Design of a dual-mode dual-band bandpass filter with a novel feed scheme

A novel feed scheme is presented to design a dual-mode dual-band microstrip bandpass filter. The proposed filter has two square loop resonators for both passbands and each resonator are fed by a feedline placed on the upper corners of resonators. Proposed structure allows controlling each passband separately in terms of mode frequencies and transmission zeros. By means of the perturbation elements such as patches or cuts, two degenerate modes can be excited. Also, wideband harmonic suppression can be achieved through this novel feed scheme. Two bandpass filters having elliptical and linear phase frequency responses have been fabricated for experimental verification of the predicted results. Measured results are in good agreement with simulated results.