A Compact Open-Loop Filter With Mixed Electric and Magnetic Coupling

A novel cascaded microstrip open-loop resonator filter with controllable electric and magnetic mixed coupling is presented to have one or more transmission zeros. For two coupled resonators, the coupled sides with the maximum magnetic field are combined with their open gaps with the maximum electric field, creating the electric and magnetic mixed coupling. By adjusting the distances between resonators and the positions of open gaps in a mixed coupled filter, the coupling coefficients can be changed, and controllable transmission zeros can be produced in the lower stopband, upper stopband, or both. Design and fabrication of the proposed second-and fourth-order filters show the advantages: such as a smaller number of resonators, higher rejection level, as well as asymmetrical response; moreover, the location of the transmission zeros can be precisely controlled.