| 摘要 |
An open transmission line system for locating am 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 multipath signals. The system may employ a variable velocity open transmission line (509), comprising a central conductor means (1100) having a permeable central element 91202) carrying a helically wound conductor (1201). The inductance of the central conductor, and hence the propagation rate of the line is varied by means of a periodically varying signal (V~) applied to the conductor. The entity causes different phase variations at the different propagation rates. The processor (109) compares these phase variations to locate the entity.
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