Re: TV Detector Vans

The message <ylEqXyU8FzBFFwyK@xxxxxxxxxxxxxxxxx>
from Ian Jackson <IanJacksonRemoveThisBit@xxxxxxxxxxxxxxxxx> contains
these words:

In message <2006091123494185168@xxxxxxxxxxxxxxxx>, Johnny B Good
<jcs.computers***@xxxxxxxxxxxxxxxx> writes



Yes the LO radiation was quite strong enough for the TV detector vans
to pinpoint the actual room the TV set was located in since the bulk of
said radiation was emitted from the chassis (or PCB) rather than the
antenna feeder.


Despite the apparent 'wide open spaces' of the UHF spectrum (471 to
862MHz - wow!), one of the restrictions placed on the allocation of
channel frequencies for multi-channel cable systems was having to avoid
the high level of LO fed back into the system. It was said that the
manufacturers could reduce this significantly, but were pressured to
'keep it up' in order to provide a good signal for the detector vans to
find. I'm not sure how true this was!

One of the benefits of having an RF amplifier stage before the first
mixer was that the RF amplifier attenuated the LO leakage from the mixer
into the antenna. This reduced the risk of interference to other nearby
TV sets tuned 5 channels higher up the band. What the detector vans
picked up was LO being directly radiated from the TV chassis itself
rather than by radiation from the TV antenna. This was very useful in
that the location of the set itself could be determined to within a few
feet of accuracy, enough to identify which room of the house it was
being operated in.

The TV detectors could also figure out which channel the set was tuned
to since the LO was offset below the channel by the common IF of (ISTR,
35MHz, but feel free to correct me on this.)

For the record, with a normal single-conversion TV, the LO is always
above the channel tuned to - originally 39.9MHz in the case of the
'traditional' UK TVs, but latterly usually the 38.9MHz of Continental
European sets.

Well, I sit corrected. The UHF tuning range covers just under an
octave, so either side could have been chosen. I just assumed they might
have chosen the low side to reduce the stretch towards the microwave
band ( and back in the late 60s, it would have been a stretch to
persuade reliable oscillation out of a transistorised tuning module).

It's quite possible that a number of the early models of UHF TV sets
might have tuned the LO to the low side for stability reasons. In any
case. The TV detection kit could easily have taken this into account
whichever side was standard for the LO offset (and might even have been
able to take both possibilities into account with operator assistance in
any cases of ambiguity - there were , after all, only 3 channels per
transmitter group back then and any sets seemingly tuned 10 channels
lower down the band were most likely low side tuned LO designs).
Whatever the reality of LO tuning was, it was a trivial matter to
identify which channel the detected reciever was tuned to.

Can you imagine the effect on the 'transgressor' when answering the
knock on the door by the TV detector man who could identify not only
which room the TV set was operating in but also which program they were
tuned to? I should think that would have settled any doorstep arguments
in rather short order.

I'm not sure but I think the policy on detections in the early days was
to give the offenders a chance to immediately remedy the lack of a
licence before proceeding to a prosecution. Nowadays, it's a totally
different world.

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

.

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