Eigen-mode analysis of a novel three-mode microstrip/slot-line resonator and the development of a compact bandpass filter with multiple transmission zeros and wide stopband property

A novel compact three-mode resonator is proposed, which consists of a parallel-coupled microstrip line resonator embedded with a slotline resonator. Resonance mechanism and properties of these three modes are analyzed and verified by using an electromagnetic simulator. Four transmission zeros are produced because of multi-paths between the input/output stub lines formed by the three resonant modes and the direct source-load coupling. A circuit model is proposed and a very compact bandpass filter is developed, which has three reflection zeros in the passband and four transmission zeros near the passband, showing sharp skirt property and a wide stopband.