The last measurements of overall system efficiency and overall rectenna efficiency were made in March of 1975 at Raytheon Company with the experimental setup shown in Figure 1- 13 The rectenna elements used in the rectenna array were those developed for the JPL RXCV demonstration at Goldstone but optimized for performance at 2.45 GHz. In order to establish a greater degree of credibility to the values of efficiency that might be obtained from the setup the Quality Assurance department of the Jet Propulsion Laboratory supervised the taking of the data. The overall DC to DC efficiency was measured at 54. 18%with a probable error of ± 0. 94%. The overall collection efficiency of the rectenna was more difficult to ascertain because of the inaccuracy in determining the fraction of the generated microwave power which is intercepted by the rectenna. The most probable efficiency, however, was 82%. A schematic of the test set up and the breakdown of efficiencies and inefficiencies is given in Figure 1'14. The last major rectenna effort ^2) reported upon is the relatively large scale reception-conversion subsystem (RXCV) for a microwave power transmission system located at the Venus site of the JPL Goldstone facility in the Mojave desert. This effort was not undertaken as a technology development as such but nevertheless gave useful output in terms of (1) confirmation of the reliability and efficiency of advanced diode design, (2) evaluation of rectenna subarray performance with incident uniform power density, (3) protective measures to be taken to guard against rectenna failure with accidental load removal or with unusual wave-forms of the envelope of the transmitted microwave power, (4) protection of the rectenna elements from the atmospheric environment. The rectenna shown in Figure 1-15 consisted of seventeen subarrays each 1.22 meters square and containing 270 rectenna elements. The rectenna element shown in Figures 1-11 and 1-12 that was designed for this application constitutes the point of departure for the technology development program being reported upon. The collection and conversion efficiency of this array was measured to be 82% at a total DC output of 3 0 kilowatts. 1.3 Progress in Rectenna Efficiency Using Progress in Rectenna Element Efficiency as an Index. The rectenna efficiency is given by the product of the microwave power collection efficiency and the rectification or conversion efficiency. The maximum theoretical collection efficiency is 100% and it has been measured at over 99% efficiency by means of VSWR measurements of a probe in front of the array. The validity of measuring collection efficiency by this means rests upon a small amount of power being reflected from the rectenna and upon a gaussian distribution of energy in the incoming wave and in the reflected wave. These conditions are closely approximated by the setup shown in Figure 1-13 where the gaussian illumination is laid down by means of a dual-mode horn. If it is assumed then that the collection efficiency can be made close to 100%, it follows then that the efficiency of the conversion of the collected power
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