RDF and RADAR

The modern application of radio signal detection for identifying and determining the presence, trajectory and ballistics of a radio reflective object, emanates from many investigations that took place in the early and middle twentieth century, and although it cannot be credited to the work of any one individual there were certainly many who were prominent.

Radio Acquired Detection and Ranging (RADAR) was preceded by Radio Direction Finding (RDF), the latter being the first proven and effective method of detecting and determining the height of incoming enemy aircraft and was employed first by the United Kingdom in 1940 as part of an integrated air defence system during the second world war.

Radiolocation by RADAR or RDF usually employs short wave radio waves transmitted in pulses. The radio reflectivity of the object determines the magnitude of the signal reflected back, and this signal is detected by a radio receiver. Depending on the transmitted/reflected beam angle and signal strength, the position and distance of the object can be determined.

The whole history of radiolocation is one of evolutionary development. In 1887 Heinrich Hertz showed that electromagnetic waves reflect much like light waves and in 1904 the German engineer Hulsmeyer patented a radio-echo device as an anti-collision method. In 1922, Marconi reported radio-echo effects and suggested it be used to ensure ships could avoid collisions

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Barrie Blake-Coleman

in bad weather and fog. In the same year two American scientists, A.H Taylor and L. Young, working for the US government service, noted that ships passing a transmitter and receiver, both tuned to the same frequency, produced definite signal interference and subsequently sketched out a radio-location apparatus able to be used at sea.

However, the first direct use of radio reflectivity was in measuring the height of the ionosphere, and the sky-wave effect of the Heaviside-Kennerly layer, when in 1924 Appleton and Barnett in the UK, and Breit and Tuve of the Carnegie Institute in the U.S. determined the relative heights of the layers. The F layer, (The Appleton layer) was discovered above 150km. Breit and Tuve were the first to use the pulse-echo reflectivity principle.

By 1939 almost most technologically advanced nations had developed Radar in one form or another. At this time, the UK, Holland, Germany and the U.S.A were some way towards developing military Radar while the first peaceful application had emanated from France. In Germany and France the developments were initially carried out by private companies, in the UK and U.S.A. by scientists in government research centres.

The French work was carried out by engineers working for the Societe' Francaise Radioelectrique and focused on metric and decimetric radio waves in order to detect obstacles at sea. The obstacle-detector development, working at short wavelengths, was sufficiently mature by 1935 for it to be installed in the Liner Normandie. Based on the pulse-echo principle it employed high frequency magnetrons and seemed to have been eminently successful - so much so that a similar system was installed at Le Havre in 1936 to detect ships entering and leaving the harbour. This venture subsequently became a military research project for detecting aircraft, but too late to help before the fall of France after Nazi invasion in 1940.

German research into radio-location began before 1935 under German Navy contracts. This was carried out by a new civilian firm (G.E.M.A.) and later augmented by contributions from Telefunken and others. As a result, by 1939 a large number of Radar sets were operational for detecting aircraft. However, although the German work was well abreast of that taking place in other countries, by the outbreak of war in 1939 it had been allowed to stagnate. German policy assumed all ground and air hostilities would be short, and that the overwhelming strength of the Luftwaffe would ensure that incoming enemy aircraft would not constitute a tangible threat to German soil.

This German policy was however diametrically opposite to that in Great Britain. Investigations in the UK began seriously in 1931 after it was reported that aircraft were seen to deflect radio transmissions and similar observations came about in 1933. At that date, a careful analysis of using the phenomenon for aircraft detection took place and H.E. Wimperis (Air Ministry Director of Scientific Research) and his assistant A.P. Rowe, made efforts to have research into long range aircraft detection accelerated. An investigating committee, made up of Sir Henry Tizard, Professor Patrick Blackett and Professor A.V. Hill concluded that the potential of radio-location offered a very superior method of detecting incoming enemy aircraft than the wholly inadequate acoustic methods then in use. A development programme was approved and Air Ministry funds set aside for its financing.

The task of leading the development programme was given to Robert. A. Watson-Watt. He was initially lecturer in physics at University College, Dundee before moving to government work as superintendent of the Radio Division at the National Physical Laboratories, Teddington.

In February 1935 he demonstrated radio-echo detection by aircraft reflecting signals from the BBC Daventry short-wave station, and the Tizard committee recommended a continuation of development along the same lines demonstrated by Watson-Watt. He, along with six assistants, designed, demonstrated and proved the first self contained and practicable aircraft radio-location system yet seen on the Suffolk coast in July and August 1935. He, and his team, solved the problems of a very high power transmitter modulated with short pulses, the construction of receivers to detect and process the incoming pulses, and the arrangement of suitable antenna for both transmitter and receiver. So effective and promising were the summer demonstrations, that the Air Ministry built a chain of five RDF stations along the south eastern coast of England.

In parallel with the installation of radio-location stations other work was started on radar based control of anti-aircraft guns and for shipping. As war commenced more coastal RDF stations were being completed along the eastern and southern coasts and more resource was being injected into radar research. EMI had independently developed a 60 MHz system by 1939.

In autumn 1939, in parallel to the extension of the RDF stations, the British admiralty asked Professor M.L. Oliphant and the physics department of the University of Birmingham to develop a high power micro-wave transmitter in order to offer better range, detection and resolving power of radar systems.

Most of those commandeered to work on the project chose to concentrate on the Varian Klystron first developed by R.H and S.F Varian at Stanford University in 1939 from work started by W.W. Hansen at Stanford in 1938. Using for the first time the principle of closed resonators, it was the best source of high frequency (very short wave) power available at the time. However, two departmental researchers at Birmingham, J.T.Randall and H.A.H. Boot realised that the Klystron was theoretically incapable of providing the power levels the Admiralty were asking for. They switched their focus to the Magnetron, an invention of A.W. Hull of the American General Electric Company in 1921. The magnetron was a medium power, microwave emitting, thermionic device using an external magnet to control electron flow. By applying the Klystron resonator principle, Randall and Boot created the cavity magnetron which in terms of performance far surpassed the specification they were working from.

Great improvements in radar performance could now be realised by the use of centimetre wavelengths. Accuracy and range were dramatically extended, as too the ability to define ground topology thereby significantly improving blind bombing.

Of note also in UK radar development was Alan D. Blumlein who, virtually single handedly, developed the Airborn Interception, Ground Control Interception and H2S systems.

In the U.S.A radar development had proceeded independently. The military had expressed interest after L.A. Hyland (an associate of A.H. Taylor) confirmed accidentally in 1931 the British report that aircraft caused interference in radio waves. Subsequently, Leo Young applied the pulse technique to prove this observation. However, irrespective of the potential for radio-location the most likely beneficiary, the US Air Force, declined interest while the US Navy board was lukewarm. However, persistent efforts by Harold Bowen, Chief of the Naval Laboratory, managed to extract $100,000 from the Navy allocations to back radar research. This came under the management of Robert M. Page head of the research section of the Naval Laboratories radio division and after a comparatively short development phase, a fully functional radar system was demonstrated in 1936. This however did not instigate any enthusiasm from the US Navy and it took another two years before grudging approval was given to fit radar to some US Navy ships. The word RADAR was coined by US Navy Commander S.M. Tucker in 1940.

As the U.S and U.K fought together after December 1941, both had already cooperated in radar development and in the U.S the Office of Scientific Research and Development established a radar division to finance and coordinate all relevant R&D. Starting with just twelve personnel it organised the Radiation Laboratory in 1940 which expanded to over 4000 research and support staff by1945. Likewise, the British Air Ministry organised a large research laboratory known as the Telecommunications Research Laboratory (TRE) where a number of significant advances in radar and radar signal processing were made.

With the modern advantage of computers and massively fast data handling and processing, current radar systems for both military and civilian use have become enormously sophisticated. The principle of radio-location is now well established and current research seems to focus (certainly as far as the military is concerned) on how to detect those objects designed not to be detectable. The onset of STEALTH technology has created the curious situation where in recognising how effective modern radar systems are, the question arises 'how can this system be defeated'. It is interesting to speculate that if radar had not been refined and improved in US and UK laboratories, STEALTH technology would not have been developed by the U.S!