{ nttoni-.3i.oi

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\ X .<> NBS TECHNICAL NOTE 1028

NATL INST. OF STAND & TECH

A111D7 EMl^Sfi

U.S. DEPARTMENT OF COMMERCE /National Bureau of Standards

Theoretical and Experimental

Investigations of

Electromagnetic Field Distortion

Due to a Perfectly Conducting

Rectangular Cylinder in a

Transverse Electromagnetic Cell
NATIONAL BUREAU OF STANDARDS

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ATIONAL BUREAU

09 OTAKDARD8

Theoretical and Experimental Investigations

of Electromagnetic Field Distortion

Due to a Perfectly Conducting

^^

Rectangular Cylinder in a

Transverse Electromagnetic Cell

Motohisa Kanda

Electromagnetic Fields Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

s

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1981
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1028

Nat. Bur. Stand. (U.S.), Tech. Note 1028, 48 pages (April 1981)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON 1981

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

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TABLE OF CONTENTS

Page

LIST OF FIGURES. iv

I

.

INTRODUCTION 2

II

.

THEORY 4

III

.

THEORETICAL AND EXPERIMENTAL RESULTS 9

IV. CONCLUSIONS 12

V

.

REFERENCES 13

-m-
LIST OF FIGURES

Figure 1. Curve of equal TEq-^ and TE^q cutoff frequencies for a transverse electromagnetic cell 14

Figure 2. Normalized TEq^ cutoff wavelength for a transverse

electromagnetic eel 1 15

Figure 3. Electromagnetic field distortion due to a metallic cut

in a transverse electromagnetic cell (electrostatic

approximation) 16

Figure 4. Coordinate system of a rectangular cylinder in a parallel plate waveguide 17

Figure 5. Magnetic field distortion due to a perfectly conducting

cylinder (width i = 18 cm, height h = 5.5 cm) in a transverse electromagnetic cell at 1 MHz 18

Figure 6. Magnetic field distortion due to a perfectly conducting

cylinder (width % = 18 cm, height h = 5.5 cm) in a transverse electromagnetic cell at 2 MHz 19

Figure 7. Magnetic field distortion due to a perfectly conducting

cylinder (width i = 18 cm, height h = 5.5 cm) in a transverse electromagnetic cell at 5 MHz 20

Figure 8. Magnetic field distortion due to a perfectly conducting

cylinder (width % = 18 cm, height h = 5.5 cm) in a transverse electromagnetic cell at 10 MHz 21

Figure 9. Magnetic field distortion due to a perfectly conducting

cylinder (width % = 18 cm, height h = 5.5 cm) in a transverse electromagnetic cell at 50 MHz 22

Figure 10. Magnetic field distortion due to a perfectly conducting

cylinder (width i = 18 cm, height h = 5.5 cm) in a transverse electromagnetic cell at 100 MHz 23

Figure 11. Magnetic field distortion due to a perfectly conducting

cylinder (width a = 18 cm, height h = 15 cm) in a transverse electromagnetic cell at 2 MHz 24

Figure 12. Magnetic field distortion due to a perfectly conducting

cylinder (width i = 18 cm, height h = 15 cm) in a transverse electromagnetic cell at 5 MHz 25

Figure 13. Magnetic field distortion due to a perfectly conducting

cylinder (width i = 18 cm, height h = 15 cm) in a transverse electromagnetic cell at 10 MHz 26

-IV-
Figure 14. Magnetic field distortion due to a perfectly conducting

cylinder (width i = 18 cm, height h = 15 cm) in a transverse electromagnetic cell at 50 MHz 27

Figure 15. Magnetic field distortion due to a perfectly conducting

cylinder (width i - 18 cm, height h = 15 cm) in a transverse electromagnetic cell at 100 MHz 28

Figure 16. Magnetic field distortion due to a perfectly conducting

cylinder (width % = 18 cm, height h = 25 cm) in a transverse electromagnetic cell at 2 MHz 29

Figure 17. Magnetic field distortion due to a perfectly conducting

cylinder (width i = 18 cm, height h = 25 cm) in a transverse electromagnetic cell at 5 MHz 30

Figure 18. Magnetic field distortion due to a perfectly conducting

cylinder (width i = 18 cm, height h = 25 cm) in a transverse electromagnetic cell at 10 MHz 31

Figure 19. Magnetic field distortion due to a perfectly conducting

cylinder (width i = 18 cm, height h = 25 cm) in a transverse electromagnetic cell at 50 MHz 32

Figure 20. Magnetic field distortion due to a perfectly conducting

cylinder (width i = 18 cm, height h = 25 cm) in a transverse electromagnetic cell at 100 MHz 33

Figure 21. Magnetic field distortion at the center of the top of the

cylinder as the ratio of its height to the separation

distance of the parallel plate waveguide at 2 MHz 34

Figure 22. Magnetic field distortion at the center of the top of the

cylinder as the ratio of its height to the separation

distance of the parallel plate waveguide at 5 MHz 35

Figure 23. Magnetic field distortion at the center of the top of the

cylinder as the ratio of its height to the separation

distance of the parallel plate waveguide at 10 MHz 36

Figure 24. Magnetic field distortion at the center of the top of the

cylinder as the ratio of its height to the separation

distance of the parallel plate waveguide at 50 MHz 37

Figure 25. Magnetic field distortion at the center of the top of the

cylinder as the ratio of its height to the separation

distance of the parallel plate waveguide at 100 MHz 38

Figure 26. Electric field distortion at the center of the top of the

cylinder as the ratio of its height to the separation

distance of the parallel plate waveguide 39

-v-
Figure 27. Simultaneous electric and magnetic field measurements

using a doubly-loaded electrically small loop 40

Figure 28. Phase degradation of the TEM mode as the ratio of

the cylinder height to the separation distance

between the center conductor and the ground

plane in the TEM cell 41

-VI-
Theoretical and Experimental Investigations of Electromagnetic Field

Distortion Due to a Perfectly Conducting Rectangular

Cylinder in a Transverse Electromagnetic Cell

Motohisa Kanda

Electromagnetic Fields Division

National Bureau of Standards

Boulder, Colorado 80303

The study of electromagnetic compatibility (EMC), that

is the electronic and biological effects due to

electromagnetic (EM) radiation, and EM calibration require

accurate EM measurement techniques for defining the EM

interference (EM) characteristics. Thus, fully enclosed

rectangular transverse electromagnetic (TEM) transmission

lines with thin inner conductors are often used for

generating standard known test fields. In all cases it is

desirable that only the dominant TEM mode should propagate.

In the EMC measurements, an object under test is placed

inside of a TEM cell. The field from the TEM mode incident

upon this scattering object is identical to that of a plane wave in a free space. However, the scattered field produced

by the object in the TEM cell is different from the

scattered field produced by the object in a free space,

because of multiple reflections from the TEM cell walls, or equivalently, the mutual coupling between the object and the

TEM cell.

The purpose of this paper is to discuss the loading

effects, i.e., the electromagnetic field distortion caused

by an object under test in a TEM cell. In the theoretical

analysis, the frequency domain integral equation for the

magnetic field, or equivalently, the current density on the

surface of a perfectly conducting cylinder in a parallel

plate waveguide is solved by the method of moments to

predict the degree of magnetic field distortion . The experimental investigations are performed by

mounting a number of electrically small half loops on the

surface of the perfectly conducting cylinder in a TEM

cell. The loading effects in terms of magnetic field

distortion are analyzed as the ratio of one of the object

dimensions (height) to the separation distance between the

inner conductor and the ground plane of the TEM cell. Also,

the response of an electrically small loop to both the

magnetic and electric components of the electromagnetic
field is used to measure the phase relation between the

magnetic and electric fields, which in turn can be used to

determine the degree of degradation of the TEM mode due to

the presence of the perfectly conducting cylinder. These

theoretical and experimental results are compared with the

available quasi electrostatic results.

Keywords: Electromagnetic Compatibility (EMC); Green's

function; integral equation; linear equation^ method of

moments; parallel plate waveguide; quasi electrostatic $ TEM

cell.

I. INTRODUCTION

The study of electromagnetic compatibility (EMC), that is the

electric and biological effects due to electromagnetic (EM) radiation,

and EM calibration require accurate EM measurement techniques for defining the EM interference (EMI) characteristics. Thus, fully enclosed

rectangular transverse electromagnetic (TEM) transmission lines with

thin inner conductors are often used for generating standard known test

fields. In all cases, it is desirable that only the dominant TEM mode

should propagate. Thus, the usefulness of these structures is limited

to a frequency region below some upper frequency bound in order to

suppress the higher order modes.

The higher order mode cutoff frequencies of the rectangular TEM

cell have been well studied by many workers [1,2,3,4]. While in a

rectangular hollow waveguide, the dominant mode is always the TEiq mode

as long as the width exceeds the height, the same conclusion does not

hold for the rectangular line with an inner conductor even if its thickness is infinitesimally small. In fact, it is found [1,2,4] that,

depending on the width of the inner conductor and the size of the TEM

cell, the cutoff frequency of the TE Q1 mode can be much lower than that
of the TE 10 mode as illustrated in figure 1. While the cutoff frequency

of the TE^q mode is simply calculated from

fc%i V 00

2 + 00

J-, (1) _ _ , 2a /ye

m = 1 n = the cutoff frequencies of the TEqj mode are much more involved and have

been calculated by many workers [1,2,3,4] as shown in figure 2. Let us

consider, for example, a typical 50ft TEM cell having a width of a = 1 m,

a height of b = 0.6 m, and a width of inner conductor of s = 0.72m.

While the cutoff frequency of the TE 1Q mode is 150 MHz, the cutoff

frequency of the TEqi mode is approximately 135 MHz according to figure

2. It can also be shown that the cutoff frequencies of all TM modes of

a TEM cell are always higher than those of their hollow waveguide

counterparts. Thus, the dominant cutoff frequency (i.e., the lowest) is

either the TEq^ mode or the TE^g mode. It is interesting to note that

the cutoff frequency of the TEq^ mode decreases as the gap between the

inner conductor and the wall of the TEM cell becomes narrower. This

phenomenon has also been observed in a ridge waveguide [5] and is associated with the infinite gap capacitance.

In the EMC measurements, an object under test is placed inside of a

TEM cell. The field from the TEM mode incident upon the scattering

object is identical to that of a plane wave in a free space. However,

the scattered field produced by the object in the TEM cell is different

from the scattered field produced by the object in a free space because

of multiple reflections from the TEM cell walls, or equivalently , the

mutual coupling between the object and the TEM cell.
Placing a test object in a TEM cell is equivalent to introducing a

capacitive discontinuity. For low frequencies, where the transverse

dimensions of the object are negligible compared to the wavelength, the

discontinuity due to the object is a pure capacitive reactance, and may

be regarded as the fringing capacitance of the corresponding electrostatic problems. Under this assumption, the ratio of the electric field

strength near the metal object in the TEM cell to the unperturbed electric field strength at the same location in a pure TEM mode has been

calculated by G. Meyer [6] and is shown in figure 3.

The purpose of this paper is to discuss the loading effects, i.e.,

the electromagnetic field distortion caused by an object under test in a

TEM cell. In the theoretical analysis, the frequency domain integral

equation for the magnetic field, or equivalently, the current density on

the surface of a perfectly conducting cylinder in a parallel plate

waveguide is solved by the method of moments to predict the degree of

mangetic field distortion. For the purpose of mathematical tractability, a parallel plate waveguide is used to model a TEM cell

structure. The results given in the paper are for the magnetic field

intensity on the surface of the cylinder. Other related quantities,

such as the electric field intensity and the poynting vector, can be

readily derived from the surface current by use of Maxwell's equations.

II. THEORY

The problem of determining the electromagnetic field scattered by a

perfectly conducting rectangular cylinder has been studied by many
workers [7,8,9]. The coordinate system used to analyze the current

density on the surface of the rectangular cylinder in a parallel plate

waveguide is shown in figure 4. The source of the TEM wave is a delta

function voltage source of the form

ffi = y Y

o

(o)) 6(2 - zo

), (2)

where V

Q

is the magnitude of an equivalent voltage source located at the

source location zQ

. Since there is no variation in the y direction and

the voltage source has only a y-component, we will have a scalar wave

equation in H , i.e.,

V H + k H = jcoe M , (3)

y y y

where v

2 = —s- + —~- and k is the free space wave number. The boundary

3X2 3Z2 condition is that the normal H field be zero on all of the reflecting

conducting surfaces, i.e.

3H

v a/= ' (4)

where n = x and z.

The solution of equation (3) may be found through the Green's

Function technique,

V

2 G(r.r') + k2 G(r.r') = -S(x-x') 6(z-z'). (5)
Multiplying eq. (3) by G (r,r x ) , eq. (5) by H , subtracting the two

results and integrating over the free space volume V,

J v [G(r,r') v

2 Hy

(r') - Hy(r') v

2 G(r,r')] dv'

= joje J v G(r,r') M

y

(r') dv' + j y

H (r 1 ) 6(x-x') 6(z-z') dv'.

(6)

Evaluating the integration of the right hand side of eq. (6) and using

Green's theorem on the left hand side, we get

/ s [^)^-H

y

(r')^Ads'

(7)

= jcoe J v G(r,r') M (r

1 ) dv' + H (r).

On the perfectly conducting surface, s, of the plates, we set the

boundary condition as

3H..(r')

y

3n t— = 0. (8)

Equation 7 then reduces to

Hy(r) = -jue / v G(r,r') M

y

r') dv' -

/ $ H y

(r')

3 G

^?

r>)

ds' , (9)

where sc indicates the surface of the rectangular cylinder. The first

term on the right hand side of eq. (9) corresponds to the incident

field, and the second term to the scattered field. The known incident
magnetic field is given by

b

V

Q

6(z'-z ) G(x,z; x',z') dx'dz'

(10)

H inc

(x,z) = -jwe j Q

l Q

V

Q

6(z'-z ) G(x,z; x',z') dx'dz'

jkz^

we ., i —i- V e cos kz.

Equation (9) is an integral equation which can be solved for Hy(r) once

Green's function G (r,r') is obtained.

The Green's function is the solution of eq. (5). By imposing the

proper boundary condition for the parallel plate waveguide, i.e.

||=0 (11)

at x=0 and x=b, then one finds [10]

G(x.z;x'.2'J - X [e^l^'l^jklz-z'l]

2kTj

. cos J^ C os^' jr

n (z+z') jrlz-z'l

n=l n

(12)

where

v

r = W k2 - ( ^)2 - (13)

n w b

Once the Green's function is obtained and the source is specified, eq.

(9) can be solved by the method of moments. The technique used is

discussed briefly below. A detailed discussion on this subject is given
by Harrington [11].

The unknown H is expanded in terms of a set of known basis functions f.,- with unknown coefficients a., i.e.

H=ja. f.. (14)

J 1=1

Substituting eq. (14) into eq. (9) we obtain

la. [f. (x,z) + / f. (x' >Z ') 3G (X ^?

X

' ,Z>) ds'] = H inC

(x,z). (15)

i=l

1 1 s'

1 8n

Equation (15) is a single equation with n unknown a^. To create at

least n linear equations, a set of testing function, w.= , is introduced,

and the inner products (integral over the surface) of both sides of eq.

(15) for each w..- are set equal. This forms a set of linear equations of

the form

[Z] [I] = [V], (16)

where the matrix element Z.-.- is given by

Z„ =

/

s [f^r) + /s-f^r 1 ) G( y' ) ds'] w (r) ds, (17)

and the column matrix element V- is given by

,inc

V. =

J H

,M" (r) w.(r) ds.

J J J (18)

The unknown coefficients a- are found by solving the matrix eq (16).
The expression for H is then given by eq (14).

III. THEORETICAL AND EXPERIMENTAL RESULTS

A number of electrically small half loops whose diameters are

1.5 cm are mounted transversely across a perfectly conducting rectangular cylinder. The cylinder is placed in a TEM cell which acts approximately as a parallel plate waveguide. A vector voltmeter is used to

measure both the magnitude and phase of the magnetic field strength and

therefore the current density on the surface of the cylinder. In the

theoretical analysis, the frequency domain integral equation for the

magnetic field on the surface of the cylinder in a parallel plate waveguide given in section II is solved by the method of moments. Other

related quantities, such as the electric field intensity and the

poynting vector can be readily derived from the surface current by use

of Maxwell's equations.

In this paper, the loading effects due to the perfectly conducting

rectangular cylinder in a TEM cell are indicated in terms of the

magnetic field distortion, which is defined as the ratio of the magnetic

field strength on the surface of the cylinder in a TEM cell to that at

the same position in an empty TEM cell. In this paper, the separation

distance of the parallel plate waveguide, i.e., the distance between the

center conductor and the ground plane in the TEM cell is chosen to be

30 cm. Three different perfectly conducting rectangular cylinders are

used, all of which have the same widths of 18 cm, but have different

heights of 5.5 cm, 15 cm, and 18 cm. The magnetic field distortion and
the corresponding phase are shown in figures 5 through 20 for

frequencies from 1 MHz to 100 MHz. The phase reference is taken to be

at the center of the top of the cylinder (x=h and j) . Figures 21

through 25 show the magnetic field distortion at the center of the top

of the cylinder as the ratio of its height to the separation distance of

the TEM cell. For a comparison, the electric field distortion due to a

metal cube reported by G. Meyer [6] is also shown in these figures. In

general, it is found that the magnetic field distortion due to a

perfectly conducting rectangular cylinder is quite small and much less

than the electric field distortion reported by G. Meyers [6].

In order to confirm the result by G. Meyer [6], a electrically

small dipole is mounted on the center of the top of the perfectly conducting rectangular cylinder. The electric field distortion (defined as

the ratio of electric field at the surface of the cylinder to that at

the same position in an empty TEM cell) is shown in figure 26 as the

ratio of the cylindrical height to the separation distance of the

parallel plate waveguide. Figure 26 indicates that the electric field

distortion is much larger than that predicted by G. Meyer [6]. The

discrepancy may be due to the fact that the Meyer calculation is based

on the quasi electrostatic approximation.

When the perfectly conducting rectangular cylinder is placed in the

TEM cell, the scattered field produced by the cylinder can be far

different from the original incident TEM mode. In order to determine

the degree of degradation of the TEM mode due to the presence of the

cylinder, the response of an electrically small loop to both the

magnetic and electric components of an electromagnetic field is

10
examined. Consider the loop loaded at each of the diametrically

opposite points as shown in figure 27. One can show that the sum of the

two load currents I is given by [12]

h - Kh V (19)

and their difference I is given as

A

3

'a

K eE x- (20)

where K^ and K e are, respectively, the loop sensitivity constant for

magnetic and electric fields. These formulas clearly show that the use

of the sum current gives a measure of the magnetic field, whereas that

of its difference gives a measure of the electric field.

While the phase between magnetic and electric field is normally in

phase for the pure TEM mode, the presence of the perfectly conducting

rectangular cylinder in the TEM cell will cause a distortion and thus a

phase degradation of the TEM mode around the cylinder. The phase degradation of the TEM mode thus obtained is shown in figure 28 as the ratio

of the cylinder height to the separation distance of the TEM cell. It

is very interesting to note that the phase degradation at the center of

the cylinder becomes predominant as the frequency approaches to the

cutoff frquency of the TE i mode and al so tne height of the cylinder

becomes comparably large compared to the separation of the parallel

plate waveguide. The phase degradation observed at the low frequencies

around 2 MHz is not well understood, but is probably due to the experi11
mental problems caused by the vector voltmeter and the hybrid junction

used in the experiments.

IV. CONCLUSIONS

This paper discussed the loading effects, i.e., the electromagnetic

field distortion caused by a metal object placed in a TEM cell. In the

theoretical analysis, the frequency domain integral equation for the

magnetic field intensity on the surface of a perfectly conducting

cylinder in a parallel plate waveguide was solved by the method of

moments to obtain the degree of magnetic field distortion. The results

given in the paper are for the magnetic field intensity on the surface

of the cylinder. Other related quantities, such as the electric field

intensity and the poynting vector can be readily derived from the

surface current by use of Maxwell's equations.

The experimental investigations were performed by mounting a number

of electrically small half loops on the surface of a perfectly

conducting cylinder placed in a TEM cell. The loading effects in terms

of magnetic field distortion were expressed as the ratio of the object

height to the separation distance between the inner conductor and the

ground plane of the TEM cell. Also, the response of an electrically

small loop to both the magnetic and electric components of an electromagnetic field was used to measure the phase relation between the

magnetic and electric fields, which in turn was used to assess the

degree of degradation of the TEM mode due to the presence of the

perfectly conducting cylinder. These theoretical and experimental

12
results were compared with the available quasi electrostatic results

V. REFERENCES

[I] Brackelmann, W., Landmann, D. , and Schlosser, W. , Die

Grenzfrequenzen von hoheren Eigenwellen in Streifenleitungen AEU,

Vol. 21, pp 112-120 (March 1967).

[2] Gruner, L., High Order Modes in Rectangular Coaxial Waveguides,

IEEE Trans. Microwave Theory Technique, Vol MTT-15, pp. 483-485,

(August 1967).

[3] Mittra, R. , and Itoh, T. , A New Technique for the Analaysis of

the Dispersion Characteristics of Microstrip Lines, IEEE Trans.

Microwave Theory Technique, Vol MTT-19, pp. 47-56,

(January 1971).

[4] Tippet, J. C, Modal Characteristics of Rectangular Coaxial

Transmission Line, Ph.D Thesis, University of Colorado,

Boulder, CO. (June 1978).

[5] Ramo, S., and Whinnery, J. R. , Fields and Waves in Modern Radio,

2nd Edition, John Wiley & Sons, Inc., New York, N. Y. (1953).

[6] Meyer, G. , Application of a Broadband Measuring Line in Field

Immunity Testing, 2nd Symposium and Technical Exhibition on Electromagnetic Compatibility, Montreux, Switzerland, pp. 241-46,

(June 1977).

[7] Mei, K., and Van Bladel, J. G. , Low-Frequency Scattering by

Rectangular Cylinders, IEEE Trans. Antennas and Propagation, Vol.

AP-11, pp. 52-56, (January 1963).

[8] Mei, K. K. and Van Bladel, J. G., Scattering by Perfectly

Conducting Rectangular Cylinders, IEEE Trans. Antennas and

Propagation, Vol. AP-11, pp. 185-192, (March 1963).

[9] Andreasen, M. G., Comments on Scattering by Conducting

Rectangular Cylinders, IEEE Trans. Antennas and Propagation, Vol.

AP-12, pp. 235-236, (March 1964).

[10] Morse, P. M. and Feshback, H., Methods of Theoretical Physics,

McGraw-Hill Book Company, Inc., New York, (1953).

[II] Harrington, R. F., Field Computation by Moment Methods, the

Macmillan Company, New York, (1968).

[12] Whiteside, H. and King, R.W.P., The Loop Antenna as a Probe, IEEE

Trans. Antennas and Propagation, Vol. AP-19, No. 3, pp. 291-297,

(May 1964).

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NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS TN-1028

2. Gov't Accession Mo, 3. ftacipienf a Acc$$si<«i No.

4. TITLE AND SUBTITLE

Theoretical and Experimental Investigations of Electromagnetic Field Distortion Due to a Perfectly Conducting

Rectangular Cylinder in a Transverse Electromagnetic Cell

5. Publication Date April 1981

#. Perfortftjnt Organization Cciie

7. AUTHOR(S)

Motohisa Kanda

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

iC f^Ka/To**^** t&tft No,

1 7233T62

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP) 13. Type of Report & Period Covered

14* Sponsoring Agency Codo

'

15. SUPPLEMENTARY NOTES

|

| Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here.)

The study of electromagnetic compatibility (EMC), that is the electronic and biological effects due to electromagnetic (EM) radiation, and EM calibration require accurate EM measurement techniques for defining

the EM interference (EM) characteristics. Thus, fully enclosed rectangular transverse electromagnetic (TEM)

transmission lines with thin inner conductors are often used for generating standard known test fields. In all cases it is desirable that only the dominant TEM mode should propagate. In the EMC measurements, an object under

test is placed inside of a TEM cell. The field from the TEM mode incident upon this scattering object is identical

to that of a plane wave in a free space. However, the scattered field produced by the object in the TEM cell is different from the scattered field produced by the object in a free space, because of multiple reflections from

the TEM cell walls, or equivalently, the mutual coupling between the object and the TEM cell. The purpose of

this paper is to discuss the loading effects, i.e., the electromagnetic field distortion caused by an object

under test in a TEM cell. In the theoretical analysis, the frequency domain integral equation for the magnetic

field, or equivalently, the current density on the surface of a perfectly conducting cylinder in a parallel

plate waveguide is solved by the method of moments to predict the degree of magnetic field distortion. The ex- perimental investigations are performed by mounting a number of electrically small half loops on the surface of

the perfectly conducting cylinder in a TEM cell. The loading effects in terms of magnetic field distortion are analyzed as the ratio of one of the object dimensions (height) to the separation distance between the inner

conductor and the ground plane of the TEM cell. Also, the response of an electrically small loop to both the

magnetic and electric components of the electromagnetic field is used to measure the phase relation between

the magnetic and electric fields, which in turn can be used to determine the degree of degradation of the TEM mode due to the presence of the perfectly conducting cylinder. These theoretical and experimental results are compared with the available quasi electrostatic results.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Electromagnetic Compatibility (EMC); Green's function; integral equation; linear

equation; method of moments; parallel plate waveguide; quasi electrostatic; TEM cell

18. AVAILABILITY (^[Unlimited

| | For Official Distribution. Do Not Release to NTIS

|X3 Order From Sup. of Doc, U.S. Government Printing Office, Washington, DC

20402

| | Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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(THIS REPORT)

UNCLASSIFIED

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UNCLASSIFIED

21. NO. OF

PRINTED PAGES

48

22. Price

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