maximum intensity and a 30 kW/m2 antenna thermal limit, while the sidelobe peaks are constrained to 0.01 mW/cm2. For this configuration, the microwave transmission efficiency is 95.4% from the output of an error-free antenna to the summed outputs of four rectennas. After including antenna errors, rectenna losses, and atmospheric attenuation, each rectenna has an output of 2.93 GW of d.c. power. The total d.c. power from the rectennas is 4 x 2.93 GW, or 11.72 GW. The associated electricity cost using the cost model discussed in Ref. 2 is 45.5 mills/kWh which is lower than the 46.8 mills/kWh for the single beam, 10 dB Gaussian reference system. By optimizing the antenna taper, it is possible to deliver more power to additional rectenna sites at a reduced electricity cost rate. Effects of Rectenna Spacing and Subarray Sizing The size of the individual subarray or power module determines the maximum allowable spacing between rectennas. Small subarrays (4 m or less) produce wide- beam antenna patterns which are less susceptible to subarray misalignment losses (see Eq. 9 and Fig. 3). The combined rectenna collection efficiencies for two beams as a function of rectenna separation are shown in Fig. 6. An optimized single beam system with an antenna diameter of 1.2 km has 96% of its energy incident upon one 9.3 km rectenna. For a separation of 150 km, two rectennas have a combined collection efficiency of 94.2% for small subarray apertures 4 m) and only 82.1% for large subarrays (> 12 m). Thus, small subarrays, or phasing to the power module level, are required in order to allow adequate rectenna separations while minimizing the efficiency degradation due to beam misalignment. (A 2% maximum degradation in collection efficiency due to multiple beams is hereby suggested as the guideline for link calculations.) If phase control is extended to the power module level, the rectenna separation can be increased to approximately 225 km with less than 2% loss in efficiency.
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