power. Microwave technology has the potential for high efficiency, large power handling capability, and close control. Other technological approaches for the transmission of power, e.g., lasers, are not considered feasible at their present state of development. There are, however, some critical microwave technology items that must be developed if SPS requirements are to be met. In the following subsections, the various aspects of microwave technology pertinent to the SPS are discussed. The system concepts are briefly described first, followed by discussions of the status and requirements in transmission, conversion, antennas, and phase front control. Since Ref. El reports on an in-depth study of microwave power transmission systems, the bulk of the discussions below are summarized from it. a. System Concepts The solar photovoltaic SPS proposed by A.D. Little, Raytheon, et al, utilizes slotted waveguide subarrays which are electronically controlled to direct the power beam to the ground receiving antenna. The subarrays consist of groups of amplitrons in series or klystrons in parallel to convert de power to microwave power. The SPS transmitting antenna would be a planar phased array, and the ground receiving rectifying/antenna (rectenna) would consist of dipole elements, each connected to a solid state diode to convert the microwave power back to de power. The operating frequency would be 2.45 GHz, and the transmission efficiency is said to be about 60%. Except for a few differences in details, this is essentially the same concept being considered by other contractors and agencies involved in SPS studies. b. Transmission A frequency of 2.45 GHz has been selected for microwave transmission because it (1) is in the USA industrial band, (2) results in nearoptimum efficiency, (3) avoids brown-outs in rain, and (4) should have minimal problems in RFI and allocation. The power density of the beam 2 2 could be held to 130 mW/in (20 mW/cm ) at the ground rectenna. Transmission will be affected by a number of propagation factors,
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