For the klystron: • Noise level exceeds radio astronomy isotropic regulations only in USA industrial band of 2.4 GHz to 2.5 GHz. • Noise level exceeds radio astronomy 60 dB antenna regulations between 2.1 GHz and 2.85 GHz. d. Antennas The SPS transmitting antenna slotted, waveguide subarray approach was selected over helical radiators, pyramidal horns, etc., because of its high efficiency. The antenna would be constructed of aluminum or composites, and would be about 0.6 miles (1 km) in diameter. The technology for the ground receiving antenna (rectenna) is more advanced than any other item related to the efficient transmission of microwave power. In recent tests at the JPL Goldstone facility, 82 percent efficiency was achieved at an output power level of 32 kWe. The rectenna for the SPS would be an array about 6 miles (10 km) in diameter, with panels tilted at right angles to the incoming wave front. With respect to the transmitting antenna, deflections in the subarrays will produce beam power loss. A graphite polyinide composite is more attractive than aluminum because it will deflect less per unit area, and also has better temperature and weight characteristics. There are questions, however, as to the extent that this material will outgas and the effect of this on the open-tube de to rf converters. Also, there are questions concerning its stability over the 30 years in the space environment. Until these questions are resolved, aluminum must be baselined and a segmented subarray approach must be taken to alleviate the deflection problem. The transmitting antenna waveguide, including its interfaces with the microwave converters and structure, has been identified as a critical technology item needing early development. With respect to the rectenna, sealing of the elements within a plastic tubing is recommended for economical environmental protection. Hailstones pose the greatest damage potential, and this must be considered in site selection. In addition, the rectenna support structure is a major item
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