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A compact modified dumbbell-shaped defected ground structure (CM-DGS) resonator on coplanar waveguide (CPW) is proposed in this letter. By adding a smaller square resonator to the conventional modified dumbbell-shaped DGS (M-DGS), the path difference for the electromagnetic wave propagation will be increased with a maximum 55.6% of the M-DGS size reduction. The scheme for the reconfigurable bandstop resonator is presented after considering the influence of the inserted short circuits on the resonant frequency. Finally, the reconfigurable bandstop resonator with mounted varactor diodes and dc biasing circuits has been fabricated and measured. Both simulated and measured transmission coefficients indicate a 24.4% tuning range centered at 1.92 GHz. The resonant frequency can be reduced further by increasing the dimensions of the smaller square, and varactor diodes with higher tunability may widen the tuning range.