Crossed beam high frequency anti-theft system
US4700179A · kind A · utility
Assignee
Inventor
Key dates
| Filing date | Jul 11, 1986 |
| Grant date | Oct 13, 1987 |
| Priority date | — |
| Expiry date | Jul 11, 2006 |
Classification
- Technology area (CPC G)Physics
- CPC primaryG08B13/24
- WIPO fieldControl
- WIPO sectorInstruments
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 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 radiating antennae located adjacent to a surveillance area. The antennae are chosen to produce circularly polarized transmission of both frequencies within the surveillance area. The two different frequencies picked up by the transponder antenna are mixed by the non-linear impedance causing the tank circuit to resonate at a single higher frequency equal to their sum, which is double the center frequency; that r…
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.