This document describes how the Sensormatic electrostatic/electromagnetic electronic alarm systems work. (record and clothing stores). The following data is provided for informational purposes only, and no guarantee is made to the accuracy of the contained information.
These systems work on a mixer principle. In the control area (typically at the exit of a store) a 915 Mhz radio signal and an approximately 100 khz modulated electric field signal is radiated into the control zone. The tags (which consist of a schottky barrier diode or PIN diode between two antenna elements) generate a signal which is the original 915 Mhz signal modulated with the 100khz signal.
This modulated signal is detected, and sets off the alarm.
Some relevant patents to the method are US 4642640 and US 4139844. The system described in patent US 5349332 describes a very sophisticated implementation of this idea.
System has high pick rate, large exit zone, controllable transition zone. Good resistance to false signals, tags can be deactivated in a manner that doesn't damage magnetic materials. Newer reusable tags are quite light and don't interfere with customer handling of product.
Use of active components in tags could increase cost of these items. Disposables possibly vulnerable to ESD.