Design of Dual-Band Bandpass Filter With High Isolation and Wide Stopband

A simple and effective method to design a dual-band bandpass filter with high isolation and wide stopband is presented and validated using symmetric open-circuited stub-loaded resonators. By adjusting the electric length and the ratio of characteristic admittance of the resonators, the first two resonant frequencies are identical, but the higher order resonant frequencies dispersed in the upper stopband to extent the bandwidth of stopband. Two transmission zeros are introduced to obtain high isolation and selectivity between the two passbands by placing them in the appropriate position. Proper feeding position is selected to further improve the attenuation of stopband. The measured results highly agree with the simulated one. Measured results demonstrate the isolation level of 40 dB, attenuation of larger than 28 dB, and a wide stopband.