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This letter proposes a double-slot-loaded resonator for exploration of a novel dual-band bandpass filter with two closely spaced passbands. A small ratio of dual-passband central frequencies is realized by positioning the double slot stubs in an offset way. The first-passband fractional bandwidth can be properly tuned by a distance between the two slots, while the second one is determined by attaching a microstrip open-circuited stub at center. After the principle of this initial filter is described, a prototype filter is designed and fabricated. Measured results satisfactorily validate the predicted ones of the proposed filter.