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From: mb@camcon.co.uk (Mike Bell)
Newsgroups: sci.electronics,sci.physics
Subject: Re: Stolen-item detectors (Was: Laser eavesdropping)
Message-ID: <498@titan.camcon.co.uk>
Date: Fri, 24-Apr-87 11:34:32 EDT
Article-I.D.: titan.498
Posted: Fri Apr 24 11:34:32 1987
Date-Received: Thu, 30-Apr-87 02:32:48 EDT
References: <12813@watnot.UUCP>
Organization: Cambridge Consultants Ltd., Cambridge, UK
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Xref: mnetor sci.electronics:620 sci.physics:1272

A local company, Cotag, produces coded tags which are sometimes used
for this purpose - mainly on pricier items.

I seem to remember the method of operation was to receive rf on one
frequency (somewhere in the 10-100 KHz region) and retransmit a coded
echo on twice the input frequency.  

Not quite as neat (or cheap) as the gdo method, but less susceptible
to false alarms and useful for other purposes (security doors/FMS systems
etc.)

[No I'm not connected with Cotag - hence the vagueness]