would be prudent to limit levels to 5 GW, or 10 GW a: most. The penalty is not large since the economy of scale is achieved at the 5 GW level. 12.2.6 FREQUENCY SELECTION Frequency selection perhaps is the most important output of this study since device development is intimately related to the choice, and system development can be severely impacted by difficulties in radio frequency interference and allocation. The DC-RF converter characteristics and the system efficiency and cost factors have been shown to be favorable in a broad range near 2 GHz so that a choice of the USA industrial band of 2, 4-2. 5 GHs centered at 2. 45 GHz appears to be straightforward. The effect on other users of the spectrum can be significant for any choice, but the 2.45 GHz selection appears to have minimal impact as discussed in an earlier section. 12. 2. 7 CHARACTERISTICS OF 5 GW AND 10 GW SYSTEMS Attention was directed to the effects of beam taper, beam collection efficiency and cost assumptions on the characteristics of 5 GW and 10 GW systems at the selected frequency of 2.45 GHz. It was assumed in these calculations that the converters would be fully packed at the center of the transmitting antenna, which as stated earlier minimizes orbital antenna diameter and ground power density. Note that these assumptions do not necessarily give minimum cost results in all cases. They do lead to lowest cost for beam tapers of 5 dB and greater if power source and transportation-assembly estimates are medium level or higher for the assumptions in Figure 12-5. The results in Figure 12-14 show that there will be favored combinations of taper and beam efficiency to make best utilization of the receiving antenna, as could be anticipated from Figure 12-3. Figure 12-15 shows that increasing taper increases transmitting antenna size, and Figure 12-16 shows that increasing taper causes higher ground power densities. The 5 dB taper 90 percent beam efficiency combination is attractive in that it has a relatively low power density on the ground with a reasonably small receiving antenna. These results are independent of cost assumptions.
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