A novel ultra-wideband bandpass filter based on a cross-shaped triple-mode microstrip resonator is proposed in this paper. The cross-shaped resonator is realized by loading two open-stubs at the center of a microstrip resonator. Circuit model for the resonator is studied and analysised. Resonance of Even and odd modes of the resonator can be fully controlled by changing the length of the resonator and the loaded stubs. An ultra-wideband bandpass filter covering the lower UWB band is designed using the proposed structure. Triple-modes of one resonator are used in the passband of the filter. An inter-digital input and output structure is applied to realize the strong coupling between resonator and ports. Experimental results validate the proposed structure and design scheme.
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