Design Methodology of High-Selectivity Waveguide Filters With Post-Type Coupling Structures

This article discusses the post-type coupling structures in the waveguide filter designs. In these filter configurations, a post structure with a resonant frequency (TEM mode) much lower than the center frequency is used to couple the two modes in the waveguide. The redundant TEM mode strongly couples with the other modes and acts as a nonresonating node (NRN). This provides a flexible bypass coupling path for configuring the transmission zero (TZ). Alternatively, the post can be considered a pure coupling structure that competes with other types of coupling to form a mixed-coupling structure that introduces TZs. An interesting conclusion can be drawn: when the redundant mode is far from the passband, a coupling path incorporating an NRN and a parallel bypass coupling path is approximately equivalent to a linear frequency-dependent coupling (FDC). This work tries to combine these two different perspectives to analyze the same structures in waveguide filters. Further exploration of the relationship between the two circuits reveals how to obtain a large slope parameter for the FDC coefficient. This methodology can also be used to analyze nonlinear FDC structures in waveguide filters, which enables their synthesis using linear FDC topologies. Two coupling structures for single-mode filters, including linear and nonlinear FDC structures, are presented to verify the analysis of introducing extremely-close TZs. Additionally, as a specific implementation of the methodology, the design of dual-mode filters with nonlinear FDC structures is presented, illustrating its utility for advanced post-type coupling structures.