Dual frequency anti-theft system
First Claim
1. A system for detecting the presence of an article within a surveillance area comprising:
- (a) transmitter means for radiating two radio frequency signals at two distinct different frequencies within the surveillance area, said radio frequencies being sufficiently close together to be received by a single transponder;
(b) said transmitter means including antenna means for each of the said two radio frequency signals arranged so that the ratio of field strengths of said two signals is substantially uniform throughout the surveillance area;
(c) transponder means removably affixed to protected articles capable of being moved with an article into said surveillance area, said transponder means having a antenna tuned to receive the radio frequency signals transmitted at both frequencies and a non-linear impedance element coupled to said antenna means, whereby said transponder means reradiates a return signal having a frequency equal to the sum of the frequencies of the two transmitted radio frequency signals;
(d) narrowband receiver means for receiving said return signal to the exclusion of the transmitted radio frequency signals and their harmonics; and
(e) alarm means responsive to the detection of said return signal by said narrowband receiver means.
3 Assignments
0 Petitions
Accused Products
Abstract
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 the metal loop forming a tuned tank circuit that resonates at twice the selected center frequency. Two transmitters generate separate tone modulated radio frequency and continuous wave radio frequencies equally displaced on opposite sides of the center frequency that are fed to respective orthogonally disposed dipole radiating antenna strips on opposite sides of a surveillance area. The dipole strips for different frequencies are at right angles on each side, while those for the same frequency are at right angles to one another on opposite sides to achieve cross polarized transmission of both signals within the surveillance area. These signals are mixed by summing in the non-linear impedance to resonate the antenna tank circuit at double the center frequency which is reradiated to the receiver antennas on each side where a very narrow band receiver detects it. The modulating tone signal derived from demodulating the detected signal produces a gradually increasing charge that is compared against a preselected threshold to trigger an alarm whenever the detected signal is of sufficient strength and duration.
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Citations
13 Claims
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1. A system for detecting the presence of an article within a surveillance area comprising:
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(a) transmitter means for radiating two radio frequency signals at two distinct different frequencies within the surveillance area, said radio frequencies being sufficiently close together to be received by a single transponder; (b) said transmitter means including antenna means for each of the said two radio frequency signals arranged so that the ratio of field strengths of said two signals is substantially uniform throughout the surveillance area; (c) transponder means removably affixed to protected articles capable of being moved with an article into said surveillance area, said transponder means having a antenna tuned to receive the radio frequency signals transmitted at both frequencies and a non-linear impedance element coupled to said antenna means, whereby said transponder means reradiates a return signal having a frequency equal to the sum of the frequencies of the two transmitted radio frequency signals; (d) narrowband receiver means for receiving said return signal to the exclusion of the transmitted radio frequency signals and their harmonics; and (e) alarm means responsive to the detection of said return signal by said narrowband receiver means. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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Specification