Design of Compact Coaxial Cavity Bandpass Filter with High Selectivity

In this paper, a new compact bandpass filter with high selectivity is presented by introducing cross coupling between five coupled coaxial cavity resonators. Owing to the adoption of cavity resonators, high unloaded quality factor (Q) and low insertion loss can be obtained. To reduce the vertical height of proposed filter, circular disk capacitive loading on open-end of the resonant rod in the coaxial cavity has been adopted. Besides, by exploiting a type of cross arrangement between the resonators, miniaturization can be further attained. Based on cavity filter design method and cross-coupling theory, two transmission zeros on both sides of the passband are synthesized by introducing capacitive and inductive cross-coupling into cascaded triplet sections. In addition, to adjust center frequency and bandwidth, tuning screws are inserted to the inner conductor of the coaxial cavities. For demonstration, a prototype of filter centered at 3.5 GHz with a bandwidth of 200 MHz is designed and simulated. Results show that the coaxial cavity bandpass filter not only exhibits a sharp roll-off with two transmission zeros located at 3.25 and 3.7 GHz, but also low insertion loss less than 0.09 dB. The overall size of the cavity was $0.2\boldsymbol{\lambda}_{0}\times 0.35\boldsymbol{\lambda}_{0}\times 0.17\boldsymbol{\lambda}_{0}$, which is more compact in comparison to the conventional fifth order coaxial cavity filters.