Scanning the Transactions - The Transmission of Power Without Wires

The transmission of power without wires has been an unfulfilled dream of scientists and engineers for more than a half century. Interest in wireless power transmission dates back to 1899, when Nikola Tesla, with the aid of a grant from John Jacob Astor, constructed a large experimental station at Colorado Springs where he attempted to transmit power over long distances at a frequency of a few hundred kilocycles. His attempts failed. No provision had been made for directing the radiated energy. Indeed, at this low frequency a directional array would have been impossibly large. Today, we are readily able to direct electromagnetic energy into narrow beams by utilizing the microwave region of the spectrum, but the generation of large amounts of power at these high frequencies poses a formidable problem. An important step forward was made in the late 1950's with the discovery that large amounts of power can be obtained from relatively small sized microwave tube structures through the use of high-efficiency designs and new cooling systems, as exemplified by the Amplitron. Average power outputs of more than 10 kilowatts can now be attained at 10 centimeter wavelengths, and 200 kilowatt outputs are now confidently predicted in the foreseeable future. As a result, the feasibility of radiowave transmission of power is once again under serious study. Among a number of potential applications being investigated is one in which a helicopter device is sustained for an indefinitely long period of time at a point above the earth's surface by energy furnished to it by a microwave beam. A typical system envisioned for this application would involve a 350-foot transmitting antenna on the ground operating at 6 kMc and a 50-foot receiving antenna on a helicopter device hovering 65,000 feet above. Recent studies have indicated that such a vehicle could hover with the desired efficiency at an altitude as high as 102,000 feet. A flying platform 10 or more miles high might provide a number of technical and economic advantages for airborne early warning systems, air route surveillance radars, educational television, etc. In any event, it appears that a 60-year dream is about to come true. (W. C. Brown “Power without wires,” IRE STUDENT QUARTERLY, September, 1962; and H. Letaw, Jr., et al., “Some comments on the transmission of power by the use of microwave beams,” IRE TRANS. ON ANTENNAS AND PROPAGATION, November, 1961.)