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From: svaldez@sdcc13.ucsd.edu (Basshead )
Newsgroups: sci.electronics
Subject: Re: Library anti-theft walk-thru device...????
Summary: defeating system
Message-ID: <19122@sdcc6.ucsd.edu>
Date: 8 May 91 08:02:57 GMT
References: <050691.224256WDBURNS%MTUS5.BITNET> <19934@slice.ooc.uva.nl>
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In article <19934@slice.ooc.uva.nl>, ropg@ooc.uva.nl (Rop Gonggrijp) writes:
> They mostly work as follows:
> 
> In the book is a strip of metal (any metal) covered by a piece of ferrite.
> The ferrite is magnetised, thus changing the frequency at which this gizmo
> resonates (it is covered in plastic, so it does resonate).
> 
> If they demagnetize the ferrite the thing goes to a lower resonating freq.
> 
> One end of the gate emits the first (high) frequency and the other end detects
> it. A sudden drop in power means something is 'stealing' energy / resonating
> and thus the alarm is activated.
> 
> These systems usually work at 915 MHz or so....... Have fun beating it
> 
> Oh, by the way, if you transmit this freq. and then play with the transmitted
> power you should be able to make the bell ring at quite considderable distance.

This is a reasonable assumption, but if there is any scientific 
argument against what I say, please correct me.

In clothing stores at the local mall, I've heard that thieves are
able to often defeat the security systems a lot of stores use --
the ones with the detectors across the entrance and the big plastic
tags on the unpurchased clothing which is removed at the counter --
anyways  they are able to shoplift merchandise by placing clothes
into a bag completely lined with foil...  

Now, I assume if you did the same to a book you could theoretically
shield the book (completely wrapped) from the RF energy emitted
by the gates.?  I'm not an EE major but this seems reasonable -- 
??????


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