Radar Recollections - A Bournemouth University / CHiDE / HLF project


Radar 'jamming'

When the CH system was being devised, Watson Watt had forseen that it would be relatively easy for an enemy to nullify a radar echo by transmitting another beam on a similar frequency. This 'active jamming' is very effective if transmission power is high enough. Other active techniques involved modulating the beam so that the returning echo became undecipherable on the display screen.

'Passive jamming' included any method that was designed to confuse the enemy's detection equipment. The use of thin strips of aluminium foil known as 'window' or 'chaff' is the best example. To specifically 'jam' the Wurzburg 50 cm transmissions it was found that strips measuring 0.125 inches by 10 inches worked best because these dimensions equate to half a wavelength. This system was devised in late 1941 primarily to deceive enemy anti-aircraft guns on coastal defences that might be radar controlled.

The problems of radar jamming were never underestimated and Dr R. Cockburn was appointed head of a 'Radio Countermeasures Unit' and his first successes was to construct a 'jammer' code-named 'Aspirin'. The transmitter was installed near Salisbury and was intended to mask the German transmissions from their large Cherbourg installation.

Furthermore, the RAF set up a special group known as 'J-Watch' and their task was to monitor and record all enemy 'jamming' transmissions; to record the frequencies used and if possible, to pinpoint the geographical location of the source. They were required to monitor every frequency from 20 MHz to 3000 MHz.

J-Watch had a number of devices they could use:

IFRU Two tunable filters within the receiver that could minimize continuous-wave interference.
IJAJ A special circuit that could suppress pulsed interference.
AJBO A special circuit to suppress frequency-modulated interference.
Transparent Screen filters These helped to minimize transitory screen interference.
W. E. Burcham
Mr S. Ratcliffe