of the main lobe will be better than 95 % (2.45 GHz-frequency and distance to geosynchronous orbit). Several antenna illumination functions have been investigated for the best performance in sidelobe and grating lobe suppression. Summarising the results, the 10 dB Gaussian taper has relatively high overall performance considering all constraints. The beam reception efficieny with the above mentioned example is more than 87 %. Fig. 2.3.3 represents the combination of two diagraphs to express the selection of the 5 GW output power level. Using klystrons £or RF-power generation, a maximum power density of 22 kW/m should not be exceeded in the antenna because of thermal problems. The maximum power-density in the ionosphere should not exceed 23 mW/cm^, higher levels might cause nonlinear heating. Fig. 2.3.3 shows, that 5 GW output power meets the restriction. 6 GW would exceed either the antenna power density limit or the ionospheric power density limit. 4 GW transmission is possible only with higher specific cost. Beam attenuation by the atmosphere is expected to be in the range of 2 % (good weather conditions) and 6 % (heavy thunderstorm with wet hail). However 10 - 13 % attenuation basing on another theoretical model were also reported. Depolarisation and deflection effects appear to be of minor influence on the beam. I.2.3.5 Receiving Antenna (Rectenna) The rectenna will be constructed by hundreds of millions of halfwave dipole rectifying elements (Fig. 2.3.4) mounted on reflecting screens (groundplanes). The rectenna elements must be placed perpendicular to the incoming beam. A feature of the reported rectenna design is that it can be continuously fabricated at high speed in different lengths. The area required for a 5 GW rectenna is 117 km^ at 36$ latitude to intercept 88 % of the beam power. Fig.2.3.4: RECEIVING ELEMENT [Source: 78/41 ]
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