Security system transmission line

摘要

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.