The techniques and results of power pattern measurements of a corrugated horn antenna designed for low sidelobes are reported. The power pattern was measured to levels 90 dB below the main beam maximum in both the E- and H-planes. The measured patterns were found to be in good agreement with predictions from existing theory for the performance of corrugated scalar feeds

Bednarczyk, S. M.

Gulkis, S.

Janssen, M. A.

Marlin, H. W.

Smoot, G. F.

English

NASA Technical Reports Server

•Y
N80-33429
TDA Progress Report 42-59 July and August 1980
Pattern Measurements of a Low-Sidelobe
Horn Antenna
M. A. Janssen and S. Gulkis
Planetary Atmospheres Section
S. M. Bednarczyk
Microwave Observational Systems Section
H. W. Marlin
Spacecraft Telecommunications Equipment Section
G. F. Smoot
University of California, Berkeley
The power pattern of a corrugated horn antenna designed for low sidelobes was
measured to levels 90 dB below the main beam maximum in both the E- and H-planes.
The measured patterns were found to be in good agreement with theoretical predictions.
I. Introduction
Requirements for antennas with high off-axis rejection are
rapidly growing, and the study of such antennas is topical in
current antenna technology. Applications requiring antennas
with low-level sidelobes exist in earth-based and satellite
communications, in radar systems, and in radio astronomy.
The work reported here was motivated by a requirement for
an antenna with very low sidelobes to measure properties of
the cosmic microwave background radiation from a satellite
(Ref. 1). The particular application calls for horn antennas of
approximately 7-deg beamwidth, operating at discrete fre-
quencies in the range 20 to 90 GHz, which have far-sidelobe
levels lower than 85 dB below the main beam peak to
adequately reject stray radiation from the earth. While
antennas that meet this requirement have been designed, the
sidelobe levels have not (to our knowledge) been measured to
levels lower than 75 dB below the main peak (Refs. 2
through 4). As our requirements exceed the capabilities of
current state-of-the-art pattern measurement techniques, we
undertook the task of extending these capabilities to meet our
objectives.
We report the techniques and results of our antenna
measurements here. The present work demonstrates that the
sidelobes of horn antennas can be measured to significantly
lower levels than has been achieved previously. The results
should be of interest in both the measurement and further
design of low-sidelobe antennas for other areas of application.
II. The Horn Antenna
A corrugated conical horn of the scalar feed horn antenna
type was employed for the measurements. The horn, shown in
Fig. 1, possesses a beamwidth of approximately 7 deg. It has
been previously employed in an airborne experiment to
measure the anistropy of the cosmic background radiation
https://ntrs.nasa.gov/search.jsp?R=19800024921 2020-03-21T15:29:43+00:00Z
(Ref. 5). The design is based on pioneering studies by Kay
(Ref. 6) and Potter (Ref. 7) of low-sidelobe antennas. Several
authors, most notably Clarricoats and Saha (Ref. 8), have
investigated the theoretical properties of this type of antenna.
The significant features of this horn are its broad bandwidth,
the 10-deg flare angle, and moderate groove spacing (two
grooves/wavelength), which permit compact and easy con-
struction. The design frequency of the horn is 33 GHz,
although the pattern was not predicted to be significantly
different at the present test frequency of 31.4 GHz. A
transition section is employed to match the circular input port
of the horn with rectangular waveguide.
III. The Measurement Technique
The strategy for the measurements was to use a conven-
tional pattern measurement technique, while taking special
precautions to eliminate the main sources of extraneous signals
that could confuse the far-sidelobe measurements. The chief
sources of background signal were anticipated to be radiation
scattered into the main beam or near sidelobes from nearby
objects, and radio frequency (RF) leakage into the receiver
behind the horn due to imperfect connectors and junctions.
Particular care was taken to obtain a clean and uncluttered test
geometry, while the RF portion of the receiver was made
extremely compact and easy to shield.
The measurement geometry illustrated in Fig. 2 was
achieved by mounting a receiver with the test horn and
transmitter on towers, illuminating the test horn on a
horizontal path. The transmitter-receiver plane was thus
removed 6 m from the ground to isolate the test system from
nearby sources of reflection. The receiver tower was located at
the edge of a mesa, the terrain beyond falling off sharply. The
transmitter employed an identical low-sidelobe horn to further
reduce potential reflection paths. The test hom was rotated in
a horizontal plane by rotating the receiver tower on an
azimuth bearing located near ground level. The axis of azimuth
rotation passed through the point where the horn axis
intersects its aperture, so that the incident signal was
uniformly sampled as the azimuth was rotated to measure the
sidelobes. The mount holding the receiver and test horn also
rotated around the test horn axis, allowing the test horn
polarization to be set at any chosen angle. The close spacing
between transmitter and test horn (4 m) gave a sufficiently
uniform illumination of the test horn while allowing for
maximum signal strength.
Fig. 3 gives a schematic of the test circuit. A fixed-
frequency Gunn oscillator supplied a continuous wave (CW)
signal at the test frequency. A calibrated attenuator in the
transmitter circuit allowed the transmitter power to be varied
from its maximum of 10 mW through a range of 50 dB. A
series 1750 receiving system manufactured by Scientific-
Atlanta Corporation was employed and consists in essence of a
narrow-band receiver that is phase-locked to the transmitter
signal. The local oscillator employs a relatively low frequency
signal (~16 GHz), the 12th harmonic of which is locked to
the transmitter frequency. The advantage of this system is that
the RF portion of the receiver is limited to the harmonic
mixer and is very compact. For the measurements, the mixer
was well wrapped with microwave-absorbing material in a
small cylindrical volume behind the test horn. The received
signal power was recorded on one axis of a two-axis plotter,
the second axis of which was synchronized with the horn
rotation angle.
IV. The Horn Pattern
After a series of measurements to guarantee proper
alignment of the test system and receiver linearity, the pattern
of the test horn was measured in both the H- and £"-planes.
The results are shown in Figs. 4 and 5, respectively. Receiver
linearity was checked by obtaining a series of//-plane patterns
with the transmitter reduced in 10-dB increments through a
range of 50 dB, making use of the calibrated attenuator. The
measured main beam maximum was found to decrease in
corresponding 10-dB increments to within ±1 dB in all cases.
The measured patterns themselves were identical to within
±2 dB down to approximately the 85-dB level shown in Fig, 4,
where system noise becomes significant. Hence, the system
linearity was demonstrated to be at least ±2 dB over the full
range 0 to 85 dB.
Receiver noise dominates the measurements below 90 dB.
In the linearity measurements the noise level seen at angles
beyond 100 deg in Fig. 4 was unchanged, while the patterns
decreased in amplitude by up to 50 dB. If this noise were due in
part to signal leakage, reflections, or receiver horn sidelobes, a
relative decrease in this noise level would have been observed.
As a further check, the receiver was wrapped with an
additional layer of microwave-absorbing material, and the
//-plane pattern was remeasured with no significant change
observed. We conclude that the pattern measurements were
limited by receiver noise alone.
The dashed curves in Figs. 4 and 5 show a theoretical
calculation of the test horn pattern based on a computer
program developed by Potter (Ref. 2). Although there are
small deviations between the calculated and measured
patterns, the overall agreement is excellent. The observed
differences may be due to machining tolerances in the actual
horn or to neglected contributions in the computations of the
theoretical patterns.
IV. Conclusion
It is demonstrated that horn antenna patterns are measur-
able to levels less than 90 dB below the central peak with
conventional range techniques. Further, existing theory for the
performance of corrugated scalar feeds appears to give
excellent predictions at these low-sidelobe levels. We believe
that several factors contribute to the measurement success.
First, the use of a relatively high frequency (31.4 GHz)
maximizes the wavelength distances of unavoidable scattering
objects in a given test environment, such as the ground and
nearby buildings. In the present measurement, the receiver was
located at the edge of a steep mesa so that the main power
lobe of the transmitting antenna essentially disappeared into
free space, with no chance for multipath reflections into the
receiver for the vast bulk of the radiated signal. The power in
such reflections was further reduced by using a low-sidelobe
hom for the transmitter, identical to that measured in the
experiment. Finally, the receiver RF path was maintained at a
minimum by employing a well-shielded harmonic mixer
immediately adjacent to the test horn output port.
The level of sensitivity reached in these measurements was
limited by receiver noise, with no evidence to indicate the
presence of extraneous signals. The test apparatus used had a
fixed short time constant for signal integration, which limited
the receiver sensitivity. A straightforward improvement could
be achieved by allowing for longer signal integrations and
incorporating a switched-reference measurement scheme to
enable the accurate determination of very low signal levels.
Acknowledgments
We gratefully acknowledge the advice and assistance of Dan A. Bathker and Gerald S.
Levy of the Jet Propulsion Laboratory. The pattern measurements were carried out at
the Mesa Antenna Test Range Facility of the Jet Propulsion Laboratory.
1
II
10
References
1. J. C. Mather, "COBE-Explorer of the Primeval Explosion," Astronaut. Aeronaut.,
pp. 60-66, Oct. 1978.
2. A. Ludwig, J. Hardy, and R. Norman, Gain Calibration of a Horn Antenna Using
Pattern Integration. Technical Report 32-1572, Oct. 1972. Jet Propulsion Laboratory,
Pasadena, California.
3. P. D. Potter, A New Computer Program for the Design and Analysis of High
Performance Conical Feed Harm. Technical Report 32-1526, Vol. XIII, pp. 92- 107,
Feb. 1973. Jet Propulsion Laboratory, Pasadena, California.
4. H. P. Coleman, R. M. Brown, and B. D. Wright, "Paraboloidal Reflector Offset With a
Corrugated Conical Horn,"IEEE Trans. Antennas Propagat., pp. 817-819, Nov. 1975.
5. G. F. Smoot, M. V. Gorenstein, and R. A. Muller, "Detection of Anisotropy in the
Cosmic Blackbody Radiation," Phys. Rev. Lett., Vol. 39, pp. 898-901, Oct. 1977.
6. A. F. Kay, A Wide Flare Horn - A Novel Feed for Low Noise Broadband and High
Aperture Efficiency Antennas. Report 62-757. U.S. Air Force Cambridge Research
Laboratory, Oct. 1962.
7. P. D. Potter, "A New Horn Antenna With Suppressed Sidelobes and Equal
' Beamwidths,"Mcrow0ve/., Vol. VI, No. 6, pp. 71-78, June 1963.
8. P. J. B. Clarricoats and P. K. Saha, "Propagation and Radiation Behavior of Corrugated
Feeds: Part II - Corrugated-conical Horn Feed," Proc. Inst. Elec. Eng., Vol. 118,
pp. 1177-1186, Sept. 1971.
11
1 I I 1 I I I I I I I I I I I 1
0 ^3- 4 9 12 15
cm
Fig. 1. Test horn—corrugated, conical scalar feed horn antenna. Flare angle is 10 deg, and
groove spacing is approximately 0.5 wavelengths. Choke grooves on rim face are to
suppress potential backlobe radiation.
12
ORIGINAL PAGE JS
' QUALITY
TRANSMITTER
TRANSMITTER
TOWER
• 4 m -
RECEIVER AND
TEST HORN
HORN
ROTATION
AXIS
MODEL
POSITIONING
TOWER
AZIMUTH
AXIS
6 m
' / / / / S / S / / / / / / / / / / V / / / / / / / / /
GROUND
Fig. 2. Schematic of pattern measurement geometry
TRANSMITTER RECEIVER
ROTARY VANE
ATTENUATOR 31.4 GHz
HARMONIC
MIXER
HARMONIC
MIXER
2.6 GHz LO 2.6 GHz LO
45 MHz IF -45 MHz IF
SCIENTIFIC-ATLANTA
SERIES 1750
RECEIVER
SCIENTIFIC-ATLANTA
ANTENNA PATTERN
RECORDER
Fig. 3. Test circuit schematic
13
-180 -150 -120 -90 -60 -30 60 90 120 150 180
Fig. 4. H-plane radiation pattern of test hom. Receiver noise dominates below 90 dB.
o
10
20
30
2 50
70 -
80 -
90
100
MEASURED
CALCULATED
I
-180 -150 -120 -90 -60 -30 0 30
ANGLE, deg
60 90 120 150 180
Fig. 5. f-plane radiation pattern of test horn
14


Pattern measurements of a low-sidelobe horn antenna

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