3. I 2R losses in antenna 4. Mechanical tolerances (pointing, subarray/power module misalignment, waveguide/slot tolerances. These are, of course, dependent on many factors, such as frequency, subarray sizes, phase control concepts, etc. As the microwave system configuration developed and converged into the present concept, system efficiencies became better defined. Some of the differences in estimated efficiencies as they evolved are shown in Figure A-9. The end-to-end efficiency for the current microwave reference system is 61 percent. As an aid to understanding the terminology used in the microwave system, please refer to Figure A-10. Microwave System Sizing - Sizing and power transfer in the microwave system is dependent on three factors: 1. System end-to-end efficiencies 2. DC power output from rectenna 3. Transmit antenna size. For a minimum cost of power at the grid, the output of the transmit antenna should be as large as possible. However, this output is constrained by thermal limits on the antenna and power density limits in the ionosphere. Thermal dissipation limits set a heat density value on the transmit antenna 2 of approximately 21 kW/m . For the operating frequency of 2.45 GHz, studies have indicated that the power density of the SPS beam should not exceed 2 23 mW/cm . If the power density exceeds this value, it is postulated that nonlinear interactions may occur between the power beam and the ionosphere. With these two limits, and a desired value of power output at the rectenna, transmit antenna size and rectenna size can be traded. The Raytheon study in 1975 (ref. 25) initially studied overall microwave system sizes. Some of the parameters established were: 1. 5 GW DC output power at rectenna 2. Rectenna size--approximately 10 km diameter (at the equator) 3. Transmit antenna size, 1 km diameter 4. Aperture illumination - 5 step truncated Gaussian with 5 to 10 dB taper.
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