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APR 2 7 1970 NBS TECHNICAL NOTE 389

U.S.

DEPARTMENT

OF

COMMERCE

National

Bureau

of Standards

Quantifying Hazardous

Electromagnetic Fields:

Practical Considerations
NATIONAL BUREAU OF STANDARDS

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NBS TECHNICAL NOTE 389

ISSUED APRIL 1970

Nat. Bur. Stand. (U.S.), Tech. Note 389, 15 pages (April 1970)

CODEN: NBTNA .

Quantifying Hazardous Electromagnetic Fields:

Practical Considerations

Ronald R. Bowman

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

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TABLE OF CONTENTS

Page

ABSTRACT . . . . 1

1. INTRODUCTION . . • - . l

2. COMPLEXITIES OF ELECTROMAGNETIC FIELDS 3

2.0 Discussion » 3

2.1 Elliptical Polarization 3

2.2 Multipath Interference 4

2.3 Reactive Near-Field Components 5

2.4 Interactions Between the Source and Nearby Objects* • 5

2.5 Complicated Time Variation of the Configuration and

Intensity of the Field °

3. APPROACHES TO QUANTIFYING EM HAZARDS. 7

3.1 The Dosimetry Approach « • 7

3.2 The Field Parameter Approach 7

3.3 The Arbitrary Approach • 8

4. CHOOSING A SUITABLE FIELD PARAMETER FOR

QUANTIFYING HAZARDOUS EM FIELDS 9

4.1 Parameters for Describing Uniform Plane-Wave Fields . 9

4.2 The Interaction of Biological Material with

Electromagnetic Fields 10

4.3 Choosing Suitable Field Parameters 11

5. INSTRUMENTS FOR QUANTIFYING HAZARDOUS

ELECTROMAGNETIC FIELDS 13

5.1 General Characteristics 13

5.2 Realization 14

5.3 Dosimetry 14

in

QUANTIFYING HAZARDOUS ELECTROMAGNETIC FIELDS:

PRACTICAL CONSIDERATIONS* f

Ronald R. Bowman

Abstract - The usefulness of power density to express the hazard potential of electromagnetic fields is limited to simple fields that are

approximately uniform and plane wave. For fields that are complicated

by having reactive components or by having multipath interference

patterns, power density is not a suitable parameter for quantifying the

potential hazards because: (a) such fields can be very strong even

though the power density is small, and (b) the power density in such

fields is very difficult to measure. Since some of the most important

hazardous fields can involve very complicated field configurations (for

instance, fields near leaking cracks in microwave ovens), it is important to establish a more rational measure for hazardous fields. A

qualitative discussion is given of the many issues involved in selecting

a suitable field parameter for quantifying hazardous electromagnetic

fields in general. It is concluded that the total energy density of the field

is the best parameter, but in many instances the electric energy density

alone will be adequate. Some general discussion is given concerning

"ideal" instrumentation for quantifying hazardous fields.

Key Words: electromagnetic fields; hazards; quantifying; field parameters; instrumentation.

1 . Introduction

The discussion by P.F. Wacker in Technical Note 391 is con- cerned primarily with analysis and with the problem of realizing an

accurate probe for quantifying hazardous electromagnetic (EM) fields

under very general conditions. In contrast, the present paper is concerned mainly with the problem of making easy, reasonably accurate

This work was partially supported by the Bureau of Radiological Health.

This paper is essentially identical to one prepared for the Symposium on

the Biological Effects and Health Implications of Microwave Radiation,

Richmond, Virginia, September 17 through 19, 1969.
survey measurements of hazardous EM fields. For general survey use,

instruments should be rugged, easy-to-use, and should be capable of

fast response as well as having long-term averaging capabilities. These,

and other considerations to be discussed, place restrictions on practical

instrument designs for general survey use.

It is very difficult to determine, even with reasonable accuracy,

the biological hazards associated with EM fields. Even in the simple

case of a plane -wave field with uniform power density, the energy absorption by biological material is quite complicated1

. At the present

time, only the hazards related to uniform plane waves in free space

(i.e., far from the source and any scattering objects) have received extensive study; and the standards, terminology, and most of the measuring

instrumentation pertain only to this simple case. Apparently, this is

why "power density" (i.e., the time-averaged magnitude of the Poynting

vector) is the presently used quantity for stating hazardous EM field

levels, despite the serious objections to this practice raised here and by

Wacker in Technical Note 391. Since the usefulness of power density to

quantify hazardous EM fields is questionable except for uniform plane

wave fields and since some of the most important hazardous EM fields

involve very complicated field configurations (for instance, fields near

leaking cracks in microwave ovens), it is important to establish a more

rational measure.

i See, for instance, H.P. Schwan, "Radiation Biology, Medical Applications, and Radiation Hazards", in Microwave Power Engineering , vol 2, edited by Ernest C. Okress, Academic Press, New York, 1968).

Poynting' s interpretation of the energy density and power flow in electrc

magnetic fields is subject to philosophical difficulties. See footnote 1 in Technical Note 391. Also, see D.S. Jones, The Theory of Electro - magnetism, MacMillan, N.Y., N.Y., 1964, pp. 51-53. These considerations are beyond the scope of this paper, and they will not involve

us in any concrete errors.

2.
Many complicated issues are involved in selecting a suitable parameter for quantifying hazardous EM fields. The discussion here will be

qualitative and is intended merely as a sketchy introduction to the complications considered. It is hoped that the discussion will clarify the

basic issues

.

2 . Complexities of Electromagnetic Fields .

2 . Discussion . Undoubtedly, another reason that the use of power density

(i.e., energy flow) has become established is that energy flow presents

a simple conceptual picture of the interaction of the field with matter.

Unfortunately, this conceptual picture is often very inaccurate. EM

fields with frequencies below about 10 GHz do not bear much analogy

with optical radiation or ionizing radiation. However, they do bear some

analogy with audible sonic fields because of the comparable wavelengths

involved. The free -space wavelength of a plane -wave 1000 MHz EM

field is about 30 cm, and the free-space wavelength of a plane -wave

1000 Hz sonic field is also about 30 cm. Of course, plane-wave EM

radiation involves transverse rather than longitudinal vibrations so that

polarization is an important additional complication of EM radiation,

but otherwise there are useful analogies between microwaves and audio

waves. Some of these analogies will be used below in discussing the

complexities that will commonly be encountered when surveying for

hazardous EM fields.

2. 1 Elliptical Polarization .

In general, the tip of the electric vector (or of the magnetic

vector) will trace an ellipse if the vector is plotted as a function of time.

Except for the rotational sense, the polarization of the field can be

specified by the axial ratio of the ellipse. Radar, telemetry, and

communications systems normally use antennas with either linear

3.
or circular far-field polarization, which are special cases of elliptical

polarization; but at points near the transmitters of these systems, the

polarization can be arbitrary. Very often, the polarization of the field

will not be known when making hazard surveys. Furthermore, the

polarization of the field may change radically from point to point within

the field. For instance, if a microwave oven has a leak from a horizontal crack and also a leak from a vertical crack, the polarization of

the resulting field will vary in a very complicated way in the region

around the oven. (It should be mentioned that polarization is usually

only defined with respect to plane wave propagation. However, it is

3

possible to define polarization for arbitrary waves , and it is this

general situation that is of interest for hazard surveys.)

2.2 Multipath Inte rfe r e nc

e

. Propagation of fields of the same frequency to a given region of

space will cause interference patterns ("standing waves"). This is a familiar phenomenon with sound waves, and the spacing between positions

of constructive and destructive interference for audible sound waves

will be roughly equal to those for microwave multipath interference

because of the comparable wavelengths involved. The net power density

in multipath fields can be less than the power density of the individual

component waves considered separately. In particular, two plane-wave

fields of equal amplitude and frequency but opposite propagation directions

combine to give a field with zero net power density. However, the combined field will have electric and magnetic field maxima of twice the

amplitude of the component waves. Clearly, if the two component waves

are each hazardous, then the combined field is also hazardous despite

the fact that the power density is zero. Similar difficulties occur in

s Born and Wolf, Principles of Optics (second revised edition),

MacMillan, N.Y., N.Y., 1964, pp. 34-36.

4.
attempting to relate the power density of multipath fields in general to

potential hazards. Therefore, power density is a poor indicator of the

potential hazards of multipath fields.

2.3 Reactive Near-Field Components.

Within distances of roughly one wavelength of some sources

(for instance, inefficient radiators), there can sometimes exist strong

EM field components that do not have a time -averaged energy flow

(though they will have instantaneous energy flows). That is, the total

field in these regions can be divided into a radiation component and reactive components (see almost any text on electromagnetic theory);

and the time -average of the Poynting vector for the reactive field components is identically zero. The radiation field represents a flow of

energy away from the source, but the reactive field has no corresponding energy loss. The energy associated with reactive fields pulses back

and forth between the source and the surrounding space during the period

of the oscillation, similar to the energy flow between capacitances and

inductances in electrical circuits. (This situation is analagous to the

operation of an unbaffled speaker cone. At low audio frequencies, i.e.,

for wavelengths larger than the dimensions of the cone, a speaker cone

operating in open air is an inefficient radiator because the air propelled

by the cone merely rushes around to the reverse side of the cone. This

produces relatively little sound radiation to points far from the cone

even though the instantaneous air flow around the cone can be very great.)

2.4 Interactions Between the Source and Nearby Objects.

A measuring instrument, subject, etc. placed near to a source

can establish multiple propagation back-and -forth between the object

and the source. This interaction can involve both scattering from the

5.
external surfaces of the source and also coupling with the internal

regions of the source. In one sense, this can be considered as a complicated case of multipath interference, but it should be remembered

that the coupling with the internal regions of the source can substantially

(even greatly) change the total amount of energy withdrawn from the

source. This interaction can be exceedingly complicated and is determined, for a given wavelength, not only by the electrical composition

of the source, the object of interest, and other nearby objects but also

by the sizes, shapes, positions, and orientations involved.

2. 5 Complicated Time Variation of the Configuration and Intensity

of the Field . We are not concerned here with the periodic variation of the field

vectors at the radio frequency. Rather, we are concerned with the longer term variation in the configuration of the field and in the amplitudes

of the electric and magnetic vectors. Consider, for instance, a microwave oven that has two widely-separated leakage cracks. Since the

microwave sources in many ovens are operated from unfiltered power

supplies, the field created by the leakage from these cracks may go on

and off at twice the power-line frequency. Further, since most ovens

have "mode" stirrers (metal vanes that rotate, typically, every few

seconds to "stir" the field configuration inside the oven), the phase and

amplitude of the field just outside each crack may change radically

every few seconds, independently of the other crack. The resulting

time variation in the multipath field outside the oven can be extremely

complicated. (It is pertinent to note that a leakage crack in an oven may

leak only for the fraction of a cycle of the mode stirrer. Therefore,

it is possible for the peak-to -average ratio to be quite high for microwave oven leakage .)

6.
3 . Approaches to Quantifying EM Hazards .

3 . 1 The Dosimetry Approach.

In surveying for hazardous EM fields, one wishes to determine as

accurately as possible the potential biological dose (e.g., the temperature rise) or the exposure (e.g., the induced currents or the EM field

established) that would result at points of interest inside a subject if

introduced into the field. In general, because of the complications

mentioned in Section 2, it is not practical to accurately predict the

biological dose or exposure from knowledge of the parameters of the

unperturbed field (i.e., the field before the biological material is introduced). This means that accurate dosimetry for EM field hazards

must be done with phantoms that simulate not only the electrical characteristics of the body but also the size and, to a lesser degree, the shape of

the body. Probably, a head- sized sphere or ovoid would suffice for a phantom in most situations of interest. At points inside the phantom,

the thermal dose could be measured by the temperature rise or the exposure measured by the currents or the EM field established. The usefulness of this type of instrument is limited because the size and weight

of the phantom would seriously hamper general survey measurements of

potentially hazardous EM fields . Therefore, even though the dosimetry

approach is most accurate, it is felt that it will probably not be used

much outside of the laboratory.

3 . 2 The Field Parameter Approach . This approach assumes that the biological effects, or at least

the possible effects, resulting from the EM field can be adequately predicted from knowledge of one or more parameters that describe the field.

7.
Because the biological effects are so difficult to predict, this approach

will always involve a conservative standard for the maximum allowable

level of the field parameter used to quantify the field. Though this

approach is somewhat arbitrary, it is felt that it is the most practical

approach to the problem of making general survey measurements.

Some discussion about the choice of a suitable field parameter and the

general characteristics of suitable instrumentation will be given later.

3 . 3 The Arbitrary Approach.

This approach would use an instrument that would respond to the

EM field but would not, in general, yield an accurate measurement of

either the dose that could be expected from the field or one of the parameters of the unperturbed field. For instance, suppose that the instrument is based on the temperature rise in a small piece of material

that simulates only the electrical characteristics of biological material.

Since a small piece of material cannot adequately mimic the energy absorption of, say, a human head, this instrument could not be expected to

yield an accurate measurement of the dose resulting from complicated

EM fields. Further, unless the piece of absorbing material is quite

small, the instrument will strongly affect the field near, say, a radiating

crack; and thus, a measurement of the unperturbed field would be practically impossible. (If the piece of absorbing material is quite small,

it could, under certain conditions, provide an accurate measurement of

some parameter of the unperturbed field; but then the instrument would

satisfy the more stringent approach outlined in Section 3.2).

The arbitrary approach is considered to be the least satisfactory

of the three approaches outlined. However, until very recently, it was

the approach that was used in practice due to a lack of better instrumentation. That is, until recently, instruments suitable only for uniform

plane-wave fields (and in some cases only linearly polarized fields) were used
to measure leakage from ovens at points close to the leaking cracks.

Very little quantitative meaning can be assigned to such measurements,

though they are probably adequate to establish rough estimates of the

hazards involved.

4 . Choosing a Suitable Field Parameter for Quantifying Hazardous

EM Fields.

4.1 Parameters for Describing Uniform Plane -Wave Fields . For a uniform plane-wave field traveling in free space, very simple

relationships exist between the magnitude E of the electric vector (i.e.,

the electric field strength), the magnitude H of the magnetic vector

(i.e., the magnetic field strength), the time -average S of the energy

flow (i.e., the power density), the time-average U of the electric field

E

energy density function (i.e., the electric energy density), the timeaverage U of the magnetic field energy density function (i.e., the mag- H

netic energy density), and the time -average U of the electromagnetic

field energy density function (i.e., the total energy density). Using

root-mean- square values, these simple relationships are as follows: — = Z Q

(the intrinsic impedance of vacuum) (1)

72

Z. = l—

J

<-& (2)

E

2

2

S = ~- = ZQ H* (3)

U 4 (4)

(

e o E2+ ^oH2

^

(

UE +Uh) — - 1 (5)

uH

For such fields, the energy density can be considered as propagating

9.
through the field point with the speed of light, c. That is,

S = cU (6)

For comparison, if a uniform plane -wave field has a power density of

10 mW per square cm, the total energy density is — pJ per cubic cm

(picojoules per cubic cm), and the electric field strength is 1.94 volts

per cm.

Because of the simplicity of relations (1) through (6), the "intensity"

of this very simple type of EM field can be adequately described by any

one of the parameters defined above. However, not all of these parameters

are adequate for describing the "intensity" of complicated EM fields.

Further, in choosing a suitable parameter for characterizing the "intensity"

of complicated fields with respect to biological hazards, one must consider the manner in which biological material interacts with EM fields.

4.2 The Interaction of Biological Material with Electromagnetic Fields .

It will be assumed here that there are no significant "non-thermal"

biological effects due to the magnetic component of the field. Then, since

biological material does not normally contain more than minute amounts

of lossy magnetic substances, there is no significant direct interaction

of the magnetic field with biological material. It is emphasized, however, that energy associated with the magnetic field will be indirectly absorbed in the biological material. That is, if the electric field penetration into the material is "damped" because of dielectric losses, energy

associated with the magnetic field will be absorbed also. This consideration could be particularly important when interference fields

or reactive field components exist since the penetration of the

electric field could be considerably greater than one would expect based

on "plane wave" absorption concepts.

10.
4 .

3

Choosing Suitable Field Parameters . As indicated in the last section, both the thermal and the nonthermal biological effects are caused by the internal electric field. The

problem is to determine the most suitable parameter or set of parameters for relating unperturbed fields to the maximum possible internal

electric fields resulting when a subject is placed at any possible position

and with any possible orientation in the unperturbed fields. Because the

resulting internal fields are generally extremely difficult to relate to the

unperturbed fields, this problem can not be resolved without introducing

simplifications, some of them rather arbitrary. Since it would go beyond

the scope of this paper to adequately justify the following statement, it

will be merely asserted that: the potential hazards of unperturbed electromagnetic fields are, in general, most closely associated with the

electric components of the fields except possibly (a) near the magnetic field

maxima in multipath fields and (b) for some reactive near-fields. For

the exceptions, the magnetic components of the fields may be equally or more important than the electric components.

The "suitability" of a field parameter for quantifying hazardous

EM fields is dependent both on the relevancy of the parameter to the

potential hazard and on the ease of measuring the parameter. Except

for very simple fields, power density is least suitable because (a) the

power density of some very strong fields can be small, and (b) power

density is a relatively difficult parameter to measure. The difficulties

involved in measuring power density follow from the fact that the power

density is given by the time average of |EXH | . Certainly, in complicated

For some of the difficulties involved in measuring power density, see

Moore School Report 63-13, "Study of power density measurement

techniques", University of Pennsylvania, Moore School of Electrical

Engineering, January 31, 1963 or

Moore School Report No. 63-23, "Feasibility studies of Poynting vector

measurements", University of Pennsylvania, Moore School of Electrical

Engineering, July 31, 1963.

11.
fields, it is much easier to measure E and H, which are scalars, than

it is to measure the time average of |EXH| . The electric and magnetic

energy densities are also relatively easy to determine since the simple

relations (4) also hold for complicated fields. That is, the electric or magnetic energy density can easily be calculated from E or H; and instruments that respond to E or H can easily be engineered to display

U or U • As shown by Wacker in Technical Note 391, it is even feasible E H

to measure the total energy density with a single sensor. Except, perhaps, for non-thermal biological effects, it is felt that U, U , and U

E H

are the most suitable parameters for quantifying the potential hazards

because: (a) the thermal heating is proportional to the squares of

E and H, and therefore, directly proportional to U, U , and U ; and

E H

(b) it is convenient to have the same units for stating the "intensities"

of the total field, the electric field, and the magnetic field. The remaining discussion will be simplified by assuming that the EM field

will be quantified in terms of energy density, though it is felt that E

and H would also be satisfactory.

At frequencies above about 1 GHz, that is for wavelengths shorter

than about 30 cm, a measurement of the electric energy density alone

is probably adequate. In part, this simplification is possible because

reactive fields are seldom stronger than radiation fields at distances

greater than about one half wavelength from the source. Also, for wavelengths shorter than about 30 cm (and longer than about 3 cm), it is

usually very easy to probe the interference patterns caused by multipath

propagation and thus locate the electric field maxima. Then U for

E

each maximum should be a reasonably good indication of the potential

hazard in the immediate region of the maximum. Magnetic field maxima

will exist between adjacent electric field maxima, but it is very unlikely

that these magnetic field maxima will represent a greater hazard than the

12.
electric field maxima.

For wavelengths longer than about 30 cm, the magnetic energy

density becomes increasingly important. At the present time, hazardous

electromagnetic fields below about 1 GHz are not nearly so common as

those at higher frequencies so that the measurement of total energy

density or of magnetic energy density is perhaps not a crucial issue.

However, there are important needs for such measurements (for instance,

around powerful radars that operate well below 1 GHz).

5. Instruments for Quantifying Hazardous Electromagnetic Fields .

5 .

1

General Characteristics . From the preceeding discussions, it is, felt that a good instrument for survey measurements of electromagnetic hazards should

have the following characteristics;

(1) the instrument should measure in terms of energy density

(or, alternatively, E and H);

(2) the sensor of the field probe should be much smaller than

the shortest wavelength of the fields to be measured;

(3) the probe should not cause significant scattering of the field;

(4) the probe should be independent of its angular orientation

in the field (i.e., independent of both' the polarization of

the field and the directions of the vectors of the field)j

(5) the instrument should be capable of reading either peak

or average values for complicated waveforms;

(6) the instrument should have a dynamic range of at least 20 dB

without having to change probes;

( 7) the instrument should be direct reading, that is it should

not need a calibration chart, nulling, or frequent re-zeroing,

In large part, this situation is due to the existence of large numbers of

microwave ovens.

13.
In addition to the above characteristics, the instrument should,

of course, be stable, rugged, lightweight, battery operated, etc.,

5.2 Realization .

It is not apparent that instruments can be realized that have

characteristics even approximating those outlined in the last section.

Recently, commercial instruments have become available that provide some of these characteristics, but it would appear that there is no

available instrument that approximates a good instrument as defined

here. However, these new instruments are vast improvements over

previously available instrumentation, and they are considered to be

very useful despite their limitations.

There are a number of possibilities for realizing "next generation"

instruments (for instance, see the references outlined in Footnote 4).

In fact, there are a number of efforts in progress to develop improved

or lower cost instruments for measuring hazardous EM fields. It is

felt likely that excellent instrumentation, at least for measuring U_ or

E, can be realized.

5.3 Dosimetry .

It is worth emphasizing that the problem of measuring the electric

field strength E or the electric energy density U in complicated unE

perturbed fields is essentially the same problem as measuring the complicated internal electric field within a phantom. Therefore, if it is

7

See the paper by P.W. Crapuchettes presented at the Symposium on

the Biological Effects and Health Implications of Microwave Radiation,

Richmond, Virginia, September 17 through 19, 1969.

As of January, 1970, a prototype instrument has been developed by the

National Bureau of Standards, Boulder, Colorado. This instrument is

limited to measuring U (or E), but otherwise it appears from preliminary tests to exceed the essential characteristics outlined in Section

5.1. Descriptions of this instrument will soon be available.

14.
possible to realize an electric field measuring instrument having the

characteristics outlined in Section 5.1 (having a very small field sensor

in particular) it should be fairly easy to incorporate the sensor of this

probe into a phantom to accurately measure the potential exposure

(i.e., the dose rate) for very complicated fields. The realization of

such a dosimeter is very important since much work needs to be done

to determine the potential hazards of complicated fields and how these

hazards relate to the parameters of the unperturbed fields.

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and industrial practice (including safety codes) de- veloped in cooperation with interested industries,

professional organizations, and regulatory bodies. Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts,

pamphlets, etc. Monographs. Major contributions to the technical

literature on various subjects related to the Bureau's

scientific and technical activities. National Standard Reference Data Series.

NSRDS provides quantitive data on the physical

and chemical properties of materials, compiled from

the world's literature and critically evaluated. Product Standards. Provide requirements for sizes, types, quality and methods for testing various industrial products. These standards are developed cooperatively with interested Government and industry groups

and provide the basis for common understanding of product characteristics for both buyers and sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other agency and

NBS-sponsored work) of limited or transitory interest. Federal Information Processing Standards Publications. This series is the official publication within

the Federal Government for information on standards

adopted and promulgated under the Public Law

89-306, and Bureau of the Budget Circular A-86

entitled, Standardization of Data Elements and Codes

in Data Systems.

CLEARINGHOUSE

The Clearinghouse for Federal Scientific and

Technical Information, operated by NBS, supplies

unclassified information related to Government-generated science and technology in defense, space, atomic energy, and other national programs. For

further information on Clearinghouse services, write: Clearinghouse

U.S. Department of Commerce

Springfield, Virginia 22151

Order NBS publications from: Superintendent of Documents

Government Printing Office

Washington, D.C. 20402
U.S. DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20230

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