D. RECTENNA POWER BEAM SCATTERING Although designed to efficiently absorb the incident power beam, the rectenna structure will inevitably reflect and scatter some small percentage of the beam power. The 2.45-GHz power density in its immediate vicinity will, therefore, be somewhat larger and more complicated in structure than is implied by the transmitting antenna’s beam pattern. This is not expected to be a large effect, although no magnitude estimates are given. E. RECTENNA HARMONIC RADIATION The rectenna can be expected to radiate harmonic energy originating in its rectifying diodes. This is recognized to be a problem both for power conversion efficiency and for EMI effects, and low-pass filters have been incorporated into the more recent rectenna designs. Details of the rectenna are still in flux, but initial measurements on one prototype (Arndt and Leopold, op. cit. p. 21) showed that power re-radiated at the second’ harmonic (4.9 GHz) was 25 dB below the incident power. This corresponds to re-radiation of 21 MW from a rectenna illuminated by a 6.7 GW beam. Third and fourth harmonics were found to be 40 dB and >70 dB below incident power, corresponding to re-radiation of 670 KW and less than 670 W, respectively. Harmonic radiation from individual dipole elements should be partially coherent across the rectenna array, leading to formation of beams. Their detailed structure will depend in turn upon the details of the partial coherence of the incident power beam, and will be time-varying. Again, the most useful statements to be made concerning harmonic radiation effects are those which are parametric in harmonic signal strength, or are effect thresholds. F. RECTENNA NOISE RADIATION The fraction of incident power absorbed by the rectenna dipoles depends on the accuracy of their impedance match with the rectifier diodes, and that in turn depends on the rectenna D.C. bus voltage and diode current. Therefore, any variation in system bus voltage, e.g., as a result of cyclic
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