British physicist James Clerk Maxwell developed equations governing the behaviour of electromagnetic waves in 1864. Inherent in Maxwell’s equations are the laws of radio-wave reflection, and these principles were first demonstrated in 1886 in experiments by the German physicist Heinrich Hertz. Some years later a German engineer Christian Huelsmeyer proposed the use of radio echoes in a detecting device designed to avoid collisions in marine navigation. The first successful radio range-finding experiment occurred in 1924, when the British physicist Sir Edward Victor Appleton used radio echoes to determine the height of the ionosphere, an ionized layer of the upper atmosphere that reflects longer radio waves.

The first practical radar system was produced in 1935 by the British physicist Sir Robert Watson-Watt, and by 1939 England had established a chain of radar stations along its south and east coasts to detect aggressors in the air or on the sea.

In 1935 Watson-Watt wrote a paper entitled “The Detection of Aircraft by Radio Methods”. This was presented to Henry Tizard, the chairman of the Committee for the Scientific Survey of Air Defence. Tizard was impressed with the idea and on 26th February 1935, Watson-Watt demonstrated his ideas at Daventry. His idea was based on the bouncing of a radio wave against an object and measuring its travel to provide targeting information. It was called radar (radio detection and ranging). As a result he was appointed head of the Bawdsey Research Station in Felixstowe.

By the outbreak of the Second World War in September 1939, Watson-Watt had designed and installed a chain of radar stations along the East and South coast of England. In the same year two British scientists were responsible for the most important advance made in the technology of radar during World War II. The physicist Henry Boot and biophysicist John T. Randall invented an electron tube called the resonant-cavity magnetron. This type of tube is capable of generating high-frequency radio pulses with large amounts of power, thus permitting the development of microwave radar, which operates in the very short wavelength band of less than 1cm, using lasers. Microwave radar, also called LIDAR (light detection and ranging) is used in the present day for communications and to measure atmospheric pollution. During the Battle of Britain these stations were able to detect enemy aircraft at any time of day and in any weather conditions. Radar was also used by ships and aircraft during the war. Germany was using radar by 1940 but Japan never used it effectively. The United States had a good radar system and it was able to predict the attack on Pearl Harbor an hour before it happened.

Britain tended to have the best radar system during the early stages of the war and in 1940 the invention of the Magnetron cavity resonator enabled more centimetric waves to be transmitted. It also enabled more compact high-frequency sets to be used by aircraft in the Royal Air Force.