From what I've read that ELINT operation foundered on the German's assumption that the British radar systems used similar frequencies to their own as a result the equipment fitted on the Graff Zeppelin failed to pick up the Chain Home transmissions while the British radar picked up the airship with no problems at all.
 
I heard of that story a long time ago and yes, it was a very stupid blunder and idiocy.
 
IIRC, the Germans were looking for microwave 'dishes' or similar, so simply did not recognise the Chain's towering masts' potential, or that transmitters and receivers were not closely twinned...
Oops...

But what really made the difference was the 'Command & Control' network that linked human spotters, the legacy acoustic system, and the new-fangled RADAR...
 
The German scientists of the GEMA obtained their first practical radar in 1934. The prototype operated in a wavelength of 50 cm.

It was followed by the early warning ‘Freya’ at the beginning of 1937, the naval radio telemeter ‘Seetakt’ in the Autumn of 1936 and the flak director ‘Würzburg’ by mid- 1939.

Military men and politicians who were in the secret thought that only Germany had the radio detection technology but the scientists, in professional contacts outside the Reich were doubtful.

To clarify this important issue, the Chief of Communication Affairs of the Luftwaffe, General Wolfgang Martini, obtained the approval from Göring to transform the ‘Graf Zeppelin II’ (LZ 130) obsolete airship into a sophisticated ELINT platform, with the objective of spying the military radio emissions of some countries that were potentially hostile.

A week before the Munich agreement, to annex Sudetenland, the LZ 130 – with a group of 30 radio operators onboard – made a reconnaissance flight of 11 hours along the Czech frontier while its crew scanned the ether for unusual signals, without any results. During the flight it was verified that the structure of the LZ 130 considerably affected the effectiveness of the receptors, equipped with simple non-directional dipole aerials. The problem was solved by installing some directional aerials in a spy basket that could be lowered as much as 1,200 m below the airship.

On 13 April and 15 June 1939, the LZ 130 made two secret flights of 30 hours, possibly along the French, Belgian and Dutch border. On 12 July the ELINT mission was made along the coast of Belgium and Holland, going up to 140 miles from the east coast of Britain. Four days later the airship looked for radio signals over the Polish borders. The flight was repeated on 22 July.

The start of the WWII was imminent and Martini could not find any signs of enemy radars. On 2 August it was the turn of the British. The LZ 130 scanned the Dutch and Belgian coasts again, and then turned northwards along the UK’s North Sea coast, again without any results. During the return flight it pretended to suffer a failure, to get at visual distance of the coast, near Aberdeen, in order to investigate strange antenna mast at the same time that emitted strong radio impulses in several frequencies. The only British answer was the visit of a Miles Magister trainer of the 612 Sqn that got around the giant to pry.

The next day there was a diplomatic protest without major consequences. That seemed final and the OKL reached the conclusion that the British antennas were used to study the long distance radio transmission, using the layers of ionized air in the atmosphere.

Actually, during its flight of 48 hours, the Zeppelin had been located by 12 radar stations of the Chain Home that operated in the 25 MHz band. The emissions were undetected by Germans that focused the main part of their search on the radio spectrum above 100 Mhz, the ‘Freya’ frequency.

Such demonstration of ethnocentrism was not only attributable to the proud Nazi military men…. Since 50 years, the researchers of the SETI project have been looking for radio emissions that prove the existence of extraterrestrial intelligence within the emission frequencies of the Hydrogen (1.2 GHz) and of the OH (1.64 GHz) because they find quite logical to associate intelligence with the presence of water!

The fact is that in 1940 the Germans captures two mobile radar stations abandoned by the British in Dunkirk, finally finding out that they operated in 4 m. They also measured the emissions in 12 m of the antennas of the Chain Home in Dover.

The conclusion of General Martini was that the German equipment was superior (‘Freya’ worked in 2.40 m, ‘Seetakt’ in 80 cm and ‘Würzburg’ in 53 cm). The excess of confidence would cost the Luftwaffe the loss of 1,887 airplanes during the summer, due to the British radar.

He also ignored that France had already built an anti-icebergs radar in 1934 for the liner SS Normandie. It worked with two emitters in 80 cm and 16 cm and was installed in Sannois in 1939, supporting the defence of Paris without being detected by the ELINT German services.

They also did not suspect the existence of the Dutch radar of 70 cm, the only prototype built by the Delft and Leiden scientists that was dismantled after the May 1940 tests to avoid its capture.
 

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