RADIO EFFECTS A. SATELLITE MAIN POWER BEAM The power transmission beam, 6.7 GW at 2.45 GHz, is the satellite's dominant microwave emission. Concern over possible ionospheric heating effects has resulted in a limitation of the microwave power densities within the transmitting beam to a maximum of 23 milliwatts/cm . At the perimeter of the rectenna array power density will be approximately 1 milliwatt/cm . Power density far from the beam center depends critically on details of the transmitting array design, but is nominally 2 microwatts/m as far as one Earth radius away from the beam center. In addition, design-dependent isolated subsidiary peaks (grating lobes) of 1 watt/m power density will occur at intervals of 440 km along an east-west line through the main beam; this line will be repeated at comparable intervals (stretched at high latitudes by projection effects) north and south. Figures 19 through 22 of the Reference System Document and the accompanying text give further details on expected power densities. An ideal isotropic antenna (0 dB gain) has an effective aperture of 12 cm^2 at 2.45 GHz. Such an antenna mounted on an aircraft or satellite passing through the center of the main beam could intercept and deliver to the receiver terminals approximately 1/4 watt of power, which is sufficient to damage many receivers. An isotropic antenna located 1 Earth radius away from a beam center would intercept 2.5 nanowatts per operating satellite. An antenna with 1/2 m effective aperture, e.g., a 26-dB radio telescope side lobe, could deliver one microwatt, which is sufficient to overload many parametric amplifiers. The frequency of the main power beam must be very stable in order to avoid unintentional frequency-scanning of the beam by the transmitting phased array. Amplitude stability is only loosely constrained by system requirement; and will depend on details of satellite D.C. power switch-gear and on the power control strategy adopted. The Reference System Report gives no values for either stability.
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