Design of A Third-order Inline Waveguide Bandpass Filter With Cross Coupling and Spurious Mode Suppression

A third-order inline waveguide bandpass filter (BPF) with cross coupling and spurious mode suppression is designed in this paper. Based on the conventional cascaded-coupled filter, the cross coupling is realized by reducing the height of the second resonating cavity and introducing a stacked cavity for cross coupling, which generates a transmission zero (TZ) on the right side of passband. In order to eliminate the spurious modes caused by the cross-coupled cavity, a short-circuited post is introduced to make TE101 and TE102 mode of the cavity introduced for cross coupling self-suppressed. Therefore, the proposed inline waveguide BPF is with one TZ on the right side of passband while preserving almost the same bandwidth of stopband when compared to the conventional cascaded-coupled filter. The design process is illustrated in detail and the measured results verify the feasibility of the proposed inline waveguide BPF.