9. 4 ASSEMBLY AND CONSTRUCTION The construction approach suggested for the rectenna is illustrated in Figure 9-16 where wire mesh is supported by a simple framework to be normal to the incoming power beam phase front. The angle is not critical due to the wide beam pattern of the dipole antenna elements. The open mesh reduces wind loads and the amount of material needed, and the relatively simple support arrangement keeps the foundation and site preparation costs as low as possible. A detail of the suggested mounting for the rectenna elements is given in Figure 9-17. dc power is collected by the elements in parallel and then summed in series as was indicated in Figure 9-H. The voltage level for summation involves a tradeoff of I R losses at low voltage and high current, versus the insulation penalties at higher voltages and lower currents. A level of 1 kV was somewhat arbitrarily selected as that level for power inversion up to 66 kV for distribution to a power grid. (An integrated rectenna industrial complex would perhaps eliminate the associated extra cost and efficiency loss. ) Environmental protection for the extremely large area of rectenna poses a unique problem in that many effective techniques are too costly to consider. The conditions to be considered are rain, wind, snow and ice, temperature extremes, hail, blown sand, salt spray, and ultraviolet solar radiation. The approaches considered were: radome over the whole assembly, exposed assembly with conformal coating, and exposed assembly with a dielectric tubing shield as shown in Figure 9-18 (top and bottom halves would be heat sealed). The radome would be too expensive; the conformal coating may pose difficulties with power loss; and the tubing may be too expensive and have cooling problems. However, the latter two concepts are proposed for further study. The main threat to damage with these methods would be the impact of large hailstones. This should be a consideration in site selection. 9. 5 ROM COST ESTIMATES Costs were generated on the basis of cost per square meter except for power distribution. It is assumed that diodes are developed to handle the full range of power densities involved and/or that power from several dipoles can be collected for a single diode at the same cost or less than for the single diode-dipole combination.
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