BTW, do those jammers send white noise, or are they smart enough to
fake a signal? Say, I build a small detonator from RC car parts, and it
depends on the transmitter sending "left, right, left..." every 30
seconds, with a timer starting on every signal, counting down until the
opposite side timer is started.

With a white noise jammer, no signal gets through anymore, 30 seconds
later you get *bang*...


But that is not a system that you would want to use for real, any kind of
failure or signal interuption would cause a detonation.


All this stuff came out at the inquest into the people shot in Gib. The
claim was made that the security forces should be able to jam the signal.

They explained in some detail why they couldn't.

They certainaly could jam the signal, what they could not do eaisly is
detonate the device when they wished.

It seems that the IRA people used a Trio Radio Amateur 144 MHz pocket
radio
system that requires a sequence of reversals transmitted by VHF FSK to
trigger an open channel. The sequence is set by a row of five thumbwheels,
each of ten digits.

So you've got a million, minus one, possible combinations.

I leave as an exercise for the reader the calculation of the linear
recursive sequence necessary to trigger the thing prematurely...

This, of course, assumes the security forces know the radio frequency and
the FSK baud rate and frequency shift.

White noise generators don't work once the bad guys get up to 'radio
amateur' levels of knowledge of triggering technology...


White noise will *allways* work as long as you can get a loud enough signal,
its just a matter of power and range.
However, the more intelligent jammers use other signal patterns to jam known
types of transmission, and "look through" their own transmissions to analyze
the target signal.

Jeff