Originally Posted by air pig View Post
...The system was actually difficult to jam as you needed to know both frequency used and how many cycles/second it was transmitted on....
Intially they were jammed by interference, but later it evolved into bending the beams so that bombs were dropped away from cities. Presumably you would need to know frequency and modulation. From Wiki:
Efforts to block the Knickebein headache were codenamed "Aspirin". Initially, modified medical diathermy sets transmitted interference, but later, on nights where raids were expected, local radio transmitters broadcast an extra "dot signal" at low power.[17] The German practice of turning on the beams long before the bombers reached the target area aided the British efforts. Avro Ansons fitted with receivers would be flown around the country in an attempt to capture the beams' location; a successful capture would then be reported to nearby broadcasters.[18]
The low-power "dot signal" was initially transmitted essentially at random, so German navigators would hear two dots. This meant there were many equi-signal areas, and no easy way to distinguish them except by comparing them with a known location. The British transmitters were later modified to send their dots at the same time as the German transmitters, making it impossible to tell which signal was which. In this case the navigators would receive the equi-signal over a wide area, and navigation along the bombline became impossible, with the aircraft drifting into the "dash area" and no way to correct for it.
Thus the beam was seemingly "bent" away from the target. Eventually, the beams could be inclined by a controlled amount which enabled the British to fool the Germans into dropping their bombs where they wanted them. A side effect was that as the German crews had been trained to navigate solely by the beams, many crews failed to find either the true equi-signal or Germany again.[19] Some Luftwaffe bombers even landed at RAF bases, believing they were back in the Reich.[20]