into DC power is really the measure of the overall rectenna element efficiency where the rectenna element is defined as shown in Figure Irll. The progress that has been made in rectenna element efficiency as determined by test equipment to be described in Section 2. 0 of this report is shown in Figure 1-16. According to Figure 1-16 the efficiency has now exceeded 90%. The validity of this figure is the subject of discussion in Section 2. 0 of this report. The progress in efficiency is closely as sociated with the use of improved diodes but the choice of circuitry is also important. 1.4 The Energy Problem and the Solar Power Satellite Concept as Factors in Determining the Extent and Direction of Rectenna Development The early stages of rectenna development were carried out in response to the need for high altitude atmospheric platforms that could stay aloft indefinitely propelled by the power beamed to them by microwaves, and for the transmission of power from one vehicle to another in space where wire transmission would be impractical. There was no generally recognized energy problem at that time and certainly no general recognition that our budding space capability could be associated with fulfilling an energy need should one exist. Now, of course, the energy problem is well recognized, as it is also recognized that the use of electrical power is growing at a faster rate than our requirements for energy as a whole and that there is a strong indication that the electrical growth rate will be further increased as energy consumers turn to electrical power as a substitute for natural gas and oil. Unfortunately, the present methods of generating electrical power pollute the environment and consume natural resources at a prodigious rate. Under these circumstances it is only natural that we turn to the sun and investigate it as an answer to our electrical energy requirements. However, two serious problems confront us when we seek to use if for this purpose. The first problem is its diffuse nature which makes it difficult to capture in large amounts without enormously large and expensive physical structures. The second problem is its low duty cycle and only partial dependability. We can be certain of its unavailability at night, but never certain of its availability in the daytime with an intensity sufficient for electrical energy producing purposes. Out in space in geosynchronous orbit, however, the sun is available over 99% of the time and its infrequent and short term eclipses by the earth can be precisely predicted and planned for. That desirable condition would be of no practical importance if it were not possible to place large energy collecting arrays into synchronous orbit and in some manner get that energy back to earth where it is needed. Dr. Glaser 2?) was the first to point out that we could combine three technologies, all developed within the past 20 years, to accomplish this task. These three technologies are (1) the new capability to transport material into space, (2) the solar photovoltaic cell which directly converts solar flux into DC electrical power, and (3) free space power transmission by means of a microwave beam. As a result of this proposal and initial feasibility study performed by a team made up of personnel from Arthur D. Little, Inc., Raytheon Company,
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