3. Slotted waveguide 4. Waveguide material - aluminum 5. Phase conjugation - to subarray level 6. No mechanical alignment of subarrays. The subsequent Boeing studies (ref. 5,7,8,9) as part of the overall, indepth trade-offs on the SPS, arrived at basically the same configuration except for the waveguide material. Doubt was expressed that mechanical tolerances such as slot sizes, subarray tilt, waveguide dimensions, etc., could be held tight enough using aluminum waveguide. As a result of that factor and the weight trade-off, graphite composites (for mechanical stability) together with thin-skin depth aluminum on the waveguide walls (for RF conductivity) was investigated and recommended. Kovar waveguides were also investigated as an alternative. The MSFC and Rockwell studies (ref. 3,4,11) resulted in basically the same microwave system configuration, with some differences in antenna structure and configuration. The basic building block was a 30M X 30M mechanical module, with approximately 1OM X 1OM subarrays as an integral part. The radiating portion of the subarrays was defined as a resonant cavity radiator to reduce wall weight of the waveguides. The reference configuration of the transmit antenna is now as listed in the fourth paragraph of this section, with the following alternate possibilities. Phase conjugation to the power module level may be desirable for more precise control and better focusing. This resulted from the LinCom effort (ref. 27) on the phase control system and will be discussed more later in this section. It may be desirable to go to resonant cavity radiators if efficiency and resonant wave stability show an effective trade-off with weight. Figure A-ll is a pictorial summary showing the transmitter design concept as evolved for the reference configuration.
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