The strangest thing I read about TV detectors
is that how they work is supposed to be a secret...

which means that any reading from them can't be used as evidence in a court of law.
( Moon Girl Technologies horrendous beanbag, Fri 2 Dec 2011, 14:53, 1 reply)

I can't really see how they're supposed to be a secret.
They use two techniques - one is directly picking up harmonics from the CRT scan coils, the other is picking up spurious emissions from the tuner (which only really works at very close range).

Since the scan coils are driven with sawtooth(-ish) waveforms at 50Hz and 15.625kHz, and they are big electromagnets, they emit a certain amount of radio interference. If you still have a CRT television or monitor, you can "detect" it with a long-wave radio - tune around until you hear a harsh buzzing, then as you move away notice how it gets quieter. The clever bit is that if you turn the radio end-on to the TV, you'll notice a sharp null as the ferrite rod aerial in the radio is very insensitive at its ends.

TV detector vans had very large loop aerials on the roof in a fibreglass cigar-shaped housing. Portable detectors worked in much the same way as the experiment above with a radio.

As for the TV channel, that comes from detecting the emissions from the tuner. To tune in a particular channel, you mix the signal from the aerial with one from an oscillator around 40MHz higher, and filter off the result - a TV channel, shifted down to about 40MHz. Some of the "local oscillator" - the bit that actually does the tuning - leaks out through the aerial socket (some say that certain makes of TV tuner were deliberately designed to do this - a conspiracy in itself!) and can be picked up with a special receiver and a directional aerial similar to a TV aerial.

None of this is particularly effective any more, with LCD TVs and modern digital tuners.
(gordonjcp, Fri 2 Dec 2011, 15:25, closed)

You do realise that detector vans..
dont exist? It was an advertising scare tactic to get people to buy licences.

What they simply do is have a database of those who have/havent got a licence. Every now and then knock on the door of someone who hasnt and see if they have a TV.
(mrbridger2010, Fri 2 Dec 2011, 15:29, closed)

They don't exist *now*
The last of them were scrapped in the early 90s. I pulled the engine and gearbox from one in a scrapyard to fit to a neighbour's ice cream van.

It was a late-70s Transit, with a single row of "minibus" seats and rear windows, then a low bulkhead, then some 19" racking bolted into the floor. The roof aerial was still mounted, although all the cabling to whatever had been in the racks had been snipped off.
(gordonjcp, Fri 2 Dec 2011, 15:45, closed)

I heard they were really just empty,
and they only had big aerials on the roof for effect.
( Moon Girl Technologies horrendous beanbag, Fri 2 Dec 2011, 15:54, closed)

I heard they were used to delivery mind-control contrail gas to the smaller airports.

( Dr. Shambolic je suis charlie, Fri 2 Dec 2011, 16:16, closed)

Wouldn't be in the least surprised
It would certainly be cheaper. Some were definitely real though ;-)

On that note, a certain local constabulary has a couple of magnetic self-adhesive signs that look like a camera window that they stick to the side of a perfectly ordinary police van then park up on a motorway flyover - *sometimes*. And sometimes it's the real camera van.
(gordonjcp, Fri 2 Dec 2011, 16:57, closed)

Steve Punt did a good program on radio four about the detector vans, fact or fiction
Can't for the life of me remember what the outcome was. Seems a lot of faff.
( Ring Of Fire A petty, drooling belming butthurt retard., Fri 2 Dec 2011, 19:51, closed)