The quasi-elliptic bandpass filter using quarter-wavelength stepped impedance resonators

A bandpass filter is designed using two end-connected quarter-wavelength stepped impedance resonators. The quarter-wavelength stepped impedance resonators are implemented by a half-wavelength stepped impedance resonator with its middle shorting to ground. Hence, the modes that the lambda/4 stepped impedance resonator supports are the (1+ 2n)/4 which also occur on the branches of a combline. An enhanced U-shape coupling feed is adopted in the filter design to reduce the insertion losses as well as suppressing the harmonics. The proposed filter has a quasi-elliptic frequency response. An equivalent circuit model is developed and the experiments are conducted for circuit design verification.