A Frequency-Variant Coupling Structure 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 TE
102
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 X
eq
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 X
eq
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.