Miniaturised bandpass filter using novel feeding structure for harmonic suppression

Presented is a very compact structure for designing harmonic-suppressed bandpass filters with high selectivity. The proposed filter utilises folded quarter-wavelength resonators to miniaturise the circuit size. A novel feeding structure is used to not only generate transmission zeros near the passband but also suppress the third harmonic. To verify the proposed idea, an experimental filter has been implemented. High skirt selectivity and wide stopband are observed in the measured results. The circuit area of the filter is only 7.9×5.4×mm (0.1×_g×0.079×_g), where λ_g is the guided wavelength at the operating frequency of 2.4 GHz.