Frequency-Variant Coupling Structures for Inline Rectangular Waveguide Filters

This paper introduces a new resonant coupling structure for inline rectangular waveguide filters. The proposed structure consists of a rectangular cavity placed on top of the main waveguide, and it exploits the TE102 mode for creating a frequency-variant coupling capable of producing one transmission zero (TZ). An equivalent circuit characterized by the frequency of the TZ and the slope parameter Xeq of the proposed stopband was used in order to realize a filter straightforwardly. Since no internal elements produce any discontinuities, very high values of the Xeq are achievable, which allows for placing the TZ closer to the passband while still proving high Q-factors. The design of a 4-pole with a TZ in the upper stopband is illustrated, and the filter was fabricated and measured for validation purposes.