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> Article <271618ef-4a7.1sci.electronics-1@vpnet.chi.il.us> From: cgordon@vpnet.chi.il.us (Gordon Hlavenka)

>Some reflect/absorb RF with a unique signature.  A detector then radiates
>some spread of RF, and looks for the signature.  This tag is deactivated by
>removing it from the goods.

The typical tags are passive resonators.  They are basically two antennas
coupled to each other, the receiver being tuned to something like 133 KHz
and the transmitter portion being tuned to 66.5 KHz, coupled through a
diode.  (The good ones are, anyway.  The cheap ones retransmit on the same
frequency as the transmitter, and are subject to more false positives, or
so I have been told.)  The antennas located at the doors transmit
(directionally) across the opening and attempt to receive the re-tuned
signal.  (I believe that they may perform some phase-shifting to eliminate
false reflections.)

I have seen two types of these tags:  reusable and disposable.  The
reusable tags have to be removed from the merchandise with a special
set of pliers that your average high school kid won't have in his
pocket.  The disposable type I have examined are aluminum foil shapes
cut to the proper dimensions, and the two halves of the circuit are
coupled with a fusible link.  The foil shapes are stuck on the back
of a label (very similar in size to an ordinary mailing label) that
has two holes in it.  The 'deactivator' is a two-pronged device that
simply applies a small voltage across the fusible link, burning it
out.  I suspect a 9 volt battery would accomplish much the same thing.

>The "magnetic" ones, I think, are probably decoys.  The whole system is to
>make you _think_ there's something going on, but even the clerks wouldn't
>know the truth.  This may be wrong in many cases, but it _is_ true in some.

The magnetic ones are very real.  I am not sure of the physics involved,
but there is a magnetic strip running between two parallel sets of wires.
I think the magnetic strip couples the two antennas together.

>One system I know of is used in books.  A strip about 1/8" wide by 2 or 3
>inches long is inserted into the binding of the book.  The strips themselves
>are pretty expensive, so stores that use them only protect some books.  I
>don't know how these are turned off.

Again, magnetic.  I've seen these in libraries.  They are reusable.  When
you czech the book out, they put the spine near a degaussing coil.  When
you czech the book back in, they simply polarize the strip inside the
binding again.
>
>I knew a guy a few years back who collected these tags.  He'd secrete the
>active portions in his wallet, pockets, etc.  And he'd set off alarms all
>the time.  This was his idea of fun -- personally I stayed away from him in
>malls.  He'd make up stories about having a plate in his head or somesuch,
>and explain this to security people.  A couple of stores tried to ban him on
>principle, but I don't know if anything ever came of it.

Sounds masochistic to me.  I personally wouldn't get much "excitement" out
of constantly getting busted...

-j, but an occasional bust now and then never helped anyone...
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