0191 -9067/80/040345-05$02.00/0 Copyright1980 SUNSAT Energy Council SPS DESIGN WITH SOLID-STATE TRANSMITTER MORRIS ETTENBERG RCA/David Sarnoff Research Research Center Abstract — The replacement of the klystrons in the microwave transmitter in the SPS leads to major redesign considerations. Several curves are presented which show the interrelations of the major design features, the power level and the transmitting and receiving dimensions. A convenient starting point for considerations of the overall SPS design is the DOE/NASA Reference System Report, October 1978, which was produced as part of the SPS Concept Development and Evaluation Program. The system described there provides 5 GW into the power distribution net. The major structures in geostationary orbit are a 10 x 5 km solar panel array to which is attached a microwave transmitting antenna 1 km in diameter. The microwave power is delivered to the transmitting antenna by an array of klystrons, each of which produces 70 kW at a design efficiency of 80%. The ground structure receiving antenna is 10 km in diameter. The microwave transmission design is based on the Goubau (1) solution for maximum power transmission between two radially- symmetric antennas which requires a Gaussian power density taper from the center to the edge of the transmitter. A similar Gaussian taper characterizes the receiving antenna power density distribution. The Goubau solution finds that the product of the diameters of the transmitting and receiving antennas has a minimum determined by the range and the wavelength: DrDt g 2KR where Dr and Dt are the diameters of the receiving and transmitting antennas, A is the wavelength (12.24 cm for SPS at the frequency 2.45 GHz) and R is the distance between antennas (36,000 km for geostationary orbit). A basic limitation of the design is the maximum power density allowable at the receiving antenna which is 23 mW/cm2 to avoid interference-producing overheating of the ionosphere. From these two limitations, the major parameters of the system are determined. The results are shown in Figure 1. For economy of construction, the product of antenna diameters is taken at the minimum value and the numerical values of the various efficiencies are taken from the Reference System Report (p. 13). Figure 1 is read by constructing a vertical line at the required system power and reading the value of the other parameters from their intersection. For example, the design parameters for the Reference System are found from the figure by drawing a vertical line from the system power of 5000 MW which intersects the Dt line to give the diameter of the microwave transmitter as approximately 1 km. The intersection with the Dr line gives the diameter of the receiving antenna as approximately 10 km.
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