Design and analysis of a compact dual-mode dual-band microstrip bandpass filter

Design and fabrication of compact dual-mode dual-band microstrip bandpass filters with a novel feed scheme is presented. The proposed filters have two square loop resonators for two passbands, and each resonator is fed by the feedlines located at the upper corners of the resonators. By using the perturbation elements such as patches or corner-cuts, two degenerate modes of each passband can be easily excited, so mode frequencies and transmission zeros of the passbands can be controlled independently. Also, the proposed feed scheme provides the harmonic suppression for the first passband. In addition, theoretical results have been obtained by using an equivalent circuit model, and a coupling scheme is also presented. The designed filters have an electrical length of for the quarter wavelength resonator, where is the guided wavelength at 1.8 GHz. Two bandpass filters having elliptical and linear phase filtering characteristics in each passband have been designed, fabricated, and tested. Measured results are in an excellent agreement with simulated and calculated ones. In all frequency responses, return losses are better than 15 dB and insertion losses are at acceptable values for both passbands.