A high selectivity dual-band filter using ring-like sir with embedded coupled open stubs resonators

We present a design of dual-band bandpass filter (BPF) using a ring-like stepped-impedance-resonator (SIR) and embedded coupled open stubs for application to a wireless local area network (WLAN). The dual-passbands are created by optimally selecting the impedance ratio (K) and length ratio (u) in the resonant behavior of a conventional SIR. Without increasing the circuit size, the performance of the second passband can be enhanced by adding the coupled open stubs in the ring-like SIR. The proposed filter was designed and fabricated. The measured results have a return loss larger than 24 dB, a low insertion loss less than 1.3 dB, and a fractional bandwidth (FBW) of 15% at 2.4 GHz, as well as a return loss larger than 12 dB, a low insertion loss less than 2 dB, a FBW of 2% at 5.7 GHz, and transmission zeros at 2.09, 2.96, 5.44, and 6.11 GHz near the passband edges. It is verified that experimental results show a good agreement with the simulated results.