Electromagnetic Fields and the Life Environment

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Electroniagnetic Fields and the Lile Environnient I

KAREL MARRA, JAN MUSIL, and HANA TUHA Institute of Industrial Hygiene and Occupational Diseases, Prague

Translated from the Czech

555 Howard Street, San Francisco, California 94105

Electromagnetic Fields and the Life Environment By

Karel Marha, Jan Musil, and Hana Tuha First published as

Elektromagneticke pole a zivotni

prost~edi

by Statnf zdravotnicke nakladatelstvf Prague, Czechoslovakia

English translation Copyright

©

1971

by

SAN FRANCISCO PRESS, INC. SSS Howard Street San Francisco, CA 9410S

Printed in the U.S.A.

SBN 911302-13-7 Library of Congress Catalog Card 72-131364

PREFACE

The primary purpose of this book is to acquaint the reader with. the fundamental problems encountered in evaluating the possible effects of radio waves on living matter, with particular emphasis on man and the protection of the human organism against such effects. Since this is the first publication of its kind, we begin by covering the topic from all possible standpoints, which in the limited space of this volume has necessarily produced a very brief summary of the several areas. To help the reader orient himself in studying the lit~rature, and especially to aid him in understanding the reasons for the frequently contradictory results of biological observations presented here, we place particular emphasis on fundamental problems in physics and biophysics. In view of the relatively large amount of literature published in this :field prior to 1965, it has been necessary to select only a few of the basic works for inclusion among the references at the back of the book. We also incorporated our own data and observations, acquired in the course of working on this problem at the Institute of Industrial Hygiene and Occupational Diseases in Prague. Several sections of the book demand concentrated attention and an understanding of basic concepts of mathematics, physics, biology and chemistry. The first two chapters are intended as a sort of introduction to the problems involved; subsequent parts were written so that physicians and electronics engineers (for whom this book is primarily intended) can obtain as much information as possible and gain valuable guidance for further study in the course of their . work. We should like to thank here in particular Prof. J. Teisinger, Director of the Institute; Prof. J. Roubal, Head of the Labor Hygiene·Department; . and Dr. P. Pachner, Head of the Scientific Methodology Division of the Labor Hygiene Department, for their kind assistance, support, and valuable suggestions during the final revision of the manuscript. We also thank all those who contributed to the successful publication of this book. The Authors

iii

TABLE OF CONTENTS Page iii

PREFACE .

1. INTRODUCTION 2. THE ELECTROMAGNETIC FIELD AS A PHYSICAL ENVIRONMENTAL FACTOR 2. 1. Physical properties of electromagnetic waves 2. 2. Properties of electromagnetic waves from the health viewpoint: Characteristics oJ fields and definitions of concepts

3. LIVING TISSUE IN THE ELECTROMAGNETIC FIELD 3. 1. Entry of electromagnetic energy into an organism . 3. 2. Effect of thickness and order of layers (constitution)

1 5 5

13 16 16 18

on absorption .

4. BIOLOGICAL EFFECTS OF ELECTROMAGNETIC WAVES AND THEIR MECHANISM . 4. L Effect on the human organism and on other vertebrates 4. 2. Effect on other organisms . 4. 3. Effect on physical and chemical prnperties of materials 4. 4,_ Mechanism of effects .

5. OccuRRENCE AND UsE OF ELECTROMAGNETIC ENERGY 5. 1. Survey of the most common sources of electromagnetic waves 5. 2. Use of electromagnetic energy in various ranges; frequencies and powers used .

5. 3. Possible sources of harmful irradiation

29

29 38 44 47 59 59 60 72

6. MAXIMUM PERMISSIBLE FIELD INTENSITY AND RADIATION AND THEIR DETERMINATION . 6. 1. Establishment of maximum permissible irradiation 6. 2. Specificity of determination of field intensity and of power density and irrndiation . 6. 3. Instrumentation and systems for field indication: analysis of individual components and methods 6. '-L Measuring devices used abroad and the possibilities for an "ideal" measurement method .

7J 7-1 78 82

9-1.

7. HEALTH AND TECHNICAL PROBLEMS INVOLVED IN WORKING WITH GENERATORS OF ELECTROMAGNETIC WAVES: ORGANIZATION OF WORKING CONDITIONS . 99 7. 1. Means o.f attaining a permissible field intensity 99 7. 2. Further means of reducing the effects of the electromagnetic field llO ' APPENDIX: Definitions . 113 Standard method of determining field intensity and electromagneticwave irradiation in the rf and uhf ranges for health purposes ll4

121

REFERENCES

v

J

INTRODUCTION

We know that the entire frequency spectrum of electromagnetic waves can affect living matter, and is thus biologically active. But the mechanism of this effect is not the same over the entire known spectrum, which currently extends from very low frequencies up to 10 24 Hz (ie waves with wavelengths greater than 3 x 10- 15 m) .

Under the influence of radiation at wavelengths shorter than 2500 one may assume ionization of molecules in the irradiated object. This radiation band is therefore referred to as ionizing radiation. The nonionizing radiation band, extending from very low frequencies up to 10 12 Hz, is referred to as the radio-wave band. The corresponding wavelengths are greater than 0.3 mm and their effective energy (owing to the relatively low frequency) is insufficient for ionization of molecules. According to another system of classification, the radio-wave band includes all coherent radiation* even at wavelengths below the abovementioned 0.3 mm, which would include laser radiation. The i>roblerri of the biological effects of electromagnetic waves in the ionizing band is generally treated in connection with corpuscular radiation. From the practical standpoint of protecting human beings exposed to it, such effects are the province of the science called radiation health physics. In our book, we shall deal exclusively with the other, nonionizing portion of the electromagnetic spectrum, radio waves. Hence, in the chapter which follow, the term electromagnetic waves (fields) will refer only to this section of the spectrum. Electromagnetic waves were discovered in 1888 by Heinrich Hertz and have - found extensive practical application, so that a great many generators of such waves can be found in every developed nation and constitute an ever-increasing environmental factor (Fig. 1) . Their practical use has attracted the attention not only of enginee1·s and physicists, but of chemists, biologists, and physicians as well, all of whom are interested in whether the longer electromagnetic waves produce chemical and biological changes similar to those caused by sources of ionizing radiation. The first experimental work in the field of investigating the effects

A,

*Coherent radiation is emitted by a radiator all of whose el- · ementary dipoles oscillate at the same frequency without rel.a ti ve displacement in time (ie they oscillate in phase).

1940

1960

1950

1970

Years

FIG. 1.--Increase in the average power of generators operating at a frequency of 3 GHz [169]. of high-frequency (rf) electromagnetic waves on living and nonliving matter dates from the end of the last century, when in 1895 Danilevskiy observed the effect of such a field on a neuromuscular preparation. After that, the number of works in this field grew steadily until the first peak of interest in this problem occurred during the decade from 1930 to 1940. During this period, much valuable work was done, mainly research on the effects on the physical and chemical properties of matter and certain simple biological systems. Substantially less attention was devoted to purely medical problems, such as the effect of rf fields on man. World War II abruptly ended the successful development of experimental projects at several laboratories. Most were not resumed after 1945; some experimental discoveries fell into complete ob 1 i vi on or have been "re dis covered" ih recent years. Following 1945 and the atomic explosions at Hiroshima and Nagasaki, workers studying the biological effects of radiation devoted their attention almost exlusively to sources of ionizing radiation, both in wave form rays, y rays) and corpuscular form (a and S particles, etc). It is only in recent years that interest has begun again to develop in rf electromagnetic radiation in the radio range as an agent that could be biologically active. This interest was stimulated by the discovery that animals and plants decline and die in electromagnetic fields of a certai'n minimum power density in the centimeter (cm) band, and by the complaints of workers at radar stations that they were experiencing certain subjective difficulties. It is therefore

ex

2

by the complaints of workers at radar stations that they were experiencing certain subjective difficulties. It is therefore understandable that until quite recently the majority of papers dealing with the effects of cm waves described experiments with animals and not with human subjects. However, a number of laboratories have recently begun a systematic study devoted to problems of basic research into the biological effects of rf fields, with particular stress on the primary biophysical mechanisms of this effect. This interest was stimulated partly by the increasing number of workers with such equipment, and partly by the discovery of possible nonthermal effects of rf fields on organisms. The largest groups of scientific workers and laboratories devoted to studying this problem are located in the US~R and the USA. In both countries, special stress has been placed on the problem of the effects of cm waves. In the Soviet Union, however, there is also a group of special laboratories where a lower frequency range (down to 30 kHz) is studied. Recently, workers in the United States have also started work on basic research connected with waves longer than cm waves. In the case of such waves, it is easier to separate thermal from nonthermal effects. In the United States, research was begun on a broad front in 1957 under the sponsorship of the Department of Defense and continued at Cape Kennedy, established as part of the rocket research base. For that reason, it is natural to expect that not all results would be published in detail. It is then understandable that other countries where such work is also conducted intensively would serve as valuable sources of information regarding new discoveries only with certain difficulties. Despite these obstacles, about 1000 articles in this new and very promising field have already been published. Along with the Soviet Union and the United States, one must also mention Poland, Italy, and recently Britain as well as countries where work is in progress on the problem of the effects of rf fields on biological systems. Here in Czechoslovakia, too, much work has been devoted to this problem. At the Institute of Industrial Hygiene and Occupational Diseases in Prague,a special high~requency department was organized. Since 1961, this department has dealt exclusively with research into the primary effects of electromagnetic fields on the human organism and the protection of individuals against these effects. The clinical aspects of the problem constitute the particular. field of interest of a group of workers at the Institute of Industr~~l Hygiene and Occupational Diseases and at the Occupational Diseases Clinic in Bratislava. The results of research in this field not only serve to protect the health of workers exposed to the effects of radio waves in the course of their daily work, but can also be used to clarify some other points which are still unclear (for example in bioclimatology and biometeorology) . The purpose of this book is to acquaint the reader with the 3

most important of the basic problems which crop up in the course of investigating the effect of electromagnetic waves on living tissue, with special emphasis on man, and to awaken a deeper interest in these questions. We have gone into somewhat more detail only in those chapters which are of basic significance for identifying the basic biophysical effects and in those sections which are of interest primarily for medical supervisors or engineers who work with generators of high or very high frequency electromagnetic waves (rf or uhf), or who build such equipment, and are therefore directly significant for practical reasons. Nevertheless, even these chapters cannot be considered (with regard to coverage) as texts or operating instructions. The clinical aspects of the problem are mentioned only for the sake of information, since very thorough publications pointed specifically in that direction have recently appeared in the USSR and Poland [163, 182, 28~]. A similar book is also in preparation here in Czecl'l:oslo_v akia. TABLE 1.--Division of electromagnetic wave spectrum into bands in the radio-wave range. Band Frequency Metric div. no. range of waveband 3 300-3000 DecamyriaHz metric 4 3-30 MyriakHz metric(mam) s 30-300 KilokHz metric (km) 6 300-3000 Hecto kHz metric (hm) 7 3-30 Dec aMHz metric (Dm) 8 Metric (m) 30-300 MHz 9 300-3000 DeciMHz metric (dm) 10 3-30 CentiGHz metric (cm) 11 30-300 MilliGHz metric (mm) 12 300-3000 DecimilliGHz metric

General Wavelength designation Frequency div. designation

Long waves Medium waves Short waves Very-short waves

Microwaves

Audio frequency Very-low frequency (vlf) (af) Low fre_9..uengr_ (lf) Medium Radio frequency (mf) frequency High (rf)* frequency (hf) Very-high frequency (vhf) Ultra-high frequency (uh f) UltraSuper-high high frequency (shf) Extremely-high frequency fre_g_ uency (eh f) (uhf) * -

*NOTE: Ultra-high frequency (uhf) is used to designate both the frequency range 300-3000 MHz and more generally the entire range above 300 MHz. In the present work, we use it in the latter sense. Radio frequency (rf) is used to designate only the range below 300 MHz, even though higher frequencies are also used for radio purposes. [T.Y'ans Z. ] 4

2

THE ELECTROMAGNETIC FIELD AS A PHYSICAL ENVIRONMENTAL FACTOR

2. 1. Physical properties of electromagnetic waves To facilitate the understanding of further discussion, we briefly summarize the fundamental aspects of the nature and characterist i cs of electromagnetic waves, naturally limiting ourselves to the absolutely necessary concepts that occur later in the text. More detailed information is readily available elsewhere, for example in Ref. [185). Despite all of the differences among the various spectral ranges of electromagnetic waves (see Table 1), the same laws hold for all of these bands: they propagate in free space with the velocity of light and are refracted, diffracted, Q.ispersed, and polarized. The reason for the specific variations may be nothing more than a different ratio of the wavelength to the dimensions of the environment or to the particles of the substance, or a different energy content as has be~n mentioned in Chapter 1. The fundamental characteristics of waves can best be explained by a simple example, the plane electromagnetic wave (in practice, this means dealing w-i th propagation in a small region sufficiently distant from the transmitter). The electromagnetic wave is characterized by the magnitude and direction of its components, electric (E) and magnetic (H). A sketch of the instantaneous magnitude and direction of both components of a wave at a certain moment in time is shown in Fig. 2a; the pattern of the electric and magnetic lines of force at points where the wave has its maximum (looking in the direction of propagation of the wave) is shown in Fig. 2b. These illustrations are suitable for explaining several important facts. The electric and magnetic components of a field are mutually perpendicular and both are also perpendicular to the direction of propagation. The magnitude of the components at each instant varies sinusoidally along the direction of propagation. The sinusoid is made up of parts which repeat periodically, hence the terms period, cycle, frequency. The number of oscillations per second is the frequency f; the length of one oscillation is equal to the wavelength A (Fig. 2a). The dependence of frequency f on wavelength A is expressed in terms of the propagation velocity v by

f where

v

= V/A

(2.1)

c//srµr

( 2. 2)

5

I i

z

H

._.--1--t-tt+---ltl----it4

II I I Direction of propogotion

(a)

(b)

FIG.2.--Plane electromagnetic wave: (a) course of electric and magnetic components of w~ve; Cb) orientation of intensity of electric and magnetic fields of plane wave. where Er is the relative dielectric constant of the medium, µr is the relative magnetic permeability of the medium, and c is the speed of light. The velocity of propagation is therefore a function of the properties of the medium through which the wave travels. In vacuum, and approximately also in air, electromagnetic waves travel with the speed of light c, ie 300 000 km/sec; in other media (water, for example), they propagate always more slowly [44]. Equation (2.2) does not contain the conductivity of the medium; in reality, however, the velocity of propagation depends on it. For example, in an aqueous conducting solution with a conductivity of 4 mho/m, a free-space wavelength of A = 2000 m is diminished to A = 4 m. Two sine waves of the same frequency are in phase when their maxima and minima occur at the same instant. The waves are said to be 180° out of phase when the minimum of one (maximum negative value) and the maximum of the other (maximum positive value) occur simultaneously. The electric and magnetic components of the field are quantities analogous to voltage and current. It is therefore possible to define the so-called impedance (apparent resistance) of a medium as follows:

z = IE!!IHI

=

/µ*/E*

(2. 3)

where s* is the complex dielectric constant of the medium and µ*is the complex magnetic permeability of the medium (Z = 377 ohms in air) . The plane in which the vector (ie the value determined by the magnitude and direction) of an electric field E lies is called the polarization plane. In Fig. 2, the electromagnetic wave is horizontally polarized. In order to explain some other features, let us move from the 6

propagation of electromagnetic waves in a free medium to their propagation along a transmission line. One of the simplest and commonest types of transmission line is one formed by two parallel conductors, the so-called two-wire transmission line. After an ac source is connected, a traveling wave is propagated along the line, carrying electromagnetic energy with it. The propagation of the wave along the line produces alternating current and voltage in it. The ratio of the amplitudes of the voltage to current is constant at all points along the line and is one of the characteristic parameters of each transmission line. It is called the characteristic impedance or wave resistance of the line. If the line is terminated by a resistance other than its own characteristic impedance, the load causes reflections, so that in addition to the incident wave traveling from the source to the load a portion of the energy travels in the opposite direction, producing standing waves along the line. The reflec~ tion is best characterized by the so-called standing wave ratio s, ie the ratio of the maximum to the minimum voltage along the conductor. From the standpoint of the source which is connected to the line, the resistance of the line at the source is also important. It is calle0 the input resistance or more generally the input impedance. The concept of impedance comprises not only the usual load composed of the real part (resistance) and the reactive. or imaginary part (inductance or capacity), but as a limiting case, its real resistance alone. In practice, use is also made of the concept of normalized impedance, which in the case of an input impedance is its ratio to the characteristic impedance of the line. The two-wire transmission line is not the only type of line, nor is it applicable to all frequencies. In the cm wave region, for example, such a line would radiate and therefore coaxial or waveguide lines are used. Even though the relations in such conductors are more complex, it is still possible to apply the above concepts. In principle, a waveguide is a tube of conducting material, usually rectangular in cross section. In addition to phenomena similar to those encountered in other types of conductors, waveguides have many specific characteristics; for example, they act as highpass filters (they do not transmit energy at frequencies lower than a certain critical frequency given by the waveguide size. A precise idea of the nature of the field within a a waveguide can be obtained from a simple concept regarding the physical process of wave propagation. The field inside a waveguide can be thought of as a consequence of the movement of a plane electromagnetic wave, propagating between the walls of the waveguide, alternately slanting upward and downward. Inasmuch as the components of the wave, which create the actual field in the waveguide, move at an angle other than 90° to the axis of the waveguide, the propagation rate of the energy along the waveguide is smaller than the speed of light. At the same time, the 7

field of the wave components is formed in such a way that the wavelength in the waveguide is greater than the wavelength in free space. At frequencies much higher than the critical frequency the waveguide may contain higher-order waves and the field within the waveguide assumes a different form. The concept of waveguide propagation as the result of succesi ve reflections of wave components from the walls of a waveguide can be used for all other waves, as well as for waveguide shapes other than rectangular (circular, dielectric, etc). Antennas are attached to conductors for purposes of radiation. In the simplest case, the conductor may be terminated for example by a dipole (Fig. 3). If waves propagate along a line to which a dipole is attached, a voltage and current arise here too and a.n

1f

FIG. 3.--Schematic representation of a transmission line terminated by a dipole. electromagnetic field forms around the dipole that propagates into the environment in the form of electromagnetic waves. Maximum radiation occurs when the length of the dipole is a multip le of half the wavelength of the rf signal being conducted. The dipole can be thought of as the fundamental element of which almost all rf antenna configurations are composed. More generally speaking, the entire radiation field produced by an antenna system can be thought of as composed of the individual fields formed by the elements of such a system, viewed as elementary dipoles. In the vicinity of the antenna, there is also an induction field in addition to the radiation field. At distances which are small relative to the wavelength (or small relative to the dimensions of the antenna, if the antenna is large) the electric and magnetic induction field will be much stronger than the field radiated by the antenna. The induction field produces mainly reactive energy, which is sent out during one half of a period and returns to the source (the antenna) during the other half. Since it decreases approximately inversely as the cube of of the distance, it is negligibly small at relatively large distances from the antenna and therefore is not considered to be a radiated field. It differs from the latter in other ways as 8

I

well: the electric and magnetic fields in the induction region are not proportional to one another, nor are they in phase with one another. At short distances from an antenna in the form of a dipole (and as a limiting case also in the radiated field of a condenser, which may be actually thought of as a dipole with a zero included angle) the electric induction field is much stronger than the magnetic induction field. On the other hand, in the vicinity of a loop antenna (or generally in the field radiated by an inductance), the magnetic induction field predominates. In view of the existence of an induction field, the region around antennas or radiating elements is divided into two parts, the near field (or induction field) for r > A/2n (and possibly an intermediate region where r is comparable with A/2n). From the above, it is clear that this division is not adequate. In the uhf band, especially for cm waves, dipoles are rarely used, mainly because other types of conductors are used. Unlike in the rf band, where the basic element of the antenna is a thin conductor (wire) through which the current flows, the effective portion of a uhf antenna is usually a flat surface from which the energy of the electromagnetic waves is transmitted into space. The simplest forms of such two-dimensional antennas are for example the open mouth of a waveguide, a horn or parabolic reflectors, or dielectric lenses like those in optical reflectors, etc. Even in the uhf band, the relationships in the vicinity of antennas are complex. This is best illustrated by Fig. 4, which Near zone

For zone

N

(l'W/cm 2)

logr(m)

FIG. 4.--Power density as function of distance from transmitting antenna [169]. 9

shows the curve of power density as a function of the distance from the transmitting antenna. Here again the division of the region around the antenna has its justifications, but owing to the large size of the antenna relative to the wavelength, other criteria must be employed. To determine the boundary between the near and far fields, we use the expression r = S/2A, where Sis the area of the mouth of the antenna [169]. From the standpoint of the radiation of antennas into space, the concept of a radiation pattern or characteristic is important. A spatial intensity pattern is a surface defined by the end point of lines radiating from the antenna in various directions and whose length is proportional to the field intensity in the various directions. Figure 5 shows the spatial intensity pattern of a horn antenna. Instead of a spatial intensity pattern, however, it is usual in engineering practice to use plane

FIG. 5.--Three-dimensional intensity diagram of a horn antenna. diagrams that show the relationship between field intensity and direction in two mutually perpendicular planes (one usually coinciding with the polarization plane (plane E) and the other perpendicular to it (plane H). In Figure 6, the plane intensity radiation patterns are in planes E and H. In addition, use is frequently made of power directional patterns; in that case the sizes of the abovementioned rays are proportional to the power output in each direction. The longest ray, whose axis is also the electrical antenna axis, is usually given the value of 1 and the sizes of the other rays "a re then measured re 1ati ve to it; the result is then a relative pattern. The important quantity of a relative power pattern is the angle between the rays whose magnitude is about 3 db less than the maximum value (ie 0.5 in the power diagram and 0. 707 in the intensity diagram). It is represented by e3db and is also called the beam width. The pattern width 8obetween the first nulls is the angle between the smallest rays (Fig. 6). It is possible to get a very good idea of the directional properties of antennas from radiation patterns. However, to avoid having to refer continually to the patterns, the term antenna gain G is used in antenna practice. It is actually a measure of the directionality and efficiency of the antenna. The directionality is given by the ratio of the powers which must be appl~ed to an isotropic and a directional antenna in order to 10

(a)

(b)~ FIG. 6.--Plane intensity diagram; (a) in plane of E, (b) in plane of H. produce an electromagnetic field of equal magnitude in a given direction from both antennas. The efficiency of the antenna is then expressed by the ratio of the radiated to the supplied power. All that we have said here about the radiation characteristics of transmitting antennas applies equally for receiving antennas. From the so-called reciprocity theorem, known from circuit theory, it is possible to derive that the radiation pattern of an antenna considered as transmitting is the same as when it is used as a receiving antenna. (This reciprocity does not apply, however, for the field configuration in the vicinity of the antenna!) It is therefore necessary to introduce still further concepts when discussing receiving antennas. From the standpoint of wire or rather single-conductor antennas, the so-called effective height (or length) of the antenna is very important. An electromagnetic field induces a so-called electromotive force (emf) in a receiving antenna, and the ratio of the combined emf to the field intensity is actually the effective height of the antenna. From the standpoint of energy reception by the antenna, it is significant that only part of the energy is transmitted to the load. It is actually necessary to consider losses in the antenna leads and losses in near~y dielectrics, as well as those losses which are produced by socalled secondary radiation, caused by the fact that when a cur11

rent passes through the antenna, radiation takes place regardless of whether the current is induced by rf waves passing through or by some other means. Therefore, the antenna actually causes a field distortion. (Other sufficiently conductive objects in the field behave similarly.) To be sure, the concept of an effective height does not apply to plane antennas. Instead, the so-called effective area of the antenna is introduced. On the basis of the previously mentioned antenna gain G, the relationship Seff;: G~ 2 /4TI 9-Pp1ies. The effective area and the effective heign't of a specific antenna are linked by rather simple conversion formulas. The actual sources or generators which produce electromagnetic energy are basically so-called oscillators, ie vacuum tubes and oscillator circuits (made up of inductances and capacitances). The oscillations generated by them are then further processed (amplified, modulated, etc). ,-Oepending on the frequency band (as well as other factors)", the generators have highly diverse designs (ie circuits and structures). Vacuum tubes of conventional types (diodes, triodes, pentodes) and oscillator circuits made up of ordinary inductances and capacitances are used up to metric and sometimes decimetric waves (ie essentially the rf band). In the uhf band, however, these components are not usually usable, mainly because of the characteristic frequency limitations of vacuum tubes (the effect of the inductances of the leads and the interelectrode capacitances makes itself evident here) and the inertia of the electrons (one of the consequences is a power loss). For these reasons, special vacuum tubes (for example, the acorn,beacon, and planar types, etc) are used in the uhf band (and frequently even at the high end of rf band as well). Planar tubes are triodes with coaxial leads, from which it follows that the oscillator circuits must have (not only for this reason) other forms (mainly coaxial or hollow resonators, ie sealed, perfectly shielded surfaces with dimensions determined by the desired frequencies). For use at still higher frequencies, vacuum tubes are made with a "built-in" oscillating circuit in one unit, so that they are associated with classical vacuum tubes by name only. They are above all the resonator magnetrons, klystrons, platinotrons, carmatrons, permactrons, carcinotrons, etc. The technology of millimeter waves extends into the field of quantum mechanics, so that the further we go toward higher frequencies, the further we depart from classical radio technology. At the sh~rtest wavelengths, so-called molecular resonance frequencies are used, where energy from outside is applied to a higher energy level; when they return to the lower level, the electrons can cause emission of electromagnetic waves. All quantum generators (masers) work on this principle, either in liquid or solid phase, as do generators of coherent radiation in the visible spectrum, the so-called lasers. 1

12

2. 2. Properties of electromagnetic waves from the health viewpoint: Characteristics of fields and definitions of concepts In the preceding chapter, we used the concept of field intensity without definition and we also stated that an electromagnetic wave is characterized in part by the magnitude of its components. The two are directly related. The intensity of electromagnetic waves is expressed in terms of the intensity of the electric field and is also from the health standpoint usually given, in accordance with current engineering practice, in volts per meter. The field intensity expressed in this manner is numerically equal to the emf induced in a conductor 1 meter long, if the waves travel through it at the speed of light. The concept of field intensity can be used in principle over the entire spectrum of electromagnetic waves under consideration. In accordance with what was stated in the preceding section, it is possible to distinguish between necessity and possibility. In the rf band, in the majority of cases, it is necessary to measure only the field intensity (more precisely, that of the electrical or the magnetic field). On the other hand, in the uhf band it is possible to measure the two components simultaneously. For this reason, in the uhf band we replace the concept of field intensity by another, namely power density, expressed in microwatts per square centimeter. From this designation itself it is clear that the power density is numerically equal to the ratio of the power passing through a surface (in microwatts)to the size of this surface (in square centimeters) . To be sure, the field intensity E and the power density N ar~ linked by the simple eq.uivalence E =

13. 77N (V /m, µW/ cm 2 )

(2.4)

which applies however, only in the far zone, where the relationship between the electric and magnetic components of the field are uniquely expressed by (2.3). Both the field intensity and the power density (in whose determination for health purposes there are certain specific difference in current engineering practice) are only a means for describing the so-called irradiation, to which a fundamental importance is ascribed from the health standpoint (cf. Chap. 6). In that connection, it is important to mention the nature of the oscillation of electromagnetic waves. Among their fundamental parameters are frequency, phase, and amplitude (size of the oscillations). What is fundamental is that one of these parameters can change in a definite rhythmic manner in the course of operation, ie undergo so-called modulation. ·From the health viewpoint, the most important is amplitude modulation (if the amplitude during the entire period of generation remains constant from the instant it is switched on until it is switched off, we have uninterrupted, continuous-wave or cw operation), and particularly its special case, pulse modulation. The classic example 13

Pov

FIG. 7.--Representation of basic parameters in pulsed transmission. is pulse modulation in radar. Figure 7 is a schematic representation of the pulse modulation cycle, which is automatically repeated as long as the apparatus is on. Among the fundamental parameters of pulsed operation is the pulse width 6 (ie the period during which oscillations are produced) and the repetition frequency f rep (ie the reciprocal of the time between pulses-the so-calleef repetition period T). Modern radars use frep values between 100 and 3000 pulses per second and values of T between 0.1 and 20 µs. The duty cycle, ie the product of the pulse width and the repetition frequency, lies in current practice approximately between 10~ 2 and lo- 5 • Another frequently used concept is the so-called pulse duty factor,ie the ratio of the pulse width to the spacing between two pulses. This applies mainly at frequencies lower than those at which radars operate (at which the duty cycle and pulse duty factor approach the same value). To characterize the oscillations, it is therefore convenient to use the concept of the pulse duty factor. From the health standpoint we may consider all equipment operating with a pulse duty factor smaller than 0 .1 as pulsed and al 1 pulsed devices operating at pulse duty factors equal to or greater than 0.1 as cw. The instantaneous radiated (pulsed) power of a pulse generator can be very high, even when the average emitted power per unit time is relatively low. This has to do with the relationship between the average and pulsed powers (the average power is given by the product of the pulse power and the duty cycle). Referring back to what we said in the preceding section (2.1), it is P.ecessary to stress the need for at least a general knowledge of the complexity of the situation in the near field. From the health stanapoint, it is important to realize that although in the far field the field intensity decreases linearly with distance and the power density decreases as the square of the distance, so that one can readily convert from one to the other, this is not the case in the near field. This is most important in measuring the power density in the vicinity of large antenna ?YStems, where one may get at a certain distance from the antenna a 14

greater power than at a shorter distance. Yet this apparently paradoxical result is completely in accord with the theoretical conclusions and corresponds to the pattern of the power density in the near field (Fig. 4).. To be sure, the situation may be complicated even in the far field by secondary radiation resulting from the presence of conducting objects in the vicinity.At the same time, if such conducting objects extend to other workplaces (especially various kinds of tubing and other installations), the secondary radiation can cause bad conditions even where direct radiation is not a problem. That may happen especially when the linear dimensions of the secondary emitter approach a multiple of half the wavelength of the oscillations produced by a nearby rf generator. In the uhf band, mainly at centimetric wavelengths, reflections are more likely, not only from ideally conducting objects (such as telephone lines), but also from various dielectrics (people, trees, etc) .

15

3

LIVING TISSUE IN THE ELECTHOMAGNETIC FIELD

3. 1. Entry of electromagnetic energy into an organism In the study of the behavior of living tissue in the electromagnetic field, the most important electrical characteristics of the tissue are the complex dielectric constant E*, the complex magnetic permeabilityµ*, and the conductivity a. In an isotropic linear medium, these constants make it possible to express the relationships between the components of the electromagnetic field. Sinceµ*= µ 0 in all ma~eria1s (except ferromagnetic ones), where v0 is the magnetic perme-a bi li ty of vacuum, E* and a remain the decisive constants, whose values for various tissues as function of frequency can be found in numerous papers, together with the methodology of their measurement [220, 230-232, 234, 236, 237, 241, 243]. It has been also established that the thermal coefficient of the electrical conductivity of tissue varies with frequency and is always negative. A more detailed discussion of the frequency dependence . of several tissues is presented later on in this chapter. One of the fundamental problems to be solved with regard to the irradiation of an organism is that of determining the socalled effective value; in other words, finding the amount absorbed by the organism or induced in it, and possibly conducted through it. Attention centers above all on absorption, although the possibility of induction along conducting paths in the organism (the nervous and cardiovascular systems) and the further spreading of the energy is also a challenging problem. Induction evidently occurs mainly at the lower frequencies, so that it would be possible in working with the latter (for example) to take the propagation of electromagnetic waves by a conductor embedded in a dielectric as a point of departure [273]. In any case, however, the solution is beyond the scope of this book, even in this simplest form. In the solution of the absorption problem, it is necessary to begin by defining the characteristic impedance of the given medium in terms of the known electrical parameters (individual parts of the body can be for the time being thought of as isotropic and homogeneous). Assuming that the time dependence of E and H in time is harmonic (sinusoidal, for example) , we can write the equations for the phase constant a and the attenuation constant S [172]: ( 3 .1) 16

where

is the real part of the complex dielectric constant and The phase constant a characterizes the shift of the phase relationships and the attenuation constant B indicates the drop in the amplitude of the oscillations during passage of a wave through a given medium. Knowing these two values, we can uniquely determine the characteristic impedance Z of the medium for plane waves propagating in a single direction; E'

w = 2nf is the angular frequency.

z

( 3. 2)

where j is the imaginary unit (j = 1-1'). These considerations may be generalized by the assumption that the wave propagating in one direction is normally incident on a configuration of plane layers (the last is semi-infinite) which form a very simplified model of a body. Even though this concept is only a highly approximate one (especially with regard to the relationship between the dimensions of the principal plane of discontinuity and the wavelength), it is adequate for obtaining some insight into the mechanisms involved. Obviously, as a result of the multiple reflections at the boundaries of the layers, two waves are always produced (an exception is the latter semiinfinite medium): one travels in the same direction as the incident wave and the other travels in the opposite direction. The problem consists in determining the relative percentages of absorbed and reflected energy. The solution can be carried out essentially by one of two methods. The first is based on an assumption of the continuity of the tangential components of fields Ey and H at all boundaries, a situation which can be described by 2n &quations (n - 1 being the number of layers). The method of solving this system of equations is given, for example, in [253]. The other method, which consists in establishing the input impedance of the system of layers, is much simpler and is valid for any type of wave. We shall not give a detailed description of it here, but advise those who are interested to consult the literature [172, 176]. Calculation of the input impedance is extremely tedious and complicated; hence, it is advantageous for a qualitative solution to employ the Smith impedance chart [185]. This procedure is a graphic one and makes it possible to get a very quick idea of the influence of individual layers on the resultant coefficient of reflection and absorption. We have already mentioned that for the sake of simplicity we shall be using a "model" of the body in the form of plane layers. In the case of normally incident energy, it is obvious that this method provides an idea of the maximum possible absorption. It is therefore the best one for health purposes. A triple-layer model to represent skin, fat, and muscle is the most realistic representation of the body. The thicknesses of the 17

individual layers can be determined from a knowledge of the weight and specific gravity of the individual parts of the body. In view of the differences between individuals, it is necessary to solve for several types of triple-layer models, ie various combinations of layer thicknesses [172, 176, 233, 240]. By making the model several layers thicker, an idea of the effect of clothing on absorption can be also gained [173]. For the sake of comparison, it is of interest to examine the case in which the model consists of a single layer having the characteristics of muscle [172, 176, 233]. The above solution methods and models understandably cannot be considered unique or exhaustive. It is always necessary to keep the specific purpose of the study in mind. In seeking qualitative data on maximum possible absorption and the effect of constituent parameters, including clothing, the above models may be considered suitable. Another problem would be to trace the dependence of absorpti-Gn on the orientation of the body in the electromagnetic field. In such a case, it would be necessary to go over to a model in the shape either of a cylinder or a rotating ellipsoid, etc [68]. Here again, however, owing to the complexity of the problem, it is impossible to determine the effect of constituent parameters (clothing, for example). For the time being, we shall consider the concept of maximum possible absorption as the governing one from the health viewpoint and give precedence to the models described above, regardless of the fact that the solution of the effect of the orientation of the body in the field js ambiguous because of the unusually complex relationships to the polarization of the field in the individual's work area.

3. 2. Effect of thickness and order of layers (constitution) on absorption To determine the specific quantity of incident energy on the basis of measurements of the field intensity, or power density, it is necessary to have some idea of the frequency dependence of absorption. For this purpose, the required electrical parameters of the individual layers must first be found. The quantities a and B can be calculated with the aid of (3.l)(Figs. 8 and 9).The values of Er and a listed in Table 2 [239, 240, 275] were used in the calculation. Average values were used for the a value of fat, taking the water content into consideration. Good agreement was found when the values of a and B so obtained were compared with the measured values [57, 58] (Table 3). The characteristic impedance of the individual layers was then determined with the ai d o f ( 3 . 2) . According to the criteria in 3.1, the equivalent layer thicknesses were selected as follows: skin lk = 0.1 to 0.5 cm; fat lt = 0.6 to 1.95 cm; muscle l + oo (in view of the considerable attenuation in muscle layers5. For the model called "muscleman,'' l = 2 3 cm and Z.~. + oo • s s 1

18

TABLE 2.--Electrical parameters of body constituents.

f

Er

(MHz) 25 50 100 200 400 700 1000 1500 3000 5000 7000 8500

Fat

8-kin o(S/m)

150 100 75 57 48 45 44 43.5 42 40.5 39 37

27 12.5 7.5 6.5 6.0 6.0 6.0 6.0 6.0 5.5 4.95 4.5

0.65 0. 70 0.75 0. 80 0.85 0.95 1.10 1. 3 2.4 4.0 5.0 7.0

Muscle. o (S/m)

Er

0.020 0.029 0.033 0.050 0.059 0.067 0.100 0 .111 0 .167 0.222 0.333 0. 370

Er

115 90 73 56 53 52.5 50.5 50.0 47 .0 44.0 42.0 40

o (S/m)

0.666 0.768 0.87 1.00 1.14 1. 31 1. 35 1.42 2.22 4. 35 6 .67 8.33

TABLE 3.--Measured values of normalized attenuation constant 6 and phase shift a [5 7, 58]. Material

A.

= 10

6 (N/ cm) Blood

0.65

Skin Fat

cm a (rad/ cm)

4.6

A. = -3 cm 6 (N/ cm) a(rad/cm

2.9

13. 5

4.1 0.81 0.65 0.73 4.5 4.6

2.65

12.1

0.21 0.20

0.49

4.0

1.45 1. 70

Muscle

-

-

2.5 2.7

11. 7 11.6

Bone marrow

-

-

0. 79

5.1

NOTE: Measurement error for 6 ± 5%, a ± (1 to2)%; temperature 37.5°C.

19

FIG. 8.--Norrnalized phase shift f3 tion of frequency.

10

101

in various layers as a func-

10'

104

f(MHz)

FIG. 9.--Norrnalized attenuation constant a in various layers as a function of frequency. (1 - muscle; 2 -- blood, 3 - skin, 4 - fat.)

20

For the triple-layer model (Fig. 10), it is possible to obtain a solution by using the Smith chart to give the values of normalized impedances at the individual boundaries and then determine the amount of energy absorbed from them. The results indicated that the dependence of absorption on frequency is of an oscillatory character in all cases, as was to be expected. The maxima and minima lie in different positions, depending on the thickness of the layers. For thicker layers, the first maximum and minimum shift toward lower frequencies, whereas for thinner layers, the phase shift is in the direction of higher frequencies. The amplitude also changes. The shift of the second maximum is in the opposite sense, likewise for the amplitude. The situation is thus quite complicated, as may be best shown by Fig. 11, which summarizes the results for all possible combinations of layer thicknesses. It is significant in this regard that the differences in absorption are considerable. Deviations up to 65% occur in extreme cases. Air

Skin

z.

Fat

Muscle

z,

z..

Z,

Direction of propagation

•• _ _ _ . 00

FIG . 10.--Representation of triple-layer model.

~

50

w

30

r '°

- + - - - --

-+--

-+------t---i

10

10'

111'

FIG. 11. - -Over- all representation of the dependence of absorption on thickness, for various layer thicknesses in arbitrary array. 21

The "muscleman" (a single-layer model with the characteristics of muscle) is represented in Fig. 12. The solution of this case yields the result that for f > 100 r-.1Hz, a layer of muscle with a thickness Z,' == 23 cm absorbs all nonreflected energy and does not allow any~hing to pass into the next medium (air). At the same time, of course, this means that for f > 100 r-.1Hz the concept of a muscle layer of finite thickness Z~ == 23 cm is identical with the concept of a semi-infinite layer. Differences in absorption for f < 100 r-.1Hz at the above-mentioned thicknesses of muscle layers are not substantial; they may be summarized by the statement that the decrease in absorption is steeper toward the lower frequencies for a semi-infinite muscle layer. Absorption as a function of frequency for a semi-infinite layer of muscle is shown in Fig. 13. From the two cases which we have discussed, it appears that the triple-layer model is the most plausible from all points of view. As shown in our own_~nal~sis, the triple-layer model is not considered invariant with regard to layer thickness; it is adequate not only for an average type of individual, but for extreme cases as well. Changes in skin thickness can be thought of as different d~grees of vascularizatioD, since skin and blood have essentially equivalent electrical parameters (see Figs. 8 and 9). The subcutaneous fatty layer introduces somewhat more complex parameters, in which it would be correct to consider, besides different thicknesses, different water levels as well.To be sure, this circumstance is not important in a qualitative analysis, so that parameters for an average water content are assumed. In view of the field perturbation for f < 1000 r-.1Hz, the results obtained for lower frequencies must be considered as suitable only for orientation purposes; still they do make it possible to draw some general conclusions. A model of the body in triple-layer form (or as a single layer) does not include the influence of clothing; it is to be expected, of course, that some changes in absorption will become evident when the model is extended in this way. Such an extend~ ed model can obviously assume a very large variety of forms, not only with regard to the different parameters of the three layers themselves, but to the clothing as well. In addition, it is necessary to take into account the various air spaces, which complicate the entire situation. In order to study the effect of the number of layers, separated by air spaces, as well as the effect of changes in the thicknesses of the cloth layers and the air spaces, it is convenient to solve two fundamental systems: (i) a system consisting of the triple layer-air space-clothing; (ii) a system consisting of the triple layer-air space-clothing-air space-clothing (Figs. 14 and 15). Once again, the investigation can be conducted qualitatively by means of the Smith impedance chart, in a manner similar to that used for the triple-layer model itself, ie on the basis of the determination of the input impedance of the system of layers [173]. 22

Air

Air

Muscle

Muscle

Air Direction of propagation •

z,

z.

Direction of propagation.

z.

l,

·~

.;

.;,

Lt

I'

Z,

·: •:. 1;

-too

-f

(a)

(b)

FIG. 12. - -Representation of a "mus cleman": (a) Finite thickness of muscle; (b) semi-infinite layer of muscle.

FIG. 13.--Absorption vs frequency for "muscleman" (semi-infinite layer) .

c

Zo

Direction of propagation

Zv

A

Triple layer

lo

z•

• 'oc

'•

1 ot

'·

1a

'o•

'•o

'·

FIG. 14.--System (i): Triple layer-airspace-clothing. 23

____.

lo

0

c

z.

zo

Triple layer

z.

zo

z.

Direction of propagation

'a•

Zoo z,

'o

~

lo

.

ZOC

'c

-i

.0 0

60

.0 0

>.

""

40

Q>

c:

w

20

104

103

f(MHz)

FIG. 17.--Envelope of maxima and minima of frequency dependence of absorption in system (ii) . absorbed energy in the triple layer itself beneath the clothing. (All data are for an average constitution.) For the sake of clarity, we ·have shown only the envelopes of the maximum and minimum values. For comparison, Figs. 18 and 19 also show the frequency dependence of absorption for the triple layer model itself, without clothing (average constitution). An analysis of the frequency dependence of absorption leads to several important findings. (1) For whatever arrangement of the system, the absorption is always lower than for the triple layer itself; (2) the character of the frequency dependence remains the same as for the triple layer itself (although that need not be a general rule); and (3) the percentage of absorbed energy decreases with an increase in the number of layers (including air spaces). It is also evident from the calculations that a decrease in absorption in the range 700-1500 MHz is more likely for thin layers, whereas around 8500 MHz thicker layers are required (especially layer D). In the vicinity of 3000 MHz, it is di±iicult 25

104

103

f(MH1)

FIG. 18.--Frequency dependence of absorption in triple-layer model (average constitution): (1) beneath clothing and air space (system i); (2) without clothing.

104

103

f(MHz)

FIG. 19.--Frequency dependence of absorption in a triple-layer (average constitution): (1) beneath two layers of clothing and air space (system ii); (2) without clothing.

26

to find any general rule. Similarly, it is possible to expect analogous results in other types of triple layers. We see by the results summarized in Figs. 18 and 19 that the frequency dependence of absorption in the triple layer by itself determines the shape of the curve of absorption of the triple layer in Systems (i) and (ii). Consequently, one cannot say that a minimum must always exist in the vicinity of 3000 MHz as would appear at first glance from the figures, because the frequency dependence of absorption for other types of triple layers are shaped somewhat differently. In general, we may conclude on the basis of the results obtained ty studying all variants of the model that the frequency dependence of absorption appears to be at first sight extremely complex and involved. The sole exception is actually the single-layer model with the characteristics of muscle, where the results obtained represent a sort of average value of absorption. Applying these findings in practice, however, would not be too safe, in view of the possibility of the considerable deviations found in other models. In fact, these variations, or more exactly the results obtained by solving the triple-layer problem, indicate that at the present time it is not practical from the health viewpoint to differentiate between the measured field intensity or power density and the effective energy actually acting on the body, if the constitutional parameters of the irradiated individual are not also measured. The only thing that could be provisorily taken into consideration is the fact that any clothing whatsoever decreases absorption by the body itself, to a degree that increases with the number of layers of clothing (and thus air spaces as well); in other words, clothing (to a certain degree) serves as a protective medium. (If the measured field intensity is represented by 100%, no more than 75% penetrates to the body; ie the measured values could be multiplied for the purpose of calculation by a factor of 0.75.) Obviously, the protective powers of clothing vary with frequency, and the coefficient might sometimes be still lower. To be sure, it would be difficult to find a combination of layers of clothing which would insure minimum absorption over a wide frequency Tange. In addition, it is impossi hle to consider such an ar..,. rangement of layers and thicknesses as static in view of body movements. The frequency dependence of absorption must be also kept in mind from another viewpoint--whether it might be one of the factors by which one can explain the difference in sensitivity of individuals to the presence of electromagnetic energy. This circumstance is already of practical significance for the use of microwave diathermy. (It would be convenient to have equipment in which not only the power but also the frequecny would be adj us tab 1e . ) In conclusion, it is now clear that the results o·f individual observations and research in the field of the effects of elec-· tromagnetic waves on man can be gene ralized only if the constituent parameters are taken into consideration. From the methodological standpoint, it follows from the results (the protective 27

power of clothing) that in order to measure the field, it is necessary to measure and record the values especially at head levels, even when they are not as high as the other levels. In that sense, the existing methodology will have to be developed further [285].

28

4

BIOLOGICAL EFFECTS OF ELECTROMAGNETIC WAVES AND THEIR MECHANISM

4. 1. Effect on the human organism and on other vertebrates The growing development of the application of radio waves for the most diverse purposes continues to increase the number of individuals who are professionally exposed . to this physical factor. Chronic exposure to electromagnetic waves can lead to subjective and objective complaints in perso~s working both with uhf [21, 81, 101, 117, 119, 126, 184, 224, 225, 274-277, 280] as well as with rf generators [51, 111, 135, 147, 157, 183, 245, 246]. The most serious is the effect of the uhf field on the eyes and on reproductive tissue (in men), since these organs lie close to the body surface and are therefore readily accessible to the effects of electromagnetic waves. Also susceptible are the nervous and cardiovascular systems, which not only lie near the surface of the body but also have conductive properties. Thermal Effects. The best known effect of rf energy absorption in biological material is its heating [34]. Heating effects result above all from relatively high rf field intensities [193]. * The most general and easily demonstrated manifestation of these effects is an increase in the total body temperature [228, 245], which increases with increasing intensity and length of irradiation [90, 279, 282]. Numerous authors have studied the influence of rf on the temperature of skin and subcutaneous tissue [40, 80, 138], the temperature of muscles [95, 142], and the temperature of the eye. Brief irradiation produces maximum heating at the body surface (skin), often even leading to local superficial burns. The temperature drops with increasing depth, But longer-wave irradiation, on the contrary, generates the highest temperature in deep-lying muscles [195]. The temperature of internal organs and of the blood flowing away from the irradiated organ also increases [282]. When fields of high intensity (40 to 100 mW/cm 2 ) are applied, blood vessels are seriously injured and there are hemorrhages in the internal organs [40, 283]. It is also possible for some organs to be seriously injured without the entire organism being overheated. This phe-

*High intensity: Order of hundreds of volts per meter up to the rf band or hundreds of microwatts per square centimeter in the uhf band. Low intensity: Order of tens of volts per meter up to the rf band or tens of microwatts per square centimeter in the uhf band. 29

nomenon arises particularly under those conditions in which some portion of the organism manifests so-called dimensional resonnance. If the dimension of some part of the object being irradiated by the electromagnetic waves is comparable with the wavelength (eg an integral multiple of half the wavelength), standding waves ma:y be created in that location. Implanted metal may also cause a concentration of rf energy [165, 195, 209]. Over a certain range of intensities, the thermoregulatory capacity of the organism can restore heat equilibrium [139, 280]. At high intensities, the thermoregulation cannot maintain the temperature of the organism within the required limit, leading to overheating of the organism and death [169, 283]. Survival times can be substantially affected by the initial temperature of the organism and its coo ling during exposure [ 7, 41]. It should be noted that results that rely entirely on animal experiments cannot be readily extrapolated to the effect of the electromagnetic field on man. ,~ The somewhat different thermoregulatory system and varioas coefficients (both thermal and rf absorption) in various organisms must be taken into account [137].

Subjective Complaints of Persons Working in Rf Field. Workers complain of headaches and eyestrain, together with a flow of tears, of fatigue derived from over-all weakness, and dizziness after prolonged standing. At n.ight their sleep is disturbed and superficial and they are sleepy in daytime. Such persons are moody, frequently irritated, even unsociable. They manifest hypochondriac reactions and a feeling of fear. Sometimes they perceive nervous tension or, on the contrary, mental depression combined with deterioration of intellectial functions (notably memory impairment) . Over a 1911ger period, definite sluggishness and inability to make decisions result. Those affected complain of a pulling sensation in the scalp and on the brow, loss of hair, pain in the muscles and in the heart region (together with a pounding of the heart), and breathing difficulties. Not infrequently they complain of difficulties in their sex life. It is moreover possible to observe slight trembling of the eyelids, the tongue, and the fingers, increased perspiration of the extremities, dermographism, * and brittleness of fingernails. A single irradiation may cause a drop in the resistance of the organism [259, 279]. With regard to the dependence of the effect of rf field on sex, women are generally more sensitive to this factor than men [64, 190, 262]. Reference has been made to a decrease of lactation in nursing mothers [186, 195]. · At a certain time after exposure had ended (sometimes as long *Dermographism (writing on the skin"): hypersensi ti vi ty to mechanical stimulation. Repeated mild friction of the skin immediately results in red spots because of local irritation and vascularization. In some persons, such increased irritability is congenital. It is quite common in neurotics. 30

as several weeks or more), the organism usually returns to its original physiological state and all subjective and objective complaints vanish. This phenomenon is usually described in the literature as a regeneration [195, 279].

Effect on the Eyes: Experiments on Animals. The most significant thermal effect of electromagnetic waves is observed in the uhf band, as has been confirmed by research on the heating of the eye as a function of frequency [195] and the distribution of heat inside the eye [39, 142]. These tests have shown that the temperature rises more steeply than does the power density [26]. Such heating leads to various degrees of eye damage [17, 107, 161, 259], sometimes extending even to cataracts on the lens and on the cornea [215]. At a sufficiently high power density, a cataract can appear even after a single exposure, with the exposure time necessary inversely proportional to the power density [137, 227, 269]. But even with single exposures at intensities at which the eye is not immediately damaged, a cataract or lens opacity can develop 1 to 60 days after exposure [195, 213]. Repeated irradiation at subthreshold intensities can lead to the same damage [14, 26, 27, 30]. This is evidence for a cummulative biological effect of uhf [132, 279]. From the viewpoint of causing cataracts, a pulsed field is more effective than a continuous field [26, 27]. All damage to the eyes results from their direct irradiation. In whole-body irradiation at near-lethal intensities the eyes are not damaged [20,31,195], not even with pulsed radiation ]142], if the radiation is not aimed at the eyes. Effects on the Eyes in Man. No eye damage was found among operators of rf installations [21], but a significant percentage of findings were made in persons working in the uhf field, especially radar operators [12, 28, 29]. In a number of cases, both uni lateral and bilateral cataracts have been described [99, 225, 279, 280]. Soviet authors warn that chronic irradiation at intensities of the order of a few milliwatts per square centimeter are sufficient to produce opacities in the human eye [195]. In such persons, a flow of tears and eye fatigue is observed first [280], combined with changes in vision, especially a decrease in sensitivity to colored light (especially blue) and defective observation of a white object. In these observations, the authors used a projection perimeter [283]. A weak rf field also lowers the threshold of sensitivity to light stimuli in the dark-adapted eye [116]. The change in the threshold of sensi ti vi ty is -the same for pulsed and continuous fields [280]. A change in intraocular pressure was also observed to result from chronic exposure to centimetric waves [279]. At subthreshold intensities, a lowering in the content of Vitamin C in the lens and in the fluid in the anterior chamber was observed [195]. During acute cataract development, a considerable lowering of the activity of adenosintriphosphatase and pyrophosphatase occurs in the lens 31

[37]. In view of these facts, warnings relating to the therapeutic application of centimetric waves are quite justified, especially where there is eye disease [99, 213]. In experiments on models seeking to determine the distribution of heat inside an eye irradiated by a uhf field, materials such as 30% gelatin or polystyrene foam are used [100]. Various opinions have been expressed as to the cause of eye damage. At higher intensities the damage is evidently of a thermal nature, connected with a coagulation of the proteins in the lens; at low intensities, there is a disturbance of metabolic processes. An important role in this process has been ascribed to glutathione. Opacities may be also caused by damage to tissue respiration and of the oxidation-reduction systems [195]. Nervous System. The subjective complaints of persons w·orking in rf fields are predomi-Jlantiy related to the nervous system.For that reason considerable attention has been paid to changes in the central nervous system under the influence of this factor; the reader will find details in several readilv accessible review articles (142, 195, 205, 279, 280, 282, 28-3]. In clinical and laboratory studies l64, 122, 123] of the effects of rf and uhf fields on the human organism, changes in the activity of the central nervous system (CNS) were studied with both large and relatively small field intensities. These changes were determined by an interpretation of EEG of subjects, the majority of whom had worked long periods of time in rf and uhf fields [64, 248]. EEG examination is at present the most convenient objective method for determining the early stages of damage 'to the CNS produced by electromagnetic waves [53, 112, 122-124]. The complex of changes in nerve functions caused by damage inflicted on the CNS by low-intensity rf and uhf fields is characterized as a syndrome of the asthenic type[47]. In only one case has a severe neurotic syndrome with further functional aberrations been described, in an individual who worked for 10 years in the field of a short-wave generator [35]. At high intensities of rf and uhf fields, the asthenic syndrome is accompanied most often by disturbances of the cardiovascular vegetative regulation [61, 102]. Functional changes, even though sporadic, have been described in individuals systematically irradiated by an rf field at wavelengths of tens and hundreds of meters [183, 194, 228]. Here, too, the neurotic symptoms are the most frequent, described as "short-wave nausea" [51, 64]. Interesting, though drastic, are experiments that describe brain activity during irradiation of the head by a strong transmitter [ll5]. When the experimental subject was emotionally disturbed or absorbed in creative work, changes in brainwave parameters occurred. At the same time it was noted that a strong rf field can even elicit halluciations. Other authors noted involuntary motor reactions when the cerebrum of healthy subjects was irradiated [142]. The functional changes mentioned 32

above are usually reversible: after removal of the source--that is, the effect of the rf field--a normal state is restored; in more serious cases, considerable improvement of the over-all condition can be obtained by appropriate treatment (35]. Only in a few individual cases may the changes be of a progressive character (46, 195]. Under the effects of electromagnetic fields, animal behavior can also be significantly changed. It is possible to observe agitation (180], excitement, and increased motor activity (142], sometimes going as far as turning tranquil animals into aggressive ones (116]. At low intensities, animals may be observed to become sleepy. Exposure of a hen's head to rf field leads to stiffness; the animal accepts neither water nor food, and when made to stand remains standing until it falls down exhausted. Also interesting are experiments in which the effects of rf field on the reflex activities of animals were studied. At intensities too low to disturb the animal, both conditioned and unconditioned reflexes were evoked in dogs (142, 208]. However; at higher intensities, conditioned-reflex activity is significantly decreased or extinguished, or else the period of time required to develop the reflex is prolonged; in some cases it is necessary to use a stronger stimulus before the reflex action takes place (11, 79, 159, 208]. For example, we observed the loss of all obedience training in a dog taught in a police school after the animal had moved about for about six months in a space subjected to a strong rf field. But these results are not unambiguous, since some dogs conditioned to have simul taneous taste and defense reflexes preserved the defense reflexes under irradiation (14]]. The EEG was used both in animals and in human subjects to detect changes in the electrical activity of the brain during irradiation by an rf field (276]. The effect of both single and chronic irradiation of the organism by an rf field on reflex activity is probably due to changes in interneural connections [143, 168, 279, 283]. It is possible to observe degeneration of neurons in the cerebral cortex and the basal ganglia, the pons, the medulla oblongata, and in some cases even the cerebellum, as well as histological and chemical changes in the vicinity of nerve fibers (20, 143, 228]. Effects similar to those produced by centimetric waves are caused also at substantially lower frequencies (12]. The reaction of the cortex to an rf field is the same as that evoked by bromides or caffein [142]. Studies have also been made of the functional state and changes in the excitability of neuro-muscular preparations in rf fields (39], and of the effect of an rf field on the rheobase and chronaxy in both animal· and human subjects; and there are descriptions of differences between the results of a continuous and a pulsed field, and of the effects of bromides and caffein on the results. The reactivity of the entire nervous system of an animal is disturbed by an rf field. For example, sensitivity to touch and 33

the threshold of pain are both reduced [223, 228]. The analgesic effects of the rf field are explained by a reduction in the conductivity of the affected nerve. Several papers examine the effect of the rf field on the threshold of sensihili ty and on the latent period of medullar nerves [142]. As mentioned previously, the visual analyzer is also affected by an rf field [51, 116, 225]. When the auditory analyzer is subjected to an rf field, even low intensities serve to reduce excitability and at the same time prolong the latent period. However, auditory acuity may be somewhat increased at such low intensities. Irradiation of the human cerebellum may manifest itself by a shortterm change in spatial sound perception without a change in the threshold of sensibility. Sensitivity also decreases (ie the level of threshold of sensibility increases) when the olfactory analyzer is subjected to an rf field [225]. This decrease in the sensitivity of smell can serve as one of the earliest symptoms of the effect of cen;imetric waves on healthy subjects working with rf installations [279]. In cold-blooded animals (a shark) the ability to "scent" or otherwise find the prey deteriorates under the effect of rf fields [287]. The effect of high intensity uhf fields may lead to damage of the interoreceptor apparatus.

Reproductive Tissue. Besides the eyes and the nervous systems, the genital organs are the most susceptible to rf fields. Perceptible changes occur mainly at high field intensities in the centimeter-wave region [17, 30, 82, 113, 137]. At these intensities, the main effect is thermal damage to the reproductive tissue. An increase in the temperature of male or female gonads results in morphological changes [83] and possible degenerative processes in these organs. The changes are similar to those produced by thermal trauma [82]. Thus the walls of the blood vessels that supply the reproductive organs may contract, or else there may be direct damage to the ovaries and the testes. Histological examinations have revealed interruption of spermatogenesis in several phases of the process [282]. These morphological changes may then manifest themselves in changes of the reproductive cycle, in a decrease in the number of offspring, in the sterility of the offspring [79], or in an increase in the number of females born. No decrease in fertility was noted in persons working in rf fields [12, 36, 280], but as far as the number of children born is concerned, girls distinctly predominate. The results of a study of the effect of rf and uhf fields on the menstrual cycles of women subjects are not uniform. Disturbance of the menstrual cycle is mentioned as one of the symptoms of the effect of electromagnetic fields on organisms [182], even though the results of some studies of women who have worked from 3 to 11 years in a uhf field have not confirmed this effect [24, 186]. It appears that rf irradiation of pregnant women and female animals increases the percentage of miscarriages [222]. The off34

spring of female rabbits subjected to the effects of an rf field showed definite functional aberrations and higher mortality when compared with individuals from a control litter (30]. The literature contains a case of embryopathy in the fetus of a mother treated by a shortwave diathermy at the beginning of pregnancy. When the child was born, it exhibited changes in the upper and lower extremities: the upper extremities lacked ossification centers (30]. Other authors have also reported that the rf field definitely impairs embryo genesis in both humans and animals, particularly in the beginning stages. The development of the fetus is retarded, congenital defects appear, and the life expectancy of the infant is reduced. The effect is cumulative and the thermal effect also plays a definite role (160].

Circulatory System. Periodic exposure to a high-intensity rf field leads to changes in the circulatory system (195, 279, 280, 283]. Disturbances in blood circulation have been described [ 80], evidenced by a change in blood flow (216] . In general it is an increase in blood flow that is described, proportional to both the intensity and duration of exposure (214]; except that in the denervated extremities, a decrease is observed. These phenomena depend on vasodilation. Clearly,a change in the blood flow and vasodilation produces a change in blood pressure (262]. At first, bood pressure rises moderately and then it drops (5, 11, 98, 224]. The drop can be very pronounced and may persist for several weeks after exposure. However, negative results in radar personnel have also been reported (224]; the heart rate also changes [141]. Depending on which part of the body is exposed, the rate may be either accelerated or retarded (11, 198, 199]. The EKG is used for objective study of changes in cardiac activity. Rf fields reduce the conductivity of the coronary circulatory system, which are then manifested as changes in the EKG, characterized as changes of the sinusoidal bradycardia type sometimes combined with sinusoidal arhythmia (245]. Further observations include aberrations in vascular reactions; for instance, oscillations of the vascular tonus (61]. Soviet authors (279] divide symptoms of chronic exposure to centimetric waves (at the level of vasometer disturbances) into three stages: (1) the initial, compensated stage; (2) the stage of moderate changes; (3) the stage of clearly discernible changes. The degree of change depends on the intensity and the duration of exposure to the uhf fields. The above changes in circulatory functions are reversible, but a case has also been described in which the change in the EKG continued after the effect of the field has been removed, even though other functions returned to normal (280]. Changes in the Blood Picture. A number of authors note that the blood picture is not noticeably affected bv the effect of an rf field (12, 36, 143, 195] .But other authors have found changes (105, 107, 224, 276], both in the white (79, 95, 98, 156, 200, 35

225, 279] and in the red blood picture [104, 224], and a drop in hemoglobin content [79]. The osmotic resistance of erythrocytes is perceptibly negatively affected [195]. When centimetric _waves act on a suspension of erythrocytes, both their volume and sh.ape change and continued exposure may even lead to hemolysis [73, 245]. Cell walls obtained in this way have electrical proper~ ties different from those of walls obtained by conventional osmotic hemolysis. After exposure to an rf field, coagulation time is decreased [195]. The prothrombin time, according to Quick, is reduced [229]. Increased coagulability, combined with the changes in the vessels,may even give rise to thrombosis. Some authors have also studied the effect of microwave radiation on the hematopoetic organs [42, 121].

Effect of Rf Fields on Other Organs. The effects on the circulatory system lead to observed acceleration (sometimes retardation) of the breathing-Tate'~ [11, 195]. Furthermore, hemorrhaging and bleeding can occur in some internal organs [20,195]. A number of authors have studied the effects on the kidneys, the adrenal glands, and the liver [20, 92, 195]. They found decreased filtration in the renal tubules, perhaps caused by degeneration of the epithelial cells in the distal and pro ximal renal tubules. In addition, there was increased activity of the adrenal cortex, hemorrhaging in the liver, and degeneration of hepatic cells. Persons working in rf fields, particularly women, exhibited enlargement of the thyroid gland, though without an accompanying clinical picture of hyperthyroidism. Increased incorporation of radioact ive iodine was detected in studies of the functioning of the thyroid [190, 262, 279, 280]. Since the dimensions of some portions of the organisms may lead to resonances, partial injury to organs may occur--for instance intestinal necrosis [20, 224]. Rf irradiation does not cause histological changes in bone marrow or changes in the incorpor ation of tracer calcium and phos.phorus into irradiated bones [56, 106, 250] ! Some histological changes in muscle following chronic irradiation have been described [158]. At the higher intensities, morphological changes are produced not only within the organism, but also in the paws and ears of experimental animals [195,259]. No histological changes were found after a single exposure [282]. BiocherrricaZ Changes. The effects of electromagnetic fields are manifested by metabolic changes in the most diverse tissues [251]. A series of experiments were made on sections of the cerebral cortex [8, 93]. Under the influence of a pulsating field, the glucose level falls and oxygen consumption rises [133, 154]. Simultaneously, there is an increase in co 2 content, in lactic acid content, and in the inorganic phosphate level,where as the content of macroergic structures decreases. Considerable aerobic glycolysis thus takes place. Changes in the alkaline reserve and in the blood pH have also been noted [33, 127]. In 36

unanesthetized rabbits, the activity of succinodehydrogenase and cytochromoxidase changes slightly;in anesthetized rabbits (whose basal metabolism is lower), the rf field raises the activity of tissue respiration almost to normal levels [118]. A study has been made of the effect of the rf field on oxidation processes in man [22]. Even at relatively low intensitites, the activity of cholinesterase in the blood and in other organs is reduced [279]. We may thus assume that the rf field causes a rise in the level of acetylcholine in some parts of the organism, which may be of great significance in the development of vegetative changes. Several changes in the composition of blood plasma have also been described [18, 252]. A number of authors have noted a decrease in total proteins, with a simultaneous decline in the ratio of albumin to globulin [9, 82, 280]. The change in this ratio is most probably caused by a substantial increase in gamma globulin [62, 82, 224, 279]. which may in turn be related to a change in the decomposition of tissue proteins. HowevAr. this observation is more of an exception than the rule. In smne cases a rise in the histamine level of the blood was detected [97,200, 280] and with it an increase in the resistance to ionizing radiation. The rf field also affects the glycemic curve [195,229, 279] and glycogen breakdown in the liver [82, 116]. In healthy individuals, only slight changes have been observed in the levels of sugar, cholesterol, and blood lipids, but there was a pronounced decrease in all three components and in improvement in subjective complaints when rf was applied to diabetics [92]. It is likely that changes in sugar and phosphorus found in the blood of rabbi ts are caused by a disruption of sugar metabolism [ 19 5] . A drop in the level of ribonucleic acid (RNA) in the spleen (and after continued irradiation, in the liver and in the brain as well) of rats was observed after chronic irradiation by microwaves. The content of desoxyribonucleic acid (DNA) remained unchanged [283, 195]. Other authors report that a single irradiation of rats led to a reduction of the activity of both ribonuclease and desoxyribonuclease, ie an increase in the level of both nucleic acids, and especially of RNA [282]. In skin cells and dermatic derivatives, a single irradiation led to an increase in the activity in both enzymes [283]. An increase in the RNA level was also observed in the lymphocytes of rf generator personnel, which corresponds in the blood picture to an increase in the number of monocytes, which contain the great majority of RNA (young cells) [245]. Also affected is fibrinolytic activity, which increases in young individuals after irradiation but is more likely to decrease in older subjects [15], although it is otherwise the same in both groups. The effect on the activity of other enzymes was also studied [91]. On the basis of biological changes (mainly in sugar metabolism) under the influence of rf fields, several authors have proposed that research in this direction might point the way toward 37

a successful cancer cure [19, 116]. Effects of rf fields on cancer patients have been actually studied [60, 164]. A group of French scientists has reported a successful cure of cancer in rats that had been subjected to short-time exposures of rf fields at various frequencies [217} 218, 219]. (However, this report was received with certain reservations by specialists in oncology.) The authors conclude from experiments made to date that rf fields cause changes in the metabolism of tumorous tissues [179], not only retarding the growth of primary sarcomas but also inhibiting the growth of secondary tumors and the production of metastases.

4. 2. Effect on other orgamsms The effect of electromagnetic waves has been studied in some invertebrates and also in unicellular organisms (bacteria and protozoa), as well as in -plants.

Effect on Invertebrates. Irradiation of various kinds of insects by the rf field produces an over-all reaction similar to that observed in experiments with mammals. The first symptom is unrest, attempts to escape, then disturbance of motor coordination, stiffening and immobility, and, after a certain interval, death [87, 195]. The exposure time necessary for a lethal reaction depends on the type of insect. Drosophila, for instance, survive over 30 min; some tropical species last only a few seconds at the same field intensity. This is not a purely thermal effect; if rf irradiation is replaced by an increase in temperature (50°C), the sensitivity of the two species is reversed. Changes in the concentration of a great variety of metabolic products have also been found. Rf radiation also affects embryogenesis; the time needed to complete metamorphosis in butterflies is increased [107], and gastrulation and development of the plutean larva is accelerated by the irradiation of fertile eggs of Paracentrotu.g lividUB. The entire life cycle and cocoon production of the mulberry boring beatle is accelerated. Effect on Unicellular Organisms. Unicellular organisms react variously to rf fields. Immobile organisms orient themselves parallel to the lines of force at the low frequencies and sometimes perpendicular at higher frequencies. Mobile organisms exhibit the same frequency dependence. Movement at the lower frequencies is in the direction of the lines of force and perpendicular to them at higher frequencies [51, 257]. When the external effect ceases, the original state is restored. In certain kinds of amoebas and some larger immovable microorganisms, a change in the outer and inner structure (in the orientation of subcellular components) has been observed. The authors designate the directional change of structure in the amoeba as a certain sort of "structural schizophrenia." In an appropriate field, experimenters have even succeeded in dividing and destroy38

ing the amoeba. Interesting experiments have been performed on paramecia. It was found that irradiating a paramecium by pulses or by series of de pulses or by ac elicited a so-called "electric-shock" reaction--a sharp braking of motion. The same type of reaction is produced by the application of microwave impulses of sufficient intensities (threshold and above threshold) . No reaction results from the effect of microwave pulses at subthreshold intensity, but the sensitivity of paramecia to the effects of pulsed de and ac is increased (ie the threshold of sensi ti vi ty is lowered) [201, 204]. The dependence of the threshold of reaction on the length of a de pulse and on the frequency of ac corresponds to the dependence observed when nerve and muscle tissues are stimulated, which only serves to support the assumption of the existence of some sort of excitable structures in paramecia. The effects of rf field on growth, viability, and some metabolic processes in unicellular organisms have been studied [66, 75, 94, 144]. Here again, frequency and intensity dependence were found. Growth is moderate at lower frequencies and is substantially limited and even halted at higher frequencies and the organism dies. On the other hand, irradiation of a culture of cardiac fibroblasts of chick embryos produced an accel~ration of growth [195]. The effect of the rf field may be used to decrease the infective power of bacteria and to inactivate certain viruses [192, 195]. However, such inactivated viruses lack the power to act as vaccines that is retained in normal inactivation by heat [181]. Conclusions as to the mechanism of the effect of electromagnetic fields on unicellular organisms have reached no consensus among the various authors [66, 94, 144, 201, 257].

Effect on Plants. During periods of maximum intensity of natural sources, an increase in the growth of wood in trees has been observed. The growth is two or three times greater than during the period of minimum intensity. On the other hand, at large field intensities, for example in the vicinity of relay transmitters and ultra-short-wave links, growth is inhibited and the so-called "vsw breaks" appear [120]. These phenomena are produced by the effect of radiation on the rate of cell di vision~' *l. Mitosis is indirect division, the most common mode of cell division. In mitosis, a complex division apparatus is formed and division proceeds in several phases. Chromosomes are morphological elements of mitotic division, found in the nucleus of every cell; among other components they contain DNA. Their structure is complex and not entirely understood in detail. Mitosis leads to the formation of two daughter cells from each cell, each with the same content of nuclear naterial (they have the same number of chromosomes. 2. Amitosis is direct division, a less common form of cell division. In amitosis, no division apparatus is formed as in mitosis; the nucleus either divides by simple constriction into two equal new nuclei, or smaller pieces separate 39

and a change in the number of individual division phases [23]. An rf field of la- 4 to 10- 12 V/m is sufficient to accelerate division, but the division rate falls above 0.1 V/m. At the same time, chromosome changes become evident [90, 94, 107]. In the cells of a growing garlic-plant root tip, various changes in the nuclear apparatus were found following irradiation by a pulsed rf field. The effects went as far as linear shortening of chromosomes, the development of pseudochiasma, links between nuclei, and the appearance of irregular chromosome walls. Daughter cells with different amounts of nuclear material were also found. Even amitotic division was produced. Similar phenomena are also caused by ionizing irradiation and some chemicals. Chromosome changes depend on the rf field frequency, power, duration of exposure, and orientation of the cell axis relative to the direction of the field; they are permanent. It thus appears that the rf field is an additional mutagenic factor, moreover one that can be well regulated, which affords an opportunity to make fine gradations in dfromosonic changes [94, 25 7] . Even very small rf field intensities, which produce no other noticeable damage, may produce genetic changes that are evidenced only after several generations. The effects on growth are also acGPmpanied by various biochemical changes produced by the effect of rf field irradiation [130, 148].

Dependence of Biological Effects upon Field Parameters. From the brief discussion above regarding the effects and action of electromagnetic fields on living organisms, and even after a detailed study of the works in the bibliography, it is still impossible to come up with a concise and (so far as is possible) unambiguous conclusion. If we keep in mind the fact that the biological effects of radio waves depend on a number of factors (the most important being intensity of the field, its nature, and the exposure time), it becomes clear that not even the final results will be unambiguous and will vary more or less (or even be contradictory) , as is frequently the case even in the above summary. From the sections that follow, which describe several important factors depending on the field parameters, it is possible to gain some idea of difficulty of getting a correct and objective appraisal of the end effect. The frequency dependence of the biological effects may vary according to the above-mentioned partial mechanism of the effects. It appears that the complex biological effect of electromagnetic waves at a relatively low field intensity is not too dependent on frequency, provided the energy acting on the organism is alfrom it, or the whole breaks up completely into several parts. Amitosis does not necessarily involve the simultaneous division of the entire protoplasm (ie the whole cell), so that giant multinuclear cells may result. Some authors think that amitosis is a pathological phenomenon; others consider it as equivalent to mitosis. 40

ways of the same magnitude, although the individual active mechanisms may exhibit a pronounced frequency dependence. There are thus basically two types of effects: thermal and nonthermal. It must be emphasized that nonthermal effects cannot be separated from the thermal, which predominate at high intensities and depend upon the energy transmitted. Thermal effects increase sharply with increasing frequency and are therefore most important in the range of the so-called microwaves. Owing to the small depth of penetration of microwaves, caused by their considerable damping in tissues, the eyes and (in men) reproductive organs are most seriously endangered. Nontherrnal effects depend primarily on the instantaneous amplitude of rf radiation. Their significance increases with repeated exposure to relatively lowintensity irradiation, especially by pulsed fields, in which the total transmitted power is relatively low but the instantaneous amplitude is quite high. In this situation, nonthermal effects tend to predominate over the thermal. On the basis of our present knowledge of the primary biophysical mechanisms of the nonthermal effects of electromagnetic waves, we can say that the primary effects involve above all the macromolecular and cellular levels. It is mainly a question of influencing the colloidal structure of the cellular contents and other colloids in the body, and also affecting the electrical conductivity of the cell, which may be significant above all for the function of the central nervous system. The frequency dependence of each of these two partial mechanisms need not be the same and our present knowledge of them is still very incomplete. We do know, however, from experiments on the effects of electromagnetic waves on isolated colloidal components, that the frequencies and intensities at which they are affected depend on the composition of the colloids . The frequency dependence of the biological effects of rf and uhf fields has been studied by many authors [17, 279]. As far as the thermal effects are concerned [41, 195, 210], the dependence is determined by the fact that the electrical characteristics of individual tissues vary with frequency [240]. This change is monotonic, ie it proceeds without any maxima or minima. For the body as a whole, however, there is a certain optimal range of frequencies at which heating reaches a maximum [69, 235]. This range is due to the finite dimensions of the body, which is not a homogeneous structure, and the frequency dependence of the dielectric constant (the so-called dispersion)of many tissues.The frequencies for maximum heating of the human body lie in the range of very short and centimeter waves. However, it may be said in general that tissue heating increases with frequency. Many other effects are also more pronounced at the higher frequencies. Thus, retardation of visual motor reaction is greater at 75 MHz than at 0.3 MHz, just as are changes in the central nervous system [280]. Changes in the growth of bacteria as a function of frequency have also been described [63]. As far as the circulation of the blood is concerned,it has been found to 41

increase in the centimetric-wave region and contrariwise to fall below normal at substantially lower frequencies [195]. In that case, however, it was more likely a matter of the effect of two different field intensities. The same assumption may be made in the previously described difference in sensitivity of the auditory analyzer between the 10 and 3 cm wavelengths, since there is no difference in the effect of these waves on the endocrine system nor on the central nervous system [279]. Far more significant than the frequency dependence is the effect of the nature of the emitted signal. The latter may be either unmodulated, so that the electromagnetic field is continuous, with a more or less constant amplitude (cw operation), or modulated. A boundaxy case of an amplitude-modulated signal is pulse modulation. Let us imagine an experiment in which we have two generators operating at the same fundamental frequency, one unmodulated and the other in pulsed operation,,. If the average radiated power of the two devices is comparable, there is no difference in the thermal response of the organism to these different fields [279]. Nevertheless,we can detect different effects. In the single exposure of rats to radiation at a power density of approximately 200 mW/cm 2 in the 10-cm range, cw operation of a generator does not produce any visible effect on the experimental animal even after 30 min, ~1ereas pulsed operation (pulse width 1 µs, repetition frequency 1000 Hz) kills the rat after 3 to 4 min [151]. Postmortem examination reveals only considerable enlargement of the spleen; the histological appearance of the principal organs (including the brain) is normal. In addition, the response of an experimental animal to pulsed-wave radiation is characteris ·tic from the very beginning and indicates the dominant influence of electromagnetic waves on the central nervous system. A pulsed field is thus biologically more effective than a cw field. This conclusion has been reached independently in the USSR, the USA, and Czechoslovakia [118, 145, 151, 254]. It may be assumed that the greater biological activity of pulsed fields is caused by nonthermal effects [118]. Mention has already been made of the difference between the effects of unmodulated and pulsed fields on the development of cataracts [26, 27, 215], morphological changes in the links between neurons [280], and on the rheobase and chronaxy [142]. The wavelengths used in these experiments were centimetric, which are usually used in pulsed operation. But even at metric wavelengths, a greater biological effect of the pulsed field has been demonstrated, for instance on oxidation processes in tissue [118]. With increasing average power density of the field, the difference between the effects of continuous and pulsed fields washes out, since the thermal effect begins to predominate [20]. It has already been stressed that the rf field, in its different manifestations, may have either a stimulatory or a damping effect; moreover either direction may be associated with a given frequency and field characteristics. It depends on its intensity 42

and period of exposure. At high powers, the effect is governed by the field intensity and period of exposure, whereas at low intensities or brief exposures the effect may oscillate. Many parameters thus affect the biological effects of an rf field.In this respect, it is evidently unimportant how the field is produced. Besided man-made rf generators, there are fields that are derived from natural sources. One such powerful source is the sun; in addition to heat and light rays, it emits radio waves over the entire width of the spectrum. The intensity of this radiation can under certain circumstances reach values that are sufficient to evoke biological effects [ 6]. In addition, the sun emits corpuscular radiation which, when it reaches the upper layers of the atmosphere, can produce short rf pulses in the range from 10 to 50 kHz [50]. Similarly, large air movements (mainly fronts with unstable moist boundaries) under certain conditions become rf field sources in the same frequency range. Many papers are available at present that deal with the biological effects of this range of frequencies [51, 187, 188, 288]. It is interesting that statistically significant evidence has been presented regarding the effect of such fields on the mortality rate of the population [51], the birth rate, traffic mishaps, and industrial accidents [6]. Laser Radiation. Following the invention of lasers, sources of coherent electromagnetic radiation in the visible region, studies of the biological effects of these rays were begun [49, 256, 284]. It was found that laser radiation, like low-frequency electromagnetic waves, has both thermal and nonthermal biological effects and has its greatest effect on biologically complex macromolecules (proteins) [62]. When pulsed laser radiation is applied to the eye, ultrasonic waves are generated in the skull, which produce vibration of the brain, the cerebrospinal fluid, and the bones of the cranium; piezoelectric recordings can be made from the bones at the base of the skull [2, 43]. Comparison with the effect of classical monochromatic light sources (whose radiation is not coherent and which can thus produce only thermal effects in practice) shows that laser radiation has specific chemical and electrochemical effects of a nonthermal origin [260]. The hypothesis has been put forward that the mechanism of this effect is similar to that of radio waves. SirrruZtaneous Action of tors. In the operation of plies are used to run the approximately 20 kV, flow tion [145]. Measurements shown that peak values up the combined influence of therefore studied; it was to a biological object is

Electromagnetic Waves and Other Facrf generators, high-voltage power supvacuum tubes in the final stages. Above of anode current gives rise to X radiamade on various uhf generators have to 50 r/min can occur. The effect of an rf field and of X radiation was found that in such a case the damage more serious than when each factor acts 43

independently [195, 264]. A reinforcement of the effect was also found for successive exposure to an rf field and ultraviolet radiation [114], as well as gamma sources [200]. The combined action of fields at different frequencies likewise appears 1 to be most dangerous [76]. Irradiation of an organism in an rf field can also significantly affect the known effects of certain chemical substances, s uch as medicines, so that the effects are increased or disturbed [116].

4. 3. Effect on physical and chemical properties of materials As long ago as 1890, two years after Hertz's work, it was found that when electromagnetic waves strike iron filings, their electrical resistance decreases [72]. This effect was examined at the time by several authors, but none could explain it. The effect nevertheless served as the basis of the so-called "coherer," the first detector-, of ~lectromagnetic waves. Later, detailed studies were made of the effect of rf fields on the behavior of colloidal systems of various compositions, and it was found that the resulting changes are partly instantaneous and partly gradual. The most sensitive instantaneous change is the migration of colloidal particles in the electrical field produced by an rf field. For example, the field from a nearby radio transmitter is sufficient to affect a colloidal solution of As 2 s 3 [270]. Comparatively weak fields cause changes in cell divi~ion [120]. Some changes do not appear immediately, however. Thus, several hours after exposure, there may be signs of disturbance of the stability of the colloidal solution, clustering of the particles, and their flocculation. Fatty emulsions behave similarly. An rf field can also speed up the curdling of milk. Inasmuch as the intensity of the rf field emitted by natural sources increases sharply before a storm, this fact is related to the well-known observation that milk curdles easily under these conditions [51]. At a certain field intensity, colloidal and larger disperse particles link up together (the phenomenon called pearl-chain formation). All of these chains are longitudinally aligned in the same direction, that of the lines of force [131]. A theory explaining this phenomenon has also been devised [132, 257]. In an aqueous solution, randomly distributed charges are bound to the water molecules. Under the influence of an electromagne tic field, these charges attempt to align themselves with it, but since each colloidal particle is dissolved, the water molecules drag the colloidal particles along with them [177]. The above effect of an rf field can also be thought of as follows. Induced dipoles are created and attract one another at oppositely charged ends, so that a chain is formed under the influen ce of electrostatic forces. The magnitude of force fields formed in this manner has been derived for aqueous and nonaqueous particles dispersed in various media [72]. It has been shown both theore44

tically and practically that even very low frequencies sufficR to produce the phenomenon of chain formation in colloids [24] . Since a suspension of unicellular organisms in a fluid medium resembles in its electrical characteristics a colloidal solution, the effect of rf fields on this system was also examined [268]. Pulsed sources are used in the majority of such experiments at present, even when not working at centimetric waves.The reason is the protection of the living organisms against the thermal effects of the rf field. It was found that both dead and living nonmotile unicellular organisms behave exactly like colloidal solutions of inorganic or organic particles, ie they form chains [94, 275]. More exact examination of an earlier result showed, however, that nonsymmetrical particles first align themselves with the lines of force and only then form chains. When motile microorganisms are irradiated, their spatial orientation, conditioned by forced motion in a certain direction,is affected as described above. It is interesting that the frequency at which the direction of the movement changes, and the minimum field intensity required for this phenomenon, depend on the type of microorganism and can be very different for two different varieties. An attempt was made to explain this phenomenon on the basis of the. theory of a nonhomogeneous dielectric [115]; at the same time, the time constant for chain formation is the same for both cw and pulsed fields and depend only on the average power [277]. The problem has not been entirely solved yet; at the present time, considerable attention is being de··· voted to this question in many laboratories, because there is a wide range of possible applications both in practical use and in many theoretical disciplines. An electromagnetic field can also evoke changes in the properties or states of some substances [116]. Thus, a persistent decrease in the surface tension of water has been observed to be caused by fields at frequencies in the vicinity of 10 kHz [189]. Waves in the 20-to 70-cm range influence the crystallization rate of certain inorganic substances. A group of materials has been found for which the acceleration of crystallization was greatest at a certain frequency [261]. The number of crystals is increased, but their sizes are smaller than would be obtained without the effect of the uhf field. No abnormal crystals were found [272]. On the basis of the behavior of crystals in an alternating field, their electrical properties can .be determined [72]. Some molecules can be set to oscillating by the action of an rf field [220]; at a certain frequency, this action can lead to resonance phenomena. Thus we have$for example, the well-known inversion of pyramidal molecules of ammonia at a frequency of 23 870 MHz (ie A= 1.26 cm). This phenomenon has been put to practical use in the regulation of highly precise clocks.Hydrogen ions can selectively absorb rf energy under certain circumstances in the ranges 2.5-8 and 16-44 MHz. In the case of glycogen, the interesting phenomenon of a change in the polarization plane of light has been observed.The 45

angle of rotation of this plane at a certain frequency is proportional to the intensity of the field [59, 115], which might be used for dosimetric purposes. Many authors have studied the effect of rf fields on protein molecules in vitro. Changes were found in the average molecular weight of a mixture of proteins. This effect is reversible, but more persistent than when heat is used [140]. As far as the specific effects of the uhf field on the denaturation of proteins is concerned, the frequency dependence probably applies here as well. Under the influence of centimetric waves, no specific effect has been observed [132], whereas in the meter-wave range such an effect was assumed [19] or was actually observed [96]. Mention has already been made of the effect of the rf field on the composition of blood serum in experiments in vivo. It is interesting that irradiation of blood in vitro also produces in subsequent electrophoretic separation a decrease of the albumin fraction and an increase of the gamma-globulin fraction in particular. This phenomenon' has been compared with the effects of ultraviolet light, which reduce6 the albumin level three times more than the rf field. The electrical characteristics of molecules, especially the dielectric constant, can change substantially with increasing frequency [137] and with the conposition of the solution (86]. The related change in the dipole moment makes it possible to explain the effect of the field on the reaction time [187]. A method has been devised that makes use of this phenomenon to facilitate the measurement of extremely rapid ionic reactions [74]. The above experiments may be to a certain extent connected with the observed increase in proteolytic activity of pepsin under the influence of an rf field at A = 8 meters, in which a pronounced frequency dependence was observed. On the other hand, it was not possible to detect any influence of an rf field on the activity of diastase at constant temperature [130]. Crystalline a--amylase can be completely deactivated at a temperature of 37.5°C when exposed to an rf field at a frequency of 12-16 MHz [129]. The activity of phosphatase (but not furnarase) can be affected similarly [188]. In experiments with enzymes, temperature may be if not the deciding at least the basic factor, so that the results must be interpreted carefully from the standpoint of nonthermal effects. It was discovered in the USSR that irradiation of tars with an rf field reduces their carcinogenic activity by as much as 95%; the investigators propose that this method be introduced in industry [45]. The increased number of experimental studies has led to the development of a methodology for irradiating test objects with both centimetric waves [195-197] and at lower frequencies [142]. If, at the same time that the test object is being irradiated, we also measure some of its characteristics (especially the electrical properties) and use electronic equipment in the experiments, it is necessary to watch out for any possible influence 46

of the results of induced rf currents on the parts of the measurement apparatus. Biological effects are studied not only by physiological and biochemical methods, but also by the modern methods of physics, for example nuclear magnetic resonance and electron spin resonance.

4. 4. Mechanism of effects There is no uniform explanation for the resultant mechanism of the effects of electromagnetic waves. The reason is that there may be a whole series of independent primary mechanisms, several of which may be acting simultaneously for a given set of parameters. The complexity of the resultant effects of electromagnetic waves on the organism is best shown by Table 4. With changes in the field parameters, the character of the individual component processes may change substantially [166, 202, 205,283]. We have already mentioned that there are two kinds of effects, thermal and nonthermal. But there is no sharp dividing line between them. This fact stems from the nonuniformity of views on the meaning of the concept of nonthermal effect (frequently label led as specific). The majority of authors understand this concept to mean the effect of electromagnetic waves of a field intensity so low that they do not produce a significant increase in the temperature of an irradiated object. That is of course a highly unobjective definition and does not provide any explanation of the basis of the phenomenon. The thermal effect is produced by an increase in ~he temperature of the 'system (or part of it) with increased molecular motion under the influence of impact and friction. It is much more accurate to consider those effects as nonthermal that stem from primary nonthermal (stationary) mechanisms of the effects of electromagnetic waves on an irradiated system, without regard for the field intensity. Because the electrically charged particles (whether ions, molecular dipoles, or colloidal particles) are always in motion in an alternating field, and because ionic and colloidal solutions including molecular dipoles are necessary components of a biological system (primarily water, but also organic compounds, such as amino acids), it is never possible in such systems to separate a thermal effect from a nonthermal one. Under suitable conditions (conductivity, dielectric constant, frequency, intensity and nature of the field), and with specific features of the organism, the nonthermal effect may definitely predominate over the thermal one and thus be the biologically effective factor. At the present time, it is precisely the mechanisms of nonthermal effects of an electromagnetic field which are the center of attention. When rf energy strikes the surface of a body, some is reflected and some penetrates to be absorbed. The reflection depends on the electrical characteristics of the tissue on which the rf 47

TABLE 4.--Some of the detailed mechanisms of the biological effects of radio waves and their relationships. The complexity of the resultant mechanisms of the effects is evident from the relationships shown. Entrance of electromagnetic waves into organism Absorption

~~

Vl

.µ

C.) Q)

4-1 4-1 Q)

4-1 0 Vl

sVl ·r-1

+

CX>

t

+

Induction

~dIonic t.~d Induce Semicon uctor current rectif~cation

/r~tation of Nonress~o~es

@

"5 Q)

s

.µ

i::: i:::

Q)

0

~

Relaxation

Dimensional

molecules

t~~

Acceleration of motion

Change of structure

of dipoles

Change in sensitivity of cells

~, Linking of dipoles

Change of reactivity

l_

Accumulation of ',colloids, formation --of _p~eudowacromolecules Resultant mechanism of effects

Thermal

~ Non thermal +

-l

Change in permeability of cell membrane

I.

0

u

Chan~e in potential of cells

field is incident. As radiation passes into tissue, its wavelength changes because there is a change in its velocity of propagation, which depends on the dielectric constant and the conductivity of the medium. A dipole located in the electrical field is aligned. If the field is alternating, the dipole begins to oscillate with the frequency of the field. The greater the dipole moment of the dipole, the greater the energy required for its oscillation. The result is an increase in temperature. The same results from an increase in frequency, since the oscillation rate of the molecular dipoles increases and the losses produced by friction likewise increase. The power thus consumed per unit volume (W/cm 3 ) is given by the following equation [247]: ( 4. 1)

where € is the average dielectric constant of the tissue,tan o is its loss factor, f is the field frequency in MHz, and E is its gradient in kV/cm. Hence the heating (and therefore the thermal effect) increases with the frequency and intensity of the field. However, practical experiments with animals have shown that because of thermoregulation the frequency dependence is not so pronounced and shows up distinctly only at high field intensities [15]]. It is thus necessary to consider another means of rf penetration into the organism. This second pathway is called "electromagnetic induction." An rf potential is induced in a conductor located in an electromagnetic field. This potential may then be carried (after some loss) by the conductor to locations where there is no direct electromagnetic field. The losses produced in a unit length of the circuit depend primarily on the resistance of the conductor, the immediate environment around the conductor, and the frequency of the rf current flowing through the conductor. That is true for a conductor of the first order. The electromagnetic field generates ionic currents in the organism. Lazarev has proposed the hypothesis [136] that the influence of the field increases the ion concentration in the vicinity of the cell membranes, which could result in largely reversible accumulation of protein molecules (some causing them so to speak to come out of solution). Moreover, he notes that a sufficiently large change in the ion concentration causes a change in the biological characteristics of the cells. The action of the electrical field on a charged particle thus leads to its forced motion. That means that the ar~angement of the system is changed. If the field vanishes, the system requires a certain time to return to its original state. This interval is called the relaxation time, which depends on the dimensions and 49

charge of the particles, as well as the characteristics of the medium and the temperature, according to tne relationship 2

T

= Ra 2kT

( 4. 2)

where R is the coefficient of friction of the particles for a given set of conditions, . a is the radius of the ionized atmosphere, k is the Boltzmann's constant, and T is the absolute temperature. If the period of the field changes in accordance with relaxation of the particles, transfer (and hence energy absorption) is at a maximum. The relaxation resonance thus appears at the frequency

f

12kT

~

( 4. 3)

where n is the viscosity of the medium. Thus, for example, in water molecules which have a relaxation period on the order of 10- 11 sec at room temperature, the absorption of energy reaches its maximum in the frequency range from 10 9 to 10 11 Hz, corresponding to wavelengths of 30 cm to 3 mm [166]. Such absorption has been observed experimentally [220]. In the case of proteins, the relaxation time in an aqueous solution is on the order of 10- 7 sec, which points to resonant absorption of energy in the range of frequencies of the order of 10 6 Hz. That agrees with the experimentally determined basic frequencies for sensitive effects of an rf field on the properties of gamma globulin, described by Bach et al. [9], al though the authors do not mention this circumstance. It should thus be possible to explain the selective frequency changes obs~rved in the electrophoretic and antigenic properties of gamma globulin under the influence of the rf field as the disturbance of the structure by the action of the relaxation absorption of energy, which in turn can lead to a shift in the reactivity of some functional groups in the molecule. An explanation of this sort also agrees with the temperature dependence of the effective frequency (Eq. 4.3), as observed by the above authors. The spatial resonance of large molecules in the microwave region is closely related to the relaxation resonance. In evaluating a possibility of this kind (disputed by many authors), it is necessary to appreciate the relationship between the wavelength and the frequency of the electromagnetic waves in the medium under consideration. We have already mentioned in Chap. 2 that the wavelength depends largely on the dielectric constant and the conductivity of the medium, but indirectly. Besides, it has been determined that some systems, among which it is necessary to include ionic conduction,, have an enormously high dielectric constant, running up 50

to many thousands [l]. We can therefore assume that in such a medium, microwaves may have a wavelength comparable with the dimensions of macromolecules. We can thus even accept the possibility of a direct spatial resonance of the molecules, which theoretically can lead to mechanical deformation or damage [166]. Direct damage to molecules by resonance phenomena nevertheless seems to be quite improbable, since a very intense field would be required. It is possible, however, to excite molecules by the absorption of a quantum of energy [195]. That increases the potential energy of molecules. A return to the unexcited state can take place either by transfer of the energy to another, unexcited molecule, which leads to an increase in its motion (and therefore to a thermal effect); by radiation of the energy; or finally by the structuring of certain parts of the molecule. The last form of energy loss leads to a change in at least some of the characteristics of the molecule. This form of absorption of electromagnetic waves in the nonionizing portion of the spectrum is known, as previously mentioned, for example in ammonia where there are 12 frequencies in the range from 2 to 4 GHz (ie A= 15 to 7.5 cm) at which molecules can be excited. Radiation of energy leads to a reversal of the tetrahedral structure of the NH 3 molecule. One more fact remains to be considered. Excited molecules very readily participate in a number of chemical reactions such as oxidation, fission, etc. This phenomenon occurs in the range of wavelengths on the order of 0.1 mm. Mention has already been made of the orientation of dipolar molecules in an electrical field resulting from the attraction between unlike electrical charges and repulsion between like charges. In the case of large dipolar molecules (for example, proteins), as the frequency rises (above the relaxation frequency), their oscillation in resonance with the field becomes increasingly more difficult; above a certain frequency, they come to rest in some intermediate position, depending primarily on the shape of the molecule and tre distribution of the charge. However, because the charge distribution of the dipole cannot be considered as fixed at a certain exact site, this "charge cloud" ~till moves about the molecule at the oscillation frequency.That can also lead to changes in the reactivity at the corresponding centers of the molecule. The group of "resonance theories" also includes the interesting idea of regarding the biological effect of electromagnetic waves as a nonthermal effect resulting from the cyclotron resonance of several notable varieties of molecules in the organism [257]. It is actually a combination of the electromagnetic field with the earth's magnetic field. This idea is still purely speculative, without any sort of experimental foundation. Charged particles (ionic or colloidal), under certain conditions, form around themselves an electrical dipole layer, composed of oppositely charged ions. The total external effect is 51

+ + + +

(a)

+

+

+ + +

8 + + +

+ + +

+

-.. + +

+

(c)

+ + +

+ + +

+

+

+ + +

+ + +

+ + +

+

+

+

+

++

'H

+

+

a+

(J +

+ + +

a

+ (b)

+

+

+ + +

+

+

+ + +

+

8 + + +

+

Fig. 20.-(a) A negatively charged particle(-), together with a double (+) layer, forms an electrically neutral whole; (b) an induced dipole which arises when a charged particle enters an electrical field; (c) linking of oriented dipoles in the direction of an electrical field (chain formation). that of an electrically neutral entity (Fig. 20a). The electrical field then contains a nucleus . (ie the original charged particle) and dipole layers, each drawn with a certain force in opposite directions, giving rise to an induced dipole which is a priori oriented along the lines of force of the electrical field (Fig. 20b). It is generally known that the greater the field intensity, the more pronounced is the separation between the two charges in the dipole. In the event of a random encounter between dipoles oriented in this manner, there is an attraction between their oppositely charged ends that leads to linking of the particles (Fig. 20c). This phenomenon, which has been known in colloids for a long time [131, 177], has recently received considerable renewed attention [257, 268] precisely because of 52

the effects of rf fields on the organism. It would appear that it may be possible to explainJat least partially, the recently discovered genetic changes evoked by high-frequency electromagnetic fields (94] in this way. The chain formation of colloidal (and more coarsely dispersed) particles might, unlike all previously suggested mechanisms,also explain certain effects of biological agents at lower frequencies. The described effect has been experimentally verified in the frequency band from 0 to 100 MHz [257]. A careful study of this phenomenon forcibly suggests the idea that· possibly not only colloidal particles form chains, but mol ecules as well. That could lead to the creation of a sort of pseudomacromolecules, whose presence would produce a change in the response of the organism. Thus, for example, if there is chain formation by ions or molecules, which are normally transported through semipermeable membranes, the process would be retarded or even prevented as the length of the chain increases. Externally, the phenomenon would manifest itself as a change in the permeability of the membrane, as one can actually conclude from some experimental reports [280]. But the permeability of a membrane can also change under the influence of an rf field on the basis of a change in polarization, which can also be produced by purely electrical phenomena, as are described later on in this chapter. All theories proposed thus far have certain shortcomings. They cannot explain, among other things, al tern a ting effects, as for instance the change in nerve-cell sensitivity, which proceeds sometimes in the positive, sometimes in the negative direction. Similar phenomena can also be explained by a change in _the electrical characteristics of nerve cells [152, 153]. Many parts of the organism can be assigned to the category of semiconductors on the basis of their electrical characteristics [66, 202], whose resistance may depend on the direction of the current flow. From this point of view, the factor of primary importance is their asymmetrically nonlinear volt-ampere characteristic, which represents current as a function of voltage (Fig. 21).

+u

-i

Fig. 21.--Typical case of asymmetrically nonlinear volt-ampere characteristic. 53

In some cases, for a certain part of this characteristic, an increase in current does not go with an increase in voltage, but rather a decrease. Such elements are said to have a region of negative resistance (segment A in Fig. 22).

Fig. 22.--Volt-arnpere characteristic with a region of negative resistance (A). A common feature of all systems with an asymmetrically nonlinear characteristic is that when an alternating signal is applied to them, it is asymmetrically distorted and a de voltage results. There is detection of an ac signal. All semiconductors have this property. It has recently been found that a great many organic compounds have semiconductor properties [109]. Many of them occur in the organism (for example, hemoglobin, desoxyribonucleic acid, etc). Finally, it has been established that nerve fibers and many other cells behave like nonlinear elements [32, 84,85], sometimes even containing a region of negative resistance [146]. At a certain value of the action potential (the so-called working p.o int) and a definite amplitude of an alternating signal, they can cause its asymmetric distortion. The biologically important semiconducting systems can thus be divided into three groups: (a) direct, ie with electronic conductivity; (b) indirect, ie with ionic conductivity; and ( c) mixed. Among the direct semiconductors we count those systems in which the nonlinear element is actually the molecule or group of identical molecules (a polymer). In such a semiconductor, the nonlinearity is of a conductive nature, and such a semiconductor (either by itself or doped with majority charge carriers) is an asymmetrically nonlinear element precisely because its resistance in a certain region of the applied potential depends on the direction of the current flow. This group includes all organic semiconductors whose semiconductor characteristics are produced 54

by molecular arrangement, especially of then electrons. The conductivity in such materials is of the electronic variety. However, there is still another group of nonlinear elements, in which ionic conductivity prevails. If such a system contains a polarizable element, then the passage of a de current (that is organized movement of the ions) leads to a polarization potential, which affects the volt-ampere characteristic of the system. In the case of a cell, the membrane acts as this kind of polarized element. If it is not itself a charge carrier, the solution-membrane-solution system (assuming similar composition of both solutions and structural symmetry of the membrane) will have the highest symmetrically nonlinear volt~ampere characteristic. If the system also contains a source of potential (as for example, the structural polarization of the membrane or unequal concentrations of some ion on the two sides of the membrane), the working point of the system shifts in such a way that the symmetry of the volt-ampere characteristic is disturbed. That produces an asymmetrically nonlinear circuit with rectification properties and a characteristic similar to that of semiconductors. As we have already seen, asymmetrically nonlinear circuits dis'tort an alternating signal in such a way as to give rise to a de component;ie rectification takes place. In direct semiconductors the rectification effect is produced by a different conductivity according to the direction of the current, whereas in indirect semiconductors, the rectification effect is produced by the polarization potential of the system; here the distortion can be thought of as adding to (or substracting from) the potential of the system, the instantaneous voltage of the alternating sig~ nal (according to the polarity of the half-wave). The lack of symmetry results in a direct current with a positive or negative sign, which changes the polarization potential of the membrane. In practice both of these factors may occur in combination so that the group of mixed semiconductors results. We know that in a physiological state, living cells have an electrical charge. Under its influence, the arrangement of ions or amphoteric compounds such as proteins is not accidental, neither within a cell nor in the immediate external environment of a cell membrane. If the potential of a cell changes, its microstructure thus changes as well; the cell is no longer in the physiological state which is likely to manifest itself in its characteristics; and if it is a controlling cell (nerve cell), the activity of other cells in the organism may be affected as well. The greater the deviation in the characteristics of the cell from the normal, the more pronounced the shift from equilibrium, and the greater the influence on the entire state and behavior of the organism. However, the effect of the absolute value of the change in potential on the behavior of the cell is largely determined by the position of the working point. This is most dramatically evident in the case of controlling (nerve) cells. More simply, we can say that the organism as a whole, and thus 55

every cell, has the ability to maintain its equilibrium to a certain degree, subject to interference both from without and within. However, this ability is limited partly by the time factor, ie the organism can defend itself for only a certain period of time; and partly by the magnitude of the deviation from equilibrium. The time factor is inversely proportional to quantity, ie the greater the disturbance of equilibrium, the shorter the period of time during which the organism can cope with this abnormal state. It should also be mentioned that the mutual relationships are not linear. As even relatively small functional uni ts of the organism are composed of a multitude of cells, for which we can assume qualitatively different electrical characteristics, the resultant volt-ampere characteristic of such a unit can have a shape which produces a different change in potential (even as to sign) according to the magnitude of the ac input signal. A more detailed discussion of this matter would exceed the scope of this book, and the interested reader-is referred to the literature [153]. On the basis of all the concepts mentioned here, the effect of high-frequency fields on the organism can thus take the form of a change in the arrangement of a number of molecules both inside and outside the cell, and consequently the passage of molecules through the cell membranes may be affected. There is no splitting of molecules and consequent production of new substances foreign to the organism, as confirmed by experimental data. Also in accord with these findings is the known reversibility of signs of damage (obviously, only to a certain degree, as long as the entire organism is not destroyed, or anyhow a part of it). Because the circulatory and nervous systems are the most conductive parts of the organism, the rf current (produced by induction and conduction) is maximum along these pathways. It is also necessary to assume the maximum possibility of changes in tissues whose cells undergo maximum asymmetric distortion and are sensitive to deviations from the normal state. It is likely that this is particularly true of the cells in the nervous system. The mechanism described earlier can change the characteristics and thus the behavior of a cell, and if it is a control cell these changes are transferred to the organs being controlled . Thus far, there have been no reports whose results could be interpreted as contradicting the above hypotheses. On the contrary, many papers present data that support these assumptions both directly and indirectly. Thus Rejzin [142], for example, found that an rf field affects a neuromuscular preparation even outside the irradiated area. He ascribes it to the so-called "diffusion of the field" by the tissue, which is nothing more than conduction. Electromagnetic selfinduction along conductive pathways in the organism has recently been put to use for measuring blood flow [271]. Both induction and conduction can be demonstrated by a simple experiment, in which the head of a rat is placed in . the fie.Id of an rf generator. The field is just strong enough (at f = 1 MHz, for exarnple)to fire 56

a glow lamp. The lengthwise axis of the body of the experi~en­ tal animal is aligned with the propagation direction of the field, so that the tail is in a field so weak that it cannot fire the glow lamp. Nevertheless, the effect of conduction suffices to fire the lamp at the tip of the tail. According to Tarusov, for example, the semiconductor nature of a cell suggested by theory can be ascribed to it on the basis of its conductivity in the resting state [254]. This is also in agreement with the finding that the thermal coefficient of resistance of tissue is always negative, which is one of the characteristic properties of semiconductors. Under the effect of an rf field the cathodic excitation of a neuromuscular preparation is increased and the anodic excitation is decreased. This result is indicative of a change in the charge of the cell in an rf field in accordance with the concepts described above. Ihe change in the charge of such a field has even been successfully measured. In accordance with the experiments cited above, an rf field produces the electrical negativity of a nerve [142]. These results have been evidently forgotten, for suggestions are only now being made that an organism or its parts could function as a detector of electromagnetic waves [202, 279]. Finally, measurements have provided the actual volt-ampere characteristics of living cells [178], which have the appearance of the characteristics of classical semiconductors: they are asymmetrically nonlinear, often with a region of negative resistance. Mention should also be made in conclusion of some consequences that stem from the theories proposed regarding the mechanism of the biological effect of the rf field. We have said that a change in the charge of a nerve cell is of the highest consequence for the entire organism, since a change in its physiological state produces a change in its controlling functions as well. The effect of the rf field on the organism must thus depend on the state of the central nervous system, as has been pointed out long ago [142, 208]. The threshold level of the rf field differs for stimulation and for inhibition of the CNS.Experiments in which rats were given a psychotone perfectly support this point, and the field intensity required to kill the experimental animals was then significantly less. On the other hand, it was found that the rf field intensity required to produce the same damage to the organism is much greater when the animal is under the influence of a narcotic. The effect of such substances, whether stimulatory or inhibitory on the CNS, may be thought of as a shift in the working point on the cell characteristic or as a change in the ability to transmit control signals in the organism. In much the same way one might assume that all other factors affecting the parameters of the compensatory system of the cell can not only weaken or intensify the effect of electromagnetic waves, but may even directly influence the functional capacity of the cell.Such a combination factor may take the form either of the effect of 57

certain chemical substances.or of physical factors, or else of changes produced by regressive structures in the organism due to influences of a psychic nature. At this point, a comparison is in order between the carcinogenic effect of certain chemical substances and their structure. All of these substances have n electrons in their molecules, closely related to the semiconductor properties of the molecule [54]. The question then arises as to whether it is precisely the semiconductor nature of these substances that play an important role in their carcinogenic effect, and whether this effect is activated by or even conditioned by the presence of an elec tromagnetic field. Nonlinear elements may also cause detection of a modulated signal, so that low-frequency component appears. Thus, we can explain the observation of Frey, who mentions the ability of persons (including the deaf!) to "hear" a pulse-modulated transmitter. The most interesting (at1d from the biological standpoint,most important) conclusions can be drawn in the case of cells whose volt-ampere characteristic has a region of negative resistance. For a favorable position of the working point (physiological state) near the peak of the characteristic curve (cf. Fig. 22), the application of a stimulus of appropriate amplitude and direction produces a sudden shift in the working point, so that even when this stimulus is removed the cell cannot return to its initial state, but remains "stimulated" to a degree. In other words, we can determine from such a characteristic curve that if we gradually increase the amplitude of the stimulus from zero we shall find a certain threshold of the effect. If we reduce the stimulus from a large amplitude, it will cease to be effective at another,usually lower, threshold value. Presman has published a hypothesis [203] according to which certain processes in living organisms at all levels of existence (from the molecular to the systemic) are initiated by internal and external electromagnetic fields. Electrical fields are doubtless an important component in the control of physiological processes in organisms, as Bassett [13] has recently demonstrated for bone growth. Naturally, these phenomena are not so simple as may have been indicated. In addition, one must keep in mind that these ideas are of a predominantly speculative nature. That should not detract in any way from their significance, however, since they are very useful for further study. A more detailed discussion of these interesting problems would go beyond the scope of this book.

58

.'J

OccuRRENCE AND UsE OF ELECTROMAGNETIC ENERGY

5. 1. Survey of the most common sources of electromagnetic waves The steady development of industry is very closely linked to the develpment of radio and electronic technology. The widest applications are to be found in the various forms of wireless communication (telegraph, telephone, television), the transmission of still pictures by radio (telephotography), detection and determination of the position of various objects and their distances (radar, radio range finding), remote control of mechanisms (radio telecontrol), long-range transmission of measured data (radiotelemetry), as well as radionavieation, radioastronomy, radiometeorology, radiospectroscopy, uses in nuclear physics, medicine, etc. Radio waves are also widely employed in industry. Their use is predicated on the production of heat through the absorption or induction of electromagnetic waves in various materials. In all these cases, the generation of rf electromagnetic energy is either an end in itself or serves as a means for further applications. In general, the sources of the rf fields by which a person is irradiated need not be limited to these artificial generators. Natural sources can play a significant role, too. Thus, for example, pulsed electromagnetic waves arise primarily ahead of a cold front or during storms. The following brief example will show that this may not be an unimportant factor in the environment. It is estimated that about 1800 storms take place on earth at any one time. They give rise to, say, 20 flashes of lightning each second, whose average power is 10 12 W. If this power were distributed uniformly over the entire surface of the earth, we should have about 2 kW of radiated power on every square kilometer. Measurements have shown that the intensity of a field at a distance of 20 km ahead of a cold front can be as high as 150 V/m. Obviously, the number of rf and uhf generators within the territory of a country such as Czechoslovakia is already considerable. Even from the incomplete data available at the end of 1963, the total installed rf power (not counting radar installations) amounted to more than 10 MW [170]. Even at that time, more than 50 types of rf industrial generators were in 0peration in Czechoslovakia, with a total number of uni ts in the thous ands, located in dozens of factories and workshops. Generators for dielectric heating were in the majority. In view of subsequent developments it is obvious that these figures have been exceeded by now and can serve only for orientation. 59

[' 5. 2. Use of electromagnetic energy m vanous ranfres: frequencies and powers used In order to obtain a better idea of the uses of electromagnetic energy, it will be convenient to detail a few of their applications, with emphasis on the most common types and on applications to which individuals are most likely to be exposed. Sources and generators of electromagnetic waves can be divided into two basic groups according to the frequency, ie rf and uhf generators. The rf group contains above all industrial generators, as well as long-wave, medium-wave, short-wave, VSW,and television transmitters (communications transmitters); the uhf group consists mainly of radars, several types of generators for dielectric heating, and generators for microdiathermy. Rf industrial generators are divided into dielectric and induction types. Although dieZ?ctric heating is the newer field, it is already outstripping (in terms of the number of units installed) induction heating despite the fact that its merits have thus far not been fully exploited [24 7]. A source of ac electrical field causes the heating, so that it is actually a matter of voltage. The sources are mainly electronic generators with a frequency range most often extending from 10 to 30 MHz. Effective dielectric heating assumes certain minimal dielectric losses ~t a given frequency (given by the loss factor tan o and the relative dielectric constants, of the material), although the losses themselves need not be the sole measure of the suitability of a material (matters of shape, size,and the like). Dielectric heating is thus a selective method and cannot be used to solve all problems or to heat all nonconducting materials. It is usually used in processes that would not be otherwise feasible, primarily in the automation of production. Dielectric heating is widely used in Czechoslovakia in the synthetic materials industry, especially in the proditction of molded bakeli te and in the plastics inductry. It is also employed for high-frequency glueing and drying of wood and of other materials, curing foundry cores and cast resins, and preheating grinding mixtures. Dielectric heating can also be employed in a number of other fields, such as textiles, ceramics, glassmaking, papermaking, and the food industry. As mentioned earlier, heat is produced in the material by means of an electric field between the working electrodes, that is a sort of capacitor. Selection of the proper electrodes depends primarily on the size, shape, and type of material; their design is generally determined by the type of heating (general or selective, etc). The electrodes are usually made of copper, aluminum, brass, or duralumin; they can be e i ther contact electrodes (thus also completing the circuit), or can have an air gap (the material can pass through). The arrangement of the equipment can be either such that the working electrodes are mounted in the same unit as the generator, or are located in a working unit separate from the main 60

generator housing. The following description will provide a good idea of some of the types of equipment. The GUR 2K high-frequency dielectric preheater (Fig. 23)* is intended for direct dielectric heating (such as is required for drying of materials), which takes place inside the cabinet. The built-in generator operates at a frequency of 27 .12 MHz with an rf power of 2 kW (adjustable). The entire operating cycle is automatic; only the initial impulse must be provided by a push button. Likewise, the drawer of the preheating chamber opens automatically when heating is completed.

FIG. 23.--GUR 2k generator for dielectric heating.

FIG. 24.--GU 6B generator for dielectric heating.

The GU 6B high-frequency dielectric preheater (Fig. 24) has a similar purpose. Here again, the soutce is built in and operates at a frequency of 20 MHz with a maximum rf power of 4 kW (also adjustable). 111e heating interval can be preset by a time re lay. The GU 7B high~frequency thermionic source serves as a unit that can supply rf energy for various installations, mainly in the woodworking industry, where it is used to cure glued joints. Rf curing takes place in a suitable press or sometimes in a clamp. The connection between the generator and the installation is usually a coaxial cable. The generator itself operates at a frequency of 17 MHz with a maximum rf power of 4 kW (adjustable in four steps). *Figures 23 to 37 were provided by Electrothermal Equipment factories (ZEZ). 61

The EDS 4A high-frequency welding press (Fig. 25) is used for joining thermoplastic materials, mainly PVC,in the most diverse branches of industry. The machine is equipped with a built-in thermionic rf generator; the welding cycle is completely automatic (the heating and cooling intervals are controlled by a timing relay). The built-in generator operates at a frequency of 2712 MHz with a maximum rf power of 500 W (adjustable).

FIG. 25.--EDS 4A generator for dielectric heating.

The movable upper electrode is controlled either by an electrohydraulic device and a push button, or (in another _operating mode) by a foot pedal. The generator shuts off automatically when the weld is complete. The EDL 1 high-frequency welding press is also intended for joining thermoplastic materials. rt is designed as an independent unit with multiple uses. It is equipped with a built-in generator operating at a frequency of 27.12 MHz with an rf power of 2 kW (with coarse and fine adjustments available). The welding cycle is completely automatic and controlled by a timer and microswitch. The movement of the upper electrode and the welding pressure are controlled hydraulically. The lower electrode can be constructed so as to facilitate access to the work by pulling out the supporting surface. The GUR 4 high-frequency dielectric gen erator is used as a unit that supplies rf energy for various installations. It operates at a frequency of 27.12 MHz with a maximum rf power of 4 kW, and has both coarse and fine adjustments. For welding thermoplastic materials, the EDL 4 welding press is connected to the GUR 4 source by means of a coaxial cable. The welding cycle is again completely automatic (as in the EDL l); the welding pressure, time, and thickness of the weld can be preset and regulated on this machine. The press can be further equipped with 62

an attachment, the S 4 two-position automatic adj us tab le tab le, which facilitates preparation of the material to be welded and shortens the handling time in assembly-line production. The entire welding installation, consisting of the GUR 4, EDL 4, and S 4, is apparent from Fig. 26.

FIG. 26.--Combination of GUR 4 generator and attachments EDL 4 and S 4. The GU 25 high-frequency thermionic source is also a selfsufficient unit intended to supply rf energy for various installations using dielectric heating. It operates at a frequency of 13 MHz with an rf power of 25 kW (coarse adjustment available). Since it is used primarily for drying, a continuous-run dryer (the EDV 2) was built for this purpose. The high-frequency distribution to the dryer is carried by coaxial cables. The most common materials treated in the dryer are textiles, wood, foam rubber, and other mineral or organic substances. The material to be dried moves on an aluminum chain conveyor through the rf field, under two electrodes whose height is adjustable. The dryer is provided with an exhaust fan. The controls for the entire dryer installation are concentrated on a single panel. The high-frequency industrial equipment for continuous dielectric heating and drying (Model GUR 100) is used for similar purposes. In order to increase the capacity several fold, several GUR 100 uni ts c.an be combined to form a production line with a common conveyor. One complete rf unit consists of an rf generator which operates at a frequency of 18 to 20 MHz with an rf power of 100 kW. Regulation of the rf power and the speed of the belt is continuous. The equipment is remote-controlled. The GU 02 portable rf source, with hand-operated tongs EDR 1 (Fig. 27), is used for packaging, for building insulation, and occasionally for making small welds in various other industries. The generator operates at a frequency of 27.12 MHz with a maximum rf power of 180 W. It is connected to the tongs by a flexible coaxial cable. The welding tongs, as is apparent, are made in the shape of a gun; the upper electrode is fixed, the lower 63

FIG. 27.--GU 02 portable generator with EDR 1 manual torrgs.

FIG. 28.--GV 6A generator for induction heating. one is spring-loaded. The source itself can also be used tor other purposes in conjunction with appropriate working equipment. Induction heating uses the effects of an alternating magnetic field to heat conductors [247]. In contrast to dielectric heating, current is involved here. The frequencies of the generators used for induction heating are substantially lower (most often 300 to 500 kHz). That has to do mainly with the so-called depth of penetration of the rf current which increases with decreasing frequency. The usual frequencies mentioned above are suitable for superficial heating, especially of objects with thin walls, layered surfaces, or small area. To be sure, it must be realized that the depth of penetration need not always be the decisive parameter. As long as the heating period is long enough to permit conduction of the heat into the material, cnly the amount of energy supplied is decisive. The combination of the values of frequency and power must thus be carefully selected. Induction heating is relatively widely used in Czechoslovakia for tempering, annealing, brazing, and sealing metal to glass. There are also many applications in molding and machin•· ing~ chemical processes, drying of enamelled metal parts, heating of powdered materials, heating of insulators by conducting heat from metal parts, etc. We have already mentioned that an alternating rnagneti c field is employed in induction heating. In64

duced currents that flow in the surface layer are produced in conducting objects placed in the heating coil or inductor. The path of these induced currents has a certain effective resistance, so that the heating is proportional to the square of the local current density. Between the heated surface layer and the cooler interior, there is a temperature gradient that causes heat conduction into the interior. The choice of a suitable inductor depends primarily on the type of operation for which it is intended, ie each inductor is basically suitable for only one purpose. It follows that there is a vast number of varieties and types, which can be nevertheless divided into four main groups: inductors that move around the outside of an object, inductors applied to the surface, inductors introduced into a cavity, and finally inductors fitted only to one part of an object. To be sure, combinations of these groups are possible. The inductors are predominantly made of copper; however, in view of the high current levels, they are cooled by water running through them, by air, spraying, or partial submersion. As in the case of induction heating, equipment is used in which the inductor either forms a single unit with the generator proper or is located outside in various arrangements. A description of several types is provided below for survey purposes. The GV6 A generator (Fig. 28) is designed for induction surface heating of metal objects or other electrically conducting objects, as well as for zone refining. It is also suitable for annealing the internal parts of vacuum tubes during evacuation, for brazing small components, etc. The maximum rf power of the generator is 4 kW (adjustable in four steps), with a working frequency of 3 to 4.5 MHz. The heating of parts takes place directly i n the heating inductor, held in a clamp in the upper part of the generator. For zone refining (of germanium, indium, aluminum, antimony, gallium, tin, etc), the EZK 2 semiautomatic attachment can be connected to the generator by a coaxial cahle. The device is operated in a horizontal position; the actual zone formation takes place in a protective atmosphere (reductive or inert). A characteristic feature of the machine is that the heating elements are movable, whereas the workpiece remains stationary during refining. The EZK 1 vertical instrument (Fig.29) is connected to the GV6A generator for suspended zone refining of silica in a hydrogen atmosphere. The type GV 10 generator can also be used for zone refining (maximum power, 10 kW; frequency, 3 to 4 MHz). A set-up ~ith an EZK 3 horizontal attachment is shown in Fig. 30; the method of shielding is particularly evident. The GV 21 high-frequency generator is suitable for various kinds of thermal processing, especially for surface and local tempering, hardening, brazing, annealing, smelting, forging, and the like. For small diameters, the material can be completeley heated through. The generator operates at a frequency of 0.45 MHz with a maximum rf power of 30 kW (again, with both coarse and fine adjustments). The parts to be heated are placed 65

FIG. 29.-- EZK 1 attachment.

FIG. 30.-- Combination of GV 10 generator and EZK 1 attachment.

either directly in the heating inductor, fastened to a clamp on the rf transformer on the front wall of the generator; or in various attachments connected to the generator. Of the massproduced attachments, the following types in particular are intended for connection to the GV 21 generator: (1) The universal semiautomatic tempering machine EKS 30 (Fig. 31), which is used for various kinds of tempering of machined components, and occasionally for brazing and annealing following initial setting of the device; the actual operating cycle is controlled automatically and an operator is required only to exchange the parts, and the device is connected to a generator by a coaxial cable. (2) R4A vertical automatic tempering machine, intended for serial and individual tempering of gears, pins, cams, and other machined parts ·, in this device, too, the entire operating cycle is completely automatic. (3} UKS 2 universal turret semiautomatic tempering machine, intended for bulk, surface, or local tempering of small- to medium-sized parts, although it can also be conveniently used for annealing or brazing; equipped with a supply bin and loader, its operating cycle may be in some cases completely automated. The GV 201 high-frequency generator is used for surface or local tempering, drawing, brazing, annealing, smelting, preheating, forging or shaping, and for other kinds of heat processing. It operates at a frequency of 0.45 MHz, with a maximum rf power of 85 kW, with coarse and fine adjustment. The GV 201 generator 66

FIG. 31.--EKS 30 attachment. takes in addition to the attachments which fit the GV 21 (EKS 30, R4A, UKS 2) several other mass-produced attachments, of which the following are the most important: the Pl semiautomatic tempering of components that can be pulled through an inductor unidirectionally; the P2 vertical semiautomatic tempering ma~ chine (Fig. 32), intended for various kinds of induction heating of long machine parts--the operator needs only to exchange the material being tempered; the EKR 2 vertical semiautomatic tempering machine, intended for both production-line and one-shot induction tempering of crankshaft parts for automobile and tractor engines--the entire operating cycle is fully automated; the EKR 3 horizontal tempering machine (Fig. 33) intended for production-line and one-shot induction tempering of main and connecting rods and multiply indented push rods in motors--the operating cycle is likewise automatic; the EKN tempering machine (Fig. 34), intended primarily for tempering gears, and occasionally for various types of pulleys; and the EPH 1 semiautomatic brazing machine, used for assembly-line brazing of laminations onto the shanks of lathe cutters--the machine has a turret-type layout, and practically the entire operating cycle is automated. Its use in conjunction with the GV 201 generator is illustrated in Fig. 35. 67

FIG. 33.--EKR 3 attachment.

FIG. 32.--P2 application apparatus.

FIG. 34.--EKN 5 attachment.

FIG. 35.--Combination of GV 201 generator and EPH-1 attachment. 68

In communications transmitters it is important to know that the main generatoT (or groups of generators, which need not all be at the same operating frequency) is usually located in a special room from which two, conductor or coaxial cables can carry the rf energy to an antenna,naturally located outside the building (or on top of it) . Similarly, in most cases the r:f energy can be also carried to so-called antenna equivalents (dummy antennas), generally located in the room. The amplifier racks (especially at the final stage) usually have glass windows so that they can be monitored during operation. The duties of the operator consist primarily of adjusting, tuning, and checking the functioning of the transmitter during operation. Transmitter sites usually also house dispatcher desks; other parts of the building are occupied by the nonparticipating personnel (in the sense that their presence at the transmitter site is unnecessary) . The operating frequencies run from tens of kilohertz to tens of megahertz, often with provisions for tuning over a rather broad band. The rf power output varies from tens of watts to hundreds of kilowatts. The operating regime, the type of operation, and the purpose and types of transmitters and antennas come in a great many different varieties, and thus cannot be described in greater detail here. Radars have actually found increasing applications in Czechoslovakia only in recent years (at least as far as civilian applications are concerned), mainly in aviation. This is understandable, since the steady growth of air traffic places heavy demands on its safety and ease. A radar emits a narrow beam of electromagnetic waves into space. When the waves strike some object capable of reflecting them, part of the energy is reflected and picked up by the receiving antenna of the radar (usually only one antenna is used for both transmission and reception, with automatic switching). The radar does not emit electromagnetic waves continuously, but only in very short pulses. From the known velocity of propagation of the waves and the time required for the reflected impulse to return, the distance of the object can be found. The successive recurrence of the pulses (repetition frequency) is set so that the individual pulses have time to return after reflection (even from a very distant object) before the next impulse is transmitted. The distance is determined from an indicator based on the same principle as an oscilloscope. Along with the distance, the device also displays (sometimes on other indicators) the direction of the antenna, so that information on both the range and location of the target can be obtained. In addition to pulse-type radars, continuous-wave installations are also used. They can operate either with an unmodulated wave (operation is based on the Doppler principle and has the disadvantage that it can be used only to track moving targets) or with a frequency-modulated wave. The transmitted and reflected signals have different frequencies owing to the delay; 69

the difference is directly proportional to the distance of the reflecting object. This principle is suitable for altimeters. The most widely used civilian radar in Czechoslovakia is a pulsed installation and radar models OR-2, RL-20, and RP-2E. The OR-2 long-range surveillance radar is intended for tracking the movement of aircraft along flight paths in the vicinity of an airport for distances up to 150 km [249]. It operates at a wavelength in the 10-cm band (frequency 2800-2900 MHz) with a pulsed uhf power of 700 to 850 kW, and gives the range, azimuth, and direction of travel of the aircraft. The pulse width is 2µs and the repetition frequency is 600 Hz. Most of the equipment is located in the antenna building at the airfield. In addition to the antenna system, the building houses duplicate transmitters and receivers and a monitoring indicator unit. The antenna system rotates at 0.5 to 1.0 rpm (with possible sector scanning through 30°, 60°, and 120°) with elevations from -5° to +15°; the beam width in the hori.~onU1l plane is 0 .92°. The RL-20 control radar is capable of guiding aircraft into the range of the landing radar, showing range, azimuth and course. It also provides a survey of the meteorological situation in the vicinity of the airport. It operates in the 3-cm band (frequency 9400-9600 MHz), with a pulsed uhf power of 150 to 200 kW. The pulse width is 1 µs and the repetition frequency is 1 kHz. The main parts of the equipment (antenna system, the duplicate transmitters and receivers, and the monitoring indicator unit) are as in the preceding type, located in the antenna building. The antenna system rotates at 15 or 30 rpm, with elevations from -2° to +10°; the beam width in the horizontal plane is 0. 7°. The RP-2E landing radar operates on the principle of rapid mechanical sector scanning of azimuth and elevation, with two identical and independent antennas, and makes it possible to bring an aircraft right down to the runway. It operates at a wavelength in the 3-cm band (frequency 9400-9600 MHz), with a pulsed uhf power of 150 to 200 kW. The pulse width is 0.5 µs, the repetition frequency is 2 kHz, the azimuth sector is 30° and the elevation sector is 10°. The antenna aperture angle in the horizontal plane is 0.8° ,and in the vertical plane it is cosec 2 ¢ up to 25°. Most of the equipment is again located in the antenna building, which in this case is generally underground; only the antenna system projects above the surface. The operation of antenna installations for Types RP-2E and RL-20 radars does not require a permanent crew (merely occasional maintenance and replacement of elements); the equipment is remotely operated from a control room [249]. Certain other types of radars are used in Czechoslovakia in various types of civilian transport aircraft, and for marine navigation, radioastronomy, college teaching, measuring the speed of vehicles, etc. In recent years, uhf dielectric heating has also begun to be used for certain applications. Thus, for example, the GU 2 MB 70

FIG. 36.--GU 2 MB generator for microwave dielectric heating.

FIG. 37.--GU 2 MS electronic range.

("Industron," Fig. 36) microwave preheater for laminated materials is used for direct heating of materials in loose or tablet fo~m. The generator is an air-cooled magnetron which operates at a frequency of 2375 MHz and a power of 2 kW (adjustable in five steps). The heating interval is governed by a time relay. Another version of this installation is the GU 2 MS electro~­ ic range (Fig. 37), which is designed for heating food by microwaves. This machine allows modern and rapid preparation of foods without a loss of nutritional values and without overcooking of fats. The cooking times in the oven of the range vary from 20 sec to 6 min. The generator is the same as the preceding model. Similar types of uhf generators are used in agriculture (to dry grain, for example). The PREMA Microdiatherm apparatus, Model 10 I4 [191], is used for microwave diathermy. It is intended for electrical therapy in laboratories for both clinical and experimental purposes. The generator is a magnetron which operates at a frequency of 2450 MHz with an rf power adjustable in eight steps from 50 to 210 W. The device is supplied with two attachments (circular and longitudinal); the irradiation time is adjustable from 0 to 40 min. In Fig. 38, the Microdiatherm is shown with the circular attachment in place. This survey of the most common Czechoslovak equipment utilizing rf and uhf energy is intended only as a brief outline. It is therefore understandable that it is far from exhaustive; the scope of this book does not permit more complete coverage. It 71

should be emphasized, however, that it is not possible in practice to forget the other, less well-known applications. In particular, it is necessary to emphasize the production or repair facilities for the generqtors, where normal operation often subjects the workers to conditions that are comparable with those obtaining in auto repair shops. To a certain extent that applies also in research and development laboratories, even though high power need not be always employed in them.

5. 3. Possible sources of harmful irradiation Despite the possible negative effects on human health, it makes obviously no sense to FIG. 38.--"Microdiatherm" 10 I4 close one's eyes to the fact generator for medical uses. that the use of rf and uhf energy is in the majority of cases of such economic advantage that it is (even at the cost of certain administrative and technical measures for personnel protection) still more convenient and profitable than the retention of older principles and production techniques and even general activities. For the communications media, notably radio and television, the sociopolitical functions must be also considered. In order to make it possible to search for ways and means that would serve to lower the level of the field where necessary, and to make their proper utilization possible, we must be aware of (among other things) the possible sources of undesirable or harmful radiation. Every installation containing an rf or uhf generator has several parts which are or can be sources of radiation and may contribute to a high field intensity at a workplace. It is above all the generator itself, in which the principal radiation sources may be primarily the oscillator, ie the vacuum tube and oscillator circuit, and possibly the power amplifier or the rf output transformer. Improper grounding can result in undesirable radiation from the metal housing of the generator. In the case of magnetrons, the cathode or filament lead may radiate. Another important element is the load, ie that part of the equipment which receives the energy that is produced. A distinction must be made here; there are loads in which radiation is an intended function (antennas), and others that are not supposed to radiate (the active elements of industrial rf genera72

tors, ie the inductor and plates of the capacitor, as well as the crystal in ultrasonic generators and various kinds of artificial or equivalent loads, such as water loads, resistive loads etc). Another important element from the standpoint of radiation is the transmission line by which energy is conducted from the generator to the load. If it is well made and maintained, it cannot radiate; unfortun~tely, such cases are quite rare exceptions. As a result, the power line is one of the biggest sources of undesirable radiation. In the uhf band, where coaxial cables and waveguides predominate as power leads, undesirable radiation may take place at connection points (flanges and connectors), not to mention the possibility of radiation from open ends, various slits, and around ionization devices. A general summary of possible sources of radiation classified according to frequency ranges is given in Table 5, together with additional data. (See also Appendix.) TABLE 5.--General summary of uses of electromagnetic waves, radiation sources, measurement units, maximum permissible doses, and protective measures. Freq.

Rf

Uhf

Indus- 1. Thermal metal processing 1. Radar trial, (tempering, smelting, 2. Radio navigation sciensoldering, etc) 3. Microwave relays tific, 2. Thermal processing of 4. Radio astronomy dielectrics (wood drying, 5. Radio meteorology & technical heating synthetic 6. Radiospectroscopy applicmaterials, etc) 7. Nuclear physics at ions 3. Communications 8. Communications, etc 4. Medicine 1. Rf transformer 1. Antennas Radia- 2. Coupling capacitor 2. Radiators ti on 3. Individual blocks 3. Connectors and power source leads, antennas 4. Slots, etc 4. Inductor or active capacitor 5. Slits, slots, etc Units Field intensity E in V/m Power density Nin µW/cm7 Maximum p. 030-30 30-300 MHz Cw operation 200 Et !Operator 400 80 Pulsed operation 80 (V/m) hr 8 hr max or Nt Others Cw operation 60 120 24 µW/cm2hr 24 hr max Pulsed Oj>_eration 24 1. Shielding of individual 1. Nonreflecting loads, Protecrf elements attenuator terminations tive 2. Shielding compartments (a) Sheets or screens 3. Open shielding: (a) metal meas(b) Coaxial cables etc ures 2. Complete shielding of rf (b) absorption covering generators 4. Protective goggles 5. Protective clothing 73

6

MAXI:MUM PERMISSIBLE FIELD INTENSITY AND RADIATION AND THEIR DETERMINATION

6. 1. Establishment of maximum permissible irradiation From the viewpoint of protecting personnel against the possible harmful effects of electromagnetic fields, the threshold values for the biologically effective field intensities are naturally important. As far as the thermal effect is concerned, the consensus is that power d€nsities from 10 to 15 mW/cm 2 lead to . warming the organism,both in animals [55] and man [195, 279, 280], which is in agreement with theoretical calculations [238]. Investigations of individual partial effects may lead to different values, however. Thus, for example, an intensity of 10 mW/cm 2 is required to produce cataracts, 1 mW/cm 2 is enough to change the sensitivity of the auditory apparatus, and fully 0.6 W/cm 2 is needed to produce pain in the skin. At centimetric waves, power densities as low as 0.1 mW/cm 2 may lead to biological consequences [125]. At lower frequencies, histopathological changes may be found at fields as low as 100 V/m [82]. Deleterious total-body effects probably result at field intensities as low as 10 V/m [135]. Many processes are even more sensitive, especially the rate of cell division, which is accelerated at field intensities as low as 10- 4 V/m; the rate of division decreases at values above 0.1 V/m [120]. The field intensity alone is not a sufficiently good measure of the threshold of effectiveness, however; the time factor doubtless enters as well. The organism has certain means of defense against hostile external influences. Its measure can be expressed by the relationship (6. 1)

This expression states that the measure of the ability of an organism to resist a given effect n is proportional to the quantitative value of that effect N (here field intensity or power density) and to the duration tnnof the effect. If the value of the function on the right-hand side of (6.1) is smaller than the appropriate value of Kn, there will be no harmful effects on the organism. The question thus remains, what is the value of the factor K for electromagnetic waves in man, and what is the form of the function f? It has been demonstrated [100] that for chemical and thermal injury , and for harmful ionizing radiation, this function is giv74

en by the product of two variables. We may thus write by analogy for the influence of radio waves in the uhf band,

K

uhf

= N't'

( 6. 2)

E't'

(6.3)

and at lower rf,

K

rf

=

The primed values are maximum for preserving equality of both expressions, so th~t K h and K represent the maximum permissible radiation in the g~vfn rang~[ The product of the actual field intensity and the duration of the effect is then called the irradiation 0. For centimetric waves, for example, ouhf = Nt

(6. 4)

It is evident that in practice it is necessary to insure that the irradiation does not exceed the maximum permissible value, so that 0 < K

(6.5)

Values of the maximum permissible radiation K for both ranges have been established on the basis of available proven knowledge of biological effects, with the realization that they can cover only a certain average sensitivity of the organism at average active field parameters. For that reason, both Czechoslovakia and the USSR have also introduced a certain safety factor. In practice it is desirable to use graphic representations of maximum values in the way they are depicted for the rf band in Fig. 39, and for the uhf and for both modes of operation in Fig. 40. In the latter, the circles represent the maximum permissible power density accepted in the USSR for 6-hr,2-hr, and 15-min exposures. Good agreement of values serves to justify our approach in fixing exposures. Table 6 surveys values of maximum permissible field intensities established or recommended throughout the world at present. The considerable differences point to the various approaches to the problem of biological effects and the protection of workers against this risk [157, 162]. In the USA, there is considerable lack of accord in establishing the maximum attainable intensity [20, 265]. Some authors advocate a value at which the heating of the organism is just at the bearable point [36, 90, 125, 137, 281]. However, others advocate that this intensity should be designated as the "tolerance unit" and that the maximum permissible value should be a power density of 1 mW/cm 2 , suggested as a standard by the Bell Telephone Laboratories [155].But some biological effects can be expected to make their appearance at an 75

&

300~~---..--....---.----r---.-----,,----,---,

~ iu

~ ~~-r---+----+-- --+--- -- + - - - i - -- ---!----

~--·----1----+---l---+---+--+----i

,00

~ .

.

~ 1 ---r--+--L-T 8

I' (hr)

FIG. 39.--Maximum permissible field intensity in the 30- to 300MHz band for various periods of exposure during the working day .

200

, -•••••••••

~

0

:>-·-·-----r----- ·········- ··- -··

1

8

l'(hr)

- - pulsed

·-·--cw

FIG. 40.--Maximum permissible power density in the uhf band for various periods of exposure to radiation during the working day as established in Czechoslovakia. Curve 1 represents continuous (cw) operation; Curve 2, pulsed. 0 = maximum permissible value in the USSR and in Poland. 76

TABLE 6.--bummary of established or recommended values of maximum permissible field intensities for electromagnetic waves. Country, author

Frequency (MHz)

USA : E1y, T. S . , Goldman,D. (1957)

3000

USA: U.S. Army (1958) All USA: Schwan,H.P. 1000 and Li, K. (1956) 1000-3000 3000 USA: General Electric 700 USA: Bell Telephone Labs.(1956) 750-30 000 USA: Mumford, W.W. (1956) NATO (1956) Sweden 87 87 Britain 300 West Germany (1962) USSR (1965) 0.1-1.5 1.5-30 30-300 > 300 Poland (1961)

>

300

Czechoslovakia (1965)

>

300

USK (1966) Canada (1966)

0.01-300 10-100 000 10-100 000

Maximum permissible intensit_x. 100 mW/cmL. 150 mW/cm2 5 mW/cm 2 10 30 10 20

mW/cm 2 mW/cm 2 mW/cm 2 mW/cm 2

Remarks Whole body Eyes Testes

-

Whole body Whole body Whole body

1 mW/cm 2

-

1 mW/cm 2

-

mW/cm 2 mW/cm 2 V/m V/m mW/cm 2

-

0.1 0.5 222 25 0.01

10 mW/cm 2 20 V/m 5 A/m 20 V/m 5 V/m 0.01 mW/cm2 0.1 mW/cm2 1 mW/cm 2 0.01 mW/cm 2 0.1 mW/cm 2 1 mW/cm 2 0.025 mW/cm 2 0.01 mW/cm2 10 V/m 1 · (mW I cm 2 ) hr 1 (mW/cm 2 )hr

6 hr daily 2 hr daily 15 min daily Entire workday 2-3 hr daily 15-20 min daily Cw operation}8*hr Pulsed op'n daiJy Every 6 min Every 6 min

*For shorter exposure, see Figs. 39 and 40. (See also Appendix.)

77

intensity of 0.1 mW/cm 2 , which is another value used in the USA [125]. This takes into account chronic exposure and the involvement of nonthermal phenomena. And that is actually the only correct viewpoint. Moreover, a safety factor of 10 has been added in the USSR and in Czechoslovakia so that the value 10 µW/cm 2 has been chosen for the maximum permissible field intensity for the entire working day in the centimetric-wave range [77, 279]. For the demonstrably less risky continuous mode, Czechoslo~ vakia has established the value of 25 µW/cm 2 , derived from the maximum intensity of the electrical component of 10 V/m. The maximum permissible intensity of the magnetic component of the field has been established only in the USSR, and even there only for a very limited frequency range.

6. 2. Specificity of determination of field intensity and of power. density an:9,. irradiation It might appear at first glance that the methodology of determining irradiation, which means actually deriving it from determination of the field intensity or the power density, is in no way difficult or new, and can generally be approached by current methods of technical practice. In reality, however, it may be shown that this view is valid only up to a point. Several examples suffice by way of demonstration. In the first place, it is obvious that not everyone (and therefore, not everything) is affected in the same way by the presence of an electromagnetic field. This point has been sufficiently discussed in Chap. 3. It should be emphasized in that connection that the frequency dependence of absorption, expressed as a ratio, need not necessarily agree with the response of the organism. The problem becomes still more complex when we realize that it is actually not possible to include the nonthermal effect in the frequency dependence of absorption, at least not entirely. Besides, the body occupies "somewhat" more space than would make it possible to consider it as a point object. However, for that it is necessary to know the total value of the energy received, that is from all directions and without regard to polarization. The fact that the body is not a point object need not be an obstacle (at least not in measurement) if it is, from the standpoint of the generator, in the so-called far field (as we shall see later on, that is of practical significance only in the centimetric wave band); but in the majority of cases it is necessary to take into consideration the presence of individuals in the so-called induction or near field, where the field itself is highly inhomogeneous and is moreover affected by the presence of the body. Even if this problem could be somehow bypassed and measurements made (for example) in the immediate vicinity of the body, it would still be likely that the configuration of the field around the antenna would be changed to some extent by bringing the body close to the antenna. That can have an effect on both the action and the properties of the antenna (changes in 78

the original characteristic radiation pattern and in the impedance). An attempt to resolve this problem at least in rough approximation has been made [110], and the results do show above all a very definite change in impedance (an increase in resistance). Another difficulty is that conventional measurements of field intensity in the near field are usually either so sensitive that measurements are impossible without a special reduction in sensitivity, or else are too low in sensitivity, quite apart from certain other disadvantages. Likewise, a measurement of radiation entirely by calculation serves really primarily for orientation, if indeed it is realizable at all (as it is, for example, in the near zone, where the complex nature of the dependence does not permit determination by calculation, especially when the radiation element is not properly defined). The summation of all these requirements or still unsolved problems thus sounds quite pessimistic and negative from the standpoint of practical applications. However, the steady development of applications for rf and uhf energy, together with the accompanying growth in the number of persons who come into either direct or indirect contact with electromagnetic fields, makes it necessary to find at least temporary methods for determining radiation levels. It is clear that the situation can be resolved only by some sort of a compromise, ie by complying with at least some of the requirements in making actual measurements, and then attending to others when taking up the methodological aspects or when evaluating the results. At the same time, efforts must be made not only to improve existing installations or methods, but also to use known mechanisms of the effects as a basis for finding other principles of measurement that could be used to replace the present imperfections. What, then, is the current state of affairs, and what approach should be taken to solve the problem as a whole? To begin with, the entire spectrum to be studied must be divided into two bands, the rf (up to 300 MHz) and the uhf (above 300 MHz). There are basically two reasons for this division: (1) in the rf band, where the body dimensions are much smaller than the wavelength, the electrical and magnetic components of the field are differentially absorbed [68]; ,and (2) there are places in the rf band at which the value of the field is customarily determined, especially in the so-called induction region, where the mutual relationship between the electrical and magnetic components of the field is not uniquely defined. From that it follows that in the rf band it is necessary to determine the E and H ·components separately, whereas in the uhf band it is the total power or power density that must be considered. To be sure either method can be used in practice from approximately 100 to 300 MHz. With regard to standards valid in Czechoslovakia, only the determination of the electrical component of the field is discussed with respect to the rf band in what follows. From the viewpoint of the complex and ambiguous nature of ab79

sorp.tion, as well as the still unresolved problems surrounding other possible pathways by which electromagnetic energy can enter the organism, and finally the unknown frequency response of the organism to the presence of electromagnetic energy, it makes no sense (especially from the health standpoint) to differentiate between the measured field or power density, and the energy actually absorbed in the body. This holds true for the measurements themselves and it is actually the principal criterion in selecting measuring instruments. In the actual evaluation,especially in the. uhf band, it is generally necessary to consider the frequency dependence of the absorption as one of several f actors by which it is possible to detect individual differences in sensitivity of personnel to the presence of electromagnetic energy. In both bands, the demand to date has been for wideband meters whose sensitivity ought not be necessarily restricted to only one polarization or directi.o.n of"" incidence of the energy. It is highly unrealistic to try to satisfy these demands under all conditions, so that the practical viewpoint and the conditions under which the measurements are usually made must be determining f actors for the present. From this it follows that the requirement for a wideband meter is justified and necessary mainly in the rf band, where it is usually required at a certain workplace to determine the field produced by the action of generators operating perhaps at various frequencies (not to mention the usual presence of higher harmonics of the basic frequency). The requirement of sensitivity to various polarizations and nondirectionality, with simultaneous maintenance of the desired sensitivity of the meter, however, is bypassed in this method by measurement of the maximum value of the field intensity at a given location. This is usually a temporary expedient; the determining considerations are economic (as is also true for all measurement arrangements in this frequency band). In the uhf band, the situation is simpler because only single generators are involved in the great majority of cases, the problem of harmonic frequencies is not involved, and the wideband-meter requirement is obviated. Usually~ even the polarization and direction of incidence of the radiation are uniquely determined. However, there are special situations where this is not true. The solution then depends on the selection of the proper antenna and method of measurement. The finite dimensions (notably height) of the body may be taken into account in making measurements at various heights. As a point of departure we note that the organs which are most sensitive to the effects of a field (besides the nervous system) are the genital organs and the eyes; the measurements are carried out at a certain working position of the body in a given location, at heights which correspond to the positions of the organs in question during work in either a sitting or standing position. Useful anthropometric data for men were obtained from the work by M. Prokopec [207]: the values for women were taken from a paper by S. Titlbachova [258]. The values, together with the given 80

Height(cm)

Men

Women

190 180 170 160 150

130 120 110 100 90 80 70 60 50 40 30 20 10

Average values for standing position (:t2s)

I:- :m:IItl :i~~r~~ep~~; ~;~~ 1(;2s i 1

s - critical error

FIG. 41.--Zones of locations of head and reproductive organs during work in standing and sitting positions. scatter, are combined in Fig. 41. It is evident that on the whole, there is no difference between men and women insofar as thes e data are concerned, and that a suitable antenna location may be determined without regard for the sex of the workers at a given site. In accordance with these data, it was found that measurements must be made at the following heights above the surface of the ground: SO, 85, 125, and 160 cm. For evaluation from the health standpoint, the highest measured value at one of these heights is then taken as the critical one (though the value at the level of the head must be taken into consideration in any case). In view of the fact that an actual antenna itself is not a point, the measurements at these heights also involve scattering among the individual workers. In the case of work exclusively in a standing or sitting position, it is permissible to make all of the measurements for the given working position, but it is necessary to have a control in the form of a check to insure that the measured values are not exceeded at other points. In making the actual measurements, it is desirable to perfonn them in the usual workplace with no personnel present in the immediate v1c1nity of the antenna (this is particularly important for the rf band). 81

The specificity of the measurement and the solution of the associated problems combine to create a task which is extensive and complex. It is evident that the information available on this subject to date has not been sufficient to allow it to be viewed from all angles, and that the task of making an accurate evaluation of radiation will require additional research and development. Hence, it is most important to arrive at uniform methods of measurement and evaluation and as far as possible uniform measurement instrumentation, so that the over-all results (including biological data) can be compared.

6. 3. Instrumentation and systems for field indication: analysis of individual components and methods Under present conditions, especially as far as the desired accuracy for health purposes is concerned (with an eye toward simplified solution), mea~prement instruments and systems are to be actually classified from the purely technical standpoint as indicators. Hence, whenever the term "meter" is used in the text which follows, it should be understood to mean an installation in the above sense. For the rf band, a device is required which will permit wideband measurement of a high value of field intensity (up to hundreds of volts per meter). From the analysis, an important part of which was the solution of the problem of quick and easy determination of the components of interest by a convenient instrument, it was found that a nonresonant rf voltmeter with an external diode probe and several attachments was the most convenient for the purpose [67, 171]. In this regard, one should avoid all arrangements that would distrub the normal operation of the voltmeter. Admittedly, certain complications do arise in connection with the nonsymmetric voltmeter input, and thus in connection with the need to use a rod antenna; but otherwise, this method is the most effective under existing conditions. If we use a BM 388 u-· niversal voltmeter (made by Tesla at Brno) together with its accessory rod antenna (100 mm long and 6 mm in diameter) at the input of the diode probe, the field intensity E is of the form

E

~

U in

kV.

-in

(6.6)

where U. is the voltage measured by the voltmeter, C is the input capli~itance of the voltmeter, l is the distance between the rod antenna and the nearest grounded object; c2 = c 1 + c{ ~· where Ci is the capacitance between the rod antenna and the voltmeter itself, and C{ is the capacitance between the rod antenna and the ground. According to this equation, the measured values on the low-frequency side of the band will be several per cent higher than the actual values, which is not a disadvantage fo~ measure82

ments for health purposes. A systematic error will be obtained when measurements are made in the vicinity of several transmitters operating simultaneously at different frequencies. It is clear that the coefficient k in a given voltmeter depends only on the size of c2 , ie on the dimensions of the antenna (diameter and length) and its distance from surrounding grounded objects. The following condition must be also satisfied: l


.. 2

Pyramidal horn (optimal)

6. lS/>.. 2

0.49S

Circular horn (optimal)

6.SS/A. 2

O.SlS

Parabolic

(6.3 to7.5) S/A.2 84

eff

( 0. 5 to·0. 6) S

The following considerations should be kept in mind in selecting an antenna: wavelength, method of dissipation of energy, and sensitivity of the entire set-up. A directional antenna should be as small as possible and have a sufficient power gain. Horn antennas are generally used for wavelengths shorter than 10 cm; dipoles (and occasionally logarithmic arrays) are used for longer waves. Nomograms for rapid design of several antennas are given in Mouser's work [167]; Fradinov [65] provides information on more accurate design of horn antennas; and logarithmic arrays are covered in [48]. Table 7 lists the power gain G and effective area Seff for the most common types of antennas. In measuring the power density it is conventional to measure the radiation at some safe distance from the generator and then gradually approach it (either bringing the meter closer to the generator or vice versa) until the maximum permissible value is obtained. It is also possible to proceed by reducing the power radiating from the device and extrapolating the results to full power. Occasionally, however,it is not possible to use either of the above methods, and an attenuator must be placed in the circuit, both to limit the input Fig. 44.--Set-up for range and to protect the measuremeasuring N (QXC 900 02 ing equipment. Various types of meter) . variable and frequency-independent attenuators with minimal power losses are routinely employed for this purpose. Most measuring (evaluating) equipment is made up of a de heating circuit for establishing the workpoint of the thermistor, an af oscillator with an automatically balanced bridge (one of whose arms is formed by the thermistor), and an af voltmeter, which indicates the changes in amplitude of af oscillations caused by uhf power incident on the termistor directly in microwatts or as power density in watts per square centimeter. A complete set of equipment for measuring the power density is shown in Fig. 44. The calibrating equipment is actually designed from the standpoint of laboratory application, where (for example) the problem of obtaining power from a wall socket or from batteries and the weight are of no concern. In some cases, however, simplified circuits can be used; the most suitable is the unbalanced bridge. In measuring constant (ie cw) processes, it is possible to use a still simpler method. If the ability of a crystal diode to detect uhf signals is used, a de microarnrneter suffices as the in85

dicator. In that case, a crystal holder (untuned if possible) serves as the transducer. Similarly, a proposal for a uniform method is also being prepared for making measurements in the rf band [286]. A practical circuit diagram for both types of simple evaluating equipment [174] is shown in Fig. 45. Typical calibration curves for the thermistor section of the appa~atus are shown in Fig. 46 and for the diode section in On Fig. 47. The over-all appearance of the equipment is shown in Fig. 48a. This equipment is not massproduced, but is easily assembled in any workshop and will serve ~ well for making preliminary measurements. D However, a suitable and simple measuring device is manufactured in Czechoslovakia for even more precise measurements. This is the field-intensity (or power density) meter. Type QXC 900 05 (made by the Tesla Works at Pardubice), with accessories (probes and antennas) that make it possible to measure power density from 1 to 150 µW/cm 2 (and higher, if an atFig. 45.--Circuit diagram of tenuator is used). With an apsimple meter. propriate probe the meter may be used in any microwave band. Figure 48 shows the equipment set up for making measurements in the 3-cm band. Among the indisputable advantages of this equipment is its transistorization, so that batteries can be used, with the resulting light weight; moreover, the meter can be calibrated directly in units of power density. In all other types of equipment, it is necessary to calculate the power density from the measured power. In the determination of the power density N from the measured power P assuming that the efficiency of the transducer n ~ 1, we haveav, (6. 8)

where the loss K is in practice caused mainly by the imperfect matching of the ~older and the resultant reflection of a portion of the received energy. Knowing the standing-wave ratio s of the holder, we can find KT from the equation 86

1

(6.9)

When using an attenuator, we must include in KT the value of the indicated loss. l(µA)

"t20

10

+--~--1------------·-~-1

I

l

~--+------l-----4----1

-!--#-JI

---~~~--1~~~~--l--~~~~+-~~~-+-~~~·---l

10 P(mW)

Fig. 46.--Calibration curves for thermistor section of simple meter.

l(µA)

..

so--1--~~~~~-+-~-J~,t» ~~~-+~~~-/~7~~--+-~~~~/'f»

__:

40 -

.1118

,1

:.-- )1 _ . /•' /'/ ,• /

,•'

,,',

,7

/~

/

30-l--~~~~~-y~~,;--~~~~-+-,+--~~~~~i~?~~~~~

,'

,,''

,•,

,,"'

...

~

,,'

~~~~--~~~~~~~--1-~~~~~-+-~~---------

101

10

10•

P (pW)

Fig. 47.--Typical calibration curves of QCV 222 diode holder (with 33 NQ 52 diode) in series with OHR 5/50 µA (1: R = O; 2: R9; 3: R10). 87

The effective area of the antenna S can be found at a known v~fbe of the gain G by means of the expression

seff

Gt..2

=4n

(6.10)

Since no conditions are prescribed in a given case for the form of the radiation characteristic of the antenna, there is no justification for combining in some manner the measurements attained in, say, two directions; only the highest measured value in each case is relevant. The same procedure is to be used when making measurements at two or more frequencies. The justification for this method is complete only if the highest measured value is substantially greater than the rest. It has already been pointed out that this case is the one most often encountered in practice. In order to take into acFig. 48.--0ver-all view of a count the possibility of more simple meter using the circuit general situations, the followof Fig. 45 (top), and batterying section deals with possible powered meter QXC 900 05 with solutions for such cases. For attachments for 3-cm band. the same block diagram of the over-all equipment (Fig. 43), the possibility of employing more general measurement methods depends primarily on the characteristics of the antenna and methods of evaluating the results. In doing so, we begin with the radiation pattern of the antenna (150]. If the radiation pattern can be approximated by the function cosPe, where p = 2, ie by a pattern symmetrical about the polar axis (direction 8 = 0), then by means of progressive measurements in six directions (in an x, y, z, system of coordinates, this would correspond to the positive and negative directions along all three axes) and arithmetic addition of the results from individual measurements, it is possible to obtain the total value of the power, whichever direction its components come from.This value is then introduced into (6.8).The case for which the radiation pattern can be approximated by the function cosPe is a special case of a more general pattern that is not symmetric 88

about the polar axis. For all other cases (ie when p f 2) ,measurement is subject to a certain error whose maximum value can be found from~ the equation 6

=

11 -

2 [ l - (p I 2 ) ] I . 10 0 %

(6.11)

For health purposes, only the case where p < 2 is of use (Fig. 49), because for other cases it is possible-to measure a value of power density lower than the actual. To be sure, it is necessary to maintain the same conditions throughout the measurements to exclude the time dependence (constant operation of generators in the area being surveyed throughout the entire measurement period). A suitable antenna,for example, is a logarithmic spiral on a conical surface with a vertex angle of 20° and a slope angle of about 69° [52], assuming that it is possible to maintain K;ri,_S ff constant within the desired frequency range by a suitable fr~quency dependence of the attenuator loss or of the standing-wave ratio of the transducer, etc. In the uhf band, it is sometimes necessary to determine the power density and irradiation '°' ' in the vicinity of rotating or scanning antennas (radars), where it is clear that the resultant values must be lower than that from a fixed antenna (depending on the form and width of the radiation pattern and the size of the size of the angle scanned). The actual determination is made 1.0 1.2 1.6 1.8 2.0 1.4 on the basis of a combination of experimenFig. 49.--Measurement error 6 tal and theoretical for p ~ 2. findings for either a stationary or a rotating (or scanning) antenna. The choice of measurement method depends (apart from additional factors) on the desired accuracy,on the assumption that the thermal time constant of the irradiated part (of the body) is long compared with the period of observation. For a stationary antenna, one may for example proceed from the measurement of the maximum value of N (ie on the electrical axis of the antenna or, in its place, with regard to possible changes in elevation) at a given spot, and then make corrections according to the equation 50-~--~-~-

p

89

(6 .12)

(6.13) where N t is the value of the power density in the vicinity of the rot~~1ng antenna, e ff is the so-called effective beam width (e 3 db