Spiral microstrip resonators for narrow-stopband filters

A rectangular spiral microstrip resonator is proposed for use in planar narrow-stopband filters. A full-wave electromagnetic simulator is used to show that this small-size resonator is suitable for realising compact narrow-stopband microstrip filters. Simulated and measured results indicate that minimal coupling between adjacent resonators exists, with the effect that the microstrip filter response corresponds very closely to that of the equivalent network response of the filter. As a comparison, results are also shown for a similar filter realised with microstrip open-loop resonators.