We consider the excitation of monofilar and bifilar modes on a transmission line in a circular tunnel by a short dipole antenna. As expected, the monofilar mode is excited more strongly by an antenna placed in the tunnel, but the bifilar mode has lower attenuation. The excessive losses in the monofilar or coaxial mode are attributed to the return current flow along the tunnel walls. In the bifilar or transmissionhyphen;line mode the fields are more confined to the region of the wire conductors.
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