Transmission of Power by Radio Waves Would Make Space Platform Possible

By WILLIAM L. LAURENCE

The transmission of substantial quantities of electric power through the atmosphere without the use of wires has been the dream of scientists for more than half a century. One of the early pioneers in this field was Nikola Tesla, the electrical genius who in 1888 invented the alternating current induction motor, which ushered in the modern age of electricity. In the early Nineteen Hundreds he built on Long Island a gigantic tower, topped with a mushroom-shaped radiator, for the development of wireless power transmission, but he was unable to arrive at a workable system with the relatively long wavelengths then available.

Transmission of electric power by radio waves longer than microwaves, in the wavelength range of three to ten centimeters, is impracticable. The longer the wavelength, the larger the antenna required to concentrate the radiation in a narrow beam. If this radiation is allowed to go out in all directions, its intensity decreases as the square of the distance, as does the intensity of light from, say, a candle flame. For example, candlelight is only one-ninth as bright at three feet as it is at one foot, and only one-sixteenth as bright at four feet.

Conventional Lengths

Conventional wavelengths are, of course, suitable for many communications purposes, such as broadcasting to listeners located in a circle or a wide arc, since the weak signals can be picked up by sensitive radio receivers and amplified thousands of times to audible levels with local power. But significant amounts of power cannot be picked out of the air in this way. Wireless transmission of power has had to wait for the development of microwave technology.

Last week came the dramatic announcement by the Raytheon Company, Waltham, Mass., that it has developed a revolutionary high-frequency, high-power microwave tube, named the Amplitron, that makes possible at last the transmission of large quantities of electrical power without wires. According to William C. Brown, Assistant Vice President and Manager of Raytheon's Advanced Development Laboratory, the Amplitron "converts nearly all of the raw input energy as supplied by a public utilities system directly to microwave energy, at efficiencies in the order of 80 percent, leaving little to be dissipated in the tube in the form of heat."

"An additional breakthrough in the past year," Mr. Brown added, "is the discovery that it is possible to design cooling systems for the Amplitron so efficient that ten times as much heat can be removed from the tube as was previously possible." A combination of the higher efficiency of the Amplitron and the cooling breakthrough, he said, "makes it possible to build tubes with power levels twenty to 100 times greater than is now available."

Power for Space

Raytheon also announced plans to use the new tube as a source of power for space-platforms that would hover at high altitudes at the end of an electromagnetic "extension cord." The super-power tube, it was stated, makes feasible stratospheric sky-stations to be used for such military and commercial purposes as missile detection, long-range communication, navigational and meteorological aids, surveillance, and global television transmission.

The unmanned helicopter-like vehicle, according to Raytheon engineers, would stay aloft for weeks through the novel ground-to-air power hook-up. It would tune in on high-power radio waves beamed from the ground and convert them into heat to drive its rotor. Informed sources said that a study contract from the Air Force "seemed assured."

The "weightless fuel" that will supply the power to keep the sky-stations aloft will be provided by high-frequency radio waves in the microwave region. The microwave power will be generated in a ground station by a battery of Amplitrons which will convert alternating current into microwave energy. It will then be beamed to the sky station by antennae grouped in a 400-foot-square array. Like a battery of searchlights aimed at a single aircraft, these beams will converge in the sky, coming to a focus in a circular spot less than 100 feet in diameter at the sky platform.

Picking It Up

At the sky-station, a circular antenna array of nearly the same diameter as the focused beam will pick up the microwave energy and conduct it through waveguides to elements which will convert the energy to heat in much the same way that a stove element converts electricity to heat. Compressed air heated by those elements will drive a heat engine. Heat turbines will operate the rotor blades to keep the platform aloft and supply other power requirements. According to the Raytheon engineers, more than 50 percent of the microwave power generated on the ground can be focused on the airborne antennae, and about 35 percent of this ground power can actually be delivered to the heat exchangers.

The platform is designed to propel itself to its sky position and to descend again to earth with energy supplied by a lightweight supplementary chemical power plant. This plant could also provide emergency power if the microwave supply were to be temporarily interrupted.

The sky-station will operate at altitudes so high that the curvature of the earth will no longer interfere with communications to distant points. This means, it was pointed out, that an efficient system of carrying volume traffic can be built between, say, the United States and Europe, using hops between Labrador, Greenland, Iceland and Scotland. To the average man this would mean that he could watch events in Paris or New Delhi on his home television screen as they happen. It would also mean that intercontinental telephone calls and messages now sent by cable could be provided at a fraction their present rates. A single microwave skypath link, it was stated, "could handle well over 100,000 telegraph messages" simultaneously.

Detection Devices

Sensitive detection devices mounted on sky platforms, it was pointed out, could remain aloft regardless of weather. They could warn of attempted enemy attack or reconnaissance and provide a constant track of the intruder.

In addition, the Raytheon engineers envisage the use of such stations as lighthouses in the sky. Large glass bulbs filled with various gases could receive some of the power that ordinarily misses the platform. Microwave radiation falling on these bulbs would be converted instantaneously to light which would be radiated in all directions. Different gases could be used to provide distinctive colors. For example, a bulb filled with neon over Chicago could give off a red-orange glow which would indicate to high-flying jet pilots as much as 150 miles away that they were approaching Chicago. A xenon bulb over New York City would shine with a blue light.

Other possible uses of the sky-station are as aerial traffic controllers, for earlier and more accurate weather detection and forecasting; and as a base from which to probe the atmosphere for its secrets. Constant sea watches to track and catalog all shipping would also become feasible.