| 摘要 |
An article surveillance system employs a label or tag containing a non linear impedance element, such as a semiconductor diode, connected to a metal antenna loop configured to pick up two distinct radio frequency transmissions displaced on either side of a selected center frequency. The non-linear impedance element connects opposing sides of a closed loop section at one end of the antenna to form a tuned tank circuit having a resonant frequency twice that of the selected center frequency. A first transmitter generates a tone modulated radio frequency displaced on one side of the center frequency, and a second transmitter generates a continuous wave radio frequency displaced from the center frequency on the other side. Both transmitter signals are fed separately to respective orthogonally disposed dipole radiating antenna strips located on opposite sides of a surveillance area with the dipole strips for the different frequencies being disposed at right angles on each side while those for the same frequency are also at right angles to one another on opposite sides, thus producing cross polarized transmission of both frequencies within the surveillance area. The two different frequencies picked up by the transponder antenna are mixed by the nonlinear impedance causing the tank circuit to resonate at a single higher frequency equal to their sum, which is double the center frequency; that resonant frequency is reradiated to be picked up by receiver antennas on each side to be detected by a very narrow band receiver responsive to the sum frequency. The modulating tone signal is derived from the received signal to produce a gradually increasing charge that is compared against a preselected threshhold level to trigger an alarm for a fixed interval only when the detected signal is of a sufficient strength and duration. |
