A critical study of the open-ended coaxial line sensor technique for RF and microwave complex permittivity measurements

An HP 8510 automatic network analyser and a six-port reflectometer have been used with a purpose-built calculable open-ended coaxial line sensor to measure the reflection coefficients of various materials, including dielectric reference liquids, in the frequency range 50 MHz-2.0 GHz. Factors crucial for calculable measurements have been identified including associated measurement uncertainties. The reference material measurements have been used in critical studies of: a commonly employed lumped equivalent circuit model of the fringing fields of the sensor; and a numerical point-matching theory of the propagating and evanescent modes, at the termination of the sensor. It is concluded that the inverse point-matching theory should enable a more widespread and accurate exploitation of the sensor technique for various applications including those in biomedicine and industrial quality control.