Radar in World War II greatly influenced many important aspects of the conflict.Brown, Louis; A Radar History of World War II, Inst. of Physics Publishing, 1999 This revolutionary new technology of radio-based detection and tracking was used by both the Allies and Axis powers in World War II, which had evolved independently in a number of nations during the mid 1930s.Watson, Raymond C. Watson, Jr.; Radar Origins Worldwide: History of Its Evolution in 13 Nations through World War II, Trafford Publishing, 2009 At the outbreak of war in September 1939, both the United Kingdom and Germany had functioning radar systems. In the UK, it was called RDF, Range and Direction Finding, while in Germany the name Funkmeß (radio-measuring) was used, with apparatuses called Funkmessgerät (radio measuring device). By the time of the Battle of Britain in mid-1940, the Royal Air Force (RAF) had fully integrated RDF as part of the national air defence.

In the United States, the technology was demonstrated during December 1934.Page, Robert Moris; The Origin of Radar, Anchor Books, 1962, p. 66 However, it was only when war became likely that the U.S. recognized the potential of the new technology, and began the development of ship- and land-based systems. The U.S. Navy fielded the first of these in early 1940, and a year later by the U.S. Army. The acronym RADAR (for Radio Detection And Ranging) was coined by the U.S. Navy in 1940, and the term "radar" became widely used.

While the benefits of operating in the microwave portion of the radio spectrum were known, transmitters for generating microwave signals of sufficient power were unavailable; thus, all early radar systems operated at lower frequencies (e.g., HF or VHF). In February 1940, Great Britain developed the resonant-cavity magnetron, capable of producing microwave power in the kilowatt range, opening the path to second-generation radar systems.Megaw, Eric C. S.; "The High-Power Magnetron: A Review of Early Developments", Journal of the IEE, vol. 93, p. 928, 1946

After the Fall of France, Britain realised that the manufacturing capabilities of the United States were vital to success in the war; thus, although America was not yet a belligerent, Prime Minister Winston Churchill directed that Britain's technological secrets be shared in exchange for the needed capabilities. In the summer of 1940, the Tizard Mission visited the United States. The cavity magnetron was demonstrated to Americans at RCA, Bell Labs, etc. It was 100 times more powerful than anything they had seen. Bell Labs was able to duplicate the performance, and the Radiation Laboratory at MIT was established to develop microwave radars. The magnetron was later described by American military scientists as "the most valuable cargo ever brought to our shores".James Phinney Baxter III (Official Historian of the Office of Scientific Research and Development), Scientists Against Time (Boston: Little, Brown, and Co., 1946), page 142.Zimmerman, David; Top Secret Exchange: The Tizard Mission and the Scientific War, McGill-Queens Univ. Press, 1996

In addition to Britain, Germany, and the United States, wartime radars were also developed and used by Australia, Canada, France, Italy, Japan, New Zealand, South Africa, the Soviet Union, and Sweden.

United Kingdom Research leading to RDF technology in the United Kingdom was begun by Sir Henry Tizard's Aeronautical Research Committee in early 1935, responding to the urgent need to anticipate German bomber attacks. Robert A. Watson-Watt at the Radio Research Station, Slough, was asked to investigate a radio-based "death ray". In response, Watson-Watt and his scientific assistant, Arnold F. Wilkins, replied that it might be more practical to use radio to detect and track enemy aircraft. On 26 February 1935, a preliminary test, commonly called the Daventry Experiment, showed that radio signals reflected from an aircraft could be detected. Research funds were quickly allocated, and a development project was started in great secrecy on the Orford Ness Peninsula in Suffolk. E. G. Bowen was responsible for developing the pulsed transmitter. On 17 June 1935, the research apparatus successfully detected an aircraft at a distance of 17 miles. In August, A. P. Rowe, representing the Tizard Committee, suggested the technology be code-named RDF, meaning Range and Direction Finding.

Air Ministry

In March 1936, the RDF research and development effort was moved to the Bawdsey Research Station located at Bawdsey Manor in Suffolk.

At Bawdsey, engineers and scientists evolved the RDF technology, but Watson-Watt, the head of the team, turned from the technical side to developing a practical machine/human user interface. After watching a demonstration in which operators were attempting to locate an "attacking" bomber, he noticed that the primary problem was not technological, but information management and interpretation. Following Watson-Watt's advice, by early 1940, the RAF had built up a layered control organization that efficiently passed information along the chain of command, and was able to track large numbers of aircraft and direct interceptors to them.Watson-Watt, Sir Robert; Three Steps to Victory; Odhams Press, 1957

Immediately after the war began in September 1939, the Air Ministry RDF development at Bawdsey was temporarily relocated to University College, Dundee in Scotland. A year later, the operation moved to near Worth Matravers in Dorset on the southern coast of England, and was named the Telecommunications Research Establishment (TRE). In a final move, the TRE relocated to Malvern College in Great Malvern.

Some of the major RDF/radar equipment used by the Air Ministry is briefly described. All of the systems were given the official designation Air Ministry Experimental Station (AMES) plus a Type number; most of these are listed in this link.

Chain Home

In 1936, following the success of the prototype Chain Home (CH) station in Bawdsey, five initial stations were built in Canewdon, Dover, Dunkirk, and High Street in Suffolk. The transmitting arrays were strung between 360 foot steel towers, with platforms at 50, 200 and 350 feet. Horizontal dipole arrays with reflector elements provided a "floodlight" radiation pattern 60 degrees wide in four selected frequencies between 20 and 55 MHz. Peak power was 200 kilowatt, with a pulse duration of 5 to 25 microseconds. Four 240 foot wooden receiver towers included crossed dipole antennas at the 95 and 215 foot level for low-level targets, while antennas at the 45 and 95 foot levels were for high elevations. An operator could select four frequencies between 20 and 50 MHz. Adjacent stations provided overlapping coverage. A goniometer helped identify and position multiple targets. CD Mk II systems were then modified to detect low-flying aircraft, creating Chain Home Low (CHL). By 1939, CH stations provided radar coverage from Aberdeen to Southampton. During the Battle of Britain, CH and CHL alerted Ground-controlled interception stations.

Ground-Controlled Intercept

In 1940, systems similar to 1.5 meter CHL were adapted to produce the Ground-Controlled Intercept (GCI). In January 1941, they became operational, and used an antenna rotated mechanically over a hemisphere to track target and interceptor simultaneously. A CRT map display was added, called plan position indicators (PPI), which rotated with the antenna.

Philo Taylor Farnsworth refined a version of his picture tube (cathode ray tube, or CRT) and called it an "Iatron". It could store an image for milliseconds to minutes (even hours). One version that kept an image alive about a second before fading, proved to be a useful addition to the evolution of radar. This slow-to-fade display tube was used by air traffic controllers from the very beginning of radar.

Aircraft Intercept

In 1936, Edward George Bowen, developed an RDF system suitable for aircraft, overcoming the limitations associated with CH, especially directing interceptors at night or inclement weather. The aircraft interception radar Watson-Watt called RDF-2A, since CH was RDF-1. Initial AI sets were first made available to the RAF in 1939 and fitted to night fighter Bristol Blenheims.

Later in the war, British Mosquito night intruder aircraft were fitted with AI Mk VIII and later derivatives, which with Serrate allowed them to track down German night fighters from their Lichtenstein signal emissions, as well as a device named Perfectos that tracked German IFF. As a countermeasure, the German night fighters employed Naxos ZR radar signal detectors.

Air-to-surface-vessel radar

While testing the AI radars near Bawdsey Manor, Bowen's team noticed the radar generated strong returns from ships and docks. This was due to the vertical sides of the objects, which formed excellent partial corner reflectors, allowing detection at several miles range. The team focussed on this application for much of 1938.

In 1939, the Air-to-surface-vessel (ASV) Mark I, using electronics similar to those of the AI Mk II, was able to spot vessels in the English Channel, including submarines. In 1940, the system was improved with the ASV Mk. II radar.

Centimetric The improvements to the cavity magnetron by John Randall and Harry Boot of Birmingham University in early 1940 marked a major advance in radar capability. The resulting magnetron was a small device that generated high-power microwave frequencies and allowed the development of practical centimetric radar that operated in the SHF radio frequency band from 3 to 30 GHz (wavelengths of 10 to 1 cm). Centimetric radar enables the detection of much smaller objects and the use of much smaller antennas than the earlier, lower frequency radars. A radar with a wavelength of 2 meters (VHF band, 150 MHz) cannot detect objects that are much smaller than 2 meters and requires an antenna whose size is on the order of 2 meters (an awkward size for use on aircraft). In contrast, a radar with a 10 cm wavelength can detect objects 10 cm in size with a reasonably sized antenna.

In addition a tuneable local oscillator and a mixer for the receiver were essential. These were targeted developments, the former by R W Sutton who developed the NR89 reflex klystron, or "Sutton tube". The latter by H W B Skinner who developed the 'cat's whisker' crystal.

At the end of 1939 when the decision was made to develop 10 cm radar, there were no suitable active devices available - no high power magnetron, no reflex klystron, no proven microwave crystal mixer, and no TR cell. By mid-1941, Type 271, the first Naval S-band radar, was in operational use.Callick, E. B.;"Metres to Microwaves", 1990

The cavity magnetron was perhaps the single most important invention in the history of radar. In the Tizard Mission during September 1940, it was given free to the U.S., along with other inventions, such as jet technology, in exchange for American R&D and production facilities; the British urgently needed to produce the magnetron in large quantities. Edward George Bowen was attached to the mission as the RDF lead. This led to the creation of the Radiation Laboratory (Rad Lab) based at MIT to further develop the device and usage. Half of the radars deployed during World War II were designed at the Rad Lab, including over 100 different systems costing US$1.5 billion.Bowen, E. G.; Radar Days, Inst. of Physics Pub., 1987

When the cavity magnetron was first developed, its use in microwave RDF sets was held up because the duplexers for VHF were destroyed by the new higher-powered transmitter. This problem was solved in early 1941 by the transmit-receive (T-R) switch developed at the Clarendon Laboratory of Oxford University, allowing a pulse transmitter and receiver to share the same antenna without affecting the receiver.