Compact microstrip tri-band lowpass-bandpass filter

A compact microstrip tri-band lowpass-bandpass filter is described. Two different sets of λ/4 uniform-impedance open stubs generate the desired transmission zeros (TZs) to divide a lowpass frequency response based on a radial stub loaded high-impedance microstrip line into a tri-band lowpass-bandpass frequency response. A lowpass passband with a 3 dB cutoff frequency of 0.66 GHz, and two bandpass passbands centred at 1.57 and 2.45 GHz for global positioning system and wireless local area network applications are obtained in this design. The fabricated tri-band lowpass-bandpass filter occupies a compact circuit area of 0.065 λ_gc × 0.085 λ_gc. Seven TZs around the three passbands result in high band-to-band isolation, good passband selectivity and a wide upper stopband.