Abstract: An open transmission line system for locating an entity (110) moving along a defined pathway uses transmissions from a remote, independent transmitter (106), conveniently a commercial radio or television station. The system comprises two receivers, both tuned to the frequency of the transmission. One receiver (101) receives signals directly from the transmitter (106) by means of an antenna (104). The other receiver (102) receives the signal from the transmitter (106) by way of an open transmission line (107). A processor (103) processes the signals from the two receivers to detect the entity. The system may use transmissions at different frequencies from plural mutually-spaced transmitters (402; 403; 404) in order to provide reliability and to minimize the effects of multi-path signals. The system may employ a variable velocity open transmission line (507), comprising a central conductor means (1100) having a permeable central element (1202) carrying a helically wound conductor (1201).

Abstract: In an intrusion detection system comprising an open transmission line (10), for example a "leaky cable", and a receiver unit (12S,12T,14S,14T,16) for processing a radio frequency signal received by the line to detect perturbations caused by an intruder (28), problems of nonlinear sensitivity due to line attenuation are resolved by combining perturbation signals (R.sub.S, R.sub.T) from both ends of the open transmission line. A processor unit (18) computes logarithmic values of the perturbation signals from the opposite ends of the line and combines them to detect presence of an intruder. The sum of the logarithmic values may be used to determine presence of the intruder and the difference between them may be used to determine the location of the intruder relative to the end of the line. The processor may also use phase information from the two perturbation signals to facilitate detection in the presence of multipath effects. Reference signals (LO,IF.sub.

Abstract: A cable for use as a distributed antenna in intrusion detection systems comprises an internal open transmission line in parallel with a periodically loaded structure which supports an external transmission line mode of operation, specifically a surface wave, and is "driven" by the open transmission line. The periodically loaded structure may comprise transmission line segments for example cylindrical conductor elements intercoupled by serial inductive coupling. There are no fewer than three and no more than fifteen conductor elements in one wavelength (.lambda..sub.S) of the external transmission line modes. Preferably the length of each transmission line segment is about one quarter of the wavelength (.lambda..sub.S) of the external transmission line modes. The inductive coupling may be provided by resistance wires extending alongside the conductor elements.

Abstract: A sensitive intrusion detection system has an RF excited antenna located within the area to be protected and a leaky coaxial cable extending around the perimeter. The presence of an intruder alters the coupling between the antenna and the coaxial cable thereby changing the signal received by the cable. The detection system is responsive to incremental changes in the in-phase and quadrature components of the received signal. When these components are plotted against each other a cardioid-like curve is obtained in the .DELTA.I,.DELTA.Q plane. By tracking both magnitude and angle of this curve as it is generated a sensitive detection mechanism is provided. When the variations in magnitude and angle exceed a threshold an alarm is sounded. To avoid the possibility of intruders using a particular path would gives a null angle response, a second cable adjacent to the first may also be employed.

Abstract: A method of manufacturing leaky coaxial cables having an array of apertures in the conductive outer layer. The total area of the apertures is a predetermined fraction of the surface area of the cable. A pair of strip conductors of particular widths are selected and wound around the inner conductor and dielectric at predetermined pitch angles. This provides apertures having a total area which is a predetermined fraction of the surface area of the cable, a predetermined shape and being of a predetermined number per unit length. By varying the pitch angle during winding the distribution of apertures and hence the coupling of the cable can be varied. By testing short sections of cables of different geometry a coupling function and an attenuation function can be calculated to provide data for winding cables with desired characteristics.