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This paper presents an accurate design technique for a low-pass filter (LPF) with a dumbbell-shaped slot (DSS) in the ground plane. In the proposed technique, a new simplified DSS inductance equation is suggested and modified to account for the stub parasitic inductance of the LPF. The compensated inductance of the DSS is determined from the modified DSS inductance equation. As a result, additional tuning is not required to achieve the desired LPF cutoff frequency. The stub section of the LPF is analyzed by transmission line theory to derive the stub parasitic inductance equation. 3-D electromagnetic field and circuit simulation validate the theoretical analysis. Two LPFs are designed, simulated, measured, and compared for further validation. One LPF is designed through a conventional method and the other through the proposed method. Both simulated and measured results show very good agreement in the proposed design.