Mode matching analysis of metallic blades in a cylinder

Scattering from a set of coplanar blades mounted on a centre shaft and located inside an open cylinder is investigated by the formally exact field matching technique. The blades are assumed to extend the entire radius of the cylinder. This allows the scattering from the blades to be treated rigorously. An incoming plane wave illuminates the cylinder. The circular guide modes induced at the aperture by this plane wave are found by field matching. These modes propagate down the cylinder and encounter first a circular guide-coaxial guide junction and then the coaxial guide-blade junction. Waves transmitted through the blades are assumed to be perfectly terminated. The set of coplanar blades is represented by a ring of sectoral waveguides of finite thickness. If the generalised scattering matrices (GSM) of the two junctions are known, they can be cascaded together with that of the intervening coaxial guide to give a complete characterization of the scattering by a set of blades. The GSM of the first junction, the circular guide-coaxial guide transition, may be readily obtained by matching the electric and magnetic fields at the interface, and then forming vector cross-products using the respective mode functions. The resulting mode coupling integrals between the circular guide modes and the coaxial guide modes may be evaluated in closed form. This procedure is straightforward. The S-matrix description of the coaxial guide-blade junction is considered next. Then results from an investigation on a set of blades in an electrically small cylinder are presented.