The Search for Extraterrestrial Intelligence Sky Survey project requires operation from 1 to 10 GHz on the beam waveguide (BWG) antenna DSS 24. The BWG reflectors are undersized in the 1- to 3.02-GHz range, resulting in poor performance. Horn designs and a method for implementing 1- to 3.02-GHz operation on DSS 24 are presented. A combination of a horn and a shaped feed reflector placed above the main reflector is suggested. The horn and feed reflector could be hidden in the RF shadow of the subreflector and struts. Results from computer analysis of this design indicate that adequate performance could be achieved

Lee, P. R.

Stanton, P. H.

English

NASA Technical Reports Server

TDA ProgressReport42-110
- 19 43 I.
August15,1992
SETI Low-Frequency Feed Design Study for DSS 24
P. H. Stanton and P. R. Lee
GroundAntennasand FacilitiesEngineeringSection
The Search for Extraterrestrial Intelligence Sky Survey project requires operation
from 1 to 10 GHz on the beam waveguide (BWG) antenna DSS 24. The BWG
reflectors are undersized in the 1- to 3.02-GHz range, resulting in poor performance.
Horn designs and a method for implementing I- to 3.02-Gllz operation on DSS 24
are presented. A combination of a horn and a shaped feed reflector placed above
the main reflector is suggested. The horn and feed reflector could be hidden in the
RF shadow of the subreflector and struts. Results from computer analysis of this
design indicate that adequate performance could be achieved.
I. Introduction
The DSS 24 34-m beam waveguide (BWG) antenna
will be used for the Search for Extraterrestrial Intelligence
(SETI) Sky Survey project over a frequency range of 1
to 10 GHz. The antenna's BWG reflectors are undersized
for low noise operation at the low end of this frequency
range. Therefore, an alternate method of feeding the an-
tenna from 1 to 3.02 GHz is presented in this article. A
corrugated, 29-dB-gain horn located at the Cassegrainian
focus would give acceptable RF performance but would be
physically large and would block the normal BWG trans-
mission path. To reduce the size of the feed horn and
facilitate swift clearing of the BWG path for normal DSN
operation, a smaller feed horn in combination with a mov-
able shaped reflector could be used instead, This feed re-
flector would sit over the BWG aperture in DSS 24's main
reflector during SETI operations (see Fig. l(a)) and would
be moved into a storage position during DSN operations.
The SETI horn-reflector combination could be hidden in
the RF shadow of the subreflector and struts as shown in
Fig. l(b). Major RF system requirements 1 are listed in
Table 1.
II. Horn Design and Analysis
Two small-aperture, corrugated horns were used to
cover the lower frequency range of the SETI sky survey
(horns number one and number two would operate over
the ranges of 1 to 1.73 GHz and 1.61 to 3.02 GHz, respec-
tively). The aperture diameter of each of these horns was
constrained to three wavelengths, at their lowest operating
frequency, in order to help limit its weight and cost. The
1G. A. Zimmerman, Search Jar Ex_ra-Terre*trial Intelligence Mi-
crowave Observinj7 Project Slcy Survey Element, 1720-4100 (inter-
hal document), Jet Propulsion Laboratory, Pasadena, California,
November 12, 1991.
246
https://ntrs.nasa.gov/search.jsp?R=19930010244 2020-03-17T08:13:54+00:00Z
output flare angles of the horns were adjusted along with
i the shape of the feed reflector to empirically arrive at an
acceptable performance across the two frequency ranges.
Once the frequency range, aperture sizel and output
flare angle were established, the detailed wideband horn
design followed the procedures Outlined in [1]. The cor-
rugation profile of the 1- to 1.73-GHz horn is shown in
Fig. 2, with its various sections labeled in accordance with
[1] (mode converter, frequency transition, angle transition,
and output flare).
The corrugated feedhorns were analyzed with a JPL
computer program 2 that uses modal field-matching tech-
niques to determine the transverse electric (TEmn) and
transverse magnetic (TMm,) scattering matrix of the
horns [2]. From the scattering matrix, the return loss and
aperture fields were known. Using the radiation integral,
the radiation patterns of the horns were calculated from
the aperture fields. Table 2 summarizes the performance
of the horns at several frequencies.
III. Feed Reflector Design and Analysis
The shaped feed reflector was designed to optimize sys-
tem performance at the lowest frequencies, where system
spillover was expected to be the highest and to result in
higher noise temperature and lower gain. The center of the
feed reflector was chosen so that the feed reflector would
be located a little above the main reflector. To approxi-
mate the location of the first geometric optics (GO) focal
point of the feed reflector, a 25-dB horn pattern was used
to illuminate the subreflector. This pattern was moved
along the axis of the subreflector, and the overall gain of
the system was calculated using JPL computer programs s
that employ physical optics (PO) techniques. At the loca-
tion resulting in the best gain, the far-field phase center of
the horn pattern was 437 cm above F1, the focal point of
the main reflector and subreflector system [see Fig. l(a)].
The first GO focal point of the feed reflector was placed
445 cm from the reflector. The second GO focal point
was placed in the desired location of the feedhorn far-field
phase center. This location was chosen so that the horns
would be close to the feed reflector without blocking re-
flected radiation. The resulting shape was an ellipsoid.
D. J. Hoppe, Scattering Matrix Program ]or Ring-Loaded Circular
Wa_eguide Junctione (internal document), Jet Propulsion Labora-
tory, Pasadena, California, August 3, 1987.
3 R. E. Hodges and W. A. Imbriale, Computer Program POMESH
for Diffraction Analysis of Reflector Antennas (internal docu-
ment), Jet Propulsion Laboratory, Pasadena, California, February
1992.
The reflector parameters, horn location, and horn flare
angle were varied until the performance was acceptable in
the 1- to 1.73-Gttz range. The performance was then eval-
uated in the 1.61- to 3.02-GHz range. The horn location
and flare angle were varied to achieve acceptable perfor-
mance. Other reflector parameters were also tried, but the
performance was not improved. Figure 3 shows the final
reflector design.
The main reflector and subreflector system was de-
signed to work optimally with a 29-dB pattern that has
its near-field phase center at F1 and using an observation
distance to the subreftector. IIow the system performs
with the GO focal point of the feed reflector so far above
F1 was investigated. The phase centers (PC's) of the 1-
GHz and 3.02-GHz patterns generated from the horn and
feed reflector combination were calculated in the far field
and at Various distances (R equals observation radius) in
the near field (see Fig. 4). As the observation distance
moved further into the near field, the phase center moved
along the z-axis in a negative direction. Defocus curves
of antenna system gain versus feed reflector location were
generated by moving the patterns along the axis of the
antenna system (see Fig. 5). At the high-frequency limit,
the far-field phase center of a focused system is located at
the focal point of tile ellipsoidal reflector, which is 508 cm
above the reflector.
At 1 GHz, the far-field phase center is 208 cm above
the feed reflector. The reflector is only about 10 wave-
lengths in diameter at 1 GHz, so the system will not focus
well in any case, but it appears to be somewhat in focus.
With the feed reflector at the design location and using
an observation distance to the subreflector, the near-field
phase center is 7 cm below F1. Using the best gain loca-
tion from Fig. 5(a), which yields only a 0.05-dB increase in
gain, the near-field phase center is 69 cm below F1. Since
a wavelength is 30 cm long, the phase centers are reason-
ably close to F1. At this long wavelength, the system is
fairly insensitive to movement of the feed reflector along
the main reflector axis.
At 3.02 GHz, the horn and feed reflector system is out
of focus, causing the far-field phase center of the reflected
pattern to be 155 cm below the reflector. With the feed
reflector at the design location, and using an observation
distance to the subreflector, the near-field phase center
is 320 cm below F1. With a wavelength of 9.9 cm at
3.02 GHz, the phase center is very far from F1. Perfor-
mance at this frequency could be improved by redesigning
the feed reflector to move the near-field phase center close
to F1. Simply moving the feed reflector does not improve
performance, as shown in the defocus curve of Fig. 5(b).
247
In an attempt to focus the system some and increase the
overall gain, the subreflector was moved along its axis. By
moving the subreflector 1.6 cm towards the main reflector,
the gain was increased by 0.16 dB.
IV. Results
Table 3 summarizes the antenna system performance
as calculated with the PO programs. In the 1- to 1.73-
GHz band, the system performs well at one horn location.
In the 1.61- to 3.02-GHz band, however, the horn must
be moved to four different locations to achieve acceptable
performance.
A study of the BWG at 1 GItz was performed using
cos"(e) patterns as the horn input. The best gain-to-noise
temperature ratio (G/T) achieved was 28.24 dB, with a
gain of 47.74 dB and a noise temperature of 83.16 K. Com-
pared to the results from horn number one plus the shaped
reflector, the noise temperature increases 56.69 K, the gain
decreases 2.02 dB, and the G/T decreases 7.30 dB.
V. Conclusion
DSS 24, operating in its normal configuration with the
BWG, performs unacceptably at the low frequencies in the
SETI 1- to 10-GHz range. By putting a horn and shaped
reflector above the main reflect-o-r_ the BWG could be by-
passed, and acceptable performan_ could be achieved.
The horn and shaped reflector would have minimal impact
on the other DSS 24 operations since the horn and reflec-
tor would be placed in the RF shadow of the subreflector
and struts.
The results presented in this article indicate that the
suggested configuration would meet SETI system require-
ments with the two exceptions of noise temperature and
beam efficiency. The noise temperature is slightly high in
the lower frequencies of both the 1- to 1.73-GHz band and
the 1.61- to 3.02-GHz band; however, the G/T is higher
than the target G/T over the entire frequency range. The
beam efficiency is lower than the system requirements over
most of the 1- to 3.02-GHz range. The minimum beam
efficiency is 81 percent at 2.21 Gttz, and the system re-
quirement is 90 percent over the entire range.
References
[1] B. M. Thomas, G. L. James, and K. L. Greene, "Design of Wide-Band Corru-
gated Conical Horns for Cassegrain Antennas," IEEE Transactions on Antennas
and Propagation, vol. AP-34, no. 6, pp. 750-757, June 1986.
[2] G. L. James and B. M. Thomas, "TEll to HEll Cylindrical Waveguide Mode
Converters Using Ring-Loaded Slots," IEEE Transactions on Microwave Theory
and Techniques, vol. MTT-30, no. 3, pp. 278-285, March 1982.
iTable 2. Horn perlormance.
Table 1. System requlrements.
Parameter Reqmred value
Noise temperature
Polarization
Instantaneous bandwidth
Aperture efficiency
Beam efficiency
Frequency, GHz Galn, dB Return loss,dB Maximum cross-
(TEll mode) polarization, clB
a Right-circular polarization.
b Left-clrcular polarization.
-_i i- to 1.73-GHz horn
1.00 17.0 --33 --33.1
< 25K ............
- 1.15 17.2 -41 -30.6
RCP _ and LCP b (simultaneous) 1.32 18.0 -51 -42.7
>_ 360 MHz
1.51 18.6 --49 --37.0
> 65 percent 3.02 19.5 --47 --37.3
> 90 percent
1.61- to 3.02-GHz horn
1.61 17.1 --65 --29.8
1.88 18.2 --46 --35.6
2.21 19.0 --50 --47.0
2.58 19.7 -47 --37.8
3.02 20.6 -48 --41.7
Table 3. System performance.
Noise G/T, Target G/T b, Aperture Beam
Frequency, GHz Gain, dB
Temperature _, K dB/K dB/K ef_ciency ei_ciency
1.0- to 1.73-GHz horn
1.00 26.47 49.77 35.54 35.18 0.747 0.861
1.15 24.68 50.93 37,01 36.40 0.739 0.871
1.32 22.06 52.08 38.65 37.59 0.731 0.915
1.51 21.10 53.11 39.87 38.76 0.708 0.935
1.73 20.55 54.19 41,06 39.94 0.691 0.945
1.61- to 3.O2-GHz horn
1.61 26.52 53.60 39.37 39.32 0.698 0.814
1.88 24.68 54.92 40.99 40.66 0.692 0.854
2.21 24.02 56.47 42.66 40.07 0.716 0.809
2.58 23.46 57.81 44.10 43.41 0.715 0.830
3.02 23.03 59.10 45.47 44.7"8 0.703 0.86"/
a The noise temperature includes contributions from the sky and atmosphere, the reflectors, the horn, and the low-noise
amplifier (LNA) assembly. Sky and atmosphere and LNA assembly values from: JPL-ARC Front-End Design Team,
NASA SETI Common Radio Frequency System Design Team Report (NASA internal report), Appendix D, p. 6, NASA,
Washington, DC, August 1, 1991.
b The target G/T is for 65-percent aperture efficiency and 25-K noise temperature.
249
ii
m_
,,=
SUBREFLECT
AXIS OF MAIN REFLECTOR
: ; :__: =__- _AND SUBREFLECTOR, AND
• CENTER OF BWG PATH-_
FEED REFLECTOR IN _.
STORAGE POSITION --_
1110.5 cm
/_15 FEED REFLECTOR2 ¢m \ IN OPERATING
/""-- _.. \ POSmON7
.....: :_ ...... __ /;_TcM /
165c_ /
LNA PACKAGE /'-../%
" _ MAIN REFLECTOR J _
cm(304,8 dtam) I ,,,,,
_- F1 FOCAL POINT OF MAIN
REFLECTOR/SUBREFLECTOR
SYSTEM
HORN !! / REFLECTOR
.<- -?-......]
k / I/ RF SHADOW-/ "_'. /
Flg. 1. SET[ low-frequency feed system on DSS 24: (a) profile end (b) RF-shadowed reglon.
250
g
c5
50 I
40
30
10
E
0 - CONVERTER .......
-10
-2O
--4O
-50 [ ....
0 20
FREQUENCY ANGLE
TRANSITION
[ lL I 1
40 60 80 100
LENGTH, cm
Fig. 2. Profile of 1- to 1.73-GHz horn.
OUTPUT
120 140
,s r _.
/ -, AXIS OF
.," "" \ \ ROTATION- 7
• ]/ " \\ --/
/ " _. /
I _
i 4_-----F = 442.9 cm _
.....I.........,;,:_ _ .........._,......
, _,_,_=83.4_3_.gj ,
', ,, _ ', /" _
>
r1 = 304.8 C / _ ELLIPSOID IS CUT
FEED _ BY A CONE WITH
A HALF ANGLE OF
REFLECTOR .... 30 deg TO FORM
THE REFLECTOR
Fig. S. Feed reflector.
251
(a)
J
J
I"
/
t
.=p.......1._- AXIS OF MAINREFLECTOR
AND CENTER
z =/"R _ _ _ - _ _ _ t" ..... OF BWG PATH
%\
\
\
OBSERVATION R \
SURFACE /
FEED HORN_ z
,4"
Z 0
J "_- FEED REFLECTOR
PC J_' OBSERVAT ON SURFACE
Z !
2
_-4
_.4
8-4
-10
-12
_(b) l I I I I I I I I I =F-F .....
-
I 1 I I I I I I I 1
U,I
> -1 | (c) I 1 I I I t I t I I t I
-
0 10 20 30 40 50 60 70 80 90 100 110 120130
R FROM FEED REFLECTOR, m
Rg. 4.: Phase center location: (s) for horn and feed reflector psi-
tern; (b) versus observation radius at 1 GHz; end (c) versus ob-
servation radius st 3.02 GHz.
@
50.45
50,40
50.35
50,30
50,25
50.20
50,15
50.10
50.05
50,00 I I I _ I I I
100 200 300 400 500 600 700
59.8
59,7 -
59.6
59,5
59.4
59.3
59.2 i59,1
59.0
584 I I I I 1 I I
-200 -100 0 100 200 300 400 500 600
DISTANCE FROM F1 TO FEED REFLECTOR CENTER, cm
Fig. 5. Gain versus feed reflector location at: (,=) 1 GHz end
(b) 3.02 GHz.
|
252


SETI low-frequency feed design study for DSS 24

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