Compact Waveguide Filters Using Novel Resonant Coupling Structures

In this article, new resonant coupling structures are proposed for designing miniaturized waveguide bandpass filters. The resonant coupling structure is constructed by adding one or two metal blocks in a relatively thick inductive iris, which can be considered as the integration of a resonator, the input, and output coupling structures. Positive or negative coupling can be realized by selecting a different number of metal blocks. The combination of different resonant coupling structures can be used to design higher-order filters with various transverse topologies, realizing different transmission zeros (TZs) distributions. In addition, the weak source–load coupling caused by the resonant coupling structures can introduce an additional TZ, which can further improve the out-of-band suppression level of the filter. For verification, two third-order waveguide bandpass filters based on all resonant coupling structures are designed and simulated to achieve different TZ distributions. Finally, a fifth-order waveguide bandpass filter employing both resonant coupling structures and classical rectangular cavities is designed and fabricated. The measured results verify the proposed method.