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Multiple-mode stub-loaded resonator with quad channel diplexer and tri-band bandpass filter are presented and analyzed theoretically in this paper. The multi-mode stub-loaded resonator employs the basic structure of a triple mode resonator. Herein, the triple-mode resonator is modified by introducing a mid-coupled line between the odd mode resonances to produce a quad-mode resonator. The proposed resonator is applied in the quad-channel diplexer design, composing of two independent dual-band bandpass filters. In turn, the triple-band bandpass filter based on a parallel quad-mode and a dual-mode stub-loaded resonator is developed. To validate the performance of the proposed resonators, two experimental examples, including a quad-channel diplexer and a triple-band bandpass filter are fabricated and measured. Each of the designed circuits occupies small area i.e. about 0.32lambda*0.22lambda and 0.18lambda*0.20lambda respectively. Good agreements between simulated and measured results are achieved.