Design of Microstrip Dual-Passband Filter Based on Branch-Line Resonators

A new type of microstrip dual-passband filter using branch-line resonators near the input and output ports is presented in this paper. The branch-line resonator plays two important roles for the proposed dual-band filter structure. One is for extracting the external quality factor of each band separately and the other is for providing the required resonance at the center frequency of each passband. By adding some additional resonators near the branch-line resonators, the coupling coefficients between adjacent resonators can also be obtained separately such that each passband response of the proposed dual-band filter structure can be designed individually.