Design of compact tri-band filter based on SIR-loaded resonator with 0° feed

In this paper, a novel compact tri-band filter based on SIR loaded is presented. By adding λ/4 resonators and loading SIR stub, the features of compact size and tri-band response are obtained. Three passbands are designed to operate at 1.85GHz, 2.575GHz and 3.05GHz, respectively. The first and second passbands are obtained using resonator loaded with SIR, which is smaller in size than UIR, the λ/4 resonators is used to generate the third passband. The three passbands can be adjusted to designed values. The skew-symmetric (0-degree) feed structure was used to get a pair of transmission zeros so the frequency selectivity was greatly improved. Due to λ /4 resonators embedded in the resonator loaded with SIR, compact size for filter can be realized. A practical filter is fabricated and the measured results show good agreement with the design requirement.