John Rumm wrote:

But there is virtually no chance of detecting a TV that does not have
the aerial lead connected.

FSVO "no" such as "quite a high" :-). AIUI the detector vans have been
/ are being replaced by hand-held TV detectors, the operating principle
of which seems to be a closely guarded secret in the Beeb - but BBC R&D
at Kingswood had a hand in the development, so it could be something
quite advanced.

The linescan output, and that of the local oscilators in a typical TV
demodulating circuit are all easily detected with the right kit. Not
only that, but the nature of the signals makes them quite conspicous.

Line scan was used by the original detectors[1] in the 50s, replaced by
LO radiation in the later vans. If they've gone back to line scan
'radiation' (pedants read "induction field") it might make it quite
harder to find non-CRT displays, which are becoming very common.

http://www.jifvik.org/tv/ also makes interesting reading on the subject
generally.


[1] "Detector: Post Office official equipped with clever apparatus which
responds to the absence of licence." M. G. Scroggie (writing as
'Cathode Ray') in Wireless World in 1950-something.

--
Andy