IMPROVED CALIBRATION OF A LARGE OPEN-ENDED COAXIAL PROBE FOR DIELECTRIC MEASUREMENTS

The accurate calibration of open-ended coaxial waveguides can be difficult because standard matched loads over a broad frequency range cannot be easily attached without connectors that will affect the measurements. An alternative termination is the open-ended probe immersed in a material with known permittivity. But this termination is often inaccurate due to the multiple sources of uncertainty in the reference permittivity. An improved calibration is obtained by measuring an open, a short, and a short-cavity. This calibration is more accurate than measurements of reference permittivities because it depends only upon the geometry of the probe. 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.