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From: per-el@obelix.UUCP
Newsgroups: sci.electronics
Subject: Stolen item-detectors (Was: Re: Laser eavesdropping)
Message-ID: <959@obelix.UUCP>
Date: Wed, 8-Apr-87 14:08:20 EST
Article-I.D.: obelix.959
Posted: Wed Apr  8 14:08:20 1987
Date-Received: Tue, 14-Apr-87 23:51:38 EST
References: <16143@sun.uucp> <2632@phri.UUCP> <1683@kitty.UUCP> <2634@phri.UUCP>
Reply-To: per-el@obelix.UUCP (Per Elmdahl)
Organization: University of Linkoping, Sweden
Lines: 33
Keywords: modulation

In article <2634@phri.UUCP> roy@phri.UUCP (Roy Smith) writes:
>	This is also, BTW, one of the ways those stolen-item detectors in
>stores work.  The big plastic clips contain some sort of passive tuned
>circuit, with a diode detector.  The gizmos on either side of the door emit
>RF at some frequency (presumably up in the several hundred Mhz range) and
>listen for harmonics comming back.....

  A girlfriend of a friend of mine found one detector in a box she had
got from a shop. I opened it and found a coil with about 10 turns and 
a capacitator in it. No diode! I was puzzled, because I did also think
it used harmonics for detection. I measured the resonant frequency
with a grid-dip meter, and found it to be about 5.5 MHz. Later I went
into a radio-shop and listened on a shortwave radio and found a strong
signal around 5.5 MHz. I started to think about what principle it worked
it used. 

 1. Send a signal and measure the loss when the resonant circuit enters
    the e-m field. (Would that work? Would it not be very sensitive for
    other things entering the field.)
 2. I talked to a service man at a shop. He was adjusting the boxes at
    the door. I looked into them but could not figure out how they worked.
    The service man did not seem to know either, only how to 
    adjust it. He talked about *two* frequencies, one at longwave and one
    at microwave, but I did not believe that. 
 3. (My favourite theory!) The boxes sends out pulses of 5.5 MHz radio
    energy. They deliver energy to the resonant circuit. When they stops
    transmitting, they start to listen. If a resonant circuit is near, 
    it will send out energy a short period of time after the excitation.
    The boxes hears that and starts the alarm.

Does anybody have other theories, or doues anybody know how this works?

        per-el@obelix.uucp