Supermarket theft detection system - uses magnetic marking strip which modifies base frequency signal from two parallel frame antennae

摘要

The theft detection system using a modulated carrier signal is used as a means to locate piffered goods within a super market system. Each article has a marking strip in the form of thin laminates of ferro magnetic material. Typically the strip is 1mm wide and 0.01 mm thick. The alarm system is based upon modification of a base frequency signal that has superimposed impulses when the marking strip is present. A transmitter (8) outputs two phase locked frequencies (f, 2f) to two parallel frame antennae (5) which have associated receiver antennae (7) coupled to the detector receivers (9).