Tapered cells are used in the calibration of electric field (E-field) probes. However, the design methodology of tapered cells has not been publicly reported in detail. This paper introduces the structure of a tapered cell and proposes a theoretical model for characteristic impedance calculation. Based on this model and developed design rules, an impedance-matched tapered cell was constructed. The measured characteristic impedances and impedance distribution along the cell show favorable agreement with the design goals, verifying the accuracy of the design methodology. The field distributions were also measured. The results prove that the cell is useful for calibrating E-field probes.