| 摘要 |
A cable for use as a distributed antenna in intrusion detection systems comprises an internal open transmission line (10, 11; 50, 51, 52, 53; 610, 611) 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 (14; 55; 655), 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 s) of the external transmission line modes. Preferably the length of each transmission line segment is about one quarter of the wavelength ( lambda s) of the external transmission line modes. The inductive coupling may be provided by resistance wires (18, 19; 57; 618, 619) extending alongside the conductor elements. The open transmission line may comprise a two-wire line (10, 11), and the transmission line segments two arrays of conductor elements, associated with respective ones of the two wire. Alternatively, the open transmission line may be an open coaxial cable (50, 51, 52, 53) and the transmission line segments a series of cylinders (55) surrounding the coaxial cable. |
