in an efficiency of 90. 5 ± 0. 5% at the 8 watt input level and to 79. 5 ± 0. 5% at the 5 0 milliwatt level. In addition to achieving improved efficiencies at the lowest power levels of expected interest in a full scale satellite solar power system, good efficiencies were obtained at still lower power levels. For example, an efficiency of 45% was obtained with an input level of one milliwatt. Despite the substantial improvements in efficiency at low power levels the resulting efficiencies do not represent the best that can be obtained. The measurements made on the diodes specifically designed for efficient operation at the lowest power levels indicated deficiencies that could probably be overcome with additional development effort and thereby substantially improve the efficiency. Althouth the emphasis on efficiency improvement was at the lower power levels some measurements in an exploring mode were made at power levels in excess of ten watts. DC output levels as high as 17 watts were obtained at efficiencies which were estimated to be as good as at levels below 10 watts. An estimate was necessary because the measurements were outside the capability of the standard measuring equipment. 2. Efficiency-Measurement Confidence and Resolution A considerable improvement in the confidence of efficiency measurements on the rectenna element was established by equating the microwave power absorbed by the rectenna element to the sum of the DC power output, the losses measured in the diode, and the circuit losses as measured experimentally and by computer simulation. The microwave power absorbed by the rectenna element is equal to the carefully calibrated incident microwave power upon the rectenna element less the reflected power which is typically less than one percent of the incident power. Each of the quantities going into the equating operation has associated with it a probable error. It was determined that the total probable error was ± 0.75% . The resulting difference between the two sides of the equation was within this probable error. Prior to the technique of balancing power input against power output and losses, confidence in efficiency measurements had to depend upon the use of the Bureau of Standards service in calibrating a secondary standard at Raytheon Company and the use of this secondary standard plus calibrated attenuators to establish the calibration of the rectenna element test set. In order to utilize the power balancing technique, however, it was necessary to carefully measure or estimate the losses in the rectenna element. This was done by developing a procedure to measure
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