On 03/08/2020 16:58, Farmer Giles wrote:
On 03/08/2020 11:17, Spike wrote:
On 03/08/2020 10:02, Farmer Giles wrote:
On 03/08/2020 10:38, charles wrote:

[detector vans] were operated by the GPO.
They existed, I've been inside one.
I've also seen the portable kit, developed later, which actually
worked, too.

Worked in what sense? The only possible thing that could be detected
would be the local oscillator in the tuner mixer. This 'beats' with the
incoming RF signal and produces the IF (intermediate frequency). This
wouldn't tell anyone if or what you were watching even if it was strong
enough to be detected from a distance, which is unlikely.

For a CRT TV or computer screen, the signal modulating the grid of the
tube was fortuitously radiated over a wide range of frequencies, This
could be picked up, usually at VHF, and used to modulate one's own CRT,
letting you see on your screen exactly what was on the target's screen.


That would be just the video frequency, and wouldn't tell you if you
were watching broadcast TV or not - nor would the signals have
sufficient energy to be picked up any distance away. Same with the line
frequency at 15.635 kHz. The TV could be being used as a monitor, or
similar, and those signals would still be present.

Some of this stuff is now a distant memory for me, but I was in the
television industry for many years.
not heard 15 Kc/s for years .......