A novel compact quad-band microstrip bandstop filter design using open-circuited stubs

A novel compact quad-band microstrip bandstop filter design is presented. Input and output ports of the filter are connected with a high impedance transmission line. Proposed filter is designed by using open-circuited stubs attached to the input and output ports. Thus, based on the f_r - 3f_r harmonic property of the stub-loaded resonators, four stopbands at 1.15-2.22, 2.94-4.56, 4.95-6.98 and 8.26-8.96 GHz are obtained. Insertion losses of all stopbands are better than 15 dB inside the stopbands. Under the even and odd mode excitations, resonance conditions of the structure are also investigated. For the experimental verification of the simulated results, proposed filter has been fabricated. Simulated and measured results are in a good agreement.