Compact quasi-elliptic bandpass filter with a very wide stopband using quarter-wavelength stepped-impedance resonators

A new compact microstrip bandpass filter with a very-wide stopband is presented using dual λ/4 stepped impedance resonators with open-stub loaded at the ends. The filter offers a quasi-elliptic bandpass response with a pair of symmetric transmission zeros created at the proximity of the passband through the couplings among loading stubs and input/output feedlines. An example circuit is designed at 2.4 GHz has a size of 0.085 λ_g × 0.128 λ_g and gives a stopband of up to 7.5 f₀ with overall rejection better than 20 dB.