Design of a novel compact bandstop filter composed on an open-loop-ring resonator and a defected ground structure (DGS) for coplanar waveguide is proposed and investigated. A simple coplanar open-loop-ring resonator has been designed and tested for the stop-band from 3.9 to 4.3 GHz with tree transmission zeroes inside the stop-band. Two almost symmetrical poles are obtained on both sides of the stopband. Then the proposed broad band bandstop filter based on the one open-loop-ring resonator has been transformed to the bandstop filter with a narrower stop-band but with improved pass-and stop-band characteristics. It is realized by adding a conventional rectangular DGS slot inside the open-loop-ring. The equivalent circuit model has been proposed to describe the behaviour of the bandstop filters. The improved bandstop filter introduces the stop-band from 3.75 to 5.3 GHz with two transmission zeroes within and more than À20 dB rejection. The total size of the proposed structure is 13.8 Â 13.4 mm 2 . Both bandstop filters have been fabricated and measured. Measurements are in good agreement with simulation results.

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