a low-level retrodirective control signal in conjunction with the microwave power beam the "bit wiggle" technique used in larger phased array radar antennas can be applied. This approach reduces the amount of control electronics required while being selfcalibrating. The transmitting antenna's tolerance losses after correction by the adaptive optics will be 4% at 2.45 GHz or 6% at 5.80 GHz because the mechanical accuracy requirement is tighter at 5.80 GHz. As reflector tolerances are even tighter, reflector tolerance losses of 5% at 2.45 GHz and 7% at 5.80 GHz are expected. Local variations in atmospheric density and moisture content affect the dielectric constant of the atmosphere, which in turn bends microwaves that pass through it. For very large antennas with very narrow microwave beam widths, this defocusing effect can be very significant. If uncorrected, defocusing will degrade efficiency, in a fashion similar to poor surface tolerances. This atmospheric defocusing effect changes quite slowly, so the same "adaptive optics" that correct for mechanical or phasing tolerances in the antenna can also correct for atmospheric defocusing. Nevertheless, as this effect is a separate source of error, it cannot be corrected perfectly. An average loss of 2% for this factor at 2.45 GHz or 5% average at 5.80 GHz is expected. Extensive work has been done on improving rectenna efficiency. With the proper power level per diode in the rectenna array, a rectenna efficiency of 88% can be achieved at 2.45 GHz. At 5.8 GHz, the efficiency is reduced to about 85%. The conversion efficiencies at the preferred beaming frequencies are as follows: Based on these values, the nominal power drawn from the hydroelectric power station to deliver 1 GW to the power grid in the Almaria region of Spain will be as follows:
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