Design of compact dual-mode dual-band filter based on frequency transformation method

In this paper, a novel dual-mode resonator is presented, which is a square loop resonator attached by four arrow-shaped patches at the corners. By adding four patches and a perturbation to the square loop resonator, the features of compact size and controllable coupling are obtained. Coupling coefficient of the low modes and that of the high modes can be controlled independently when the dual-mode resonators cascade. Based on the resonator, a dual-band filter is designed using frequency transformation method. The building block is realized by a dual-mode resonator in stead of two common resonators. Due to the decrease in the number of resonators, the size of the designed filter can be reduced by half. A practical filter is fabricated and the measured results show good agreement with the design requirement.