With the advantage of historical perspective, we realize that these experiments were decades ahead of the unfolding of a technology that could accomplish his objective. This new technology was based upon the early microwave experiments of Hertz (4), but had to await the development of efficient generators of microwave power. This capability began to emerge with the microwave generators developed for the radar of World War II and later for microwave communications. The event which directly precipitated interest in the use of microwaves for power transmission was the support of the development of super-power microwave tubes by the Department of Defense in the early I960's, This program resulted in high-efficiency tubes with such high power handling capability (several hundreds of kilowatts) that the use of microwaves for the efficient transfer of large amounts of power became a distinct feasibility. The first demonstration of the efficient transmission of meaningful amounts of power by microwaves took place at the Spencer Laboratory of Raytheon Co. in May 1963 In this first demonstration, shown in Figure 1-1, the means used for collecting the power at the receiving end of the system utilized conventional antenna technology in the form of a pyramidal horn. The means used for rectifying the microwave power to DC power was a close-spaced thermionic diode. Neither of these technologies was completely satisfactory. The receiving horn was highly directive and because of the difficulty of matching its antenna pattern to that of the incoming beam its collecting efficiency was only 87%. The rectifier efficiency was only 50%. Nevertheless, as a result of this demonstration the Rome Air Development Center of the Air Force became interested in the concept of a microwave powered platform for communication purposes. The Raytheon proposal of a microwave powered helicopter to accomplish this objective and the resulting contract became crucial in determining the evolutionary path of the collection and rectification of microwave power from a free-space microwave beamj'Tt was recognized that the pyramidal horn would not be satisfactory because of a combination of its high directivity with the natural roll and pitch of the vehicle. It was also recognized that the limitations of the close-spaced thermionic rectifier would place severe limitations on the practicality of the platform. The "rectenna" device was proposed to the Air Force as a solution to this problem. The rectenna device made it possible to simultaneously solve the directivity and antenna pattern matching problems of microwave power collection and at the same time make practical use of the semiconductor device whose power handling capability had prevented it from being seriously considered for a system in which significant amounts of power were being handled. The Raytheon Company actually demonstrated a microwave powered helicopter using a rectenna prior to active work on the Air Force contract. But the Air Force contract was the basis for an extension of the effort and several notable demonstrations, including a ten hour continuous flight of the vehicle' Figure 1-2 shows the helicopter in flight. It was necessary, of course, to use laterally constraining tethers to keep the helicopter on the microwave beam but this limitation was later removed by a study and experimental confirmation
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