Novel ultra-wideband bandpass filter with notched band using stubs loaded multi-mode ring resonator

A novel compact microstrip ultra-wideband (UWB) bandpass filter (BPF) with narrow notched band using stepped-impedance open stub and short stub loaded multi-mode ring resonator (MMR) is proposed in this letter. By tuning the length and width of open stub, short stub and ring resonator, the center frequency and bandwidth of the UWB passband can be easily adjusted to the desired value. The narrow notched band was introduced by using a new technique which involves embedding stepped-impedance open stub have wide and narrow parts, which are placed on the symmetrical plane to perturb the ring resonator. The center frequency of the notched band can be controlled by tuning the stepped-impedance open stub. A prototype of UWB with 3 dB fractional bandwidth (FBW) of passband is (2.8-11.05 GHz) about 119%, notched band at 5.4 GHz with the FBW is 4.6% is designed and fabricated for demonstration, indicating good agreement with the theoretical expectation.