Compact Tunable Bandpass Filters Based on Lambda/4 Coaxial Resonator and Capacitive Loading Sleeve

This paper presents a novel design methodology for compact tunable filter with constant absolute bandwidth (CABW). In the resonator design, a tuning element (a capacitive loading sleeve) is driven by a dielectric rod to tune the resonant frequency of a λ/4 coaxial resonator. Apart from a wide frequency-tuning range, the resonator maintains a stable unloaded Q (Q
u
) during the process. Another advantage is that the variation of the physical coupling coefficient between two resonators is close to the theoretical requirement without any optimization. In order to further simplify the tuning mechanism, a method using a coupling post to control the input/output (I/O) coupling is proposed. Therefore, in this design, only the resonators need to be tuned without any electrical contact. Based on the proposed resonator and coupling structures, a compact four-pole filter configuration is proposed. The filter has a wide tuning range from 1.55 GHz to 2.55 GHz (48.8%), over which the 3-dB ABW variation is 103.85±6.45 MHz (±6.2%).