We see that trends expected from the relations previously described are present, including the effects of converter packing limitations. The latter produces discontinuities in the higher power transmitting antenna diameter trends and as a result there are gradual increases in the capital cost near 2 GHz for the higher powers. Decreasing efficiencies also contribute to a leveling off of costs for the lower power cases. The trends for klystron configurations of both aluminum and graphite composite materials shown in Figure 12-12 follow the same pattern as for the amplitron-aluminum cases of Figures 12-7 and 12-8. There is a slight shift to minima at higher frequencies for the klystron. 12.2. 5 GROUND POWER DENSITY AND POWER LEVEL SELECTION The microwave power density at the ground has implications for both environmental and biological effects and so is a key parameter in describing the MPTS. The peak level at the center of the beam is of primary interest and its magnitude, Pn, is given by: The peak levels are plotted in Figure 12-13 for the ranges of power levels and operating frequencies of interest. Maximum converter (amplitron) packing at the transmitting antenna is assumed which results in the minimum ground power density, i. e, , the smallest antenna gives the lowest peak ground power density for a given overall power level and beam taper. Also plotted are the approximate level for ground solar radiation (100 mW/cm ), the threshold estimated for onset of self-induced irregularities in the ionosphere, and the USA standard for continuous exposure (10 mW/cm ). We see that power levels above 5 GW increase the potential for environmental distrubance in the ionosphere and for potential difficulties in adequately safeguarding the air space above the receiving antenna. It is quite probable that ionospheric effects will be so localized that other users will not be disturbed, and that aircraft and bird fly-throughs will be too rapid to cause damage, but it
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