Dipole-mode propagation in hollow tubular waveguides

Guided dipole-mode propagation is best known in its role as a surface wave supported by a dielectric rod isolated in space. A development of that arrangement consists in the provision of an outer coaxial surface satisfying the boundary conditions at a finite radius. The field is found to take the form of a dual-dipole mode comprising a counterpart field on the inside of the outer surface. A further step eliminates the inner surface altogether and allows the same basic mode to propagate within a hollow tubular structure. The requirements are investigated and some experimental results are given.