Loop antenna for security systems

• US 4,373,163 A
• Filed: 07/14/1980
• Issued: 02/08/1983
• Est. Priority Date: 07/14/1980
• Status: Expired due to Term

• Current Assignee: Id Systems Incorporated (PowerFleet, Inc.)
• Original Assignee: I D Engineering Incorporated
• Inventors: Vandebult, Jan
• Primary Examiner(s): Lieberman, Eli
• Application Number:
  US06/168,484
• Time in Patent Office: 939 Days
• Field of Search: 343/842, 340/572
• US Class Current: 343/842
• 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

Disclosed is a loop antenna for the propagation of electromagnetic radiation with minimal electrostatic radiation for use in security systems. At least a single loop of conductor forms the radiating surface of the antenna. However, the loop is disposed in a tubular electrostatic shield which serves to minimize electrostatic radiation while not significantly interfering with the desired electromagnetic radiation. In preferred embodiments the electrostatic shield may be electrically grounded or a resistor may be placed in the conduction loop of the antenna to broaden the frequency response thereof. Additional preferred embodiments include twisting the single loop antenna one or more times such that two or more lobes are provided forming a twisted loop antenna. Far-field cancellation of transmitted signals is achieved and there is self cancellation in this antenna of received common mode signals. Additionally, the twisted loop antenna in a preferred embodiment has an electrostatic shield comprising a single unbroken loop of metal tubing around the periphery of the loop with at least one gap occurring in the electrostatic shielding where the shielding covers the conductor along the common side of adjacent lobes.

Claims

1. A loop antenna for propogating or receiving electromagnetic radiation but not electrostatic radiation in conjunction with a security system for identifying the presence or absence of at least one resonant tag circuit in the presence of said antenna, said security system including at least one transmitter and at least one receiver, said antenna defined as a twisted loop antenna comprising:

a conductor formed in at least one twisted loop wherein a portion of a single loop is twisted at least 180°

to form at least two adjacent, separate lobes of the same loop such that with a given current flow in said loop, said current circles in opposite directions in said adjacent lobes, said twisted loop having a perimeter and a common side between adjacent lobes;

electrostatic shielding surrounding said perimeter of said twisted loop and surrounding said common side, a first insulative gap formed in said shielding surrounding said perimeter, and a second insulative gap formed in said shielding surrounding said common side, wherein said conductor and said shielding comprise a coaxial cable; and

further comprising a cylindrical form around which said coaxial cable is wrapped to form said twisted loop in the form of a figure "8" with a cross-over between adjacent loops traveling circumferentially around said cylindrical form to produce said common side, and said shielding of said cable at said cross-over being isolated against electrical contact to form said second gap.

2. The loop antenna according to claim 1 and further including cylindrical means for covering said coaxial cable and said cylindrical form, said covering means including a base for mounting said cylindrical form.

3. The loop antenna according to claim 1 wherein said first gap comprises an opening in the shielding of said coaxial cable.

4. The loop antenna according to claim 1 wherein said shielded cable extends around opposite halves of said cylindrical form to form said cross-over.