| 摘要 |
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 receivin g antenna, one of which is a leaky cable, and connected to a transmitter unit and a receive r unit, respectively. A control unit controls both the transmitter and the receiver, and causes the receiver to scan a preselected radio band, with the transmitter not transmit ting, to detect one or more relatively quiet portions of the band in which instant receiv ed signal levels are lower than a predetermined threshold. The control unit selects a plur ality of disparate frequencies in such one or more relatively quiet portions and subseque ntly causes the transmitter to transmit signals by way of the transmitting antenna at the disparate frequencies. The receiver unit receives signals corresponding to the t ransmitted signals and the control unit detects perturbations in the received signals cause d by an intruder in the vicinity of the leaky cable to determine the presence of an intr uder. The disparate frequencies may have a bandwidth equal to at least 5 per cent and pref erably about 10 per cent of their mean frequency. Operation in the quiet bands allows frequency diversity to be employed, reducing the effects of standing waves and a llowing the leaky cable to be deployed without a surrounding electrically-lossy medium, for example above ground, while providing uniform detection sensitivity.
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