Dipole-type-mode propagation in a rectangular wave guide

The paper describes experimental evidence, supported by a simple theoretical analysis, of a new form of wave inside a rectangular-guide structure, two opposite walls of which present to the field anisotropic impedances. The mode concerned is of dipole pattern, belonging to the surface-wave family, and is not subject to cutoff. It is shown to be a slow wave, and to propagate along a guide of cross-sectional dimensions that are insufficient to provide for the support of any of the so-called waveguide modes. In the case considered, two walls of the guide facing each other consist of grids of spaced parallel wires running in the direction of propagation, while the other two surfaces comprise the usual continuous bare metal plates. The distinctive feature of the electromagnetic field over the transverse plane within this structure is its wave character between the opposite smooth metal walls, and its highly evanescent behaviour in the perpendicular direction. The spacing between adjacent conductors of the wire-grid surfaces is shown to affect the velocity of propagation of the wave along the guide, and the attenuation is found to be of the same order as that of TEM wave at the corresponding frequency. There are, in fact, prospects of reduced transmission losses at high frequencies using this dipole-type mode, which may be regarded as a perturbed TEM wave transforming more closely to that pattern as the frequency is reduced.