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X-632-64-243 






,N64 33566 



TM X-55094 




(NASA CR OR TMX OH AD .NUMBER) 



(CATEGORY) 



A VERTICAL TEST RANGE 

FOR 
ANTENNA RADIATION MEASUREMENTS 



•a;- 




BY 

JOHN STECKEL 

AND 

WILLIAM KORVIN 



JULY 1964 



NA$A 



GODDARD SPACE FLIGHT CENTER 

GREENBELT, MD. 



A VERTICAL TEST RANGE FOR ANTENNA RADIATION MEASUREMENTS 



4 



By 
John Steckel and William Korvin 



July 1964 



Spacecraft Technology Division 
Spacecraft Electronics Branch 



Goddard Space Flight Center 
Greenbelt, Maryland 



ABSTRACT ,f 

93*** 

In order to facilitate the measuring of satellite antenna radiation patterns 
under a controlled RF environment the concept of a "Vertical RF Test Range" 
was devised. The vertical test range configuration is shown in Figure 1. 

Of primary importance was the test range's capability to adequately absorb 
and/or suppress undesirable electromagnetic energy as low as 125 Mc which 
would normally be reflected from the chamber walls and floor. The satellite 
antennas of concern are of the broad-pattern, near- omnidirectional type (i. e., 
dipole, turnstile, etc.). 

The preliminary study, investigation, and l/9-scale chamber model meas- 
urements indicated that a feasible reflection coefficient level of the order of 
20 db to 25 db can be expected in an electrically small chamber of approximately 
2K x 2k x l\in size. Subsequent full-scale measurements in the completed 
chamber have verified these reflection coefficient levels. 

At higher frequencies of operation it was anticipated (and also indicated by 
1/9- scale chamber model measurements) that the reflected energy level would 
be lower. The dominant controlling factor at higher frequencies is the structural 
weather protective radome. At the lower frequency the radome appears as a 
very thin wall (approximately 0.05 k thick). But as the frequency is increased, 
the presence of the radome becomes increasingly significant. 



m 





Figure 1 -Vertical Anechoic Chamber 



IV 



It should be remembered that an anechoic chamber is by no means free 
space. Therefore, it is of utmost importance that the RF anechoic chamber be 
"calibrated" not only as a function of frequency, but also as a function of illumi- 
nating or source antenna characteristics. Then, indeed, the confidence level at^. 
which the anechoic chamber may be used is defined. 



ce level at 

2& 



CONTENTS 

Page 

Abstract i 

I. INTRODUCTION 1 

A. Antenna Test Range 1 

B. Definition of R-f Anechoic Chamber 1 

C. Testing Criteria as a Function of Type of Test: 

Impedance, Back Scatter and Radiation Pattern 2 

H. DESCRIPTION OF VERTICAL RF TEST RANGE 12 

A. Physical Description 12 

B. Electrical Description 15 

C. Advantages 19 

m. ANALYSIS OF CHAMBER 19 

A. Scaled Chamber Measurements 22 

B. Full Scale Chamber Measurements 25 

IV. CONCLUSIONS 44 

V. RECOMMENDATIONS 44 



VII 



LIST OF ILLUSTRATIONS AND TABLES 
Figure Page 

1 Vertical Anechoic Chamber ii 

2 Effect of Reflected Energy on Typical E-Plane Pattern 

of Horn Antenna H 

3 Photograph of Wall Mount Inside Chamber 13 

4 Photograph Illustrating Size of Absorbent Material 14 

5 Photograph of 9 Panel Radome 16 

6 Photograph of l/9th Scale Chamber 23 

7 Equipment Set-Up for l/9th Scale Measurements 24 

8 Standing Waves in l/9th Scale Chamber Measured from 

Aperture of Chamber to Floor 26 

9 Standing Wave Curves Measured Across l/9th Scale Chamber 
Aperture 27 

10 Photograph of Track and Cart Arrangement 29 

11 Interpretation of Standing Wave Curves 30 

12 Full Scale Chamber Standing Waves at 125Mc, 400Mc 31-34 

13 Radiation Plots Showing Effect of Radome on Reflection 
Coefficient at 1200Mc 38 

14 Effect of Radome on Parallel and Perpendicular Polarization . 38-39 

15 Pattern of Horn Antenna (E -plane) Measured Outside and 

Inside of Chamber 42-43 



vm 



I. INTRODUCTION 

A. Antenna Test Range 

The most important feature of an antenna test range is the control and 
reduction of reflections to the extent that they do not introduce significant 
errors in the measurement of the system under test. In order to achieve 
this, most test range sites are chosen in open fields clear of reflecting ob- 
jects. Satisfactory operation is obtained as long as the system under test 
is sufficiently directive to discriminate against ground reflections and re- 
flections from the test tower. Antennas for small scientific spacecraft are 
operated in the UHF refion and designed to be as nearly omnidirectional as 
possible and thus are a "worst case" for conventional test ranges. The ideal 
solution is to provide a reflectionless invironment for the system under test 
that still permits convenient operation. 

Recent advances in r-f absorbent material has improved the perform- 
ance of r-f anechoic chambers to where they are comparable to the best 
outdoor range sites. 

B. Definition of R-f Anechoic Chamber 

A r-f anechoic chamber may be defined as an enclosure, suitably lined 
with an electromagnetic energy suppressing (absorbing and scattering) 
material, which may be used to measure such electrical characteristics as 
impedance, radiation pattern, and back scatter of a body within the chamber. 



Further, the degree of suppression of reflected undesirable electromagnetic 
energy shall be such as to approximate free space. The allowable divergence 
from free space is a function of the type of test and the tolerable error 
which the disigner places on the test. For instance, a higher level of reflec- 
tions may be tolerable when measuring impedance than when attempting back 
scattering measurements. 

C. Testing Criteria as a Function of Type of Test: Impedance, Back Scatter, 
and Radiation Pattern 

A primary requirement for measuring "free space" impedance of an 
an antenna is the exclusion of all external, foreign sources of reflection. 
This requirement at first thought appears extremely stringent, if not im- 
possible. To meet the requirement explicitly would mean that an impedance 
measurement would have to be made at distances miles from the surface 
of the earth and from within the antenna system. Fortunately, the require- 
ment can be relaxed to a degree governed by the tolerable error permitted 
in the system impedance expressed as a Voltage Standing Wave Ratio. 
Consider a perfectly matched antenna system radiating energy. Under a 
condition of no electrical mismatch from external sources there would be 
no standing wave and VSWR = 1.00:1. In practice, all antenna systems ex- 
hibit some mismatch from reflection resulting from electrical discontinuities. 
The vector addition of the incident and reflected voltages results in a stand- 
ing wave, the magnitude of which is expressed in the familiar form: 



VSWR db = 20 log 10 -^ 
when 

Ej = incident voltage 

E 2 = reflected voltage 
This VSWR is an indication of the power transfer of an antenna system. 
For instance, a system with a VSWR of 6.0.1 would reflect 1/2 of the avail- 
able power which is intolerable in most applications. Below is a brief table 
indicating the percentage of r-f power transmitted as a function of system 
VSWR. 

VSWR R-f Power Transmitted 
1.00:1 100 % 

1.04:1 99.6 % 

1.22:1 99.02% 

Since a perfect impedance match is seldom achieved the question then 
becomes how large a reflection due to the environment of the test site is 
tolerable while still permitting acceptable measurement of reasonable accu- 
racy of the system impedance. Consider first, a worst case condition, i.e., 
all the incident energy striking a reflecting surface is reflected back to the 
antenna under test. If the power radiated P Q from a perfectly matched an- 
tenna of unity gain, travels a distance R to a reflecting wall and is completely 



reflected back to the antenna a standing wave will result. However, the 
inverse square law for the attenuation of electromagnetic energy will reduce 
the amplitude of the reflected wave such that the SWR induced will be less 
than infinite. For example, the attenuation of a 136 Mc signal to a reflecting 
wall 8 feet away can be computed from: 

a = 37 + 20 log f + 20 log R 



where 



a = attenuation (db) 
f = frequency (mc) 
R = distance (feet) 



then 



<* = 23.53 db 
Assuming perfect reflection from the wall, the signal will be further 
reduced by 6 db in the return path to the antenna. Thus, the returned signal 
will be about 29.5 db below the incident signal. The resulting mismatch to 
the perfectly matched antenna is computed from: 

r - 1 



r 



+ i 



where 



|T| = reflection coefficient = 29.53 db 

= .0335 



••• VSWR = r =1.07:1 



From the analysis above it is concluded that the impedance of a low 
gain antenna system may be measured with confidence if the walls of the 
enclosure are at least eight feet away and are less effective as a reflector 
than a metallic wall. Thus, the requirement on the absorption of unwanted 
reflections from the environment is easily met when making impedance or 
VSWR measurements on low gain antenna systems. 

By definition, a , the scattering cross section of a body is the ratio of 
the power scattered per unit solid angle to the power incident per unit area. 
In terms of the radar equation, 

P R 16t7 2 R 4 



„ square meters, 
P T G T G R \» 

where 

k = the wavelength in meters. 

R = the distance between the source antenna and the scattering cross 
section body in meters. 

P = the transmitted RF power. 

P D = the received (reflected) RF power. 

G T = the transmitting antenna power gain with respect to an isotropic 
radiator.* 

G R = the receiving antenna power gain with respect to isotropic radiator.* 
The problem which presents itself in measuring scattering cross sec- 
tions in an anechoic chamber is the suppression of the energy from chamber 



*An isotropic radiator is a fictitious antenna, used as a reference, which radiates energy truly omnidirectional. 
Further, assuming this antenna is radiating an RF power of one watt, the magnitude of the electric field 
strength measured at a radius of 1 mile from the source is equal to 3.40mw/meter. 



walls reflected back to a receiving antenna. The necessary chamber per- 
formance, as in the case of VSWR measurements in a chamber, is relative. 
For example, a large scattering cross section would reflect an energy level 
which could be many orders of magnitude greater than energy contributed 
by the reflected energy from the chamber walls (including ceiling) . In this 
case a percentage tolerance error can be established. Unfortunately, many 
radar cross section measurements are made of objects which have a small 
scattering cross section. Now, although the walls of the chamber absorb and 
suppress the RF energy, the fact that the walls are many orders of magni- 
tude in size larger than the object being measured results in a chamber wall 
reflection coefficient which is as large as or larger than the object to be 
measured. In other words, the scattering energy of the body to be measured 
is hidden by the reflections from the chamber. 

There is one technique which is described by Elery F. Buckley (Emerson 
and Cuming, Inc.) which allows a calibration of a chamber for measuring 
radar cross sections in a controlled manner. 

Very briefly, a transmitting and receiving antenna are placed at one 
end of a chamber. Two or more conducting spheres of known physical size 
(and hence known radar cross section) are placed at the opposite end of the 
chamber one at a time. The spheres, one at a time in the chamber, are 
rotated eccentrically about an axis producing an in-to-out of phase response. 
There will be two amplitude components for each of the different size spheres 



which contribute to the recorded interference pattern. One of the amplitude 
components is from the sphere; the other is the constant field equal to the 
energy return from the chamber plus electric field from transmitter- receiver 
cross coupling. The voltage ratio of these magnitudes produce an ambiguity. 
That is to say, the chamber scattering cross section will be either of two 
values, hence, the second measurement is made using a different conducting 
sphere with a known a. The second measurement results again in two val- 
ues of the chamber cross section. The true chamber reflecting cross section 
is that cross section which is common to both of the measurements. 

An example which illustrates the magnitude of energy suppression which 
an anechoic chamber must exhibit for the measurement of radar cross 
sections follows: 



TRANSMIT 

a 

RECEIVE ANTENNA 



R,~ .9R 2 



R2 = 10 meter 



W 



<r, <r 2 



-B 



W 



Let the scattering cross section be that produced by a sphere of radius 
a = 0.1 meter when the wavelength ^ = 3cm (i.e., a A > 1). Then the scat- 
tering cross section o- j = na 2 = 3.14 x 10~ 2 meter 2 

Now, the return power received by an antenna G r from a scattering 
body of cross section o- when the transmitted power P T is radiated by an 
antenna G T is, 

_ P t Gr Gt ^ 
Pr 16tt 2 R 4 a 

Consider first the return power received from the spherical scattering body 
and letting P T = 1 watt, R x = 9 meters, 

G x = G D = 63 and \ = 3 cm. 

P P T G R G T ^ 

Pri 16tt 2 R 4 ^ 

P D , = .109 x 10" 6 watts 
Next, assume the wall w-w in the figure on page 7 to be a perfectly re- 
flecting surface 3 meters on a side so that the wall radar cross section is 

Arr A 2 
" 2 = — 

where A = area of wall and X. = wavelength at the operating frequency. 
Then, 

477 A 2 



a 2 



\ a 



a 2 = 12.56 X 10 4 M 2 

The return power received from the back wall w-w is (the side, top and 
bottom walls may be neglected in this case since the predominant contribu- 
tion to erroneous back scatter measurements is the back wall), 
and, P p = .284 watts 
By comparison P R /P R = -64.2 db 

Or, the energy normal to the back wall W-W must be suppressed by 
64.2 db to allow the spherical body (.1 meter radius) to present an equal 
amplitude to the receiver antenna G R . Of course, further suppression of the 
back wall energy must be attained before the energy from the spherical body 
is discernible from (i.e., above) the back wall scattered energy. 

The example outlined above now allows a feeling for the nature and 
magnitude of energy suppression and/or absorbing characteristics that may 
be required of an RF anechoic chamber designed to measure radar cross 
sections. 

Measurement of antenna radiation patterns generally requires a better 
anechoic chamber than one for the measurement of antenna impedance. But 
the requirements are not as critical as for radar cross section (back scatter) 
measurements . 

As an illustration, let us examine a typical E -plane pattern of a horn 
antenna and then show the effects of various magnitudes of reflected energy 
interfering at various aspect angles in the chamber. Figure 2 shows the 



E -plane pattern as measured in a controlled environment at an outside 
antenna range. The pattern approaches the theoretical pattern for a horn 
antenna exhibiting uniform distribution across the aperture. Figure 2 also 
shows a ray outline of reflected energy when measuring the E -plane pattern 
of this antenna in an anechoic chamber. Let sources of reflection be loca- 
ted at angles defined by 8 = 50° and 60°. Then the resulting perturbations 
from these reflecting sources are shown in Figure 2 as dashed lines. The 
deviations in the pattern are identified as (T) , (2) , and (Jj) . The magnitude 
of the perturbation at 1 is 1.6 db. The magnitude of the reflected energy 
from point I which produces this 1.6 db is equal to 21 db since the reflection 
coefficient in db = 20 log (S - 1/S + 1), where S = standing wave ratio. 
Refering to the figure, it can be seen that the perturbations take place at 
-14.2 db on the radiation pattern. Therefore, the actual reflection coefficient 
(magnitude of the interfering reflected energy) is equal to 35.2 db. At 
point 2 the deviation from point II is 1.1 db or a reflection coefficient of 
25 db. Since the radiated energy is down 15.5 db at this point, the actual 
reflection coefficient is 40.5 db. Now consider point (§) (i.e., db down on 
the pattern). A refection coefficient of 40 db will cause a 0.175 perturbation 
in the pattern at this point. 

From the above exercise one can now see clearly the effect of reflected 
energy on a typical pattern from arbitrarily chosen points in an anechoic 
chamber enclosure which is 35 db to 40 db down from the incident energy. 

10 




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H. DESCRIPTION OF VERTICAL RF TEST RANGE 
A. Physical Description 

The vertical test range is as shown in Figure 1. The walls and floor of 
the structure are reinforced concrete. Attached to the chamber through a 
common wall is the control room which houses all the electrical measuring 
devices. Opposite the control room side are two doors which when opened 
allow the chamber to be used as one end of a horizontal antenna range in 
conjunction with available antenna towers. The roof of the chamber is an 
A- sandwich type RF transparent radome. Also, shown in this figure is the 
outside azimuth - elevation mount and fiberglas mast approximately 35 feet 
above the chamber controlled from inside. This facilitates the changing of 
source antennas. Inside the chamber (see Figure 3) is a wall-mounted an- 
tenna mount capable of rotating and allowing measurement of a satellite 
antenna system at or near the center of the chamber quiet zone. 

The walls are lined with pyramidal absorbent material 70 inches in 
length on 2 feet square bases (Figure 4). In the area of the side wall mount 
smaller pyramidal absorbent material is used allowing the rotation of an 
off- set arm containing the fiberglas mast. The floor of the chamber is 
lined with foam structure 70 inch pyramids on top of which is cemented a 
smooth floor - decking material of 1/2" thick sheets of semi-rigid vinyl 
foam which can be walked upon in setting up experiments within the chamber. 



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Figure 4-Photograph Illustrating Size 
of Absorbent Material 



14 



The radome is a structural body of the A- sandwich type. It is approxi- 
mately 4" thick and the 9 sub-pieces are assembled into a continuous weather 
protective roof (Figure 5). 

Uniform chamber lighting is achieved by directing four flood lamps 
toward the radome. The four lamps are located at the four upper corners 
of the chamber such that they are well hidden by the absorber on the walls. 
This technique of lighting results in two very favorable conditions; the 
lighting is uniform and avoids the visual problem of looking into the lamps; 
a minimum of RF reflection from the fixtures is obtained since they are 
well shadowed by the 70" absorbent material. 

Finally, the chamber is temperature controlled to prevent large varia- 
tions of temperature which may effect antenna measurements. 

B. Electrical Description 

Electrically the chamber was lined on 5 surfaces with an RF absorbent 
material and the 6th side enclosed by an RF transparent (radome) roof. 

The RF absorbent material was supplied by the B. F. Goodrich Com- 
pany, Shelton, Connecticut. Each individual piece of the material, VHP-70 
is a 70" high pyramid shaped absorber on a 2 feet square base. All of the 
absorbent material exhibits a minimum reflection coefficient of 28 db at 
120Mc, 40 db at 400Mc and 50 db at 1,000 to 10,000Mc. 



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The tests on the absorber at the manufacturer' s plant were made in a 
closed loop technique at 120Me and 400Mc. At 1,000, 5,000 and 10,000Mc an 
open loop technique was used. The closed loop technique consisted of meas- 
uring four 2'x2' absorber pieces at a time in a flared waveguide system in 
which the absorbent material serves as a load under test. The absorber is 
moved inside the waveguide resulting in a standing wave that moves in con- 
junction with the physical movement of the absorber. A fixed probe inside 
the waveguide located between the absorber and RF oscillator detects the 
standing wave which is converted to power reflected. 

The open loop technique of testing consisted of a horizontal version of 
the NRL type arch method. A reference was used which consisted of a flat 
2'X8' metal plate. The reflection level of four 2x2 pieces was measured and 
compared to the flat metal plate. The reflectivity of the absorbent material 
is the difference in return power in db between the metal plate and the 
absorbent material. 

The radome was supplied by Raymond Development Industries, Inc., 
Huntington Park, California. The electrical characteristics of the A- sandwich 
radome material were specified as: 

(a) Transmission loss ^ 7.5% over the frequency range of 120Mc to 
10,000Mc. 

(b) Refraction less than 10% for angles of incidence from 0° to 45°. 

(c) Loss tangent of glass laminate = .005. 



17 



(d) Dielectric constant of glass laminate = 4.0. 

(e) Loss tangent of foam core = .0005. 

(f) Dielectric constant of foam core = 1.12. 

Although tests at the manufacturer' s plant confirm the meeting of the 
specifications on a flat 40"x40" sample of the radome, measurements indi- 
cate that the actual radome does exhibit less than specified performance 
particularly at frequencies above l.OOOMc. This can be easily explained 
since the actual completed radome psssesses a curved surface and the nine 
pieces comprising the total radome are assembled with beefed-up glass 
laminate flange sections. These two conditions increase the detrimental 
effects (increased diffraction and reflection). 

As previously stated the frequencies of primary interest are in the 
120Mc to 400Mc region. But some tests were performed in the chamber at 
higher frequencies. 

Normally, in evaluating the chamber a quiet zone is defined. That is, a 
volume within the chamber in which known (measured) reflectivity levels 
exist. Then in this zone antenna systems can be evaluated being fully aware 
of the limitation of the chamber. Therefore, by definition, the quiet zone is 
a 10 foot diameter sphere which is tangent to the chamber floor and centered 
elsewhere within this chamber. 



18 



C . Advantages 

Ground reflections are the most serious source of error when using a 
conventional test range for measurements of low frequency, low-gain an- 
tennas. The error may be refuced to a degree by additional height to the 
towers, but towers over 100' high are expensive and rather inconvenient 
to use. Likewise a conventional anechoic chamber designed for operation 
at low frequency is expensive and requires a large building to provide even 
a modest size test range. The vertical test range provides an attractive 
compromise between tall towers and a large anechoic chamber. The cham- 
ber portion need only be large enough to prevent reflections from the ground 
and nearby reflecting surfaces and the tower need only be tall enough to hold 
an antenna out of the near field of the antenna under test. Furthermore, the 
length of the test range may easily be varied by adjusting the height of the 
outside antenna. Operation of a vertical range is especially convenient. The 
model under test, the test and control equipment, and personnel are all at 
ground level and thus avoid the need for hoists and elevators. 

HI. ANALYSIS OF CHAMBER 

The magnitude of unwanted reflections that can be tolerated in an antenna 
test range have been shown to be a function of the parameter being measured. 
Since the site is never perfect, the results obtained may be interpreted in terms 
of the known site imperfections provided the reflection coefficients and in some 



19 



cases phase are accurately specified. However, measurement of reflection co- 
efficient of absorbent material is difficult and techniques for its evaluation have 
not been standardized. Currently the Pattern Comparison Technique 1 and the 
Free Space VSWR Technique 2 are favored in evaluation of r-f anechoic chambers. 
In both techniques the significant result is the comparison of the incident to the 
reflected energy from an absorbing wall to determine its reflection coefficient. 

Briefly, in the Pattern Comparison Technique, the pattern of a directive 
antenna (15 to 20 db gain) is measured successively at closely spaced points 
along the radii of the chamber quiet zone. The quiet zone may be defined as the 
volume within an anechoic chamber in which an antenna under test will be meas- 
ured. Then the patterns are superimposed on each other with pattern peaks 
coincident. The deviations in the patterns are read at different aspect angles 
and VSWR curves vs aspect angle are constructed. The curves may then be con- 
verted to reflection levels within the chamber. 

The Free Space VSWR Technique is a method of continuously recording the 
amplitude variations produced by reflections. Two directive antennas are used, 
with one being moved continuously across the chamber quiet zone at a discrete 
aspect angle for each recording. The amplitudes recorded are reduced to inci- 
dent and reflected energy levels, thus allowing the reflection coefficient vs aspect 
angle of a chamber to be determined. 

From the brief summary above (and more so from the referenced literature) 
it can be seen that both techniques rely on use directive antennas. This is not 

20 



a disadvantage in the usual situation where the chamber is large in terms of 
wavelength and directive antennas are usually evaluated. However, in the 
vertical test range more emphasis is placed on measurement of nearly omni- 
directional antennas in a termination chamber that is electrically small, 
i.e., approximately 1A. in depth and 2X. on a side. Although evaluation of chamber 
performance using directive antennas will indicate both a direction and reflection 
coefficient for sources of reflections, unless considerable effort is made to 
integrate the reflection levels from all directions the chamber will appear better 
than when used with an omnidirectional antenna. Therefore most of the analysis 
of both the scale model and the full scale range was made using a dipole antenna 
to probe the energy levels within the termination chamber. In this method, a 
dipole antenna is moved throughout the quiet zone and energy levels vs position 
recorded. The difference between the levels recorded and calculated free space 
levels are converted into VSWR and finally a reflection coefficient computed. 

It is common practice to try to reduce the large number of different reflection 
coefficients that are measured in evaluating a chamber to a single number that is 
then used to define the chamber's performance. In general, it will be found that 
this number is not really a common denominator and that the relative perform- 
ance of two different chambers should not be judged on this alone. For instance 
the quiet zone of chamber A may be only 1/3 the volume that was included in 
evaluation of chamber B, yet chamber B may be considerably better than A over 



21 



the same quiet zone. Unfortunately, the performance rating of anechoic chambers 
is like the evaluation of radio receivers in 1940, i.e., not complete unless the 
test conditions are known as well as the results. 

A. Scaled Chamber Measurements 

Reflection coefficient measurements were made in a 1/ 9th- scale chamber. 
A photograph of the scale chamber is shown in Figure 6. Figure 7 defines 
the test set-up. No attempt was made to design and test a l/9th-scale ra- 
dome because the radome would appear as a thin wall structure in the fre- 
quencies of most interest (up to around 400Mc) in the full scale chamber. 
Although no scaled radome measurements were made in this particular 
chamber design it is felt that general comments on scaled radome measure- 
ments are in order. 

Scaling a multi-panel, sandwich radome is, in general, difficult. Extreme 
care must be taken in scaling the ribbing, flange design and radome curva- 
ture. Serious errors can be expected if this precaution is not taken. Thin 
wall structural radomes of dielectric constants of approximately 3 or 4 may 
be scaled for measurements with confidence and also thick wall foam radomes 
of low dielectric constants ( e n = l . 1 to 1.4) are practical for scaling purposes. 

Measurements were made at frequencies from near l,080Mc to 3,600Mc. 
This corresponds to full scale chamber measurement of approximately 120Mc 



22 




Figure 6-Photograph of 1 /9th Scale Chamber 



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to 400Mc. It is to be noted that a dipole probe is used inside the scaled 
chamber to deliberately prevent discrimination against any reflected energy. 
A directive type antenna would have been too selective. 

The measurements at l,080Mc in the l/9th-scale chamber indicated a 
reflection coefficient of 20 db and at 3,600Mc 30 db reflection coefficients 
were measured. 

Figure 8 is typical of the standing waves measured by probing the 
scaled chamber with dipoles. The dipole probes were moved in small 
increments in terms of wavelengths from the aperture of the chamber to 
the back. Figure 9 depicts standing wave curves resulting from probing 
across the chamber in small increments. This data is typical of that ob- 
tained in the 1/ 9th- scale chamber. 

It was from these measurements that the full-scale chamber perform- 
ance was predicted. 

As was previously mentioned no attempt was made to determine the 
effect of an A- sandwich type radome on the full scale chamber at scaled 
frequencies. That the radome effect would be negligible at the low fre- 
quencies of interest was predicted on the basis that the radome would 
appear as a thin wall (approximately k/8 thick or less) at 400Mc and lower. 

B. Full Scale Chamber Measurements 

The quiet zone of the chamber was probed with a dipole using an 
illuminating antenna which was directional in nature. 

25 



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1/9 SCALE CHAMBER 
f * 1080 mc 
DIPOLE PROBE 
S/A HORN 



1/9 TH SCALE CHAMBER 
f=3600 mc 
S/A GAIN STANDARD 
DIPOLE PROBE 



(db) 




POSITION NO. I 
REFL. COEFF ~ 21 db 




POSITION NO. I 
REFT.. COEFF ~ 31 db 



(db) 




POSITION NO. 2 
REFL.C0EFF~2ldb 




POSITION NO. 2 
REFL COEFF ~ 32 db 



(db) 




5 432 I 01 2345 

POSITION NO. 3 
REFL.CC€FF~2ldb 






POSITION NO. 3 
REFL.C0EFF~32db 



POSITIONS 



PROBE 




S/A HORN 



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Figure 9-Standing Wave Curves Measured Across 1/9th Scale Chamber Aperture 



27 



Specifically, at 125Mc and 400Mc the magnitude of the reflection coeffi- 
cient was determined by moving a dipole horizontally inside the chamber in 
directions normal to each wall and across the diagonals. With the dipole 
fixed to a cart made of low dielectric constant foam material, the cart was 
moved along tracks which could be oriented as desired within the chamber. 
The track, cart and dipole support are shown in Figure 10. There horizontal 
measurements were made at discrete heights of 2 (and/or 3), 4, 5, 6 and 8 
feet above the floor of the chamber. The reflection coefficient in the verti- 
cal direction was determined by measuring the standing wave as a function 
of vertical movement of the dipole from 2 to 10 feet above the floor. Result- 
ing standing wave curves are converted to an equivalent reflected energy 
level (reflection coefficient). Figure 11 is a typical curve. The dashed 
curve of Figure 11(b) represents the probed energy level as measured in 
Figure 11(a) in the absence of reflected energy. The solid curve super- 
imposed on the dashed curve represents the effect of the reflected energy. 
Peak to peak value of the standing wave is 0.5 db. This standing wave of 
0.5 db results from a reflected energy level of 30 db below the incident sig- 
nal level. Or, Reflection Coefficient = 20 log VSWR-l/VSWR+1 . Measured 
curves are shown in Figures 12(a) through 12(d). 

At l,200Mc reflected energy levels greater than expected were measured 
(approximately 32 db average). Also, the reflected energy levels were polari- 
zation sensitive (i.e., E -perpendicular vs E-parallel) with differences in 

28 




Figure 10- Photograph of Track and Cart Arrangement 



29 



SOURCE 




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(PLAN VIEW OF CHAMBER) 



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Figure 12d— Ful I Scale Chamber Standing 
Waves at 125 Mc, 400 Mc 



34 



reflection coefficients between 6 and 15 db as a function of aspect angle. 
This obvious deviation from the 45 to 50 db levels anticipated was attributed 
to the radome for two reasons: 

(1) The chamber at l,200Mc is electrically large (15 X. x 15X. 10\) and 
therefore the chamber would be expected to be much better than at 
400Mc (reflection levels %32 db). 

(2) This A- sandwich radome (4 inches thick, with solid fiberglas panel 
flanges) appears as a relatively large discontinuity. 

Measured magnitudes of reflected energy for various linear polarization 
orientations when probing the chamber at 125Mc and 400Mc are tabulated in 
Tables 1 and 2. Some inconsistency is apparent, especially in the higher 
coefficient values measured at 125Mc. This is attributed to instability of 
the measurement equipment; however, retests of selected orientations indi- 
cate no serious differences from the data presented or the conclusions 
drawn from the data. 



35 



Table 1 
125Mc Chamber Reflectivity Levels 



Track 
Orientation 

9 T 


Dipole 
Orientation 


Dipole Height Above Floor 


3 ft. 


4 ft. 


5 ft. 


6 ft. 


8 ft. 


0° 

0° 

90° 

90° 

45° 

45° 

135° 

135° 


0° 

90° 

0° 

90° 

45° 

135° 

45° 

135° 


24 db 

25 db 
*27db 

26 db 
29 db 
29 db 
26 db 

*22db 


31 db 
24 db 

*26 db 
27 db 
22 db 
31 db 
21 db 

♦21 db 


29 db 

27 db 
*28db 

28 db 
24 db 
31 db 
21 db 

*24db 


29 db 
26 db 

*24db 
29 db 
23 db 
32 db 
21 db 

*24db 


29 db 
25 db 

*20db 
25 db 

*25db 
27 db 
22 db 

*21db 


*Measurements performed at 123Mc using NASA 
battery operated signal generator. 



Table 2 
400Mc Chamber Reflectivity Levels 



Track 
Orientation 

6 T 


Dipole 
Orientation 

6 » 


Dipole Heights Above Floor 


2 ft. 


4 ft. 


5 ft. 


6 ft. 


8 ft. 


o 



o 



o 

90 

o 

90 

o 

45 

o 

45 o 

135 o 

135° 


0° 

90° 

0° 

90° 

45° 

135° 

45° 

135° 


31 db 
39 db 
31 db 
31 db 

31 db 

32 db 
35 db 
32 db 


39 db 
39 db 
32 db 
32 db 
39 db 

31 db 

32 db 
32 db 


31 db 

31 db 

32 db 
32 db 
39 db 

30 db 

31 db 
30 db 


35 db 
32 db 

31 db 

32 db 

34 db 
31 db 

35 db 
31 db 


32 db 
31 db 

30 db 

31 db 

32 db 
32 db 
30 db 
39 db 



36 



Data taken at l,200Mc are not presented because later tests prove 
that the radome is the major contributor to standing waves within the 
chamber and therefore does not truly represent the capability of a vertical 
test range. 

To verify the fact that the radome was the major contributor to the 
problem (i.e., larger reflection coefficients than anticipated and polari- 
zation sensitivity) measurements were performed with the source an- 
tenna inside the chamber just under the radome and compared to the 
pattern taken through the radome (Figure 13(a)). Thus, an interference 
pattern (see Figure 13(b)) between incident and reflected energy was 
produced by essentially removing the radome from between the source 
and receive antenna. In both cases a horn antenna was moved across 
the chamber at a 45° angle (from the vertical). The +45° refers to the 
horn antenna being tilted toward the chamber wall containing the wall 
mount. And -45° refers to an angle toward the wall without the wall 
mount. 

Plots in Figure 14 compare the standing waves of the A- sandwich 
radome for the two polarization conditions E-perpendicular and E-parallel 
as defined below. 



37 



FREQ. - 1200 mc 




PATTERNS MEASURED THRU RADOMEl 

Figure 13a-Radiation Plots Showing Effect of Radome on Reflection Coefficient at 1200 Mc 



38 

























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Figure 13b-Radiation Plots Showing Effect of Radome on Reflection Coefficient at 1200Mc 



39 



















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VSWR = 6 db 
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LOG PERIODIC ANTENNA OVERHEAD 



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40 



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Except at angles far off normal incidence reflected energy from E- 
perpendicular is nearly always greater than from E-parallel because of 
the zero at the Brewster angle occurring only with parallel polarization. 
From Figure 14 it can be seen that the difference in reflection coefficients 
is 14 db. The Brewster angle (where the reflection value goes to zero) is 
defined as follows: 



tan 



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An important point which is illustrated in Figure 15(a), (b) is the degree 
of accuracy with which the antenna radiation petterns can be measured 
through a radome which exhibits significant reflection characteristics. In 
this case the radiation patterns of an antenna (gain standard at l,200Mc) was 
measured first in a free space pattern range method and then the measure- 
ment was repeated inside the vertical chamber. To measure the pattern 



41 



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Figure 15a— Pattern of Horn Antenna (E-plane) Measured Outside and Inside of Chamber 



42 






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43 



inside the chamber, the antenna was rotated about a fixed axis and not moved 
across the chamber. Therefore, the "available" reflections which exist 
across the chamber are not probed and therefore, a rather respectable 
pattern can be achieved. As a matter of interest Figure 15(b) shows the 
effect the wall mount can produce in terms of reflected energy. 

IV. CONCLUSIONS 

Measurements taken from the 1/ 9th- scale model of the test range are in 
reasonable agreement with the measurements of the full scale range. The 
structural A-sandwich radome definitely reduces the performance of the facility 
at higher frequencies, but does not affect operation in the frequency range of 
primary interest (125— 400Mc). 

The concept of a vertical test range composed of an electrically small 
termination chamber with a r-f transparent radome has been found feasible 
and provides at moderate cost a convenient, quasi-all-weather, facility for 
accurate measurement of antenna characteristics. 

V. RECOMMENDATIONS 

Considerable confidence may be placed on the results of measuring scale 
models of anechoic chambers. Since the instrumentation is not difficult and the 
cost is small it is recommended that more extensive use be made of scale models 
to check chamber and anechoic material performance. 



44 



A multipanel sandwich radome has definite frequency limitations when 
used as part of an antenna test range. Therefore it is recommended that a 
thick, low-dielectric- constant foam radome be used. 



45 



REFERENCES 

1. Buckley, E. F., "Outline of Evaluation Procedures for Microwave Anechoic 
Chamber", Microwave Journal, August, 1963. 

2. Emerson, W., "Chamber Information", unpublished report of B. F. Goodrich 
Company. 

3. Jasik, "Antenna Engineering Handbook", New York: McGraw-Hill Book 
Company, 1961. 

4. Harvey, "Microwave Engineering", New York, Academic Press, 1963. 

5. Electronic Space Structures Corporation, "Ground Radomes", 1964. 



46