[Milsurplus] LO radiation

[J. Forster]

Well, part of it  has to do with the improvements in receivers since WW II. Another part is because (as I mentioned earlier) WW II DFing used nulls of the receiver antenna, and it is
easier to detect a signal than to DF it.

Also, in WW II raster scan displays were not used. It turns out that TV type displays are rather easy to detect at a distance, and it is possible to get information off them. Consider (by
Fourier Analysis) the nature of the signal they emit. It's basically an intensity modulated comb with a fundamental frequency of the horizontal sweep rate, typically between 15,746 and 80
K Hz.
Problem is, you need a lot of receiver BW to make much of the display, and you get all kinds of noise, but, you can realize a wide band comb filter with feedback and a delay cable. It
works.

The scanning electron beam on a CRT can easily be detected. Try this: set your scope to free run and put a probe near the CRT face. Depending on the polarity selected, the beam will avoid
the probe, by moving either down or up (electrons are negative). Put the probe near a TV screen, and you will see video. Tempest tries to stop these kind of leaks. Tektronix sells a
metalized CRT screen to stop this, BTW.

Tempest was a reaction to all this and is likely an over design, but I don't really know. If I'd been setting the spec, I would likely have tried to estimate what might be done 30 years
in the future, and set the spec accordingly. Many military systems are around for ages, remember.

FWIW,
-John



Radioman390 at cs.com wrote:

> What, may I ask is all the brouhaha about "Tempest compliant" receivers?
> There's actually a spec for the degree of attenuation of internal signals at the antenna input.
> And this is still current!