Field strength uniformity control system for article theft detection system

Claim 2

In an article theft detection system the combination of at least two antenna windings mounted in the vicinity of a checkpoint along an article egress path, said antenna windings lying in different planes, an oscillator for generating high frequency electrical signals, signal transmission means connected between said oscillator and said antenna windings to energize said windings from said oscillator, said signal transmission means including means forming separate signal paths to the antenna windings lying in the different planes and phase shift means in at least one of said signal paths for producing a relative phase difference between the signals applied to the different antenna windings.

Claim 3

An article theft detection system according to claim 1 wherein the planes of said two antenna windings are arranged traversely to each other and wherein said phase shift means is operative to produce a net relative phase difference of 90* between the signals applied to the different antenna windings.

Claim 4

An article theft detection system according to claim 2 wherein at least one antenna winding encircles said egress path and lies in a plane perpendicular to said egress path.

Claim 5

An article theft detection system according to claim 3 wherein two additional antenna windings lie in planes parallel to said egress path.

Claim 6

An article theft detection system according to claim 4 wherein said one antenna winding is connected to be energized from one of said signal paths and wherein both of said two additional antenna windings are connected to be energized from another of said signal paths.

Claim 7

An article theft detection system according to claim 2 wherein separate phase shifters are provided in said one and another signal paths.

Claim 8

An article theft detection system according to claim 6 wherein one of said separate phase shifters is operative to produce a leading phase shift of approximately 45* and the other phase shifter is operative to produce a lagging phase shift of approximately 45*.

Claim 9

An article theft detection System according to claim 6 wherein said phase shifters have complementary frequency sensitivity characteristics whereby the same net phase difference is maintained at different signal frequencies.

Claim 10

An article theft detection system according to claim 6 wherein one phase shifter comprises a resistor and a capacitor connected in parallel with each other along one of said signal paths and said other phase shifter comprises a resistor and an inductor connected in parallel with each other along said another signal path.

Claim 11

Apparatus for generating an electromagnetic interrogation field at a given checkpoint through which protected articles must pass, said apparatus comprising at least two antenna windings lying in different planes in the vicinity of said checkpoint and means for applying electromagnetic signals of the same frequency but in phase shifted relationship simultaneously to said antennas.

Claim 12

Apparatus for generating an electromagnetic interrogation field at a given checkpoint through which protected articles must pass, said apparatus comprising at least two antenna windings lying in different planes in the vicinity of said checkpoint and means for applying electromagnetic signals of the same frequency but in phase shifted relationship simultaneously to said antennas.

Claim 13

Apparatus according to claim 10 wherein said antenna windings lie in planes which are substantially perpendicular to each other and wherein said means for applying electromagnetic signals includes phase shifter means operative to maintain a substantially 90* relative phase shift between the signals applied to the antennas in different planes.

Claim 14

An article theft detection system comprising a variable frequency oscillator, a tuning oscillator connected to said variable frequency oscillator for causing same to produce an output signal whose frequency varies cyclically at a rate corresponding to the frequency of said tuning oscillator, means defining an output signal path including a first junction connected to the output of said variable frequency oscillator and means forming a pair of branch signal paths leading from said first junction to associated antenna junctions, separate antenna windings connected respectively to said antenna junctions, said antenna windings being positioned at a given checkpoint and lying in different planes, detector means connected to said antenna junctions, said detector means being responsive to the changes in energy level at the junctions which occur at a rate related to the frequency of said tuning oscillator to produce an output signal, alarm means arranged to be actuated by said output signal from said detection means and phase shifter means arranged in at least one of said branch signal paths between said first junction and one of said antenna junctions.

Claim 15

An article theft detection system comprising a variable frequency oscillator, a tuning oscillator connected to said variable frequency oscillator for causing same to produce an output signal whose frequency varies cyclically at a rate corresponding to the frequency of said tuning oscillator, means defining an output signal path including a first junction connected to the output of said variable frequency oscillator and means forming a pair of branch signal paths leading from said first junction to associated antenna junctions, separate antenna windings connected respectively to said antenna junctions, said antenna windings being positioned at a given checkpoint and lying in different planes, detector means connected to said antenna junctions, said detector means being responsive to the changes in energy level at the junctions which occur at a rate related to the frequency of said tuning oscillator to produce an output signal, alarm means arranged to be actuated by said output signal from said detection means and phase shifter means arranged in at least one of said branch signal paths between said first junction and one of said antenna junctions.

Claim 16

An Article Theft Detection System according to claim 12 wherein said antenna windings include a first winding lying in a plane transverse to an egress path and a second winding lying in a plane extending laterally of said egress path.

Claim 17

An article theft detection system according to claim 13 wherein said phase shifter means is operative to produce a net phase difference in signals applied to said first and second windings of substantially 90*.

Claim 18

An article theft detection system according to claim 12 wherein said phase shifter means includes a pair of phase shifters each connected along a different one of said branch signal paths, one of said phase shifters being operative to produce a leading phase shift and the other being operative to produce a lagging phase shift.

Claim 19

An article theft detection system according to claim 15 wherein one of said phase shifters comprises a resistor and a capacitor connected in parallel with each other along one of said branch signal paths and the other phase shifter comprises a resistor and an capacitor connected in parallel with each other along the other of said branch signal paths.

Claim 20

An Article Theft Detection System according to claim 12 where said detector means comprises separate detector devices connected, respectively, to each of said antenna junctions.

Claim 23

A method according to claim 18 wherein said coils lie in planes which are mutually perpendicular and wherein said coils are energized 90* out of phase with respect to each other.