Dual-mode and three-transmission-zeros bandstop filters with closed-loop ring resonators

In this paper, ring-resonator bandstop filters (BSFs) with three attenuation poles are designed and analyzed using a two-port ABCD matrix of the transmission-line model. The symmetrical geometry of the ring-type BSFs allows us to explain the resonance operation via even and odd modes. Moreover, perturbation elements are employed to control the attenuation poles and obtain good performances. In order to validate the analytic method, three different BSFs are built using the microstrip-ring resonator. The simulated and measured results are presented and compared with the theory over a wide frequency range. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 44: 114–118, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20562