NBSIR 81-1638

NBS

PUBLICATIONS

A111D4 5 5 1 5 2 M

EVALUATION OF A REVERBERATION CHAMBER

FACILITY FOR PERFORMING EM RADIATED FIELDS

SUSCEPTIBILITY MEASUREMENTS

Myron L. Crawford

Electromagnetic Fields Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

*QC—111 100

. U 56

81-1638

1981

c.2

February 1981

NBSIR 81-1638

ATtONAL BUREAU

OF STANDARDS

LIBRARY

JUl 2 0 1981

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EVALUATION OF A REVERBERATION CHAMBER

FACILITY FOR PERFORMING EM RADIATED FIELDS 7

SUSCEPTIBILITY MEASUREMENTS

Myron L. Crawford

Electromagnetic Fields Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

February 1981

U S. DEPARTMENT OF COMMERCE, Malcolm Baldridge, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
. > '

'
TABLE OF CONTENTS

Page

List of Figures and Tables...... iii

1.0 Introduction. .1 2.0 Reverberating/Mode Tuned Chamber Theory of Operation.. ...2

3.0 Description of TEMEC Measurement Setup and Procedure and NBS Evaluation

System 5 4.0 TEMEC Analysis/Evaluation Results ......7

4.1 Transmit and Receive Antennas VSWR.. 7 4.2 Chamber Coupling Efficiency/Insertion Loss.......... .8

4.3 Test Zone E-Field Uniformity.................... ..8

4.4 E-Field Amplitude Calibration 9 5.0 Conclusions 10

6.0 Recommendations. .11

7.0 Acknowl edg ments 12

8.0 References 13

LIST OF FIGURES AND TABLES

Page

Figure 1. Cross sectional views of TEMEC (Mode Tuned Chamber) 14

Figure 2. Photograph of field tuner/stirrer installed in 2.4m x 3.0m x 7.6m

shiel ded enclosure. 15

Figure 3. Two dimensional graphic interpretation of modes inside resonant

chamber 16

Figure 4. Mode tuned enclosure EMC measurement system 16

Figure 5. Block diagram of NBS modified mode tuned enclosure evaluation

system for EMC measurements 17

Figure 6. NBS isotropic E-field probe and scanning system ...18

Figure 7. Input VSWR of long wire transmit antenna as a function of tuner

position 19

Figure 8. VSWR of transmitting and receiving antennas. Tuner in fixed

position 22

Figure 9. VSWR of 10 dB attenuator-power detector used to measure received

power from reference antenna.... ...23

Figure 10. Minimum and average insertion loss of TEMEC (mode tuned chamber)

with and without correction for net input power changes.

Uncorrected based on incident power. Corrected based on net

input power 24

Figure 11. Block diagram of system for evaluating E-field uniformity in test

zone of TEMEC 25
Figure

Fi gure

Figure

Fi gure

Figure

Figure

Figure

Figure

Table

Table

Table

Table

Table

12. Cross sectional views of TEMEC showing location of transverse and

longitudinal scans 26

13. Results of E-field uniformity measurements taken along transverse

scans through center of TEMEC test zone. Results are maximum

amplitudes obtained from measurements made for complete revolution

of tuner (unless otherwise indicated) as a function of probe

position 27

14. Results of E-field uniformity measurements taken along

longitudinal scans through center of TEMEC test zone 0.914 m above

floor. Results are maximum amplitude of E-field obtained from

measurement of hermitian magnitude of E-field using NBS probe 30

15. Results of E-field uniformity measurements made using NBS

isotropic probe at 200 MHz. Computer based system used to correct for changes in net input power as a function of tuner

and probe position. Scan made along length of chamber

(longitudinal) through center of TEMEC test zone 0.914 m above

floor 31

16. E-field amplitude inside TEMEC as determined from NBS calibrated

isotropic probe measurements. Probe located at center of chamber 32

17. E-field amplitude inside TEMEC as determined from receiving

antenna output power measurements 34

18. Maximum and average E-field in TEMEC chamber as determined by NBS

calibrated probe and receiving antenna power measurements. E„

determined from NBS probe measurements. E^ determined from H receiving antenna power measurements 36

19. Difference between E-field inside TEMEC as determined by NBS probe

and receiving antenna power as a function of probe position along

a longitudinal scan at center of test zone one meter above

floor. Probe motion is from door end (receiving antenna location)

toward transmit antenna (back) 37

Potential resonant modes inside TEMEC (2.44m x 3.05m x 7.62m

chamber) below 125 MHz

Summary of maximum variation in E-field along scans through TEMEC

test zone. Scans + 1 meter from center of chamber. E-field

measured using NBS isotropic probe

Comparison of maximum E-field determined from NBS probe

measurements made with a single dipole switched on and with all 3 orthogonal dipoles switched on (isotropic). Data gives maximum

and minimum values of maximum E-field determined for a complete

rotation of the mode tuner at discrete points along a transverse

scan across the width of TEMEC (i.e., data shows influence of

polarization of field in chamber)..

Difference between E-field determined from probe and receiving

antenna measurements with and without correcting for net input

power changes to transmitting antenna (i.e., error possible from

failure to normalize probe and antenna output to net input power

changes)

Summary of difference between E-field amplitude determination

inside TEMEC using NBS calibrated probe, E„ and receiving antenna,

c a c - r r ^

38

38

39

39

39

-TV-
Evaluation of A Reverberation Chamber Facility for

Performing EM Radiated Fields Susceptibility Measurements

Myron L. Crawford

Electromagnetic Fields Division

National Bureau of Standards

Boulder, Colorado 80303

This report describes measurement procedures and results for

evaluation of a large 2.44m x 3.05m x 7.62m reverberating chamber

facility in the frequency range (100-1000) MHz. This facility,

referred to as a translational electromagnetic environment chamber,

"TEMEC" , is a mode tuned chamber developed for use in measuring

electronic equipment susceptibility to EM radiated fields. A brief

description of mode tuned cavity theory is given along with a description of the TEMEC measurement setup and the National Bureau

of Standards modification of this setup for analysis and evaluation

purposes. Measurements described include: (1) evaluation of the

chamber's transmitting and receiving antenna voltage standing wave

ratio; (2) measurement of the chamber's insertion loss or coupling

efficiency versus frequency; (3) determination of E-field uniformity

in the chamber test zone versus frequency; and (4) determination of

the absolute amplitude calibration accuracy of the test fields in

the chamber based upon the receiving antenna received power

measurements.

Conclusions given indicate the chamber may be useful for

performing electromagnetic susceptibility measurements at frequencies down to 200 MHz. E-field variations (time averaged) in

the chambers test zone decrease from approximately 10 dB at 200-300

MHz to less than 3.7 dB at 1000 MHz and are anticipated to be less

than 2 dB above 2 GHz. The uncertainty in establishing the absolute

E-field amplitude in the chamber test zone is estimated to be less

than 10 dB.

Key words: Electromagnetic radiated susceptibility measurements;

mode tuned chamber; resonant cavity.

1.0 Introduction

Use of a mode tuned (reverberating) chamber for establishing electromagnetic (EM)

fields is a relatively new method which appears to offer some unique advantages and

potential for performing EM radiated susceptibility testing of electronic equipment.

However, significant unanswered questions exist concerning the credibility of results

obtained using this new method relative to correlation of the results to a free-space

envi ronment

These questions basically fall into two categories. The first category relates to how

well the chamber operates as a reverberating or highly moded chamber to establish time

averaged (statistically) uniform, susceptibility test fields whose amplitude can be

accurately determined as a function of test zone location and frequency. Measurements to

evaluate this category of questions are performed with the chamber essentially empty (that

is, no electronic equipment or test objects are placed inside which significantly interact

or modify the EM field established inside the chamber). These measurements consist of:
(a) evaluating the chamber's transmitting and receiving (reference) antennas voltage

standing wave ratios (VSWRs); (b) measuring the chamber loss or coupling efficiency versus frequency; (c) determining the E-field uniformity averaged over tuner position in the

chamber test zone versus frequency; and (d) determining the absolute amplitude calibration

accuracy of test fields in the chamber. Results of these measurements performed in the

TEMEC chamber are contained in section 4 of this report, following a brief discussion of

reverberation/mode tuned chamber theory of operation (section 2), and the description of the

TEMEC measurement system, (section 3). Conclusions arrived at from the results given in

section 4 are contained in section 5. The second category of questions for evaluation relate to the interaction effects that

exist between an equipment under test (EUT) placed inside the chamber and the ability, after

inserting the EUT into the test chamber, to accurately characterize the test fields.

Obviously, inserting an EUT into the chamber will influence the chamber quality factor or Q

and its boundary condition, and hence will alter the field that was present before

introducing the EUT. An additional question concerns the ability of the enclosure's complex

field to couple to the EUT as compared to how a planar (far-zone) EM field would couple to the EUT in free-space. Obtaining answers to these questions are beyond the scope of this

contract and are proposed in section 6 as important follow-up work.

boundary conditions are continuously and randomly perturbed by means of a rotating

conductive tuner or stirrer. The time average field inside such a cavity, when sufficient

modes are excited, is assumed to be uniform (in level) and formed by uniformly distributed

plane waves coming from all directions [1,2]. Resonant modes can exist in the cavity when

any of the waveguide modes appropriate to the cavity cross section are a multiple of a half

wavelength of the cavity length. The expression for calculating the frequencies of the

cavity modes is given as:

mode numbers, and a, b, and d are the cavity dimensions in meters.

Dimensions for the TEMEC chamber are shown in figure 1. Table 1 gives the frequencies

for the first few modes up to approximately 125 MHz. Inside a reverberating chamber, the EM field can be characterized by a succession of

independent steady state fields that are excited by the long wire transmitting antenna and

stirred or tuned by a metal tuner shown in the photograph of figure 2. The efficiency with which energy can be injected into the chamber is determined by the

VSWR or impedance match between the rf source and the transmitting antenna, and by the

2.0 Reverberating/Mode Tuned Chamber Theory of Operation

A reverberating/mode tuned chamber is a large (in terms of wavelength) cavity whose

( 1 )

where c is the speed of light = 3 x IQ

8 meters/sec, m, n and p are integers representing

2
ability of the transmitting antenna to couple energy into the particular modes at the

appropriate frequencies. This coupling is enhanced by the low-directivity characteristic of

the antenna and by the tuner's ability to change the chamber boundary conditions, and hence,

shift the mode frequencies to coincide with the source excitation frequency. A simple way

to explain the tuner influence is by the graphical interpretation for a rectangular twodimensional TE case shown in figure 3 [3]. The basic resonance equation is given as

where m and n are mode numbers, a and b are cavity dimensions, in meters, and k's are the wave numbers.

Figure 3 gives the modes intercepted for particular frequencies (or wavelengths)

2tt determined by k mn

= . The modes can be shifted, depending upon the Q factor of the

mn chamber and the geometry of the tuner, as indicated by the shaded area. The relationship

for determining the boundaries for this area is given as

Of course, not all modes in the shaded area (figure 3) will be present, but only those modes

which are excited by the source transmit antenna. Also, not all modes that are excited will

be influenced by the tuner, but only those which are influenced by its motion.

The efficiency with which power is received by the receiving antenna is influenced by a number of factors in addition to those mentioned above. These include power loss in the

chamber walls, power leakage from the chamber, and the coupling characteristics of the

receiving antenna relative to the existing modes in the chamber. The ability to measure the

power available at the receiving antenna is also influenced by the receiving antenna and

power detector VSWRs (i.e., the impedance mismatch that occurs between the two).

Obviously, if the excitation frequency is high enough, the modal density (number of

modes in the shaded area) is so large that omnidirectional, uniform fields, averaged over

all tuner positions, can be achieved inside the chamber, even with a small tuner. In such a condition, directivity characteristics of the transmit and/or receive (reference) antenna

are washed out and the antennas appear as if they were omnidirectional at these frequencies

(for example above 1-2 GHz for a chamber the size of the TEMEC). Continuous modal coverage

(at least one mode for each tuner position) is obtained. One would expect field

uniformities (time averaged) of better than +2 dB. If the number of modes is large, N can

be calculated from the expression [4].

and the mode density is given by

( 2 )

(3)

dN 4* _V _ 3N

df f *3 " f

( 5 )

3
V is the cavity volume in equations (4) and (5).

For the TEMEC chamber, V = 56.7 m

3

. At 1 GHz there could be as many as 8796 modes with

a mode density of 26 x 1CT 6. This means that if the tuner could shift the chamber's modal

characteristics equivalent to a frequency shift of 1 MHz at 1 GHz, twenty-six modes could be

influenced, (i.e., aN = 26). In comparison, at 200 MHz, only approximately 70 modes can exist with a mode density of 1.06 x 10 _6

» This means that if the tuner could shift the

chamber's modal character!' sties equivalent to a frequency shift of 1 MHz at 200 MHz, only

one mode would be influenced (i.e., aN = 1).

At lower frequencies (e.g., below 1-2 GHz for the TEMEC) continuous modal coverage is not achieved and the directional characteristics of the particular modes excited start to become apparent. This means that only some EM waves coming from fixed directions are present. Hence, there is a need to design a tuner with three-dimensional tuning

characteristics, such as the tuner employed in the TEMEC facility. Also, the transmitting

antenna must have omnidirectional properties as much as possible. Again, the long wire

antenna used in the TEMEC chamber appears to give satisfactory performance. An additional

way to enhance the low frequency modal coverage is to lower the Q of the chamber by

increasing chamber loss or leakage. From cavity theory, the Q of a chamber or cavity is given as [5]:

Q “ S6

where V is the cavity volume in meter3

, S is the cavity internal surface area in meters^,

and 6 is skin depth = / —

This expression must be considered a maximum or upper bound for a chamber because it assumes the chamber losses are due only to finite wall conductivity.

For most chambers, significant loss occurs from leakage as well as from finite

conductivity in the enclosure walls; hence, a more meaningful determination of the average

Q, (Q

1 ), can be obtained from measuring the chamber's insertion loss [6]. Insertion loss is defined as the ratio of the power radiated from the chamber transmitting antenna, P

t

, to the

power received by the chambers reference antenna, P r

. Assuming uniform energy distribution over the volume of the cavity, the expression for

Q' is then:

V P

Q' - 16’2 13 p“ < 7 >

If we assume continuous modal coverage, as required for eq. (7) to be valid, the bandwidth

of each mode must be larger than the modal distance (separation in frequency between modes)

or

f df fx3

Q

> dN ~ 4tt V

’ ( 8 )

If Q is large for a particular chamber, the bandwidth of the modes will be small and

4
continuous modal coverage will be obtained, only if frequency is sufficiently high. For

example, insertion loss for the TEMEC chamber is very small indicating a high Q, hence,

continuous modal coverage is not obtained, as indicated by the results discussed in

section 4, at frequencies below (200-300) MHz.

3.0 Description of TEMEC Measurement Setup and Procedure

and NBS Evaluation System

The basic TEMEC susceptibility measurement system is shown in figure 4 [7]. The EUT

may be placed anywhere convenient within the chamber provided no point on the EUT is closer

than one meter to any wall, the floor, or ceiling. Test, power, and control cables are routed to appropriate monitors, etc., outside the enclosure via filtered feedthrough as required to prevent leakage of fields to the outside environment. The susceptibility test

fields are generated inside the enclosure by coupling rf power to the transmitting antenna

from an rf source. The electric field strength, E^, is determined by measuring the pickup

power on the receiving antenna through one complete rotation of the field tuner/stirrer for

a signal generator power large enough to provide sufficient measurement sensitivity. The

received power, P^, is sampled while the tuner/stirrer is rotating a minimum of 100 times

and the average and peak power are determined.

The power density, Pp, measured in free space using an impedance matched, lossless,

receiving antenna is given by [8]

P 4rrP

P D

= TR

= UI WaMS/m2 (9)

GX2 where A

R (the effective aperture of the receiving antenna) = — G is the antenna gain, and

A is the wavelength in meters. If the receiving antenna is subjected to a plane wave in

free space at a sufficient number of different aspect angles, an antenna power pattern can

be obtained from which the average response may be determined. The average gain over 4ir solid angle is unity and hence eq. (9) can be rewritten in terms of average quantities as:

4tt P

P D

=

- y2-r watts/m2 (10)

where P is the average received power over a 4tt solid angle.

In the mode-stirred chamber case, a similar relationship exists if the field

distribution at each point in the antenna aperture plane is assumed to be a composite of

randomly polarized plane waves. This implies, of course, that the orientation of the

receiving antenna will not influence the measured response and hence the effective gain of

the antenna is unity.

For the mode-stirred chamber, the "equivalent" average power density is given as [8]

4ttP

1

P^ = —^2~~ watts /m2 (11)

5
where is the average received power in the chamber determined from the receiving antenna

response, P^, at a sufficient number of different mode tuner positions.

A peak or maximum response can also be determined from the set of measurements

resulting from the different mode tuner positions. It is interesting to compare the peak

response or peak power density as well as the average power density as a function of

location inside the chamber for a fixed input power to the chamber. This may be important

because an EUT may respond to the peak of the exposure field as opposed to the "average"

field.

In performing susceptibility tests, the input power, , is adjusted as required to obtain the test level power density desired as determined from eq. (11), and the measurement

of P^ with the EUT in place. The equivalent electric field, E^, is then found by using the

expression

where n is the intrinsic wave impedance in the chamber. Equation (12) assumes planar farfield conditions. Obviously, such conditions do not exist inside a multimded chamber;

hence, the equation's validity is questionable. However, since data to determine E A

is

statistical ly averaged from a set of measurements made at many different tuner positions

over a complete rotation of the tuner, using an average value of 120 tt ohms for the wave impedance may be reasonable.

One of the major objectives of this evaluation program (as stated earlier) was to experimentally determine the validity of eq. (12) (i.e., the absolute amplitude calibration

accuracy of the test fields in the chamber). To accomplish this, the TEMEC test system was modified slightly by NBS for evaluation purposes. A block diagram of the modified system is

shown in figure 5. The purpose of using a calibrated bidirectional coupler is to measure

the net power flow to the transmit antenna as opposed to measuring only the incident

power. This allows corrections to be made for changes in the net input power resulting from

antenna-rf generator impedance mismatch and rf generator output variations that occur during

a complete rotation of the field tuner. The bidirectional coupler was also used to evaluate

the VSWR of both the transmit and reference antennas. This was achieved by measuring the

return loss from the antennas when rf power was applied at their input/output terminals

through the coupler.

A precision 10 dB attenuator and power detector were used in place of the calibrated

receiver to measure the receiving antenna power. This was done to minimize impedance

mismatch with the receiving antenna.

Mapping of the field distribution and determination of the absolute field levels were made using the NBS calibrated isotropic E-field probe [9] and scanning system shown in

figure 6. The system is made of dielectric material and is designed for a minimum

perturbation/interaction with the test fields established in the chamber. The field probe

for evaluating the chamber serves as a standard EUT and causes minimal perturbation of the

test field.

( 12 )

6
4.0 TEMEC Analysis/Evaluation Results

4.1 Transmit and Receive Antennas VSWR

VSWR measurements were performed to determine potential error resulting from changes in net input power occurring during a complete rotation cycle of the tuner. These measurements

were also made for the transmitting antenna and receiving antenna as a function of frequency

for a fixed tuner position. Results of these measurements are shown in figures 7 and 8.

The long wire transmitting antenna maximum VSWR is high (35:1) at 100 MHz but decreases

substantially as the frequency increases. In contrast the maximum VSWR of the receiving

antenna remains high (>10:1) over the measured frequency range of 200 MHz to 1.0 GHz.

The received power available at the antenna terminal was measured using a 10 dB

precision attenuator and power detector with a combination VSWR shown in figure 9. If

actual power delivered to the load (power meter) is to be compared with the maximum

available from the source (receiving antenna), a conjugate impedance match must exist.

Power transfer between a source and a load of reflection coefficients r<. and is

given as

fraction of maximum

P = available power absorbed

by the load

(1 - r s

2

) (1 - f

L

csT I

1 . r r S L 2 (13)

where and r L denote reflection coefficient magnitude and phase.

|r<.| and jr^| can be obtained from the appropriate VSWR by the expressions

i VSWR - 1 |r S' VSWR + 1

'

If the load (power detector) VSWR is 1.065 (worst case) and the receiving antenna

(source VSWR) is as great as 10.0,

10 - 1 10 + 1 .8182

1.065 - 1 1.065 + 1 .0315

P F

= .3219 = -4.92 dB

mi n

P

F = .3479 = -4.59 dB max

or an error as large as -4.92 dB could exist in the measurement of the receiving antenna

power. Obviously this error could be reduced, at least at frequencies above 300 MHz, if the

receiving antenna VSWR was improved to be comparable with the transmitting antenna.

Considerable error can also exist, as mentioned earlier, in the determination of the chamber

test field level if the net input power is assumed constant during a complete revolution of

the tuner. The normal calibration procedure appears to make this assumption. This error is

discussed in more detail in section 4.4.

7
4.2 Chamber Coupling Efficiency/Insertion Loss

The chamber coupling efficiency or loss was determined at a number of discrete

frequencies as shown in figure 10. These data were obtained at each frequency by measuring

the net input power ( Pj nc i dent “ ^reflected) t0 t *ie transmitting antenna, and by measuring

the receiving antenna power, for 100 stepped positions equal to one full rotation of the

tuner. Changes in the net input power were normalized to give a receiving antenna output

proportional to a constant net input power. These data for the minimum and average

insertion losses are shown as the solid curves in figure 10. The dashed curves show the

minimum and average losses determined without normalization or correction for net input

power changes. It is interesting to note that even though large point by point differences

occur between the normalized and raw data, statistically the raw and normalized data

minimums and average loss values agree quite well.

4.3 Test Zone E-Field Uniformity

E-field uniformity in the test zone of the TEMEC chamber was evaluated as a function of

position along two orthogonal scans using the measurement system shown in figure 11.

Location of the two scans are shown in figure 12. Measurements were made of each orthogonal

component of the E-field (E v

= vertical, Ey = transverse, and E^ = longitudinal) and the

hermitian magnitude, E^ = / E^ + E^. + E^ . Measurement results obtained along the

transverse scan (across the width of the chamber), at 5 frequencies are given in

figure 13. These data were obtained by measuring the maximum E-field components and

magnitude at (5-10) cm increments in position along the 2 meter scan. No attempt was made

to normalize these measurements to correct for net input power changes as a function of

tuner or probe position. The minimum of the E-field magnitude, E , at 500 MHz and

"min

700 MHz is also given. At 1000 MHz, only E p maximum is given. Notice that as the frequency

increases, the variation in E with position decreases. The measurement results are nnax summarized in Tables 2 & 3. The difference between E v , E T , E, , and E n (shown in

v max 'max L max Fmax Table 3) indicates the chamber discrimination to EUT radiation directivity and

polarization. Recall that if sufficient modes or mode tuning exists, the chamber will

couple the EUT radiation independent of its polarization. Obviously, this is not happening

at the lower frequencies but is becoming less polarization dependent as the frequency

increases. An analysis of the data of figure 13, especially at 200 MHz, gives some indication of which modes exist inside the chamber. The relationship between E n and nnax

E n in figure 13c and 13d is also worth noting. At 200 and 300 MHz, the E n was less

Pmin nnin

than the resolution of the probe and instrumentation. This indicates the tuner capability

to influence the amplitude of the excited modes at positions along the scan. As the

frequency increases, the number of modes excited increases rapidly and the tuner has more effect on coupling the fields to the probe (EUT). Recall from section 2 that at frequencies

for which continuous modal coverage exists, minimal tuning is required. At these

frequencies (above approximately - 2 GHz) E will approach E n in amplitude and the

nil in nnax

8
time averaged field variations with position will approach zero. E-field measurements were also made along longitudinal scans (along the length of the

chamber) indicated in figure 12. These results, shown in figure 14, give the variations in

E n with measurement position for 100 MHz, 200 MHz and 300 MHz. The mode characteristic

^max

at 100 MHz is very evident. Again, these data were not corrected for changes in net input

power that occur as a function of tuner position and probe location.

Figure 15 gives the results of measurements made at 200 MHz using the computer based

system (figure 5) to correct for variations in the net input power. Results are given for

both the maximum E p and the average E

p as a function of position. The maximum E p can be

compared to figure 14 to give an indication of errors that exist (at 200 MHz) due to failure

to correct for input power variations. An estimate of the error anticipated by failure to normalize the measurements of E p and E A to an equivalent constant net input power can be

obtained from figures 16 and 17 and the results are summarized in Table 4.

4.4 E-Field Amplitude Calibration

Measurements were made to determine the absolute amplitude of the E-field inside the

TEMEC chamber relative to planar E-field calibrations. The NBS probe used to make these

measurements was calibrated using a TEM cell [10] and anechoic chamber to determine its

output response as a function of absolute E-field (far-field) level at the calibration

frequencies. The probe was placed at the center of the TEMEC test chamber with all three

channels operating to give an isotropic response to the field. This measured E-field

amplitude, Ep, was then compared to the E-field amplitude, E^, determined from a simultaneous measurement made of the power available at the terminals of the receiving

antenna and calculated from eq. (11). The results of these measurements as a function of

tuner position are shown in figures 16 and 17. The solid curves gives the E-field corrected

for net input power variations (i.e., the probe and receiving antenna output response were adjusted to give the equivalent for a constant (normalized) net input power.) The dashed

curves are the results obtained from raw data (i.e., before a correction was applied for net input power variations). The maximum and average E-field values obtained from the data

shown in figures 16 and 17 are summarized in the graphs of figure 18. The differences in the E-field as determined from the probe measurements and the receiving antenna received

power measurements are summarized in Table 5. The accuracy of these results is dependent on the ability to determine Ep and E^ from

their appropriate measurements and computations. The calibration accuracy for the probe

E-field measurements is estimated at approximately + 1 dB [9]. The uncertainty in the

E-field determination based on measuring the maximum and average available power from the

receiving antenna is difficult to estimate. This uncertainty is caused by the

contribution in the uncertainties of the measurement parameters and the validity of the

assumption made in equation (11). The purpose of Table 5, of course, is to provide the

basis for estimating the accuracy in E^ by comparing E^ with the calibration probe field

measurement.

9
It is interesting to compare the normalized (corrected for input power fluctuations)

difference between the measured E-field inside the TEMEC chamber as determined using the NBS

probe and the chamber receiving antenna as a function of the NBS probe's position in the

chamber. Figure 19 gives an example of the results obtained at 200 MHz for maximum and

average E p and EA along a longitudinal scan. Note that as the probe moves away from the

receiving antenna toward the transmitting antenna, the measured field increases and the

difference between E p and E^ increases.

5.0 Conclusions

The following conclusions can be drawn from the results of the evaluation of the TEMEC

chamber.

(1) Modes are excited and effectively tuned in the chamber at frequencies down to

100 MHz. VSWR of the transmitting antenna is high at low frequenices (approaching

200 MHz and below) but improves rapidly with increasing frequency. This antenna

design appears suited for this application. The tuner employed in TEMEC also is

effective and appears to work satisfactorily.

(2) The long wire used as the receiving antenna has a very high VSWR across the test

frequency range. This can contribute to large errors (up to 5 dB) in received,

available power measurements due to mismatch. It would be preferable to use an antenna similar to the transmitting antenna with improved VSWR characteristics.

(3) Mode structure is strongly present at 100 MHz (field variation of 15.3 dB in test

zone) and at 200 MHz (field variation of 6.5 dB in test zone), but decreases as frequency increases (3.7 dB at 1000 MHz).

(4) The E-field is also polarization dependent inside at the lower frequencies. This

effect is apparent up to at least 700 MHz but is decreasing with frequency (i.e.,

figure 13 and table 3 indicate average differences between E

p

, Ey, E[_, and E^ of

approximately 6 dB at 200 MHz to approximately 4 dB at 700 MHz).

(5) The insertion loss is low, (average loss of 4.5 dB at 100 MHz to 9.5 dB at 1000 MHz, see figure 10) indicating the chamber has a high Q. This will limit modal coverage at lower frequencies where the mode density is small. It may be advisable to increase

the chamber loss by a few dB to improve low frequency field uniformity.

(6) The potential difference in determining maximum and average E-field in the chamber

with or without correcting for net input power changes is 4.7 dB at 100 MHz,

decreasing to 0.35 dB at 1000 MHz (Table 4) by using the receiving antenna.

(7) The difference in determining the absolute level of the E-field in the test zone using

the NBS calibrated, isotropic probe as compared to using the chamber's receiving

antenna is between 3.9 dB - 8.2 dB (Table 5). This corresponds to an apparent average

systematic offset error of 6.2 dB (i.e., the measured E-field inside the chamber is

10
6.2 + 2.3 dB greater than the E-field calculated from eq. (12) using the chamber's

receiving antennas received power measurements). It is important to note this

apparent offset error results from a limited set of data for this particular chamber

and should not be considered conclusive for application to mode tuned chambers in

general. Additional research is needed to substantiate these results at additional

frequencies and with other chambers. Some suggested reasons for an offset error,

however, include, but are not necessarily limited to:

a) Errors in measuring the total power available at the chamber's receiving

antenna. This can result from the impedance mismatch that exists between the

antenna and its power detectors as a function of the chamber's mode-tuner

position;

b) The fact that maximum coupling of power from the source connected to the

transmitting antenna, thence to the excited modes in the chamber and thence to

the receiving antenna, is not likely to occur all simultaneously for any modetuner position; and

c) Evanescent modes, as well as propagating modes, may exist inside the chamber

which may contribute to the total field. Such modes are not included in eq.

(IDThe mode tuned concept (TEMEC), using a computer based system for control, data

acquisition, and data analysis, is superior to continuous mode stirring. This is apparent

due to the ability to monitor and normalize output to input parameters variations and to

control test-field variation time constants. The field exposure time constant is important

when performing equipment susceptibility tests and must be sufficiently long to allow the

EUT time to respond.

The TEMEC facility can be operated as a mode tuned chamber at frequencies down to

100 MHz. Accuracy in establishing the test field level in the chamber as determined by

receiving antenna power measurements may be reasonable, assuming a systematic offset

correction before inserting an EUT. However, the influence of an EUT on the test field was

not determined in this study. This effect could be substantial and should be analyzed

before the uncertainty in establishing test fields with the EUT present can be determined

(see section 6, Recommendations).

6.0 Recommendations

Results of this study, coupled with other research into the mode tuned/stirred

measurement technique, indicate that this technique may have considerable potential for

performing EM susceptibility measurements. Unresolved questions, referred to in section 1 of this report, emphasize the need to perform the following tasks.

(1) Perform measurements to evaluate interactions between the EUT, the test field, and the

field generation equipment. This would require measurements to determine the ability

11
to establish the absolute level of test field in the chamber after insertion of the

EUT. These results should be carefully analyzed to determine if a systematic offset error exists as suggested by the results of Table 5 and conclusion 7 of this report,

(i.e., if the actual test field is consistently greater than the field calculated from

the receiving antenna power measurements).

(2) Analyze the ability of the enclosure's complex field to couple to the EUT as compared

to open-space planar field coupling. This may require measurements of rf currents

coupled onto EUT input/output and power line cables and on EUT enclosures.

(3) Analyze, where feasible, the boundary value problem associated with inserting an EUT

inside the enclosure.

7.0 Acknowledgements

Work described in this report was sponsored by the Pacific Missile Test Center, Point

Mugu, California, with Mr. Ken Canoga as project monitor. The facility evaluated is located

at McDonnell Douglas Astronautic Corp. East, St. Louis, Missouri.

The author wishes to acknowledge the assistance of James Roe and Ed Gardner of

McDonnell Douglas Astronautics Company for their assistance and cooperation in performing

the measurements discussed in this report. The author further recognizes the benefit of the

discussions he had with Dr. Mark T. Ma (NBS) in both developing approaches to evaluate the

facility and in analyzing and explaining the results of data obtained. He also appreciates

the assistance of Drs. Carl F. Stubenrauch (NBS) and David C. Chang (University of Colorado)

in reviewing this report.

12
8.0 References

[1] Ramo, S. and Whinney, J. R., Fields and Waves in Modern Radio, 2nd ed., (pp 418-428),

(Wiley, New York, November 1960).

[2] Corona, P., Latmiral, G., Paolini, E., and Piccioli, L., Use of a Reverberating

Enclosure for Measurements of Radiated Power in the Microwave Range, IEEE Trans, on EMC

EMC-18, [2] (May 1976).

[3] Corona, P., Instituto Universitario Navale, Napoli Italy - Private Communication.

[4] Mendez, H. A., A New Approach to Electromagnetic Field-Strength Measurements in

Shielded Enclosures, Wescon Technical Papers (1968).

[5] Argency, E., and Kahan, T., Theory of Waveguides and Cavity Resonators (Hart Publishing

Co., New York, DATE?).

[6] Corona, P., Latmiral, G., and Paolini, E., Performance and Analysis of Reverberating

Enclosures with Variable Geometry, IEEE Trans, on EMC EMC-22, [1], (Feb 1980).

[7] Cumming, J. R., Translational Electromagnetic Environment Chamber, A New Method for

Measuring Radiated Susceptibility and Emissions, IEEE International Symposium of EMC,

San Antonio, Texas, IEEE 75 CH 1002-5 EMC (October 1975).

[8] Bearn, J.L. and Hall, R.A., "Electromagnetic susceptibility measurements using a modestirred chamber," IEEE International Symp. n EMC, Atlanta, GA, June 20-22, 1978,

78-CH-1304-5 EMC.

[9] Bowman, R. R., Larsen, E. B., Belsher, D. R., and Wacker, P. F., Second Progress

Report: Electromagnetic Hazards Project, NBS Report, unpublished (September 18, 1970)

Private communication.

[10] Crawford, M. L., Generation of Standard EM Fields for Calibration of Power Density

Meters 20 KHz to 1000 MHz, NBSIR 75-804 (January 1975).

13
u

ino

co

a

(NID

-H

Q< > , W W \ ffi m

•J

!=> CQ

(XoH

T&W

i-( >w

mo® CO

JL

14

Figure

1.

Cross

sectional

views

of

TEMEC

(Mode

Tuned

Chamber).
Figure 2. Photograph of field tuner/stirrer installed

in 2.4m x 3.0m x 7.6m shielded enclosure.

15
Figure 3. Two dimensional graphic interpretation of modes inside resonant chamber.

TUNER CONTROL

Figure 4. Mode tuned enclosure EMC measurement system.

16
17

Figure

5.

Block

diagram

of

NBS

modified

mode

tuned

enclosure

evaluation

system

for

EMC

measurements.
S3*

mm wmwi

18

Figure

6.

NBS

isotropic

E-field

probe

and

scanning

system.
Figure 7. Input VSWR of long wire transmitting antenna as a function of tuner position

19
HMSA

20.0

300 MHz

Tuner Position (*)

20
HMSA

N

H

”

HMSA

21
22

Figure

8.

VSWR

of

transmitting

and

receiving

antennas.'

Tuner

in

fixed

position.
HMSA

23

Figure

9.

VSWR

of

10

dB

attenuator-power

detector

used

to

measure

received

power

from

reference

antenna.
o

(OP) ssoq aaqureqo

24

Figure

10.

Minimum

and

average

insertion

loss

of

TEMEC

(mode

tuned

chamber)

with

and

without

correction

for

net

input

power

changes.

Uncorrected

based

on

incident

power.

Corrected

based

on

net

input

power.
25

Figure

11.

Block

diagram

of

system

for

evaluating

E-field

uniformity

in

test

zone

of

TEMEC.
81m

T

T

COY £

•H>

COCD

T~\ \ V \

<DU

•H cS Es cc

be CD \ c +j o a J < V\ M

X

G rH

•H CS P > G 03 •H G

CD CD

CD 4-> <D G

PS < K £

lOo

CO

-><E-

•s

<D •H>

-aGw o

26

Figure

12.

Cross

sectional

views

of

TEMEC

showing

location

of

transverse

and

longitudinal

scans.
Figure 13a.

Figure 13b.

Figure 13. Results of E-field uniformity measurements taken along transverse scans through

center of TEMEC test zone. Results are maximum amplitudes obtained from

measurements made for complete revolution of tuner (unless otherwise indicated)

as a function of probe position.

27
E-field

(dB

V/ra)

Figure 13c.

Figure 13d.

28
-field

(dB

V/ra)

Figure 13e.

29
o

30

Figure

14.

Results

of

E-field

uniformity

measurements

taken

along

longitudinal

scans

through

center

of

TEMEC

test

zone

0.914m

above

floor.

Results

are

maximum

amplitude

of

E-field

obtained

from

measurement

of

hermitian

magnitude

of

E-field

using

NBS

probe.
30

^20

CQ

X)

<D •H<Ww

a

10

Probe Position (m)

Probe Position (m)

Figure 15. Results of E-field uniformity measurements made using NBS isotropic probe at 200

MHz. Computer based system used to correct for changes in net input power as a function of tuner and probe position. Scan made along length of chamber

(longitudinal) through center of TEMEC test zone 0.914m above floor.

31
E-field

(dB

V/m)

,

E-field

(dB

V/m)

E-field

(dB

V/m)

i i i ,

i

; 0 90 180 270 360 Tuner Position (°

32
Figure 16. E-field amplitude inside TEMEC as determined from NBS calibrated isotropic probe

measurements. Probe located at center of chamber.

33
l-field

(dB

V/m)

,

E-field

(dB

V/m)

i

,

E-field

(dB

V/m)

34
E-field

(dB

V/m)

E-field

(dB

V/m)

E-field

(dB

V/m)

Figure 17. E-field amplitude inside TEMEC as determined

from receiving antenna output power measurements.

35
o

36

Figure

18.

Maximum

and

average

E-field

in

TEMEC

chamber

as

determined

by

NBS

calibrated

probe

and

receiving

antenna

power

measurements.

E

p

determined

from

NBS

probe

measurements.

EA

determined

from

receiving

antenna

power

measurements.
Freq =

200AC-I

37

Figure

19.

Difference

between

E-field

inside

TEMEC

as

determined

by

NBS

probe

and

receiving

antenna

power

asa

function

of

probe

psition

along a

longitudinal

scan

at

center

of

test

zone

one

meter

above

floor.

Probe

motion

is

from

door

end

(receiving

antenna

location)

toward

transmit

antenna

(back).
TABLE 1

Mode

Resonant Frequency

MHz

f011 52.97

f 012 63.00

f 101 64.55

f 102 73.00

o

r~H oo 76.85

f 110 78.73

f lll 81.15

O CO 85.25

f 112 88.02

f 113 98.41

f 02

100.31

f 022 105.95

f023 114.73

f 120 116.00

f 121 117.65

f 201 124.52

Potential resonant modes inside TEMEC

(2.44m x 3.05m x 7.62m chamber) below 125 MHz

TABLE 2

Frequency

MHz

Transverse Scan E- Field Variation dB

Longitudinal Scan E- Field Variation dB

100

200

300

500

700

1000

15.3

6.5 3.5

5.7 9.3

5.9

4.3

3.7

Summary of maximum variation in E-field along scans through

TEMEC test zone. Scans +1 meter from center of chamber.

E-field measured using NBS isotropic probe.

38
TABLE 3

Sd

= E v 2+E

L 2+E

t 2

E

v (dB V/m) E

L (dB V/m) E

t (dB V/m)

Frequency (dB V/m)

MHz Max. Min. A Max. Min. A Max. Min. A Max. Min. A

200 30.1 23.6 6.5 26.920.1 6.8 25.5 17.8 7.7 27.5 15.8 11.7

300 29.5 23.9 5.6 27.920.3 7.6 23.2 18.0 5.2 26.5 17.0 9.5

500 31.6 25.7 5.9 24.921.0 3.9 29.6 20.9 8.7 26.0 20.0 6.0

700 31.1 26.7 4.4 28.322.5 5.8 27.8 21.9 5.9 27.5 22.4 5.1

Comparison of maximum E-field determined from NBS probe measurements made with a single dipole switched on and with all 3 orthogonal dipoles switched on (isotropic).

Data gives maximum and minimum values of maximum E-field determined for a complete

rotation of the mode tuner at discrete points along a transverse scan across the

width of TEMEC (i.e., data shows influence of polarization of field in chamber).

Frequency

MHz

E

p Max

dB

TABLE 4

E

p Ave

dB

E a Max

dB

Ea Ave

dB

100 4.8 0.1 4.7 1.0

200 0.1 3.2 0.8 2.7

300 0.2 1.6 1.7 1.6

500 0.4 0 2.2 0.25

700 0.2 0.8 0.7 0.3

1000 0 0.15 0.35 0.2

Difference between E-field determined from probe and receiving antenna

measurements with and without correcting for net input power changes to transmitting antenna (i.e., error possible from failure to normalize

probe and antenna output to net input power changes).

Frequency

TABLE 5

AEmax

AE ave MHz dB dB

100 4.3 4.5

200 8.0 4.5

300 7.6 7.5

500 3.9 4.0

oo 6.8 7.0

1000 7.5 8.2

Summary of difference between E-field amplitude determination inside TEMEC using

NBS calibrated probe, E p and receiving antenna, EA

. aE = E

p

- EA

39
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

N BSIR 8 1-16 3_8_

2. Performing Organ. Report No. 3. Publ ication Date

February 1981

4. TITLE AND SUBTITLE

Evaluation of A Reverberation Chamber Facility for Performing EM

Radiated Fields Susceptibility Measurements

5. AUTHOR(S)

Myron L. Crawford

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State. ZIP) U. S. Navy

Pacific Missile Test Center

Pt. Mugu, California 93041

10.

SUPPLEMENTARY NOTES

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

11.

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

bi bliography or literature survey, mention it here)

This report describes measurement procedures and results for evaluation of a large 2.44m x 3.05m x 7.62m reverberating chamber facility in the frequency range

(100-1000) MHz. This facility, referred to as a translational electromagnetic

environment chamber, "TEMEC", is a mode tuned chamber developed for use in measuring

electronic equipment susceptibility to EM radiated fields. A brief description of

mode tuned cavity theory is given along with a description of the TEMEC measurement

setup and the National Bureau of Standards modification of this setup for analysis

and evaluation purposes. Measurements described include: (1) evaluation of the

chamber's transmitting and receiving antenna voltage standing wave ratio; (2)

measurement of the chamber's insertion loss or coupling efficiency versus frequency;

(3) determination of E-field uniformity in the chamber test zone versus frequency;

and (4) determination of the absolute amplitude calibration accuracy of the test

fields in the chamber based upon the receiving antenna received power measurements.

Conclusions given indicate the chamber may be useful for performing electromagnetic susceptibility measurements at frequencies down to 200 MHz. E-field

variations (time averaged) in the chamber's test zone decrease from approximately

10 dB at 200-300 MHz to less than 3.7 dB at 1000 MHz and are anticipated to be less

than 2 dB above 2 GHz. The uncertainty in establishing the absolute E-field amplitude

in the chamber test zone is estimated to be less than 10 dB.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Electromagnetic radiated susceptibility measurements; mode tuned chamber;

resonant cavity.

13. AVAILABILITY

Uni imited

| | For Official Distribution. Do Not Release to NTIS

: Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[X] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

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15. Price

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