An example of the first qra'ting lobe peak for an antenna tilt of 2 arc-minutes and a random subarray tilt of 3 arc-minutes is shown in figure 5. Other simulations have established a mechanical aline- ment requirement of 3 arc-minutes for both the subarray and array mi salinements in order to keep the grating lobe peaks below .01 mw/cm^. The systematic tilts have a greater impact than the random subarray tilts on the peaks. The alinement requirement can be written °TOTAL (3 min) = °ARRAY + aSUBARRAY (1) Present studies into the antenna structural design and into techniques for subarray alinement indicate that the 3 arc-minute requirement should be attainable (ref 3). RECTENNA PATTERNS 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. A wire mesh screen with 75-80% optical transparency is used for the ground plane. The circular antenna pattern from a geosynchronous SPS microwave antenna is an ellipse at latitudes off the equator. For economical reasons the rectenna is a series of serrated sections perpendicular to the incident beam rather than a continuous structure. 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 edge diffraction patterns at the top edge of each rectenna section. To provide an estimate of the power levels around the rectenna, studies indicate that the dipoles have a 98% collection efficiency under normal loading conditions (Ref 4). The 2% reflected microwave power is directed upward and towards the southern horizon. This reflected energy at 2.45 GHz is only partially coherent since the regions of coherence for the incident beam are limited due to phase irregularities in the heated ionosphere and atmosphere. Harmonics of 2.45 GHz will be generated within the half-wave rectifying diodes and will be reradiated back through the low-pass filters and dipoles. Initial measurements of the harmonic levels relative to the fundamental indicate the 2nd, 3rd, and 4th harmonics are down by -25 dB, -40 dB, and <-70 dB respectively for the normal dipole/ diode rectifier configuration (Ref 4). These harmonics are radiated in the direction of the incoming radiation plus all other spatial directions where the energy adds in-phase, provided the separation of the dipole elements are in excess of one wavelength. Since the present configuration has multiple dipoles feeding one diode, the separation between radiating elements will exceed one wavelength, thus producing multiple harmonic lobes off-axis. There will also be leakage power through the rectenna. For a ground plane transparency of 80%, approximately 1% of the incident power appears as leakage through the wire mesh (Ref 5). This energy will have a maximum of .2 mw/cm^ at the center of the rectenna and decrease to .01 mw/cm? at the edge. Since the rectenna's receiving surface appears serrated with individual sections perpendicular to the incident radiation, there will be diffraction losses at the top edge of each section. An analysis of the knife edge diffraction pattern has been made to determine the variation in power density incident upon the adjacent rectenna section (Ref 6). The power density will vary in the shadow region (area behind the rectenna section) as shown in figure 6. It may be necessary to extend the size of the rectenna section to intercept part of the shadow region. There will also be energy lost as heat in the rectenna due to I?R losses in the receiving elements and in the diodes. Approximately 7% of the incident energy is expected to be lost as heat. Studies into possible weather modifications due to this heat dissipation indicate very minimal effects may be expected (Ref 7). The rectenna is equivalent to a surburban community as a heat source, with little or no detectable changes in weather patterns. The expected power levels around the rectenna for reflected energy, leakage energy, harmonics, diffractions, and heat losses are summarized in f i gure 6 • •• Munn imii
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