna connected to your receiver. This is because there is a harmonic relationship between the bands. The third harmonic of the 2m transmission will fall within the receiver band-pass filtering of the 70 cm receiver. The problem can be somewhat improved by mounting the 70 cm antenna for vertical polarization, maximizing the cross polarization loss between the antennas. The minimum stacking distance for antennas of different frequencies is one half of the stacking separation between two of the low-frequency antennas. For example, the stacking distance for two 2m antennas is 2.95 meters so your 70cm antenna should be at least 1.47 meters above the 2m Yagi. The high band antenna is inside the capture area of the low band antenna. This is a compromise but acceptable. No part of the low band antenna should be inside the capture area of the high band antenna. The stacking distance for antennas of different frequencies must also be greater than the stacking separation between two of the high-frequency antennas. Both rules apply! Use whichever separation distance is the greater. ½ of 2m antenna stacking separation is 1.47 meters. 1 x the 70cm stacking distance at 435 MHz is 0.98 meters. The 70cm antenna should be at least 1.47 meters above the 2m Yagi.

If you do have an elevation rotator, it is common to use a cross beam and mount a 2m antenna on one side of the rotator and a 70cm antenna on the other side. It is best if the crossbeam is non-metallic and the spacing between the antennas should be at least 1.47 meters. If you are using linear, i.e. not circular, polarization, it is best to mount one antenna with vertical polarization and the other with horizontal polarization. Most people mount the 2m antenna horizontal and the 70cm antenna vertical.

CIRCULAR POLARIZATION OR CROSSED YAGIS? EME operators use crossed Yagi antennas and switch to whichever polarization has the best signal. You can do the same when you are receiving a satellite downlink. Circular polarization doesn’t work very well for EME at VHF and UHF frequencies because reflected signal from the Moon has the opposite polarization to the transmitted signal. Also, there would be a 3 dB cross polarization loss between linear transmitted signals, and the circularly polarized receive antenna. However, circular polarized antennas are great for working satellites. Even though there will be a 3 dB signal strength penalty between the linear antenna on the satellite and the circular polarized antenna on the ground, you don’t experience the deep nulls caused by linear cross polarization as the polarization of the satellite’s antennas changes with respect to your position. Overall you will be able to hear a better signal, for more of the time.

EIRP Effective isotropic radiated power (EIRP) is the actual RF power being radiated by the antenna in the direction of the main beam. It is the sum of the transmit power going into the antenna (in dBm) and the antenna gain in dBi. Since it is a power level, the result is measured in dBm. EIRP is often specified by radio licensing authorities because it is the actual power that is being radiated from the antenna system. If they licensed a particular amount of transmitter power, you could use a very high gain antenna and achieve a very large amount of radiated power in the direction that the antenna is pointing. By defining the allowed amount of radiated power, the radio licensing authority can perform calculations to determine the likelihood of the transmission interfering with other radio services. The site operator can use any combination of transmitter power, feed line loss, and antenna gain, to achieve the mandated EIRP amount. EIRP may used to govern the licensed power for fixed link ‘point to point’ radio systems or your local ham radio repeater. EIRP (dBm) = Transmit power dBm + Antenna gain dBi For example, transmitting 1 watt of power (+30 dBm) into an antenna with 30 dBi of gain results in an EIRP of +60 dBm (1000 watts of effective power). Alternatively, transmitting 100 watts of power (+50 dBm) into an antenna with 10 dBi of gain also results in an EIRP of +60 dBm (1000 watts of effective power).

PATCH ANTENNAS Patch antennas are very commonly used for short low power microwave links of the type used to extend WiFi service or Ethernet LANs. They are small, lightweight, and flat which makes them easy to mount. They can be and often are mounted in arrays for extra gain. A patch antenna consists of a usually square ‘patch’ of metal mounted over a larger piece of metal which acts as a ground plane. The space between the plates can be air or any non-conductor at the RF frequency in use. The dimensions determine the frequency of operation and the spacing determines the bandwidth. Placement of the feed point on the plate determines the resonant impedance and the polarization. The feed can be achieved using a pin soldered through the radiating plate and passing through a hole in the ground plane layer, or in the case of a printed circuit board antenna using a microstrip etched into the radiating plate layer. At high frequencies, they can be made by selectively etching double-sided printed circuit board. You can design patch antennas for either linear or circular polarization. The little antenna used for GPS reception on those little GPS boards for Arduino boards is a patch antenna and many cell phones use them as well. There are variants, including one where a corner of the radiating plate is connected to the ground plane. This apparently improves the loading of the antenna. Another variant is the printed monopole antenna, in which the ground plane is etched so that the antenna can work on two bands. Some printed circuit board designs have a middle layer to allow operation on two bands or to extend the bandwidth within a single band. High-frequency antennas from 2.5 GHz up, often have many patch antennas on the same printed circuit board with a common ground plane. The antennas are connected in phase using strip lines etched into the printed circuit board. A linear polarized patch antenna works like a stacked pair of two dipoles. It has a maximum gain of 9dBi. The placement of the feed point on the patch radiator determines the polarization. Of course, you can also rotate the entire antenna to change its polarization from horizontal to vertical. To make the antenna work with circular polarization it is made so that the horizontal plane resonates at a slightly different frequency to the vertical plane. This is done by making the radiating plate slightly too long in one plane and slightly too short in the other. Often by clipping the corner off two diagonally opposite corners of the square. It introduces a phase change between horizontal and vertical. At a particular frequency, there

will be a 90-degree phase difference. If the patch is fed so that each plane gets the same power, the antenna will radiate a circular polarized signal. This is the same technique that was used to feed the circular polarized Quadrifilar Helix antenna. One element is cut long and the other is cut short and they are both fed at the top of the loop. Placement of the feed point determines whether the antenna will right hand circular polarized or left-hand circular polarized.

I built this K3TZ designed 2.4 GHz patch feed for a microwave dish used to receive signals from AO-40.

DISH ANTENNAS The dish part of a microwave dish is not the complete antenna, it is just a reflector that concentrates the signal to a focal point where the actual antenna ‘driven element’ is located. Think of it as the reflector behind the bulb in a torch concentrating light in one direction. The antenna ‘feed’ may be a dipole, an element inside a resonant cavity, a waveguide, a helix, a patch antenna, or even a small Yagi. It is usually mounted at the focal point of the dish facing the dish reflector i.e. away from the direction that the dish is pointing. An offset fed dish has the feed point mounted on an arm usually below the dish. In this case, the dish is just a small section of a full sized parabola. The advantage of the offset feed is that the LNB and its supporting arm do not get in the way of signals arriving at the dish. The disadvantage is that the small dish section has much less gain than a full-sized parabolic dish would have. Radio telescopes and very large satellite dishes used for the commercial operation of geostationary satellites often use a Cassegrain feed which works on the same principle as the Cassegrain reflecting telescope. In that case, the antenna is in the center of the back of the dish. It emits a signal towards a small convex reflector mounted at the dish’s focal point. The signal reflects from the small reflector into the dish and from there the dish works the same as if the antenna was mounted at the feed point. When receiving the reverse occurs. Signals hitting the dish surface are reflected towards the small convex reflector at the focal point and from there down to the antenna feed point. This method makes it easy to change the frequency of operation by changing the antenna feed behind the dish. It is much easier than suspending a lot of equipment way out at the focal point of a very big dish. The shape of the dish is critical. It must be a parabolic reflector so that all of the received signals that touches the dish will be reflected onto the antenna mounted at the focal point. Bad placement of the antenna ‘feed’ or irregularities in the surface will reduce the gain of the dish. Wire mesh or ‘grid’ dishes are much lighter and easier to manage. They have significantly less wind loading so you are less likely to have it blow down. At low microwave frequencies between 1 and 2 GHz, the mesh looks like a solid surface because the wavelength of operation is significantly larger than the gaps in the mesh. The width of the slots or holes in the reflective metal mesh should be less than a tenth of the wavelength (λ/10). At 2 GHz one wavelength is 150

mm, so the gaps between the mesh wires have to be smaller than 15 mm. At frequencies above 5 GHz, the mesh size becomes impractical so microwave dishes use solid reflectors. For example, one wavelength at 10 GHz is 30 mm so the holes in the mesh would have to be less than 3 mm.

GAIN OF A MICROWAVE DISH The gain of a microwave dish antenna is proportional to the diameter of the dish and also to the frequency of operation. That means that at low frequencies a dish has to be very large if you want to get very much gain. Although dish antennas can, and sometimes are, used for VHF and UHF frequencies they are most commonly used for frequencies above 900 MHz. Gain G = 10 log eA ( D / λ)2 G is the gain over an isotropic antenna in dB eA is the aperture efficiency factor. It is always between 0 and 1, generally between 0.3 and 0.6 for amateur radio antennas (30% and 60% efficiency), and between 0.7 and 0.8 (70% to 80% efficiency for big high-efficiency dishes. D is the diameter of the parabolic reflector in meters λ is the wavelength of the signal in meters Doubling the diameter of a dish increases the gain by 6 dB Doubling the frequency increases the gain by 6 dB Large gains can be achieved if you have a sufficiently large dish reflector. However, when an antenna has a large gain, the beamwidth becomes very narrow, possibly only a degree or two wide. A narrow beam antenna requires very careful control over its position, or you may miss the satellite completely. Computer controlled tracking systems are required if you want to track satellites with a big dish. It is much easier with geostationary satellites because you don’t have to move the dish. For frequencies up to 11 GHz, dishes less than 3m in diameter don’t have to use continuous tracking to receive signals from geostationary satellites. They can be pointed at the satellite and then fixed. The small dishes used to receive satellite TV have quite wide beam angles. Once they are pointed in the right direction they can be fixed in place. The efficiency factor eA in the above equation is a bit of a problem since we don’t know what it is. Small dishes used and made by hobbyists usually have fairly low-efficiency factors around 30 – 50 %. The only real way to find it is to build the dish and then measure its gain. The main factors making up the efficiency factor (eA) are

Aperture taper efficiency is a measure of how evenly the dish is illuminated by the radiating antenna. The antenna at the focal point of the dish probably will not radiate exactly the same density of signal to all parts of the reflector. If it does manage to create an even density, it will almost certainly spill some of the power over the edge. Spillover Efficiency. Some of the power being radiated from the antenna feed point will miss the dish, and not be reflected towards the target. Surface loss. A good microwave dish has to be a perfect parabola. Any dings or irregularities will reflect the signal in the wrong direction and reduce the gain of the antenna. Many hobbyists dishes are made from wire mesh and this can never be as good as a smooth parabolic surface. Mesh dishes are OK at low microwave frequencies because the mesh looks like a solid surface at the longer wavelengths. At frequencies greater than 5 GHz, a solid reflector is required. Cross polarization. The polarization of the transmitted and received signals must match otherwise there is polarization loss. Unintended skew in the alignment of the transmitting (or receiving) antenna element at the dish feed point reduces the efficiency of the dish. Aperture blockage – The feed arrangement, or the LNB on a receiving dish, blocks the reflector to some of the incoming signals. It creates a shadow in the same way that it would block light from the Sun illuminating the entire dish. This naturally reduces the efficiency and hence the antenna gain. It is especially important when you have a small diameter dish because a greater percentage of the dish surface is blocked. Many small dishes (such as small satellite TV dishes) use offset feed arrangements so that the dish is not shadowed by the LNB. Non-Single Point Feed - the dish reflector focuses the incoming radio signals onto a focal point. But if the antenna is relatively large, i.e. for a low microwave frequency, some of the antenna at the dish feed point may extend outside of the dish’s focal point. In that case, some of the antenna is not being exposed to the full density of the received signal being captured by the dish reflector and the dish gain is affected.

ANTENNA BEAMWIDTH The beam width, or beam angle, of an antenna, is twice the angle from the center of the main lobe of the radiation pattern to the point where the power is half (3dB less) than the power in the center of the beam. It is called the 3 dB beamwidth. The beam width may be different in the vertical orientation to the horizontal. Ideally, for a circular polarized antenna, the vertical and horizontal beamwidths will be the same. Otherwise, the radiation pattern would be oval rather than circular. Beamwidth is a measurement of how directional an antenna is. As the signal is concentrated into a narrower beam, there is more gain in the direction of the main beam. High gain antennas have a narrow beamwidth. They are very directional and must be aligned precisely or you may miss the intended target. Lower gain antennas like a three element Yagi have a wide beam width. They are much less directional and so you can point them approximately at the satellite. Omnidirectional antennas are not directional, at least not in the polarization plane of interest, so you can’t point them in any specific direction. Omnidirectional antennas like a ¼ wave or a collinear have 360-degree beamwidth in the horizontal plane, they transmit or receive from all directions equally, but they have less beamwidth in the vertical plane. If the antenna is in free space i.e. not near the ground or any other reflector, it can have anything up to 360 degrees of beamwidth in the vertical plane. An antenna that is near the ground can’t radiate into the ground so the maximum beam angle is limited to the 180 degrees that are above the ground. You can measure your antenna beamwidth by transmitting a constant signal to a remote receiver over a ‘line of sight’ path. Start by pointing directly at the receiver’s antenna and then slowly rotate the transmit antenna until the received power drops by 3 dBm. Double the amount you rotated the antenna is the 3dB beamwidth. For a receiving station, you can measure the received signal from a distant signal source and rotate your antenna until the signal drops by 3 dBm, or 6 dB if your received signal strength indication is calibrated in terms of received signal voltage. Again, the 3dB beamwidth is double the amount you rotated the antenna.

As with all directional antennas the higher the gain, the narrower the beamwidth will be. The actual relationship between antenna gain and beam angle depends on the efficiency of the antenna including factors such as how much energy the antenna radiates that is not in the main lobe. Some power will be in small side lobes or out the back of the antenna. The angle between the center of the main lobe and the 3dB point is approximately equal to the angle from the 3 dB point and the first major null in the signal. Put another way the angle between the center of the main lobe and the first signal null is the same as the beamwidth. For example, if you have a high gain antenna with a 10-degree beamwidth and it is misaligned by 10 degrees, you would receive no signal from the wanted station. It becomes quite a big challenge when you are using a 1.5 m microwave dish at 10 GHz. Even with a modest 40% efficiency, you should have a gain of 40 dB. The beamwidth is only 1.4 degrees and the target is a small satellite 35,786 km away. It may be easier to use a smaller dish and sacrifice some of the gain.

BEAMWIDTH OF A MICROWAVE DISH The beamwidth of an antenna with a parabolic reflector (microwave dish) is: Beamwidth in degrees Ψ = 70 *λ / D Where: D is the diameter of the parabolic reflector λ is the wavelength of the signal

YAGI GAIN AND BEAMWIDTH The beamwidth of a Yagi antenna is dependent on its height above ground, its design (boom length and element spacing) and of course its gain (which is linked to the number of elements). These design factors affect the size and number of side lobes. It also affects how much energy is sent out the back of the Yagi, (front to back ratio). Stacking Yagi antennas at the correct spacing increases the overall gain by decreasing either the vertical or the horizontal beam width. Gain and 3dB beamwidth for a typical DL6WU design Yagi antenna. Elements Gain Gain Beamwidth dBi dBd 3 6.9 9.0 92 4 8.6 10.7 75 5 9.9 12.0 65 6 11 13.1 57 7 11.9 14.0 52 8 12.7 14.8 47 9 13.3 15.4 44 10 13.9 16.0 41 11 14.4 16.5 39 12 14.9 17.0 37 13 15.3 17.4 35 14 15.7 17.8 34 15 16 18.1 32 16 16.3 18.4 31 17 16.6 18.7 30 dBi = antenna gain compared to an isotropic antenna dBd = antenna gain compared to a dipole antenna Beamwidth = 3dB beamwidth in degrees Elements = number of antenna elements. It assumes one reflector, one driven element and a number of directors.

QUICK QUIZ QUESTIONS 1. A preamplifier is a really great way to improve the performance of fixed home stations with long coaxial feeder runs. A. The best location for the preamp is on the antenna mast near the antenna B. The best location for the preamp is in the shack where it is protected from the weather C. The best location for the preamp is mounted directly to the antenna as near as possible to the antenna connector 2. The most important factor when buying a preamplifier is; A. its gain or amplification in dB. You need as much gain as possible. B. it's noise factor. It should be low but not too low. C. it's bandwidth. It needs to be very wideband to cover VHF and UHF frequencies. 3. The best antenna polarization for receiving signals from a LEO satellite; A. is vertical polarization if the downlink is in the 70cm band. B. is horizontal polarization if the downlink is in the 2m band. C. is circular polarization 4. Fixed stations use coaxial feeder cable. A. For a vertical polarized antenna the feeder cable should be 75 Ohms. B. Feeder cable loss is unimportant if you use a preamplifier and turn up your transmitter power. C. Feeder cable loss affects the noise performance of your station. You should use coax cable with a low loss at the frequencies that you are using. 5. Is high SWR on your coax cable a problem?

A. No, because all of the transmitter power will get radiated eventually B. Yes, because it could damage my transmitter C. Yes, it reduces the amount of signal that I receive from the satellite and the amount of power I radiate from the antenna on transmit.

QUIZ ANSWERS 1. The correct answer is A. The best location for the preamplifier is on the mast close to the antenna. Placing the preamplifier in the shack dramatically reduces the overall noise factor and the signal to noise improvement that you want from a preamplifier. Placing the preamp on the antenna may affect the antennas performance. It might be OK mounted on the boom or behind the reflector on a Yagi. But the best method is to connect a short flexible tail from the antenna to the masthead preamplifier and then high quality coax or Heliax cable from the mast to the shack. 2. The answer is B. For weak signal EME work you want a very low noise figure, but for satellites, a medium noise figure is acceptable (and cheaper). You don’t want too much gain. Just enough to overcome the coax cable loss plus a margin of about 10dB. Too much gain will overload your receiver with noise, reducing its dynamic range and also makes the preamplifier more susceptible to overload from strong signals. It is best to use preamplifiers with bandpass filters for the band that you want to receive. They are less likely to be overloaded by ‘out of band’ signals especially the signal that you are transmitting on an adjacent antenna. 3. The answer is C. Even though the satellite is not transmitting into an antenna configured for circular polarization, it is best to use a receive antenna with circular polarization. Circular polarization minimizes cross polarization losses (signal nulls) as the attitude of the satellite relative to your location changes during the pass. The convention that 2m linear polarized antennas are mounted horizontal and 70m linear polarized antennas are mounted vertically is to maximize crosspolarization isolation and make the elements of the 2m antenna s high above the ground as possible. 4. The answer is C. Feeder cable loss affects the noise performance of your station and it reduces the RF power that you are radiating towards the satellite. You should use coax cable with a low loss at the frequencies that you are using. Most antennas have a 50 Ohm impedance, so you should match that by using coax with a 50 Ohm impedance. Option B is a bad approach. Buying an expensive amplifier rather than better coax cable is a poor investment.

5. The answer is C. High SWR reduces the amount of signal that you receive from the satellite because of mismatch loss. It also reduces the amount of power radiated from the antenna on transmit because some of the reflected power is lost as heat due to cable loss.

DOPPLER SHIFT The frequency of both your uplink and downlink signals is affected by the relative velocity between the satellite and your location. This is called the Doppler Effect or Doppler shift. Dealing with Doppler shift is a fundamental requirement for all satellite operation, other than geostationary satellites. Higher radio frequencies are subjected to more Doppler shift. Also the faster the satellite is traveling and hence the higher the relative velocity, the more Doppler shift occurs. Satellite tracking software can provide automatic control of your receiver and, if required, your transmitter frequencies. Doppler shift is named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague. It is commonly heard when a vehicle sounding a siren or horn approaches, passes by and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by and lower during the recession. Doppler shift affects all frequencies from sound waves, through the radio spectrum, up to visible light and beyond. For a given relative velocity, the amount of Doppler shift increases with increasing frequency. Range rate The range rate is the rate that the satellite is directly approaching or receding from your location, not its actual orbital velocity. If it is negative, the satellite is coming closer and if it is positive, the satellite is going away. A higher range rate means higher relative velocities and therefore more Doppler shift. At AOS, the satellite is at maximum distance and the range rate is at its highest negative value. The satellite is approaching at maximum relative velocity for the particular pass, so the Doppler Effect causes the maximum positive frequency shift for the particular pass. Near maximum elevation, the Doppler shift is at its highest rate of change. At peak elevation, the satellite is neither approaching nor departing. The range rate is zero and there is no Doppler shift.

At LOS, the satellite is at maximum distance and the range rate is at its highest positive value. The satellite is receding at the maximum relative velocity for the particular pass, so the Doppler Effect causes the maximum negative frequency shift for the pass. Passes with higher peak elevation have a higher maximum range rate and a higher maximum Doppler shift.

The picture above shows the typical Doppler shift curve as the satellite passes from AOS through peak elevation and away to LOS. This particular chart is for a satellite pass directly over the observer’s location. The frequency is 145 MHz and the satellite is at 350 km. It was calculated for the ISS. Doppler shift starts at about +3.8 kHz at AOS, drops through zero as the elevation peaks, and then reduces to – 3.8 kHz at LOS. For a more typical pass where the peak elevation is less than 90O, the Xaxis would be revised so that the maximum elevation is in the middle i.e. the elevation scale would go from zero to max elevation and back down to zero. The maximum amount of Doppler shift would be less because even at AOS the satellite is not coming straight towards your location. Rate of change

The rate that the Doppler Effect changes the frequency is not linear. The amount of Doppler shift changes faster near the middle, higher elevation, part of the pass. This means that you can’t just apply a fixed amount of correction every few seconds. The Doppler shift has to be continually calculated from the range rate as it changes throughout the pass. Doppler shift at your location vs Doppler shift at a different location Doppler shift does not change the frequency that is being transmitted. It only changes the frequency that is being received at a particular location at a particular time. The amount of Doppler shift is directly related to the relative velocity between the satellite and the operator’s location. At a different location, the relative velocity is different so the amount of Doppler shift being experienced is different. This means that the frequency displayed on your receiver will be different to the frequency displayed on the receiver of another operator who is listening to the same satellite downlink at a different location. The satellite may be rising at your location and you will be correcting for positive Doppler shift. At the other location, the satellite may be nearing LOS. In that case, the other operator will be correcting for a negative Doppler shift. This is an important point, which is commonly misunderstood by satellite operators. The table shows max Doppler shift for a LEO satellite at an altitude of 800 km. Band Freq. (MHz)








21.280 29.400 145.900 435.070 1269.000 2401.000 10250.000

+/+/+/- 3.27 +/- 9.76 +/- 28.5 +/- 53.8 +/- 230 477 659 kHz kHz kHz kHz kHz Hz Hz Table 8: maximum Doppler shift for popular amateur radio satellite bands. Geostationary satellites Geostationary satellites orbit at the same speed as the Earth rotates, so the satellite stays above a fixed point on the equator. This means that there is no range rate because the satellite never gets any closer or further from your location. A range rate of zero means that there is no Doppler shift at any frequency. Max Doppler

FM and digipeater satellites

If the satellite has an FM repeater or a digipeater, its receiver and transmitter frequencies are fixed. You have to adjust your transmitter frequency so that the satellite will receive the Doppler shifted signal at the correct frequency. You also need to adjust your receiver frequency to compensate for the amount of Doppler shift on the downlink. FM is more tolerant of off-frequency signals than SSB, so the Doppler correction does not have to be exact. You can pre-program three or five channels on your FM radio and switch through them as the pass proceeds. Beacons, telemetry, and commercial satellites Satellites with beacons, telemetry, or data, downlinks have transmitters on fixed frequencies. You must change your receiver frequency to account for Doppler shift as the satellite passes over. This is true for commercial orbiting satellites such as weather satellites as well. The only exception is when you are receiving signals from a geostationary satellite. Geostationary satellites stay in the same position relative to the observer on Earth. Therefore, the range rate is zero and there is no Doppler shift. SSB / CW linear transponder satellites If you are using single sideband or CW through a linear transponder any frequency error due to Doppler, changes the tone of the received signal. Doppler shift can make an SSB signal sound like unintelligible ‘duck talk,’ or move the signal completely out of your receiver’s passband. SSB signals absolutely must be corrected for Doppler shift. Without correction, the Doppler shift would cause your uplink signal to drift across the transponder pass-band as the pass progresses. This could cause your signal to move over other conversations, which will make you unpopular. If you are not careful, it can move your signal right off the satellite! Satellite tracking software with CAT control adjusts the uplink and downlink frequencies on your radio(s) so that the signal stays in the same part of the satellite transponder bandwidth and your receiver remains tuned to your downlink signal. If the other operator is also correcting both uplink and downlink, you should not have to touch the radio’s VFO knob at all. If your software supports a ‘both’ tracking mode you should definitely use it. The only exception is in the rare case where the station that you are working is making manual frequency adjustments. It is virtually impossible to perform both uplink and downlink Doppler correction by manually adjusting your transmitter and receiver frequencies. If you don’t have computer control of your radios, you should

follow the ‘one rule,’ established by early amateur radio satellite operators before computer control of the radio became the norm. The ‘one rule’ works on the theory that there is more Doppler shift on higher frequencies, so the higher frequency either uplink or downlink should be adjusted. The ‘one rule’ For a UV transponder, the uplink is the higher frequency. You should adjust your transmitter frequency so that your downlink signal sounds normal. For a VU transponder, the downlink is the higher frequency. You should adjust your receiver frequency so that your downlink signal sounds normal. If you are using computer control but working a station that is manually correcting for Doppler shift using the ‘one rule’, you can set your computer tracking to correct the higher frequency either uplink or downlink. Even then, you will probably have to adjust your receiver frequency a bit, because the Doppler shift that he or she is compensating for at his or her location is different to the Doppler shift that you are experiencing at your location. Manual correction for Doppler shift is difficult especially if you are a ‘newbie.’ My strong advice is to download a tracking program that has ‘both’ uplink and downlink correction and connect your radio to your PC for CAT control.

CALCULATING DOPPLER SHIFT Once you know the range rate, you can work out what effect it will have on your transmissions. The amount Doppler shift changes as the satellite pass proceeds. Initially, the satellite is coming towards your location. Almost always, it will not be coming directly towards you. It will be approaching at an angle. That means that only part of the satellite’s velocity is in your direction. The range rate will be less than the satellites orbital velocity and will be a negative number. Doppler shift causes the uplink frequency received at the satellite and the downlink frequency received by your station to increase. After peak elevation, the satellite recedes toward your horizon. The range rate becomes positive and Doppler shift causes the radio frequencies to decrease. The formula for calculating the amount of Doppler shift given a known range rate is included in the Formulas section. There is also a simple formula that does not need the range rate but it only applies, when the satellite is in a circular orbit and will pass directly over your location. That is not a very likely ‘real world’ scenario. Satellite tracking programs use vector mathematics to calculate the instantaneous range rate as the satellite passes over. For the 2m band, we can expect Doppler shift to be less than ± 3.8 kHz and for the 70cm band, it will be less than ± 11.8 kHz. For FM satellites, you can use pre-determined channels rather than computer frequency control. For SSB satellites computer control is preferable but manual tuning can be used. The higher the transmitted frequency the more Doppler shift occurs. LEO satellites have high orbital velocities so there is too much Doppler shift for microwave frequencies to be easily used. MEO and HEO satellites have a lower orbital velocity because they are at higher altitude, so they can use microwave frequencies. Geostationary satellites stay in the same position relative to the observer on Earth. The range rate is zero so there is no Doppler shift and microwave frequencies can be used.

DOPPLER SHIFT ON LINEAR TRANSPONDERS There is a big advantage in using reverse tracking linear transponders. The way that they work partly offsets the net effect of the uplink and downlink Doppler shift. For an approaching satellite, there will be positive Doppler shift on both the uplink and the downlink transmissions. With a reverse tracking transponder, the positive Doppler shift on the uplink frequency causes the downlink frequency to be a little lower than it would be if there were no Doppler shift. This reduces the effect of the Doppler shift on the downlink. Conversely, when the satellite is receding, the negative Doppler shift on the uplink causes the downlink frequency to be a little higher, reducing the effect of the negative Doppler shift on the downlink. For a normal tracking transponder, the overall Doppler shift is equal to the uplink Doppler shift plus the downlink Doppler shift. In the example below, the operator will have to tune from + 7 kHz at AOS down to - 7 kHz as the satellite travels to LOS. For a reverse tracking transponder (lower diagram), the operator would only have to tune from + 3 kHz at AOS to - 3 kHz at LOS.


1. You transmit an upper sideband signal to the satellite on a frequency of 145.905 MHz 2. Because it is near the start of the satellite pass, there is a positive Doppler shift of 2 kHz. That means that your signal arrives at the satellites linear transponder at 145.907 MHz 3. The satellite transponder converts and amplifies the signal. It is transmitted from the satellite on a frequency of 435.807 MHz. 4. At the higher 70cm band, frequency there is more Doppler shift. By the time the signal arrives at your receiver, it has been shifted 5 kHz higher at 435.812 MHz. That is + 7 kHz higher than the signal would be in the middle of the satellite pass where there is no Doppler shift. 5. As the satellite rises to peak elevation, the Doppler shift reduces from + 7 kHz to zero.

6. After the peak elevation, the Doppler shift the amount of Doppler shift increases from zero to – 7 kHz. 7. If there is no uplink Doppler correction, the frequency arriving at your receiver will change by ± 7 kHz during the pass.


1. You transmit a lower sideband signal to the satellite on a frequency of 145.905 MHz 2. Because it is near the start of the satellite pass, there is a positive Doppler shift of 2 kHz. That means that your signal arrives at the satellites linear transponder at 145.907 MHz 3. The satellite transponder converts and amplifies the signal. It is transmitted from the satellite as an upper sideband signal on a frequency of 435.893 MHz. 4. At the higher 70cm band, frequency there is more Doppler shift. By the time the signal arrives at your receiver, it has been shifted 5 kHz higher at 435.898 MHz. That is only + 3 kHz higher than the signal would be in the middle of the satellite pass where there is no Doppler shift. 5. As the satellite rises to peak elevation, the Doppler shift reduces from + 3 kHz to zero.

6. After the peak elevation, the Doppler shift the amount of Doppler shift increases from zero to – 3 kHz. 7. If there is no uplink Doppler correction, the frequency arriving at your receiver will change by ± 3 kHz during the pass.

FORWARD AND REVERSE TRACKING TRANSPONDERS For any forward tracking transponder, the overall Doppler shift is equal to the uplink Doppler shift plus the downlink Doppler shift. ∆F = ∆FUPLINK + ∆FDOWNLINK For a VU mode reverse tracking transponder, the downlink frequency is higher than the uplink frequency. The overall Doppler shift is equal to the downlink Doppler shift minus the uplink Doppler shift. ∆F = ∆FDOWNLINK - ∆FUPLINK For a UV mode reverse tracking transponder, the uplink frequency is higher than the downlink frequency. The overall Doppler shift is equal to the uplink Doppler shift minus the downlink Doppler shift. ∆F = ∆FUPLINK - ∆FDOWNLINK

RANGE RATE The range rate is the relative velocity between your location and the satellite. It may seem odd to say that the velocity of your location is a factor. Surely, that is always zero? But of course, from a reference point out in space, your location is moving due to Earth’s rotation and this has to be taken into account. Doppler shift is calculated from the range rate and the frequency of the signal. Range rate is a measurement of convergence (how fast two objects are moving together), or divergence (how fast two objects are moving apart). It is measured in meters per second, (or kilometers per second). For purely historical reasons to do with the plotting of sailing warships on maps, range rate is considered to be negative when objects are converging (coming towards you) and positive when objects are diverging (moving away from you). This makes sense if you are considering ships that are approaching each other, but it makes the Doppler calculation a bit tricky. The Doppler shift calculations actually use negative range rate (RR). A satellite may be traveling around the world at 7,000 meters per second, but only the speed that the satellite is approaching or receding from your location contributes to the Doppler shift between you and the satellite. The relative velocity or ‘range rate,’ is a complex vector calculated from the satellite’s orbital speed, the elevation angle, and the azimuth angle. If the satellite was coming directly towards your location, on a path that would cause an overhead pass with a maximum elevation of 90 degrees, the relative velocity, and the range rate could be calculated simply. The satellite’s direction of travel would be one vector axis and a vector pointing straight down towards the Earth’s center would be the other. The range rate would be approximately equal to the orbital velocity when the satellite was at zero degrees of elevation and would be zero when the satellite was at 90 degrees of elevation. Assuming a constant velocity (circular orbit), the Doppler shift during the satellite pass would be just like the ‘Doppler shift curve’ drawing shown above. But of course, most satellites don’t pass directly overhead, they pass to one side of your location and the maximum elevation will be less than 90 degrees. In that case, the relative velocity at zero degrees of elevation will be less than the orbital speed since some of the motion is not directly towards your location, so there will be less Doppler shift. The

relative velocity and the range rate will still be zero when the satellite is at maximum elevation. Let’s go back to the “train whistle example” most often used to describe Doppler shift. You are on a railway platform when the express train rattles through without slowing down. It blows its whistle as it passes by. You hear the pitch of the whistle start high and reduce to normal as the train passes you and then the pitch drops further as the train recedes into the distance. This is the classic example. Now consider what you would hear during the same incident if you were standing 500 meters from the track. You would hear the same descending pitch and it would be at the normal frequency as the train passes you. But the frequency of the whistle would never be as high or as low as when you were standing on the platform. That is because the relative velocity is different. If you were driving in a car that was keeping pace with the train, the whistle would sound the same (normal) throughout, because the relative velocity would be zero.

ATMOSPHERIC REFRACTION - EFFECT ON DOPPLER SHIFT The Doppler calculation is accurate in the vacuum of space. However, at elevations of less than 5 degrees, the radio path through the Earth's atmosphere is not perfectly straight. It is refracted a little by the atmosphere. Actually, it is refracted differently by the troposphere and the ionosphere layers. Dry air has little effect, causing less than 1 Hz additional change in frequency, but air containing ice crystals or water vapor can make a difference of 50 to 100 Hz. The refractive index of air is about 1.0003 so the speed of light in air is about 299700 km/s (about 90 km/s slower than in a vacuum). Refraction in the troposphere is due to the refractive index of air, gasses, and water vapor that the signal passes through. The amount of refraction is independent of the frequency of the radio signal. Refraction in the ionosphere is due to ionization and is inversely proportional to the frequency squared. The effect on the radio signal is small, less than 1 Hz change in frequency for signals above 100 MHz and negligible for signals above 1000 MHz.

EFFECT OF REFRACTION ON APPARENT SATELLITE ELEVATION At low elevations, the refraction occurring as the signal travels through the atmosphere (troposphere and ionosphere combined) also has an effect on the perceived elevation of the satellite, making it seem a little higher above the horizon than it actually is. Atmospheric refraction causes an error of about 35.4 arc seconds, (approx. ½ degree), at zero degrees elevation. The amount of refraction reduces rapidly as the elevation increases. At 10 degrees if elevation the amount of refraction has dropped to 5.3 arc seconds, (0.08 degrees). On the horizon, refraction is slightly greater than the apparent diameter of the Sun, so when the bottom of the sun's disc appears to touch the horizon, the sun's true altitude is negative. If the atmosphere suddenly vanished at this moment, one couldn't see the sun, as it would be entirely below the horizon. Refraction of the radio signal as it passes through the atmosphere is not an issue at the low frequencies used for LEO satellites because the beam angle of our antennas is quite wide. But it can become a problem when you are using narrow beam dish antennas to receive signals from HEO or GEO satellites and the elevation is very near to the horizon. Some tracking programs correct for the effect of refraction on the elevation and some do not. If the elevation indication it is not corrected, the refraction effect contributes to the belief that you can ‘work’ satellites that are below the horizon. Thanks to refraction, you may be able to access a LEO satellite like FO-29 about nine seconds before AOS until about nine seconds after LOS. Higher orbit satellites travel more slowly, taking longer to cover half a degree of arc, so the effect becomes more pronounced. The variable nature of the amount of delay caused by signal refraction in the atmosphere is one of the main sources of error in GPS navigation systems. It is compensated for by ‘triangulating’ signals from several satellites. The GPS satellite constellation is structured so that at any given time at least two GPS satellites will be at high elevation and therefore unaffected by refraction on the signal path.

SATELLITE PREDICTION AND TRACKING SOFTWARE There are dozens of satellite prediction programs available for all common computer operating systems and for iOS and Android smartphones. There are also several online prediction programs. Prediction programs usually produce tables showing essential data for the next few passes of the nominated satellite. They may also show the satellite's current position on a map and some applications will remind you when a nominated satellite is due to rise over your horizon. Typical information from phone applications includes: Satellite name Current Azimuth and Elevation AOS time for the next pass LOS time for the next or current pass Maximum elevation for the current or next pass Current range Current height above ground Lists from online sources typically show a table like this one from the AMSAT web page: It contains all the information you need for the next few satellite passes. AMSAT Online Satellite Pass Predictions - AO-07 View the current location of AO-07 Date (UTC)



AOS Maximum Max El LOS Azimuth Elevation Azimuth Azimuth


26 Sep 16








26 Sep 16








26 Sep 16








26 Sep 16








26 Sep 16








26 Sep 16








26 Sep 16








DOWNLOADABLE TRACKING SOFTWARE Tracking programs provide all of the information supplied by the prediction programs with the added feature of being able to control your antenna rotator and radio. The ‘CAT’ (‘CI-V’ on Icom) control adjusts the receiver and transmitter frequencies to correct for Doppler shift as the satellite passes over. There are many satellite tracking programs available. I use SatPC32, which is available as a download from and also from the AMSAT-NA, AMSAT-UK, and AMSAT-DL websites. SatPC32 is written and supported by Erich Eichmann, DK1TB. He has very kindly donated all the proceeds from the software to AMSAT, to support amateur satellites. The software is available in English or German language versions. If you use the dk1tb link, you can download a trial version for free and register it later via any of the above mentioned AMSAT websites. The website lists 23 Windows applications, 13 Macintosh applications, 8 Linux applications and several Android and iOS applications for smartphones. Some can track the Moon, Sun, and planets as well as satellites. Unfortunately, some of the listed sites seem to be inactive so you might have to check out more than one. ‘Gpredict’ by OZ9AEC seems to be current and up to date. It works with Windows, Linux, and Mac OS operating systems. It is free software licensed under the terms and conditions of the GNU General Public License. “Gpredict is a real-time satellite tracking and orbit prediction application. It can track an unlimited number of satellites and display their position and other data in lists, tables, maps, and polar plots (radar view). Gpredict can also predict the time of future passes for a satellite, and provide you with detailed information about each pass.”

ONLINE REAL-TIME SATELLITE PREDICTION: The N2YO website at has current data as well as the individual two line TLE for any selected satellite. It shows you the current location of the satellite and details about the next pass over your location. The ISS fan club website at has a lot of great information about the International Space Station.

ONLINE ORBITAL PREDICTIONS AMSAT-NA has a page that displays a table showing data for the next few passes of a selected satellite. The Heavens Above website is dedicated to visible satellites, It has an online calculator to show you when and where to look.

CAT (CI-V) CONTROL OF YOUR RADIO If you are building a serious satellite ground station and you plan to use satellites with linear transponders, having CAT (CI-V on Icom) control of your transceiver or receiver frequencies is a must. It is easy to achieve and it should not cost more than the price of a connecting cable between your radio and your computer. If you have your station set up for digital mode operation or automatic contest logging, you probably already have CAT control working. Refer to the radio manual and help screens in the satellite tracking program for connection details. CAT commands are text strings sent from the computer to control settings on the radio. The radio responds with messages indicating that the command has been executed or by supplying the requested information. Usually, the radio sends information out every time you adjust the radio tuning or other controls, and the PC software does the same. In that way, the computer software is aware of any changes that you make on the radio and the radio is aware of any changes made by the satellite tracking software. Each of the major amateur radio manufacturers uses a completely different format for CAT commands. Smaller vendors and SDR software developers usually pick one of the three dominant formats; Icom, Yaesu, or most commonly Kenwood. To compound the problem, the various radio models often support only a subset of the full CAT command set. Or they use variations of the standard CAT commands for specialized functions. The satellite tracking software will probably have a setting for your radio model. If it does not, pick a similar model from the same manufacturer and hope for the best. Flex Radios and ANAN SDRs can be set to use the Kenwood CAT command structure. You can select either ‘Kenwood’, or ‘TS-2000.’ Older radios including the Yaesu FT-847 and the Kenwood TS-2000 use an RS-232 serial cable for CAT control. This may be a problem if your computer has no 9-pin D connector serial ports. However, a USB to RS232 adapter cable can be used instead. Newer radios feature USB CAT connection to the computer. If you are using an SDR the serial cable is replaced by a virtual Com port pair using a utility program like com-0com. A virtual com port pair links two programs running on the same computer in the same way that an external cable links the computer to a physical radio. There is no need to use a very fast serial connection. 9600 bauds or any higher data rate will be fine. The important thing is that the radio and the

computer must be using the same data rate, number of data bits, stop bits, and parity settings. 9600,8,1,N is typical. Whatever rate you use for other applications will work OK for satellite operation. RTS and CTS flow control are not required unless the signals are being used for PTT control or for sending CW.

AUTOMATED ROTATOR CONTROL Computer control of the antenna system is a very nice addition to your satellite station. Having the computer steer the antennas as the satellite passes over leaves you free to operate the radio and makes overall station operation much easier. One less thing to worry about. I am a firm believer in using as much automation as possible. You can buy ‘azimuth’ or ‘azimuth plus elevation’ rotators with RS-232 or USB ports on the control box. It is as easy as setting up CAT control to connect these to your computer. The satellite tracking software supports most of the satellite rotators that have data ports. If like me, you have an older rotator, which has no computer control facility you can build a controller to do the job or you can buy a rotator controller. SatPC32 and other satellite tracking programs can control a wide range of antenna rotators and commercially available rotator controllers. You have to change a menu setting to select the device. SatPC32 has setup instructions in the subfolders relating to the different rotators and controllers. Be careful before you make a purchase. Apparently, if you run SatPC32 on a 64-bit operating system such as Windows 10, not all of the listed rotators are supported because of a file problem, (IOPort.sys not supported in 64-bit error). Also, some of the rotator controllers listed in the SatPC32 menu were designed for connection over Centronics parallel printer ports and PCs have not had those for many years. I have built three different rotator controllers over the years. The difficulty is not in building the controller but in writing the software to interface it to the satellite tracking software. My latest controller for my Yaesu G-5400 rotator uses an Arduino microcontroller board and an Arduino compatible relay board with four relays. The relay switch contacts are wired across the up, down, left, and right control switches on the Yaesu control box. The relays are controlled via Arduino digital output lines. Four LEDs wired to other Arduino output ports are used to indicate when the box is actively controlling the rotators and which way the antenna is tracking. The box can change the antenna elevation and azimuth simultaneously. The antenna position is determined by reading the voltage across the azimuth and elevation position sensors using two of the Arduino’s analog to digital converters. The rotator position sensors are wire-wound potentiometers inside the rotator bodies. The voltages from these ‘pots’ are also used to drive the position meters on the Yaesu control box. The whole controller cost much less than $100 including the box. It is powered by the computer

USB connection. Two pieces of software are required. A program written in the C computer language is used to control the Arduino. Don’t be scared! The Arduino software is pretty simple to write. Hundreds of radio hams use Arduino boards for all sorts of projects, so help is easy to find. The second piece of software runs on the PC and acts as an interface between the Arduino controller and the SatPC32 or Orbitron software. These applications constantly output the tracking data using a DDE (direct data exchange) format. Unfortunately, DDE is considered obsolete by Microsoft and Visual Studio no longer supports it. This is a real shame because there is no real replacement. The beauty of DDE is that is an ‘unconnected’ service. A DDE server program just sends out data and has no idea if any other application is using it. It means that the program won’t crash if the connection to the rotator is lost or the rotator software is closed. You can get around the DDE not supported problem by using NDDE.dll Since 2006, NDDE.dll is also no longer supported but it is still available. There is an example program for the DDE interface in a SatPC32 program subfolder. Software that is more modern probably uses a .dll or API to interface with the rotator controller software. To establish a DDE link with SatPC32 the Client program has to set the DDE parameters as follows: Server name: SatPC32 DDE Topic: SatPcDdeConv DDE Item: SatPcDdeItem The DDE interface is constantly active and can be used by any third party client program to receive the name of the selected satellite, the azimuth and elevation bearings for a rotator, the downlink and uplink frequencies, the transmitter and receiver modes (FM, USB, LSB) and the MA value calculated by SatPC32. The DDE string is updated every second. The DDE string for Orbitron and SatPC32 has the following format. You can use the field codes to parse the string. Not all data is sent every time. For example, mode data is only sent if it is changed. SNxxxxx AZxxx.x ELxx.x DNxxxx UPxxxx DMxxxx UMxxxx MAxxx Fields: SN - tracked satellite name (spaces are converted to "_") AZ - azimuth in degrees 0.0 to 360.0 EL - elevation in degrees -90.0 to +90.0 DN - downlink frequency including Doppler, in Hertz


- uplink frequency including Doppler, in Hertz - downlink mode (optional field) - uplink mode (optional field) - mean anomaly (optional field added to SatPC32 data)

Examples: SNISS_ZARYA AZ182.1 EL36.3 DN145800000 UP145800000 UMFM-N SNAO-07 AZ180.3 EL-10.1 DN145950000 UP432150000

TYPES OF ORBIT There are dozens of different types of orbit. I will explain the basic parameters of the more common ones. The orbit that a satellite is placed into is chosen to meet specific mission objectives. In the very common case where many satellites are being deployed from the same launch vehicle (rocket), the orbit will be chosen by consensus or more likely by the party responsible for the primary payload. In other words, by the group that paid the most. Orbit LEO MEO HEO

Description Low Earth orbit Medium Earth orbit High Earth orbit

Altitude 160 – 2,000 km 2,000 - 35,786 km

Entirely greater than 35,786 km HEO Highly Elliptical Apogee greater than orbit 35,786 km Geostationary Geostationary 35,786 km orbit Luna The Moon’s 384,401 km orbit

LEO – LOW EARTH ORBIT The low Earth orbit is the most common orbit for small satellites. Most Hamsats and Amsats are in polar, low Earth orbit with an inclination just over 90 degrees. Low Earth orbit extends from 160 km, which is the lowest altitude that a satellite can complete a single orbit without being additional thrust being applied, out to 2,000 km. Advantages of Low Earth Orbit 1. Low-gain antennas and low transmission power can be used. Because the satellites are close, there is less path loss. 2. Low power requirement on the satellite means that LEO satellites can be small and relatively cheap to build and launch. 3. It is cheaper to place satellites into low orbit. Some are launched from the International Space Station. 4. LEO satellites usually use VHF and UHF bands because there is a large amount of Doppler shift caused by the high orbital speed of the satellites. Luckily 2m and 70cm radios are relatively cheap. 5. Simple construction – no motors or thrusters are required. 6. Low latency (signal delay) because the satellites are close.

Disadvantages of Low Earth Orbit 1. The satellite has a small footprint, limiting the communications range. 2. Low orbit means high speed. Each satellite pass is short, only 10 to 15 minutes. 3. The high speed creates more Doppler shift, limiting the frequencies that can easily be used. 4. Satellite antennas need to be able to track the satellite.

MEO – MEDIUM EARTH ORBIT The medium Earth orbit extends from 2000 km all the way out to the geostationary orbit at 35,786 km. This range is mostly used by satellites performing radar and visual mapping of the Earth’s surface and by the navigation (GPS) satellites. It is not used by any of the current Hamsats and few (if any) of the Amsats. Most MEO satellites orbit at approximately 20,200 km, which gives them a ‘semi-synchronous’ orbit with a period of 12 hours. That means that an overhead pass will last about five hours. The 31 GPS satellites are arranged in six orbital planes at a distance of 20,200 km. You should be able to see signals from at least four GPS satellites, anywhere on the planet, at any time. Other MEO satellites include the Galileo navigation satellites at an altitude of 23,222 km and the Glonass satellites at 19,100 km. Communications satellites designed to cover the North and South Poles are also placed into either MEO or Molniya orbits. The Glonass satellites are a Russian equivalent to the GPS satellites. They are used for navigation purposes by the Russian military. Some GPS receivers use signals from both the GPS and the Glonass satellites. Receiving multiple satellites improves the location accuracy. The Galileo satellite constellation currently has 16 satellites in MEO with the intention of increasing this to a total of 29 satellites. The Galileo satellites are another GPS alternative known as the European ‘global navigation satellite system’ (GNSS). The Galileo satellites also provide the MEOSAR search and rescue service, receiving and forwarding signals from emergency locator beacons. Advantages of Medium Earth Orbit 1. A MEO satellite has an orbital period of about 12 hours so you can cover the Earth’s entire surface with as few as 12 satellites. 2. Relatively simple construction 3. Satellite footprint of a satellite at 20,200 km is 16,930 km in diameter 4. Satellite pass can last 5 hours for a satellite at 20,200 km 5. Lower velocity so less Doppler shift. Higher frequencies can be used Disadvantages of Medium Earth Orbit

1. It is much more expensive to get a satellite up to a MEO orbit. The satellite may have to use a transfer orbit to boost from LEO to MEO. On board, propulsion or positioning thrusters are usually required. This significantly increases the complexity and price of the satellite. Commercial MEO satellites are often launched directly into MEO orbits, but this is also expensive. 2. MEO satellites require higher transmission power to overcome the additional path loss and there is more signal latency. 3. Larger high gain antennas are required on the satellite. 4. You need more uplink power (EIRP) to overcome the increased path loss to the satellite 5. Satellites pass through the Van Allen belts and may suffer radiation damage, especially when passing through the lower Van Allen belt.

HEO – HIGH EARTH ORBIT A high Earth orbit is defined as being entirely further out than the geostationary orbit at 35,786 km. This orbit is seldom used and it is never used by Amsats or Hamsats. The acronym ‘HEO,’ is more commonly applied to satellites in very elliptical orbits, with an apogee out past the geostationary orbit and a perigee fairly close to Earth.

HEO - HIGHLY ELLIPTICAL ORBIT A highly elliptical orbit is an elliptical orbit with a low perigee, often around 1000 km and a high apogee, often outside of the 35,786 km geostationary orbit. The most commonly used highly elliptical orbit is called a Molniya orbit, named after the Molniya Soviet communication satellites, which used them. Satellites in highly elliptical orbits spend a lot of time traveling out to their apogee and returning towards Earth then they zip around the Earth in a couple of hours at their perigee. This means that they give excellent coverage over a large geographic area for most of the time, with short periods just before and after they reach perigee where they are unusable. Advantages of the Highly Elliptical Orbit 1. The satellite spends most of the orbital period out near apogee providing a full hemisphere of coverage of the Earth’s surface. If the satellite is a Hamsat, this allows transcontinental communications via amateur radio satellite. 2. The satellite is available for about eight hours at a time. 3. For most of the pass, the velocity is much slower than a LEO satellite, so there is less Doppler shift. Microwave frequencies can be used, except during the short periods when the satellite is approaching or leaving perigee. 4. For most of the orbital period, not much antenna tracking is required. Disadvantages of the Highly Elliptical Orbit 1. It is much more expensive to get a satellite up to a HEO orbit. The satellite will have to use a transfer orbit to boost from LEO to HEO. On board, propulsion is required. This significantly increases the complexity and price of the satellite. 2. HEO satellites require higher transmission power to overcome the additional path loss and there is more latency. That means more solar cells and a larger, heavier satellite. 3. High gain antennas are required on the satellite and at the ground station. This makes working the satellite marginal with average home station setups.

4. Higher power uplink is required to overcome the increased path loss 5. The satellite is exposed to radiation from the Sun. This killed the memory chips on AO-10.

MOLNIYA ORBIT A Molniya orbit is a highly elliptical orbit with an inclination of 63.4 degrees and an orbital period of half of a day. Molniya orbits are named after a series of Soviet/Russian Molniya communications satellites, which have been using this type of orbit since the mid-1960s. Satellites in highly elliptical orbits spend a lot of time far out near their apogee and hardly any time at all zipping around Earth at their perigee. Much of Russia is at high geographic latitudes. Satellites in geostationary orbit do not provide full coverage of the northern regions. But a satellite in a near-polar Molniya orbit can provide great communications range over high latitudes for extended periods. As the apogee altitude can be as high as 40,000 km, the satellite will have excellent coverage of Russia, northern Europe, and Canada. To get a continuous coverage at least three Molniya spacecraft are needed. There have been three Hamsats placed into Molniya orbits: AO-10, AO13, and AO-40. Unfortunately, none of them is still operational. AMSAT OSCAR 10 was launched on 16th June 1983 but it suffered some damage when it was nudged by the Ariane rocket booster during deployment. A stuck thruster control resulted in a longer thruster burn than intended and the satellite ended up with a low inclination highly elliptical orbit. Its perigee was around 3846 km and its apogee was about 35,608 km. In spite of being a bit deaf due to a broken antenna, the satellite was very popular with amateur radio operators. On June 15th, 1988, OSCAR 13 was launched into an orbit with a perigee of 2414 km and an apogee of 35,405 km. After eight and a half years in orbit, the perigee had degraded to around 300 km. Eventually, AO-13 succumbed to orbital drag and burned while re-entering the atmosphere. AO-40 is the most complex Hamsat ever built, with transponders ranging from 21 MHz up to 24 GHz. It was launched on November 16th, 2000 at a reported cost of 4.5 million dollars. A mistake in failing to remove a protective cap before the launch caused an explosion when the main motor was fired leaving the satellite in an unintended equatorial orbit. During 2001, gas from the ‘Arcjet’ motor was vented to raise and stabilize the orbit of the satellite. The spacecraft got a lot higher than expected. The final orbit has a perigee of 1100 km and an apogee of 58,700 km. AO-10 and AO-40 are still in orbit but they are inactive.

GSO – GEOSYNCHRONOUS ORBIT A geosynchronous satellite is in an orbit that matches the Earth’s rotation. The orbit is usually near circular and is at the same average distance as the geostationary orbit, but the inclination is greater than zero degrees. In other words, the orbit is not in the equatorial plane. The distance above Earth is 35,768 km (radius from Earth center is 42,164 km). It takes a geosynchronous satellite one sidereal day to complete an orbit (23 hours, 56 minutes and 4 seconds). This ensures that it will be over the same spot on the surface at the same time every day. From the Earth, the satellite moves in a sort of figure 8 pattern known as an ‘analemma’ shape. The orbit can be configured so that the satellite spends most of its orbit positioned over a continent, which makes it ideal for weather satellites. Satellites in inclined geosynchronous orbits are much less common than geostationary satellites. A geosynchronous orbit does not have to be circular. It can be highly elliptical. The Infrared Space Observatory was in a highly elliptical geosynchronous orbit at an orbital height of apogee 70,600 km and perigee 1,000 km. The elliptical geosynchronous orbit ensured that the observatory would be at perigee at the same time, over the same place, every day, allowing easy download of the data captured by the satellite. The rest of the time, the satellite was out in deep space taking Infrared photographic images of distant stars and galaxies. Satellites can also be placed into a semi-synchronous orbit, which has an orbital period of half a sidereal day, (11 hours and 58 minutes). The satellite travels in the same direction as the Earth’s spin and appears from the ground to complete one orbit a day. The circular MEO orbit at 20,200 km used by GPS satellites is a semi-synchronous orbit. The first operational geosynchronous satellite was Syncom 2. It was launched on a Delta rocket B booster from Cape Canaveral July 26, 1963. A few months later Syncom 2 was used for the world's first satelliterelayed telephone call, which took place between United States President John F. Kennedy and Nigerian Prime minister Abubakar Tafawa Balewa. Syncom 2 was placed into a geosynchronous orbit over the Atlantic Ocean and Brazil, with 33 degrees orbital inclination.

GEO - GEOSTATIONARY ORBIT The geostationary orbit is a special case of the geosynchronous orbit. It is always circular with an inclination at or very near zero. All geostationary satellites are directly above the equator. The geostationary orbit is at 35,786 km. At that distance, the orbital period is 1436.1 minutes or one sidereal day. This means that the satellite orbits the Earth in exactly the same time as it takes the Earth to complete a rotation. The satellite stays above the same point on the equator. This has huge advantages for communications and TV distribution satellites because they can service the same areas of the Earth all the time and the ground stations do not have to track a moving target. At that orbital distance, the satellite can cover almost a complete hemisphere although it can’t cover all of the Polar Regions. Most geostationary satellites have high gain antennas called ‘spot beams’ aimed at specific countries or regions. The satellites used for TV broadcasting, consumer Internet to remote areas, and international telephone & data are all geostationary satellites. Because the satellites don’t move relative to the Earth’s surface, you don’t have to track them. You can use a fixed dish antenna. Since all geostationary satellites are at zero degrees latitude, they are listed according to their longitude, for example, ‘37O West.’ To point your dish at the satellite you only need to know the bearing from your location to the satellite. The elevation is similar to your latitude. This means that if you are located on the equator your dish points virtually straight up. At high latitudes, the dish will be almost vertical, pointing towards the horizon. This is skewed a little because most small dishes used for TV are offset fed. They point high, the beam angle is not parallel to the face of the dish. The idea of placing telecommunication relay satellites into geostationary orbit was invented by Science Fiction author Arthur C Clarke on May 25th, 1945, well before any man-made satellites had been launched. The first geostationary communication satellite was Syncom 3. It was launched on August 19, 1964, on a Delta D launch vehicle from Cape Canaveral. The satellite was placed into geostationary orbit approximately above the International Date Line and was used to telecast the 1964 Summer Olympics in Tokyo to the United States. On 1st of January 1965, the operation of the satellite was handed to the USA Department of Defense who used it until April 1969.

The Hughes Aircraft Company (now Boeing) built the first fully commercial communications satellite to be placed in geostationary orbit. The satellite was operated by COMSAT. It was often called, ‘Early Bird’ but was officially named Intelsat 1. The satellite was launched on a Delta D rocket from Cape Canaveral on, April 6th, 1965. Intelsat 1 was mostly used for television and telephony traffic between Europe and the USA. It weighed 34.5 kg. A modern Intelsat such as Intelsat 31 weighs about 6,350 kg. The cost of putting a satellite into a geostationary orbit is prohibitive for amateur satellite constructors, however, adding an amateur radio transponder as a hitchhiker on a commercial satellite is possible. Es'hail-2 is a planned geostationary communication satellite owned and operated by Es’hailSat, the Qatar Satellite Company. It will include an amateur radio payload, which makes it a Hamsat as well as being a commercial satellite. Es’hail-2 will carry two ‘Phase 4’ amateur radio non-inverting transponders operating in the 2,400 MHz and 10,450 MHz bands. Advantages of geostationary orbit 1. It is possible to cover almost all parts of the earth with just three geostationary satellites. 2. 3.

The signal from the satellite is available 24 hours a day. Ground station antennas can be fixed (no requirement for tracking) and they can be small because the satellite uses spot beams to concentrate the downlink power.

Disadvantages of geostationary orbit 1. Larger antennas are required for northern/southern regions of the earth where the distance to the satellite is greatest. 2. High uplink power is required because the satellite is a long way out. 3. There is significant latency because the satellite is a long way out. This limits the number of satellite ‘hops’ that are allowed for international telephone traffic. It is not a problem for TV transmission where the small delay in transmission is not noticeable. The round trip delay is 253 ms, about a quarter of a second. It does not seem like much but you can tell there is a delay when you are talking to someone via a geostationary satellite. In a conversation, the delay is about half a second by the time you say

something and then the other party responds. If the call uses two satellite links, the one-second delay becomes very noticeable. 4. Placing a satellite into geostationary orbit is extremely expensive. The satellites are very large and very heavy. Intelsat 31 weighs 6,350 kg. With the solar panels stowed for launch, the satellite measures 8.6 x 3.4 x 3.1 meters. 5. Geostationary satellites need a lot of power because they have several very wide-band transponders. For example, Intelsat 31 has four C-band transponders and seventy-two Ku-band transponders. The downlink beams are focused using horn or dish antennas but the wide bandwidths mean that high power transponders are needed. The high power requirement means that geostationary satellites need to have large solar panel arrays. Recent geostationary satellites like Intelsat 31 can generate 20 kW of electrical power.

HOHMANN TRANSFER ORBITS Rather than launching a rocket all the way up to the intended MEO, HEO, or GEO orbit, it is more efficient to place the satellite into a low earth orbit and then transfer from that orbit to the intended high orbit. A Hohmann transfer orbit is an elliptical orbit that is used to transfer a spacecraft between two circular orbits in the same plane. The orbital maneuver to perform the Hohmann transfer uses two engine impulses, one to move a spacecraft onto the transfer orbit and a second to move off it. This maneuver was named after Walter Hohmann, a German scientist who published a description of it in 1925. If the satellite is in a low circular orbit and you apply thrust in the direction of the satellite’s motion around the Earth it causes the orbit to become elliptical with the perigee being at the altitude that the spacecraft was at when the thrust was applied. The new apogee is higher and on the opposite side of Earth. A second burst of thrust is applied when the satellite is at the apogee, lifting the perigee so that a circular orbit at the higher altitude is achieved. To move from a high circular orbit to a lower one the thrust is applied in a direction opposite to the direction of movement. This keeps the apogee at the same altitude but reduces the perigee to a lower height. The second burn against the direction of motion is applied when the spacecraft reaches perigee. This reduces the apogee and a lower circular orbit is achieved. The amount of thrust applied in the two engine burns is critical. Too much power on the first burn will cause the new apogee to be higher than desired. Too much power on the second burn will cause the new orbit to become elliptical rather than the desired circular orbit. These problems can be rectified using more thruster burns but this uses additional fuel. The energy required is called ‘delta V’ because it is the amount of thrust required to change the velocity by a calculated amount. You can achieve a transfer orbit using a smaller amount of thrust for a longer time, but the total energy required is as much as 40% higher. The same technique is used to shift a spacecraft from an orbit around the Earth to a new orbit around the Moon or a different planet.

GTO - GEOSYNCHRONOUS TRANSFER ORBIT A geosynchronous transfer orbit or geostationary transfer orbit (GTO) is a Hohmann transfer orbit that is used to move a satellite from an initial low Earth orbit up to a geosynchronous or geostationary orbit, using high thrust chemical engines. The geosynchronous transfer orbit is highly elliptical with an apogee of 35,786 km. If the final orbit is to be geostationary, the initial LEO orbit has to be at an inclination of zero degrees. The perigee can be at any height, but it is usually quite low. This is so that the rocket booster stages end up in orbits where the orbital drag will pull them down fairly quickly. The satellite and its booster are not at the low altitude long enough for drag to be a problem. If the satellite is using low-thrust engines such as electric propulsion to get from a GTO transfer orbit to a geostationary orbit, the transfer orbit can be super-synchronous, having an apogee greater than the final geosynchronous orbit. The low thrust continuous burn method takes much longer to achieve due to the low acceleration that can be achieved. The satellite's low thrust engines are run continuously as the satellite travels around the geostationary transfer orbit, with the direction of thrust changed to suit wanted vectors the as the orbit progresses. Spacecraft using a Hohmann transfer orbit take only a few days to reach the geosynchronous or geostationary orbit. But using low thrust engines or electrical propulsion, it can take several months to slowly change the geostationary transfer orbit into a circular geostationary orbit.

SSO - SUN SYNCHRONOUS ORBIT The Sun-synchronous orbit, also known as a heliosynchronous orbit, combines inclination and altitude in a way that makes a LEO satellite pass over a particular location at the same time every day. This is useful for control stations and it has other advantages. An SSO orbit can be used to ensure that the satellite is illuminated by the sun without ever going into eclipse, which is useful for small satellites with small or no batteries. Sun-synchronous orbits are good for satellites carrying out scientific imaging, oceanic research, weather imaging, ionosphere sounding, or atmospheric testing. Anything that needs the satellite to be over a particular region at the same time on consecutive days. Typically, the orbit would be arranged so that the satellite has twelve ascending crossings of the equator a day so that each crossing occurs at the same local time each pass. “Typical sun-synchronous orbits are about 600–800 km in altitude, with periods in the 96–100-minute range, and inclinations of around 98° (i.e. slightly retrograde compared to the direction of Earth's rotation).” Some Amsats and Hamsats are put into sun-synchronous orbits so that they will be over the builder’s country in the evening, a convenient time for ham radio operators to use it. A ‘dawn’ or ‘dusk’ orbit is arranged so that the satellite crosses the equator (ascending) at dawn or dusk. A satellite in this orbit will follow the day/night ‘gray line’ terminator around the world. A ‘noon’ or ‘midnight’ orbit always crosses the equator (ascending) at noon or at midnight.

RETROGRADE ORBITS A retrograde orbit is an orbit where the satellite is traveling in a direction partly opposite to the spin of the Earth, i.e. traveling, East to West rather than West to East. It is indicated by a satellite inclination greater than 90 degrees. Retrograde orbits are used when the time that the satellite will pass over a given location is important. The satellite orbit is not affected by its retrograde orientation but the local time that the satellite appears above the horizon is affected. It takes more energy to place a satellite in a retrograde orbit. Especially if the launch site is near the equator. The launch site, and therefore the rocket on the launch pad, is already traveling at the speed of Earth’s rotation. This means that if the rocket is fired in the same direction as Earth’s spin, it already has some of the speed that it needs to reach orbit. To launch a satellite into a retrograde orbit the rocket must overcome the speed of the Earth’s rotation plus the additional speed required to achieve orbit. Almost all of the satellites that are placed in retrograde orbits are put into polar orbits with only a few degrees of retrograde motion, so the additional fuel requirement is not significant. Most commercial Earth-observing satellites use retrograde sunsynchronous orbits to ensure that observations are performed at the same local time for each pass over any given location. Almost all communication satellites use prograde orbits. Geostationary satellites must use prograde orbits or they wouldn’t remain stationary. Many Amsats and Hamsats use sun-synchronous or normal LEO retrograde orbits with about 98 degrees inclination. A sun-synchronous orbit makes the satellite appear over your location at the same time every day. Non-sun synchronous retrograde orbits precess, so each day the satellite appears above your horizon a few minutes earlier (or later). FO29 gets about five minutes earlier each day. AO-07 gets about an hour earlier each day.

POLAR ORBITS Polar orbits have a high inclination, often slightly retrograde orbits at about 98 degrees. The advantage with a polar orbit is that they provide satellite passes all over the planet. Because the footprint of LEO satellites is so small, the latitudes covered is basically the same as the inclination. A satellite with an inclination of 45 degrees will cover all areas of the Earth that lies between 45 degrees north and 45 degrees south. A satellite in a polar orbit at 98 degrees covers latitudes between 98 degrees north and 98 degrees south. Because the designers want to provide worldwide access to their satellites, most LEO Hamsats and many Amsats are placed into polar orbits.

EQUATORIAL ORBIT The equatorial plane is mostly used for geostationary satellites. The inclination of a satellite in an equatorial orbit is either 0 or 180 degrees. A 0O degree inclined orbit indicates a prograde equatorial orbit, the satellite is orbiting in the same direction as Earth is rotating. A 180O inclined orbit indicates a retrograde equatorial orbit. In that case, the satellite would be orbiting in a direction opposite to the direction of the Earth’s rotation.

ECLIPTIC ORBIT The ecliptic plane is the plane of the Sun’s spin. All of the planets of our Solar System, except Pluto, orbit the Sun in the ecliptic plane. On Earth, the ecliptic plane is tilted at 23.5 degrees from equator because the Earth’s axis of spin is tilted. It is this tilt that causes the seasons as Earth orbits the Sun. A satellite in an ecliptic orbit around the Earth has an inclination of approximately 23.5 degrees. Its orbit is aligned with the planets and stars, rather than the equator. This is useful for satellites that look outwards, such as observatories and satellites that study the Sun, measuring solar radiation, solar flares, and sunspots. The Hubble space telescope has an inclination of 28.47 degrees, fairly close to the ecliptic plane.

SPACE AND ORBITS THE EDGE OF SPACE The official definition states that Space begins at 100 km above the Earth’s surface. It is known as the Kármán line. As you get higher, the air becomes thinner. Kármán calculated that an aircraft flying at about 100 km above the ground would have to travel so fast in order to get an aerodynamic lift from the atmosphere that its curved path around the planet would cause it to go into orbit. The Kármán line is approximately at the ‘turbopause,’ above which atmospheric gases are turbulent and not well mixed. It is also close to the coldest part of the atmosphere called the ‘mesopause,’ which features very cold regions as low as -100O C in two bands at altitudes around 85 km and 100 km. At altitudes above 160 km, there are very few atmospheric gasses or air molecules and the sky above looks completely black. The Kármán line is named after Theodore von Kármán (1881–1963), a Hungarian-American engineer and physicist. His research was primarily in the fields of aeronautics and astronautics. In the final chapter of his autobiography, Kármán addresses the issue of the edge of outer space: “Where space begins…can actually be determined by the speed of the space vehicle and its altitude above the earth. Consider, for instance, the record flight of Captain Iven Carl Kincheloe, Jr. in an X-2 rocket plane. Kincheloe flew 2000 miles per hour (3,200 km/h) at 126,000 feet (38,500 m), or 24 miles up. At this altitude and speed, aerodynamic lift still carries 98 percent of the weight of the plane, and only two percent is carried by [centripetal] force, or Kepler Force, as the space scientists call it. But at 300,000 feet (91,440 m) or 57 miles up this relationship is reversed because there is no longer any air to contribute lift. Only [centripetal] force prevails. This is certainly a physical boundary, where aerodynamics stops and astronautics begins, and so I thought why should it not also be a jurisdictional boundary? Haley has kindly called it the Kármán Jurisdictional Line. Below this line, space belongs to each country. Above this level, there would be free space.” Source Wikipedia. Technically, it is possible to put a satellite into orbit at any altitude. However, due to atmospheric drag, the lowest altitude at which an object in a circular orbit can complete at least one full revolution without propulsion is about 160 km. Most satellites in ‘low Earth orbits’ are at

altitudes between 500 and 1500 km. The International Space Station is comparatively low at about 400 km. When you think about it, space is not very far away. The edge of space is about 100 km up. You probably travel further when you go on holiday. But it is a lot harder to get to and so far, there are no hotels. At an altitude of 160 km, the density of atmospheric gasses is so thin that it is indistinguishable from a vacuum. Compared to the 12,742 km diameter of the Earth, the 160 km thickness of the atmosphere is very thin. That tiny layer of atmosphere is all that protects our planet from meteors and solar radiation. We should look after it.

GEOCENTRIC AND OTHER DEFINITIONS All of the artificial satellites that we are interested in are in Geocentric orbits. Geocentric means ‘Earth-centered.’ So a Geocentric orbit simply means that the object in question is in orbit around the Earth. All of the planets in our solar system orbit the sun so their orbits are Heliocentric. The solar system orbits the black hole in the center of the galaxy in a Galactocentric orbit. An orbit around the Moon is known as a Luna orbit. It is Luna-centric. There are many classes of Geocentric orbits including the circular, elliptical, geosynchronous, geostationary, sun-synchronous, polar, equatorial, and ecliptic orbits discussed in the last chapter. There are also classifications based on the altitude of the satellites; LEO, MEO HEO etc. and transfer orbits, which are used when a satellite is transitioning from one orbit to another.

GOING INTO ORBIT You can’t just fire a rocket straight up into space to put a satellite into orbit. If you do that, there are only two possible outcomes. Either the rocket will achieve escape velocity and travel away from Earth forever, or it will run out of fuel, gravity will gradually slow it down and it will fall back to Earth, hopefully burning up in the atmosphere. A rocket or satellite must be ‘inserted’ into an orbit. This is achieved by applying force (rocket thrust) parallel to the Earth’s surface. You may recall videos of Space Shuttle launches, which clearly show the shuttle transitioning to the orbital mode by rolling onto its back and curving away to the horizon. The Hitchhiker’s Guide to the Galaxy, by Douglas Adams, talks about flying. “There is an art; it says, or rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss.” In many ways, this is what any object that is in a stable orbit has achieved. Satellites fall towards Earth, but they don’t collide with it. In the early days of space exploration, the weightless feeling of being in orbit was called “free fall.” Achievement of an orbit is governed by centripetal force. Put any ideas about centrifugal force away in a dark box. The boffins all agree that centripetal force is the only force required to keep an object in orbit. For a satellite in orbit around the Earth, the force of gravity is the centripetal force pulling the satellite down towards the Earth. We all know the feeling of whirling a bucket full of water around and around at the end of a string. The bucket and the water defy gravity and fly out to the end of the string. The faster we spin the bucket, the more centripetal force we must exert to hold onto the string. If we let go, the bucket flies off at a tangent. It does not fly outwards from the center point in the direction that the string was. It flies in the direction that the bucket was traveling when you released the string. The ‘string’ holding a satellite in orbit is gravity. If gravity suddenly stopped working, all of the satellites circling the planet would fly off into space. Imagine that you are standing at the top of a high mountain, with Valerie Adams, Olympic shot-put gold medalist, a cannon, a rocket, and a supply of shot-put balls. You throw a shot put as far as you can towards the horizon. It does not go far, falling to the ground a few meters away. Because the force of gravity is pulling the ball downwards throughout all of the balls short flight, the course or ‘trajectory’ is a curve. Next, Valerie throws the shot-put ball. It flies much further, a personal best, but it still

follows a curved course to the ground. Now we try the canon. The ball travels much faster and flies much further eventually falling to the ground more than a kilometer away. In all three examples, the shot-put has achieved a sub-orbital trajectory. Finally, we attach a ball to the rocket and fire it horizontally straight at the horizon. This time the ball disappears from view. Maybe it completes a complete orbit of the Earth. Gravity is still acting to pull the ball downwards but the speed is so high the ball never reaches the ground. If the shot-put ball travels fast enough to reach an orbit, it has achieved an orbital trajectory. If it traveled fast enough to escape Earth’s gravity completely and disappear forever into space, it would have achieved an escape trajectory. Note that we have neglected the effect of air resistance, which would slow the ball as it flies.

Sir Isaac Newton’s laws of motion state that an object will travel in a straight line unless some other force acts upon it. In other words, if there was no gravitational force, the shot-put balls would fly away from Earth in a straight line like the bucket when you let go the string. The force of gravity pulls the balls downwards as they travel forward, creating a curved trajectory. If a ball is traveling faster, it flies further, still curving down towards the center of the Earth. If the ball is traveling fast enough, it will curve towards the ground but never land. At this point, the ball is in orbit. So long as the speed remains constant, with no air resistance slowing it down, and the force of gravity remains constant, the ball will continue to

orbit, and no other force is needed to keep it there. The Moon has been in orbit around the Earth for millions of years. Amateur radio satellite OSCAR 7 has been in orbit since 1974. Neither satellite has any motors or mechanism for changing their orbit. Actually, there are other forces acting on orbiting satellites. The Earth is not completely round. It is a bit squashed. Like many of us, it is a little fatter around the Equator than around the Poles. This means that the gravity experienced by the satellite varies as the satellite orbits. Especially if the satellite is in a polar rather than an equatorial orbit. The Earth’s magnetic field also has some effect and so does Solar radiation pressure. But the major cause of orbital decay is atmospheric drag, caused by the resistance of the spacecraft impacting gas molecules. As the satellite loses energy, it falls to a lower altitude where it encounters more atmospheric gasses. The increased drag causes it to drop more and eventually the satellite re-enters the atmosphere. As the satellite drops to lower altitudes its speed increases. The friction of high-speed collisions with air molecules and gasses causes extreme heat and in most cases, the satellite burns up before reaching ground level. In some cases, satellite components survive the extreme temperatures and crash into the Earth’s surface, usually into the sea. Important low orbit satellites like the Hubble telescope and the ISS have to be boosted back into their proper orbit periodically to compensate for orbital drag. The ISS has a huge cross-sectional area. It can lose up to ninety meters of altitude per day. At altitudes below 200 km, spacecraft orbits decay rapidly and the satellites usually de-orbit and burn up within a week. If a satellite is very big, its own gravity can cause a tidal effect on the larger body. That is why the Moon creates tides on Earth. If the satellite has an orbital period that is shorter than the primary's rotational period or is in a retrograde orbit i.e. orbiting in the opposite direction to the spin of the larger body, the ‘tidal bulge’ in the gravitational force between the two bodies saps momentum and causes orbital decay. Phobos a moon orbiting Mars, and Triton a moon orbiting Neptune, exhibit this decay in their orbits. They will both eventually crash into their respective planets. Luckily, our Moon is not in a retrograde orbit and its orbital period is much longer than the time that the Earth takes to complete a rotation. So, the tidal drag is minimal and the Moon will not crash into the Earth. Artificial satellites in retrograde Earth orbits do get slowed down a very small amount by the tidal effect that they have on the Earth.

ORBITAL VELOCITY AND ALTITUDE If an object is in a stable orbit, its velocity is governed by its distance from the center of the Earth, i.e. the radius of the Earth plus the altitude of the satellite above the ground. For a circular orbit, the velocity stays constant. For elliptical orbits, the velocity changes depending on the distance from Earth. When the satellite is at perigee close to Earth it is traveling at its fastest. It slows down as it travels out to the apogee and then speeds up again as it returns to perigee. Satellites in low orbits travel faster and satellites in high orbits travel slower. This can be a bit counter-intuitive. If you apply force (thrust) in the direction that the satellite is moving it will move to a higher orbit and slow down. A short burst of thrust will create an elliptical orbit with the apogee, furthest point from Earth, 180 degrees around the orbit from the point where the thrust was applied. Orbit type

Satellite Orbital period Planetary Moon 27.321 orbit days Geostationary GEO 24 hours * High Earth HEO 11.3 Orbit hours Low Earth LEO 1.8 orbit hours

Orbital velocity 1.10 km/sec 3.07 km/sec 3.94 km/s ** 7.3 km/s **

Mean distance 384,401 km 35,786 km 36,000 – 2,500 km 1323 - 400 km

* One sidereal day; 23 hours, 56 minutes and 4 seconds. ** Average velocity during the elliptical orbit. Rendezvous with an object in space is a tricky business. You can’t just fly up to a satellite or another spacecraft. The way that orbital velocity interacts with orbital altitude complicates things. If you accelerate the spacecraft, it moves up to a higher more elliptical orbit and slows down relative to the target object. If your fire your engine forward, so that the satellite decelerates it will move into a lower orbit and speed up. If the target is ahead of you, firing a thruster to move the craft upwards, makes you slow down, so you drift back away from the other craft. Moving down, makes you speed up, so you drift towards to the other craft. The easiest method if the other object is ahead of you in the orbit is to position your spacecraft slightly lower. This will let you catch up with the target. Then when you are very close, you can move up to the correct orbit and your velocities will be the same. If the target object is behind you in the orbit,

you would position your spacecraft into a slightly higher orbit and drift back to meet it. Rendezvous and docking with another spacecraft such as the International Space Station usually takes several hours and it can take several days.

SHAPE OF THE ORBIT Kepler defined that all orbits are ‘conic sections’ i.e. forms of ellipse that can be derived from the shape of a cone. Some satellites orbit in a circular path at a constant distance from Earth (altitude). Other orbits are described as elliptical, parabolic, or hyperbolic depending on their eccentricity, (how non-circular they are). A satellite in an elliptical orbit around the Earth follows a path around two focus points (foci). One is at the center of the Earth and the other is the point in space that the satellite turns around before returning towards Earth.

The distance from the satellite to the focus point at the center of the Earth, plus the distance from the satellite to the other focus point is the same at any point on the satellite’s orbit. It is equal to the length of the major axis, or twice the semi-major axis (SA). A + B = 2 x SA Orbital eccentricity is the distance between the foci, divided by the length of the major axis. It is a number between zero for a circular orbit, and one. The Perigee is the point in the orbit where the satellite is closest to Earth. i.e. the left side of the ellipse above. The Apogee is the point in the orbit where the satellite is furthest from to Earth. i.e. the right side of the ellipse above.

THE SEMI-MAJOR AXIS The semi-major axis is used in many orbital calculations because it is the average distance between the orbital focus point at the center of Earth and the satellite. It is usually calculated from the Mean Motion, which is one of the numbers supplied in the TLE data. Mean Motion is the average number of orbits that the satellite completes in a day. Two methods of calculating the semi-major axis are included in the Formulas section. The longest and shortest lines that can be drawn through the center of an ellipse are called the major axis and minor axis, respectively. The semimajor axis is half of the major axis and represents a satellite's average distance from the center of the Earth. The semi-minor axis is half of the minor axis. But it is not used to calculate anything useful. The major axis is line A-A’, the minor axis is line B-B.’ The semi-major axis is the half the major axis, i.e. line A-C.

The semi-major axis is also equal to the distance from the center of the Earth to the satellite when it is at perigee (point A), plus the distance from the center of the Earth to the satellite when it is at apogee (point A’), all divided by two. Semi-major axis SA = (RSAT at apogee + RSAT at perigee)/2

If the orbit is circular, the distance between the two foci is zero i.e. there is only one focus, so the orbital eccentricity is zero. The semimajor axis is equal to the Earth’s radius plus the altitude of the satellite. If the orbit is elliptical, the eccentricity is greater than zero and less than one. If the eccentricity becomes equal to one, the path is termed parabolic and the focus points become infinitely far apart. In other words, this is the point where the satellite has reached escape velocity. It will break out of orbit and fly away. Hyperbolic paths are used to accelerate a spacecraft by passing close to the Earth or another planet. In this case, the eccentricity is greater than one and the semi-major axis is negative. For a sub-orbital path, where the satellite does not reach an orbit but crashes into the ground. The semi-major axis is less than the Earth’s radius plus the altitude of the satellite. Mathematically, sub-orbital, parabolic, and hyperbolic curves are still considered to be ‘orbits’ even though they do not result in the satellite going completely around the Earth and returning to the point of origin. These paths are really parts of orbits rather than complete orbits. Eccentricity and the semi-major axis are two of the six orbital elements defined by Kepler. They describe the shape of the orbit, the apogee, and the perigee.

SATELLITE FOOTPRINT AND MAXIMUM DISTANCE Many illustrations show satellite orbits like this.

These images give a false impression. They are OK for illustrating high or medium Earth orbits, but they are grossly out of scale for low Earth orbits, which are really much closer to Earth’s surface. The ISS maintains an orbit with an altitude of between 330 and 435 km, which is only about 3% of the diameter of the Earth. The area on the ground that the antennas on a satellite can cover is called its footprint. On satellite tracking programs, the footprints of LEO satellites are drawn as a roughly circular image on the map of the Earth. Satellites that are high can cover a full hemisphere of the Earth, but low satellites like the ISS, Hamsats, and Amsats, are too close. Their antennas can only cover an area about 7000 km across. The diameter of the satellite footprint defines the maximum distance that you can communicate via a LEO satellite. As an experiment, shine a torch onto a basketball. At a distance, the beam covers one complete side of the ball, but as you get close to the ball, the circle of illumination reduces to a small circle on the surface. Assuming simple antennas rather than targeted spot beams, the footprint is a circle. When the footprint is drawn onto a flat map or computer screen, the shape looks distorted especially when the satellite is near the

poles. The footprint is larger when the satellite is at apogee (furthest from Earth) and smaller when the satellite is at perigee (closest to Earth).

The diameter of the footprint and hence the maximum distance that you can communicate via the satellite is calculated from the radius of the in Radians. However, Earth multiplied by two times the Earth angle since you don’t usually know the Earth angle, it is more usual to calculate the footprint from the Earth’s radius in km (RE) and the Satellite’s radius (RS), measured from the center of the Earth, i.e. its altitude plus RE, in km. Diameter of coverage footprint in km = RE x ACOS(RE/RS) x 2. The maximum possible communication distance between ground stations is equal to the diameter of its footprint in km. The record for communication via the FO-29 (JAS-2) satellite is 7599 km. The calculated footprint at apogee is only 7586 km, so this accomplishment represents communication when the satellite is technically below the horizon at both sites. The record was assisted by transmitting from a high site, which effectively extends the horizon. At very low elevation angles, the signal is refracted a little by the atmosphere. Working a satellite that is slightly beyond the horizon is not uncommon. However, it is unusual when both stations are doing it. You probably won’t need to know how many square km the satellite's footprint covers, but it is interesting to know what percentage of the Earth’s surface is included in the footprint.

The area of the footprint in square km, ASAT = Pi x RE2 x 2 x 1-Cos(


The surface area of Earth in square km AEARTH = 4Pi x RE2 Percentage of Earth covered by the satellite is ASAT / AEARTH %. For a typical LEO, the footprint is about 8.6% of the Earth’s surface when the satellite is at apogee and about 5.5% when the satellite is at perigee. If foreground obstructions such as buildings dictate a minimum elevation for your antenna, the footprint and maximum distance are reduced. See the mathematics section for a calculation that includes minimum elevation.

SATELLITE INCLINATION Satellite inclination is important if you live at high or low latitudes. It indicates the plane of the satellite orbit. An inclination of zero degrees indicates an orbit around the equator. Geostationary satellites travel in orbits with zero inclination. An orbit with an inclination of 90 degrees passes over both poles. An orbit with an inclination of more than 90 degrees is in a retrograde orbit (see the next section). I live at a latitude of 42 degrees south. A LEO satellite must have an orbital inclination of at least 42 degrees for it to pass over my house. A satellite at an inclination of 35 degrees will never come far enough south to pass directly over my location but I can still get a radio signal to it.

Most LEO satellites are in ’retrograde polar’ orbits with inclinations of 97 or 98 degrees, so over a few orbits, they cover everyone. But a few satellites are different. For example, LAPAN-A2 (IO-86) has an orbital inclination of only 6 degrees. The ISS orbits at an inclination of 52 degrees, primarily so that it covers the United States and Russia. It is ‘workable’ from most populated places.

PROGRADE AND RETROGRADE ORBITS An inclination between 0 and 90 degrees indicates that the satellite is in a ‘prograde’ or ‘direct’ orbit. This means that the satellite is orbiting in the same direction as the planet’s rotation. An inclination between 90 and 180 degrees indicates that the satellite is in a ‘retrograde’ or ‘indirect’ orbit. This means that the satellite is orbiting in the opposite direction to the rotation of the planet.

PIONEERS OF ORBITAL MATHEMATICS The idea that the Earth revolves around the sun is credited to a Greek scholar called Aristarchus of Samos who wrote of the idea in the 3rd century BC. But Nicolaus Copernicus (1473-1543) was probably the first to publish a mathematical model that placed the Sun rather than the Earth at the center of the universe. Italian scientist and astronomer Galileo Galilei (1564–1642) invented the telescope and discovered the moons orbiting Jupiter. Before his time these moons were considered to be bulges on the surface of the planet. He also carried out observations of sunspots, which are important for HF propagation. Galileo did not invent the idea that the Earth revolved around the Sun, but he did get into considerable trouble by supporting the theory. He was tried by the Roman Inquisition, found "vehemently suspect of heresy", and forced to recant. He spent the rest of his life under house arrest. As a result of his experiments in physics, he determined that objects with different weights (masses) were affected by gravity the same and that they fall at the same rate. His theory was famously demonstrated by Apollo 15 Commander David Scott when he performed a live demonstration for the television cameras. He held out a geologic hammer and a feather and dropped them at the same time. Because they were in a vacuum, there was no air resistance and just as Galileo had predicted, the feather fell at the same rate as the hammer. All objects released together fall at the same rate regardless of mass. This is important because it means that the speed that a satellite travels is not affected by the mass of the satellite. A big satellite will orbit at the same velocity as a small satellite at the same altitude. This makes it much less likely that satellites will collide with each other. The theory around how objects orbit the Sun, the Earth or any other object, was developed by Johannes Kepler (1571-1630). He calculated the laws of planetary motion and defined the six numbers that we use to define where a satellite is at any given time. To send an amateur radio transmission through a satellite we need to know when it will be above the horizon and where to point our antennas. Kepler data provides the numbers that we need to for these calculations. These days we use a satellite-tracking computer program to download the Kepler data from the Internet as a TLE file and to automatically calculate the tracking parameters.

Isaac Newton (1643-1727) discovered the laws of gravity and motion and invented calculus. He worked out that gravity kept the planets in orbit around the Sun and the moons in orbit around the planets. His calculations were very nearly correct and they are still used for most calculations involving force, motion, and gravity. Newton calculated the paths that objects will take when they are subjected to various forces, including gravity. He created a gravitational constant to make the mathematics work, but he didn’t know how gravity worked. The gravitational constant G = 6.674 x 10-11 N.m2/kg2. It is a very small number but it acts on every object that has mass, so it is very important. Newton’s laws of universal gravitation state that the attractive force between any two bodies is directly proportional to the product of their masses. In our case, the mass of Earth (ME) times the mass of the satellite (MS). And inversely proportional to the distance between the objects (r). Actually, it is the distance between the ‘center of mass’ of each object i.e. the center of the Earth and the center of the satellite. What this means is that big or massive objects have more gravitational attraction and objects further apart have less gravitational attraction. The net gravitational force between two objects F = (G ME MS)/r2. Because the Earth is very big and the satellite is very small, we usually consider that the satellite is orbiting the Earth rather than each body orbiting the other. It makes the mathematics much easier. Two objects of similar mass, orbit each other in a more complex way. Binary stars are an example of this type of orbit. Three similar size objects orbiting each other is very difficult to calculate and apparently, four similar sized objects orbiting each other creates so many variables that it is virtually impossible to calculate the paths of the four objects. The path of four smaller objects such as planets orbiting a larger body such as the Sun is easier to calculate, but each planet affects the orbits of all of the others. These ‘permutations’ or wobbles in the orbit of planets in our Solar system have been used to predict the existence of outer planets that had not at that time been observed by telescope. Albert Einstein (1879-1955) published his theory of gravitation in 1916 following his general theory of relativity published in 1915. These papers provided a unified description of gravity as a geometric property of space and time. In particular, he predicted the curvature of ‘spacetime’, which explained how gravity distorts space so that objects traveling in a straight line follow a curve in space.

Essentially, Newton said that gravity pulls objects away from the straight line that they want to travel in when a force has been exerted on them. Einstein said that the objects continue to travel in a straight line but that the path appears curved because space is curved by the presence of the mass of the Earth (or any other mass). In Newtonian physics, free fall is any motion of a body where gravity is the only force acting upon it. In the context of general relativity, where gravitation is reduced to a space-time curvature, a body in free fall has no force acting on it and moves along a geodesic. Source Wikipedia. Although Einstein’s theory has been proven correct and this disproves Newton’s theory, the differences are only apparent in situations where the gravitational effect is very large or the objects are traveling at extremely high relative speeds such as an object orbiting a black hole. Newton’s mathematics are much simpler and they are adequate for our calculations.

TIME DILATION ‘Time is an illusion, lunchtime doubly so’ (Douglas Adams) Einstein predicted that time is not absolute. It is affected by gravity and by the speed that you are traveling. For example, time passes faster on the Moon than it does on Earth. Any object with mass causes distortion of spacetime. Large masses like the Sun or the planets cause a larger distortion or ‘curvature.’ We sense this change in the curvature of space as gravity. A clock that is close to a large mass runs slower than a clock at a distance. You don’t have to go into space to prove this. Atomic clocks are accurate enough to confirm this effect with as little as a one-meter difference in height. This effect is not anything to do with the clock’s construction. It is a real difference in the way that time works. Also, if you travel faster, time passes more slowly. This was confirmed by experiments in which atomic clocks were flown in airliners, in both directions around the world. The time indicated on the clocks that had traveled was then compared with a reference clock that had not been moved. Time on the International Space Station ‘ISS’ is slowed by the fact that the space station is orbiting at a velocity of around 7700 m/s and it is sped up a little because of the station’s altitude. The relative speed has more effect than the distance above Earth, so the net effect is that the astronauts experience a slightly slower time compared to people on Earth’s surface. Satellites in higher orbits experience a slightly faster time. ‘The nature of spacetime is such that time measured along different trajectories is affected by differences in either gravity or velocity – each of which affects time in different ways.’ Source Wikipedia. No matter where you are, or what speed you are traveling, time seems normal to you. It’s those other people that are experiencing a slower or faster time. Of course, as far as they are concerned, their time is normal and it is you that is experiencing a slower or faster time. You might think that a few microseconds here and there are insignificant and for most satellite applications, they are. But time variations are very important for satellites doing mapping or providing data for location services. The accuracy of the positioning information from GPS satellites is dependent on comparing very accurate time signals received from three

or more satellites, and the GPS satellites experience time differently to us on Earth. The 31 GPS satellites are in Medium Earth Orbits at an altitude of about 20,200 km with an orbital speed of about 3,874 m/s. Time shift due to the satellite’s velocity is about -7.2 µs (microseconds) per 24 hour day. Time shift due to the satellite’s altitude is about +45.7 µs per day. The resulting time shift is +38.5 µs per day. It does not seem like much, but a onemicrosecond error is enough to create a 300-meter error in the accuracy of the distance calculation. Not so good for navigation of self-drive vehicles! To correct for the time dilation effect, the clock generators on GPS satellites are tuned to 10.22999999543 MHz before the satellite is launched. This results in an apparent clock signal of 10.230000 MHz from the satellite when it is in orbit. The clock oscillators can be even more finetuned while the satellite is in orbit to account for the very slightly noncircular orbit and for temperature drift. Why does gravity affect time? It is actually the same reason as the reason that velocity affects time. Mathematically, gravity is an acceleration towards the center of mass of the Earth. Time passes slower for anything undergoing acceleration. As gravity is an acceleration towards a center of mass, then time will move more slowly relative to something undergoing a lesser acceleration i.e. subject to less gravity. Note that while astronauts in space feel weightless, they are still being affected by gravity. Their forward velocity balances the effect of gravity so they don’t fall to Earth and they feel weightless. The time dilation that occurs is a result of both their orbital velocity and the acceleration of gravity. The ‘time slowing down’ and ‘time speeding up’ effects cancel each other at an orbital height of 50% of the planet's radius (3185.5 km). Time dilation due to reduced gravity at an altitude of 3185.5 km is +20.047 µs per day. Time dilation due the velocity required for that orbit is -20.047 µs per day. Also, assuming no slowing down due to atmospheric drag, at this altitude, the orbital speed of 6458.19 m/s is equal to the speed with which you'd hit the ground falling from that height. No kidding, it’s true! For a description of the formulas for calculating time dilation, refer to ‘Calculation of time dilation’ at ystem

KEPLER’S FIRST LAW OF PLANETARY MOTION Johannes Kepler published three laws of planetary motion. The first two in 1609 and the third in 1619. These laws govern the movement of any object that is in orbit around any other object. They are the basis for all satellite prediction programs. Kepler’s First Law, the ‘law of Ellipses,’ states that ‘The orbit of a planet is an ellipse where one focus of the ellipse is the sun.’ This law states that all orbits are ellipses, even circular orbits, which are an ellipse with only one focus point. The longest and shortest lines that can be drawn through the center of the ellipse are called the major axis and minor axis, respectively. The semi-major axis is half of the major axis and represents a satellite's mean distance from its primary. It is also equal to the distance to the satellite at perigee plus the distance at apogee. The semi-minor axis is half the minor axis. The orbital eccentricity describes how elliptical the orbit is.

The distance from the satellite to the focus point at the center of the Earth, plus the distance from the satellite to the other focus point is always the same. It is equal to the length of the major axis, or twice ‘a’ the semi-major axis. A + B = 2 x a

Orbital eccentricity is the distance between the foci divided by the length of the major axis and is a number between zero for a circular orbit, and one.

KEPLER’S SECOND LAW OF PLANETARY MOTION Kepler’s Second Law, the ‘law of equal areas,’ states that ‘A line from the planet to the sun sweeps out equal areas in equal amounts of time.’ This law determines that the speed of a satellite is governed by its distance from the mass that it is orbiting. Satellites speed up as they get closer to Earth and travel slower as they reach apogee. Planets orbiting close to the sun travel faster than planets that are further out.

In the drawing below. If area 1 is equal in size to area 2, the satellite will take the same time to travel from p1 to p2 as it does to travel from p3 to p4. Since the distance between p1 and p2 is further than the distance between p3 and p4, it follows that the satellite is traveling faster between p1 and p2 than it is between p3 and p4.

KEPLER’S THIRD LAW OF PLANETARY MOTION Kepler’s third law, the ‘law of harmonies,’ states that the period of a planet's orbit squared is proportional to its average distance from the sun cubed. The third law states that if ‘a’ is the semi-major axis and ‘T’ is the orbital period, then T2 = a3. Note that ‘a’ must be in ‘astronomical units’ (AU) and ‘T’ must be in years. T2 x a-3 is a constant that applies to all planets in the solar system. The formula explains why the spacing of the planets orbiting the sun gets bigger as you go further from the Sun. The outer orbits have longer orbital periods and every time the orbital period is squared, the distance is cubed.

Earth Mars Jupiter

Period Distance Years AU 1 1 1.88 1.52 11.8 5.2

Kepler’s third law means that if you know the time that it takes for a satellite to complete an orbit you can work out its orbital path. The semimajor axis is the average distance between the orbital focus point at the center of Earth and the satellite. If you know the semi-major axis, calculated from the apogee and perigee, you can calculate the orbital period. A more accurate formula for satellites orbiting the Earth takes the ‘standard gravitational parameter’ µ = G x ME into account. G · ME · T² = 4 · π² · a³ µ · T² = 4 · π² · a³ To find the orbital period of the satellite, the formula can be rearranged to; T = √((4π²/ µ) x a3) or T=2 √(a3 / µ ) To find the semi-major axis the formula can be rearranged to;

a = (T2 µ / 4π²)(1/3) G is the universal gravitational constant 6.674E-11 ME is the mass of the Earth 5.9722E+22 kg (we ignore the tiny by comparison, weight of the satellite) µ = G x ME = 3.986004418 x 1014 m3s-2 T is the orbital period in seconds a is the semi-major axis in meters Source for this


SIX KEPLERIAN ELEMENTS AND TWO MORE FOR LUCK The traditional orbital elements are usually referred to as the six Keplerian elements, after Johannes Kepler and his three laws of planetary motion. These six numbers are used to precisely, locate any object in orbit. If you know exactly where the satellite was in its orbit at a particular (epoch) time, you can work out where it will be at any future time. Two elements determine the shape of the ellipse: Eccentricity Semi-major axis Two elements describe the orbital plane: Inclination Argument of perigee Two elements describe the time it takes to complete an orbit: Right ascension of the ascending node Mean anomaly Two additional elements let us work out precisely where the satellite will be at any future time. They are essential for satellite prediction and tracking programs. They are: Epoch, and Drag Due to atmospheric and other drag factors, satellite orbits change with time. To track the satellite we need to know when and where the satellite will appear in the sky. Your calculations, or more likely the ones done by your satellite tracking software, need the six Kepler numbers. The ‘Kepler’ data for LEO satellites should be updated every couple of weeks. If you are intending to track the ISS, you should always use new data because the satellite is regularly re-boosted to a higher orbit. CubeSats launched from the ISS are in low orbits with consequently high orbital drag. Their orbital parameters change quite fast so you should download new satellite data regularly. The calculations for geostationary satellites are much easier because they have been placed into circular orbits, which make them stay above the equator at a fixed longitude. Their inclination is zero and their eccentricity is very small. The orbital period matches Earth’s sidereal day, 23 hours, 56 minutes, and 4.09 seconds. This means that we already know three of

the six numbers. To find a geostationary satellite you only need to know its longitude and your own longitude and latitude. Eccentricity The orbital eccentricity is the distance between the two orbital focus points, divided by the length of the major axis. It is a number between zero, for a circular orbit, and one. If the eccentricity is greater than one, it means that the satellite is in a parabolic or hyperbolic orbit and will break out of stable orbit. If the orbit is circular, the distance between the two foci is zero i.e. there is only one focus point and the orbital eccentricity is zero. In that case, the semi-major axis is equal to the Earth’s radius plus the altitude of the satellite. Semi-major axis The semi-major axis is half of the major axis. It is the average distance to the satellite measured from the center of the Earth. If you know the eccentricity and the semi-major axis, you can calculate the apogee and perigee and also plot the elliptical path that the satellite will follow. Inclination The inclination is the difference between the satellite’s orbital plane and the equator. It ranges from 0 degrees to 180 degrees. A satellite in orbit around the equator, for example, a geostationary satellite, has an inclination of zero. A satellite that passes directly above both poles (real, not magnetic) has an inclination of ninety degrees. Most LEO satellites of interest to amateurs are in polar orbits with around 97 to 98 degrees inclination. An inclination greater than 90 degrees indicates a retrograde orbit meaning that the satellite is traveling in a direction that is partly against the direction that the Earth spins. An inclination less than 90 degrees indicates a prograde orbit where the satellite travels in the same direction as the Earth as it rotates. If a LEO satellite has significantly less inclination than your latitude (north or south), you may not be able to hear signals from it, because it will never rise over your horizon. Argument of perigee We know the shape of the ellipse from the eccentricity and the semi-major axis. However to track the satellite we need to know where in the orbit the

satellite will be when it becomes available. Will it be at its farthest point (the apogee), or at its closest (the perigee), or somewhere between? The perigee of a satellite’s orbit is the point where the satellite passes closest to the Earth’s surface. For a LEO satellite, the perigee will be at a different longitude every orbit. But the latitude where the satellite reaches perigee stays the same all the time. It is always at a certain number of degrees around the orbit from the ‘ascending node,’ which is the point where the satellite crosses the equator traveling from south to north. That might be confusing! Let’s try it another way. An orbiting satellite with an inclination greater than zero (i.e. not orbiting the equator), will cross the equator twice every orbit. One time it will be traveling in a north to south direction and later it will cross in a south to north direction. We use the point where the satellite crosses the equator heading in a south to north direction as a reference point. It is called an ‘ascending node.’ At 180 degrees further around the orbit, the satellite will cross the equator traveling north to south. That point is called a descending node. When the satellite has completed 360 degrees around the orbit, it will have circumnavigated the world and will cross the equator heading south to north again. At some point during each orbit, the satellite will pass through its highest point (apogee) and its closest point (the perigee). The number of degrees around the 360-degree orbit from our reference point (the ascending node) to the point where the satellite is at its closest to Earth (perigee), is called the ‘argument of perigee.’ It can be anything from 0 to 360 degrees. The descending node is not used for any calculations. It is simply 180 degrees around the orbit from the ascending node. If the satellite is in an equatorial orbit (0 degrees inclination), or a circular orbit (no perigee) the argument of perigee is undefined. Right ascension of the ascending node (RAAN) From the perspective of the ground, a LEO satellite passes over the equator as many as thirty times a day. Half of those crossings are in the south to north ‘ascending’ direction and the other half are in the ‘descending’ direction. These crossings of the equator occur at different longitudes every orbit. But if you looked at the satellite and the Earth from a point far away in space, you would see that, in reality, the satellite is following the same path around and around, while the Earth turns below it. This is why the reference to the ‘right ascension of the ascending node,’ is a defined point in space rather than a point on the surface of the Earth.

The equatorial inertial coordinate system (GEI) is a way of describing the position of a satellite using the Earth's equatorial plane as the reference plane and the ‘First Point of Aries’ as the origin of longitude. There is more about coordinate systems later, but here is the short version. The bearing to the ‘First Point of Aries’ that is used for the reference direction, is an imaginary line from the center of the Earth to the position that the sun is in at a specific time that only happens once a year, called the ‘spring’ or ‘vernal’ equinox. For more on this please see the section titled ‘Vernal equinox and the first point of Aries.’ The equatorial inertial coordinate system (GEI) coordinates are also known as ‘celestial’ coordinates because they are fixed to a point in space rather than a point on the Earth. The RAAN (right ascension of the ascending node) angle is measured eastwards (i.e. right if you are facing north) from the ‘First Point of Aries’ reference line to the first ascending node. An ascending node is a point where the satellite crosses the equator while moving south to north. RAAN can range from 0 degrees to 360 degrees. Although it is sometimes called a longitude because it measured in degrees around the equator. It actually has nothing at all to do with the latitude or longitude shown on maps or the globe. The RAAN and the argument of perigee are used to locate where the satellite is in its orbit and its position above the Earth’s surface. Epoch The Kepler satellite position data in the TLE file is a snapshot taken at a particular and very accurate time called the ‘epoch.’ The epoch number is made up of the reference year, day, and time accurate to eight decimal places. By knowing what the six Kepler values were at the Epoch time we can ‘run the clock forward’ to any time in the future and calculate exactly where in its orbit the satellite will be. Tracking programs run the calculations every few seconds to predict the satellite’s position throughout the pass over your location. Mean anomaly (MA) The mean anomaly number in the TLE data file indicates how far around the orbit the satellite was at the TLE epoch time. It is measured from the perigee and is based on a circular orbit at the average, or ‘semi-major axis’ distance. MA can range from 0 to 360 degrees. It is 0 degrees when the satellite is at perigee and 180 degrees when the satellite is at apogee.

To find the mean anomaly at a future time, you add the mean anomaly at the epoch time (from the TLE file), to the ‘mean motion’ multiplied by the time that has elapsed since the epoch time. M(t) = M0 + n (t - t0) M(t) = the Mean Anomaly at the time you want, i.e. now. M0 = the Mean Anomaly at the epoch time from the TLE data; n = the satellite orbit's Mean Motion from the TLE data; t = the time chosen; and t0 = epoch time. In the ‘early days,’ the mean anomaly was sometimes referred to as the satellites ‘phase.’ The measurement was revised so that there were 256 time periods called ‘ticks’ around the orbit rather than 360 degrees. If you see reference to MA being between 0 and 256 ‘ticks’ with the apogee occurring at 128 then the scale is in terms of phase. I assume that this was done so that the MA could be described using a single 8-bit computer byte. Computers have advanced since then and nobody uses phase to describe MA anymore. Drag term In a perfect world, any satellite in orbit would continue to orbit forever. But the real world is more complex than the Kepler model suggests. The drag term is used to describe how susceptible the satellite is to atmospheric drag. It is calculated as a ratio between the size of the Earth compared with the surface area and mass of the satellite. Needless to say, it is a very small number. The drag on a LEO satellite increases over time as the satellite orbit degrades, eventually causing orbital decay and re-entry. The change in the drag parameter over time is the main reason that you should use reasonably recent TLE data files. The lower the satellite, the more recent the data file should be. In addition to atmospheric drag, there are other minor influences on the motion of satellites. The Earth is not a perfect sphere and gravity is not evenly distributed. Also, there is the pressure of solar particles and interactions between the gravitational fields of the Sun and the Moon. But all of these forces are so small that they are ignored by most of the software that amateurs use. Professional satellite tracking programs compensate by introducing minor corrections known as perturbations.

QUICK QUIZ QUESTIONS 1. What height is required before an object can be placed into an orbit around the Earth? A. 100 km above the surface B. 160 km above the surface C. Any height that clears local obstructions like mountains 2. To put an object into a stable orbit you must; A. fire a rocket up to orbital altitude B. apply a force to make the object move parallel to the Earth’s surface C. use the atmosphere to provide lift 3. The speed of a satellite in a stable circular orbit; A. balances the centripetal force of gravity so that the satellite follows the curvature of the Earth. B. is faster the higher the orbit is above the Earth’s surface C. causes any clocks on board to run faster than a clock on the Earth’s surface 4. How does something that weighs as much as a bus stay up there? A. Once the satellite is in orbit no additional force is required to keep it in the same orbit B. The satellite carries fuel to provide additional thrust when the satellite begins to fall out of orbit C. All satellites fall into the atmosphere eventually 5. Einstein and Newton both produced theories about gravity and calculated how the planets orbit the Sun. A. Einstein’s theory replaced Newton’s so now we always use his formula to calculate the orbit of satellites B. Newton’s formula is easier to use and accurate enough for us to calculate the orbit of satellites

C. The Earth is very big compared to the satellite so, neither theory is relevant to how satellites orbit the Earth

6. Gravity and the speed that the satellite is traveling both affect time at the satellite relative to the time on the Earth’s surface. A. The small amount of time difference is of no importance for GPS satellites B. The clock signal of navigation devices is changed to compensate for the time difference C. The clock signal of GPS satellites is set to compensate for the time difference 7. A satellite in a stable circular orbit travels at a speed; A. that is determined by the orbital altitude B. that depends on the mass of the satellite C. if you add more force you can make the satellite orbit at any speed you want

QUIZ ANSWERS 1. The best answer is B. A satellite can be put into orbit at any height that clears local obstructions like mountains. But actually, this is only true on airless places like the Moon. On Earth, the drag of air hitting the satellite makes it impossible to achieve a stable unpowered orbit below about 160 km. 2. The answer is B. You have apply a force to make the object move parallel to the Earth’s surface. The satellite will orbit when its velocity parallel to Earth is high enough to keep ahead of the effect of gravity pulling towards the Earth’s center of mass. Firing the rocket straight up will cause it to fall straight back down. You must curve the flight path so that the spacecraft starts by going straight up but ends up flying parallel to the Earth’s surface at the wanted orbital altitude and speed. Rockets don’t usually use atmospheric lift, they have no wings. At the speed required to get a satellite up to the wanted altitude wings would create too much drag. The space shuttle had small wings but they were not used to assist the spacecraft into orbit. 3. The answer is A. The speed of a satellite in a stable circular orbit balances the centripetal force of gravity so that the satellite follows the curvature of the Earth. The satellite wants to travel in a straight line, but gravity pulls it back towards Earth, forcing the satellite to follow a curved path. The orbital speed is directly related to the altitude. When the speed is too low, gravity wins and the satellite crashes. When the speed is just right the satellite falls but never hits the Earth. An even higher velocity would cause the object to go into an elliptical orbit. If the speed is very high, the satellite will achieve escape velocity and travel away from Earth forever. The speed that a satellite travels is slower, not faster, at higher orbits. Velocity causes any clocks or oscillators on board the satellite to run slower than they would on the Earth’s surface (not faster). Altitude (distance from Earth), causes any clocks or oscillators on board the satellite to run faster than they would on the Earth’s surface. 4. There is an element of truth in all three answers, but A is the best answer. Once the satellite is in orbit no additional force is required to keep it in in the same orbit. Some satellites such as the space station and the Hubble space telescope do carry motors and fuel to provide additional thrust when the satellite begins to fall out of orbit as a result

of atmospheric drag. But this is the exception. Satellites in higher orbits don’t require any additional thrust to stay in the same orbit indefinitely. I guess that orbital drag will claim most of the satellites orbiting the Earth eventually but it will take thousands, maybe millions of years in the case of high satellites in geostationary or geosynchronous orbits. 5. The answer is B. While Einstein did provide a more complete understanding of what actually causes the effect that we know as gravity, Newton’s formulas are easier to use and they are accurate enough for most situations. You only need Einstein’s formulas when orbital speeds and masses are extreme such as calculating the orbit of an object circling a black hole. Newton calculated the effect of gravity without understanding what causes it. He used a gravitational constant to make the unexplained effect of gravity work in his formulas. Option C is completely wrong. Both Einstein’s and Newton’s apply to any object, which is orbiting another. 6. The answer is C. Although the small amount of time difference is of no importance for communications satellites, it is very important for GPS satellites. GPS based mapping requires reception and triangulation of very accurate time signals from three or more GPS satellites at once. Even a small timing error can cause large errors in navigation. It is the satellite clock that is compensated because the receiver on the ground has to receive signals from multiple satellites. GPS satellites use extremely stable clock sources that are many times more accurate than the clocks of portable navigation devices. 7. A is the correct answer. A satellite in a stable circular orbit travels at a speed that is determined by its orbital altitude. The orbital velocity is completely independent of the mass of the satellite, so that rules out answer B. If you add more force, you will push the satellite into a higher orbit and it will, in fact, slow down. So that rules out answer C.

UNDERSTANDING TLE FILES The data files required by your satellite tracking or prediction program are called TLE ‘two line element’ files. They are issued by NORAD in plain text format. Each TLE file contains the data for a group of satellites such as the amateur satellites, navigation, or weather satellites. You can download the TLE datasets for most satellites from or or Most satellite tracking programs remind you to download and update TLE files automatically. The text for each satellite in the file looks like this. NOAA 6 1 11416U 84123A 86 50.28438588 0.00000140 00000-0 67960-4 0 5293 2 11416 98.5105 69.3305 0012788 63.2828 296.9658 14.24899292346978

Name of the satellite: The name is an eleven character field, containing the name associated with the satellite. In the example, it is weather satellite NOAA-6.

Celestrak TLE files contain more detailed names than the AMSAT files do. For example, ‘OSCAR 7 (AO-7)’ instead ‘AO-07’ and ‘JAS-2 (FO-29)’ rather than ‘FO-29.’ The two lines of numbers following the name are the same. Line one Line one of the TLE data contains information about the date and time of the dataset, the satellite’s catalog number, its launch date, and drag coefficient. Satellite Number The NORAD catalog number. In the above example, it is 11416U. A catalog number is issued for all of the objects in space that NORAD tracks. The U means that the data is unclassified, not secret or restricted. International designator The international designator contains the launch year, the launch number in that year, and a letter indicating the object resulting from the launch. The format is YYLLLXX. In this example, the designator is 84123A. Indicating that the satellite is part A of the 123rd launch in 1984. If other satellites were launched on the same rocket, or associated bits of space junk like rocket housings ended up in orbit rather than burning up, they would be objects B, C, and D etc. Epoch date and time The epoch time is the reference year, day, and time of the particular TLE data set. The time is UTC from midnight. The format is YY DDD.TTTTTTTT. Epoch time is very important because it is the reference time for the calculations that work out where the satellite is now or at any future time.

In our example, the epoch is 86 50.28438588. This equates to Year = 1986 Day number 50 = 19th February, (day 1 is New Year’s Day) Time = 0.28438588 of a day = 6.82526112 hours after midnight UTC. Ballistic Coefficient

The ballistic coefficient is also called the ‘first derivative of mean motion.’ It is the daily rate of change in the number of revolutions of Earth the object completes each day, divided by 2, expressed in revs/day. The ballistic coefficient is an indication of how much the satellite is slowing down due to atmospheric drag. Ballistic coefficient is not used by most modern satellite prediction programs. They use the BSTAR number instead. Second Derivative of Mean Motion The ‘second derivative of mean motion’ measures the second time derivative in daily mean motion, divided by 6. The units are revolutions per day cubed. It is not used by most modern satellite prediction programs. Drag Term (BSTAR) or (B*) The BSTAR is a measurement of how susceptible the satellite is to orbital drag. It is a measure of the satellites mass and surface area referenced to the radius of the Earth so the numbers are small. It is a denominator and the last two characters define a power of 10 so, 67960-4 means 0.67960E-4. BSTAR drag is used in place of the ballistic coefficient and second derivative of mean motion to calculate the rate that the satellite is slowing down due to atmospheric drag. Ephemeris type Ephemeris type is no longer used. It is always set to ‘0.’ Element Set Number The element set number was a running count of all ‘two line element’ sets that had been generated for the satellite. In the example, it is 529, but it is now no longer used and it is always set to 999. Checksum The last digit of the line is the checksum for line one. The checksum is a Modulo 10 number. It is used to ensure that line one of the TLE data is valid. Line two Line two of the TLE data contains most of the information required by satellite prediction and tracking programs. The first digit of line two is 2. Satellite Number The NORAD catalog number is repeated, without the classification letter. Inclination (0 – 180 degrees)

The inclination is the angle between the equator and the orbital plane. The value provided is the mean (average) inclination. An inclination of 0O or 180O indicates an equatorial orbit. An inclination near 90O indicates a polar orbit. If the inclination is between 0 degrees and 90 degrees the satellite is in a prograde orbit (satellite traveling in the same direction as Earth’s rotation). If the inclination is between 90 degrees and 180 degrees the satellite is in a retrograde orbit (satellite traveling in the opposite direction to Earth’s rotation). For NOAA-6, the inclination is 98.5105 degrees, a fairly typical retrograde polar orbit. Right Ascension of the Ascending Node (0 - 360 degrees) The angle between the vernal equinox and the point where the orbit crosses the equatorial plane (going north). For NOAA-6 the RAAN = 69.3305 degrees. Eccentricity A constant defining the shape of the orbit (0 = circular,