Novel compact tri-band bandpass filter using square ring short stub loaded resonators

This letter presents a new method to design tri-band filters is used to the square ring short stub loaded resonators. Based on the odd- and even-model analysis, it is found that the first three resonance frequencies of the square ring short stub loaded resonators can be conveniently controlled. Benefiting from this feature, the resonator can be utilized to design a tri-band bandpass filter (BPF). To verify the proposed approach, a novel compact tri-band BPF is designed and fabricated with microstrip technology for demonstration, indicating good agreement with the theoretical expectation.