Design of compact quad-band bandpass filter using semi-lumped resonators

A compact quad-band bandpass filter using semi-lumped resonators is presented. The proposed filter is composed of four pairs of semi-lumped resonators, which consist of microstrip sections and lumped inductors. Compared with the conventional half-wavelength uniform-impedance resonator, the proposed resonator achieves a compact size due to the lumped inductor employed. Moreover, transmission zeros appear in the stopband to improve the passband selectivity. The proposed quad-band bandpass filter achieves not only compact size but also good passband selectivity.