Electromagnetic Modal Analysis of Circular-Rectangular Waveguide Structures for Combline Filter Design

Abstract

<p>The rapid growth in mobile and satellite communications has intensified the requirements for good performance, compact structure, high quality and low cost waveguide filters and diplexers. This dissertation is devoted to the full wave analysis and modeling of various circular-rectangular (C-R) coaxial waveguide structures which are commonly used to develop combline filters and diplexers. Specifically, models that can be cascaded to simulate the system performance of the filters and diplexers are being sought in the dissertation. The research includes three parts: (1) modal analysis of the higher-order modes in the C-R waveguide; (2) modal analysis of the TEM mode in the CR waveguide; and (3) the scattering characteristics of the right-angle bend and the T waveguide junctions loaded with a generic post. A rigorous analysis, which combines the orthogonal expansion method and the Galerkin method, is performed to obtain the higher-order eigenmodes in the C-R waveguide. The Bessel-Fourier series is employed to merge the circular and rectangular coordinate systems used in the analysis. The cutoff frequencies of the higher-order modes are determined using the singular value decomposition (SVD) technique. The modal solution of the TEM mode in the C-R waveguide is obtained by superposition of the resonant modes in an equivalent rectangular cavity loaded with a conducting post. The characteristic impedance and attenuation coefficient of the waveguide are derived from the solution of the TEM mode. Analytic models of the right-angle bend and T -junctions loaded with posts of varying heights are derived. A novel technique of the extended eigen mode functions is developed to deal with the complex boundary conditions in the junction structures. The general scattering matrices of the right-angle bend and T - junctions are obtained.</p>