Compact hairpin resonator bandpass filters with dual-band response and wide upper stopband

Compact bandpass filters are devised with a dual-passband response and a wide upper stopband based on hairpin resonators with three shunt open stubs; one is tapped at its center and the other two are allocated on both sides symmetrically. By adjusting the stub lengths, all resonators are designed to have two identical leading resonant frequencies but different higher-order ones. To enhance the rejection level and extend the upper stopband, the transmission zeros created by the tap structures and the coupled-line stages are used to eliminate the remaining leading unwanted peaks. In addition, a method for reducing the radiation loss in upper stopband is investigated. Measured results show that the fifth-order filter has a rejection level below 30 dB can be extended to six times the first frequency. Good agreement between measured and simulation is obtained.