Miniature dual-mode bandpass filters using hexagonal open-loop resonators with E-shaped stubs loading

A miniature dual-mode microstrip bandpass filter by using novel dual-mode hexagonal open-loop resonators with an E-shaped stub loading is proposed in this work. The loading E-shaped stub perturbs and reduces the high-order mode frequency of the single-mode (operated) hexagonal open-loop resonator, consequently, changing the second-mode frequency of the single-mode resonator into the second operating mode of the dual-mode resonator. The field patterns of the dual modes and the dual modes changing characteristics are described. The proposed filter shows a size reduction of about 55% compared with the conventional dual-mode microstrip hexagonal loop filter. Both theoretical and experimental results are presented.