This was complicated by domestic radios in people's homes also leaking radiation.
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.
Soviet short-wave transmitters were extensively used to broadcast messages to clandestine agents, the transmissions consisting simply of number sequences read aloud and decoded using a one-time pad.
In his book Spycatcher, former MI5 officer Peter Wright related an incident in which a mobile RAFTER unit was driven around the backstreets in an attempt to locate a receiver, but the search proved futile.
Initially, MI5 believed interference and the effects of large metal objects such as lamp posts in the surrounding frustrated the search.