Compact bandstop filter using coupled-line section

A new compact bandstop filter with a coupled-line structure is proposed. Stopband rejection depth and bandwidth can be controlled by the configuration of the coupling section, and good return loss is realised by properly adjusting the impedances of two open-circuited stubs. Design equations are obtained using a lossless transmission line model. To validate the analysis, a bandstop filter with about 72% fractional bandwidth at 5 GHz has been fabricated. The implementation area is 0.32λ_g × 0.18λ_g.