A general study of the leaky-feeder principle

The principle of using the leakage fields of a transmission line to provide mobile radio communication in tunnels has been studied theoretically and experimentally. A mode-conversion theory is developed to account for the fields observed round imperfect bifilar and coaxial lines, and predictions from it are confirmed by observation: the internal velocity ratio of the cable used is an important factor in the coupling achieved, and standing-wave effects are normally to be expected at both ends of such a line regardless of impedance matching. For any given line and operating conditions an optimum frequency can be shown to exist. Systems aspects are considered briefly and a proved technique for providing duplex two-way communication using one-way line repeaters is described.