Compact Narrowband Bandstop Filter with Wide Upper Passband

This paper presents a compact narrowband bandstop filter (BSF) with wide upper passband to reject any unwanted signal in an ultra-wideband system. The proposed structure consists of sections of transmission lines and a lumped capacitor. A lossless transmission line model is used to obtain the design equations. Then, full-wave simulation is used to obtain the filter layout in microstrip technology. Analysis shows that the second undesired stopband is far away from the first stopband frequency. If the microstrip lines and capacitor are chosen properly, the second stopband is at least forty times the first stopband frequency. A prototype BSF is fabricated with the stopband at 1.04 GHz. The upper passband of the filter extends up to 12.11 GHz. At the same time, significant size reduction is obtained. Occupied rectangular area is $0.0009 \lambda^{2} g\left(\lambda_{g}=\lambda_{0} / \sqrt{\varepsilon_{r}}\right)$ at 1.04 GHz.