Small.
Giant.
German Würzburg Radar
The conviction that Germany did not possess radar as an early warning technology had stemmed partly from the absence of anything resembling the high radio towers so prominent on Britain’s coastlines. Watson-Watt, in fact, had spent his vacation in the summer of 1937 looking for tell-tale aerials in Germany; finding none, he assumed that Nazi Germany had not plumbed radar’s secrets. After studying the Oslo Report, which mentioned the use of short wavelengths which would have precluded large aerials, R. V. Jones thought otherwise. His suspicions were confirmed in the summer of 1940 when he received information that British aircraft had been intercepted as a result of a German system called ‘Freya-Meldung-Freya’. Jones reflected that Freya was a Nordic goddess whose necklace was protected day and night by Heimdall, a watchman who could see vast distances in every direction.
Meanwhile, in the autumn of 1940, a radar expert with TRE detected radio signals on a wavelength of 80 centimetres in the English Channel which came into operation when British ships were being fired on. He had stumbled on another highly secret German radar development, known as Seetakt, an advanced system integrated with gunfire control.
Jones realized that this gunfire control radar was not the Freya early warning system, which threatened to be of a quite different order of technology. The breakthrough finally came in February 1941, when Jones was shown certain ‘curiosities’ caught in reconnaissance photos of the Hague peninsula — two circular dishes some 20 feet in diameter on the edge of a field. Further scrutiny revealed that they were being rotated. It was indeed a Freya station. Work could now start on discovering the frequencies on which they operated in order to jam them, as well as the task of finding the sites of all the other Freya stations stretched out across the continent. In the meantime Jones had begun to concentrate on a type of equipment associated with the Freya system, referred to in the Oslo Report and occasionally mentioned in coded German messages as a Würzburg.
The messages spoke of these systems being transported to Romania and Bulgaria, where, in time, British receiving stations traced transmissions on wavelengths of 53 centimetres (the Oslo Report had given the lengths as 50 centimetres). Photo reconnaissance failed to turn up a visual clue as to the nature of the Würzburg equipment until November of 1941, when a small object was detected on a photograph of a Freya station at Cap d’Antifer. Further photographs revealed a parabolic reflector some 10 feet in diameter close by. Thereafter Jones began to deduce the full complexity of the Germans’ early warning system, which was composed of the Freya long-range detectors and two types of associated Würzburgs — smaller systems that controlled the operation of searchlights and larger systems, Würzburg Riesen, which directed the German night fighters on to incoming RAF raiders. He did not rest until he had captured an actual model of a Würzburg, when a Combined Operations raid of 119 British paratroopers dropped on Cap d’Antifer early in 1942 and brought most of the equipment back to Britain with the aid of a small naval force. British intelligence now had all the important components of a Würzburg in their hands and were in a position to calculate the range of wavelengths in order to devise appropriate jamming counter-measures.
German intelligence had shown no such foresight, persistence and cunning during the period in which Reichsmarschall Goering planned, and attempted to carry out, the destruction of Britain’s air power by daylight raids as a prelude to invasion. When the Battle of Britain commenced in earnest in July 1940, Britain’s early warning radar system was composed of twenty-one Chain Home stations (some of these structures towering 350 feet into the sky) and a further thirty Chain Home Low units giving early warning of invaders approaching the British Isles across the English Channel and the North Sea. Royal Observer Corps units acted as back-up inland, although their visual sightings depended on daylight and the absence of cloud cover.
German intelligence had neglected to identify the purpose of Britain’s highly visible radar towers and aerials, even though the structures were recorded two years before the outbreak of war. Unarmed German aircraft had criss-crossed Britain under the pretext of making weather observations for the Lufthansa airline. In 1939 a Zeppelin codenamed LZ 130, under the command of Luftwaffe signals section, flew along Britain’s radar chain attempting to pick up radio signals; but nothing was learned. From July to September the Luftwaffe failed to defeat the Royal Air Force, largely due to the ability of British radar defence to anticipate incoming raiders and respond with interceptors. Although Goering eventually guessed the purpose of the radar towers, he never understood the importance of their function and did not attempt systematically to put them out of action. As described earlier, following the bombing of London in an accidental raid on the night of 24 August 1940, Churchill ordered a bombing raid on Berlin and Hitler retaliated by pounding London night after night for a week. The onslaught on London, destructive as it was, gave the RAF and its bases a much-needed respite. When Goering returned to his original strategy, Fighter Command was ready for a decisive battle. The climax of the battle is customarily dated 15 September, when some seventy-nine Luftwaffe aircraft were downed for the loss of thirty-six RAF fighters. Thereafter Goering switched again to night raids on London and the industrial heartlands, while Hitler postponed his invasion of Britain indefinitely.
Meanwhile, as Britain increased its night raids against targets in Germany and occupied Europe, the Reich’s engineers worked on improving radar defences. With headquarters at Zeist in Holland, Major General Josef Kammhuber had by 1941 divided the mainland into a grid of boxes 27 miles wide and 21 miles deep, each containing a Freya early warning dish and two Würzburgs — one to detect individual enemy planes, the other to direct a Luftwaffe fighter. By the end of 1941 the system could detect incoming raiders 200 miles out, and estimate the altitude of aircraft from 150 miles away.
WW2 Radar CV56 Magnetron
This Magnetron was the heart of the type 271Q naval radar use to detect U boats in the battle of the Atlantic. The Magnetron was one of the most important scientific developments of WW2.
Made in 1942/43 this example came back from Canada in 2005 where it had been a naval spare part. The original cardboard box is marked “Admiralty Signals Establishment, Oldham”.
The Magnetron
What gave Britain an inestimable technological advantage in radar, however, an advantage that would be shared with the Americans a year before they entered the war, was the development of a small device, about the size of a saucer, known as a ‘resonant cavity magnetron’. Its development tells an instructive tale of military science and technology resulting in incalculable advantage to the Allies, and disaster for Germany.
At stake was the development of forms of radar that would reduce the bulk of equipment needed, and provide flexible and highly accurate information for the users. What was needed was a type of airborne radar small enough to be installed in a fighter aircraft, working on about 10 centimetres wave length so as to pinpoint Luftwaffe night raiders, and yet generating sufficient power to achieve long-range detection. The main goal was to achieve a narrow radar beam so as to pinpoint the target. This is determined by the width of the antenna compared with the wavelength. Hence the shorter the wavelength the smaller the antenna for the same beam width.
German radar historian Kroge claims that in the summer of 1935 German engineers at the private company GEMA were working with a rudimentary form of ‘magnetron’, originally invented by Albert W. Hull of the American firm General Electric in about 1920. The magnetron consists of a vacuum tube placed in a magnetic field so that electrons follow curved paths while travelling across the tube. German researchers found the magnetron unstable, preventing the expected maximum range of 20 kilometres from being attained consistently: ‘only by constant tuning of the receiver could respectable distances be observed at all’. In the end it was abandoned for other solutions.
The British research path to a useable magnetron had started at Birmingham University in 1939 with experiments for generating microwaves, which led to the construction of a ‘cavity magnetron’, a radical modification of Hull’s device. Robert Buderi, the historian of radar, states that the invention came about ‘accidentally on purpose’. Two physicists, John Randall and Henry Boot, conducting experiments under Marcus Oliphant at Birmingham University, put together the combined virtues of two devices — the traditional magnetron and a klystron, an American invention (from the Greek kluzo, meaning the breaking of waves on the seashore) discovered as part of an attempt to find a microwave power source for blind aircraft landing.
As Buderi puts it, ‘The challenge rested in adapting the klystron’s doughnut-shaped cavities to the cathode and node structure of the magnetron, which depended on cylindrical symmetry.’ As the American physicist I. I. Rabi described it a year later, when an early model was taken to the United States, the cavity magnetron was a kind of whistle operating under electric and magnetic fields. Buderi explains the technicalities:
Not unlike the way air flowing in front of a whistle hole causes a tone to be emitted — the frequency of which is largely dependent on the instrument’s size and shape — the electrons oscillated at a specific radio frequency determined by the cavity dimension.
The first test of the completed device was conducted on 21 February 1940, and three days later the researchers could confirm that it operated on 9.5 centimetres and generated an output of 400 watts. By May the output had increased to a prodigious 12-15 kilowatts. It was the dawn of microwave radar, but hard-pressed, battered Britain badly needed a major partner in its development and mass production.
Anglo-American Technological Collaboration
In early August of 1940 Churchill asked one of his top technology advisers, Henry Tizard, to travel to the United States with a basket of top secret discoveries; the cavity magnetron was among them. The device in its completed form appeared like a clay pigeon and could fit in the palm of a person’s hand. Tizard and his team demonstrated the operation of the cavity magnetron on 19 September in Washington, DC, to a dumbfounded audience composed of members of America’s National Defense Research Committee. In the course of the presentation it was revealed that the tiny device was capable of generating a thousand times more output than America’s most advanced instrument on a wavelength less than 10 centimetres. Within a month a decision had been taken to establish a Radiation Laboratory at MIT, Cambridge, Massachusetts, later known as the Rad Lab.
By 1943 centimetric radar, employing the resonant cavity magnetron, was being used by the Allies not only for night interception in the air but in modified form for locating bombing targets ‘blind’ in Germany. It was only a matter of time before the Germans would discover the guidance secrets of the Allies in a crashed aircraft. On 2 February an aircraft belonging to a Pathfinder force (used to guide bombers on to their targets by dropping flares ahead of the bomber formations) was shot down near Rotterdam and a damaged cavity magnetron was retrieved from the wreckage. German engineers, referring to the device as the Rotterdamgerät, soon confirmed that it was a centrimetric radar device and that it aided night-fighter search or warning as well as bombing guidance. Herman Goering, on learning of the equipment, declared:
We must frankly admit that in this sphere the British and Americans are far ahead of us. I expected them to be advanced, but frankly I never thought that they would get so far ahead. I did hope that even if we were behind we could at least be in the same race.
Professor Leo Brandt of Telefunken was ordered to reconstruct the device so that it could be employed by Germany’s radar, but on 1 March the equipment was destroyed in a bombing raid. That same night, however, another magnetron was found in a bomber shot down over Holland and Brandt went to work again.
German scientists hastened to incorporate cavity magnetrons into their systems for bomb guidance, anti-aircraft detection and gunnery and airborne radar, but by the time they were ready to be deployed the war was approaching its end. In the meantime Germany’s researchers found methods of countering the benefits of cavity magnetrons, in particular a detector called Naxos and another device known as Korfu, designed to pick up 10-centimetre radar wavelengths. Naxos was particularly useful for U-boats detecting search planes operating on microwave to detect the presence of periscopes. But as fast as the Reich engineers devised countermeasures the Allies were developing 3-centimetre wavelength radar to catch their quarries.
