GBPPR 58 kHz EAS Jammer

Overview

Electronic Article Surveillance (EAS) is a technique certain stores use to prevent people from shoplifting.  One common method used by Sensormatic Retail Solutions involves a high-Q 58 kHz resonant tank circuit made using an amorphous-magnetic alloy inside a little sticker tag which is discretely attached to the item being monitored.

When the resonant tank circuit inside the sticker tag is remotely energized by a high-power pulsed 58 kHz illumination signal, the tank circuit will resonant (at 58 kHz) for a few milliseconds even when the illumination signal is removed.

This decaying resonance signal is fairly specific and easy to detect during the "receive cycle" of the EAS monitoring system.  If a decaying 58 kHz signal is detected, it's safe to assume an unauthorized item is leaving the store and the the anti-shoplifting system will signal an alarm.

Now, that's just a quick overview...  U.S. Patent 5,841,348 "Amorphous Magnetostrictive Alloy and an Electronic Article Surveillance" goes into much more technical detail on the actual construction and operation of the EAS tags themselves.  Also in that patent is the below diagram.  The diagram visually explains how the 58 kHz illumination signal is pulsed for about 200 milliseconds and then there is a series of 200 milliseconds receive windows where the system is listening for the decaying 58 kHz resonance "echo" caused by an anti-shoplifting tag.

As you can see, if one were to use a simple 58 kHz Continuous Wave (CW) jammer during the receive cycles, the system wouldn't be able to "listen" for a decaying resonance signal from a passing EAS tag.  Unfortunately, newer EAS systems have a "jammer detector" mode which will sound an alarm when remotely jammed or will silently notify security personal.  A remotely operated high-power "stand off" jammer may be used to trick the store personal into thinking something is wrong with their fancy EAS system...

Pictures & Construction Notes

Overview of an old induction telephone amplifier circuit.

The large coil of red enameled magnet wire will be used for the transmitting antenna in the 58 kHz EAS jammer.

The stock coil measured 87 millihenries.

To make it resonant at 58 kHz, solder a 82 pF capacitor in parallel with the coil.

You can make your own coil by winding a few hundred turns of #30 gauge enameled magnet wire in a similar fashion.  Use an inductance meter to check the coil until you get to around 87 mH or so.

Avoid using a ferrite core for the transmitter antenna coil as these are designed for receive-only applications.

Mounting the 58 kHz EAS jammer antenna.

It's mounted on a small piece of perfboard using 1/4-20 nylon hardware.

A BNC connector is used to connect the antenna to the transmitter circuit.

Overview of the 58 kHz clock generator and transmitter circuit.

The clock generator is based around a standard 1.856 MHz crystal and CD4049 hex buffer configured to make an oscillator.

This then feeds a series of CD4013 "divide-by-2" flip-flops.  The CD4013s eventually divide the 1.856 MHz signal by 32 to generate a final 58 kHz clock signal.

This 58 kHz clock signal then drives an IRF510 MOSFET with the 58 kHz resonant coil/capacitor antenna in series with the IRF510's drain line.

This isn't technically a real "transmitter," but it is a very simple way to generate and transmit a 58 kHz signal.

Mounting the 58 kHz clock generator and MOSFET driver in an old printer switch case.

The banana jacks on the upper-left are for the +12 VDC power input.

Above the banana jacks are a panel-mounted LED for a power indicator and a SPST power switch.

The panel-mounted isolated BNC jack is on the upper-right.  The BNC needs to be isolated to prevent shorting the drain line of the IRF510.

An easy way to generate a "higher power" jamming signal is to run the IRF510 at up to 100 volts.  There is an optional voltage select switch and banana jack input on the front-panel to allow applying a higher external voltage to the IRF510 drain.

If you don't have the ability to directy monitor 58 kHz, you can use these techniques.

Test the 58 kHz EAS jammer with a shortwave receiver tuned to 232 kHz, the 4th harmonic of 58 kHz.

The 9th harmonic is at 522 kHz which can be monitored on a standard AM radio.