Highly-selective and Compact Bandpass Filters Using Microstrip — Coaxial Resonator

This paper presents two compact high performance bandpass filters using high quality $\boldsymbol{Q}$-factor hybrid microstrip - coaxial resonator. In design, the hybrid resonator of proposed filters is composed of a microstrip line and a short-circuit coaxial line, which utilizes great use of space in the shielding box. Additionally, the unloaded quality factor of the proposed resonator is larger than 500, shows that a high value in comparison to the conventional microstrip resonator. Furthermore, by employing source-load coupling and mixed electromagnetic coupling scheme, two transmission zeros are generated to enhance the selectivity and the out-of-band rejection simultaneously. Finally, two second-order filters show the advantages such as small in size, high $\boldsymbol{Q}$-factor and good rejection level as well as easy to integrate with other printed circuits, are designed, fabricated, and measured. The measured results are in good agreement with theory and simulations.