Synthesis Design of Box-Section Bandpass Filter With Hybrid $\lambda/4$ and Dual-Mode Resonators

In this letter, a box-section bandpass filter consisting of hybrid λ/4 and dual-mode resonators at X-band is presented. Compared with the conventional even-odd mode method, the dual-mode resonator in the proposed filter is equivalent to separate λ/2 and λ resonators. A generalized synthesis approach is firstly proposed with extensive discussions on designing asynchronously tuned resonators. One prototype centered at 8.65 GHz, with 0.9 GHz passband is fabricated. Measured results of this filter demonstrate very high single-sided selectivity with 40 dB skirt slope steepness of 1.98 and minimum in-band insertion loss of 1.43 dB.