station, provided the reactor satellite is in a stable, tethered orientation within 5-10 km of the target station. ANTENNA ALIGNMENT REQUIREMENTS Microwave beam patterns and collection efficiency for a nuclear satellite microwave system are influenced primarily by frequency, range and sizing considerations, and secondarily by the antenna taper. In this section, a third set of microwave beam features are considered, specifically the grating lobe peaks distributed across the far field. Each subarray on the antenna acts as a distinctly phased unit. The ensemble of subarray patterns are in phase not only at the mainbeam location, but also at other regularly spaced angles wherever the pathlength shift between adjacent subarrays is an integral number of wavelengths. Thus rectangular phasing periodicities on the antenna project a rectangular grating lobe pattern across the far-field (Fig. 14). Using 2 m square subarrays in a 20 m diameter antenna, grating lobes will occur at spacings of 129 m around the central mainbeam at the space station. If the satellite antenna alignment is precisely controlled (within a few arc minutes), these phasing peaks will coincide exactly with null positions of the individual subarrays’ gain patterns, thereby greatly reducing grating lobe magnitudes (8). This nulling of grating lobes is a necessary condition for high efficiency power transmission; high intensity grating lobes not only impose environmental hazards, but also severely reduce the power content of the mainbeam. When the microwave antenna is allowed axial tilts, the main beam and grating lobe positions are preserved by the phase control system. However all subarrays become aligned along a different orientation offset from the target station. The resulting shift in subarray pattern null positions allows a rapid increase in the grating lobe peaks. The case showing the microwave mainbeam and grating lobes with 10 arcminutes antennna tilt is given in Fig. 15 for the 5.8 GHz configuration (A).
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