A compact wide stopband microstrip bandpass filter using quarter-wavelength shorted coupled-lines

A compact microstrip bandpass filter using quarter-wavelength (λ/4) shorted coupled-lines is presented in this paper. The shorted symmetrical coupled-line using two λ/4 resonators is employed to introduce the fundamental resonant modes to meet the limits of the passband operation. Besides, the shorted asymmetrical coupled-lines at both sides of the shorted symmetrical coupled-line have two functions: 1) harmonic suppression in the stopband, 2) bandwidth enhancement in the passband. The design is then verified by experiment, which exhibits a good insertion loss better than 2.11 dB, and a wide stopband from 2.58 to 10.8 GHz with a satisfactory rejection of more than 20 dB.