WAVE-PROPAGATION IN A 3-DIMENSIONAL DOUBLY PERIODIC PARALLEL PLATE WAVE-GUIDE

The stopband structure of a three-dimensional waveguide with doubly periodic corrugations is studied. A preliminary investigation using straightforward perturbation theory gives a formula for the resonant frequencies whereas the main part of the paper is devoted to the null-field approach. In the numerical examples the waveguide is assumed to be hard-walled with symmetric, antisymmetric or one-sided corrugations which are, moreover, doubly sinusoidal. The interaction pattern and the stopband widths are determined for the lowest modes. The resonances are found to be of stopband or crossover type depending on the relative directions of propagation of the resonating modes and it appears that geometrical symmetries of the boundaries suppress well-defined sets of resonances.