Compact and high selectivity dual-mode microstrip BPF with frequency-dependent source–load coupling

In this letter, a dual-mode microstrip bandpass filter (BPF) with compact size and high selectivity is designed with a short-stub centred loaded resonator. Owing to the frequency-dependent source–load coupling and the intrinsic characteristics of the short-stub loaded resonator, three transmission zeros that are located near the passband edges and in the upper-stopband can be generated to improve the selectivity. The compact BPF with fractional bandwidth of 3.5% located at 2.4 GHz for WLAN application has been designed and fabricated, and good agreement between simulation and measurement verifies the validity of this design methodology.