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
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
J-Watch had a number of devices they could use:
||Two tunable filters within the receiver
that could minimize continuous-wave interference.
||A special circuit that could suppress pulsed
||A special circuit to suppress frequency-modulated
|Transparent Screen filters
|| These helped to minimize transitory screen