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From: crandell@ut-sally.UUCP (Jim Crandell)
Newsgroups: net.physics
Subject: Re: anti-shoplifting devices
Message-ID: <326@ut-sally.UUCP>
Date: Tue, 27-Nov-84 12:37:48 EST
Article-I.D.: ut-sally.326
Posted: Tue Nov 27 12:37:48 1984
Date-Received: Fri, 30-Nov-84 07:45:13 EST
References: <179@ihnet.UUCP>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 19

> Anybody know how those anti-shoplifting devices work?
> You know, those little plastic bars they stick on clothes and things,
> and terrible alarms go off if you sneak one past the store exit.

There may be numerous systems, but the one I've read about uses VHF
radio waves, and the ``tag'' itself is fairly passive, usually
containing just a resonant circuit, a diode and a small antenna.  The
alarm unit radiates a rather low-powered field in the vicinity of the
store exits, and the tag receives this (usually) dead carrier, but the
diode serves to distort the originally sinusoidal waveform so that it
contains numerous nonfundamental frequency components, which the
antenna then radiates, albeit weakly.  The alarm unit's receiver is
tuned to two or three times the transmitter's frequency, and so when a
tag is brought near it, it senses the radiated harmonic, and the alarm
goes off.
-- 

    Jim Crandell, C. S. Dept., The University of Texas at Austin
               {ihnp4,seismo,ctvax}!ut-sally!crandell