The all-metal aircraft gave new impetus to radar research, both because of the metal's radio-reflecting quality and because of the new threat that high-speed metal bombers presented to the defences. During the 1930s many countries carried out experiments with radio-detection devices. In Japan, the Yagi short-wave directional aerial was invented. In America, the army's Signal Corps Laboratories worked on radio-detection, and the US navy sought the co-operation of American industry. The Yagi aerial's wide use, and the US navy's name 'radar' that was adopted by all concerned, gave chauvinists in those countries an opportunity to claim that it was a national invention. In fact the Germans had an even better claim.
Dr. Rudolf Kьhnold, head of German naval signals research, was prompted by early work on sonar (underwater detection) to rig up a crude radar set, using a brand-new powerful valve from Philips. By 20 March 1934, before Watson-Watt in England had even proposed experiments, Kьhnold's apparatus was beamed across Kiel Harbour and obtained a 'picture' of the battleship Hessen. By October of the same year, a test was arranged to show that his apparatus could detect a ship at seven miles. Aircraft flying through the beam provided an unexpected demonstration of the application of radar to air defence.
By the end of the 1930s Germany was producing detection equipment that was superior to the equivalent British sets. The German battleship Graf Spee had a gun-ranging radar, Seetakt, as early as 1937. When the Graf Spee was scuttled in Uruguay in 1939, after the Battle of the River Plate, a British radar expert took a rowing boat out to the half-submerged ship and climbed up to examine the aerial. He reported on it as a gun-laying radar but so reluctant was anyone in Britain to admit that the Germans might have such a device that the report was filed away and forgotten.
By July 1938, the Germans had the Freya, a good early-warning radar, in action. It detected a Junkers Ju 52 transport aircraft at 90 kilometres, although it had no altitude-finding ability. However, the German anti-aircraft guns had radar accurate enough for gunnery. Their Wьrzburg-A demonstrated in July 1939 was very sophisticated. A Telefunken product, it used 8 kilowatts on 50 cm and had the same parabola for sending and receiving. Its initial range was 30 kilometres, and when afterwards it was equipped with a rotating dipole to give an overlapping beam, it was accurate to half a degree and 100 metres.
And German radar sets proved excellent in use. In the winter of 1939-40, Freya radar units on the German islands of Wangerooge and Heligoland had detected incoming RAF raids so efficiently that Bomber Command formations were decimated. In May 1940, an anti-aircraft battery at Essen-Frintrop used a Wurzburg radar, and was able to shoot down an RAF bomber which they could not see. This so impressed Göring that he made a public announcement that no enemy aircraft would ever fly across Germany. It was a remark he was never allowed to forget. After the Dunkirk evacuation there was great consternation in Whitehall when it was realized that some British mobile radar units had been left behind in France. Furthermore, British radar secrets had all been shared with their French allies. There was considerable relief when the crews who had abandoned the radar sets all swore that the equipment had been totally destroyed.
In fact, the French armed forces proved faithful to their British friends even in those years of misery. No such secrets, of radar or anything else, were passed to the Germans. However, at least one British radar set, more or less in working order, was captured during the German advance. But the British need not have worried. German experts gave it no more than a perfunctory look before declaring it rather primitive by German standards, which it was.
Berlin and London shared a determination to believe that the other side knew nothing of radar technology, even though there existed plenty of evidence otherwise. And Milch had shown a German radar set to a visiting French air force delegation in August 1938. In the autumn of 1937, during an official visit to England with Udet and other officers, he had boasted about German radar to an astonished audience of high-ranking RAF officers during a formal lunch held in Milch's honour.
In the ante-room of the Officers' Mess at Fighter Command HQ Milch suddenly addressed a loud question to the assembled officers. "Now, gentlemen, let us all be frank," he said. "How are you getting on with your experiments in the detection by radio of aircraft approaching your shores?"
There was some embarrassed laughter, and an attempt to change the subject, but Milch persisted. "Come gentlemen, there is no need to be so cagey. We've known for some time that you were developing a system of radiolocation. So are we, and we think we are a jump ahead of you."
Perhaps, in technical terms, Milch was correct but radar could never be a practical warning system for any frontier except a coastline. But the system itself was born out of a unique British ability to compromise.
It has been said that the motto of the team at Bawdsey, Suffolk, where practical radar was developed, was "Second best tomorrow." This meant that they could not afford to spend a year or two in search of perfection but must get radar working soon, even if its performance was below peak. Although an egoist, Watson-Watt never promised more than he knew he could deliver.
It was this restraint that caused Watson-Watt to name their experiments RDF (Radio Direction Finding). It was a deliberate attempt to deceive the curious, because although they believed they could crack the problem of range-finding, they thought the problem of direction-finding would be difficult or even impossible.
Watson-Watt's original memorandum of 27 February 1935 is a remarkable document. It not only set out the alternative paths of research but guessed rather accurately what might be achieved. At the end it noted the importance of identifying aircraft, and suggested a radio method by which friendly aircraft could give a coded reinforcement of the reflected radio wave. He added a note about the need that all this would bring for really good radio-telephone communication with the fighter pilots, realizing that his radar would be measured by how efficiently it placed fighter planes in a position to attack enemy bombers. (At this time the envisaged equipment was very large and few men dreamed that it would ever be made small enough to fit inside even the largest aircraft.)
But the most remarkable thing about radar is that no one had invented it long before Kьhnold's experiments. There were hundreds, perhaps thousands, of scientists and experts paid by their governments to advise on such scientific matters. The phenomenon of radio waves re-radiating from distant aircraft was repeatedly mentioned in professional journals, and the Post Office got endless complaints about the way aircraft spoiled radio reception. Yet these were simply treated as problems to be solved. None of the experts was able to link these problems' of interference with the threat of the bomber, which continued to get tremendous publicity.
Watson-Watt's first guesses set British radar off to a good beginning. His decision to treat an aircraft as though its wings were a horizontal antenna started him off using a wavelength of 50 metres, calculating this to be twice the wingspan of the average bomber. Almost immediately Watson-Watt changed to half that wavelength to avoid commercial radio signals. His decision to treat the bomber as if it were a horizontal wire led him to horizontal polarization and to stacked aerials. His experience with cathode-ray tubes (which had been improving rapidly at this time) contributed to the presentation. It was essential that only existing parts could be used: there was no time to start inventing new components. It was upon these basic decisions that rapid British progress depended. Britain was an ideal country for a radar defence chain, for the sea provided no obstructions to the signals transmitted.