Answer Geek: How Electronic Tags Foil Theft

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.

The only way to disarm this system is to remove the tag. Occasionally, someone forgets to remove it and the tag fails to trigger the alarm. That’s one of life’s more annoying retail moments: getting home with a spiffy new sweater only to realize you can’t wear it because it still has that clunky tag on it. Actually, it happens more than occasionally: these systems fail about 20 percent of the time. But I did mention that this is an old technology?

Radio Systems

More common these days, especially in the United States, are radio frequency (RF) systems. Detectors in these systems use two pedestals which serve as gates that you pass between when you leave a store. One is a transmitter that sends out a signal (the normal range is 7.4 to 8.8 MHz). The other pedestal is a receiver. RF systems use small flat, usually square-shaped, labels that have an electronic circuit containing a capacitor and an inductor. When the label receives a signal from the detector, it energizes the capacitor and the inductor, which causes them to send energy back and forth at a specific frequency. The tag then transmits a signal that is picked up by the receiver. The receiver also picks up the signal sent by the transmitter in the other gate. It compares the two signals and if the difference is correct, the alarm goes off. When you pay for an item with an RF tag, the salesperson exposes the tag to a very strong RF pulse that burns out the components of the electronic circuit, rendering the tag incapable of transmitting a signal.

In Europe, many stores use an electromagnetic (EM) system in which a low frequency electromagnetic field is generated between the two pedestals or gates. That field changes strength and polarity, switching back and forth from positive to negative. The labels in EM systems have a small embedded wire that changes magnetic state when it is exposed to the changing electromagnetic field. Each time the wire flips from positive to negative, it generates a signal which is picked up by a receiver in the detector. EM labels are easy to deactivate — exposing them to a magnet does the trick — and reactive, a feature that makes them ideal for use in libraries hoping to cut down on book shrinkage.

AM Systems

A fourth system, called acousto-magnetic, or AM for short, sends out a radio frequency signal (58 kHz is typical) in short bursts, transmitting anywhere from 50 to 90 pulses per second. AM system labels have a resonator and a magnetic strip. The radio frequency burst makes the resonator vibrate, initiating a signal that is tuned to match the original signal from the transmitter. The resonator works like a tuning fork, sending out a sustained signal and if the receiver continues to detect the proper radio frequency in between pulses, the alarm is triggered. The label can be turned off by demagnetizing it.

Until recently, adding sensor tags and labels was left up to the store. The newest trend is called source tagging. With source tagging, a security label is added by the manufacturer, embedded in the packaging, say, or placed under a label, or even woven directly into an article of clothing. There is talk that these tags will eventually be used to help with inventory control as well as shoplifting.

So how useful is “loss prevention” in reducing “shrinkage”? Industry estimates are that EA systems can reduce theft by up to 60 percent. EAS systems are such a strong disincentive to would-be shoplifters that some stores have taken to putting fake detector gates and fake tags on items, which provides at least some of the deterrence at a much reduced cost. Such dummy systems also increase the likelihood that you’ll end up with a brand new article of clothing with a plastic tag still attached. I don’t know about you, but I hate it when that happens.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.