Quasi-Elliptic-Type Inline Waveguide Filters With Mutually-Coupled Rotated-Dual-Post Frequency-Variant Coupling Inverters

This work presents the design techniques to realize quasi-elliptic-type inline waveguide bandpass filters based on mutually-coupled rotated-dual-post (MCRDP) nonlinear frequency-variant coupling structures. A pair of closely-spaced partial-height posts placed inside the waveguide allows to pro- duce a first-order bandpass filtering transfer function with two transmission zeros (TZs). The pole can be moved either below or above the TZ pair by separating the posts laterally along the length of the waveguide and by controlling the sign of the mutual coupling between the posts. The mutual-coupling sign can be reversed by placing both posts either at the same or at opposite broad walls of the waveguide. EM simulated results for two third-order filters with a pair of TZs located above and below the passband are provided to validate the proposed design theory.