The JSC study in August 1976 (ref. 2) expanded on the previous studies, and defined a 10-step truncated Gaussian, 10 dB taper which approximated a continuous taper and provided for side-lobe suppression. Other developments and trades regarding configurations and subsystems are discussed in the following sections. Transmit Antenna Configuration - The early in-depth study of the SPS microwave system conducted by Raytheon in 1975 (ref. 23) took the system sizing as discussed above and developed a configuration for the transmit antenna with the following characteristics: The transmit array consisted of 18M X 18M subarrays, with a structure fabricated out of graphite polyimide, based on a 1 percent beam power loss and material coefficients of expansion, and sized to be effectively packed into the Shuttle cargo bay. Because of questions of outgassing of the graphite polyimide, an aluminum configuration was suggested with independent 5M segments within the 18M X 18M subarray. Motor-driven screwjacks were used on each subarray to adjust for subarray deflections and tilt during operation. The JSC study (ref. 2) investigated 4M X 4M and 10M X 10M subarrays in order to widen the beamwidth and eliminate the requirement for screwjacks. It was determined that the effects of coefficients of expansion on surface tolerances would allow a 10M X 10M subarray, and the larger size (10M X 10M) would reduce the complexity of the phase control problem. Raytheon also performed a trade-off study of transmit antenna types (including space-fed and cylindrical arrays) and on subarray types, including helical radiators, parabolic dishes, pyramidal horns, and slotted waveguides. The latter was chosen because of its potential for high efficiency and the efficient means of distributing the RF power from the power amplifiers. Thus, as a result of the Raytheon and JSC studies (ref. 23,2) the configuration evolved into: 1. 10M X 10M subarrays (7850 total) 2. Subarray structure - graphite composite
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