A definition is derived for an antenna configuration fixed-mounted high in the payload bay on the hybrid OFT-2 pallet which is compatible with Orbiter interface requirements. Tests showed that the combination of the selected panels and the designed corporate feed meets SIR-A performance requirement of 33 db gain. The effects of Orbiter structure proximity on performance were determined by scale model tests to be negligible. The potential for improved performance during subsequent reflights includes a multiple-beam capability and dual polarization

Kierein, J. W.

Metzler, T. A.

Nitschke, H. A.

English

NASA Technical Reports Server

DEFINITION STUDY OF THE SHUTTLE IMAGINC RADAR-A (SIR-A) 
ANTENNA ON THE SECOND SPACE SHUTTLE MISSION (OFT-2) 
HAROLD A. NITSCNKE, NASA JOHNSON SPACE CENTER, HOUSTON, TX 
JOHN W. KIEREIN, BALL BROTHERS RESEARCH CORPORATION, BOULDER, CO 
THOMAS A. METZLER, BALL BROTHERS RESEARCH CORPORATION, BOULDER, CO 
SUMMARY 
The study resulted in the definition of an antenna configuration fixed-mounted high in 
the payload bay on the hybrid OFT4 pallet which is compatible with Orbite# interface 
requirements. Tests showed that the combination 0.’ the selected ‘&neb and the 
designed corporate feed will meet the SIR-A performance requirement of 33 dB gain. 
The effects of Orbiter structure proximity on performance were determined by scale 
model tests to be negligible. The potential for improved performance during 
subsequent reflights includes a multiple-beam capability and dual pokrization. 
1.0 INTRODUCTION 
A study was conducted to define the concept and preliminary design of the SIR-A 
antenna. Operational Flight Test Number 2 (9FT-2) is the first mission to include 
scientific experiments in the payload bay and SlR-A is the highest priority experiment 
approved for flight. It is presently scheduled for flight during the second half of 1979. 
The primary consideration of the study was to provide the concepts for the antenna, 
supporting structure, and corporate feed at the lowest possible cost, while meeting the 
performance requirements and Orbiter interface requirements. The antenna was 
constrained to fit in the area forward of the Development Flight Instrumentation (DFI) 
without protruding from the payload bay envelope. The antenna points to the north 
side of the orbital track while the Orbiter flies with the payload bay opening facing the 
earth. This orieqtation is intended to thermally quulify the Orbiter to fly in this 
attitude for long durations. Results of the study included definition of the location in 
the payload bay, selection of antenna panels, design and selection of the corporate 
feed, design of supporting structure, and performance projections. The study has been 
followed by a program of detailed design, manufacturing, and test of the system. 
2.0 CONFIGURATION 
The SIH-A antenna design relies heavily upon previous SEASAT technology and the 
utilization of standard components in an ..*fort to minimize cost and risk factors while 
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https://ntrs.nasa.gov/search.jsp?R=19780022517 2020-03-22T03:40:32+00:00Z
optlmiziq system performenee. Seven honeycomb miaostrip antenna paneb, 
identical to tha panel ae~eropea and tested for the SWAT Bngineering MW unit 
(SMU), are arrayed to form a 9.38m x 2.08111 planer, longitudinally polarized, rt?diating 
8Vfqtce. A unique coaxial corporate feed distributes power to each panel in-phase, but 
with a stepped amplitude taper such that the center three panels each radiate one- 
sixth of the total power and the outer four panels each one-eighth. Power to each 
radiate petch on the panel is carried by a microstrip feedline network etched on the 
facq of the panel. 
The antenna panels and corparate feed, as shown in Figure 1, are mechanically 
slpp#ted and aligned to a planar surface by a fixed stm@ac& truss. An installation 
tcm. with adjustable fittings to the OFT4 spacelab-hybrid pallet, supports the 
mtcrma/stranebad< in a fired high-in-thepeyload bay location and allows vernier 
aajtgtment of the antenna boresight pointing angle to 47 degrees from nadir. 
Initial truss collcepfs were  constructed with graphite epoxy material, but reduction in 
the pallet-mounted, structural fundamental frequency requirements has permitted the 
use of less costfy, aluminum trusswork. With either material, thermal and mechanical 
distortions of the truss structure are sufficiently small  to cause the array to deviate 
less than - +0.635cm from a planar surface during on-orbit operations. Multilayer alumi- 
nized mylar on the trusswork and teflon-impregnated quartz cloth on the radiating 
panels provide passive thermal control. The multilayer insulation is covered y i t h  the 
quartz cloth to reduce the surface reflectivity, thus avoiding potential glare to the 
C R W .  
3.0 ANTENNA PANELS 
The 1.34m x 2.08m EMU panel combines microstrip array techniques with amique 
fiberglass honeycomb construction [ 1 J to yield an efficient, lightweight, monolithic 
antenna. All radiating elements and feedlines are precision photo-etched on a copper 
laminate and supported above a bonded capper ground plane by the honeycomb struc- 
ture, thus significantly reducing dielectric losses. The complete EMU array is 
illustrated in Figure 2, with nominal measured performance for individual panels 
tabulated below. Low development and manufacturing costs, rugged construction, and 
adequate performance made the EMU design particularly attractive for application to 
the SIR-A antenna. Only minor attachment fitting modifications have been made in 
this SEAS-4T design for the SIR-A program. 
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PARAMETER 
Gain 
Sidelobes 
E-Plane 
H - Plane 
Beam w idth 
E- Plane 
H-Plane 
VSWR 
TABLE 1 
EMU PANEL CHARACTERISTICS 
PERFORMANCE 
25.9 dB 
( -12 .5  dB 
(-16.0 dB 
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6.20 
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4.0 CORPORATE FEED 
A seven-way corporate feed, composed of standard air-loaded coaxial line and reactive 
power dividers, efficiently distributes power to the array panels and implements the 
reguired phase and amplitude distribution. As depicted in Figure 3, the  power divider 
configuration theoretically yields a amplitude taper across the array. If 
required, minor power distribution variations due to load mismatches in the reactive 
network can be compensated by modifying the power split of the first reactive divider, 
the only nonstandard electrical component. Equal phase illumination is ensured by 
trimming similar lines to equivalent electrical lengtb and critically phasing the 127 
inch line.)shown in Figure 3, such that the electrical length from input to panel 
terminations is equivalent for all ports. Multipcctor breakdown considerations dictate 
the use of TNC-type connectors an& dielectrically loaded power dividers. A 3 dB 
multipactor margin is maintained throughout the system. Based on vendor data, 
system insertion is calculated as less than 0.7 dR. 
A prototype SIR-- feed system composed of Andrews H5550 7/8" coaxial cable with 
N-type connectors and Microlab DZTN power dividers was constructed and tested 
during this study in order to establish confidence in the proposed design. The feed 
exhibited good control over the amplitude distribution and total system insertion loss 
was 0.3 dB vith input VSWR less than 1.15 < 1. Theoretical insertion loss for the 
prctotype system was 0.2 ,' dB. 
5.0 SYSTE C PERFORMANCE 
~ ~~ ~ 
Projected nominal performance of the SIR-A antenna is shown in Table 2. When the 
array of seven EMU panels is uniformly illuminated, the E-Plane beamwidth (-8 dB) is 
IV-2-5 
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IV-2-6 
theoretically 1.Y9 degrees with 13 dB sidelobes, but the amplitude taper implemented 
by the corporate feed increases this beamwidth to 2.07 degrees and potentially reduces 
sidelobes to -15.2 dB. Array distortions, and particularly phase d amplitude errors in 
the feed, rill  cause deviations from these nominal values [ 2 1. 
The panel gain (25.9 a i ) ,  plus the seven4ement array factor gain of 8.45 dB, yields a 
theoretical 34.35 dBi gain for the SIR-A antenna array; however, several nominal 
directivity arrd efficiency losses must be corrsidered (Table 2). 
TABLE 2 
DIRECTiVITY AND EFFICIENCY LOSSES 
E-Plane Taper 
Feed Network 
-0 . 02 
-0 . 70 
Mechanical Distortions -0 . 08 
Feed Phese and Amplitude Errors -0.13 
Thermal Blanket -G. 05 
-0.2  Frequency and Temperature Effects 
N E T  LOSS -1. 18 
Thw, the nominal projected gain of the seren-panel SIR-A antenna array is 33.17 dBL 
Due to the proximity of the SIR-A antenna to potential reflecting surfaces within the 
Shuttle payload bay, a 1/10 scale model radiation pattern test at 12.75 GHz *as 
conducted to assess the impact of the Shuttle structural surfaces on the SIR-A antenna 
performance. The test configuration is depicted in Figure 4. This figure shows the 
antenna in a scale model payload bey with the DFI pallet installed. The Orbiter 
radiator panel on the side of the bay is also mocked up. Radiation patterns with the 
.antenna mounted high in the bay, as in Figure 1, indicated no measurable bearnwidth 
variation from free space conditions and only a minor increase (0.8 dB) in one H-Plane 
sidelobe. Shuttle structural surfaces will not adversely affect SIR -A performance. 
6.0 POTENTIAL FGR IMPROVEMENT 
Because the SIR-A experiment remains inside the Shuttle vehicle and returns to earth, 
the potential for evolution in system configuration during subsequent reflights is 
possible. Probablt reflights, as yet unscheduled, will provide greater ground coverage 
than is possible during the  S-day OFT-2 Mission. Improvements in performance could 
be achieved, with small additional costs, by adding an H-Plane multiple-beam 
capability to the antenna. Dual polarization could also be added with some 
IV-2-7 

modification to the antenna circuit art- and transmitter. The antenna could have 
an on-orbit pointing adjustment capabili. neorporeted. Another improvement would 
be to fold the antenna so that it occupies approximately the width of one Spacelab 
pallet during launch and descent, and deploys over the aft  payload bay doors during 
operation. This change would reduce the  length of payload bay occupied, without 
affectiq Orbiter thermal radiator performance, thus allowing a larger variety and 
number of additional other payloads to share the  mission. In order to accomplish this, 
a foldable feed system, as impiemented on the SEASAT SAR antenna and a DoD SAR 
antenna, would be required. Such changes would result in additional scientific data and 
provide evolutionary improve men t. 
7.0 REFERENCES 
i 1; Brejcha A. G., Keeler L. G., Sanford G. G., "The SEASAT-A Synthetic 
Aperture Radar Antenna," Synthetic Aperture Radar Technology 
Conference - 1978. 
121 Huisjen M. A., "Prediction of Antenna Array Performance froin Subarray 
Measurements," Synthetic Aperture Radar Technoiogy Conference - 1978. 
IV-2-9 


Definition study of the Shuttle Imaging Radar-A (SIR-A) antenna on the second space shuttle mission (OFT-2)

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