MODAL ANALYSIS OF A LEAKY FEEDER CABLE MODELED BY A SHEATH HELIX WITH A PERFECTLY CONDUCTING CORE

Leaky feeders have applications in restricted areas where wireless communications are difficult such as mines and tunnels or when only localized broadcast is desired. Methods of analysis of the electromagnetic modes for typical problems usually involve approximation techniques. The cable is replaced by simple parameters derived from solution of the cable in an infinite medium.;Results agree with those of previous studies involving the sheath helix and also with Hill and Wait's results using the wire helix model. Properties of modes found for the cable in infinite free space, buried in an infinite lossy medium, and centered in a tunnel are consistent with those found by investigators for other types of cables.;An interesting property of the sheath helix is the variation of its surface transfer impedance with helix angle. Most of the results are for a fixed helix angle. From the interpretation of the numerical results however it is suggested that study of this model is useful beyond its direct application to modeling of the helical sheathed cable.;In this work modes of a leaky feeder cable in tunnels and buried in earth are investigated by modeling one type of cable as a sheath helix with a perfectly conducting core. The sheath helix model is a good representation for a helical shield when the helical period and radius is much less than the modal period. This model allows the rigorous formulation of the eigenmode problem of the cable centered in a tunnel or buried in earth. Numerical techniques are used to find the modes and disperson curves are plotted.