On 15 Jan 2009 10:16:20 GMT, Huge wrote:

On Wed, 14 Jan 2009 14:37:19 -0800, john wrote:

Huge wrote:

It is entirely
possible to detect radio (including TV) receivers by 'listening' for
the local oscillator signal that leaks back up the antenna feed and is
radiated. This is, for example, how radar detector detectors work. The
military go to a great deal of effort to minimise this leakage in their
comms systems, since you do not want your forces detectable by the
local oscillator leakage from their radios.

Since the L/O frequency changes according to the channel being watched,
the assertion that it is possible to detect this is also true.

--
"Please try to understand, the one you call Messiah is a lie."
[email me at huge {at} huge (dot) org dot uk]

It's a neat idea and seeing as I've a sensitive lab' spectrum analyser
racked about 30cm away from the TV aerial (haven't we all), I took a
look.
Nothing, nada, zilch! is coming back out of the aerial.

Ho, hum. I'm just going on what I was told by my friend who designs front
ends for military radios....

Sounds like what I read many moons ago. Quite detailed info on the
operation and technology of the detector vans were published in the
Post Office Engineering Journal - that's when they had things inside
the vans rather than just a roof rack with a Yagi bolted on.

The issue was somewhere from the mid 70's to the mid 80's. Not sure
if a local reference library will still have them and I'll bet no one
has got round to scanning them and putting them online.

--