Design of coaxial quasi-elliptic filter using mixed electric and magnetic coupling

A coaxial quasi-elliptic bandpass filter (BPF) with controllable mixed electric and magnetic coupling is proposed in this paper. Adding a ridged transmission line section between the posts of inner resonator cavities is applied to enlarge the magnetic coupling between the adjacent resonators. On the other hand, the metallic caps on the top of the inner posts increase the capacitance loading of the coaxial resonators, thus not only decreasing its resonant frequencies and reducing the filter size, but also enhancing the electric coupling between the cavities. The proposed three-order BPF has two independently tunable transmission zeros (TZs). By simply adjusting the heights of the ridges and the distances between metallic gaps in a mixed coupled filter, the magnetic and electric coupling coefficients can be changed and two TZs can be tuned in the lower stopband, upper stopband, or both. The filter carries many advantages, such as compact size, low insertion loss, and high rejection level. A three-order mixed coupling filter has been simulated, fabricated, and measured. The measured results show good agreement with the simulation response showing the validity of the presented method.