A cross-shaped stepped-impedance resonator bandpass filter with wide stopband

A compact microstrip bandpass filter is developed using a cross-shaped stepped impedance resonator. The latitudinal stepped impedance resonator is designed to determine the first resonant mode of the filter. Two stepped impedance open-stubs are loaded at the center of latitudinal stepped impedance resonator and form a new longitudinal stepped impedance resonator, which produce the second resonant mode. Wide stopband is realized by using stepped impedance resonator. A transmission zero is obtained at the upper stopband and excellent selectivity is achieved. This filter shows the return loss of greater than 20 dB in passband, and insertion loss below 20 dB from 2.5 GHz to 7 GHz.