Design of UWB microstrip bandpass filter using stub-loaded quintuple-mode resonator

Design of a novel ultra-wideband (UWB) microstrip bandpass filter is presented by using a square loop quintuple-mode resonator (QMR) having open-circuited stubs. The proposed resonator has five resonant modes in UWB band range and is coupled to Input/Output (IO) ports by parallel-coupled feed lines. The first three resonance modes are resulted from the stub loaded square loop resonator, whereas the remaining two modes can be formed by means of the coupling between the open-circuited stubs and the resonator. The designed UWB filter has totally seven poles including two poles resulting from the nature of the coupling sections and five poles from the QMR. Theoretical analysis of the resonator is realized by defining the equivalent circuit model. Coupling scheme and coupling matrix of the designed filter are also extracted to clarify the filter topology. In addition, two patch perturbation elements are used to control the in-band return loss levels. A seven pole UWB bandpass filter is designed, analyzed, and fabricated. Measurement results show a good agreement with the predicted results. (c) 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:662-666, 2016