NisrUNITED STATES

DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS

AND TECHNOLOGY

NATL INST. OF STAND & TECH R.I.C.

Allia3 M3mMb

NIST

PUBLICATIONS

NIST Technical Note 1342
NATIONAL INSTITUTE OF STANDARDS &

TECHNOLOGY

Research InformatkMi Center

Gaitharsburg, MD 20899

DATE DUE

Demco, Inc. 38-293
Measurement and Evaluation of

a TEM / Reverberating Chamber

Myron L. Crawford

Mark T. Ma

John M. Ladbury

Bill F. Riddle

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

Prepared for

Electromagnetic Environmental Effects Division

U.S. Army Electronic Proving Ground

Fort Huachuca, Arizona 85613-7110

%

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director

IssuedJuly 1990
National Institute of Standards and Technology Technical Note 1342

Natl. Inst. Stand. Technol., Tech. Note 1342, 116 pages (July 1990)

CODEN:NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1990

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
CONTENTS

Page

List of Figures and Tables v

Abstract 1

1. Introduction and Background 1

2. Anticipated Advantages and Limitations 3

3. Chamber Design and Theoretical Considerations 4

3.1 General Considerations 4

3.2 USAEPG 1/10 Scaled TEM/Reverberatlng Chamber 6 3.3 Proposed Full -Size TEM/Reverberatlng Chamber 7

4. CW Evaluation of the 1/10 Scaled TEM/Reverberatlng Chamber 8

4.1 Measurement Approaches 8 4.1.1 TEM Mode 8

4.1.2 Reverberating Mode 8 4.2 NIST CW Evaluation System - 9 4.3 CW Evaluation and Calibration Results 10

4.3.1 TEM Line and Antenna VSWR and Chamber Coupling

Efficiency 10

4.3.2 Chamber Quality Factor 11

4.3.3 E-Fleld Amplitude Calibration 11

4.3.4 Effects of RF Absorber Loading and Multiple Driven

Plates 13

4.3.5 Tuner Effectiveness 14

4.3.6 Test Zone E- field Uniformity 15

4.3.7 Test Results from 20 GHz to 40 GHz 16

4.3.8 Summary of CW Calibration and Evaluation Results 17

5. Pulsed RF/Tlme Domain Evaluation of the 1/10 Scaled

TEM/Reverberatlng Chamber 18

5 .

1

Background 18

5.2 NIST Pulsed RF Evaluation System 18

5.3 Pulsed RF Evaluation Results 19

5.4 Comments on Pulsed RF Measurement Results 20

6. Projecting the 1/10 Scaled Chamber CW and Pulsed RF Measurement

Results to a Full -Scale TEM/Reverberatlng Chamber 21

7. Performing EMC/V Measurements Using the TEM/Reverberatlng Chamber 22

7.1 Performing EMC/V Measurements Using Manual Operations 22

7 .

2

Automated EMC/V Measurements 25

m
CONTENTS (cont.)

7.2.1 TEM Mode Measurement Procedure 25

7.2.2 Reverberating Mode-Tuned Measurement Procedure 25

7.2.3 Reverberating Mode -Stirred Measurement Procedure 26

7.2.4 Comparison of TEM/Reverbe rating Chamber Measurement

Results with Anecholc Chamber Tests - Some Examples -- 27

8. Summary and Conclusions 28

9

.

Acknowledgments 30

10. References 30

TV
List of Figures and Tables

Figure 3.1 Cross sectional sketches of the USAEPG 1/10 scaled

TEM/reverberating chamber -- - 33

Figure 3.2 Photographs of the USAEPG 1/10 scaled

TEM/reverberating chamber -- 34

Figure 3.3 Theoretical mode distribution for the 1/10 scaled

TEM/reverberating chamber (1.31 mx 2.41 mx 3.87 m) 35

Figure 3.4 Cross section view of a TEM cell for determining

the field distribution 36

Figure 3.5 Cross sectional sketches of a proposed full scale TEM/

reverberating chamber (13.1 m x 24.1 mx 38.7 m) 37

Figure 3 . 6 Theoretical mode distribution for the proposed full scale

TEM/reverberating chamber (13.1 mx 24.1 mx 38.7 m) 38

Figure 3.7 Theoretical mode distribution for a large TEM/reverberating

chamber (16.2 m x 34.4 m x 38.7 m) 39

Figure 3.8 Theoretical quality factor of proposed full scale TEM/

reverberating chamber (13.1 m x 24.1 m x 38.7 m) assuming:

(a) galvanized (zinc) steel and (b) enamel painted

(cold rolled) steel 40

Table 3.1 Comparison of test fields in a galvanized (zinc finish)

steel reverberating chamber of 3.7 mx 5.2 mx 9.8 m with

that in an anechoic chamber at 3 m separation distance

for same transmitted power. Test volume definition in

anechoic chamber is (± 3 dB) 41

Table 3.2 Comparison of test fields in a galvanized steel

reverberating chamber with that in an anechoic chamber

for same transmitted power. Separation distances in

anechoic chamber = 1 and 3 m 42

Table 3.3 Test volume for radiated fields in anechoic chamber at 1 m

and 3 m separation distances with selected transmitting

antenna gain. Test volume definition is based on halfpower points of the radiation pattern 43

Table 3.4 Theoretical mode calculations of frequency and gap

between adjoining modes for 1/10 scaled TEM/reverberating

chamber, a = 1.31 m, b = 2.41 m, and d = 3.87 m 44

Table 3.5 Normalized electric-field components in the 1/10 scaled

TEM cell with 100 Q characteristic impedance, (a) back

plate and (b) side plate 45
List of Figures and Tables (cont.)

Table 3.6 Relative permeability of cold rolled 11 gauge steel to be

used in construction of welded shielded enclosures as a function of frequency 46

Figure 4.1 Block diagram of the instrumentation used in the cw evaluation of the USAEPG 1/10 scaled TEM/reverberating

chamber - 47

Figure 4.2 Photograph of the NIST equipment used to evaluate the

USAEPG 1/10 scaled TEM/reverberating chamber 48

Figure 4.3 Measured VSWR of (a) top plate, (b) back plate, and

(c) side plate TEM line, from 1 to 2000 MHz, transmitting

into the 1/10 scaled TEM/reverberating chamber 49

Figure 4.4 Measured composite VSWR of the top plate (0.03 to

1.0 GHz) line and the broadband horn antenna (1.0 to

18 GHz) transmitting into the 1/10 scaled TEM/

reverberating chamber 50

Figure 4.5 Measured coupling efficiency between the top plate line

transmitting) and log periodic antenna (receiving, 0.1

to 1.0 GHz), and the broadband horn antennas (transmitting

and receiving, 1.0 to 18 GHz) in the 1/10 scaled

TEM/reverberating chamber 51

Figure 4.6 (a) Theoretical and experimental Q of the USAEPG 1/10

scaled chamber and (b) ratio of theoretical to

experimental Q, 0.2 to 18 GHz - 52

Figure 4.7 E-field components measured at the center of the 1/10

scaled chamber using a NIST isotropic (5 cm dipole)

probe (position #3 in figure 4.14) with chamber excited

by: (a) top plate, (b) back plate, and (c) side plate.

Net input power is 1 W 53

Figure 4.8 Maximum and average E-field in the 1/10 scaled chamber

measured with (a) calibrated 1 cm dipole probe (0.03 to

18 GHz), and (b) log-periodic receiving antenna (0.1 to

1 GHz) and broadband horn receiving antenna (1 to 18 GHz)

.

Net transmitted power is 1 W - - 54

Figure 4.9 Input power required to generate a 20 V/m maximiom

E-field at the center of the 1/10 scaled chamber using:

(a) back plate (Hor. Pol.), (b) top plate (Vert. Pol.),

and (c) log periodic and broadband horn 55

Figure 4.10 Cross sectional sketches of the USAEPG 1/10 scaled

TEM/reverberating chamber showing placement of 15

pieces 0.2 mx 0.6 mx 0.6 m rf absorber 56

VI
List of Figures and Tables (cont.)

Figure 4.11 E- field components measured at the center of the 1/10

scaled chamber using a NIST 5 cm dipole, isotropic,

probe with the chamber excited by: (a) top plate, no absorber; (b) top plate, absorber loaded; (c) back

plate, no absorber; (d) back plate, absorber loaded;

(e) side plate, no absorber; (f) side plate, absorber

loaded; (g) back and side plates driven simultaneously

in phase, no absorber; and (h) all three plates driven

simultaneously in phase, no absorber 57

Figure 4.12 E- fields calculated from receiving antenna power

measurements in the 1/10 scaled chamber with 1 W net

input power. Same transmitting and receiving

antennas used as in figure 4.8 59

Figure 4.13 Measured tuner effectiveness in the 1/10 scaled

chamber using: (a) receiving antenna power measurements

(same receiving antennas as figure 4.5), and (b) 1 cm dipole probe E- field measurements 60

Figure 4.14 Cross sectional sketch of the USAEPG 1/10 scaled

TEM/reverberating chamber showing placement of isotropic

probes for measuring spatial E-field uniformity 61

Figure 4.15 Spatial distribution of the E-field components in the

1/10 scaled chamber measured with 7 NIST isotropic

probes (5 cm dipoles) when transmitting 1 W net input

power from top plate: (a) maximum and (b) average values 62

Figure 4.16 Smoothed average and maximum of measured E-field

components in the 1/10 scaled chamber generated by

the top plate with 1 W net input power, determined

from data in figure 4.15 64

Figure 4.17 Spatial distribution of E-field components in the 1/10

scaled chamber measurh 7 NIST isotropic probes

(5 cm dipoles) when transmitting 1 W net input power

from back plate: (a) maximum and (b) average values 65

Figure 4.18 Smoothed average and maximum of measured E-field

components in the 1/10 scaled chamber generated by

the back plate with 1 W net input power, determined

from data in figure 4.17 67

Figure 4.19 Spatial distribution of E-field components in the 1/10

scaled chamber measured with 7 NIST isotropic probes

(5 cm dipoles) when transmitting 1 W net input power

from side plate: (a) maximum and (b) average values 68

Figure 4.20 Smoothed average and maximum of measured E-field

components in the 1/10 scaled chamber generated by

the side plate with 1 W net input power, determined

from data in figure 4.19 70

vii
List of Figures and Tables (cont.)

Figure 4.21 Measured coupling efficiency (minimum loss) of the horn

antennas (transmitting/receiving) placed inside the

1/10 scaled chamber at 20 to 40 GHz 71

Figure 4.22 Maximum E- fields in the 1/10 scaled chamber determined

from power measurements of the receiving antenna, 20

to 40 GHz, with a net input power of 1 W 72

Figure 4.23 Calculated normalized propagation attenuation in

air for 5 humidities 73

Table 4.1 Spatial variations in E- fields measured inside the

1/10 scaled TEM/reverberating chamber over test volume

defined by probes shown in figure 4.14 74

Table 4.2 Summary of operation characteristics of 1/10 scaled

TEM/reverberating chamber 75

Figure 5.1 Block diagram of instrumentation used in pulsed rf

evaluation of the 1/10 scaled TEM/reverberating chamber 76

Figure 5.2 Maximum values of transmitted (positive) and received

(negative) rf pulse waveforms in the 1/10 scaled TEM/

reverberating chamber determined with increasing

amounts of rf absorber loading at . 9 GHz 77

Figure 5.3 Maximum values of transmitted and received rf pulse

waveforms inside empty (no absorber) 1/10 scaled TEM/

reverberating chamber determined at selected frequencies

Rf input pulse width is 5 /zs , duty cycle is 0.001

Figure 5.4 Maximum values of transmitted and received rf pulse

waveforms inside 1/10 scaled TEM/reverberating chamber

determined with: (a) no absorber, and (b) 1 piece of

20.3 cm X 61 cm X 61 cm (8" x 24" x 24") rf absorber

at selected frequencies 79

Figure 5.5 Maximum and average values of received rf pulse

waveforms inside the 1/10 scaled TEM/reverberating

chamber determined by mode -tuned approach. Chamber

empty (no absorber) . Measurements were taken at 10

selected frequencies and 3 pulse widths 82

Figure 5.6 Time required for rf signal transmitted into the 1/10

scaled TEM/reverberating chamber to rise 63% of steadystate amplitude using: (a) no absorber, and (b) 1 piece

of 20.3 cm X 61 cm X 61 cm rf absorber • 86

vm
List of Figures and Tables (cont.)

Figure 5.7 Time required for rf signal transmitted into the 1/10

scaled TEM/reverberating chamber to rise 90% of steadystate amplitude using: (a) no absorber, and (b) 1 piece

of 20.3 cm X 61 cm X 61 cm rf absorber

Figure 5.8 Estimated correction factors for amplitude response of rf

test pulses in the 1/10 scaled TEM/reverberating chamber

at selected frequencies as a function of input pulse

width using: (a) no absorber, and (b) 1 piece of

20.3 cm X 61 cm X 61 cm rf absorber

Figure 5.9 Estimated correction factors for amplitude response of rf

test pulses in the 1/10 scaled TEM/reverberating chamber

at selected input pulse widths as a function of frequency

using (a) no absorber, and (b) 1 piece of 20.3 cm X 61 cm X 61 cm rf absorber 90

Table 5.1

Table 5.2

Figure 6 .

1

Figure 6 .

2

Figure 6 .

3

Table 6.1

Figure 7 .

1

Figure 7.2

Figure 7 .

3

Summary of influences of rf absorber loading on loss,

quality factor, and charge/decay time of the 1/10 scaled

TEM/reverberating chamber at . 9 GHz

Comparison of measured and calculated charge/decay time

of the 1/10 scaled TEM/reverberating chamber,

determined from chamber average loss measurements

(figure 4.5) and from data shown in figure 5.5

Projected coupling efficiency (minimum loss) of 13.1 m

X 24.1 m X 38.7 m TEM/reverberating chamber

Projected E- fields in 13.1 m X 24.1 m X 38.7 m TEM/

reverberating chamber for 200 W input power determined

from (a) projected chamber losses and Q factors, and

(b) measurements made in RADC chamber and NSWC chamber

91

92

93

94

Projected charge time for 13.1 m X 24.1 m X 38.7 m TEM/reverberating chamber 95

Summary/c one lus ions - 3 regions of operation - 13.1 m X 24.1 m x 38.7 m TEM/reverberating chamber

(zinc coated) - 96

Block diagram of swept frequency EM radiated immunity

and shielding effectiveness measurement system using

TEM/reverberating chamber 97

Block diagram of swept frequency EM radiated emissions

and shielding effectiveness system using TEM/

reverberating chamber 98

Measured SE data of 3 cm X 6 cm TEM cell with 15 mm

circular aperture using 1/10 scaled TEM/reverberating

chamber and swept frequency measurement system 99

IX
List of Figures and Tables (cont.)

Figure 7.4 Block diagram of automated susceptibility measurement

system using the TEM/reverberating chamber in the TEM

mode of operation 100

Figure 7.5 Block diagram of automated radiated susceptibility

measurement system using the TEM/reverberating chamber

in the transition and the reverberating modes of

operation 101

Figure 7.6 Maximum responses of NIST 1 cm dipole probe to an

E- field of 37 dBV/m generated in (a) the 1/10 scaled

TEM/reverberating chamber, (b) NIST 2.74 m X 3.05 m X

4.57 m reverberating chamber, and (c) NIST 4.9 m X

6.7 m X 8.5 m anechoic chamber 102

Figure 7.7 Maximum powers received by a broadband (1 to 18 GHz)

horn antenna in an E- field of 37 dBV/m using: (a) 1/10

scaled TEM/reverberating chamber, and (b) NIST anechoic

chamber 103

Figure 7.8 Comparison of broadband horn antenna response in 1/10

scaled TEM/reverberating chamber with its response

minus free -space gain measured in the NIST anechoic

chamber 104

Figure 7.9 Comparison of SE of 3 cm X 6 cm TEM cell with 15 mm

circular aperture: (a) theoretical calculations,

(b) measured in RADC chamber, (c) measured in 1/10

scaled chamber with mode -tuned approach, and

(d) measured in 1/10 scaled chamber with modestirred approach 104
MEASUREMENT AND EVALUATION OF A TEM/REVERBERATING CHAMBER

Myron L. Crawford

Mark T. Ma

John M. Ladbury

Bill F. Riddle

Electromagnetic Fields Division

National Institute of Standards and Technology

Boulder, CO 80303

This report summarizes the measurement and evaluation of a 1/10 scaled model TEM/reverberating chamber developed as a single, integrated facility for testing radiated electromagnetic compatibility / vulnerability (EMC/V) of large

systems over the frequency range, 10 kHz to 40 GHz. The

facility consists of a large shielded enclosure configured

as a transverse electromagnetic (TEM) , transmission linedriven, reverberating chamber. TEM mode test fields are generated at frequencies below multimode cutoff, and modestirred test fields are generated at frequencies above

multimode cutoff. Both the chamber's cw and pulsed rf

characteristics are measured and analyzed. The report also

discusses the basis for such a development including the

theoretical concepts, the advantages and limitations, the

experimental approach for evaluating the operational parameters, and the procedures for using the chamber to perform

EMC/V measurements. A full-scale chamber that will pro/ide

a test volume of 8 m x 16 m x 30 m is proposed. Some projections of the full-scale chamber's estimated characteristics and operational parameters are also given.

Key words: cw and pulsed rf testing; radiated EM com- patability and vulnerability measurements; reverberating

chamber; TEM cell

1. Introduction and Background

The ability to simulate an operating environment for accurately

measuring the performance of electronic equipment is fundamental to ensuring

the electromagnetic compatibility (EMC) of the equipment. In recent years,

the Fields and Interference Metrology Group of the National Institute of

Standards and Technology (NIST) in Boulder, CO, has been developing improved

methods and facilities for EMC testing. Two such facilities are the

transverse electromagnetic (TEM) cell and the reverberating (mode-stirred)

chamber. Each has its advantages and limitations.

The TEM cell [1,2] is an expanded 50 Q rectangular coaxial transmission

line, with tapered sections at both ends, that operates ideally with only

the TEM mode to generate the equivalent of a planar field. The cell's use

is limited to frequencies below a few hundred megahertz, depending upon the

size of the equipment under test (EUT) and hence the size of the cell. This
limitation is due to the requirement that only the fundamental TEM mode

exist in the cell at frequencies for which tests are to be made.

The reverberating chamber [3,4,5,6] is a shielded room with one or more rotating stirrers to mix the fields transmitted from an antenna placed

inside the chamber. The generated test field is determined statistically

(maximum and average values) from the multimoded, complex fields. This

technique has significant advantages for testing large EUTs efficiently and

cost effectively. However, the method is restricted to frequencies above a few hundred megahertz, again depending upon the size of the test chamber.

This limitation is a consequence of requiring the chamber to be electrically

large enough that modes exist inside the chamber for adequate mode mixing

and spatial averaging to ensure that relatively uniform fields will be

generated for testing.

Combining these two measurement techniques into a single facility will

allow EMC testing over the combined frequency range of the TEM cell and the

mode-stirred chamber. This would result in significant cost and time

savings with a possible improvement in measurement accuracy in performing

essential EMC tests. A combined TEM/reverberatlng chamber will have three

distinct regions of operation: (a) TEM, (b) transition, and (c)

reverberating. The TEM region will cover the low frequency range near dc to

the first multimode cutoff frequency. The transition region covers the

range above the first multimode frequency, up through the frequencies for

which the first few higher modes are excited In the chamber, but

Insufficient in number to provide adequate spatial averaging. The test

field will be a combination of the TEM mode and a limited number of higher

order modes that yield a complex field that is rather difficult to define

both in polarization and amplitude. Techniques such as absorber loading to

suppress the quality factors of the higher order modes, and/or field

averaging over the volume using a matrix of isotropic E- field probes must be

used to define the test field parameters. The reverberating region of

operation will cover the higher frequency range for which sufficient modes

exist to ensure adequate mode mixing and spatial averaging. This test field

simulates a complex, real -world field environment.

This report summarizes the measurement and evaluation results of a particular chamber designed to determine how well a combined ("hybrid")

TEM/reverberatlng chamber could be used to operate as a radiated EMC/V test

facility. The chamber has the dimension of 1.31 m x 2.41 m x 3.87 m, which

represents a 1/10 scaled model of the future full-size facility (13.1 m x 24.1 m X 38.7 m). Both the cw and pulsed rf characteristics of this chamber were evaluated. Anticipated advantages and limitations of this combined

chamber are outlined in Section 2. A description of the chamber Itself is

given in section 3. Section 4 describes the system used for performing the

continuous wave (cw) evaluation/calibration of the chamber and presents the

results obtained. Section 5 describes the pulsed rf, time -domain evaluation

systems and the associated measurement results. Section 6 provides some projections, based on the results contained in Sections 4 and 5 for the

scaled chamber, of the characteristics for the large, full-scale

TEM/reverberatlng chamber. Section 7 outlines the measurement procedures

for performing EM susceptibility and shielding effectiveness tests with this

chamber by both manual and automated operations . This section also gives

sample results obtained using control standard EUTs for comparison with data
obtained in an anechoic chamber and other reverberating chambers to show

correlation factors. Finally, Section 8 gives a sximmary and some conclusions derived from this study and from previous work in developing the

TEM cell and reverberating chamber measurement techniques

.

2. Anticipated Advantages and Limitations

Motivation for developing a TEM/reverberating chamber arises from its

potential for realizing a number of significant advantages. These include:

a. Electrical isolation from the environment;

b. Accessibility (indoor test facility);

c. The ability to generate high test fields over a large volume

efficiently. For example, 1 W of rf power applied to the

transmitting antenna inside this 1/10 scaled chamber, operating in

the reverberating mode at 10 GHz , generates approximately 100 V/m

maximum test E-field;

d. Broad frequency coverage (10 kHz > 40 GHz)

e. Cost effectiveness. The cost of a TEM/reverberating chamber is

estimated to be less than half that of an equivalent anechoic

chamber, primarily due to the cost of rf absorbers and fireprotection system costs associated with an anechoic chamber. Also,

rf source power requirements are significantly less for a TEM/reverberating chamber than for an anechoic chamber as indicated

in section 4. Test time for using the hybrid chamber is anticipated

to be approximately 1/5 of that required when using an anechoic

chamber to characterize the maximum response of an EUT

;

f. Reciprocity (the TEM/reverberating chamber can be used for both

immunity and EMI/TEMPEST emission testing)

;

g. No rotation of the EUT. This results from the three separate

orthogonal plates designed for TEM mode operation and the inherent

test field characteristics of the chamber operating in the

reverberating mode;

h. Physical security, since the chamber is an enclosed facility; and

i. The potential broad application for cw and pulsed rf testing.

There are also disadvantages or limitations when using such a hybrid

chamber. Some of these are:

a. Loss of directional or polarization characteristics of the EUT

placed inside the chamber when operating in the reverberating mode;

b. Limitations of the test field to planar fields at lower frequencies

(TEM mode of operation) , and to complex multimode fields at higher

frequencies (reverberating mode of operation)

;
c

Measurement of only the total power radiated from the EUT in the

reverberating mode of operation as opposed to determining both the

total power radiated and the directional characteristics of the

emissions for TEM mode of operation;

d. Less response of an EUT measured in the reverberating mode than when

measured inside an anechoic chamber (free -space) , if the EUT is

exposed to the same level of test field in each chamber. This

reduction in response is proportional to the EUT's free -space

directivity [5]. Hence, the susceptibility criteria determined for an EUT measured in a TEM/reverberating chamber operating in the

reverberating mode must include an additional factor proportional to

the EUT's estimated free-space maximixm gain, if the results are to

be correlated to the worst case free -space conditions; and

e. Relatively large measurement uncertainty (as great as ± 10 dB) for

establishing the test field in the frequency range of the transition

region between TEM mode of operation and reverberating mode of

operation of the chamber.

3. Chamber Design and Theoretical Considerations

3.1 General Considerations

Factors which influence the chamber's electrical and operational

characteristics include: (a) the size and materials used in the chamber's

construction, (b) the characteristic impedance of the transmission lines

used for TEM mode operation of the chamber, and (c) EUT's mechanical and

electrical operational requirements

The size, shape, and materials used in the chamber determine the test

volume, quality factor, and the shielding effectiveness characteristics of

the chamber. The size and shape also influence the frequency distribution,

number of modes, and density of the modes that are excited inside the

chamber. At low frequencies, only the fundamental or TEM mode can exist in

the chamber. As the frequency increases, higher order modes may begin to

propagate. The resonant frequencies of these modes can be determined by

where i, n, and p are integers, and f. is in megahertz when the

rectangular chamber's internal dimensions a, b, and d are expressed in

meters. By carefully selecting the chamber dimensions, we can minimize

redundant modes (modes occurring at the same frequency) , resulting in a better mode distribution.

The quality factor of the resonant modes in the chamber determines the

input power requirements for establishing test fields when the chamber is

operated in the reverberating mode. The theoretical composite quality

factor of a reverberating chamber is approximately [7, 8],
Q = J_ J_ 1 (2) "^ 1

-^ 16 ^ a-^ b-^ a

)

where V is the chamber's volume in m^ , S is the internal surface area in m^

,

5 = J 2/ {(x>iJ.a) is the skin depth of the chamber wall in m, cr is the wall

conductivity in S/m, /i = fj, /j. is the wall permeability in H/m, /j, = Un(lO)

H/m is the free -space permeability, jj, is the relative permeability, and A

is the wavelength in m. This composite Q is determined by averaging the Q

values of all possible modes within a small frequency interval around the

frequency of interest [7]. The composite Q estimated from (2) is considered

an upper bound because it does not take into account losses other than that

due to wall conductivity. In reality, other losses will also occur during

the measurement, for example, due to leakage from the chamber, antenna

support structures, and the chamber's wall coatings. An alternative means

of determining the chamber's Q is also available by measuring the chamber's

loss [9]. Chamber loss is determined experimentally by measuring the

difference between the net input power, P , delivered to the chamber's

transmitting antenna or transmission lines and the power available, P , at a reference antenna or an EUT. Both P and P are measured in the same units

t r (W or mW) .

If the energy were uniformly distributed over the volume of the

chamber, an empirical value of the chamber's quality factor (Q') could be

obtained using the following equation [9],

V P

Q'= 16 7r2 -^ -^ , (3)

A t

where V and A are as previously defined.

If a chamber factor, K = Q/Q' , is determined, the "equivalent" power

density, Pj' , in a reverberating chamber may be determined by: [5]

,

47rP P^' = -^ W/m2 . (4)

With the aid of (3), we can relate P ' to the net input power, P , by

3AP

s '^r

As can be seen from (5) , the material used in constructing the chamber as well as the chamber's dimensions influence the input power requirements for

establishing a specified test field in the chamber. Typical K factors for

reverberating chambers are in the range of 2 to 5.
Obviously, the electrical properties of the materials also influence

the shielding characteristics of the chamber. Since the chamber is

anticipated for use at frequencies as low as 10 kHz, steel with a relativelyhigh permeability may be required to obtain adequate shielding from magnetic

fields. If such metal is used, it may be advisable to treat it with zinc

(i.e., galvanize it) or line the chamber with a metal such as aluminum or copper to improve the surface conductivity and hence the chamber's quality

factor.

It is of interest to compare the power densities of test fields

anticipated inside a reverberating chamber with that anticipated inside an anechoic chamber assuming the same available input power. The power

density, P., generated inside an anechoic chamber, assuming far-field

conditions, can be determined by

P G

P^ = ^

2 W/m2, (6) d

k'K r

where G is the free -space, far -field gain of the transmitting antenna and r is the separation distance between the antenna and the test location in m. A comparison of the test fields given as the ratio, '?'/'?, obtained from

(5) and (6) for a comparable size, zinc -finish, reverberating chamber of 3.7 m X 5.2 m x 9.8 m and an anechoic chamber is given in Table 3.1 at

frequencies from 100 MHz to 500 MHz when the tuned dipoles are used. The

table also gives the size of the test volume that falls within ± 3 dB region

in the anechoic chamber at r = 3 m. The ratios, P//P., for r = 1 m and 3 m

d d

obtained using open-ended waveguide and horn transmitting antennas in an anechoic chamber, at frequencies from 0.2 to 18 GHz, are given in Table 3.2.

The test volume definitions for the radiated fields in the anechoic chamber

as a function of antenna gain at 1 m and 3 m separation distances are given

in Table 3.3. These tables demonstrate that significantly less power is

required to generate the same maximum amplitude test fields in a reverberating chamber over large volumes than it is generated in an anechoic

chamber over comparable volumes

3.2 USAEPG 1/10 Scaled TEM/Reverberating Chamber

The 1/10 scaled chamber is the model of a proposed large

TEM/reverberating chamber anticipated to be built at the USAEPG, Ft.

Huachuca, AZ . It is a rectangular shielded enclosure, 1.31 m x 2.41 m x

3.87 m (4.3 ft x 7.9 ft x 12.7 ft) in size, constructed from sheet steel

welded together at the edges. The interior finish is of flame -sprayed zinc.

Cross -sectional drawings of the 1/10 scaled chamber are shown in figure 3.1.

Photographs showing different views are shown in figure 3.2. Calculated

resonant-mode frequencies for this scaled chamber using (1) are given in

table 3.4. The theoretical mode distribution obtained from these data is

shown in figure 3.3 in terms of the percentage frequency interval (gap)

between adjoining modes as a function of frequency.

Three orthogonal plates are mounted in the chamber, each designed to

operate as a 100 f2 characteristic impedance TEM transmission line terminated
in its characteristic impedance. One of the cross -sectional views along

with the necessary equations [10] used for determining the electric -field

distributions inside the chamber are given in figure 3.4.

The test volume in the chamber is the region centered beneath the top

plate and the floor, and between the back and side plates and their

corresponding front and end walls. The size of the usable test volume is

determined by the mode of operation of the chamber. For TEM mode of

operation (approximately from 10 kHz to 70 MHz), the maximum recommended

test volume is from 1/2 to 1/3 the region between the plates and opposite

walls. For reverberating mode of operation (from 300 MHz up), the test

volume is the total region between the plates and opposite walls minus

approximately 30 cm separation distance from the plates and walls (or 1/6

wavelength at frequencies above 300 MHz) . For reverberating mode of

operation, this corresponds to approximately 0.7mxl.6mx3.0m for the

1/10 scaled chamber.

The theoretically estimated electric- field amplitude distribution

inside the chamber, operating in the TEM mode, is given in table 3.5 for the

back plate and side plates driven as TEM transmission lines. The electricfield amplitudes are given in terms of the orthogonal components, defined as X (tangential to the plate) and y (normal to the plate) relative to the

plate of interest. The distribution inside each cross section is relative

to the center value at x = and z = h./2 (back plate) and y = and x = h./2 (side plate). The theory used to obtain these estimates [10] is not

valid for determining the field distribution for the top plate driven as a transmission line due to the large gap distance between the plate's sides to

the chamber end walls. This field distribution was determined

experimentally, as described in Section 4.

Two tuners are used for stirring or redistributing the reflected energy

associated with modes excited inside the chamber for the reverberating mode

of operation. The tuners consist of 4 rectangular blades, 0.2 m x 0.4 m in cross section, made from plastic foam covered with heavy (> 25 mm) aluminum

foil. The blades are mounted at offset angles (30 to 45 ) with respect to

the steel drive shafts that are connected to precision stepping motors.

Each tuner motor operates independently, under computer control, so that the

tuner revolution rates and positions can be controlled individually or in

combination.

The chamber is equipped with a bulkhead access panel located on the

back and a 0.762 m high x 2.286 m wide access door located on the front side

of the chamber.

3.3 Proposed Full-Size TEM/Reverberating Chamber

The proposed full-scale TEM/reverberating chamber will have the overall

dimensions of 13.1 m x 24 . 1 m x 38 . 7 m as shown in figure 3.5. It has an approximate test volume of 8 m x 16 m x 30 m. There are two orthogonal

plates (top and back) , each designed to operate with 75 Q characteristic

Impedance for the TEM mode of operation. The third (side) plate is deleted

because of the large gradients expected in the field distribution as
evidenced in the scaled chamber results that were considered excessive. The

reason for using 75 Q instead of 100 Q for the characteristic impedance is

to improve the uniformity of the TEM field distribution in the test volume

as compared to what was realized in the 1/10 scaled model. This will,

however, reduce the field strength by an amount proportional to the square

root of the ratio of the impedances /(100/75) for the same input power to the

transmission lines. As with the scaled model, the plates will be terminated

in their characteristic impedances and driven independently from a highpower rf source when operated as TEM transmission lines. For the

reverberating mode of operation, the percent gap between predicted modes for

this proposed full-scale chamber is given in figure 3.6. Figure 3.7 gives

the percent gap that would exist if the dimensions of the chamber were increased to 16.2 m x 34.4 m x 38.7 m. This larger chamber could be used,

in the reverberating mode, at frequencies down to approximately 20 MHz as opposed to 30 MHz estimated for the chamber proposed in figure 3.6. The

theoretical composite Q for the full-scale chamber is estimated in figure

3.8 for both cold rolled steel and zinc finishes. The values of u used in

r calculating the theoretical composite Q for a steel chamber are given in

table 3.6. The value of fi for a galvanized steel (zinc coated) chamber is

assumed to be 1 . The conductivities (a) used for steel and zinc are 0.4 x

10 S/m and 1.74 x 10 S/m, respectively. The zinc increases the Q by

approximately the square root of the ratio of the conductivities of the two

metals at frequencies where fj, is 1.

4. CW Evaluation of the 1/10 Scaled TEM/Reverberating Chamber

4.1 Measurement Approaches

4.1.1 TEM Mode

For TEM mode of operation in the frequency range from 10 kHz to 70 MHz,

two techniques are employed in determining and monitoring the E- field being

generated in the chamber. The first technique uses measurements of the

incident and reflected powers on the sidearms of a bidirectional coupler,

the impedance of the TEM transmission lines, the separation distance between

the plates and ground plane, and the location between the plates

(anticipated placement of the EUT) to determine the E- field normal to the

driven plate. The second technique is to place a calibrated isotropic Efield probe in the area where the field is to be measured. Using these two

techniques gives a double check to help assure consistency and validity of

measured data while providing a real-time display of the field strength as measurements are being made.

4.1.2 Reverberating Mode

For reverberating mode of operation at 300 MHz or higher frequencies,

the chamber was evaluated using two different operational approaches

referred to as mode -tuned and mode -stirred [5].

For the mode- tuned tests, the tuner is stepped at selected, uniform

increments. The net input power supplied to the transmitting antenna, the

8
power received by a reference antenna, the field-measuring probe responses

and the EUT response at each tuner position are measured and recorded.

Corrections are then made for the changes in the transmitting antenna's

input impedance as a function of tuner position and frequency. The

measurement results are then normalized to a constant net input power value.

The number of tuner steps used per revolution was 200 (increments of 1.8 ).

For the mode-stirred tests, the tuner was continuously rotated while

sampling the power received by the reference antenna, the field probe

response, and the EUT response at rates much faster than the tuner

revolution rate. These measurements were made using a spectrum analyzer and

"smart" voltmeters with diode detectors. The "smart" voltmeters are capable

of data storage and calculation of various arithmetic functions. Large data

samples (up to 9999) are obtained for a single tuner revolution. Tuner

revolution rates are adjusted to meet the EUT output monitor, diode probe

response time, and sampling- rate requirements of the instrumentation.

Typical rates used were approximately 2 to 4 minutes per revolution. The

input power was measured only at the beginning of each measurement cycle.

For the cw tests, a calibrated bidirectional coupler was used to measure the

net input power to the transmitting antenna.

The field level inside the chamber is determined using two techniques

similar to TEM mode of operation. One technique uses a reference antenna to

measure the power density of the field inside the chamber by use of (4)

given in section 3.1. The other technique uses one or more calibrated

probes to measure the E- field. These measurements are described in more

detail later.

4.2 NIST CW Evaluation System

The block diagram of the system used in the cw evaluation is shown in

figure 4.1. A photograph of the NIST equipment used to perform the chamber

evaluations is shown in figure 4.2. The test field is established inside

the chamber by means of an rf source connected to the appropriate

transmitting antenna or plate. Placement of an EUT should fall within the

test volume as defined in section 3.2 except, possibly, relative to the

floor. The separation distance between the EUT and the floor may be less

than 30 cm depending upon the intended EUT configuration rel^ive to the

ground plane. (For some applications, the EUT is mounted close to or directly on a ground plane. In these cases the EUT should be mounted the same in the chamber for test purposes.) Test leads and cables are routed to

appropriate monitors, for example, outside the chamber through shielded

feed- through connectors. Coaxial cables are used for rf signals, and highresistance lines are used for dc signals. This is done to prevent leakage of

the EM energy to the outside environment or into the instrumentation room. A 10 -dB attenuator is used whenever possible between the receiving antenna

and the 50 Q power sensor or spectrum analyzer. This is done to minimize

impedance mismatch with the receiving antenna and measurement system

overload.

The transmitting and reference receiving antennas used for the cw tests

are the top, back and side plate transmission lines (1.0 MHz to 1 GHz

transmitting), a log periodic antenna (0.2 to 1.0 GHz receiving), and
broadband horn antennas (1.0 to 18.0 GHz transmitting and receiving). A

reference receiving antenna is not used at frequencies below 200 MHz. For

these frequencies the field was determined only using calibrated probes,

4.3 CW Evaluation and Calibration Results

4.3.1 TEM Line and Antenna VSWR and Chamber Coupling Efficiency

The efficiency with which energy can be injected into or coupled out of

the chamber by the transmitting transmission lines or antennas and the

receiving antennas is determined by: (a) the VSWR of the lines or antennas

[that is, the impedance match between the rf source and the transmitting

line or antenna (load) or between the receiving antenna (acting as the

source) and its output detector (load)], and (b) the ability of the lines or antennas to couple energy into or out of the particular modes (TEM or higher order modes) that exist at the test frequencies of interest. For the

reverberating mode of operation, rotating the tuner changes the

characteristics of the field inside the chamber, which in turn influences

the effective in-situ VSWR of the antennas. Hence the net input power to

the chamber and the received power detected by the reference antenna vary as a function of tuner position. That is, the impedance matches between the rf source and transmitting antenna, and between the receiving antenna and its

termination, affect the power transfer between the two antennas. This can result in errors in determining the amplitude of the field inside the

chamber if no correction is made. The equation for calculating the

magnitude of the error caused by the impedance mismatch is

2 2 fraction of maximum (1 - |r| ) (1 - |r| ) P„ = available power absorbed = ^r

, (7)

by the load 1 1 - T T \

where F and F denote the complex reflection coefficients for the sources and j(p.oads respectively as defined above. Their magnitudes, |F | and |F |, can be obtained from the appropriate VSWR by the expression,

F. = (VSWR. - 1)/(VSWR. + 1), i = S or L.

The VSWRs of the transmission lines (plates) and the broadband ridged horn

antennas used to excite the chamber are shown in figures 4.3 and 4.4.

Figure 4.3 gives the VSWRs of the top, back and side plates; and figure 4.4

is the composite VSWR of the top plate (1.0 MHz - 1.0 GHz) and the broadband

ridged horn antenna (1.0 - 18 GHz) in the transmission mode. The receiving

antenna's (log periodic, 0.2 - 1.0 GHz, and broadband ridged horn, 1.0 - 18

GHz) VSWRs are similar to figure 4.4. The figures show the maximum,

average , and minimum VSWR obtained by rotating the tuner through a complete

revolution (360°). Large variations and high values of VSWR exist,

especially at the lower frequencies. At higher frequencies (> 2.0 GHz) the

VSWRs' variations become less and their values are lower, approaching the

free-space VSWR of the antennas. At frequencies below approximately 200

MHz, the VSWR of the lines is unaffected by the tuner rotation. At

10
frequencies above approximately 10 MHz the high VSWR results from moding

effects in the chamber and line impedance mismatch.

The coupling efficiency of the chamber is defined as the ratio of the

net input power delivered to the transmitting plate or antenna to the power

available at a 50 fi matched terminals of the receiving antenna. These

ratios, called the chamber loss, are given in figure 4.5. The curves show

the average and minimiom losses measured by the mode -tuned approach, using

the top plate (0.1 - 1.0 GHz) and broadband ridged horn (1.0 - 18 GHz)

antennas transmitting, with a log periodic (0.1 - 1.0 GHz) and broadband

ridged horn (1.0 - 18 GHz) antennas receiving. Impedance mismatch between

the receiving antenna and power detector used to measure the received power

is not accounted for in these measurements . The magnitude of the error

(uncertainty) resulting from this is discussed in [2, 5, 11] and is included

in table 4.2.

4.3.2 Chamber Quality Factor

The chamber's quality factor, Q, Influences the operation of the

chamber in a number of ways for the reverberating region of operation.

Examples are tuner effectiveness, rf input power requirements and the

accuracy with which test field levels can be established inside the chamber.

Chamber Q also influences the time required for the chamber to charge up to

a steady-state condition after the input signal is applied. This affects

the chamber's response characteristics for pulsed rf testing, as is

discussed later.

Results obtained by using (2) to calculate the composite Q with the

relative permeability values given in table 3.6, and by using the chamber

minimum loss data shown in figure 4.5 together with (3) to determine the

experimental Q' are shown in figure 4.6. Figure 4.6 (a) gives the

theoretical and experimental Q for the chamber, and figure 4.6 (b) gives the

ratio of theoretical to experimental Q. The * and + points indicate the

data determined from calculations and experimental measurements , and the

solid lines are estimated curve fits. At the higher frequencies the ratio

approaches roughly 2.4. This indicates that at frequencies where (2) and

(3) are valid, there are relatively small losses other than the loss in the

chamber walls and in the transmitting and receiving plates or antennas

(antenna efficiencies)

.

The value of 2.4 is determined from using the conductivity of steel,

instead of zinc, in (2) as expected for the zinc flame -spray finish in the

chamber. Apparently, the flame -spray finish is too rough compared to the

skin depth thickness, giving either the effect of a greatly increased

surface area or increased loss, thus making the effective finish equivalent

to cold rolled steel. Apparently, this type of finish will not give the

desired improvement in Q as anticipated for the higher conductivity zinc over steel.

4.3.3 E-Field Amplitude Calibration

The E- field strength in the chamber can be determined either from

measurements of the input power to the transmission lines (TEM operation) or

n
from the power received at the terminal of the receiving antennas

(reverberating operation) . It can also be determined by measuring the Efield with calibrated probes.

For low frequencies (TEM region of operation) , the electric field can

be calculated by using the expression,

J P.R

1 E = ^ , V/m (8)

where b is the separation distance between the plate and the opposite wall

or floor in m, P. is the net input power to the line in W, and R is the

transmission line's impedance (~ 100 Q)

For high frequencies (reverberating region of operation) , an equivalent

electric field in the chamber may be calculated using the expression,

^a = J~^ " ^ J ^° ^r ' ^/"^ ^^^

where 77 is the average wave impedance of the chamber (377 Q) , P ,

' is the

equivalent power density in W/m^ , in the enclosure, A is the wavelength in

m, and P is the measured average received power in W. The validity of (9)

has been verified and discussed in [5]

Results of the measured electric fields in the chamber for 1 W net

input power are shown in figures 4.7 and 4.8. Figure 4.7 gives the average

and maximum electric field components, obtained with a calibrated, 3-,

orthogonal -dipole probe placed at the center of the test zone, with the

chamber excited by one of the three transmission line plates. The x, y, and

z components are the horizontal longitudinal (end to end) , the vertical (top

to bottom) , and the horizontal transverse (front to back) , respectively. In

addition, the total magnitude (RSS value of the three components) is also

included in figure 4.7 as marked by T. Figure 4.8 gives the maximum and

average electric fields determined from received power measurements together

with (9) and from measurements that used a calibrated 1 cm dipole probe.

For these measurements the chamber was excited using the top plate at

frequencies from 0.03 to 1.0 GHz and using a broadband ridged horn antenna

at frequencies from 1.0 GHz to 18 GHz. For both figures, the tuners were rotated through one complete cycle (360 )

Three distinct regions of operation are noted for this scaled chamber.

At frequencies below approximately 70 MHz the field is singly moded (TEM

mode) and the tuner rotation has little or no effect upon the field's

amplitude. Hence, the maximum and average E- field are the same. Also, the

dominant component, the one that is polarization-matched with the TEM mode

E- field, is evident. The amplitude of the component corresponds to the

calculated value determined from measuring the net input power and (8). At

frequencies between approximately 70 MHz and 300 MHz, the chamber undergoes

12
a transition region where higher order modes exist in addition to the TEM

mode but in insufficient numbers for the chamber to reverberate properly.

Large variations in the field strength exist in this region. At frequencies

above approximately 300 MHz, sufficient modes are generated so that the

chamber is reverberating effectively. In this region the effect of Q on the

resonant modes is evident and the tuner is effective in redistributing the

fields within the chamber. Thus the maximum amplitude of the field is

substantially higher than that for the TEM mode of operation (as great as 25

dB) for the same net input power of 1 W, and the polarization properties of

the components are lost. Note that the amplitudes of the three components

in figure 4.7 in this frequency region are approximately the same, and that

the total magnitude (RSS value) is about 4.8 dB higher than each individual

component.

The 3 orthogonal dipole probes and the 1 cm single dipole probe used to

measure the fields in figures 4.7 and 4.8 were calibrated in a planar field

using a TEM cell [12] at frequencies up to 500 MHz and in an anechoic

chamber at frequencies from 500 MHz to 18 GHz [13]. The assumption is made

that the average field over the aperture of the probe inside the

reverberating chamber will approximate the planar field used to calibrate

the probe. This is reasonable, at least at frequencies for which the probe

is electrically small. Also, the open-space far-field gain of an electrically small dipole is small (1.76 dB) . Thus, the probe-measured

fields should be equivalent, within approximately 1.76 dB, to the E- fields

determined using a receiving antenna. This is true if the polarization

properties of the probe and receiving antenna are effectively eliminated in

the chamber. The variations in the fields determined by using received

power measurements and by using the probes (figure 4.8) are typical of the

random variations in the data used to determine the field strength inside

reverberating chambers. The agreement between the two methods is considered

reasonable

.

Using the results of figures 4.7 and 4.8, we can calculate the input

power requirements for establishing a given field level in the chamber. An

example is shown in figure 4.9, which gives the net input power required to

generate a 20 V/m field in the 1/10 scaled chamber using the top and back

plates and a combination of log periodic and ridged horn antennas.

4.3.4 Effects of RF Absorber Loading and Multiple Driven Plates

Tests were made to determine the effect of lowering the Q in the

chamber by adding rf absorbing materials, or of driving more than one plate

simultaneously, upon the field in the test zone. The transition region of

operation is of particular interest.

Fifteen pieces of 0.2 m x0.6mx0.6m pyramidal rf absorber were placed inside the chamber as shown in figure 4.10. Measurements were then

made using a 5 cm dipole, isotropic probe placed at the center of the test zone similar to the measurement for figure 4.7 with each of the plates

driven with 1 W net input power. The results are shown as (b) , (d) , and (f)

of figure 4.11 for the frequency range, 30 to 300 MHz (includes the entire

transition region of operation). Results of driving all the plates and just

the back and side plates, simultaneously from the same source, without

absorber loading, are also shown in figure 4.11 as (h) and (g) . These are

13
compared to the results obtained in figure A , 7 , which are reproduced in

figure 4.11 for the 30 to 300 MHz region, as (a), (c) and (e)

A comparison of (a) with (b) , (c) with (d) , and (e) with (f ) shows the

effect of the absorber loading, lowering both the amplitude and the

variation in the amplitude of the measured E-field components. The

absorber's effect is most pronounced at the higher frequencies where the

absorber is most effective in lowering the Q of higher order mode

resonances. Thus, use of absorber would be helpful in reducing the

variation in the E-field and hence the uncertainty in determining the

amplitude of the test field. However, this must be balanced against the

reduction in the field level and the cost and inconvenience of placing the

absorber in the chamber. At frequencies above 300 MHz, the absorber would

have to be removed from the chamber for proper operation of the chamber in

the reverberating region.

Comparisons of figure 4.11 (a) [top plate, no absorber] with (h) [all

plates, no absorber] and (c) [back plate, no absorber] with (g) [back and

side plates, no absorber] indicate similar field levels and variations in

the components. Apparently, little is gained by multiple excitation of the

plates

4.3.5 Tuner Effectiveness

Proper operation of the chamber in the reverberating mode is dependent

upon the effectiveness of the tuner to obtain randomness in the distribution

of the test field inside the chamber. To achieve this the tuners must be

electrically large and shaped or oriented to distribute energy equally into

all chamber resonant modes as much as possible. A test to determine how

well the tuners are functioning is to measure the ratio of the maximum to

minimum E-field in the test volume in the chamber as a function of tuner

position. This is done by either measuring the ratio of the received power

at the terminals of the receiving antenna or by measuring the electric field

using a calibrated probe, relative to a constant net input power to the

transmitting antenna. A large ratio (greater than 20 dB) indicates that the

tuner is effective in redistributing the scattered fields inside the

chamber

Measurement results for the scaled chamber are given in figures 4.12

and 4.13. Figure 4.12 gives the maximum, average, and minimum E-field

based on the receiving antenna power measurements for a complete revolution

of the tuners as a function of frequency. Figure 4.13(a) is the ratio of

the maximum to minimum E-field, or the tuner effectiveness, determined from

figure 4.12, and figure 4.13(b) is the corresponding tuner effectiveness

determined from E-field measurements made using a calibrated 1 cm dipole

probe. The measurements were obtained by using the same transmitting and

receiving antennas as noted in figure 4.5. This ratio is also influenced,

in addition to the tuner size and shape, by the chamber Q. For example,

placing rf absorber and/or an EUT inside the chamber will lower the

chamber's Q, referred to as the loading effect. A minimum ratio of 20 dB is

recommended to assure proper operation of the chamber [5]. As seen in

figure 4.13, a tuner effectiveness of approximately 30 dB is achieved at

frequencies above about 300 MHz for the 1/10 scaled chamber.

14
4.3.6 Test Zone E-Field Uniformity

Tests were made to determine the E- field uniformity in the chamber as a function of spatial position and frequency. Seven small NIST isotropic

probes [14, 15, 16], designed to operate at frequencies up to 2 GHz, were placed at various locations inside the chamber (positions 1, 2, 3, 4, 5, 6,

8; no position 7 was designated) as shown in figure 4.14. Each probe has

three orthogonally oriented dipoles which are aligned with the three axes of

the chamber, earlier called the horizontal longitudinal (E ), vertical (E ),

and horizontal transverse (E ) . Measurements were made of the field

z strength of each orthogonal component at the seven locations for each tuner

position (200 steps of 1.8°) for frequencies from 1 to 2000 MHz. These data

were normalized for a net input power of 1 W applied at the input terminals

of the transmission lines or transmitting antennas. The maximum and average

values for each component and the RSS value were then determined from the

complete data sets . The results of these measurements obtained when driving

the chamber with the top, back, and side plates are shown in figures 4.15

thru 4.20. Figures 4.15, 4.17, and 4.19 give the individual data obtained

for all seven probes. Figures 4.16, 4.18 and 4.20 give the averaged values

determined from the 7 individual data sets of figures 4.15, 4.17, and 4.19.

Again, the three regions of operation are apparent. The spread in the data

shows spatial field variations inside the chamber at the indicated

frequencies. Note that the dominant components match with the TEM mode Efield polarization for each driven plate in the TEM mode of frequency region

(below 70 MHz). For example, the E component is dominant for the top

plate; E is dominant for the back plate; and E is dominant for the side

plate. These results are in agreement with figure 4.7, as expected.

Gradients in the E- fields between the plates are significant. This is due

to the placement of the plates close to the top, back, and side walls as necessary for a clear test volume. The vertical E- field difference for the

top plate, figure 4.15, over 1/2 the vertical separation distance (positions

2 to 3 and 3 to 4) , is approximately 12 dB (± 6 dB) . The corresponding

values of E , at positions 5 and 8 (off the edges of the plate) are significantly lower (~15 dB) than at the positions directly under the plate.

The E values at positions 1 and 5 are approximately the same as at position

3, indicating the field is relatively uniform along the length of the plate.

The horizontal transverse component, E , for the back plate (figure

4.17) has a total difference in magnitude of approximately 20 dB (± 10 dB

from positions 1 to 3 and from positions 3 to 6) . This is in good agreement

with the theoretically predicted values shown in table 3.5. Again, E along

the plate, positions 5, 3, and 8 are approximately equal. The E values at

positions 2 and 4 off the edges of the plate are lower than at position 3 by

approximately 2 to 5 dB . The field distributions across the width and along

the length of the back plate are reasonably uniform in the described test

volume. The cross polarized component, E , however, is almost as strong as the dominant E . z

15
The horizontal longitudinal component, E , for the side plate (figure

4.19) has a difference In magnitude between positions 5 and 3 of

approximately 14 dB (± 7 dB) . The difference In E between positions 3 and

8 could not be measured due to Insufficient probe sensitivity and rf Input

power. We presume, from the results shown In table 3.5, that this

difference Is approximately the same as for positions 5 and 3. The

theoretically predicted difference from table 3.5 Is 18 dB . Variations in

E along the length and across the width of the side plate are relatively

small, essentially In agreement with table 3.5. However, because of the

excessively large differences along the length of the chamber associated

with rf applied to the side plate, use of the side plate to generate TEM

fields is not recommended; and for this reason, a similar plate is not to be

included in the design for the full-size chamber.

At frequencies above approximately 300 MHz, the differences between the

averaged maximum and average amplitudes approach about 8 dB in figures 4.16,

4.18, and 4.20, typical of reverberating operation of the chamber. This is

another indication that the chamber is operating properly in the

reverberating mode at these frequencies.

The amplitudes of the field components in figures 4.15, 4.17, and 4.19,

at frequencies above 300 MHz, are approximately the same. The total

magnitude or RSS value of the average field in these figures is

approximately 4.8 dB (a ratio of J 3) greater than the individual components.

This will occur if E = E ~ E , and indicates that the field components are X y z

^ randomly polarized In the chamber. The total magnitudes of the maximum

amplitudes in these figures, however, are only about 3 dB greater than the

individual components. This indicates that the components' maximxam values

are not completely Independent of each other. This is similar to the

results obtained for other reverberating chambers and appears to be inherent

in the measurement method. The implication is that if multiple receptors

are used (for example an Isotropic probe whose output is a function of all

three orthogonal dlpoles) in establishing the test field amplitude inside a reverberating chamber, the results will be biased by either 3 dB for maximum

response data or 4.8 dB for average response data.

A summary of the spatial field distribution obtained from figures 4.15,

4.17, and 4.19 are given in Table 4.1. As mentioned earlier, the E- field

amplitude in the chamber is Influenced by the chamber's Q. Inserting the 7 probes into the chamber with their lossy transmission lines lowers the

chamber's Q and hence the field strength in the chamber compared to what

would be present without the loading effect of the multi-probe system.

4.3.7 Test Results from 20 GHz to 40 GHz

Measurements were also made to determine the chamber's coupling

efficiency (loss) in the frequency range from 20 GHz to 40 GHz. The method

of measurement and the definition of coupling efficiency are the same as those described in sections 4.3.5 and 4.3.1 except that appropriate

waveguide hardware and Instrumentation were used. The results are shown in

figure 4.21, Using these results and K ~ 2.4 in (5), we can predict the Efield to be generated in the chamber by a net input power of 1 W. These

16
results are shown in figure 4.22. Each figure gives the results of two sets

of measurements, one made at NIST in Boulder, CO, and the other at a later

date at USAEPG in Sierra Vista, AZ. The second set of measurements was made

at USAEPG because of the large departure in the first data (between 24 GHz

and 26 GHz) from expected results. The initial reaction to the first set of

data was to question their validity. However, after some research, it was realized that the data were correct and the differences were due to watervapor absorption in the frequency range between 20 GHz and 26 GHz as shown

in figure 4.23. The curves in figure 4.23 give the calculated normalized

propagation attenuation/kilometer in air for 5 relative humidities (RH) [17,

18]. The propagation attenuation is also influenced, to a lesser degree, by

temperature and barometric pressure. A check of the weather conditions on

the day the measurements were made at NIST verified that the relative

humidity was high, accounting for the large loss between 24 GHz to 26 GHz.

The relative humidity in Sierra Vista, in contrast, is much lower, about 20

%. These results imply the need to control the environment inside the

chamber, especially for measurement at frequencies above 20 GHz.

4.3.8 Summary of CW Calibration and Evaluation Results

Results of the CW calibration and evaluation of the 1/10 scaled chamber

indicate three distinct frequency regions of operation. The TEM mode region

extends from 10 kHz to approximately 70 MHz, slightly lower in frequency

than where the first higher order mode becomes resonant. Test fields

established in this region simulate planar fields with the E- field normal to

the driven plate and the magnetic field tangential around the plate. Large

differences in the fields exist in the test volume between the plates and

ground due to the close proximity of the plates to one of the chamber's

inside surfaces. These changes in E-field amplitude vary from ± 6 dBV/m

about the center location for the top plate, ± 10 dBV/m about the center

postion for the back plate, and up to ± 14 dBV/m about the center position

for the side plate, for the volume occupying 1/2 the distance between the

plates and outside walls or floor. The accuracy with which the E-field

within this unperturbed volume can be established is estimated to be

approximately ± 2 dB. Input power requirements to establish 200 V/m test

field are determined by the plate driven. For vertical polarization (top

plate) , about 640 W are required. For horizontal transverse polarization

(back plate) , about 4000 W are required. Use of the side plate is not

recommended.

The second frequency region, referred to as the transition region,

extends from approximately 70 MHz to 300 MHz. The test fields are a combination of TEM plus an increasing number of higher order modes, as a function of increasing frequency. Thus the field is no longer a simple

equivalent plane-wave simulation, but now includes higher order mode

components which combine with the TEM mode to produce a more complex field.

For this reason, the amplitude determination of the test field has errors

that can be as large as ± 10 dB. (See figures 4.11 and 4.17 for example.)

The input power required to generate 200 V/m using the top plate is about

200 W at 70 MHz decreasing to about 20 W at 300 MHz. Polarization of the

test field changes from vertical E-field at 70 MHz to undefined

(reverberating field) at 300 MHz. The input power required to generate 200

V/m using the back plate is about about 1500 W, decreasing to about 20 W at

17
300 MHz. Polarization of this test field changes from horizontal transverse

at 70 MHz to undefined (reverberating field) at 300 MHz.

The third (reverberating) region extends from approximately 300 MHz to

40 GHz or possibly greater. The test fields are complex multimoded fields

simulating a near -field environment. The uncertainty of determining the

maximum or average amplitude of the test field varies from an estimated ± 8 dB at 300 MHz down to ± 4 dB at frequencies above 2 GHz. The input power

required to generate a maximum test field of 200 V/m varies from about 20 W

at 300 MHz decreasing to about 3 W at 2 GHz, then increasing again to about

6 W at 18 GHz. (See figure 4.9 for reference). This discussion is

summarized in table 4.2.

5. Pulsed RF/Time- Domain Evaluation of the 1/10 Scaled TEM/Reverberating

Chamber

5 .Background

Parameters of EMI signals that can contribute to upset in electronic

equipment include: (a) total energy, (b) peak amplitude, and (c) transient

characteristics. All these parameters are different inside a reverberating

chamber than in free space. Their characterization inside a reverberating

chamber, particularly for pulsed rf fields, provides information required

for making necessary analysis to determine correction factors as a function

of the input pulse parameters. The term "pulsed rf" refers to pulse

modulated cw with characteristics similar to a pulsed radar signal. It also

provides insight into the inherent limitations associated with using this

complex environment for pulsed rf EMS/V testing. Obviously, these

parameters are influenced by the chamber's Q factor, since the time required

for a pulsed wave's amplitude to rise to its steady- state value inside the

chamber and to decay to zero after the input signal is removed is a function

of the chamber's Q. These charge and decay times can be reduced by

artificially lowering the chamber's Q, for example, by inserting small

amounts of rf absorber. However, this reduces the accuracy of determining

the test field amplitude. Results of work to evaluate the response

characteristics of the chamber when excited by pulsed rf of various pulse

widths and frequencies, and with the chamber loaded with a small amount of

absorber, are presented and discussed in this section.

5.2 NIST Pulsed RF Evaluation System

A block diagram of the system used for the pulsed rf evaluation

measurements of the chamber is given in figure 5.1. A test field is

established inside the chamber by means of a pulsed rf source connected to a broadband ridged horn antenna. The input signal is monitored by a calibrated diode detector connected to the side arm of a calibrated

directional coupler. The output of this detector is connected to one channel of the transient digitizing oscilloscope. The field established

inside the chamber is monitored by a broadband ridged-horn receiving antenna

and calibrated diode detector connected to the second channel of the same oscilloscope. The receiving antenna is similar to the antenna used for

transmitting the signal. As in the cw tests, the tuner is rotated to

redistribute the rf energy inside the chamber.

18
The mode -tuned approach was used when it was necessary to optimize the

measurement accuracy and to obtain complete statistical information for

evaluating the time-domain response characteristics of the signal detected

by the receiving antenna. The mode -stirred approach was used for relative

measurements with the digitizing oscilloscope placed in its maximum-hold

mode of operation. Mode stirring is much faster than the mode tuning

and results in much larger data sampling; however, it provides only maximum

values. The mode -stirred approach was used to analyze relative information

such as charge and decay time and to determine correction factors applied to

the test field amplitude when input pulse widths are too short for the

chamber to charge up to its steady- state or cw- response values. The

digitizing oscilloscope used is capable of measuring signals with rise times

of approximately 30 ps, at a sampling rate of 50 kHz, with sample sizes up

to 1024 per scan.

5 . 3 Pulsed RF Evaluation Results

Measurements were made, using the mode -stirred approach, to determine

the effect of lowering the reverberating chamber's Q on the charge/decay

time of the pulsed rf field excited inside the chamber. Varying amounts of

pyramidal rf absorber were placed in the center of the chamber's test zone resting upright (cones up) on the floor. Measurements were first made at

900 MHz (the lowest pulsed rf test frequency) , since the response time is

the longest at this frequency. Results of these measurements are given in

figure 5.2. The graphs of each figure have two traces. The upper trace is an envelope of the input pulses applied to the chamber transmitting antenna.

It represents the "painted" envelope derived from all the input pulses

during one complete rotation of the tuner. The signal shown is the reduced

voltage values measured on the side arm of the directional coupler using a diode detector connected to one channel of the digitizing oscilloscope. The

lower trace is the time -domain envelope of the pulses received by the

chamber's reference receiving antenna and measured with a second diode

detector connected to the second channel of the digitizing oscilloscope.

The polarity of this pulse has been reversed to separate the two traces.

The diode detectors have built-in 50 Q load resistors for impedance matching

to 50 Q. The spread in the top trace (input signal) is due to variations in

signal amplitude caused by variations in antenna VSWR (input mismatch) as a function of tuner position. The magnitude of this variation is frequency

dependent, decreasing as the frequency increases as shown in figure 5.3.

Figure 5.2 (a) was obtained with the chamber empty (no absorber). Figures

5.2 (b)-(h) are for increasing amounts of absorber starting with 1 piece of

7.6 cm thick, 61 cm x 61 cm rf absorber, ending with 8 pieces of 30.5 cm

thick, 61 cm x 61 cm rf absorber. The effects on both the amplitude and

response time are obvious. Figures 5.3 and 5.4 show similar results but at

selected frequencies up to 18 GHz. Figure 5.3 gives the maximum received rf

pulse waveforms for the empty chamber (with no absorber) excited with 5 /zs wide rf pulses. These measurements were used for determining the chamber

rise/decay time as a function of frequency, assuming that the input pulse

width is wide enough to allow the field in the chamber to charge up to its

steady state value. Figure 5.4 shows the effect of a single piece of

absorber to reduce the chamber's Q on the maximum received pulse waveform.

The input pulse width for these tests was 1 /xs

.

19
It is interesting to compare the chamber's response time calculated

from the experimentally determined, average Q values, with the measured

response time. The response time is approximately 2Q/w for the signal to

rise or decay to 63 % (1 - 1/e) of the steady- state amplitude. These

results are shown in tables 5.1 and 5.2. Table 5.1 gives the measured

minimum chamber losses , the estimated Q factors , and the measured

charge/decay times determined at 900 MHz for varying amounts of absorber.

The measured rise/decay times were extracted from the results shown in

figure 5.2. Table 5.2 gives the measured average chamber losses, the

associated calculated Q factors, the predicted charge-decay times, and the

measured charge -decay times at the selected frequencies when the chamber is

empty. The calculated and measured response times differ somewhat, but are still within the margin of error expected for these type of measurements.

Mode tuning was used to accurately determine: (a) the ratio of the

received pulse amplitude versus input pulse amplitude as a function of time

after the input pulse is turned on, and (b) the time required for the

chamber to charge up to its steady- state amplitude and then to decay to zero after the pulse is turned off. These measurements were made with the

chamber empty (no absorber) . Results of these measurements are given in

figure 5 . 5 for three input pulse widths , 1 fj-s , 3 fis and 10 /is , and for the same 10 frequencies. Each graph shows two curves, one for the maximxim and

one for the average received signals. By examining these data at the

selected frequencies, we can determine the approximate charge-decay times as

a function of frequency. These results are shown in figure 5.6. Similar

results showing the time required for the received signal to rise to 90

percent of the maximum amplitude are given in figure 5.7. The implication

of figures 5.6 and 5.7 is that the input pulse width, at the frequencies of

interest, should be equal to or longer than the values shown by the curves

for the pulsed rf test field amplitude to reach 63 or 90 percent of the

maximum steady- state amplitude. The steady- state amplitude is achieved if

the input pulse width is sufficient for the chamber to charge up to 100

percent, or to its maximum output for a given input pulse amplitude.

If the transmitted input pulse width is shorter than the charge time of

the chamber, an error will result in terms of establishing a known peak

amplitude of the test signal in the chamber. An estimate of a correction

factor to apply for this condition can be found by calculating the ratio of

the received signal amplitude to the steady- state transmitted signal

amplitude as a function of time after turning on the transmitted pulse.

Figure 5.8 gives these results, determined from the data of figures 5.4 and

5.5 with the mode -stirred approach. The data are projected down to input

pulse widths of 0.1 /zs . There is a significant reduction in the correction

by loading the chamber with even a small amount of rf absorber. Figure 5.9

shows another way to display the data given in figure 5.8. The correction

factors are given, for discrete input pulse widths, as a function of

frequency rather than for discrete frequencies as a function of input pulse

width. The symbol, *, indicates the actual data extracted from figure 5.8,

and the solid curves are the smoothed approximations for these data.

5.4 Comments on Pulsed RF Measurement Results

Mode -tuned data are generally considered to be more accurate than modestirred data for absolute measurements of chamber losses and test field

20
levels . This is so because the measurements are taken for each of the

(stopped) tuner positions and sampled at each instant of time during the

pulse, so that a maximum value (or minimum or average) can be extracted from

the total data taken. Each pulse is digitized at 512 samples in time, for

each of the 200 angular positions of the tuner during a complete revolution.

Then corrections are made to each measurement for cable loss, coupling ratio

of the directional coupler, attenuator calibrations, and diode detector

readings according to previous calibration of power response versus signal

frequency and indicated voltage levels. Mode -stirred data cannot be

corrected in the same manner; hence, only representative maximum values are obtained. However, results derived from mode -stirred data usually have

better definition and are thought to be more accurate for determining

relative parameters such as the charge time and test field amplitude

correction factors. This is because larger data sampling approaching

continuous sampling as a function of tuner position is possible, compared to

the limited (200 tuner position) sampling for the mode-tuned approach.

For part of the data, the maximum amplitude of the pulsed rf field

strength, when measured immediately after the pulse turn- on, appears to be

slightly greater than the steady- state value. This overshoot is thought to

be due to the imperfect leading edge of the pulse produced by the pin diode

switch in the pulse source. The overall trend, however, is for the shorter

pulses to have a smaller value of peak detected voltage. This is because

the on- time is not sufficient for the EM energy in the reverberating chamber

to reach its steady- state value. Thus, data were taken for a range of pulse

widths to determine the correction factor to be applied when using the

chamber to take EMS/V data for very short radar pulses.

6. Projecting the 1/10 Scaled Chamber CW and Pulsed RF Measurement Results

to a Full -Scale TEM/Reverberating Chamber

The theoretical projected composite quality factor for a full size

chamber (13.1 m x 24 . 1 m x 38 . 7 m) is given in figure 3.8. The K factor,

defined as the ratio of the theoretically determined composite Q by (2) to

the experimentally determined Q' by (3), for the 1/10 scaled chamber, is

shown in figure 4.6 (b) . This factor approaches a constant value of

approximately 2.4. This value is consistent with K factors determined for

other reverberating chambers of different sizes but with similar materials.

Thus, assuming the same K factor for the full-scale chamber, we can project

the minimum loss for the full-size chamber. These results are shown in

figure 6.1 for both steel and galvanized steel. The shape of the curve for

steel, at frequencies below 1 GHz, is due to the frequency dependence of the

permeability of steel as was indicated in table 3.6.

The results of figure 6.1 can be used to predict the E-field in the

full-size chamber by using (9) from Section 4.3.3 and the definition of loss

(Loss = P /P_) . These results are shown for steel and zinc coated steel in

r t

figure 6.2 (a) when the input power is 200 W. The curves for aluminum and

copper were calculated from the zinc data in accordance with the theory that

the power density in the chamber is proportional to the square root of the

conductivity of the metals used for the wall surface . Thus , aluminum and

21
copper surfaces should result in approximately 1.54 dB and 2.63 dB higher

fields, respectively, than the zinc surface of the same chamber size.

Figure 6.2 (b) gives the projected E- field to be generated in the fullsize chamber by an input power of 200 W, based on the data obtained for a 3.69 m X 5 . 18 m X 9 . 79 m galvanized steel chamber located at RADC , Griffiss

AFB, NY, and for a 3.51 m x 5.18 m x 10.82 m welded-steel chamber located at

the Naval Surface Weapons Center (NSWC) , Dahlgren, VA. A comparison of the

appropriate curves indicates excellent agreement for the steel -chamber data

[figures 6.2 (a) and (b) ] but only limited agreement for zinc. Differences

are due to the assumption that the chamber surface is of pure zinc. In

reality, however, the actual chamber coating is not pure zinc.

Figure 6.3 gives the projected charge time for the full-scale, empty,

chamber (no absorber loading) . These results are based on the theoretical

determination of Q for steel and zinc chambers and an assumed K- factor of

2.4 for reduction in Q, which is further reduced by approximately 10 dB to

give an equivalent average chamber

The projected operational characteristics for the full-scale chamber

are summarized in table 6.1. Again three regions of operation are expected

as defined earlier. The results are scaled by a factor of 10 from table 4.2

except for the input power requirements for a given E field.

7. Performing EMC/V Measurements Using the TEM/Reverberating Chamber

7 . 1 Performing EMC/V Measurements With Manual Operations

An especially useful application of the TEM/reverberating chamber is to

perform shielding effectiveness (SE) measurements. A block diagram of a simple, manually operated system, which does not require the use of a computer controller, is shown in figure 7.1. The system uses a sweep

oscillator and spectrum analyzer to cover approximately octave bandwidths in

the frequency range of interest. Two measurement approaches are required

depending upon the region of operation (TEM or reverberating mode)

For cable/connector SE measurements , in the TEM mode region of

operation, a reference cable is placed in the chamber oriented to give

maximum coupling to the magnetic field of the TEM mode. This cable

typically consists of a 72 fi unshielded parallel wire configured, as nearly

identical as possible, to the cable/connector under test (CUT). Reference

measurements are made by establishing the test field in the chamber at the

frequencies of interest and recording the power coupled to the output of the

reference cable in channel A of the analyzer. The net input power for these

measurements is recorded on the Y.. channel of the xy recorder using the time

base output for the x axis. The time base is set the same as the oscillator

sweep rate and is triggered at the same time the oscillator starts its

sweep. The CUT is then placed in the same location and orientation as the

reference cable and the test field is reestablished at the same test

frequencies. The power coupled to the output of the CUT is recorded in

channel B of the analyzer at the same time the net input power is recorded

on channel Y- of the xy recorder. Impedance mismatch errors are corrected

by using (7) as described in section 4.3.1. The SE is then determined by

22
SE = 10 log !£HLlii£ie , (10)

ref i (CUT)

where P^tt't ^^ ^^^ power coupled to the CUT; P. .p,,^,^ is the net input power

applied to the chamber when measuring P^,,„; P ^ is the power coupled to the •^^ ^ CUT ref '^ ^ reference cable: and P. , ,-. is the net input power applied to the chamber

1 (ref) r r rr when measuring P r-- " ref

Measurements are made in the reverberating region of operation in much

the same manner as for TEM mode of operation. The exceptions are that now

the tuner(s) are rotated at revolution rates fast (about 1/s) compared with

the sweep oscillator sweep speed (100 s/sweep) . The spectrum analyzer is

set up to measure in its maximum hold mode and tc scan the same frequency

band as the sweeper at fast scan rates (20-30 ms/scan) . Resolution and

video bandwidths compatible with the scan rate, dynamic range requirements

are selected to ensure that the analyzer is operating in calibration. If

the SE is anticipated to be high (> 40 dB) , a calibrated attenuator is used

between the output of the reference antenna and the analyzer. The reference

antenna replaces the reference cable at frequencies in the reverberating

mode region of operation.

The steps for performing the measurements proceed as follows: First,

connect the reference antenna to the analyzer and measure its received power

in dBmW for the fields established in the chamber by the power applied to

the transmitting antenna or plate. Take a sweep and store the measurement

results in one of the channels, for example channel A, of the analyzer while

also recording the net input power on channel Y- of the xy recorder. Next,

connect the output from the CUT up to the analyzer and repeat the

measurement, being sure the net input power remains the same or is measured

and recorded on channel Y- of the xy recorder. It may require up to 3 or 4

sweeps to fill in the maximum received power across the frequency band.

Store the results in dBmW in the analyzer's second channel, for example

channel B. Then subtract channel B from A and add the calibrated attenuator

factor in dB, if used, to get the SE in dB . This assumes that the cable

lengths between the reference antenna and the CUT to the analyzer are the same. If limited sweeps are made, use the envelope of the CUT data to

subtract from the reference data so that the minimum SE values are obtained.

Using this procedure will ensure adaquate accuracy without requiring long

sweep times or a large number of repeated sweeps. (Normally it is not

necessary to wait for a maximum recording at all frequencies.) These data

give nearly continuous frequency coverage measurements of SE. Measurements

of EUT response to determine the susceptibility or shielding effectiveness

of enclosures proceed in much the same manner as above.

For some applications, it may be advisable to determine the shielding

factor (SF) of the EUT. This is defined as the ratio of the EUT's response

expressed in power to the test field's power density,

SF = 10 log (Peu^/P^') , dB/m2 (11)

23
where P^Tirr ^^ ^^^ power coupled to the EUT in W, and P ' is the power

EUT O- density inside the chamber in W/m^ . The SF is similar in concept to the

well known antenna factor used in antenna calibrations.

An alternate approach to these measurements is shown in figure 7.2.

This block diagram gives the setup for measuring radiated emissions or SE by

driving the CUT in the radiating mode or by measuring the radiated emissions

from the EUT. (For radiated emission measurements, the EUT is not connected

to the rf source, but, rather, becomes the source itself.) For SE

measurements , mismatch errors caused by large VSWRs of the CUT are minimized

by using the bidirectional coupler to measure the net input power to the EUT

and to the chamber's transmitting antenna, while measuring the power

received by the reference antenna for each case. Use of a bidirectional

coupler to measure the net input power gives the equivalent of an impedance

match source for the measurements. For the case of cable, the SE is then

determined by

SE = 10 log 'n(CUT) ^ref(t)

^^ ^^2)

^n(t) ^ref(CUT)

where P ,^^_, is the net input power to the the cable; P j~/^TTrr.x is the

n(CUT) ^ ^ ref(CUT)

power received by the reference antenna when power is applied to the CUT;

P , . is the net input power to the chamber's transmitting antenna or plate;

and P r-,^. is the power received by the reference antenna when power is ref(t) r y r applied to the chamber's transmitting antenna.

For the case of an EUT in general, the power radiated from it excites

the chamber and is coupled to the reference antenna. This coupled power is

measured on one channel of the analyzer. The EUT is then turned off and

power is applied to the chamber's transmitting antenna or one of the plates.

The output power from the oscillator is adjusted to give approximately the same coupled power to the reference antenna. This may require the use of a calibrated attenuator between the reference antenna and analyzer to keep

from reducing the oscillator output power too low. These measurements are recorded on another channel of the analyzer. The radiated power from the

EUT is then determined by:

= P

^^^f(EUT)

3 rad(EUT) n(t) ' ^ ^

ref(t)

where P j~,„TTmN is the power received by the reference antenna when the EUT ref(EUT) ^ ' radiates

An example of SE data obtained using the block diagram of figure 7.1 is

given in figure 7.3. The EUT is a TEM cell having a cross -section area of 3 cm x 6 cm with a 15 mm circular aperture. This EUT is a control standard

device developed at NIST for SE measurement standardization. Two traces are shown in figure 7.3. The top trace is the measurement of the reference

24
antenna received power recorded, in dBmW, on channel A of the analyzer. The

bottom trace is the received power coupled to the EUT recorded, in dBmW, on channel B of the analyzer. A 10 dB attenuator was used between the

reference antenna and analyzer in performing this measurement. This 10 dB

must be added to the difference, P ^ - P„-_, , in calculating the SE of the

ref EUT ° EUT.

7 . 2 Automated EMC/V Measurements

7.2.1 TEM Mode Measurement Procedures

Detailed step -by- step measurement procedures using TEM cells for EMC

measurements are given in [1]. Measurements using the combined chamber in

the TEM mode region of operation follow the same steps. These consist of:

(a) Determining the EUT test parameter requirements in terms of test

field amplitude, waveshape, modulation, frequency range and interval,

exposure polarization, exposure time requirements, and EUT performance

degradation criteria;

(b) Placing the EUT inside the cell in one of two locations for

testing. The first location is close to the ground plane to minimize

exposure of the EUT's input/output, power, and monitor leads to the field

generated inside the cell. The second position is midway between the plate

and ground plane. This position provides greater field exposure to the

EUT's leads and hence gives an indication of how energy is coupled to the

EUT;

(c) Accessing the EUT as required for operation and performance

monitoring. The EUT input/output and ac power leads should be as nearly the same as in its intended use. Leads should be the same length, if possible,

and be terminated into their equivalent operational impedances so as to

simulate the EUT in its operational configuration. Care must be taken in

routing the leads including monitor leads (if not transparent to the rf

field) inside the chamber for obtaining the most meaningful, repeatable

results. Placement of the leads, wiring harnesses, etc., should be recorded

for future reference so that if repeat measurements are required, the leads can be placed as nearly identical to the original tests as possible. Fiber

optics or high- resistance lines should be used for monitoring the operation

of the EUT if possible. Such leads minimize coupling to and distortion of

the test fields;

(d) Connecting the measurement system as shown in figure 7.4; and

(e) Applying power to the chamber's TEM lines with the EUT in operation

and recording the EUT's monitor responses as a function of the test field

level, modulation parameters, polarization, and frequency to determine

immunity.

7.2.2 Reverberating Mode -Tuned Measurement Procedure

A detailed step-by-step measurement procedure and flow diagram for

performing automated radiated susceptibility tests using a reverberating

25
chamber is contained in [5] . The procedure is the same for the combined

chamber used in the reverberating region of operation. It is summarized

briefly as follows:

(a) Determine the testing requirements for the EUT as referred to in

section 7.2.1 item (a) above;

(b) Place the EUT inside the chamber and access it for operation and

monitoring;

(c) Connect the measurement system as shown in figure 7.5;

(d) Input the measurement and calibration parameters into the computer.

These include frequency ranges and increments for the tests, number of tuner

steps to be used, signal generator output levels to generate the required

test field amplitudes, EUT output level maximum limit to prevent damage, EUT

response time, and cables, directional couplers, and sensors calibration

factors ; and

(e) Perform the measurements in automated sequence starting at the

lowest frequency and level, increasing the frequency in increments at a constant exposure level and then increasing the levels until the maximum

test level is achieved or the EUT maximum allowable response is reached.

Correct the measurement data using the appropriate calibration factors

previously put into the computer. (This can occur simultaneously with making

and recording the measurements.) The optimum technique for obtaining modetuned data is to step or increment the frequencies at each tuner position

before stepping the tuner to its next position. When the tuner is stepped,

it should be allowed to stabilize while power meters, voltmeter, monitors,

etc., are being zeroed and residual offsets measured. Obviously the rf is

turned off while this is done. The rf power is then turned on; the test

field is established; the measurements are made at each frequency; and the

data are corrected (with respect to a given input power) and stored. The

corrected data are then configured at the end of the run, after the tuner

has been stepped through all positions to complete one revolution (360°) and

compiled by frequency. Various calculations of averages are then made and

the data are presented in the final form.

7.2.3 Reverberating Mode -Stirred Measurement Procedure

Although the mode -stirred measurement approach begins in a similar

manner to the mode -tuned described above, the automated measurement sequence

of events is somewhat different, and hence some parameters necessary for

performing the measurements differ from the mode -tuned approach. The

procedure is summarized briefly as follows, after determining the EUT test

parameters

(a) Place the EUT inside the chamber and access it for operation and

monitoring;

(b) Connect the measurement system as shown in figure 7.5;

(c) Input the measurement and calibration parameters into the computer.

These include the frequency ranges and increments, the signal generator

26
output levels , range and increments , the EUT output level minimum and

maximum response limits, the EUT response time, the cables, directional

couplers, and sensors calibration factors, and the tuners rotation rates;

(d) Perform the measurements using the automated sequence, which

includes setting the generator to its first frequency, starting the tuners

in motion, and monitoring the EUT response (s) while slowly increasing the

test fields. When the EUT minimum response is exceeded or the maximum test

field is reached, the tuners are stopped; the system is initialized with the

monitor voltmeters; and the spectrum analyzer is set up to start taking data

when triggered. The rf power is turned on, the net input power is measured

and the tuners are placed into motion as the voltmeters and spectrum

analyzer are triggered into performing their measurements;

(e) Make calculations of averages and corrections for calibration

factor while the measurement proceeds. The results are then Q,<5mpiled at the

end of each frequency run after one complete revolution of the tuner and

stored in memory for presentation at the conclusion of the measurements. If

the maximum EUT response is reached during the measurement, the generator

output is reduced by an interval of 1 dB and the measurement is repeated

starting with a complete new initalization and a full 360° revolution of the

tuners. This ensures accurate results without damaging the EUT.

7.2.4 Comparison of TEM/Reverberating Chamber Measurement Results with

Anechoic Chamber Tests - Some Examples

Comparisons of the EUT response obtained using a reverberating chamber

with anechoic chamber data are typically made in terms of maximum or peak

values. The main reason for this is that the EUT's worst- case performance

or susceptibility is normally what is desired. An additional practical

consideration is that it is very difficult to obtain a true average EUT

response from anechoic chamber data. Even determining the EUT's peak

response in an anechoic chamber may require considerable effort involving

complete pattern measurements

.

The benefit of making this type of comparison is to obtain a "correlation factor" between results obtained in the TEM/reverberating

chamber and those obtained in an anechoic chamber. This was done for three

reference standard EUTs . These were: (a) a 1 cm dipole probe, (b) a small

broadband horn antenna similar to what is used at frequencies from 0.8 to 18

GHz as the transmitting and receiving antennas in the chamber, and (c) a small TEM cell with 3 cm x 6 cm cross section and a 15 mm diameter circular

aperture. The measurements were performed in a combination of facilities

including the RADC's small 1 . 41 m x 1 . 57 m x 1 . 75 m aluminiam reverberating

chamber located at Griffiss AFB, NY; the NIST 2 . 74 m x 3 . 05 m x 4. 57 m steel

reverberating chamber located in Boulder, CO; the USAEPG 1/10 scaled 1.31 m

x 2.41 m X 3.87 m TEM/reverberating chamber located presently at Ft.

Huachuca, AZ; and the NIST 4.9mx6.7mx8.5m anechoic chamber located in

Boulder, CO.

The maximum responses obtained using the 1 cm dipole probe measured in

the 1/10 scaled TEM/reverberating chamber, the NIST reverberating chamber,

and the NIST anechoic chamber are given in figure 7.6. The probe response

in the anechoic chamber is greater than its response in the reverberating

27
chamber and TEM/reverberating chamber by an average of about 2 dB. This is

expected, since 2 dB corresponds approximately to the free-space gain of an "electrically small dipole. Thus, the correlation factor between the

reverberating chambers' and the anechoic chamber's results is the free-space

gain of the EUT [5]

Measured maximum responses of a broadband horn antenna obtained using

the 1/10 scaled TEM/reverberating chamber and the NIST anechoic chamber are shown in figure 7.7. The horn was aligned on the boresight (main beam) of

the transmitting antenna for the anechoic chamber measurements to obtain the

maximum response. As expected, the horn response is greater in the anechoic

chamber than in the TEM/reverberating chamber by approximately its freespace gain. This is shown in figure 7.8 where the horn's calibrated freespace gain has been subtracted from its measured maximum response. Clearly,

the results are in general agreement.

^ Figure 7.9 shows a comparison of the shielding effectiveness of a small

^apertured TEM cell obtained from theoretical calculations, from measurements

made using the RADC small reverberating chamber, and from measurements made

using the 1/10 scaled TEM/reverberating chamber. Both mode -tuned and models tirred approaches were considered using the scaled TEM/reverberating

bhamber. The RADC chamber measurements were performed using the mode- tuned

^approach. The mode- tuned results for the two chambers are in good

agreement, but are greater by a few decibels at the higher frequencies than

the theoretically determined values (straight line). The mode-stirred data

agree better with the theoretically determined values. The limited number

of tuner positions (200) used for the mode-tuned measurement may explain the

higher SE results, since a limited sample at higher frequencies does not

allow a true measurement of the peak response of the EUT. In general,

however, the results agree within predicted measurement uncertainties.

8. Svumnary and Conclusions

The designed hybrid chamber has three distinct regions of operation:

(a) TEM mode, (b) transition, and (c) mode-stirred. For the 1/10 scale

chamber, the TEM region covers the frequency range from approximately 10 kHz

to 70 MHz; the transition region covers the frequencies from 70 MHz to about

300 MHz; and the mode -stirred region covers the frequency range from 300 MHz

to > 40 GHz. In the transition region, the first few higher order modes are excited inside the chamber, but in insufficient number for the chamber to

reverberate properly. Thus, the test field is a combination of the TEM mode

and a limited number of higher order modes that yield a complex field that

is difficult to accurately determine both in polarization and amplitude.

Techniques such as absorber loading, to suppress the Q of the higher order

modes, and test field averaging over the test volume, using multiple probes,

may be helpful in reducing errors in establishing the test field and in

defining the test field parameters.

Test fields are excited in the chamber at frequencies up to

approximately 1 GHz by using the transmission lines. At frequencies above

approximately 1 GHz, ridged horn antennas are used. The parallel plate

lines should be unterminated (open circuit) with the bulkhead feed- through

connectors capped to prevent rf leakage to outside the chamber when they are

28
not in use. Other important characteristics may be summarized as follows,

together with some recommendations:

(a) Spatial variations of the maximum and average electric fields in

the test volume of the chamber are given in table 4.1 at a few selected

discrete frequencies. These data were obtained using the NIST multiprobe

system and the mode -tuned approach with 200 tuner increments per revolution

at frequencies from 0.03 to 2 GHz. The limitation for determining the

spatial E-field variation depends on the increasing mode density and hence

field complexity in the chamber as a function of increasing frequency. The

spatial E-field variations should decrease as frequency increases above 2 GHz to less than the result of ± 2 dB shown in the table for 2 GHz.

(b) Expected field levels to be generated in the three frequency

regions by a given input power are presented in table 4.2, together with

estimated uncertainties.

(c) The maximum E-field established inside the chamber at frequencies

above 300 MHz is about 7 to 8 dB greater than the average E-field. This is

consistent with the previous results obtained for other reverberating

chambers such as the RADC large chamber , the Naval Surface Weapons Center

(NSWC) chamber, and the NIST reverberating chamber.

(d) Reference standard EUT responses determined in this scaled chamber

are approximately the same for the same EUT at the same field levels as using RADC, NSWC and NIST reverberating chambers.

(e) The directional characteristics of an antenna or EUT placed inside

this hybrid chamber operated in the reverberating region of frequencies are

lost, resulting in an equivalent gain of dB in the highly complex field

environment.

(f) The correlation factor between free-space (anechoic chamber) and

reverberating chamber measurement results for the same EUT is the free -space

(far-field) gain of the EUT. This implies that susceptibility criteria

determined for an EUT using a reverberating chamber should include an additional factor proportional to the EUT's open-field estimated gain as a function of frequency.

(g) If the chamber is used for pulsed rf EMS/V testing, and the input

pulse widths are shorter than the chamber's charge time, an error will

result in establishing a known maximum amplitude of the test signal. An

estimate of the correction factors to apply for shorter pulses is given in

figures 5.8 and 5.9.

(h) The use of rf absorbing material inside the chamber significantly

reduces the Q and charge time of the chamber, and hence should be considered

as an effective means to improve the fidelity of the test pulse waveform to

be applied to the chamber. However, care must be exercised to ensure that

the absorber would not excessively reduce the Q so that the reference

antenna received power drops by more than 6 dB , or the tuner effectiveness

is reduced to less than 20 dB.

29
(i) The use of rf absorbers also reduces the amplitude and variations

In the E- field generated In the transitional frequency region, and thus the

uncertainly In establishing the test field level, at perhaps the expense of

Increased rf Input power requirements

( j ) The mode -tuned approach Is recommended for use at frequencies from

70 MHz to 2 GHz with at least 200 tuner positions per revolution, if

absolute levels of the test field are required. The mode -stirred approach

is recommended for use at frequencies above 2 GHz for absolute measurements

(with more than 3000 samples per revolution for frequencies between 2 to 4

GHz, and more than 5000 samples per revolution for frequencies above 4 GHz).

The mode-stirred approach is also recommended for all frequencies above 300

MHz if only relative measurements are made.

(k) Antennas for transmitting rf energy into the chamber and for

measuring the test E- field should not be used outside their specified

frequency range. For example the 0.8 to 18 GHz ridged-horn antennas should

not be used outside their specified band.

(i) Additional work is needed to verify experimentally the correction

factors shown in figures 5 . 8 and 5.9. This could be accomplished by

comparing the measured responses of a well -characterized EUT to cw and

pulsed rf fields as a function of frequency and pulse widths using both an anechoic chamber and the TEM/reverberatlng chamber.

(m) If corrections are not made for mismatches of the transmitting and

receiving antenna, absolute amplitude measurements of the test field inside

the chamber will be lower than the actual values. This will cause an error

in determining the EUT response, which will also be too low. Experiences in

performing this type of measurements indicate that these errors may be as great as 5 dB when VSWRs exceed 15 to 1,

(n) The average wave impedance in the reverberating region of

operation, when the maximum response of an EUT is measured, appears to be

higher than 377 Q. This is especially true for frequencies lower than 2 GHz. This means that if a wave impedance of 377 Q is assumed when

determining the maximum amplitude of an exposure field, there will be a systematic error resulting in too low a calculated E-fleld, and thus also in

too high an EUT response. If the field strength is determined by using a calibrated E-field probe, there is still some uncertainty, since the

environment in which the probe is calibrated is different than the

reverberating chamber environment.

9

Acknowledgments

Work described in this report was sponsored by the U. S. Army

Electronic Proving Ground, Ft. Huachuca, AZ , with Robert Weeks as project

officer. The authors acknowledge his assistance in making this project

possible. In addition, the authors express their appreciation to Motohisa

Kanda for his support and to J . W. Adams for his helpful comments.

10. References

[1] Crawford, M. L. Generation of standard EM fields using TEM

transmission cells. IEEE Trans. on Electromag. Compat. EMC-16

(no.4) : 189-195; 1974, November.

30
[2] Crawford, M. L. ; Workman, J. L. Using a TEM cell for EMC measurements

of electronic equipment. NBS Tech. Note 1013; Revised 1981 July.

[3] Corona, P.; Latmiral, G. ; Paolini, E. ; Piccioli, L. Performance of a reverberation enclosure for power measurements in the microwave range.

2nd Symp . Tech Exhibition on EMC; 1977; Montreux, Switzerland.

[4] Bean, J.L.; Hall, R.A. Electromagnetic susceptibility measurements

using a mode -stirred chamber. IEEE Int. Symp . on EMC; 1978 June;

Atlanta, GA.

[5] Crawford, M.L.; Koepke , G.H. Design, evaluation and use of a reverberation chamber for performing electromagnetic

susceptibility/vulnerability measurements. NBS Tech. Note 1092; 1986

April.

[6] Ma, M.T. ; Kanda, M. ; Crawford, M.L.; Larsen, E.B. A review of

electromagnetic compatibility/interference measurement methodologies.

Proc. of IEEE; Vol. 73, No. 3; 1985 March.

[7] Liu, B. H. ; Chang, D. C; Ma, M. T. Eigenmodes and the composite

quality factor of a reverberating chamber. NBS Tech note 1066; 1983

Augus t

.

[8] Dunn, J. M. Local, high-frequency analysis of the fields in a modestirred chamber. IEEE Trans, on Electromag. Compat. EMC-32 (no. 1):

53-58; 1990 February.

[9] Corona, P.; Latmiral, G.; Paolini, E. Performance and analysis of

reverberating enclosures with variable geometry. IEEE Trans, on EMC;

EMC -22 (no. 1): 2-5; 1980 February.

[10] Wilson, P. F. On slot-coupled waveguide and transmission line

structures. Ph.D. Dissertation, Dept. of Electrical Engineering, Univ.

of Colorado, Boulder; 1983.

[11] Crawford, M. L. ; Koepke, G. H.; Ladbury, J. M. EMR test facilities - Evaluation of reverberating chamber located at RADC , Griffiss AFB,

Rome, NY. NBSIR 87-3080; 1987 December.

[12] Crawford, M. L. Generation of standard EM fields for calibration of

power density meters: 20 kHz to 1000 MHz. NBSIR 75-804; 1975 January.

[13] Bowman, R. R. Calibration techniques for electromagnetic hazard

meters: 500 MHz to 20 GHz. NBSIR 75-805; 1976 April.

[14] Larsen, E. B.; Ries, F.X. Design and calibration of the NBS isotropic

electric-field monitor [EFM-5], 0.2 to 1000 MHz. NBS. Tech. Note 1033;

1981 March.

[15] Kanda, M. ; Driver, L.D. An isotropic electric-field probe with tapered

resistive dipoles for broad-band use, 100 kHz to 18 GHz. IEEE Trans, on Microwave Theory and Tech. MTT-35 (no. 2); 1987 February.

31
15] Bensema, W.D.; Reeve, G.R.; Koepke , G.H. A multisensor automated EM

field measurement system. Proc . IEEE IMTC ; 1985; Tampa, FL.

17] Liebe, H. J. An updated model for millimeter wave propagation in moist

air. Radio Science 20 (no. 5): 1069-1089; 1985 May.

18] Liebe, H. J.; Layton, D. H. Millimeter-wave properties of the

atmosphere: Laboratory studies and propagation modeling. NTIA Report

87-224; 1987 October.

32
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Table 3.4 Theoretical mode calculations of frequency and gap between

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Mode Indices Frequency Gap Size

# MHz MHz

1 Oil 73.36 73.36

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8 102 138.19 2.28

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16 113 174.60 1.07

17 122 186.06 11.46

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19 104 192.65 1.80

20 024 198.86 6.21

21 032 202.31 3.45

22 114 202.47 0.17

23 015 203.52 1.04

24 123 205.25 1.73

25 130 219.12 13.87

26 033 220.08 0.96

27 131 222.52 2.44

28 105 225.01 2.49

29 124 229.43 4.42

30 025 230.35 0.92

31 201 232.09 1.74

32 132 232.42 0.33

33 115 233.47 1.05

34 210 237.16 3.69

35 211 240.30 3.14

36 016 240.70 0.39

37 202 241.60 0.90

38 034 242.79 1.19

39 133 248.05 5.26

40 212 249.50 1.45

44
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45
Table 3.6 Relative permeability of cold rolled 11 gauge steel

to be used in construction of welded shielded

enclosures as a function of frequency.

Frequency (GHz) Relative Permeability

0.03 1000

0.05 1000

0.10 1000

0.15 350

0.20 100

0.30 30

0.50 8

0.70 4

1.00 2

1.50 1.5

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Figure 4.3 Measured VS'JR of (a) top plate, (b) back plate, and (c) side

plate TEM line, from 1 to 2000 MHz, transmittina into the

1/10 scaled TEM/ reverberating chamber.

49
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Figure 4.6 (a) Theoretical and experimental Q of the USAEPG 1/10 scaled chamber

and (b) ratio of theoretical to experimental Q 0.2 to 18 GHz

52
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Figure 4.7 E-field components measured at the center of the 1/10 scaled chamber using a NIST isotropic (5 cm dipole) probe (position r3 in figure 4.14) with chamber

excited by: (a) top plate, (b) back plate, and (c) side plate. Net input

power is 1 W.

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73
Table 4.1 Spatial variations in E fields measured inside the 1/10 scaled

TEM/ reverberating chamber over the test volume defined by probes

shown in figure 4.14.

Frequency, MHz E-Fields Variation, dB

0.01-200 +10

300 ± 8

500 t 6

700 + 4

1000 ±3

2000 + 2

74
Table 4.2 Summary of operation characteristics of the 1/10

scaled TEM/reverberating chamber.

1. TEM Region (10 kHz to 70 MHz)

Planar Field Simulation

1 kW Input Power Yields Approximately

250 V/m vertical polarization and 80 V/m horizontal

polari zation

Uncertainty < ± 2 to 9 dB

2. Transition Region (70 MHz to 300 MHz)

TEM Field Plus Limited Higher-Order Modes

1 kW Input Power Yields Approximately 250 to 1000 V/m

Polarization Variable

Uncertainty = ± 10 dB

3. Reverberating Region (300 MHz to 40 GHz or Higher")

Complex Field Simulation

10 W Input Power Yields Approximately 100 to 300 V/m

Uncertainty = ± 4 to 8 dB

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1 Pc. 7.6 cm Absorber

(b)

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(c)

1 Pc. 20.3 cm Absorber

(d)

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(f)

4 Pc. 30.5 cm Absorber

(g)

8 Pc. 30.5 cm Absorber

h'

2.5 Time (ps)

5

Figure 5.2 Maximun vaijes of transmitted (positivo) ar.d received (neqstive) r-^ pulse waveforms in the 1/10 scaled TEM/reverberating chamber

determined with increasing amounts of rf absorber loading at 0.9 GHz

77
Time (ps) Time (ps)

Figure 5.3 Maximum values of transmitted and received rf pulse waveforms

inside empty (no absorber) 1/10 scaled TEM/reverberating chamber

determined at selected frequencies. Rf input pulse width is

5 ys, duty cycle is 0.001.

78
2.5

Time (us)

Figure 5.4 Sheet 1 of 3

79
1.5

Tiine (ps)

Figure 5.4 Sheet 2 of 3.

80
Figure 5.4 Sheet 3 of 3

Figure 5.4 Maximum values of transmitted and received rf pulse waveforms inside

1/10 scaled TEM/ reverberating chamber determined with: (a) no absorber,

and (b) 1 piece of 20.3 cm X 61 cm X 61 cm (8" X 24" X 24") rf absorber

at selected frequencies.

81
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Figure 5.8 Estimated correction factors for amplitude response of rf

test pulses in the 1/10 scaled TEM/reverberating chamber

at selected frequencies as a function of input pulse width

using (a) no absorber, and (b) 1 piece of 20.3 cm x 61 cm

X 61 cm rf absorber.

89
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u

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Figure 5,9 Estimated correction factors for amplitude response of

rf test pulses in the 1/10 scaled TEM/reverberating

chamber at selected input pulse widths as a function

of frequency using (a) no absorber, and (b) 1 peice

of 20.3 cm X 61 cm x 61 cm rf absorber.

90
Table 5.1 Summary of influences of rf absorber loading on loss,

quality factor, and charge/decay time of the 1/10

scaled TEM/reverberating chamber at 0.9 GHz.

Absorber

Loading

Min. Chamber

Loss, dB

Max. Oual

Factor

ity Measured

Charge/Decay

Time, yS

None 7.0 10 418 0.29

1 pc. 7.6 cm 10.8 4 333 0.22

1 pc. 12.7 cm 11.1 % 043 0.19

1 pc. 20.3 cm 11.7 3 522 0.18

1 pc. 30.5 cm 12.2 3 139 0.14

2 pc. 30.5 cm 15.6 1 435 0.10

4 pc. 30.5 cm 15.9 1 339 0.10

8 pc. 30.5 cm 21.4 377 0.10

*Charge/decay time is measured from figure 5.2, based upon 63% rise

to steady-state field amplitude inside chamber after excitation

source is turned on, or 63% decay from steady-state amplitude

after source is turned off.

91
Table 5.2 Comparison of measured and calculated charge/decay time

of the 1/10 scaled TEM/reverberating chamber, determined

from chamber average loss measurements (figure 4.5) and

from data shown in figure 5.5.

Frequency

GHz

Average Chamber

Loss, dB

Average

Quality

Factor

X(10)4

Calculated

Charge/Decay

Time*

ys

Measured

Charge/Decay

Time

ys

0.9 18 0.083 0.29 0.56

1.3 22 0.099 0.24 0.68

2.0 26 0.144 0.23 0.60

2.9 30.5 0.155 0.17 0.56

4.2 35 0.167 0.13 0.36

5.65 37.5 0.229 0.13 0.37

8.9 42 0.318 0.11 0.52

12.0 45 0.391 0.10 0.45

16.0 47 0.584 0.12 0.43

18.0 48 0.661 0.12 0.30

*Calculated charge/decay time is from 2Q/c

92
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Figure 6.1 Projected coupling efficiency (minimum loss) of 13 . 1 m x 24 . 1 m x m TEM/reverberating chamber.

93
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Figure 6.2 Projected E-fields in 13.1 m X 24.1 m X 38.7 m TEM/reverberating chamber

for 200 W input power determined from (a) projected chamber losses and

Q factors, and (b) measurements made in RADC chamber and NSWC chamber.

94
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Figure 6.3 Projected charge time for 13.1 m X 24.1 m X 38.7 m TEM/

reverberating chamber.

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Figure 7.7 Maximiom powers received by a broadband (1 to 18 GHz) horn antenna in an E-fieid of 37 dBV/m using: (a) 1/10 scal ed TEM/reverberating

chamber, and (b) NIST anechoic chamber.

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TEM/reverberating chamber with its response minus free-space gam

measured in the NIST anechoic chamber.

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Frequency (GHz)

Comparison of SE of 3 cm X 6 cm TEM cell with 15 mm circular

aperture: (a) theoretical calculations, (b) measured in RADC

chamber, (c) measured in 1/10 scaled chamber with mode-tuned

approach, and (d) measured in 1/10 scaled chamber with modestirred approach.

104
BL-114A

(5-90) U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATASHEET

PUBUCATION OR REPORT NUMBER

NIST/TN-1342

2. PERFORMINQ ORQANIZATION REPORT NUMBER

PUBUCATION DATE July 1990

4. TITLE AND SUBTITLE

Measurement and Evaluation of a TEM/ Reverberating Chamber

5. AUTHOR(S)

Myron L. Crawford, Mark T. Ma, John M. Ladbury, and Bill F, Riddle

6. PERFORMINQ ORQANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BOULDER, COLORADO 60303-3328

7. CONTRACT/QRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

9. SPONSORINQ ORQANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP) Electromagnetic Environmental Effects Division

U.S. Army Electronic Proving Ground

Fort Huachuca, Arizona 85613-7110

10. SUPPLEMENTARY NOTES

11. ABSTRACT (A 200-WORO OR LESS FACTUAL SUMMARY OF MOST SIQNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOQRAPHY OR

UTERATURE SURVEY, MENTION IT HERE.)

This report summarizes the measurement and evaluation of a 1/10 scaled model TEM/reverberating chamber developed as a single, integrated facility for testing radiated electromagnetic

compatibility/vulnerability (EMC/V) of large systems over the frequency range, 10 kHz to

40 GHz. The facility consists of a large shielded enclosure configured as a transverse

electromagnetic (TEM), transmission line-driven, reverberating chamber. TEM mode test

fields are generated at frequencies below multimode cutoff, and mode-stirred test fields are generated at frequencies above multimode cutoff. Both the chamber's cw and pulsed

rf characteristics are measured and analyzed. The report also discusses the basis for

such a development including the theoretical concepts, the advantages and limitations,

the experimental approach for evaluating the operational parameters, and the procedures

for using the chamber to perform EMC/V measurements. A full-scale chamber that will provide

a test volume of 8 m x 16 m x 30 m is proposed. Some projections of the full-scale

chamber's estimated characteristics and operational parameters are also given.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS) cw and pulsed rf testing; radiated EM compatibility and vulnerability measurements;

reverberating chamber; TEM cell

13. AVAILABIUTY UNUMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).

X

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161.

14. NUMBER OF PRINTED PAGES

116

IS. PRICE

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