Cross-Coupled Waveguide Bandpass Filters With Inline Footprint and Extremely Flexible Transmission Zeros

A new class of cross-coupled waveguide bandpass filters (BPFs) with inline footprints is proposed in this letter. The cross coupling is realized by a thin resonant cavity, whose resonant frequency is offset from the center frequency. Due to the cross coupling, a transmission zero (TZ) is produced on either side of the passband without negative coupling structures, even extremely close to the passband. The spurious spikes resulting from the cross-coupled cavity are suppressed by increasing the filter order. Consequently, a fifth-order BPF with a pair of TZs outside the passband and spurious-mode suppression is designed, fabricated, and measured. The measured results demonstrate the feasibility of the proposed BPF.