Figure 46 SPS Capital Cost vs Frequency - 300 $/kg Figure 47 Peak Ground Power Density vs Frequency The significant trend is to reduced cost at higher power levels which is due to the fact that the rectenna is better utilized as power densities on the ground increase. The relative improvement slows above 5 GW as the basic power source costs dominate. Cost at low power levels trends lower with increasing freouencv because the transmitting antenna gain increases with reduced microwave wavelength. At the relatively economical pow’er levels of 5 GW and above, however, there are broad minima around 2 GHz, caused by the dc-rf converter packing limit resulting in a larger transmitting antenna than would otherwise be optimum at higher frequencies. A critical factor in addition to direct cost to be considered in selecting frequency and power is ground power density, which has implications for biological and environmental effects. Figure 47 shows peak ground power density assuming dc-rf converters are fully packed at the center of the transmitting antenna, an approach which minimizes antenna diameter and thus minimizes ground power density. Reference values are 100 mW/cm2 for sunlight at ground level and 10 mW/cm2 for the USA standard for continuous exposure to microwaves. An estimate for onset of ionosphere modification effects, based on scaling from experiments at much lower frequencies [Meltz, 1974j, is also shown. Levels above the biological standard could be accommodated (restricting rectenna area and its air space to fly throughs would limit exposure to short periods), and ionosphere modifications probably will be localized and have negligible effect on other users; nevertheless it would be prudent to limit power to levels as low as can be economically useful, such as 5 GW or 10 GW at most, for planning purposes.
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