In the old days TV's had a "local" oscillator that one tuned to the carrier frequency of the channel you wanted to watch, what those coily things on top of the van were meant to do was to pick the signal generated by them up and triangulate (manually) the location of the signal and the frequency then polaroid the oscilloscope with the recieved signal. The reality was that they had records of who had licences or not and called on the houses without licences and let guilt do the rest, oh, coupled with powers of entry and siezure. The vans were to intimidate people into suggesting they were caught or would cruise around areas to panic people into buying licences.