to prevent non-linear heating interactions between the ionosphere and the microwave beam; and (3) microwave transmission efficiency considerations (particularly the RF levels into the rectenna). Studies into the microwave beam/ionosphere interactions indicate that non-linear thermal self-focusing instabilities in the F-region (200 to 300 kilometers altitude) and thermal runaway conditions in the D-region (100 kilometers) may limit the maximum power density at the center of the beam to approximately 23 mW/cm at the 2450 MHz operating frequency (references 26, 29, 30 and 31). Above this threshold power density level (which is a theoretical number, not yet verified by experiments), non-linear interactions between the power beam and the ionosphere begin to occur. These non-linear heating interactions are of concern because of possible degradations to existing HF and VHF communication and VLF navigation systems due to RFI effects and multipath degradations. The heated ionosphere may also introduce phase jitter and/or differential phase delays on the uplink pilot beam signal. These ionosphere/microwave beam interactions are now being studied, both theoretically and experimentally. Antenna Characteristics The microwave antenna has both a primary and a secondary structure composed of a graphite composite material. The primary structure is an open truss, 130 meters deep, with an octagonal shape over 1000 meters in width and length. The secondary structure is a deployable cubic truss, 9.93 meters in depth, which provides support for installation of the microwave subarrays. The aperture illumination function across the 1-kilometer transmit array was optimized to provide the maximum amount of RF power intercepted by the ground rectenna and to minimize the sidelobe levels. A number of different illumination functions, operating in the presence of phase and amplitude errors and element (subarray) failures, have been studied (see Appendix A). The 10- decibel Gaussian taper has the best overall performance of the optimized illumination functions after considering the maximum power density constraints in the transmit array and rectenna.
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