Abstract: A detection system for detecting intruders moving in the vicinity of a defined path comprises a distributed antenna, for example an open transmission line, extending along the path, and an array of discrete antennas extending alongside the distributed antenna and within a predetermined distance therefrom. The discrete antennas and the distributed antenna define a plurality of detection zones. A radio frequency transmitter is connected to each of the discrete antennas, and a complementary receiver in a control unit at a remote location is connected to the distributed antenna. The control unit also controls the transmitters, and the array of antennas to exchange radio frequency energy between the distributed antenna and the discrete antennas and analyzes the energy received from the discrete antennas so as to detect perturbations caused by an intruder moving adjacent said path and adjacent a particular antenna.

Abstract: In a detection system for detecting intruders in the vicinity of a predetermined path or line defined by a distributed antenna, for example an open transmission line, an array of discrete antennas are provided alongside the distributed antenna and within a predetermined distance therefrom. The antennas are spaced apart from each other and the distributed antenna and define a plurality of detection zones. A radio frequency transmitter is connected to one of the distributed antenna and the array of discrete antennas, and a complementary receiver is connected to the other of the distributed antenna and the array of discrete antennas. A control unit controls the transmitter, receiver and array of antennas to exchange radio frequency energy several times via the distributed antenna and selected ones of the array of antennas and to detect perturbations caused by an intruder moving adjacent the path and adjacent a particular antenna.

Abstract: In an intrusion detection system of the kind which comprises a “leaky cable” or open transmission line and is used to determine the presence of objects, things or people moving in the vicinity of the leaky cable, a transmitting antenna and a receiving antenna, one of which is an open transmission line/leaky cable, are connected to a transmitter unit and a receiver unit, respectively. A control unit controls both the transmitter unit and the receiver unit, and causes the receiver unit to scan a preselected radio band, with the transmitter unit not transmitting, to detect one or more relatively quiet portions of the band in which instant received signal levels are lower than a predetermined threshold. The control unit selects a plurality of disparate frequencies in such one or more relatively quiet portions and subsequently causes the transmitter unit to transmit signals by way of the transmitting antenna at the disparate frequencies.

Abstract: Apparatus for monitoring opening of sealed enclosures, especially, but not exclusively, sealed containers containing goods-in-transit, such as goods vehicles, maritime containers or transport containers of the kind carried by tractor-trailers, ships, trains or aeroplanes comprises a radio receiver unit having at least one antenna for reception of radio signals. The receiver unit is housed within the enclosure and operates to scan a predetermined band of radio frequencies, periodically or continuously, and detect a sudden change in signal level. When it detects such a change in radio signal level, the receiver generates an alarm signal. Alternatively, the receiver may detect the difference between internal and external radio signal levels and determine the container to have been opened if the difference is less than a preset threshold.

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.