The mobile manual for radio amateurs. [2d ed.]

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The MOBILE •

MANUAL /or RADIO AMATEURS -

-...

COPYRIGHT 1960 BY

THE AMERICAN RADIO RELAY LEAGUE, INC.

I Copyright securl!d under the Pan·Americon Convention

lnternollonol Copyright secured

This work lo Publication No. 21 of The Radio Amalour'e

Librmy, publuhod by lho Looguo. All righta rosorvod . N o pciri of thjs work may ho rpr oduced in =Y form oxcopl by wril1on pormiuion ol lho p\lbUsher. All righb ol lrt signal, and it is helpful if the signal cnn be sci. aL J.A6 ::\fc. \YiU1 C1 at half C!tpacitauco, Co is adjusted l)ntil Lbe test signal is hoard. It is adviMblo Lo check the frequency or the high-rrcqucmc'y oscillfl.tor n.t, this point to make 1>"Ure tJmt iL is 1ttljuslcd w Ll10 low-frequency side of the in put 1nixcr circuit. C 1, La, Lo and L; should be tu11cd for m:l..ximum converter sensitivit.y. The variable inductors of the original model rc!sona.Led wi th the Luning slugR nl.>ouLhalfway in. The frcq11cncy n.nd s!;ability of the crystal-con Ll'ollc0 d11al-lriodc amplifier, buL lht' (l switch on the auto rc>r:r.iver. This 1'ncrgizly \\'it h some of the more fldvnnced or co rnplic~led 1111its. A ty pe 1.281\7 pc11k1gri1l cou vcrt K1A of the pus.h-t,o-talk rcla.r, .I{1. Crystal freq ue11cy is :~000 kc:. which pli\ces !J1c 75-meter pho11c hand bcl,w1•u11 800 aud 1000 on t11e b roadcast. receive r dial. When operating on 40 meters, the second hnrmouic or the crysh1l is used ond I.he hroaduasl. tfin.I is t.uued from 1200 to 1300 kc. lf crystal ac:Livity appears to be sluggish, t.he 250-1,µI'. c11pm:il.or mr1y he replll.ced wilJi one ol' high c1· Vt\ lue to s(.n •11i;l.he1l for U!'e u.s a r.f. or i.L amplifier, unc.1 tlrn l2A01i is a hepl,oclc in tended for prmtagricl-co11verl~~ r app licn.l,ions. Bot.h l,ype>< a.re prinrnrily i11 tc11dl'd for 1Jpen1.ti on :d, l)l'(mdcast frequcncio~, hu t (Jt1r 011·11 work wilJ1 U1e tubes vroves Lhem to he quite aclnpLal>le l.! several iLdvant~1ges. l'ir;;L, L11ey nfTl'r a worlhwhile reduction in ctu-reut. drain :1s tumparcd lo olltl'r tubes. For example, 1J1c

21-21.:ow, robll-c U1 to appro:1.im11.tcly lialf rn.pacil:111ce nnc.J t he11 ad.inst L2 to resonance (ufic 1.ltcl g;rid-dip mc lc•1· 11s U1e indi r..ator) aL t.he low end of the band. MovP t he grid-dipper over to IJ10 plate circu_it of tlw amplifier :tnd peak 14 a.t the cen ter of t.hc hnnd. Next, couple the mrter to L1 of t.l1c oscilln.tor and t.une the coil to the rrcqucnc:y of t he cryst11.l in uise. AHcr t h(•sc initial adjustrnc1rt.s, p l:~to p Oll'l' t'

44 m:·1y l>c· nppli1•d lo tl1C' rc111v(•rla11J~. \ 'oh11t.:r n•adi11g, th1u i11clir:1lo normal opera-

MOBILE FRtJQURNCY

Band Jfc.

CnART P'Oll ·r1rn Co~Vlmrnn

I

---

Crystal Frcg., Jfc.

I .I'. lfonyc J\r.

:? ti

1;:i0-1 JOO tiOO !JOO liOO !l:>O liOO 1050 GOO 1500 GOO 1500

3.5-1 7- i.3

Ii .J

H-1-1.35 21- 21.-!5

1:1 I :?O J

28-28.9 28.8-29.7

,\ J 01111.11

:?7 . 1

:? 2

---

Kote: I.i. range i11dicnks hro:idra~L receh-er tuning range nrcossary for cowring the ::issocinted nmntr ur fr, mounted on smotl b rackets fastened to the side wotls of lho converter hous· ing is 4 15/ 16 inches long, 2 Inches wide o nd l Y2 Inches deep. V1 is mounled on lhe chouis lo lhe rlghl o f V: os seen in this rear view. J1, J~ and h ore in lino in thol order from lefl lo right acrou the reor wotl of tho thouls. An interior view of a coil comportmenl Is shown In 1ho fore· ground. Terminals of the coils are soldered direclly lo the soclcet terminals. Notice thol the crystal for the osciHalor is mounted odjacenl 10 lc

J

Receiving

45

» '/'his l'i

cryslal-conl.rolled lmnsisl-Or c01u.'erler powered by dry tis ca/fa for a minimwn of i11!Jlallatio1i problems. Only t1co

n11111rcJiom1 are necessary.

Transistor Mobile Converter M . F. DE MAW,• W8HHS

cr_r,.t.'11-conlrolled cou,·ert.cr 1lestribed io this nrticle has m:iny fcnturc:1 tJ1at should rnake it especially appenlini.: to the mobile operator, ns woU ns to tho experimenter who is interested in trnoi;istor circuitry. 0110 of tho most intcn'~ling chnmctcriFtics of the circuit is its ~implicity . n i cryst.al-controlled, fi xed-tuned convert C' r whirh can be mndo very compact a11d exhibits exccllcn~ performance when used in conj1111ct,ion with tho n11Lo mobilo rcccivol'. With ~ligh t motlific11Lio11 of t ho oaoillaLor froquonc:y, it, r.n.1 1 bo used cffec:Livcly in co11ju110Lio11 wi Lh the popular Q5-cr from t.ho Conunnnd l::iot series. TILis should also hrJ a p:~rLiculnr nLlrneLion Lo tho Novice who desires ndc..litio1111l b1111dsprcn1l for SO a1ttl '10 meters. All of t he componcnt:1 for tho convert.er nrc boui:cd in ·~~mull .\li11ibox U1nt CAil bo co11cou.lcd behind the cla.shbo:ird of the car. This N1ntribut.cs to much better fomily relations in ccificd for the 4.-Mc. eil• are clilficolt. to obtain t.hc ncxt.-P.-oxima.tcly the 8'llilC inducto.uee. Altcrn11tively, c1>mmerei.ully-made slug-tuned coils such as t.ltu North Hillil 120 orories or the CTC LS3 series may be used for this ill\cl other frequency ranges, wi1h link-,, as dc~cribocl (•bovc added a11 wai: Uu· oppflrlu11it.v ror :i liWe "oatmctil type" l'l'l cycle. TJ1c 11sm1lly-n:commended v:1l110 of o:willaL-Or uljor.t ion vollnge tot.he mixer omiLll!r is i11 Uie ncighbod1oud of 250 rniHivolt.s peak. I. sclcclcd 300 millivo!Ls peak o~cillalo r i11jr.cLio11 1·nl t.a gc 1

It, is ku own that; for the oscillt~l.or vc1l u1ge

Test setup used by W6ZNM for the meo•urements described in the text.

\

Receiving

51

but, ~ubscq11rntl_v di~l'Overed that this v:1l11c rar PXceeJcd llw -meter inst.allatioo it wns discovered thnb the ordinary r.f. choke (l prefer the iron-core or fe rrite type) provided the moi;t, effeclfre h;ol:\tion be t.ween tJ1e desired signal and t.he u11wnnkd 11oiM. The capacit-0r made the poorest showing ror two l'easons, (1) the low irnpedanr,e or the cm 11oise circuit., and (2) t.he su~­ ceptibilit.y trn111>i~tors to signals of very low mnplit.udn. 111 some cases, "bypass" capaoit;ors acLtta.I Jy f11nd io11r,J 11$ (:Oll)Jli.ng capacitot'S to bring i11 addit., io11:d 11oiso.

nr

TRANSISTOR CONVERTER uowN in Fig. 4 is :i circuit dingr.im of a. -:10m cler tmnsistorized crystal controlled conv.-rlcr. Tt is n i;imple circuit, yet. works surprisingly well considering lhe small numhcr of parts it uses. The circuit is bal"icallv a. Pierce oscillator wit h an unL11ned collector. Tl1e transistor used is one of t.he lCS!\ expensive types nnd the crystals can be s11rpl11s. 13nsically, the circuit oscillates as n c ryst.al-controllcd oscilltitor while the input signal is inject.cu into the bnse. After mixing, the dePircd sig1111l is picked out by o. tunable receive r. I 11~c u ;j to G Mr.. ARC/ 5 receiver witJ1 my cv11vcrLC'r. Hcsistor R1 may M.ve a value from 100,000 ohms Lo J megohm. However, I fow1d 3!l0,000 ohma worked besL in my circuit. Wilh 3 volL'l 011 tJie collector, t.h e unit draws about JOO µo.. Vvl1011 tl1e crystal is removed, t,bc; collector current. will jump lo 500 to JOOO µa. Thill is a ~ood indicator to show when t.lte circtti L is not, osc:illnting. The converter cru1 he u~ed to cover othe r bn11ds by choosing I.he proper cryst:al Mel clrnnging the input circui t to Lt111e to U1e new frc·qtt tl pbtr and srrC'cn rnrrem is 50 ma. 1f it is impo•rilJle to reduce the plate current :it. re:.:ou:rnte lo this fibTUre, extra l:ixed c:ipac:it.wr!' (('4) '1ho11ld lw add!!d n:> l'l'qllircd . Receiver

For reception, a convc rlcr

: t·an be i:el u p for IJ)C• h:Lm band. R emember, howew r. t1111l this process involves sacrifiting some of ll1e 1·overag, it mu.1· :S " portion of tht• modulator c:1U1o'Qt.:kl·I i~n't, reallv ucccs.~:1 rv: n similar c·oil f11rm without pu1.;; cot~ld be mou;ato.'d

This view shows lhe chonge. ove r rela y mo unle d alongside lhe outp ut lank cc ii o nd co pocitor. Th e resislors ond copocifors ot the up· per left a re R, , Rn, C 111 ond Cu in the mod ulolor circuit. RFCt is wired b etween on i.uulo te d tie point ond the 6C4 socket. The resistor lo the lefl of the choke is R~.

63

Transmitting

The 10-wott 75-meter mobile transmitter is fastened under the instrument panel by means of bolts through holes drilled in the top of the transmitter box.

uircctJy 011 a space r, of COlll'SC. A double-pole antenn:t/power change-over n~­ by, K t, is mounted in the space lefi a.lo ngsidc the tank coil aocl ca pacitor. One pole swi tches ihL· COfLX line to ihe a11ten 11a, while the other one may be used to switch powe r between t ransmi tter autl rcceivr r or to turn [1 transmitter supply on

and off. The rein.\' used here is a sk1ndurd. 6-volt tt.c:. relay wit.h a· 20-0 11111 coil. It is nct,uated by the push-Lo-talk swi tch on the T-17 rn icrophone. 'l' he micropho ne t.rn.nsformer is suspended from the top of Ll1c box. Small components are grouped ::~round t hP tuhc sockets witJ1 whi1;h tJ1ey are assoeiated. AMP 6AQS

4MC.

~

- 2bOV. t-

AN T, BAfl. COAX TO RCV~.ANT.

TERMINAL

Fig. 1-Circuit diagram of the 10-wo ll 75-meler mobile transmitter. All resistors ore y, watt unless otherwise specified . All copocitonces le ss than 0.001 µf. are in µµf. Polarized capacitors a re electrolytic; other fixed capacitors may be ceramic or mica. L2-2 turns insulated hook-up wire over ground e nd of Li . Ci- Cs- Cera mic or mica. C~ -140-µµf. midget variable (Hammarlund MC- 140-S R1-Rs-See above. o r similar). S1-S.p.s.t. togg le. C10, C11 -Electralytic. Ti - Microphone transformer, single -button carbon {Triad Ji-Open-circuit jock. A-5XJ. Ji-Coa xial receptacle (S0-2391. T~ - Combination modulation and speaker output transKi -See text. Li - 32 turns No. 20 enam., 1-inch diam., close-wound. forme r-see text (Triod M- 4ZI.

64

MOBILE The amplifier tonk coil hos been removed in this view lo obta in on unobstrucled view of the components on lhe rear wall of lhe box. RFCo is mounted on o small cone insulator at the upper right. The two resistors immediately below lhe choke ore R1 to the left a nd R3 against the'rrear wa ll. The two mica cbj:)'a cltors nea r the coii socket are Co below a nd Cs a bove. Suspended from lhe top wall of the box is the micro· phone transformer. The mounting tabs may have to be trimmed with tin shears, so that the transformer will fit inside the box.

P o wer Supply and Adju stment

Ji'or nn input of 10 w:i.tts to the lion.I, n power supply delivering 260 voHs at 100 ma. will supply hut h r.f. a nd audio sections. The most-common types of vibrator supplies will meet these requ irements. The power unit of U1e orr in tho cathode r· il'c11it, i R i.o obt.ai11 (,lw rcq11 irc:d l>iu.!< 011 the moduln t.nl', anti apply the proper ~m·e1•11 volt.itj!O (,o Lhe GC L(i. (Th c: 1110c usHd to l'ull ca.p a bility wit h Lh is Ly p e o r sup ply . A 12-volt equ iva lcut {GH7) th1i.t mn.y bu subsi iL11tcd wit,h out modification of the circu i(, is av:iilub lc. The exciter n.11d t.he audio L111Jes mn.y he wired for eithe r 6- or 12-volt operatio n. H migM be well Lo poin (, ou t tJ1at t h e e xciter is h usky enough to d rive a hrger amplifie r t ub thl' cc11L

... 0

Sl c::

50

::;

..J OJ

·)!~

I I

I

~

...

u

I

~

c

2

a.

::.

«

J,.o--o+ Q

I

~

>

>I4 I"'-

!i

~

w UJ n.

"'

¥

2

.,,.: = ov ., o>..,.,

0:

g

...

CJ

VJ

"

u

-E

:t

~

g_.

.2.

~

s

~ CV E

g

g

_.,

e>

ar ~~~-:.:o

& ~ c:oE E ..g_. g- o~o

a2

·- .2.. C

-c.=t:

.:~]::·g 'It

N

o

g~~o~o-;

z

..::!

z

-2

~6: s_;g ~~ .~~~ c o

0 ~ ~-

"{( '

...,

E T·:0 0

1...·- - < 0 V O:>O Cl)E a

~ ~'1~ ·''[ ~1 ~

....

::s

~ a. -

0

¥

~I

•

a."

0,, -gJi . . ._. ec u"'. --;: 0 c

,., , I

w

G.l

-

I ~ ~ ·~ ~: ...: nmc wil.l wonde r why Lim t wo-cryst:il method of td i~nmenL is prefe rred to just plain pea.king at a mid-li:t11d frequency. The n.nswor is quite simpll•: If yo u have t.o go out and huy a. 3750-kc. crysl,;tl, you end up "'ith a rock that. c:tn be used for :llignment of the rig, 80-mcwr c.w. work a nd not,hi11g else. T he:. baud swiwh may uow be sci, su c:cossion~tll y Lo t.l1 CJ 7- 1.rnd 28-Mc. positions, a nd t.hc doublcr:i.nd mult;iplier-sta.gc coil.s resonatc:d. /,2 must reso nate nrotmd 3.6 lVlc. cvc11 t.hough 8 1 is set for 7-Mc. ope ration. L3 is connected i.nto l;hc double r pliil;c t:i rcui t in the 14-, 21- a nd 28-Mc:. posit;ions or th1; band switch and the coil should rcso11.~1te al. approximately 7.2 Mc. IL will probably be found IJ1:1t, e!xcitatiou fo r t.J1c final is considerably highe r at 7 iVCc. and above Uii:u1 it is :it 3.5 i\lc. TJ1is is because Va is dri vt:n hy !1, tn nctl :;tage at frc:q1u:11cics i1bovc a.s !vie. [n fact, the grid c11rrn11t. fo r I.he nmpli-

This view of the mobile transmitter shows the audio section at the letl, seporoled from the r.f. comportment by a partition (2 ~6 inches high) that runs betwee n the front and rear pane ls. Meter shunts ore mounte d on lhe term•· nols of $ 3, and C1 is supported by a Y2-inch cone insulator at the right of RFCs. Components mounted on the chassis and rear pan91 are identified in Figs. 3 and 4, respe ctively. This view shows a type 6883 (12-valt version of the type 6146) inserted in the subrnounted amplifie r-lube socke t.

91 fier may run as high as lO ma. in t.l1e 7- and 21-l\ic. range and, of course, it is a good idea t.o back off on l,he exciLation control whe ncw r Uu: currcn L e xceeds 4 or 5 ma. There is a. possibi li ty that initial tuni ng of I.he exciter st;ugcs will resu lt in inadequate clrivt~ for the fin nl nt 28 lVfo. Should this be t he case, ilt,Lunc L 0 ~o. th e low-iud11ckrnc.:c side of -the previo11"' adjust;mc nL (l,J 1(~ pe:r.kcd setting for rnttximum 7-1\fo. excitation). f•:vc n Urnugh t he diugram s how:; /,,r, to be floating rrcc with .S1n set for 28 Mc., the coil and Lhe ~Lnt.11 Citpacitance of the swil.ch mtiy form a 28-Mc. series I.nips huuting the grid circuit of t.he amplifier. B ecause of I.be abundance of drive avaifoble at 7 Mc., it should be possible to detune enough to break up the 28-Mc. series resonance wiLl1out reducing the 7-1\fo. drive below an acceptable le vel - around 5 ma . Plate-currc n I; readings for the exciter tubes wi ll be depeJJdcnl. 011 l;he power-supply output, vol(,age, the sel.t.ing or t.ho oxcitation control, a11d Uw band or operntion. With n power-supply ou tpu1, of 300 voll:6H6 IMEG.

MOO fUBES

.'~:ip C4z1·~~s

To 11.v S,. P!IOH£ ..,__ }

Sia

~;~

,- .__c_·_v_ _______ ~

.... ___

B Fig . 1 - Circuit diagra ms sho wing the connections to o key ja ck (A) and o p hone-c.w. switch (B) for the Viking mobile rrartsmitter. Component. othe r tha n C1, Ji a nd St ore oil origina l part. of lhe tra nsmitter and ore labeled with Johnson idenrificotions (see opera ting ma nual for the tronsmillc r}.

floU1 v.f.o. a nd crysln.I control ltavc bceu used nnd, even under the most. trying rol\d couclitions, the stabili ty leaves 11othi11g t.o ho de~ire1!. - C,fr. (!ny B. 11 filoiu.~, jr., l'lltN.fP Editor• no1t: "Itlt improper wire tlre1a, i~ is conc~i""ble UmL the i1.Ullill:1tion of Ji will ~uU ln oitlier trnn1imitter in.wbility or decrel>Sed outpui lrotu tlw buffer/multiplier. Siiould either oi these difficulties occur, it "'ill be ny Lhc usua l " t.unc for maximum !tigrml " tech nique. To gel Lho 8!i!n-kc. crystal osrill:ttor o::i the proper frequency, leav1• t.hc unil in tho rec-Jive condition, h11t, connect the microphone. You will now be uble 1;o hc;1.r in t he receiver .iusL wl1at, your t,1·a,n~mi l.Lcd si~nn.l will souud like on the o~hCJ· e nd . Adj111;1t C:x. unti l y onr voice sountls most uaturnl. Tho freque ncy will p robably he abun t 200-·500 cyc:lc!< :\hOV() the edge or I.he fil ter pa,.~s­

Alignment

T l1ere is ve ry liWe tilill:nmenL dilficuny. Tho first. step is to 'CJ}.ICll U1e filn.meut or

n+

lead"

U1e st.ages which rn11st be neu tralir.cd (173, Vo, TT10, 1'u). 1'blJn appl,v pow())· tu ·~he rema ini11f,!; stages. The statiou recciver S m et.e r c~\n I.Jc usocl ror essentially a ll of the alignmen t . Couple Lhc (;o

l't.'Ce ivGr lighUy t-0 t he COttX output of tin: final (or the L2BY7) and tunCal«•d for a ma xi mum readin g. Now rn·il.rh d.ow11 lo 15 meters a nd Lu11 e in 'Lho t:arricJ'. You can set, R2 to one e nd of it.s ro.ngc 'Lfl :wsmc Lhi~L ~orno CillTior will come thro ug h. Pci:i:k the i. I'. tn111sforme1'S :rnd all Llrn othe r hn~11~mi ttor tuned cirrnit~ (C1, C4 tli rnug li C;) for a rnt~ximw11 reading. 'T'o ncubrali:te V3 , adj11st.

C1-Cn, inc.- 3-22-µµf. lrlmmers from ARC-5 i.f. Irons· formers. Cio- 3-12-µµf. ceramic trimmer. C11- 7-45 -µµ f. ceramic trimmer. C1 2 - 35-Mµ f.-per-section varia ble. C1n, C1o1-Neutro lizing capacitor; see text. Cx- 3- 12 µµf. ceramic trimmer (see text). Ji, J2-Coaxiol connectors, chassis mounting. Kr - D.p.d.t. relay, 1 2-volt coil (operated by send·receive switch). li-25 turns No. 34 o n Ye·inch d iam. form (ARC·5 i.f. transformer form). Adjusl to resonate a l 8550 kc. wi!h C1 ond the shloldod le ad to V.1D. l 2-8 lurns wound on some form a s Li . LJ- Bifilo r wind ing on loroido l ferrile core, total inducta nce 50 t•h. (See footnote 2 ). l .1- 40 turns No. 24, Yi-inch diam., 32 t.p.i. (Miniductor 3004). Ls-35 turns some as l 4. lo-22 turns same as Li, topped 8 turns from bollom. l1- 1 8 turns some a s 1.,. Lf- 3 turns so me as Lo, spoced 2 turns from L7.

bn11d.

ln- 27 turns No. 28 enom. on 1Ye-inch diam. form, 24 t.p.i. (BC-458 oscillator coil form). Topped a t 13th lurn. Coil should be a djusted to functio n with crysta l used in VXO. l io-1 3 lurns sa mo as L1. l 11-7 turns same os L,. spaced 3 turns from ground end of Lw. l 10-7 turns same os b . lu - 11 turns some os L., spaced 3 turns from ground end of Ln. L,. - 19 turns some a s 4. L15- l 9 turns some os L., spaced 3 turns from Liislor. Yr-8553-kc. cryslol. Y2, Y1-8550-kc. crystal._ v., Ys- 8551.5-kc. cry.101. Yc-30-Mc. cryslol (for VXO )•.

•

S.S.B. Equipment

109

lliJ

@ REC.

fig. 4 -Lo yout diagram of th~ top o f the exciter chassis. The lower edge in this dra wing is the front as seen in the front view of the unit.

ANTEN N AIN

CARRIER

BALANCE

6

a

© ~

Modifications A:; wa.s poini.ed uui earl ier, t he choicn of the VX O and fil ter frcq11 cnci1·~ ll'llS haiilK.

l 11 /.,1 llw rolor of Uw c:i.pacitor is grou11!1cd to th1• cnn, um l llU' :l.'i.liilil)· wns J!rt'nlly l'.61t

A

From QS1', August, 1%9. I Viuifo, "Cheap and Ea.,,>• S.S.13.." QS1', ~l11reh. 1!156.

improvct!. T o g..t. t11 L2 l\Ic., the G-!\ fc. pickup l\' inding 11·u,: fod, a:; !"+s (fo r l e.• c

So

A -

nnd

R, = 0.01215(GA)Z • 0.5 ohm.

llr = 0.01215(9.57) 2

•

I. II ohme

It. is clear lbnt a coo11idcr.iblc incrcaac in the radiation resistance is obluincd by placing U1e induct-0nce in the ccnwr portion of the rndinLor. Ground-Loss R esistan ce The current flowing at the b1186 or the antenna must be returned lo the bnac o r tho nnlcnnn by current.a induced in tho ground bonoot.h tho rodiat-0r. These currents must be collected hy lho cnr body aud through tho cnpacitanCC of tho C1\r hod y to the ground . Since tbe urou or tho CM body is considembly less thtm 1~ q u1t1•J.er wavolcngtb, only a portion of these curronts will be oolloclcd by tho car frame itself, and tho rest wi ll be collected by ground currents fl owlug through tho (:iLpnuitnnce of the car to the ground. 8 inr.e tho ground ie not,

l)

= G.4 de~rcc-amµcrcs,

Now consider the effect of introducing a series loading coil, ae shown in Fig. :3. If lhe inductance or the looding roil is zero, the current distribution will be curYe (I), as shown in Fig. 3. This is obviously tbn.~ or a simple base-loaded radiator, as shown in Fig. 2. As the inductn..nce is increased Crom zero, the current distribution ia modified lo thnt. or curve (2). At some value or inductance, Lo, the input impedance of lhe antenna as seen between the base or the antenna and ground is a pure resistance with no rc.'lctive component. For this value of induct:mcc, Lo, we have the maxi-

Fig. 3 -CYrrent distribu tion on o iectlonoli zcd or centerloodod ontenno.

mum current area on porlion h2. This is shown as curve (:1\ or Fig. 3. For this condition, the current flowing through Lo,

l1

where

-

I . cos

n•.

T unin g.Coil-Loss Resistance In order that n short u.ntennn wi ll take power, the capacitive reactaoco of t.hc anten na must bo tuned out by means of c~ suit.uhlo tuning coil. To estimate the inductance required, we need to know the capacitive rcack1ncc of the nnlennn at the operating frequency. The rcaclance of n short vertical antenna, such M @hown in Fig. 2, is

Gt,

-

·x•

h1

= hi -

- J

electric."ll height. of where

For example, assume that 55 inches (I ..t meters),

Gi • Gt "' 6.4 deg"Cea (at 3.81

~fc.).

-1

lossless, quite a large loss resistance, is found. The current path is shown in Fig. •I. In the past, writers have neglected this loss resistance. This resistance will be n function or the positioning of the radiu.lor, t.l1e t.,vpo or cnr on which the an tenna ie mounted, nnd Lo some extent on the ground beneath t.bc rndiuLor. A vnluc of 10 lo 12 ohms h!lll been mc11s11rcd hy the author for 8- to 16-foot untonnne nt, :~ .8 l\ lo.

a, = el- ohms. 300

of

=

=

Q

For example, suppose we have the antenna sysLcm of Fig. 6. 24 Here h = llO + = 67 inches

2

'

a Zo

= 0.125 inch, = 418 ohms (as before), and

G1

=

G2 = 6.'1° (at 3.81 Mc.)

(Add 5 per cent for spurious end effects.) jXr,lj = j4:18 (cotan 6.71° - tan 6.71°) = j3500 ohms, or 146.3 µh. at 3.8 1 Mc.

fo:l, cycles.

x.

R,

+

l.(i.:~5

base-loaded whip described, Rt + R. = 0.5 + 10 + 5.85 ohms. 6

= 3.81 (10) 16.35

1752

= 35.4 kc.

For the center-loaded a.ntenna described, Rt= l.11 + 10 + 11.7 = 22.81 ohms. l':lf = :1.81 (10)6 22.81

3500

= 24.8 kc.

It must be remembered that a baudwidtb of

5 kc. is required for double-sideband a.rn. phone operation. The bandwid th of both autennas described above is adequate. H owever, the proxirn-

Now consider the sectionalized antenna, as shown in Fig. 3. Firstly, we calculate the reactance of t he top portion. F rom this we subtract the lumped reactancc of the loading coil, and fina.l ly the input reactance of the antenna is calculiited by a..o.suming tho lower portion an openedout tnmsmission line, terminated in t.he resultant rnnct.auce of the load ing coil and the top scctiou. If the antenna is resonated so that the base reactance is zero, it can be shown that the reactance or tbe inductor required is jXLo = j Z 0 (cotan G1 - tan G2) ohms, where Zo = characteristic impedance or anwnna (as before), G1 = electrical length of top po1tion of antenna., aud G2 = electrical length of bottom portion of antenna.

=

vertic;~l

For the

r

and

~

t:.f =

. I me ies,

Fig . 6-Center-lood ed whip mounted on o cor. 55"

ity of oear-by metal objects can cause considerable detuning of the circuit . Also, very little shift in operating frequency can be allowed without retuning the nuteuna. Tl1eoretical Radiation Efficiency The radiation efficiency of an antenna was shown to be the radiation resistance divided by the total rcsist;ance, or 11

=

R,(100)

R.

+ R, + R.

%

For tbe 110-inch base-loaded whip, 0.5(100) = 3 13 0.5 + 10 + 5.85 . O· For the 110-tnch center-loaded whip, 11

-

" =

1.11

1.ll

+ 10 + ll.7

= 4.86o/, .

°

MOBILE

128 H is clear that a small but worthwhile improvement is obtained by center-loadillg the antenna. H owever, this is g-ained at the expense or reduced opemt.U.1g bandwidth, and increased meclum icnlco11slruction problems.

D eter mination of the Op timum Location of tlie Loading Coil Su ppose we hnvo a 16-foo t whlp ant.cnna. T he aotcnna is bumpor-mounlecl, the bilRc insulntor hcing 2 foet from the ground. The 1Wcrag hz/h1 = 0 is, or course, the case of n lmsoloadcd nntennn.. H is seen that. the hcst. location for the coil is approximately in the l'C'lll.11timum location of the coil is shifted toward the fc'cd point ns the gro1111d-loss rcsist.anco is reduced.

Field M eosu,.ements of R adiation Resistance

Tbe nct.ual m ens11romenl of rndiaLion rc8i~r anco of 1t11 a nl cn M nt 3.81 ;\1c. i s difficult, n nd involves equipment not normally ll.\'nilnble lo Lho avor:1gc 11m:1teur. However, to show that me~tsuremcnts Cfln ho taken to prove the theory we hnve developed, I think a short discussion of tho principles involved would IJe in order. The su ..faoo-wnvo ticld intensity (l h nL is, for irrounded ra.diat-0rs) from a short mdi:1t-0r t:illl he (•xprcssed in Lcnns of nu:linted power, distirnce, :1ml prop:ig1u ion fact-0r for the ground hcLween t ht' I r:tnsmitlcr and receiver by tho following t•x pn·~sio ns: P, =

and

1"112

- waLts, 34.ti fl'o = Fkd mv. / m.,

l:'!

~...,

...

""

.:::, Lo_ O I'-~~~~-'-~-'-~~~~~'--~~~~

01

O.S

d

I (MILES)

10

20

Fig. 8 - Graph showing voriotion of ground-wave field infenoily with distance for poor 10 good ground.