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....




Well LO does leak, but a lot less than it used to as better designs with
more stages between antenna and LO keep it to a minimum.

Remember detector vans date from the days of valve sets: valves were
expensive, and using as few as possible was a design goal.