Lately, I've been curious about those security scanners commonly used in libraries and department stores. It appears to be a long range metal detector and yet is in-sensitive to belt buckles, keys and other metal objects that one carries. I don't think that it is RFID technology. So what is it that gets interrogated by the scan antennas?

Cheers, DPW [ Everything has limitations...and I hate limitations.]

 

One system used metalic strips which emitted a weak RF burst when hit with a magnetic pulse. Strip placed in book spline.

 

They are pretty neat. They are basically EM antenna that absorb a particular frequency. there is a weak magnet inside that augments or retards the shape of the antenna causing a short between legs of the antenna, allowing it to react in an allowable frequency. Degaussing, after the book is returned, re-sets the antenna to its non shorted, reactive frequency.


The permanent reference books that are not allowed to be taken out of the library, have no ferrous short-creating shunt-type jammie in the tag, so it can not be "deactivated".

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Here you go, wikipedia has a pretty good page on the devices:
http://en.wikipedia.org/wiki/Electronic_article_surveillance
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