Design of the inline filter with improved box-section topology based on dual-mode waveguide loaded air slots resonator

This paper presents the dual-mode waveguide loaded air slots resonator (DM-WLASR) that can exhibit a non-degenerate mode feature. That is, there is almost no coupling between the dual-modes. As a result, either mode could be the fundamental mode by adjusting the resonator dimensions, enabling a straightforward control of places of transmission zeros (TZs). Two second-order filters were designed for validation, each of which produces one TZ below/above the passband. Then the DM-WLASR cascades two coaxial resonators to form the novel “box section” topology. As an improvement, for the first time, the proposed topology could generate three TZs by introducing mixed-coupling and cross-coupling. In summary, all filters developed in the work provide the advantages of adjustable TZs and compact configurations. For fast demonstration, 3D metal printing technology was employed to manufacture the four-pole filter, and the tested results agreed well with the simulated results.