Ultra-wideband bandpass filter using symmetric stub-loaded resonator and stepped impedance resonators

A novel planar ultra-wideband (UWB) bandpass filter with multiple transmission zeros based on a stub-loaded resonator and two stepped impedance resonators (SIR) is presented in this paper. The bandwidth of the fifth-order passband can be tuned by adjusting even/odd-modes resonant frequency. Six transmission zeros are realized by the resonators. Four of them are produced by the stub-loaded resonator, and another two transmission zeros are realized due to the stepped loaded resonators, which can be adjusted by changing the ratio of characteristic impedance of the stepped impedance resonator. In addition, the SIR structures can achieve better stopband suppression without affecting the previous good performance. The prototype of the proposed filter with 3-dB fractional bandwidth 75.6% is designed and fabricated.