[1] A thermal limitation of 23 kW/m2 in the transmitting antenna; [2] A peak power density of 23 mW/cm2 in the ionosphere; [3] Cost effectiveness (the larger the power system, the more cost effective). The thermal limitation at the antenna is a function of the amount of heat generated by the d.c.-RF power converter tubes and of the effective radiator area. The reference configuration has 70 kW klystron tubes operating at 85% conversion efficiency and cooled by passive heat pipe radiators. From thermal considerations, larger antennas or lower transmit powers are desirable; however, as the antenna size increases, beam focusing is enhanced and the power density in the ionosphere increases. At some threshold power density level, which is dependent on the operating frequency, nonlinear interactions between the ionosphere and the power beam could begin to occur. These nonlinear heating effects are of concern because of possible disruptions produced in low frequency communications and navigation systems by RF interference (RFI) and by multipath effects. Theoretical studies of the ionosphere completed during the early phases of the SPS evaluation program indicated the power density should be limited to 23 mW/cm2 in order to prevent such nonlinear heating effects (3,4). This theoretical value was taken as the SPS design guideline. Based on these two opposing constraints, the reference system was sized to produce 5 GW of rectenna output power with an antenna 1-km in diameter. A Gaussian taper was chosen for the reference design as previous studies had shown this type of illumination function was a good approximation to an optimum aperture distribution (4,5). An evaluation of the relative performance of various Gaussian tapers can be obtained from a comparison of their rectenna collection efficiencies, i.e., the percentage of the transmitted power intercepted by the rectenna. For the SPS concept, only a portion of the main beam will be collected; the sidelobe energy occupies a very large area at minimal power density levels and is not economically feasible to collect. The rectenna collection efficiencies for a number of Gaussian tapers are shown in Fig. 1. Increasing the taper produces a lower main- beam gain, a wider beam, and reduced sidelobes. In summarizing, the 10 dB Gaussian taper was initially selected for the best overall performance (maximum power delivered at a high efficiency) when considering the power density constraints at the transmit array and in the ionosphere. SPS COST MODEL A detailed analysis of subsystem costs and masses for the reference 5 GW satellite with silicon solar cells for photovoltaic conversion is developed in Ref. 2. That cost model, figured in 1977 dollars, is used in this paper in optimizing the antenna design to achieve minimum electricity costs (minimum mills per kWh). Future changes in the absolute costs of the reference system should not have a great impact on the conclusions stated herein since this analysis is based upon the relative costing of various illumination functions. The principal elements in the SPS recurring costs are |1] Satellite hardware, [2] Transportation, [3] Space construction and support, [4] Rectenna, [5] Program management and integration, [6] Cost allowance for mass growth.
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