Investigation of quasi-elliptic hairpin dual-mode filters under different perturbations

This article presents a dual-mode resonator developed from a hairpin resonator with a reactive perturbation tapped at the mid-point. The dual-mode resonance characteristics are investigated intensive under both inductive and capacitive perturbations. Using this resonator, a method of synthesising the filter is also developed to determine the circuit layout directly according to the specified filter characteristics. Moreover, to improve the filter's performance, cross-coupling technique to realise quasi-elliptic responses is also discussed for both types of perturbations. Finally, two quasi-elliptic dual-mode filters were designed, fabricated and measured to verify the proposed theory.