Multiple frequency theft detection system
First Claim
1. A swept frequency detection system for detecting resonant circuit targets attached to articles of merchandise located in an interrogation region, said targets being resonant, respectively, at different frequencies, said system comprising means for supplying, simultaneously, a plurality of swept frequency alternating electrical signals centered, respectively, at different frequencies, a plurality of transmitter antennas connected respectively, to receive an associated one of said alternating electrical signals centered at an associated one of said different frequencies and to produce corresponding electromagnetic waves, simultaneously, in an interrogation region, each transmitter antenna being formed in a plurality of loops offset from each other along a diagonal line such that the loops along each diagonal line produce electromagnetic waves centered about an associated one of said different frequencies, the diagonal lines of the respective transmitter antennas crossing each other, and a receiver arranged to detect disturbances to said electromagnetic waves produces by the presence in said interrogation region of a resonant circuit which is resonant within the frequency sweep of any of said alternating electrical signals.
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Abstract
A swept frequency theft detection system for detecting different resonant circuit targets which are resonant at different frequencies. The system comprises arrangements to generate swept frequency transmitter signals centered at different frequencies but which are swept in synchronism. Also provided are antennas formed by offset loops, with the loops of different frequency antennas lying along different diagonal lines.
83 Citations
27 Claims
- 1. A swept frequency detection system for detecting resonant circuit targets attached to articles of merchandise located in an interrogation region, said targets being resonant, respectively, at different frequencies, said system comprising means for supplying, simultaneously, a plurality of swept frequency alternating electrical signals centered, respectively, at different frequencies, a plurality of transmitter antennas connected respectively, to receive an associated one of said alternating electrical signals centered at an associated one of said different frequencies and to produce corresponding electromagnetic waves, simultaneously, in an interrogation region, each transmitter antenna being formed in a plurality of loops offset from each other along a diagonal line such that the loops along each diagonal line produce electromagnetic waves centered about an associated one of said different frequencies, the diagonal lines of the respective transmitter antennas crossing each other, and a receiver arranged to detect disturbances to said electromagnetic waves produces by the presence in said interrogation region of a resonant circuit which is resonant within the frequency sweep of any of said alternating electrical signals.
- 10. A swept frequency detection system for detecting resonant circuit targets attached to articles of merchandise present in an interrogation region, said targets being resonant, respectively, at different frequencies, said system comprising signal generating means for generating a plurality of swept frequency alternating electrical signals centered at different frequencies and swept together in synchronism, transmitter antena means arranged to receive said alternating electrical signals and to generate coresponding elecromagnetic waves in an interrogation region, and a receiver system arranged to detect disturbances to the electromagnetic waves produced by the presence, in the interrogation region, of a resonant circuit which is resonant within the frequency sweep of any of said alternating electrical signals and to generate an alarm in response to said detection.
- 13. A swept frequency detection system for detecting resonant circuit targets attached to articles of merchandise located in an interrogation region, said targets being resonant, respectively, at different frequencies, said system comprising a plurality of variable frequency oscillators, each being responsive to an applied sweep signal to shift its output frequency in accordance therewith, each variable frequency oscillator having a different center frequency, a sweep signal generator connected to apply sweep signals simultaneously and in synchronism to said variable frequency oscillators, transmitter antenna means connected to the output of said variable frequency oscillators to generate corresponding electromagnetic waves in an interrogation region, and a receiver system arranged to detect disturbances to said electromagnetic waves produced by the presence in said interrogation region of a resonant circuit which is resonant at any of the frequencies produced by any of said variable frequency oscillators and to generate an alarm in response to said detection.
- 18. A swept frequency detection system for detecting resonant circuit targets attached to articles of merchandise in an interrogation region, said targets being resonant, respectively, at different frequencies, said system comprising a swept frequency signal generator, transmitter antenna means connected to receive signals, via a plurality of signal channels, from said swept frequency signal generator, a frequency converter connected along at least one of said signal channels to convert the frequencies received from said signal generator to frequencies different from the frequencies in the other signal channels while maintaining the frequencies swept together in synchronism and a receiver system arranged to detect disturbances to electromagnetic waves produced by the presence of a resonant circuit target in the vicinity of said transmitter antenna means and to generate an alarm in response to said detection.
- 25. A swept frequency theft detection system for detecting resonant circuit targets attached to articles of merchandise located in an interrogation region, said targets being resonant, respectively, at different frequencies, said system comprising means for generating a plurality of swept frequency electrical signals each having a different center frequency, said signals all being swept in frequency simultaneously and in synchronism, a signal amplifier and summer connected to amplify and combine said electrical signals, a transmitter antenna connected to receive the amplified and combined electrical signals and to generate corresponding electromagnetic waves in an interrogation region, and a receiver system arranged to detect disturbances to said electromagnetic waves produced by the presence in said interrogation region of a resonant circuit which is resonant at a frequency within the frequency sweep of any of said swept frequency electrical signals, and to generate an alarm in response to such detection.
Specification