A Simple Design of Planar Harmonic-Suppressed Filters Using Shunt Open Stubs

For harmonic suppression, equivalent T-shaped transmission lines are used to replace series connecting transmission lines of conventional planar filters. The added shunt open stubs in the T-shaped lines work like bandstop filters at the unwanted harmonic. The proposed filters have improved stopband rejection and compact size. In this paper, General design equations are given to transform arbitrary series connecting lines of filters into the relevant equivalent T-shaped lines to suppress any unwanted signals. Experiments have been done to validate the design concept.