ELECTRONIC THEFT DETECTION SYSTEM HAVING UNIFORM FIELD STRENGTH

摘要

1446259 Theft detection systems KNOGO CORP 9 April 1974 [13 April 1973] 15684/74 Heading G4N [Also in Division H4] A theft detection system comprises at least two windings 24, 36, 38 lying in crossed planes and an arrangement 10-18 for applying signals to the windings at the same frequency but differing in phase. The windings are placed at or around the exit through which protected articles having transponder tags must pass. An oscillator 12 is periodically swept around its centre frequency by a low frequency oscillator 10 and applied to detectors 26, 40 via phase shift networks 16, 18. A rotating magnetic field is produced at the monitored exit. In the presence of a tag or transponder having a resonant frequency at the centre frequency of oscillator 12, energy normally coupled to detectors 26, 40 is partially absorbed via windings 24, 36, 38 and this reduction of energy is detected by 26, 40 to trigger an alarm.