separate resistor to obtain a much greater range of data on the behavior of the rectenna. This latter decision involved a change in the manner in which the DC power was collected and instrumented/^) The output of each rectenna element was brought back through the reflector plane. This arrangement shown in Figure 1-8 provided such an enhanced capability to study and understand the performance of the rectenna that it was retained in the further development of the rectenna. (See Figure 1-9 for the later adaptation to a more recent MSFC rectenna. ) The construction, however, is not economical and is not recommended for most applications. The changed collection geometry as shown in Figure 1-8 improved the collection efficiency to about 93%. Other changes improved the overall transmission, collection, and rectification considerably . Because the diode rectifier is such an important element in the collection and rectification process, a search for diodes which would improve the efficiency and power handling capability of the rectenna has been a continuing procedure. In 1971, Wes Mathei suggested that the Gallium Arsenide Schottky-barrier diode that had reached an advanced state of development for Impatt devices might be a very good power rectifier and provided a number of diodes for testing.(^» ') These devices were indeed much better. Their revolutionary behavior in terms of higher efficiency and much greater power handling capability rapidly became the basis for the planning of improved rectenna performance. The knowledge of the superior performance of this device was coincident with the advancement of the concept of the Satellite Solar Power Station by Dr. Glaser of the A. D. Little Co. (21) The earliest investigation of a rectenna design for this concept indicated that the economics of its construction would be crucial and that mechanical and electrical simplicity of the collection and rectification circuitry would be of paramount importance. This factor, combined with the fact that no harmonic filters had existed in previous rectenna element designs but would be necessary in any acceptable microwave power transmission system, motivated a completely new direction of rectenna element development. This new direction was the development of a rectenna element employing a single diode in a half-wave rectifier configuration with adequate wave filters to attenuate the radiation of harmonics and to store energy for the rectification process. The construction of such a rectenna element and its insertion into a DC bus collection system is shown in Figure 1-9. This rectenna element was used, in the last phase of the MSFC sponsored work at Raytheon to construct a rectenna 1. 21 meters in diameter which was illuminated by a gaussian beam horn (Figure 1-10). The combined collection and rectification efficiency of this rectenna was measured at 80%. A lower cost and slightly more efficient form of this rectenna element was developed for the RXCV work sponsored by NASA at JPL. This element is shown in Figures 1-11 and 1<-12, together with a greatly simplified equivalent electrical circuit of the device. The same electrical circuit applies to the MSFC rectenna element of Figures 1-9 and IrlO.
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