History of radar
The history of radar started with experiments by Heinrich Hertz in the late 19th century that showed that radio waves were reflected by metallic objects. This possibility was suggested in James Clerk Maxwell's seminal work on electromagnetism. However, it was not until the early 20th century that systems able to use these principles were becoming widely available, and it was German inventor Christian Hülsmeyer who first used them to build a simple ship detection device intended to help avoid collisions in fog. True radar which provided directional and ranging information, such as the British Chain Home early warning system, was developed over the next two decades.
The development of systems able to produce short pulses of radio energy was the key advance that allowed modern radar systems to come into existence. By timing the pulses on an oscilloscope, the range could be determined and the direction of the antenna revealed the angular location of the targets. The two, combined, produced a "fix", locating the target relative to the antenna. In the 1934–1939 period, eight nations developed independently, and in great secrecy, systems of this type: the United Kingdom, Germany, the United States, the USSR, Japan, the Netherlands, France, and Italy. In addition, Britain shared their information with the United States and four Commonwealth countries: Australia, Canada, New Zealand, and South Africa, and these countries also developed their own radar systems. During the war, Hungary was added to this list. The term RADAR was coined in 1939 by the United States Signal Corps as it worked on these systems for the Navy.
Progress during the war was rapid and of great importance, probably one of the decisive factors for the victory of the Allies. A key development was the magnetron in the UK, which allowed the creation of relatively small systems with sub-meter resolution. By the end of hostilities, Britain, Germany, the United States, the USSR, and Japan had a wide variety of land- and sea-based radars as well as small airborne systems. After the war, radar use was widened to numerous fields, including civil aviation, marine navigation, radar guns for police, meteorology, and medicine. Key developments in the post-war period include the travelling wave tube as a way to produce large quantities of coherent microwaves, the development of signal delay systems that led to phased array radars, and ever-increasing frequencies that allow higher resolutions. Increases in signal processing capability due to the introduction of solid-state computers has also had a large impact on radar use.
Significance
The place of radar in the larger story of science and technology is argued differently by different authors. On one hand, radar contributed very little to theory, which was largely known since the days of Maxwell and Hertz. Therefore, radar did not advance science, but was instead a matter of technology and engineering. Maurice Ponte, one of the developers of radar in France, states:But others point out the immense practical consequences of the development of radar. Far more than the atomic bomb, radar contributed to the Allied victory in World War II. Robert Buderi states that it was also the precursor of much modern technology. From a review of his book:
In later years radar was used in scientific instruments, such as weather radar and radar astronomy.
Early contributors
Heinrich Hertz
In 1886–1888 the German physicist Heinrich Hertz conducted his series of experiments that proved the existence of electromagnetic waves, predicted in equations developed in 1862–4 by the Scottish physicist James Clerk Maxwell. In Hertz's 1887 experiment he found that these waves would transmit through different types of materials and also would reflect off metal surfaces in his lab as well as conductors and dielectrics. The nature of these waves being similar to visible light in their ability to be reflected, refracted, and polarized would be shown by Hertz and subsequent experiments by other physicists.Guglielmo Marconi
Radio pioneer Guglielmo Marconi noticed radio waves were being reflected back to the transmitter by objects in radio beacon experiments he conducted on March 3, 1899, on Salisbury Plain. In 1916 he and British engineer Charles Samuel Franklin used short-waves in their experiments, critical to the practical development of radar. He would relate his findings 6 years later in a 1922 paper delivered before the Institution of Electrical Engineers in London:Christian Hülsmeyer
In 1904, Christian Hülsmeyer gave public demonstrations in Germany and the Netherlands of the use of radio echoes to detect ships so that collisions could be avoided. His device consisted of a simple spark gap used to generate a signal that was aimed using a dipole antenna with a cylindrical parabolic reflector. When a signal reflected from a ship was picked up by a similar antenna attached to the separate coherer receiver, a bell sounded. During bad weather or fog, the device would be periodically spun to check for nearby ships. The apparatus detected the presence of ships up to, and Hülsmeyer planned to extend its capability to. It did not provide range information, only warning of a nearby object. He patented the device, called the telemobiloscope, but due to lack of interest by the naval authorities the invention was not put into production.Hülsmeyer also received a patent amendment for estimating the range to the ship. Using a vertical scan of the horizon with the telemobiloscope mounted on a tower, the operator would find the angle at which the return was the most intense and deduce, by simple triangulation, the approximate distance. This is in contrast to the later development of pulsed radar, which determines distance via two-way transit time of the pulse.
Germany
A radio-based device for remotely indicating the presence of ships was built in Germany by Christian Hülsmeyer in 1904. This has been recognized by the Institute of Electrical and Electronics Engineers as the invention of the first working radar system by inauguration of an IEEE Historic Milestone in October 2019.Over the following three decades in Germany, a number of radio-based detection systems were developed but none were pulsed radars. This situation changed before World War II. Developments in three leading industries are described.
GEMA
In the early 1930s, physicist Rudolf Kühnhold, Scientific Director at the Kriegsmarine ''Nachrichtenmittel-Versuchsanstalt in Kiel, was attempting to improve the acoustical methods of underwater detection of ships. He concluded that the desired accuracy in measuring distance to targets could be attained only by using pulsed electromagnetic waves.During 1933, Kühnhold first attempted to test this concept with a transmitting and receiving set that operated in the microwave region at 13.5 cm. The transmitter used a Barkhausen–Kurz tube that produced only 0.1 watt. Unsuccessful with this, he asked for assistance from Paul-Günther Erbslöh and Hans-Karl Freiherr von Willisen, amateur radio operators who were developing a VHF system for communications. They enthusiastically agreed, and in January 1934, formed a company, Gesellschaft für Elektroakustische und Mechanische Apparate, for the effort. From the start, the firm was always called simply GEMA.
Work began in earnest at GEMA. Hans Hollmann and Theodor Schultes, both affiliated with the prestigious Heinrich Hertz Institute in Berlin, were added as consultants. The first apparatus used a split-anode magnetron purchased from Philips in the Netherlands. This provided about 70 W at 50 cm, but suffered from frequency instability. Hollmann built a Barkhausen-Kurz tube regenerative receiver connected to Schultes dipole antenna array, while von Willisen used a 48 cm transmitter, first connected to a Yagi antenna and then a parabolic antenna at 13.5 cm and 48 cm. In June 1934, a large steamer was detected by Doppler-beat interference at a distance of about with the 48 cm equipment, and 4 km with the 13.5 cm equipment. In October, strong reflections were observed from an aircraft that happened to fly through the continuous-wave beam. Yet, keeping the transmitted signal out of the receiver continued to be a problem.
Kühnhold then shifted the GEMA work to a pulse-modulated system. A new 52 cm Philips magnetron with better frequency stability was used. It was modulated with 2- μs pulses at 2000 per second. The transmitting antenna was an array of 10 pairs of dipoles with a reflecting mesh. The broad band heterodyne receiver used Acorn tubes from RCA, and the receiving antenna had three pairs of dipoles and incorporated lobe switching. A Braun tube was used for displaying the range.
The equipment was first tested at a NVA site at the Lübecker Bay near Pelzerhaken. During May 1935, it detected returns from woods across the bay at a range of. It had limited success, however, in detecting a research ship, Welle, only a short distance away. The receiver was then rebuilt, becoming a super-regenerative set with two intermediate-frequency stages. With this improved receiver, the system readily tracked vessels at up to range.
In September 1935, a demonstration was given to the Commander-in-Chief of the Kriegsmarine. The system performance was excellent; the range was read off the Braun tube with a tolerance of 50 meters, and the lobe switching allowed a directional accuracy of 0.1 degree. Although this apparatus was not put into production, GEMA was funded to develop similar systems operating around 50 cm and 2.4 m. These became the Seetakt for the Kriegsmarine and the Freya for the Luftwaffe respectively.The navy needed the shorter wavelengths for surface targets, while the air force needed the extended range with the longer wavelengths. The 50 cm magnetron was replaced with the GEMA TS1 triode, while the magnetron replaced the Barkhausen tube.
Kühnhold remained with the NVA, but also consulted with GEMA. He is considered by many in Germany as the Father of Radar. During 1933–6, Hollmann wrote the first comprehensive treatise on microwaves, Physik und Technik der ultrakurzen Wellen'', Springer 1938.