(3) Relatively nondirective aperture. (4) High absorption efficiency. (5) Passive radiation of waste heat. (6) Low radio frequency interference (RFI). (7) Capable of large aperture sizes. (8) Low mechanical tolerance requirements. (9) Low cost. Of the several candidate antennas which have been chosen for analysis, each is required to interface with the RF/DC converter which shows the most promise, the diode rectifier. The diode may be used singly or in groups for each candidate antenna. If large numbers are grouped together then it will be necessary to provide for some type of auxiliary cooling. There are four types of receiving antenna arrays which have been considered. The array of contiguous horns, as well as the array of contiguous reflectors and feed horns, both with the disadvantage that they cannot collect the impinging energy with as much efficiency as the other two approaches considered. The phased array of small elements with a common MW load can increase this efficiency by matching individual elements to the incident radiation, but this makes the phased array directive and the common MW load will require auxiliary cooling. The four approaches considered are shown in Figure IV-D-1-1. With the exception of the rectenna approach, all other approaches fail to meet the requirements in at least four ways. Because of the above factors, the rectenna approach has been selected as the reference for continued analysis. It appears to be the most promising approach, and actual tests on some initial rectenna designs have gone far in proving the feasibility of this concept. In the latter part of 1975 JPL demonstrated the concept at their Goldstone Deep Space Facility in California. A level of 323 KW was transmitted at a frequency of 2.388 GHz and a 12 x 25-foot array of rectenna dipole/rectifier elements intercepted approximately 11.4 percent of the beam. Of the 36.8 KW intercepted, 30.4 KW of DC power was generated with an efficiency of conversion of 82.5 percent. The power density at the rectenna elements of the above series of tests, however, was high compared to what can be expected at an operational SPS rectenna site, especially on the periphery. This low power density at the rectenna elements requires further investigations to determine how the output efficiency may be increased. This will be discussed more under the section on "Operating Characteristics" and "Technology Status, Criticality."
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