Explanation
Since most radio receivers are of the superhet design, they typically contain local oscillators which generate a radio frequency signal in the range of 455 kHz above or sometimes below the frequency to be received. There is always some radiation from such receivers, and in the initial stages of RAFTER, MI5 simply attempted to locate clandestine receivers based on picking up the superhet signal with a quite sensitive receiver that was custom built. This was not always easy because of the increasing number of domestic radios and televisions in people's homes.
By accident, one such receiver for MI5 mobile radio transmissions was being monitored when a passing transmitter produced a powerful signal. This overloaded the receiver, producing an audible change in the received signal. Quickly the agency realized that they could identify the actual frequency being monitored if they produced their own transmissions and listened for the change in the superhet tone.
Read more about this topic: Operation RAFTER
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