General rotationally symmetric coupled feeding structure and its application to single- and dual-band balanced filters

This paper proposes a new general rotationally symmetric coupled feeding structure and its applications to the design of single- and dual-band balanced filters for high common-mode (CM) suppression. The balanced feeder consists of two coupling lines coupled with a ring resonator, which can be either a uniform impedance ring resonator or a stepped impedance ring resonator. Under differential-mode (DM) excitation, the voltage distributions of the coupling-line and resonator are both rotationally even-symmetric and the total coupling coefficient adds up, which indicates that the proposed feeding structure can achieve DM transmission characteristic. Under CM excitation, their voltage distributions are rotationally even- and odd-symmetric, respectively, so the total coupling coefficient cancels out to zero and CM suppression property is achieved. For demonstration, four single- and dual-band balanced filters with wide range of CM and harmonic suppression are designed, fabricated, and measured, and the simulation and measured results match well.