significant as,sumptions and groundrules be identified and discussed with regard to their impact. The following paragraphs provide that discussion and point out some of the important results arising from the case studies. Possibly one of the most far-reaching groundrules applied to the case studies is the focus on 5.S GHz as the frequency of choice for power transmission in the new concepts, whereas previous studies have concentrated on 2.45 GHz. The choice of the higher frequency results from a desire to limit the size of the in-space transmitting array for a given beam spot size on the Earth, while at the same time keeping the frequency low enough for the beamed power to pass through the atmosphere without substantial attenuation (even under moderate rain conditions). Also, although both 5.8 GHz and 2.45 GHz fall within the Industrial, Scientific and Medical (ISM) spectral bands accessible to wireless power transmission, the chance of obtaining a spectral slot for this application appears to be better at the higher frequency where there is currently less competition. Another study groundrule that follows closely the choice of frequency is the proposed use of advanced solid-state FET devices for the power transmitting arrays as opposed to the use of magnetron tubes. The literature6 suggests that these advanced devices could ultimately offer efficiencies approaching 80% at a power level of 10 Watts per device and a frequency of 5.8 GHz. This compares with 90% efficiency and 5 kW for magnetrons operating at 2.45 GHz, as reported by Bill Brown7. However, in his white paper found in the appendix to this report, Brown suggests the difficulty of designing magnetron arrays that would operate at higher frequencies such as 5.8 GHz. This might be especially true in closely packed arrays needed for electronic beam steering, where heat removal from these high powered devices could be a significant problem Unlike the Reference System, the new SSP concepts under study are generally not situated in GEO, beaming power to a single ground site below them Except for SolarDisc, the power satellites and relays associated with the new concepts/architectures, are transmitting their power from LEO and/or MEO and consequently require electronic beam steering to provide service to a multitude of ground sites along their orbital path. Even in GEO, it is desired that a SolarDisc be able to service many ground sites with a single transmitter, and electronic beam steering would be preferable to mechanically slewing the large transmitting array. The choice of electronic beam steering as a study groundrule has several implications that should be pointed out. First, the ability to electronically steer a beam over a wide angular range requires that an array element have a maximum dimension that is small with respect to the wavelength of the transmitted power, and that the array elements be closely spaced to maximize efficiency. The maximum element size is, in fact, inversely related to the transmission frequency and the maximum beam slew angle. Due to the element packing requirement, the element size, along with the power produced by each element/device, determines the maximum array size for any specified output power. What this implies is that low power transmission requirements lead to small planar arrays and, assuming reasonably-sized ground site rectennas, low beam-coupling efficiencies and poor performance for systems in higher orbits. Conversely, systems transmitting high power levels with (or without) beam steering require large planar 6 Sachihiro Toyoda, “High Efficiency Amplifiers for 8 GHz”, 1996 IEEE MTT-S Digest, p. 689 William C. Brown, “The SPS Transmitter Designed around the Magnetron Directional Amplifier”, Space Power, Vol. 7, No. 1, 1988.
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