Direct-Coupled TE–TM Waveguide Filters

This article presents a new class of bandpass filters, which extends the state-of-the-art of direct-coupled waveguide cavity structures. The new filter class possesses the strengths of the most advanced pseudo-elliptic nonresonating mode TM cavity filters while maintaining the versatility of realizing both narrow and wide passbands as is for the most conventional inductive iris direct-coupled waveguide filters. The key characteristic of the proposed filter class is that TE10 and TM11 sections are directly coupled to each other without using any dedicated coupling element while still making it possible to fully control all the coupling coefficients by solely adjusting the position and orientation of the sections. After an initial design based on coupling matrices, conventional and/or mixed lumped-distributed equivalent circuits, coupling curves, and simple design guidelines, the optimization of these structures can be carried out using well-known mode-matching and integral equation-based techniques, as well as the more recent full-wave model of the electromagnetic coupling matrix. Several configurations and filter examples, including the experimental results of an ${X}$ -band SATCOM bandpass filter, are presented to validate the effectiveness and versatility of this new general class of waveguide bandpass filters.