power density distribution of the horn or reflector and horn aperture. A number of steps may be taken to improve this efficiency but they will increase the cost of the collector and in any event will not make it possible to approach closely to 100% capture efficiency. The phased array with common microwave load can improve upon this situation since the matching of its individual elements can be tailored to a uniform incident illumination. However, the common microwave load makes the phased array highly directive and would involve auxiliary cooling for the common microwave load. A comparison of the various approaches with the requirements for reception and rectification of space-to-earth power transmission is given in Figure 9-6. It will be noted that all of the approaches with the exception of the rectenna approach fail in four or more ways to meet the criteria for ground collection and rectification. The rectenna approach meets them all. There is one condition, however, in which the rectenna approach may be unfavorable. That condition is where the density of the illumination is so low that a single dipole cannot collect enough power to operate efficiently. Under these conditions it may be necessary to use one of the other collection schemes such as an array of dipoles which would feed enough power into a single diode to make it operate efficiently. Under these conditions the increased directivity may be acceptable. The variation of power from the center of the receiving antenna to the edge for various system power levels is given in Figure 9-7, and the variation in efficiency with input power of a rectenna element using presently designed diodes is given in Figure 9-8. It may be seen that for a 10 km, 5 GW case that only the elements at the very center provide high efficiency. Under these circumstances it is appropriate to undertake development of a rectenna element with higher efficiency at lower power levels. Several design parameters are involved in this development. These include an increase in the circuit impedance of the rectenna element to increase the dc voltage at a given power output, a reduced junction area in the diode to optimize efficiency at the lower power levels, and finally a change in the junction materials from GaAs-Pt to GaAs-W which will produce less
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