Radar could hardly be kept a secret. The Chain Home aerials were 364 feet high and right beside the coast; the 20 - 30MHz signals used were easily detected on a short-wave radio and, of course, Hurricanes and Spitfires kept turning up unexpectedly, just where they were least wanted.
Nevertheless, the Luftwaffe seems not to have realised just how crucial Chain Home was to the UK's defence. Although a few sporadic raids were made on the stations, the little damage that was done was easily repaired. In retrospect a concerted attack on Chain Home could have seriously hampered the RAF, but instead the Luftwaffe attempted to avoid the system, rather than destroy it.
The first technique was to fly below the Chain Home beam: to get the necessary range it was directed quite high in the air. This was countered by the introduction of smaller Chain Home Low (CHL) systems which effectively filled in the gaps under Chain Home's surveillance, albeit at a rather shorter range.
With that option gone Luftwaffe techniques turned to nullifying the advantage given by radar. Chain Home only navigated RAF pilots to the general vicinity of enemy aircraft; they then relied on seeing them to make an attack. German pilots therefore took to flying in bad weather and at night, when the chances of being seen were much smaller.
And so started a game of measure and counter measure which has been played to the current day. Radar designers do their best to spot aircraft; aircraft designers do their best to hide from the radar and so it goes on.
Gliders could fly under the radar
Perhaps the first stealth aircraft were gliders. They were used to great effect by the Germans in the capture of the Belgian Fort Eben Emael in May 1940, taking advantage of their near-silent approach to surprise the fort's defenders. No radar was used there, but it was realised that as small, light aircraft with a minimum of metal parts (no engines, of course, and wooden fuselages), gliders would be much harder to detect with radar.
The British therefore set up a major research programme based at Worth Matravers in Dorset to investigate the detection of gliders by radar. A small team of glider pilots - most of whom had gone to work for the Air Transport Auxiliary, ferrying planes around - was recruited. A few sporting gliders were brought out of storage and a couple of WW1 Avro 504 biplanes, all that could be spared, were used as tugs.
The gliders were towed to around 10,000 feet, as near France as possible, and then flew back towards the Worth radar sets, trying to fly as low as possible to evade detection. Quite apart from the flying, this was a hazardous business: the French coast was in German hands and enemy aircraft were constantly around. Philip Wills describes one encounter in his book "On Being a Bird":
One day, in the darkness of the tracking room, I watched [Mungo] Buxton on the screen take off, and commence his seaward climb. As he started to gain height, we saw the track on an enemy machine leave the French coast and flying directly towards our crawling train. Remorselessly the two points of light approached each other and for one agonising moment they fused. Then they separated again, and each proceeded unhurried on his divergent course. Buxton saw nothing, and it occurred to us that if the German pilot in 1940 looked out over the side of his aircraft and saw in the no-man's-land of mid-channel and last-war Avro 504 towing a sporting German sailplane at 10,000 feet and 45 m.p.h., he probably put his hands before his eyes and drove madly away.
(On Being a Bird, Philip Wills, 1953, pub. Max Parrish & Co, London, pp 24-25)
The Plan Position Indicator
The reference to points of light on a screen shows that Wills was watching a radar system already significantly more advanced than Chain Home in its display. Instead of requiring adjustment of a goniometer to locate a single target, the Plan Position Indicator (PPI) used a cathode ray tube to display a picture of the sky derived from a rotating aerial: this type of radar display is still standard today.
The introduction of the PPI was crucial to the development of effective airborne radar systems. Although the cavity magnetron allowed small transmitters to be built, the information had to be presented in a way which was instantly comprehensible to pilots.
Development of stealth aircraft
After WW2, the development of radar and countermeasures continued throughout the Cold War and afterwards. Radar designers used more transmission power, higher frequencies and shorter wavelengths to improve the range and accuracy of detection; aircraft designers attempted to thwart them. The best known results are the Stealth fighters and bombers used by the American Air Force. These evade detection by using flat surfaces rather than curves, so there is rarely any surface at right angles to approaching radar signals, which are bounced off at an angle and not back the way they came, to a detector.
Such sophistication comes with its downsides. Flat surfaces are none too aerodynamic, and the flying performance and handling of stealth aircraft are, in general, abysmal. They are also not completely invisible to radar, and various techniques have been developed to detect them in flight. One of these is to use a longer wavelength than modern millimetric radar, for which stealth designs are optimised.
It's interesting to reflect that the Chain Home systems, if any were still operational, would have no difficulty at all in detecting the world's "stealthiest" aircraft...
This article is part of the Radar: What happened next? collection, which looks at different aspects of radar technology since the introduction of Chain Home, the first functioning radar defence system developed during the Second World War. This collection was inspired by the OU/BBC drama Castles in the Sky, featuring Eddie Izzard as Robert Watson-Watt, the father of radar.