A compact planar dual-band bandpass filter with asymmetric bandwidths

Two different lengths of open-loop rectangle-ring resonators were parallelly positioned at two sides of input/output microstrip lines with the same coupling gap and length. The longer open-loop rectangle-ring was designed to resonate at 1.23 GHz and 2.31 GHz. The shorter open-loop rectangle-ring was designed to resonant at 2.52 GHz to improve the bandwidth of 2.4 GHz band. The proposed filter revealed great filtering properties and was suitable to used for the applications of GPS (L2-band, 1.227 GHz), and WLAN (IEEE802.11b/g, 2.4~2.4835 GHz).