Electronic goods monitor detecting presence of indicator

摘要

To detect the presence of a marker 60, an electronic article-surveillance apparatus includes a transmitting circuit 10 and a transmit antenna, 12 and a receiving antenna 16 next to the transmit antenna 12. An electromagnetic signal is generated within a controlled area, the signal being received by the receiving antenna 16. When the marker 60 is in the controlled area, a disturbance is created in said electromagnetic signal and is picked up by the receiving antenna 16. The disturbance is the marker signal. An electronics module 18 receives the electromagnetic signal picked up by the receiving antenna, and a feedback conduit, linking the electronics module 18 with the transmitting circuit 10, controls the amplitude of the electromagnetic signal so as to maintain the marker signal at a constant amplitude and shape. A detection module 20 analyzes the marker signal. An alarm circuit 22 is actuated by the detection module when the latter confirms the presence of the marker signal having predetermined features.