Electronics: Flight by Microwave

The 9-ft. bowl-shaped antenna pointed straight up. Above it floated an object that looked for all the world like a small, square bedspring with a tiny helicopter attached. The rotor blades whirled with a thin whine, and the helicopter strained at the guy wires that kept it from climbing more than 50 ft. There it hovered, its blades spinning sturdily, drawing their power out of invisible microwaves shooting up from below. This was Raytheon Co.’s first public demonstration of an aircraft powered solely by radio energy.

Dream of Power. The experimental flight at Burlington, Mass., last week was an impressive achievement. Transmitting power by radio waves has long been a gleam in the eyes of electrical engineers, but the process is normally too inefficient for anything except radio and TV communication. Raytheon started years ago to work out an improvement (TIME, May 25, 1959).

The big break came recently with the discovery that small silicon diodes, working much like the cat’s-whisker crystals of early radio sets, can pick microwave energy out of the air and turn it into direct current with reasonable efficiency. Thousands of diodes, strung like glass beads on a network of wires, are needed to intercept Raytheon’s beam. In the model helicopter demonstrated last week, they feed direct current at about 100 volts to a small motor taken from an electric drill. The beam of 2,450-megacycle microwaves starts out with three kilowatts of power; the diode antenna turns it into electricity with an efficiency of about 10% .

50,000-Ft. Station. Raytheon’s microwave power transmission was developed under a contract with the Air Force, which confidently expects that later helicopters will be built to operate without guide wires. There should be no trouble making them hover at 50,000 ft. carrying a payload equal to their own weight. At this height they will be able to send line-of-sight communication beams to similar vehicles 600 miles away. As TV stations they will be capable of serving an area 50 times larger than can be reached by a 1,000-ft. tower. As radar stations, they could each guard a large part of the U.S.