Radar, Origins to 1939

Reflection was an important part of Heinrich Hertz’s 1887 demonstration of the existence of electromagnetic waves, and the idea of using that property to ‘‘see’’ in darkness or fog was developed shortly afterwards.

Christian HUlsmeyer constructed a device in 1902 that he hoped might prevent collisions at sea. It used two cylindrical paraboloid reflectors to transmit and receive waves of decimeter length generated from spark oscillations with the reflected signals detected by coherer.

The equipment was demonstrated successfully at a conference of ship owners in Rotterdam, but it was only capable of showing the direction of an object and not its range, as this required accurate timing of signals at the microsecond level, a technique that lay years in the future. Thus a liner eight kilometers distant was indistinguishable from a tug at 500 meters, and the remarkable device aroused no enthusiasm among seamen.

Hulsmeyer’s set was soon forgotten, although it was adequately patented and demonstrated to numerous witnesses. The idea recurred to no effect in World War I but seems to have been discussed informally among engineers. Guglielmo Marconi proposed using reflected radio waves for the location of objects in a paper delivered at a meeting of the Institute of Radio Engineers in New York in 1922.

By 1920 the vacuum triode had revolutionized the generation and detection of radio waves, and broadcast radio was transforming many aspects of everyday life. However the circuit elements needed to measure the time between the emission and reception of a pulsed train of waves—the key to radar ranging—were still missing.

Triodes were able to work at high frequencies, and in the early 1930s various experimenters built equipment capable of measuring the speed of an automobile, if not its range. If the target were moving, the reflected wave would have its frequency shifted by the Doppler effect and through interference with the transmitted signal within the receiver would produce an easily recognizable signal, the beat frequency of which was proportional to the speed.

Police radar was therefore possible before air-warning radar. A revival of the Hulsmeyer idea returned in 1935 in equipment designed by Camille Gutton and mounted in the new transatlantic liner Normandie. The equipment did not do well at sea, and the watch officers were not impressed because there was still no range information.

Triodes were unable to follow the rapid changes in signal amplitude that characterize a short wave train. Furthermore, operators required a device that presented the time elapsed between emission and reception. It was recognized that a cathode-ray oscillograph would perform this function, but those available before 1930 were inadequate for a variety of reasons.

Both of these functions were also vital for television of sufficiently high definition to rival the cinema, and both were the subject of research in the electronic industry, which demonstrated all-electronic television in 1930. The necessary elements were multigrid amplifier valves and high-vacuum, low-voltage cathode-ray tubes. When these two circuit elements became available for the video amplifier and for the picture tube, respectively, serious radar work could begin.

The radar sets first envisioned were to use wavelengths of a few centimeters, which allowed the beam to be shaped into a form of ‘‘radio searchlight’’ with a reflecting dish of practical size. This approach faced a serious obstacle: the absence of any generator working at these short wavelengths with sufficient power or frequency stability.

There were, however, many observations (through the Doppler effect) of aircraft and ships seen at wavelengths of a few meters, an effect that was particularly pronounced in experiments studying the propagation of waves intended for the transmission of television. At these frequencies, antennas of manageable size could be fashioned from arrays of dipole radiators so that the individual radiations would constructively interfere to form the desired radio searchlight.

By the early 1930s, serious efforts were underway in the U.S., Germany, and Britain to construct radio-location devices using relatively long wavelengths. (Russian efforts were ahead in the early 1930s, but they yielded little as a result of serious organizational problems and purges that sent key engineers to the gulag.)

The German company GEMA built the first device that can be called a functioning radar set in 1935 with Britain and America following only months behind. Two groups in the U.S.—the Signal Corps and the Naval Research Laboratories—proceeded independently but on lines very similar to those of the Germans in using dipole arrays. They had airwarning and searchlight-pointing prototype sets ready for production in 1939.

The British physicists Robert Watson Watt and Arnold Wilkins proceeded along a different line using wavelengths of tens of meters with broadcast rather than ‘‘searchlight’’ transmission. This equipment, although inferior to that working on shorter wavelengths, was seen by Air Vice-Marshal Hugh Dowding as the key to the air defense of Britain from expected German attack.

As commander of the newly created Fighter Command, he created a system of radar stations and ground observers linked by secure telephone lines to the fighter units. He drilled Fighter Command to use the new technique, and when the Luftwaffe came in the summer of 1940, the attacking squadrons were ambushed by defending fighters positioned by radar.

Triodes capable of generating significant amounts of power at decimeter wavelengths had been developed by the late 1930s, and by 1940,the Bell Laboratories in the U.S., the Royal Navy in the U.K., and the Telefunken Company in Germany had sets that worked at 50 centimeters.

All these designs went into production with the onset of World War II and furnished, with various modifications, the radar used by the combatants until 1943, when centimetric-wave equipment was developed, which used electronics of a completely different nature.