Abstract: A dual leaky coaxial cable comprising a pair of parallel elongated conductors, a dielectric surrounding each of the conductors, separate first external conductive shield apparatus surrounding at least the major portion of each of the dielectrics, separate second external shield apparatus surrounding each of the first external shield apparatus, apparatus associated with the shield apparatus for selectively coupling magnetic fields which may surround each of the elongated conductors through the first and second surrounding external shield apparatus, and apparatus for maintaining the individual respective second external shield apparatus separated by a distance which is a fraction of the diameter of either of the second external shield apparatus.

Abstract: The invention relates to a leaky cable intrusion detection system comprising a pair of spaced, parallel, buried, leaky coaxical cables. A radio frequency signal is applied to one of the cables, whereby an electromagnetic field outside said one cable is established, and a radio frequency signal from the field penetrates and is received from the other of the cables whereby disturbances in said field can be detected. The cables are spaced apart a distance such that they are intermediately coupled, being coupled to a greater degree than loosely coupled and to a laser degree than tightly coupled. In the preferred embodiment a longitudinal shield is spaced from and is below the cables. By varying the spacing between the cables and the shield, variations in field shape and sensitivity of the system can be substantially reduced.