Electronic article surveillance system

• US 5,825,291 A
• Filed: 03/25/1997
• Issued: 10/20/1998
• Est. Priority Date: 04/10/1996
• Status: Expired due to Fees

• Current Assignee: Sentry Technology Corp.
• Original Assignee: Sentry Technology Corp.
• Inventors: Paul, Christopher Reinard, Platt, Victor, Zhou, Peter Y., Nicolette, Thomas A.
• Primary Examiner(s): TONG, NINA C
• Application Number:
  US08/827,041
• Time in Patent Office: 574 Days
• Field of Search: 340/572, 340/571, 343/742, 343/842, 343/867, 343/744, 343/841, 343/853, 343/741
• US Class Current: 340/572.7
• CPC Class Codes: G08B 13/2474 Antenna or antenna activator geometry, arrangement or layout loop antennae H01Q1/22
• CPC Class Codes: H01Q 7/04 Screened antennas H01Q7/02, H01Q7/06 take precedence

Abstract

An electronic article surveillance system capable of reliable operation at high frequencies such as about 8 MHz, has an arrangement of transmitter and receiver antennas each lying in a flat, horizontal plane in substantial alignment with each other, with the receiver antenna positioned at the floor of a passageway at an interrogation zone and the transmitter antenna positioned overhead. Each antenna takes the form of a pair of parallel-connected, coplanar loops with the transmitter-connected loops being positioned such that electrical current flows around each loop in parallel in the same direction (a ++ configuration) and in the receiver antenna electrical current flows around each loop in parallel in mutually opposite directions (a +- configuration). The antennas are approximately of the same size and overall configuration, and they are positioned in substantial alignment so that the currents induced in the receiver antenna by the field from the transmitter antenna will effectively cancel. Current variations caused by the passage of a target through the interrogation zone will be greater in some of the conductors of the receiver antenna than in others and so can be detected.

Claims

1. An electronic article surveillance system for producing an alarm signal when an article of merchandise having a responder attached thereto is carried through an exit passageway, said system comprising an interrogation signal generator connected via a first pair of terminals to a transmitter antenna for generating electromagnetic waves in an interrogation zone and a receiver connected via a second pair of terminals to a receiver antenna for receiving electromagnetic waves present in said interrogation zone, said receiver including signal processing circuits for detecting predetermined characteristics in the received waves to generate an alarm, said transmitter antenna extending in a horizontal plane above said passageway and said receiver antenna extending in a horizontal plane below said passageway, each of said antennas being formed of a plurality of loops which extend over different, non-overlapping areas in a common plane and which are electrically connected in parallel to its respective pair of terminals, the loops of said receiver antenna being connected such that currents flow in parallel therein in mutually opposite directions and the loops of said transmitter antenna being connected such that currents flow in parallel;

therein in the same direction.

2. An electronic article surveillance system according to claim 1 wherein said one of said antennas is formed of two loops and wherein its respective pair of terminals is located between said loops.

3. An electronic article surveillance system according to claim 2 wherein each loop of said one of said antennas is mounted on a member in such manner as to be rotatable about that member about a line that extends between said loops.

4. An electronic article surveillance system according to claim 1 wherein said transmitter antenna and said receiver antenna are each formed of two parallel connected loops, each of said loops comprising a single turn.

5. An electronic article surveillance system according to claim 1 wherein said one of said antennas is said transmitter antenna, each of the loops of which is a single turn, each loop being formed of a metal pipe.

6. An electronic article surveillance system according to claim 1 wherein said one of said antennas is said receiver antenna, each of the loops of which is a single turn, each half of each loop being twisted along its length with a separate grounded conductive wire.