This paper presents a tri-band bandpass filter (BPF) using a novel stub-loaded resonator. Different from other stub-loaded resonator, this resonator utilized a short-ended main transmission line and a centerly-loaded open stub. The resonator is theoretically analyzed. The first three resonant frequencies can be individually adjusted and this enables the convenient designs of tri-band BPFs. For demonstration, a tri-band BPF is implemented. Transmission zeros are created close to each passband edge, resulting in high skirt selectivity. Comparisons of the measured and simulated results are presented to verify the theoretical predications.
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