tions between the power beam and the ionosphere. Subsequent tests conducted by the Institute for Telecommunication Sciences (ITS) at Plattville, Colorado, and by Rice University at Arecibo, P.R., show no evidence that 23 mW/cm2 is an upper limit (9). The effects produced by simulated SPS heating are many times less than natural ionospheric disturbances created by solar flares. Although the actual upper limit is unknown since the present ground heating facilities at Plattville and Arecibo will have to be upgraded to achieve higher ionospheric heating levels, it is reasonable to expect that the 23 mW/cm2 constraint can be increased. However, levels above 40-45 mW/cm2 are somewhat academic if the maximum sidelobes are constrained to 0.01 mW/cm2 as seen from Fig. 5. (d) System Characteristics Optimal system characteristics including antenna size, rectenna size, and transmit power, as functions if ionospheric power density limits and antenna thermal limits are shown in Figs. 6 and 7. As the ionospheric density limit increases, the antenna size and transmit power
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