A dual-frequency (1.668 and 5.01 GHz) feed was designed for the Deep Space Station (DSS) 14 70-m antenna to support the Soviet Phobos Mission. This antenna system was capable of supporting telemetry, two-way Doppler, and very long baseline interferometry (VLBI). VLBI and two-way Doppler information on the Phobos spacecraft was acquired with this antenna in 1989

Reilly, H. F., Jr.

Stanton, P. H.

English

NASA Technical Reports Server

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TDA Progress Repot42-107
November 15, 1991
N92-14246 '
sI_<_tB, _ ....
.
The L-/C-Band Feed Design for the
DSS 14 70-Meter Antenna
(Phobos Mission)
P. H. Stanton and H. F. Reilly, Jr.
GroundAntennasand FacilitiesEngineeringSection
A dual-frequency (1.668 and 5.01 Gltz) feed was designed for the DSS 14 70-m
antenna to support the Soviet Phobos Mission. Th& antenna system was capable
of supporting telemetry, two-way Doppler, and very long baseline interferometry
(VLBI). VLBI and two-way Doppler information on the Phobos spacecraft was
acquired with this antenna in 1989.
I. Introduction
Two Soviet spacecraft were launched in 1988 to observe
Phobos and Mars. In cooperation with this Soviet project,
the United States agreed to provide tracking and telecom-
munications from the DSS 14 70-m antenna at Goldstone.
A dual-frequency (1.668 and 5.01 GHz) L-/C-band feed-
horn was designed and installed on the antenna to support
this project. This antenna system was capable of handling
telemetry and two-way Doppler and was part of a very long
baseline interferometry (VLBI) system. VLBI and two-
way Doppler information on the Phobos 2 spacecraft was
acquired with the antenna early in 1989. Unfortunately,
both the Phobos 1 and the Phobos 2 spacecraft ceased
functioning properly before the mission was completed.
To handle a simultaneous downlink frequency of
1.67 Gttz and an uplink frequency of 5.01 GHz, a dual-
fi'equency feed was designed which was composed of an
L-band dual-mode horn [1] enclosing a thin, coaxially
mounted, C-band surface-wave antenna [2]. A photograph
of this dual-frequency feed is shown in Fig. 1.
II. Dual-Frequency Feed Design
A dual-frequency feed, composed of a horn enclosing
a coaxially mounted surface-wave antenna, is described in
[3]. This type of feed has the ability to efficiently illumi-
nate a Cassegrain antenna at two frequencies simultane-
ously. The design of this feed is facilitated by the rela-
tively low radio frequency (RF) interaction obtainable be-
tween the horn and the surface-wave antenna. A frequency
spread of 3:1 for the Phobos mission's feed allows nearly
independent adjustment of the beamwidths and phase cen-
ters. Beyond the general requirements of simultaneous
L-/C-band operation and low impact on the existing 70-m
system, Table 1 lists the primary RF requirements of the
70-m antenna with this dual-frequency feed. A drawing of
the L-/C-band feedhorn is presented in Fig. 2.
88
https://ntrs.nasa.gov/search.jsp?R=19920005028 2020-03-17T14:38:49+00:00Z
A. Surface.Wave C-Band Feed
In the design of a C-band feed for the 70-m antenna at
Goldstone, the following needs were addressed:
(1) A common L-/C-band phase center for simultaneous
operation.
(2) A suitable RF radiation pattern; e.g., beamwidth
(10 dB) ,_ 26 deg, approximate Gaussian shape, and
low cross-polarized radiation.
(3) Low interference with the L-band horn's radiation
pattern.
(4) Ability to accommodate 15 kW continuous RF
power.
(5) Circular polarization.
A surface-wave antenna that could be mounted coax-
ially inside the L-band horn was selected to satisfy the
above needs. This C-band feed was composed of two main
parts: an artificial dielectric rod, in this case a disc-on-
rod [2] that supports a surface wave, and a launching horn
that excites a surface wave oll the disc-on-rod. The major
challenge of this design was to obtain a high surface-wave
(HEn-mode) launching efficiency. IIigher launching effi-
ciency allows greater isolation from the L-band horn and
greater control over tile C-band RF radiation pattern. Im-
proved launching efficiency was accomplished in two ways.
First, the TEll-mode smooth-wall launching horn used in
[2] was discarded and an HEH-mode corrugated horn was
used for a better match with the HEal-mode surface-wave
antenna. Second, the surface-impedance taper at the in-
put of this antenna was optimized for high surface-wave
launching efficiency.
Over most of tile length of the disc-on-rod, the surface-
impedance profile, controlled by varying the disc diameter
along a constant diameter rod, was designed to balance two
conflicting needs: first, to couple the surface wave tightly
enough to minimize its interaction with the surrounding
L-band horn and, second, to couple the surface wave as
loosely as practical to minimize the resistive loss. Over
the length of this disc-on-rod, after the input launching
taper, the disc diameter was gradually made smaller in a
series of straight sections and tapers. Finally, the surface
wave was coupled loosely enough to achieve the needed
effective aperture at the radiating end of the disc-on-rod.
The length of the disc-on-rod was adjusted to collocate
its phase center with that of the surrounding L-band horn
(approximately 20 in. inside the aperture). Details of the
disc-on-rod and launching horn are shown in Figs. 3 and 4.
The 15 kW of continuous C-band power transmitted
on the surface-wave antenna presented three problems.
First, the removal of the disc-on-rod's resistive heat loss
of approximately 600 W was accomplished with an inter-
nal water flow (0.6 gallons per minute) through the entire
length of the rod. Second, the disc-on-rod was primarily
supported (in a high-power field) by aramid string ten-
sion members at regular intervals along its length. This
dielectric string was small in diameter (0.025 in.) and
nearly transparent to the L-/C-band propagation. Third,
in order to protect the L-band low-noise amplifier from the
transmitted C-band power, isolation was provided by tight
coupling of the surface wave near the base of the L-band
horn and the addition of a bandpass filter in the L-band
waveguide.
Two additional components were designed as part of
the C-band feed: first, a coaxial waveguide, TEll-mode,
quarterwave-plate polarizer, and second, a rectangular-
waveguide-to-coaxial-waveguide transition in which an in-
ductive post was mounted to impedance-match the C-band
feed to the transmitter.
B. L-Band Feed
The L-band feed was made up of a dual-mode "Pot-
ter" horn [4], a four-arm waveguide network to bypass the
C-band input, a suppressor for unwanted modes in the cir-
cular waveguide, and a eircular-to-rectangular-waveguide
transition. The L-band feedhorn was a sheet metal dual-
mode antenna with a TE11/TMll-mode converter step
(see Fig. 2). The L-band received signal was extracted by
a symmetric, four-arm network just forward of the C-band
launcher, and the signal was recombined just behind the
C-band input. This L-band network not only made room
for the C-band input, but also served as a circular polar-
izer by the fact that one opposing pair of arms was one
quarter of a guide wavelength longer than the other pair.
The L-band horn was impedance-matched over a 5 per-
cent bandwidth primarily by adjusting the axial position
of the C-band launch horn relative to that of the L-band
horn and placing inductive posts in the four waveguides
of the bypass network near its junction with the L-band
horn.
The L-band mode suppressor consisted of diagonally
opposed probes inserted into a circular waveguide orthog-
onal to the desired TEll mode's E-field and phased to
couple higher order propagating modes (TMox and TE21)
to a resistive load. Without this suppressor, higher order
mode resonances occurred within the feed at L-band.
89
III. Results
The dual-frequency feed was put through a series of
RF tests 1 including pattern, voltage standing wave ratio
(VSWR), L-/C-band isolation, high power, and noise tem-
perature. Typical amplitude and phase RF radiation pat-
terns for this L-/C-band feed are shown in Figs. 5 and 6.
The VSWR was less than 1.14:1 at L-band and less than
1.08:1 at C-band. The isolation provided by this feed ge-
; For a detailed account of these tests, see M. Gatti, C-/L-Baud Feed
Test Summary, JPL Interoffice Memorandum 3334-88-0_;2 (inter-
hal document), Radio Frequency and Microwave Subsystems Sec-
tion, October 18, 1988.
ometry between the C-band input and the L-band output
was greater than 38 dB. The high-power tests included a
15 kW run for 4 hr continuously with no anomalies. The
L-band feed noise temperature was 8.15 K.
The dual-frequency feed was mounted on the DSS 14
70-m antenna and system tests were performed. Details
of the 70-m system test with this L-/C-band feed in place
are given in [5]. Table 2 summarizes the test results. The
system was in compliance with the requirements listed in
Table 1 except that the L-band system temperature was
35.3 K instead of the required 35 K or less.
References
[1] J. Withington, "DSN 64-Meter Antenna L-Band (1668-MHz) Microwave System
Performance Overview," TDA Progress Report _2-9_, vol. April-June 1988, Jet
Propulsion Laboratory, Pasadena, California, pp. 294-300, August 14, 1988.
[2] S. A. Brunstein and R. F. Thomas, "Characteristics of a Cigar Antenna," JPI,
Quarterly Technical Review, vol. 1, no. 2, Jet Propulsion Laboratory, Pasadena,
California, pp. 87-95, July 1971.
[3] S. Narasimhan and M. S. Sheshadri, "Propagation and Radiation Characteris-
tics of Dielectric Loaded Corrugated Dual-Frequency Circular Waveguide Horn
Feeds," IEEE Trans. Antenna PropagaL, vol. AP-27, no. 6, pp. 858-860, Novem-
ber 1979.
[4] P. D. Potter, "A New Horn Antenna with Suppressed Sidelobes and Equal
Beamwidths," Microwave J., vol. VI, no. 6, pp. 71-78, June 1963.
[5] S. Gatti, A. J. Freiley, and D. Girdner, "RF Performance Measurement of the
DSS-14 70-Meter Antenna at C-Band/L-Band," TDA Progress Report _P-96,
vol. October-December 1988, Jet Propulsion Laboratory, Pasadena, California,
pp. 117-125, February 15, 1989.
9o
Table 1. RF requirements
Parameter C-band L-band
Bandwidth, MHz 5010 4-10 1668 4- 40
Gain, dBi -- > 57
Effective __- +138
isotropic
radiated
power, dBm
Noise
temperature, K -- < 35
Polarization Right circular Left circular
Ellipticity, dB < 2 < 2
Power, kW >_ 15 --
Table 2. Measured RF performance
Parameter C-band L-band
B_ndwidth, bJHz 5010 4-10 1668 4- 40
Gain, dBi -- 59.5
Effective +138.3 --
isotropie
radiated
power, dBm
Noise
temperature, K -- 35.3
Polarization Right circular Left circular
EUipticity, dB < 2 < 2
Power, kW > 15 --
91
_LACK
ORIGINAL PAGE
AND WHITE PHOTOGRAPH
C-BAND DISC-ON-ROD
STEP
FEEDHORN
ONVERTER
Fig. 1. L-/C-band feed.
COMBINER/
POLARIZER
TRANSITION
(RECTANGULAR
TO CIRCULAR)
_ C-BAND LAUNCHER
QUARTERWAVE
P LATE
S JUNCTION
(COAXIAL TO
,,"1 RECTANGULAR)
I_ ¢ BENT WAVEGUIDE
/{_//" (7 in. x 7 in, x 90 deg)
wR43oy __ I
(1 in.) --J
Fig. 2, L-/C-band duel-frequency feed system.
92
0.697 in._
0.225 in. TYPICAL--_/
0,225 in. TYPICAL_
0.501 in, diam
0.710 in. diam
0,884 in. diam --
0,884 in. diam
1.010 in. diam
NOTE: NOT TO SCALE
m
m
m
u
m
I
Am
48,492 in.
m
6.667 in, diam
Fig. 3. C-band disc-on-rod and launching horn.
52,084 in,
17.960 in.
28.736 in.
tP-
93
0.048 in.
_[
•,=-0.406 in, dlam--=,"
0.449 in.
Adia m _!
SEE FIGURE 3 FOR DETAILED
MEASUREMENTS OF DISC
DIAMETER Adtam
Fig. 4. Element detail of disc-on-rod.
Y
10 20 3O
THETA, deg
\
-BAND
L-BAND
40
Fig. 5. Amplitude of L-/C-band feed pattern.
/
/
\
60
94
180
9O
0
o_
-90
-180
0
I I I I L I I
Z _"
Y
X
I I I I
5 10
THETA, deg
\ / \
,,j C-BAND
1 I I
15 20
Fig. 6. Phase of L-/C-band feed pattern.
95


The L-/C-band feed design for the DSS 14 70-meter antenna (Phobos mission)

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