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In this paper, a novel tunable bandstop filter has been proposed and fabricated. This filter is compact because the resonator is located under the microstrip line not to occupy much space. The equivalent circuit model of the proposed structure is proposed. By employing a varactor diode which has variable capacitance, the tunable bandstop characteristic is achieved. The fabricated bandstop filter shows good tunability( 1.04-3.13 GHz).