Electromagnetic detection system of high sensitivity.

摘要

An electromagnetic detection system of the field disturbance type, in particular suitable for use as an electronic anti-shop-lifting system. The system comprises a transmitter/receiver (10,40) including a transmitting/receiving circuit having a coil (20) for generating an interrogation field in a zone to be guarded, and a sweep signal generator (12) for periodically varying the frequency of the interrogation field within a pre-determined frequency range. The system further includes a plurality of detection plates (30) provided with at least one resonant circuit whose resonant frequency is within said frequency range. According to the invention the frequency tuning of the transmitting/receiving circuit is variable and means (24) are provided for causing the attunement of the transmitting/receiving circuit, in operation, to follow the varying frequency of the interrogation field in synchronism therewith.