Waveguide Filters With Central-Post Resonators

In this letter, a class of rectangular waveguide filters with central-post resonators is introduced. By introducing a central post in an oversized rectangular cavity, the resonant frequency of TE301-mode can be brought close to that of TE201-mode. The two resonant modes are coupled to adjacent fundamental mode resonators by inductive irises. A doublet coupling topology can be used to model the physical configuration, and flexible filter responses with finite-position transmission zeros (TZs) can be realized. Four 4-pole filters with different TZ positions are designed and simulated. A seven-pole filter with two cascaded central-post resonators is fabricated and measured. In the seven-pole filter, each central-post resonator independently controls one TZ on the lower side and very close to the passband. The measurement results of the seven-pole filter agree with the simulated counterparts very well, thus validating this filter design technique.