Resonant-mode behavior of a new slotline-embedded three-mode microstrip-line resonator and its filter application

This paper presents detailed investigations on resonant-mode behavior of a new three-mode resonator, which consists of a parallel-coupled microstrip line resonator embedded with a slotline resonator. Electric-field distributions and modal resonant frequencies of three modes are analyzed by using an electromagnetic simulator, thereby showing that the three modes are controllable with the ratio of the microstrip-line length to the slotline length. Based on the results of the resonant-mode behavior, a very compact bandpass filter and its circuit model are developed to confirm the effectiveness of the resonator. The developed filter has three reflection zeros in the passband and four transmission zeros near the passband as well as a sharp skirt property and a wide stopband. The validity of the multipole response is supported by the calculated results of the circuit model formed by a multi-path configuration.