A simplified equivalent circuit model of a dumbbell-shaped defected ground structure (DGS) is developed. The equivalent circuit model is derived from the equivalent inductance and capacitance, which develop because of the perturbed return current path on the ground and the narrow gap, respectively. The theory is validated against the commercial full-wave solver CST Microwave Studio. Finally, the calculated results are compared with the measured results. Good agreements between the theory, the commercially available numerical analyses and the experimental results validate the developed model. © 2007 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2007.