Mode perturbations of a ring resonator for wideband and multiband filters

In this paper, we study the mode perturbations of a planar ring resonator for proposal and unified design of wideband and multiband microwave bandpass filters. Compared to a planar quarter-/half-wavelength transmission line resonator, a one-wavelength resonator based on a closed ring structure owns two orthogonal degenerate modes. By introducing a small perturbation along a ring, the two degenerate modes can be excited simultaneously, which can be designed to be either a narrow-band two-pole filter or a circularly polarized antenna. We can further excite and stretch the high-order pairs of degenerate modes, which could be designed to work together with two fundamental degenerate modes for applications in wideband and multiband filters. Different topologies and designs are discussed as examples to provide physical insight into their working principle and to demonstrate their advanced features in applications.