Intruder detection system

Abstract

An intruder detection system comprising a line adapted to be located substantially at ground level and consisting of an elongated high permeability magnetic core on which is wound a coil in the form of successive sections of respectively opposite winding handednesses, a high gain low noise amplifier coupled to the line and a detector coupled to the amplifier and adapted to be coupled to an alarm system.

I claim: 
     
       1. A passive line for an intruder detection system operable in the earth's magnetic field, comprising: a. a plurality of elongated high-permeability wires arranged in lengthwise contact to define a flexible high-permeability magnetic core un-magnetized except for the effect thereon of the earth's magnetic field, the cross-section of the wires being very much smaller than the cross-section of the core; and   b. a coil wound on the core having at least one layer of insulated conductor wound on the core in the form of many successive lengthwise serially connected sections of respectively opposite-winding handedness, the sections being of substantially equal length and homogeneously wound.   
     
     
       2. An intruder detection system using the line of claim 1 comprising a high-gain, low-noise amplifier, and an impedance matching transformer coupling the line to the amplifier for detecting transient inhomogeneous magnetic fields in the vicinity of the line. 
     
     
       3. A line according to claim 1 including: a. a shield of braided metal encasing the coil and core to effect flexing of the line; and   b. an outer plastic envelope encasing the shield for waterproofing the line.   
     
     
       4. A passive line for an intruder detection system operable in the earth's magnetic field, comprising: a. a plurality of elongated high-permeability wires arranged in lengthwise contact to define a flexible high-permeability magnetic core un-magnetized except for the effect thereon of the earth's magnetic field, the cross-section of the wires being very much smaller than the cross-section of the core;   b. a coil wound on the core having at least one layer of insulated conductor wound on the core in the form of many successive lengthwise serially connected sections of respectively opposite-winding handedness, the sections being of substantially equal length and homogeneously wound; and   c. a second coil wound on the core and comprising at least one layer of insulated conductor wound on the first mentioned coil in the form of successive lengthwise sections of respectively opposite handedness, the sections of one coil being shifted lengthwise by about a half a section with respect to the sections of the other coil.   
     
     
       5. A passive line for an intruder detection system operable in the earth's magnetic field, comprising: a. a plurality of elongated high-permeability wires arranged in lengthwise contact to define a flexible high-permeability magnetic core un-magnetized except for the effect thereon of the earth's magnetic field, the cross-section of the wires being very much smaller than the cross-section of the core; and   b. a coil wound on the core having at least one layer of insulated conductor wound on the core in the form of many successive lengthwise serially connected sections of respectively opposite-winding handedness, the sections being of substantially equal length and homogeneously wound;   c. said coil comprising a plurality of layers of conductors, overlying sections in different layers having the same winding handedness.