Detection of unauthorised objects passing through secure area e.g. for shop theft prevention by selectively receiving HF radiation to detect security element

摘要

The method involves one or more high frequency transmitters which generate a directional HF field in a secure area. A security element in this area generates a resonant HF oscillation and is excited to radiate at the excitation frequency or a mixture of excitation frequencies. At least one HF receiver and a signal analyser are arranged so that HF radiation within the region to be secured are selectively received, detected and analysed. The security element consists of a plastics card (1) on which one or more high frequency oscillation circuits are applied using an electrically conductive material. Alternatively, the element may include dipole antennas or individual radiators.