Magnetic antenna

• US 4,890,115 A
• Filed: 02/10/1988
• Issued: 12/26/1989
• Est. Priority Date: 02/10/1988
• Status: Expired due to Term

• Current Assignee: Sensormatic Electronics Corporation (Johnson Controls International plc)
• Original Assignee: Monarch Marking Systems, Inc. (Avery Dennison Corporation)
• Inventors: Hartings, Michael F.
• Primary Examiner(s): Hille, Rolf
• Assistant Examiner(s): Wimer, Michael C.
• Application Number:
  US07/154,655
• Time in Patent Office: 685 Days
• Field of Search: 343/842, 343/866, 343/867, 343/741, 343/742, 343/841, 343/870, 343/788, 343/728, 340/551, 340/552, 340/561, 340/572
• US Class Current: 343/742
• CPC Class Codes: H01Q 7/04 Screened antennas H01Q7/02, H01Q7/06 take precedence

Abstract

A magnetic antenna particularly usable for deactivating electronic article surveillance tags that are attached to articles of merchandise to prevent unauthorized removal from a protected area. The antenna comprises a driven element in the form of a transmitting loop, preferably a pair of loops driven in opposite phase, and shield grids disposed on opposite sides of the driven elements. The shield grids serve to prevent electric field radiation from the driven element from propagating past the shield, but permit the passage of magnetic field radiation therethrough. The antenna may also be used at a protected exit to detect the presence of a tag.

Claims

1. A magnetic antenna, comprising:

means including an active element having two loops disposed adjacent each other in a generally coplanar relationship for generating a strong field that has a limited propagation distance near the antenna, said loops being connected together in a parallel connection;

means connected to the parallel connection of said loops for applying signals to said parallel connection to thereby cause currents to flow in opposite directions in said loops; and

means including a shield structure having a pair of generally planar shields disposed on opposite sides of said active element in a generally parallel relationship therewith for preventing the passage of the electric field of an electromagnetic radiation therethrough while permitting the passage of magnetic field radiation.

2. A magnetic antenna as recited in claim 1 wherein said shield comprises a structure fabricated from a plurality of individual spaced conductors.

3. A magnetic antenna as recited in claim 1 further including means for connecting the shield to a source of common potential.

4. A magnetic antenna as recited in claim 1 wherein said generally planar shields have portions thereof that extend beyond said loops, and the portions of the generally planar shields that extend beyond the loops are turned to extend toward the loops.

5. A magnetic antenna as recited in claim 1 wherein said loops and said shields are fabricated from a plurality of self-supporting members.

6. An antenna as recited in claim 5 wherein said self-supporting members are spaced approximately 1/4" apart.

7. An antenna as recited in claim 6 wherein said shields have horizontal and vertical dimensions of approximately 9 inches for an antenna designated to operate at approximately 8 MHz.

8. A magnetic antenna, comprising:

means including an active element having two single loops disposed adjacent each other in a generally coplanar relationship for generating a strong field that has a limited propagation distance near the antenna, said loops being connected together in a parallel connection;

means connected to the parallel connection of said loops for applying signals to said parallel connection to thereby cause currents to flow in opposite directions in said loops; and

means including a shield structure having a pair of generally planar shields disposed on opposite sides of said active element in a generally parallel relationship therewith for preventing the passage of the electric field of an electromagnetic radiation therethrough while permitting the passage of magnetic field radiation.

9. A magnetic antenna, comprising:

means including an active element having two loops disposed adjacent each other in a generally coplanar relationship for generating a strong field that has a limited propagation distance near the antenna, said loops being connected together in a parallel connection at a point generally centrally located on the antenna;

means connected to the parallel connection of said loops for applying signals to said parallel connection to thereby cause currents to flow in opposite directions in said loops; and

means including a shield structure having a pair of generally planar shields disposed on opposite sides of said active element in a generally parallel relationship therewith for preventing the passage of the electric field of an electromagnetic radiation therethrough while permitting the passage of magnetic field radiation.

10. A magnetic antenna as recited in claim 9 wherein each of said two loops is a single loop.