"Pietro" schreef in bericht
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I was wondering if it is possible to detect the presence of inactive
cell phones/wifi devices by detecting radio signals they emit.
If inactive means 'switched off', then 'no'.

I
figure wifi would be the best bet because I guess cells dont
broadcast
all that much when not in use, right?
If switched 'on' they DO exchange info with their 'cell' from time to
time.
If left stationary they will do so less, but if in motion they will do
so more frequently.

Would there be enough output to find them?
2 Watts (for a cellphone) is more than enough!

Would it be strong enough to detect over a distance of say 100m?
Yes.

Could signals from multiple devices be distinguished?
That could be troublesome. I have only experience with analogue
signals.

Would the equipment be very expensive.
Analogue can be home-build

Would I get thrown in prison?
Depends on your country. Not here ( in the Netherlands ).

Gerard.