NBSIR 74-352 ^K)

EXPERIMENTAL EVALUATION OF THE U.S. ARMY, WARRENTON

TRAINING CENTER ANECHOIC CHAMBER

M.L. Crawford

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

November 1973

Prepared for

U.S. Army

Warrenton Training Center

Virginia

NBSIR 74-352

EXPERIMENTAL EVALUATION OF THE

U.S. ARMY, WARRENTON TRAINING CENTER

ANECHOIC CHAMBER

M.L. Crawford

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

November 1973

Prepared for

U.S. Army

Warrenton Training Center

Virginia

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

CONTENTS

Page

1. INTRODUCTION 1

2. SOME THEORETICAL ANALYSIS OF THE TAPERED CHAMBER 5

3. DESCRIPTION OF MEASUREMENT SYSTEM AND TECHNIQUES 10

4. EXPERIMENTAL RESULTS 14

5. CONCLUSIONS AND RECOMMENDATIONS 17

6. REFERENCES 19

LIST OF FIGURES

Figure Page

1 Cross -Sect ional View of Tapered Chamber. 20

2 Simplified Plan Geometry of the Tapered Section of

the Chamber Showing Quantities used in Calculation, 21

3 Three Dimensional Sketch of Chamber Geometry Showing

Image of Reflection from Upper Wall to Field Point

on Horizontal Plane. 22

4 Calculated Amplitude Taper Across the Quiet Zone in

both E and H-Planes of the Transmit Antenna as a Function of Distance from the Center. 23

5 Simplified Sketch Illustrating Total Aperture Field

Variation Caused by a Single Specularly Reflected

Wave . 24

6 Reflection Coefficient Chart. 25

7 Block Diagram of NBS "Free Space VSWR" Measurement

System. 26

8 Probe Mounted on Probe Traversing Mechanism Inside

Chamb er . 27

9 Instrument Circuit Functional Diagram. 28

10 Probe Response Pattern. 29

iii
LIST OF FIGURES (Continued)

Figure Page

11 Probe Sensitivity. 30

12 Photograph o£ Chamber Evaluation Probe and Meter. 31

13 Field Probe Traversing Mechanism Detail. 32

14 Recording o£ Field Amplitude Across Quiet Zone Vs.

Probe Position for Xy (Transverse), Yy (Vertical),

and Zy (Longitudinal) Scans at 160 MHz. (Vertical

Polarization) 33

15 Recording of Field Amplitude' Across Quiet Zone Vs.

Probe Position for Xj^ (Transverse), (Vertical),

and (Longitudinal) Scans at 160 MHz. (Horizontal

Polarization) 34

16 Recording of Field Amplitude for Extended Scans

Centered on Quiet Zone Vs. Probe Position at 160 MHz. 35

17 Recording of Field Amplitude Across Quiet Zone Vs.

Probe Position With Model Tower in Place at 160 MHz.

(Horizontal Polarization) 36

18 Recording of Field Amplitude Across Quiet Zone Vs.

Probe Position at 2.6 GHz. 37

19 Recording of Field Amplitude for Extended Scans Centered on Quiet Zone Vs. Probe Position at 2.6 GHz. 38

20 Recording of Field Amplitude Across Quiet Zone Vs.

Probe Position With Model Tower in Place at 2.6 GHz. 39

21 Recording of Field Amplitude for Extended Scan Centered on Quiet Zone Vs. Probe Position With Model

Tower in Place at 2.6 GHz. 40

22 Recording of Field Amplitude Across Quiet Zone Vs.

Probe Position at 4.0 GHz. 41

23 Recording of Field Amplitude for Extended Scans Centered on Quiet Zone Vs. Probe Position at 4.0 GHz. 42

24 Recording of Field Amplitude Across Quiet Zone Vs.

Probe Position at 8.0 GHz. 43

iv
LIST OF FIGURES (Continued)

Figure Page

25 Recording o£ Field Amplitude for Extended Scan Centered on Quiet Zone Vs. Probe Position at 8.0 GHz. 44

26 Summary of Chamber Quiet Zone Reflectivity and

Amplitude Distribution. 45

27 Chamber Polarization Measurements at 160 MHz. 46

28 Chamber Polarization Measurements at 2.6 GHz. 47

29 Chamber Polarization Measurements at 4.0 GHz. 48

30 Chamber Polarization Measurements at 8.0 GHz. 49

31 Summary of Chamber Symmetry/Polarization

Measurements. 50

32 Location of Source of Reflections Based Upon Period

of Amplitude Ripple For X (transverse) Scans. 51

33 Location of Source of Reflections Based Upon Period

of Amplitude Ripple For Y (vertical) Scans. 52

34 Location of Source of Reflections Based Upon Period

of Amplitude Ripple For Z (longitudinal) Scans. 53

V

EXPERIMENTAL EVALUATION OF THE

U.S. ARMY, WARRENTON, VIRGINIA, TRAINING CENTER,

ANECHOIC CHAMBER

This report describes the experimental performance

evaluation of a 50-foot tapered anechoic chamber located

at U.S. Army, Warrenton Training Center. Measurements

of chamber reflectivity and polarization properties

were made at four frequencies, 160 MHz, 2.4 GHz, 4.0 GHz,

and 8.0 GHz. The measurement system developed by NBS

for measuring chamber reflectivity utilizes a new NBSdesigned isotropic probe mounted on a polyfoam track

and support structure for probing the chamber's quiet

zone in all three orthogonal planes. The probe is omnidirectional within 1 dB and has sufficient sensitivity

to permit evaluation of reflectivity levels as low as -59 dB. Complete scattering information is obtained

without receive antenna pattern correction with only

three orthogonal scans per frequency required. The

report presents a brief treatment of the theory involved for evaluating the chamber, a description of

the measurement system and technique used, the measurement results , and conclusions and recommendations

including location of sources of reflection within

the chamber.

Key words: Anechoic chamber evaluation; omnidirectional probe; polarization; reflectivity;

three-dimensional scanning.

1. INTRODUCTION

This report describes the experimental performance

evaluation of a tapered anechoic chamber 50 feet long located

at U.S. Army, Warrenton Training Center. The measurements

were made by the National Bureau of Standards, Boulder,

Colorado (NBS) , with support and assistance from Warrenton

Training Center personnel. Funds were provided by Warrenton
under contract WTC-5 38-74. Evaluation was performed to determine chamber reflectivity and polarization properties at four

frequencies; 160 MHz, 2.4 GHz, 4.0 GHz, and 8.0 GHz.

Section 2 of the report presents a brief treatment of

the theory involved in evaluating anechoic chambers. The

system and measurement technique employed in evaluating the

Warrenton chamber are described in section 3. Section 4 discusses the measurement results and presents the data in

graphical form. Conclusions and recommendations are presented

in section 5

.

There has always been a need to know what limitations

exist in the use of anechoic chambers. Improvements in chamber

design and absorber quality have extended their application

and frequency range and placed new emphasis on accurate determination of the credibility of measurements made inside the

chambers. The evaluation technique described in this report

was developed at NBS in response to the need for an improved,

accurate means of determining chamber performance.

Following the invention of the anechoic chamber, pattern

comparison techniques have been used to determine performance.

This technique, which has been reported in numerous sources

[1,2,3] is cumbersome and limited in completeness and accuracy.

Because of different averaging techniques and different antennas used in this technique, chamber reflectivity measurement

2
results varied considerably from chamber to chamber. A

second evaluation technique, known as the "Free-Space VSWR"

technique [4], utilizes field probing to determine signal

strength within the chamber quiet zone as a function of probe

position. By coupling the linear motion of the receiving

antenna and the response of the receiver to a XY recorder,

an interference pattern similar to a standing wave curve is

obtained. The amplitude and period of these oscillations are

then used to determine the amplitude and arrival angle of the

interfering signal. Since the reflected or interfering signal

in a chamber is the vector sum of many components arriving

from different parts of the chamber, the results of each scan

through the quiet zone are a function of the orientation and

pattern of the receiving antenna. Typically, a number of

scans are made with different aspect angles to obtain complete

interference pattern information. Corrections are then made

to the received data to account for the receive antenna pattern. This "VSWR" technique is usually considered more accurate for practical reasons than the pattern comparison

technique [5], but it is still limited by the discrimination

of the receive antenna to incoming signals arriving at angles

within the pattern nulls. Experiments using the "VSWR" technique indicate that the use of receiving antennas with medium

to high gain for reflectivity evaluation gives results that

always favor the chamber performance [6].

3
The technique briefly described in this report utilizes

a new NBS-designed receiving probe* consisting o£ three orthogonal dipoles designed to have essentially an isotropic

receiving pattern. This type of probe responds equally (within ± 1 dB) to signals arriving from any direction and with

any polarization. Thus complete scattering information is

obtained without receive antenna pattern corrections with only

three orthogonal scans per frequency required. Sensitivity

of the probe is sufficient to measure "free-space VSWR's" as

low as 0.02 dB (chamber reflectivity = 59 dB) , using typical,

moderate gain (- 10 dB) source antennas driven by 10 -watt

power oscillators. The probe's main limitation is its restricted use for pin-pointing sources of reflections within

the chamber. This handicap is partially overcome by the

probe's added feature of switchable dipole selection. This

allows the use of the probe as a single dipole or combination

of dipoles appropriate to the particular measurement need.

If additional resolution for the location of the source of

reflection is required, one can always replace the probe by

higher gain receiving antennas oriented toward the suspect

area.

*This probe was designed and built by R.R. Bowman and W.E.

Jessen of NBS and was modeled after the NBS electromagnetic

hazard probes but with greater sensitivity [7].

4
2. SOME THEORETICAL ASPECTS OF THE TAPERED CHAMBER

2 . 1 The Tapered Chamber as a Ground Reflection Range

A cross -sectional view of the Warrenton chamber is shown

in figure 1. It is a truncated rectangular cone terminated

in a right parallelepiped. At the lowest frequency of

160 MHz, the cone's absorber represents a fairly smooth

reflecting surface and reflects a significant amount of

energy into the chamber's quite zone. To examine this effect

we consider first-order reflections as though they originated

from image antennas located 90 degrees apart around the apex

of the cone (see figure 2) . By determining the path lengths

of the reflected waves as the distances from the field point

P to the respective images, and with some simplifying assumptions, the field at point P, E(P), can be expressed in

terms of the direct-path field amplitude E^.

E(P) = E

D

-j2^Rj^/A

e + C

fR

1

D

R

R.

^ + p

^

R^ -Ut\R /X

o (1)

R„, R„ , R„ are as shown in figure 2; C, and C- are the

complex reflection coefficients from side walls and topbottom walls in the taper; X is the wavelength at the test

frequency; and R^ is the path length as shown in figure 3

5
between the image of reflection from upper and lower walls to

the field point on a horizontal plane. Details of the theoretical analysis are treated briefly in [2] and more completely

in an evaluation report by T.G. Hickman and T.J. Lyon of

Scientific Atlanta, Inc. [8].

The validity of eq. (1) is dependent upon a number of

assumptions and approximations including a simplified fourimage reflection in the cone of the chamber, equal approximations of the reflection coefficients C^^ for the two side

images, equal approximations of the reflection coefficients

for the top and bottom images, etc. Needless to say,

calculations using eq. (1) yield only first-order approximations, but do give theoretical results typical of what is

achieved in practice. Tapered chambers, by virtue of their

design, typically have field amplitude uniformity across the

quiet zone of a few tenths of a dB . The report referred to

above [8], for example, contains the calculated amplitude

distributions for the quiet zone of an 84-ft. tapered chamber.

Figure 4 is a reproduction of these curves. Calculations for

the Warrenton chamber were not made but should produce results similar to those shown in figure 4. Experimental

measurements made of the amplitude distribution across quiet

zone of the Warrenton chamber are contained in section 4.

6
2 . 2 Chamber Reflectivity Level Evaluation

Determination o£ the reflectivity level and angle of

arrival of the interfering signal within the chamber's quiet

zone is best understood by considering the simple case of a direct-path plane-wave signal of amplitude combining with

an interfering plane-wave signal of amplitude E^, arriving

at the test point in the quiet zone at angle 9 from the normal.

Figure 5 gives a graphic illustration. A maximum occurs at

points and X2 produced by two reflected wave fronts whose

phases differ by 27t while assuming the phase of E^ is constant. The resultant amplitude is

E = E , + E . (2) —max —d —r ^

^

Halfway between these two points the waves are in phase opposition and the resultant amplitude is

E . = E, - E . (3) —mm —d —r ^

^

The maximum amplitude variation is thus given by

—

E = E - E . = 2E . (4) —max —mm —r ^

^

The angle 6 can be determined by

e = sin"-^(A/d) (5)

where d is as shown in figure 5.

The field at any point in the quiet zone may also be

represented as the sum of two phasors , E^ and E^ as illustrated in figure 5(c) where E^ rotates relative to E^.

7
The phase o£ the field across the quiet zone will vary as the

phase of the resultant of the direct-path and reflected

phasors . The maximum phase variation for this case is

2 sin"-*- E /E, (6) —r —

d

where is less than E^. The stronger signal is assumed

generally to be the direct signal. Figure 6 is a graph of

the magnitude of the resultant field amplitude ripple (Free

Space VSWR, E /E . ) as a function of the ratio E /E,. ^ ' —max —mm^ —r —

d

The preceding example represents an idealized reflection.

The actual ratio of E_^/E^ within a chamber will vary from

point to point due to the complex variation of the reflected

signal(s). The reflected or interfering signal is the vector

sum of many components arriving from different parts of the

chamber. Also in a general test situation, neither the direct

nor the extraneous waves will be strictly planar; however,

the idealized representation does demonstrate the manner in

which extraneous signals distort an otherwise planar wave

front. For the case of tapered chambers, the region around

the chamber axis is clear of reflecting objects with perhaps

the exception of a model tower and head. In the evaluation,

it may be necessary to remove the tower to evaluate the

chamber's reflectivity. This is evident from reflectivity

data obtained in the Warrenton chamber with and without the

model tower in place. If the region around the chamber axis

8
is clear of reflecting objects, the potential sources of

extraneous energy are dissimilarities in the absorber on

the walls, points of discontinuity at wall junctions and

absorber transition regions, the positioner pit, and other

objects placed in the chamber.

A typical recording in a chamber will show periodic

amplitude variations (attributable to the chamber's imperfections) superimposed on the basic variations produced by

changing path distance and beamwidths of the source and

receive antenna patterns. Use of an isotropic receiving probe

simplifies interpretation of the recorded results by eliminating the need for correcting for the antenna pattern and

avoids discrimination against reflected signals arriving at

angles aligned with nulls in the receive probe pattern. After

the interference pattern is recorded, the periodic component

is enveloped by drawing a line connecting the maximums and

another connecting the minimums . The spacing between these

two lines is the "Free Space VSWR" in dB . The worst case

VSWR is indicated by the maximum vertical width between the

two envelope curves.

2 . 3 Chamber Polarization Properties Evaluation

The use of anechoic chambers to determine antenna patterns requires a knowledge of the polarization properties of

the chamber. In the absence of reflections and assuming

9
chamber symmetry, rotation o£ the source (receiving) antenna

about the chamber's axis for incremental rotations of the

receiving (source) antenna should produce identical patterns.

The degree and nature of the deviation in the measured polarization pattern is indicative of the polarization error introduced by the chamber. For these measurements, linear polarized

antennas are typically used. In this case the received

signal of interest is given by

= K cos(a-T^) (7)

where K is the received signal amplitude, a is the angle of

rotation of the test antenna, and is the tilt angle of the

degenerate polarization ellipse of the received wave. The

receive antenna is mounted on the model tower head axis and

patterns are taken by rotating the probe antenna through 360

degrees for systematically stepped rotational displacements

of the source antenna about the line of sight. The measurements can be made for different placements of the transmit

antenna along the axis of the chamber and for different receive antenna positions in the quiet zone.

3. DESCRIPTION OF MEASUREMENT SYSTEM AND TECHNIQUES

• Determinat ion of Distortion o f the Incident Field in Quiet Zone

The technique used by NBS to evaluate the Warrenton chamber

utilizes the "Free-Space VSWR" concept. Figure 7 gives a

10
block diagram of the measurement system. Since the chamber

performance is influenced by the source antenna characteristics

and positioning, the chamber was evaluated using source antennas provided by Warrenton and used in their testing programs. These antennas were mounted in the throat of the

chamber according to the procedure prescribed by the chamber

manufacturer and adopted by Warrenton. The test probe developed by NBS was mounted on a polyfoam track attached to an

NBS designed polyfoam tower. Figure 8 shows a picture of

the probe traversing mechanism assembled inside the chamber.

The test probe developed by NBS consists of three small,

orthogonal dipole-diode antennas that separately sense the

corresponding orthogonal components of the electric portion

of the electromagnetic field. A functional diagram of the

probe and metering circuit is given in figure 9. The three

detected signals are transmitted away from the sensor by means

of specially fabricated high-resistance lines that are essentially transparent to the electromagnetic field. Three preamplifiers buffer these signals, which are then summed in a scaling amplifier. This provides a composite signal that

2 2 2 is proportional to|E|^=(|E| +|E| +|E|)at the position

of the sensor. Switches are installed on each channel to

allow display of the response of each channel separately or

combined. The instrument is easily calibrated by placing the

11
probe in either a "free-space" field or in a contained field

(e.g., in a waveguide) of known intensity and polarization.

The response of each channel and the composite are given in

figure 10. The composite response is isotropic within ± 1 dB

Probe sensitivity is given in figure 11. By using generators

(with the source antennas) that are capable of producing test

fields in the quiet zone of a few microwatts/cm^, this sensitivity was adequate for reflectivity measurement resolutions

of greater than 59 dB . The limiting factor was the noise

level generated by movement of the high- resistance transmission line connecting the probe dipoles to their sensor circuits. A high- resistance static shield around these lines

accounted for the excellent noise factor allowing for the

59 dB plus resolution. A picture of the probe and meter is

shown in figure 12.

The probe traversing mechanism consists of a six-foot

polyfoam track mounted on a polyfoam telescopic tower. A

small carriage with the probe attached moves along the track

via a system of pulleys, teflon guides, and counterweight.

The movement is accomplished by pulling a polyester cord

connected to the probe carriage and threaded through the

pulleys, around the guides, and down to a plastic drum that

is attached to a ten-turn potentiometer. The cord is wrapped

around the drum, then through guides at the base of the tower

and on outside the chamber. Figure 13 gives the traversing

12
mechanism detail. The counterweight returns the probe to its

starting position and resets the ten-turn potentiometer at the

conclusion of each scan. The mechanism is designed for both

vertical and horizontal operation. Thus, scans can be made

through all three orthogonal axes of the chamber's quiet zone.

Ideally, when designing such a probing device, material would

be used that is transparent to the field to be measured.

Practically, however, this is impossible. This traversing

mechanism and probe represent a reasonable approximation to

the ideal. The probe is small with essentially a transparent

(to RF) transmission cable, and the scanning mechanism scatters

or absorbs very little of the test field (dielectric constant

of 1.045). Care must be used in assembling the tower and

track. Experiments verified that some glues are not suitable

for this type of application. They retain too much moisture.

3. 2 Test Procedure

The probing mechanism is first assembled and oriented in

the chamber with the probe pulled to one end. Next, the probe's

metering circuit output is zeroed with the source (test field)

turned off. A constant dc voltage is applied across the tenturn potentiometer to generate a voltage proportional to the

probe location along the track. This proportional voltage is

fed to the X-axis of an XY recorder and calibrated to indicate

13
feet o£ probe movement. The amplitude output response from

the probe-meter system is connected to the Y-axis of the

recorder and the source or test field is turned on. Calibration of the amplitude response (Y-axis) in dB is done by

adjusting the source generator output using the power meter

and directional coupler monitor. The probe is then moved

along the track, and the interference signal is recorded on

the XY recorder.

Determination of the polarization properties of the

Warrenton chamber were very restricted because the model tower

head was not operative. Rotation of the test antenna around

the head axis had to be done manually at discrete steps, and

the ability to align the antenna accurately at each step

was limited. The procedure at the higher test frequencies

(4.0 and 8.0 GHz) required very careful trimming of the

source antenna axial alignment to locate the received cross

polarization null. Variations in the depth of these nulls

as a function of axial rotation were used to determine the

polarization properties for the chamber. Results of these

measurements are contained in section 4.

4. EXPERIMENTAL RESULTS

The measurement results, using the techniques outlined

in section 3, are presented primarily in graphic form.

Figures 14 to 26 give the results of scans made to determine

14
chamber reflectivity and amplitude distribution. The amplitude distribution of the received signal in an extended

region including the quiet zone was determined by making

overlapping scans. This was done by moving the NBS probe

tower at 3-foot intervals in the direction of the scan through

the quiet zone. Since each scan was 6 feet long, three overlapping scans in each (X and Z) direction covered a 12-foot

length. This procedure was followed to provide curve alignment and verification of measurement results as indicated in

figure 16. Figures 16, 19, 21, 23, and 25 present this data.

Each of the figures 14 to 25 gives pertinent data describing

the source or transmit antennas used, its polarization, the

calculated reflection level, p, and the angle of arrival, 6,

of the dominate reflected signal for each scan axis, X (transverse) , Y (vertical) , and Z (longitudinal) . The scan orientation is given on each graph. A summary of the chamber reflectivity and quiet zone amplitude distribution, extracted

from figures 14 to 25 is given in the table of figure 26,

Figures 2 7 to 30 give the results of chamber symmetry/polarization measurements. In each case the axial ratio was obtained by rotating the source antenna for various fixed

orientations of the receive antenna. Data were obtained

for the four frequencies, 160 MHz, 2.6 GHz, 4.0 GHz, and

8.0 GHz, and are summarized in figure 31.

15
Finally, a few comments are in order regarding interpretation o£ the measurement results. Measurements at 160 MHz

required a large scan area to obtain enough o£ the interference pattern in the quiet zone to determine the reflected

signal level. Variations in the amplitude distribution somewhat limit the ability to resolve interference resulting from

reflections. Part of this problem was solved by taking data

over a 12-ft. area rather than over the designated 6-ft.

quiet zone. Also, insight into the anticipated amplitude

distribution due to spatial loss of the source signal (Z

scan) or taper roll off (X scan) helped to resolve this.

It should be noted that the measured amplitude distribution

in the quiet zone is not as smooth as one would expect for

this type of chamber. This, it is believed, is due to misalignment of the cone at the apex of the chamber. This

misalignment was visually evident when mounting and aligning

the source antenna with the chamber axis. It was also apparent, see figures 20 and 21, that reflections from the

model tower were important, especially as the test frequency

is increased. This necessitated the removal of the model

tower to obtain an accurate determination of the chamber's

reflective properties. Since the chamber is normally used

with the model tower and head in place, it is well to be

aware of its potential influence on the measurement results

.

16
The scatter of the model tower head may not be serious if the

antenna under test (mounted to the head) presents blockage

to the head or has sufficient forward gain. However, if this

is not the case, it would be well to use non-metallic material

in the head construction.

Scatter or reflections from the probe transport tower

become more pronounced at higher test frequencies. This

was especially apparent when the source antenna was horizontally polarized. Evaluation of the reflectivity data

at 8.0 GHz disclosed that more reflections were resulting

from the probe support structure than from the chamber. This

limited the measured results at this frequency to approximately -45 dB. At lower frequencies, the scatter from the

probing structure becomes less apparent while the chamber's

reflections increase.

5. CONCLUSIONS AND RECOMMENDATIONS

One primary concern is the large signal amplitude (as

much as 2.0 dB) variation through the quiet zone. This is

very similar to what one would expect from operation in a rectangular chamber or if the source antenna was removed

far enough from the tapered chamber apex to allow spectral

reflection to occur from the side walls, floor, and ceiling.

The quiet zone amplitude distribution should be less than a

few tenths dB for uniform illumination in hi gh - accuracy antenna

tests

.

17.
Calculations of chamber reflection arrival angle indicate the largest contribution regions are:

1. the junction of floor and taper transitions;

2fi the junction of ceiling and taper transitions; and

3, the back wall, especially at 160 MHz.

Figures 32 to 34 give a graphical interpretation of these

reflection areas for each scan axis.

Because of the limitations in determining the polarization properties of the chamber fo.r this evaluation, it is

recommended that these measurements be repeated by Warrenton

after the model - tower -he ad ' s axial rotation drive is operative. This will allow complete axial rotation patterns to be

compared for stepped axial rotation position of the source

antenna and should give a more accurate chamber symmetry

evaluation.

A grounding problem in the transmit room was discovered

during the initial measurements. The ground connection between the source antenna and its generator is not isolated

from the antenna support structure. Thus, if balanced antennas

with baluns are used, RF is conducted along this structure

and reradiates into the transmit room. Movement of personnel

inside the room was detected by the receive probe mounted in

the quiet zone of the chamber. This problem was avoided by

clearing all personnel out of the transmit room during the

measurements , but it is suggested that this problem be

corrected.

18
6. REFERENCES

R.H. Hiatt, E.F. Knott, and T.B.A. Senior, "A Study of

VHP Absorbers and Anechoic Rooms," Radiation Lab., Univ.

Michigan, Tech. Rep. 5391-1-F., Feb. 1963.

R.R. Bowman, "Prevalent Methods for Evaluating Anechoic

Chambers: Some Basic Limitations," presented at 1966

Measurement Seminar, Session III, Lecture 3 of High

Frequency and Microwave Field Strength Precision,

NBS Report (unpublished)

.

"Antenna Pattern Range Evaluation by Pattern Comparison

Method," Emerson § Cuming, Inc., Canton, Mass . /Gardena

,

Calif. , Aug . 1963.

W.H. Emerson and F.W. Brownell, "Measurement of Various

Chambers," Various contractor and agency reports from

MR-0 7 through MR-80. Emerson § Cuming Inc., Canton, Mass./

Gardena, Calif.

J. Appel -Hansen , "Reflectivity Level of Radio Anechoic

Chambers," IEEE Trans, on Ant. and Prop., Vol. AP-21,

No. 4, July 1973.

H. Hollman, "Design and Function of Anechoic Funnel and

Rectangular Chambers for Antenna Tests," Fermeldetech

,

Z. der Deutschen Bundespost, A 454 TBr 13, Nov. 1971.

R.R. Bowman, E.B. Larsen, D.R. Belsher, and P.W. Wacker,

"Second Progress Report: Electromagnetic Hazards Project," NBS Report (unpublished), Sept. 18. 1970

T.G. Hickman and T.J. Lyon, "Experimental Evaluation of

the Massachusetts Institute of Technology, Lincoln

Laboratory Anechoic Chamber," Scientific- At lanta , Inc.,

Atlanta, Georgia, Report submitted under Contract

AF 19(628) 5167, June 1968.

19
20.
21.
/MAG£

oc

Figure 3. Three Dimensional Sketch of Chamber Geometry Showing Image of

Reflection from Upper Wall to Field Point on Horizontal Plane.
E-Plane Cut

Ci =0. 4

C2 = 0. 8

^ 1 1 h

L 1 1 1 , , , , 0 1 2 3 4 5 6 7 Distance from Center of Qui-^t Zone (feet)

Figure 4, Calculated Amplitude Taper Across the Quiet Zone in

both E and H-Hanes of the Transmit Antenna as a Function of Distance from the Center.

23
)
0.05

0 10 20 30 40 50

REFLECTED ENERGY BELOV/ DIRECT ENERGY (db)

Figure 6, Reflection Coefficient Chart.

26
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31.
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Figur'" 13. Fiold FVobe Traversing Mechanism Detail,

32
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38.
I 39
40.
41.
4

42.
43

Reflectivity Quiet Zone Field Amplitude Variation (dB)

Freq (MHz) VSWR (dB) X (transverse) Y (vertical) Z (longitudinal)

160 24-27

2600 36-38.5

4000 41-46

8000 # 44.5-46

# Reflections, introduced by probe

ability to measure chamber refle

0.7 0.9 2.3

1.3 0.7 2.2

2.4 2.3 1.5

1.3 1.6 2.0

ransverse mechanism at 8.0 GHz limits

tions to < 45 dB.

Figure 26. Summaiy of Chamber Quiet Zone Reflectivity and Amolitude Distribution,

45
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49
Cros- Polarized Antenna

Freq (MHz) Axial Ratio Variation (dB)

160 25.5-30

2600 30-43

4000 31.6-43

8000 38.6-48

Figure 31. Summary of Chamber Symmetery/Polarization Measurements.

50

52.
53.
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 74-352

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Experimental Evaluation of the U.S. Army, War renton

Training Center Anechoic Chamber

5. Publication Date November 1973

6. Performing Organization Code

7. AUTHOR(S)

M.L. Crawford

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS , Boulder Labs

DEPARTMENT OF COMMERCE

Botilder, Colorado 80302

10. Project/Task/Work Unit No. 2727484

11. Contract/Grant No. WTC - 538-74

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

U.S. ArmyWarrenton Training Center

Virginia

13. Type of Report & PerioH Covered Oct. -Dec. , 73

NBS Interagency Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.) This report describes the experimental performance evaliiation of a 50 foot tapered

anechoic chamber located at U.S. Army, Warrenton Training Center. Measurements of chamber reflectivity and polarization properties were made at four frequencies, 160 MHz, 2,4 GHz, 4,0 GHz, and 8,0 GHz, The measurement system developed by NBS for measuring chamber reflectivity utilizes a new NBS designed

isotropic probe moiinted on a polyfoam track and support structure for probing the

chamber's quiet zone in all three orthogonal planes. The probe is omni-directional

within 1 dB and has sufficient sensiti-vity to permit evaluation of reflectivity levels as low as -59 dB, Complete scattering information is obtained without receive antenna

pattern correction with only 3 orthogonal scans per frequency required. The report

presents a brief treatment of the theory involved for evaluating the chamber, a description of the measurement system and technique used, the measurement results,

and conclusions and recommendations including location of sources of reflection

within the chamber.

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

)

Anechoic chamber evaluation; omni-directional probe; polarization; reflectivity;

three-dimensional scanning.

18. AVAILABILITY Unlimited

[X For Official Distribution. Do Not Release to NTIS

Order From Sup. of Doc, U.S. Government Printing Office Washineton. D.C. 20402. SD Cat. No. CI3

1

1 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74

ISCOMM . lU.