(2) What should be done if thermal radiation problems at the center of the transmit array cannot be solved without using complex, active cooling systems? One solution is to reduce the taper to 5 dB and increase the antenna size. The collection efficiency for a 1.25 Km, 5 dB taper antenna is shown in Figure IV.A.2 -21. It would be more desirable to increase the diameter to 1.25 Km and keep the 10 dB taper. The data given in Figure IV.A.2-1 indicates the boresight power density would decrease approximately 30% by going to the larger diameter and keeping the 10 dB taper. (3) Klystron versus amplitrons - The model configuration uses 50 Kw klystrons for the power converters in contrast to previous studies which employed 5 Kw amplitrons. It is felt that klystrons would have a higher reliability. From a system viewpoint, there are two critical issues which may determine whether amplitrons or klystrons should be used. There are (1) high efficiency with reduced power - the DC input power to the transmit antenna will decrease by about 30% over the 30-year lifetime. As the current slowly decreases to 70% of its initial value into the power converters, the klystron or amplitron must maintain a high DC-to-RF conversion efficiency. One method to reduce this 30% reduction is to add solar cell arrays periodically to the SPS. (2) High reliability - the rectenna collection efficiency is very dependent upon the failure rate. The % collection efficiency as a function of failure rate for a transmit antenna with 10° phase error and + 1 dB amplitude error is shown in Figure IV.A.2-22. The slope of the curve gives a 2% loss in efficiency for each additional 1% failure rate. As discussed previously, this is one billion dollars loss in revenue over the 30-year life for 1% failure rate. Thus, the power converters must operate efficiently and reliably in a high temperature environment. IV.A.2(1) BASIC MICROWAVE SYSTEM PERFORMANCE SUMMARY The performance characteristics and requirements for the model microwave system may be summarized as follows: (1) Output DC power - 5GW at the rectenna (2) Transmit Array size - 1 km in diameter (3) Array aperture illumination - a 10 step, truncated Gaussian amplitude distribution with a 10 dB edge taper (4) Subarray size - 100 m^ (approximately 10m X 10M) (5) Number of subarrays - 7,850 (6) Error budget - Total RMS phase error for each subarray - 10° (for the phase control system) Amplitude tolerance across subarray - +/- 1 dB Failure rate (total) - 2% over 30 year lifetime
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