LUNAR MICROWAVE SYSTEM CONSIDERATIONS Any lunar solar power system faces three configurational difficulties arising from lunar orbital characteristics. First, as has been stated, the moon's monthly phases cause lunar power stations to lie dark and inactive for two weeks of each lunar month. A nearly constant Earth power supply can only be achieved by constructing multiple lunar power stations at peripheral locations transmitting microwave energy at highly oblique angles. A second set of problems arises in the lunar microwave system due to the tenfold increase in required beam transmission distance. If a uniform antenna taper is assumed, the rectenna power collection efficiency (power incident upon the rectenna divided by the transmitted power) is (4): It can be shown that a high efficiency lunar power transmission system requires a value of Urg approximately equal to 4.0, which yields a collection efficiency of 89% for a uniform taper, or 96% for an antenna taper optimally shaped to concentrate power in the main beam and reduce sidelobes. For high efficiency operation, the argument Urg (Eq. 1) indicates the interdependent linear relationship between antenna radius (RT), rectenna radius (rg), and transmission distance (Ro). Since the frequency remains fixed at 2.45 GHz for both the lunar based and GEO transmission systems, the only way to counteract a tenfold increase in transmission distance (Ro) without greatly enlarging land requirements for the receiving antenna, is to provide a tenfold increase in antenna radius (RT). This corresponds to a hundredfold increase in required lunar antenna area to produce a beam pattern on the Earth essentially identical to that from a geosynchronous SPS. Additional complications arise when the effects of physical antenna tilt are considered. The retrodirective phased array antenna is mechanically and electrically divided into subarrays. Each subarray would have its own radio frequency (RF) receiver for phase processing an uplink pilot beam signal. The subarray surfaces may have the form of slotted waveguide radiators fed by single or multiple power conversion tubes as in the case of the SPS, or of parabolic reflectors (either a solid dish or an open-faced steel mesh) fed by a single high power tube. In the SPS 1 km antenna it was cost effective to utilize relatively expensive aluminum waveguides in order to achieve a high antenna transmission efficiency. The trade-off between attainable efficiency and expenditures for subarray surface radiators in not as straightforward for a lunar system, since the LPS antenna would be 100 times larger than the SPS array. The Appendix addresses this trade-off issue in terms of a wide range of microwave radiator efficiencies. The outputs from these subarrays combine to form a single coherent beam at the center of the ground rectenna where the pilot beam transmitter is located. Because of
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