The basic idea
Radar, short for radio detection and ranging, works by sending out a radio pulse and listening for the echo from an object such as an aircraft. The delay between pulse and echo gives distance, since radio waves travel at the speed of light, and the direction the antenna is pointing gives bearing. The principle is simple, but making it work in practice required strong transmitters, sensitive receivers and ways to display the result. These developed during the 1930s in several countries at once, often in secret, which is why no single inventor can be named.
Earlier steps
Heinrich Hertz showed in the 1880s that radio waves reflect off metal objects. In 1904 the German engineer Christian Hülsmeyer patented the “telemobiloscope,” a device to detect ships by radio echoes to prevent collisions in fog, but it attracted little interest. After the growth of wireless telegraphy and radio broadcasting, observers noticed that aircraft and ships passing between a transmitter and receiver disturbed the signal. In the United States, Albert Hoyt Taylor and Leo Young at the Naval Research Laboratory noted such effects in the early 1920s. In 1925, Gregory Breit and Merle Tuve used pulses of radio waves to measure the height of the ionosphere, which showed the pulse-and-echo method in a scientific setting.
Britain’s air defense system
In Britain, concern about bombers led the Air Ministry to ask scientists whether a “death ray” could be built. Robert Watson-Watt, of the Radio Research Station, and his colleague Arnold Wilkins calculated that such a weapon was unrealistic but that detecting aircraft by radio reflection should be possible. In February 1935, a test near Daventry used a BBC shortwave transmitter, and a receiver picked up the echo from a bomber flying through the beam. The government rapidly funded research.
The result was Chain Home, a network of tall radio masts along the coast that began operating in the late 1930s. It could detect aircraft at distances of many miles, and its information was passed to a control system that directed fighters. Operators and plotters turned raw echoes into usable tracks, so the system was as much about organization as about equipment. Chain Home is widely regarded as having helped the Royal Air Force use its limited fighters effectively during the Battle of Britain in 1940, though it was one part of a larger defense system that also included observers and communications.
Parallel work elsewhere
Radar development was international. In Germany, companies such as Telefunken and GEMA developed systems including the Freya early-warning radar and the Würzburg gun-laying radar, used for air defense and fire control. German radar worked well, though its use was less integrated with fighter control early in the war. In the United States, the Naval Research Laboratory and the Army Signal Corps built working sets, and a radar at Pearl Harbor detected the approaching Japanese aircraft on December 7, 1941, though the warning was not acted on effectively. France, the Soviet Union, Italy, the Netherlands, Japan and others had experimental radars too. Japanese researchers, including Hidetsugu Yagi, whose antenna design became widely used, contributed to radio technology, although radar received less priority in Japan’s forces early in the war.
The cavity magnetron and microwaves
Early radars worked at long wavelengths, requiring large antennas and giving limited detail. A key advance came in 1940 at the University of Birmingham, where John Randall and Harry Boot built a cavity magnetron, a vacuum tube that produced powerful microwave pulses. The radar magnetron allowed smaller antennas and sharper images. In late 1940, Britain shared this and other technology with the United States through the Tizard Mission, and American laboratories, notably the MIT Radiation Laboratory, developed it into production sets.
Microwave radar could fit into aircraft and ships. Airborne radar helped hunt submarines in the Atlantic and let night fighters find bombers, while shipboard radar improved navigation and gunnery. The name “radar” came into use in the US Navy in 1940. Wartime displays relied on cathode ray tubes, which turned echoes into visible blips on a screen.
Countermeasures and limits
Radar was not perfect. Both sides developed jamming, deception such as dropping metallic strips to create false echoes, and techniques for flying low to evade detection. Early sets suffered from clutter, false echoes and limited reliability. Interpreting the displays required trained operators, and false alarms were common.
After the war
By the end of the Second World War, radar was used for air defense, naval warfare, bombing navigation and aircraft interception. Its success encouraged peacetime uses, notably in civilian radar for aviation, shipping and weather. Because so much development was secret, the credit for radar remains shared among many scientists, engineers and military officers, and national accounts have sometimes emphasized their own contributions. A balanced reading is that radar was a collective achievement of the 1930s and 1940s, with Britain’s integrated system and the magnetron among its most influential steps.