to be the smallest entity for phase control, has the peaks of the grating lobe patterns at the ground as shown in figure 21. Since the distance between maxima for the grating lobes is inversely proportional to the spacings between subarrays, a 10.4 meter square subarray has peaks every 440 km. If the phase control system is extended down to the power module level, the grating lobes will be spatially smeared and the peaks greatly reduced in amplitude. This improvement in grating lobe pattern would be due to differences in spacings between the power tubes within the antenna. There are two types of mechanical misalinements: (1) a systematic tilt of the entire antenna structure, and (2) a random tilt of the individual subarrays. The systematic tilts have a greater impact than the random subarray tilts on the grating lobe peaks. An example of the first grating lobe peak for a total antenna/subarray tilt of 3.0 arcminutes is shown in figure 22. Other simulations have established mechanical alinement requirements of less than 1 arc^minute for the antenna tilt and less than 3-arc-minutes for the random subarray tilts. The near-field antenna pattern for distances close to the transmit array is shown in figure 23. A peak density of approximately 32 kk/m occurs at a distance of 1600 km. Rectenna Characteristics - The present ground receiving antenna (rectenna) configuration, which receives and rectifies the downlink power beam, has half-wave dipoles feeding Schottky barrier diodes. Two-stage low-pass filters between the dipoles and diodes suppress harmonic generation and provide impedance matching. For economic reasons, the rectenna is a series of serrated panels perpendicular to the incident beam rather than a continuous structure. Each panel has a steel mesh ground plane with 75-80% optical transparency. This mesh is mounted on a steel framing structure, supported by steel columns in concrete footings. Aluminum conductors are used for the electrical power collection system. The rectenna will produce RFI effects due to rescattered incident radiation and harmonic generation within the diodes. There will also be a small amount of RF energy leakage through the ground screen as well as knife eoge diffraction patterns at the top edge of each rectenna section. The general
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