Direct-Coupled TE–TM Waveguide Cavities

This letter presents a novel structure where TE10 and TM11 waveguide sections are directly coupled along an inline configuration. Specifically, the basic configuration here reported consists of TE–TM–TE three sections that generate third-order filtering functions having one transmission zero. No dedicated coupling discontinuities (such as irises or slots) are included at the TE–TM interfaces. Hence, due to the higher impedance that the larger TM11 Section shows at one end of the smaller TE10 sections, TE modes resonating with a quarter-wavelength variation along the longitudinal direction become feasible. The adjustment over position and orientation of the TE sections allows to achieve a vast range of independent coupling coefficients, thus guaranteeing some of the largest design flexibility in terms of achievable bandwidths and full control of the transmission zero location. The experimental results of a three-section prototype validate the proposed concept while paving the way for the introduction of an innovative class of direct-coupled TE–TM waveguide filters.