Loop antenna for security systems

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.