antenna with 30 m rectenna will give the same performance as a 15 m antenna with 40 m rectenna. The 5.8 GHz transmission frequency was chosen based on: (1) the availability of this frequency within an existing Industrial-Medical-Scientific (IMS) band; (2) the existence of present day technology to mass produce microwave tubes at this frequency and upgrade efficiency potential to 75%; and (3) no water vapor absorption problems since the beam does not traverse the atmosphere. (The SPS frequency was chosen to be 2.45 GHz in order to avoid atmospheric water vapor absorbtion losses which increase sharply above 3.5 GHz.) Microwave frequencies higher than 5.8 GHz reduce the beam spreading effect (larger U*) and allow longer transmission ranges, but introduce additional hardware complexities. Power tubes from 10-20 GHz would likely experience degradations of 5% or more below the nominal 75% conversion efficiency allowance for 5.8 GHz tubes. This study therefore will investigate only frequencies of 5.8 and 11.6 GHz; beam performance at higher frequencies may be estimated by scaling according to the U* beam spreading factor. The microwave system sizing options at 11.6 GHz are shown in Fig. 13. Doubling the beam frequency allows antennas to transmit power twice as far for a given rectenna, or alternately, to transmit the same distance using a half-sized rectenna. Since near-field zones for the beam are also extended, a 20 m diameter antenna is no longer appropriate for the 5 km transmission range. A balanced 11.8 GHz microwave system providing minimal near-field beam degradations and moderate antenna sizing is best met by: (C) a 20 m antenna and 30 m rectenna separated by 10 km, or (D) a 10 m antenna and 30 m rectenna at 5 km spacing. Such microwave configurations offer good potential for the transportation of power from a nuclear reactor to a space
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