Novel design of dual-band microstrip bandpass filters with good in-between isolation

Novel dual-band microstrip bandpass filters (BPFs) are proposed with good in-between isolation. A half-wavelength stepped-impedance resonator (SIR) is characterized, aiming at producing the two resonant frequencies at 2.4 and 5.2GHz. Two types of coupled microstrip lines in the parallel and anti-parallel formats are then investigated in terms of unified equivalent J-inverter network. Extensive results are derived to show their frequency-distributed coupling performances under the different coupling lengths. The parallel coupled line is modeled to bring out a promising transmission zero between the two resonant frequencies so as to achieve a good in-between isolation. In this way, a two-stage BPF is designed to exhibit their dual-band frequency responses with good isolation. A three-stage dual-band BPF is in final optimally designed and its predicted performance is further confirmed in experiment.