In the prelude to the war, several countries did some researches to investigate the possibility of detecting distant objects using radio waves.
In the period of 1934-1939, eight nations developed, independently and in great secrecy, such systems : United States, Great Britain, Germany, USSR, Japan, Netherlands, France and Italy. Great Britain additionally shared knowledge of their research with other Commonwealth nations: Australia, Canada, New Zealand and South Africa, and these countries have also developed their own models of radar. During the war, Hungary was added to that list.
According to pre -war theories, it would be possible to locate objects with the production of rapid discharges of radio waves. These waves colliding with the solid objects would be reflected and received by a receiver. The size of the object would be determined by the intensity of the reflected signal and the distance by the elapsed time between emission and reception.
In the late 19th century, the German physicist Heinrich Hertz, had already conducted studies demonstrating that solid objects could reflect radio waves.
In the early '30s, the Germans were developing a radio apparatus for detection called Funkmessgerät für Untersuchung (Measuring System for Recognition Radio, or Wireless Device for Research) .
In 1934, German engineers had designed a device capable of detecting a ship or a plane approximately 12 kilometers away.
From this model, two machines quite advanced for its time were produced: the Seetakt for the Kriegsmarine and Freya for the Luftwaffe (German Air Force). However, the German Navy was not interested by the apparatus, although determining the installation of radio rangefinders in all its large surface ships.
But Luftwaffe, in contrast, showed a strong interest in the technology being in a short time able to develop other very effective equipment comparing with Freya: the Wüzburg.
The year de1939: German and English vied for the lead in the race for the quiet development of radar.
The Freya, large fixed antennas, with a range of 120 kilometers, worked together with Wüzbug whose mobile antenna, dish-shaped was able to detect aircraft flying at high speed and reached 40 kilometers.
The Radio Rangefinders installed on ships of the Kriegsmarine proved themselves very accurate at the beginning of the war, being of great value to control the fire of large caliber cannons, but with the course of the conflict, the Allies developed the most advanced naval radar.
The Germans were also responsible for the first system to incorporate a display Plan Position Indicator (in English acronym, PPI). In Germany, this screen was called Panorama. The Panorama worked with the Early Warning Radar Jagdschloss.
On the British side, the engineer Robert Watson-Watt, made the first demonstrations with a radar system in 1935.
In September of that year, there were already operating equipment with a range of up to 80 kilometers, a fact that led the British authorities to consider seriously the possibility of installing a Network of Coastal Radar Stations.
In 1938 the network began to be built, forming a chain, called the Chain Home. Initially 20 stations were installed, being this number expanded shortly thereafter.
In the same year, tests of a new radar system designed to detect ships at sea were initiated, and in 1939, were made demonstrations of how to detect aircraft in flight.
The British also developed mobile stations called Chain Home Low (CHL), capable of detecting aircraft at low altitudes.
Moreover, it also put into operation a system for the identification friend-enemy (Identification Friend or Foe), which showed up on the radar screen planes or ships were friend or foe.
Earlier, radars, for the Allies were named with the acronym RDF (Radio Detection Finder) and the course of the war the name was changed to RADAR (Radio Detection And Ranging).
The Radar has proved able to identify data related to the four dimensions of a target: distance (the time elapsed between the emission and reception of the echo), size (according to the shape and behavior of the visual signal), position (with a directional antenna called goniometer), altitude (achieved with the connection of antennas, which is the data more difficult to obtain).
For the system to work at maximum efficiency, skilled operators were needed in order to quickly handle the equipment and perform the necessary calculations to obtain the coordinates of the target. Difficulty to determine the altitude of the aircraft would persist throughout the war, for both sides.
The Radar, besides the fact that played a vital role during the Battle of Britain, was also of great value in the Battle of the Atlantic, especially after the introduction of a device, the Magnetron, which operated a system of 10 cm wavelength, rather than previous 100cm. With him it was possible to detect even the periscope of a submarine submerged several miles away.