An improved method for calibrating an open-ended coaxial sensor is proposed. The calibration utilizes the measurements of an open, a short, and a short-cavity. These terminations are inexpensive to make and result in a calibration that depends only on the geometry of the sensor. The short-cavity is modeled by waveguide modal techniques and the theoretical reflection coefficient determined to any accuracy. Consequently, an improvement in the accuracy of the measurements is obtained over a large bandwidth. The calibration was tested by comparing theoretical and experimental values of the complex permittivity for common liquids, and the discrepancy was seen to be small. >