Experimental Investigation of Dual-Mode Microstrip Ring Resonators

The general behavior of microstrip ring resonators has been known for some time. In particular, the possibility of exciting two degenerate resonances on a single ring has already been established. In the technical literature, however, one can not find (to the authors knowledge) a complete and detailed description of actual microwave filters implemented by explicitly using the dual-mode nature of the microstrip ring. In this paper we present computer simulations and measured results describing a dual-mode filter cell implemented by using a single microstrip ring resonator. The microstrip dual-mode filter presented allows the implementation of two transmission poles and two transmission zeros using only one dual-mode ring resonator. This result is of particular interest because it has already been demonstrated experimentally that a similar behavior can be also obtained with circular-waveguide resonators. The presence or the absence of the transmission zeros is explained in simple terms. In addition, a procedure is presented to predict the position in frequency of the transmission zeros.