Patent ID: 7479871

Claim:
A ferromagnetic threat warning system, comprising: a first radio unit mountable to a ferromagnetic threat object; a second radio unit mountable at a door to an area to be protected against entry of a ferromagnetic threat object, at least one of said first radio unit and said second radio unit being adapted to transmit radio frequency signals, said signals being adapted to indicate whether said signals are transmitted by said first radio unit or said second radio unit, at least the other of said first radio unit and said second radio unit being adapted to receive radio signals; a signal strength sensor in at least said other of said first radio unit and said second radio unit, said signal strength sensor being adapted to measure the strength of received radio frequency signals; and a processing device in at least said other of said first radio unit and said second radio unit, said processing device being adapted to generate a ferromagnetic threat proximity alarm when the following conditions are met: (1) said measured strength of a received radio frequency signal reaches a predetermined level, said predetermined signal strength level being selected to indicate approach of said first radio unit within a preselected distance of said second radio unit; and (2) said other of said first radio unit and said second radio unit identifies said received signal as being transmitted by said one of said first radio unit and said second radio unit.