Answer Geek: How Electronic Tags Foil Theft

<br> -- Q U E S T I O N: How do those little sensor strips that are embedded in the cover of books in libraries and in products in the store work?

— Tyson B.

A N S W E R: Welcome to the world of “loss protection,” an entire industry dedicated to the art of preventing something called “shrinkage,” better known to the rest of us as “shoplifting.” How big a problem is shrinkage? Estimates of the dollar value vary widely, but here is a nice round number for the total value of goods that are secreted out of stores in the United States by nonpaying customers each year: $26 billion, according to a University of Florida study. That’s a lot of shrinkage!

So what’s a store owner to do? Watching every customer — either in person or with a closed circuit video camera — isn’t practical, and putting all of your goods in cases and behind counters isn’t good for business.

Electronic Tags

Instead, a lot of stores (and libraries) use something called Electronic Article Surveillance, or EAS. All EAS systems have three components: labels or hard tags that are attached to the merchandise; detectors within the tag or label that trigger an alarm if you try to leave a store without paying for an item; and deactivators, which allow a sales person to turn off or remove a tag or label once you’ve paid for an item.

There are four basic EAS systems used in stores today. The oldest type uses microwave technology. In these systems, the detector consists of two transmission antennas which send out two different signals: one is a high-frequency signal between 902 and 906 MHz, the second is a much lower frequency, around 111.5 kHz. The detector also includes a high-frequency receiver. The tags in these systems — which are usually those big hard plastic things that can make it a pain to try on an article of clothing — consist of a microwave diode and an antenna that can receive both the high and low frequency signals and also send a high frequency signal. If you are attempting a little “shrinkage” action with a piece of clothing sporting a microwave tag, when you walk by a detector the tag will read the two frequencies, combine them, and then retransmit them as a single signal. The detector then checks to make sure that the signal it is receiving is the right frequency (say, 903 MHz plus 111.5 kHz). If it is, the detector triggers the alarm.