A Novel Method to Simplify the Structure of Dual-Band Bandpass Filters: Design the Resonators on Combined Substrates

Two dual-band bandpass filters are investigated both using two resonators parallel positioned along with microstrip lines. The proposed dual-band bandpass filter was based on λ _g/2 open-loop rectangle-ring resonator (Resonator L). A shorter open-loop rectangle-ring resonator (Resonator S) was parallel positioned at another side of the input/output microstrip lines with the same coupling conditions to the original longer resonator. If phase difference for signals in two paths was equal to 180°, the energy cancellation has happened, and no passband would be found. The longer resonator resonates at 1.23 and 2.4 GHz band. The shorter resonator resonates at 2.4 GHz band to enhance the bandwidth and out-of-band rejection by phase difference method. The bandwidth of 1.23 GHz band is produced by λ _g/2 fundamental response of Resonator L. The bandwidth of 2.4 GHz band is produced by combining 1 λ _g first spurious response of Resonator L and λ _g/2 fundamental response of Resonator S. The proposed filters are suitable to use for GPS (L2-band) and WLAN.