Direct-Coupled TE-TM Waveguide Cavities

This paper 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 a TE-TM-TE three-section that generates 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, thanks 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. Position and orientation of the TE sections allow 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 concept while paving the way for a new class of direct-coupled TE-TM waveguide filters.