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Radio Age, September, 1922, pages 9-10:
Problem of Radio Power Transmission
By CHARLES P. STEINMETZ
Chief Consulting Engineer, General Electric Company
(Address delivered at International Radio Congress, Chicago)
THE successful development of radio communication by telegraph and telephone raises the question of the possibility, or impossibility, of radio power transmission.
In some respects, radio power transmission exists today, for the message which you receive by radio has been carried by the power of the electro-magnetic wave from the sending to the receiving station. However, while the sending station sends out electro-magnetic waves of a power of several kilowatts, or even hundreds of kilowatts, this power scatters in all directions, and it may be only a fraction of a milliwatt, which we receive, that is, less than a millionth of the power sent out. But this small power is sufficient, when amplified, to give us the message.
The problem of power transmission essentially differs from that of the transmission for communication, that in power transmission most, or at least a large part of the power, sent out by the generating station, must arrive at the receiving station, to make it economical to transmit the power.
Hence, the problem of radio power transmission is that of directing the radio waves so closely that a large part of their power remains together so as to be picked up by the receiving station. Much successful work has been done in directing radio waves, and for instance our Trans-atlantic stations send out most of their power Eastwards. But still even as directed the power scatters over the coasts of Europe from Norway to Spain, so that it is impossible to pick up an appreciable part of it.
The limits of impossibility of concentrating a beam of radio waves may be illustrated by comparison with a beam of light. Light is an electro-magnetic wave, differing from the radio wave merely by having a wave length many million times shorter. While usually the light scatters in all directions, like the wireless wave, we can direct it in a concentrated beam by the searchlight. But there is inevitably a scattering of the light in the searchlight beam, and when the beam starts perhaps with a square-yard section at the searchlight mirror, at 10 miles distance it has at the very best scattered to a diameter of 2,000 feet, and at 100 miles distance the beams cover a section of 16 square miles. If it were a beam of radio power, it would thus require at 100 miles distance a receiving station covering 16 square miles--about four miles wide and, what is still more difficult, four miles high, to pick up a large part of the power.
The cause of this scattering is two-fold. First, the inevitable imperfections of any apparatus. No matter how perfect a reflector, there are slight imperfections, and at 100 miles distance, they seriously count. Furthermore, even with an absolutely perfect reflector the beam of light would stay together only if the light came from a mathematical point. As it must, however, come from a small area, this causes an inevitable scattering, which at best gives an angle of scattering of about two degrees. This is about 100 times as much as would be permissible to economically transmit power a hundred miles by a direct radio beam.
Thus the probability of power transmission by directed radio is very small, except perhaps in very special cases, where the distances are moderate and the efficiency of transmission of secondary importance.
The second possibility of radio power transmission--at least theoretically--is by resonant vibrations or standing waves. Suppose we had a very large sending station sending out electro-magnetic waves not of hundreds, but of hundred thousands or millions of kilowatts, and suppose we could find a wave length, where the absorption in the passage of the wave through space is sufficiently small so as to be negligible compared with the amount of power.
Assuming first there were no receiving stations. Then the waves issuing from the sending station would circle the globe and return to the sending station, and if the wave length is adjusted so that the return wave coincides with the out-going wave, it would return its power, and little power would be required from the sending station to maintain such a system of high power standing waves--only enough to supply the losses--just as little power is required in an electric wire transmission system, to maintain the voltage wave, as long as no current is taken off.
Suppose now we erect a second station, tuned for the same wave length as the sending station. It would resonate with the standing electro-magnetic wave issuing from the sending station, thereby stop its passage by absorbing its energy. It would, as we may say, punch a hole in the standing wave sheet coming from the sending station. Power would then flow into this hole; the sending station would begin to send out additional power to maintain the wave sheet, and this power would be received by the receiving station. This would give a real radio power transmission.
Any receiving station of suitable design would then be able to pick up power from the universal power supply carried by the standing wave sheet covering the earth. Also, several sending stations may send out power. These may either have different wave lengths, then would not interfere, and the receiving station could be tuned to receive power from any of the generating stations. Or--what would be preferable--all the generating stations would be tuned to the same wave length, that is, the same frequency. Then they would have to be synchronized and operate in synchronism, just as different electric generating stations on the same transmission line are operated in synchronism.
Theoretically, this is an interesting speculation, but whether it could ever become a possibility, would depend on the question, whether a radio wave of such length could be found, as to make the losses of power by absorption, etc., economically permissible, and whether stations for such wave length and power would be economically feasible. Furthermore, it would have to be an international development. Therefore, even if such radio transmission by a stationary electro-magnetic wave sheet were possible, its realization at best is rather distant, so that the present outlook for radio power transmission is very remote. I thought it of interest, however, to bring this before you as an interesting speculation of future possibilities.