Evaluation of modelling routines for on-line implementation of the open-ended coaxial probe

Three models giving the relationship between the complex reflection coefficient and complex permittivity have been evaluated for use with an open-ended coaxial probe in an online measurement system: Cole's bilinear model; and the more sophisticated models of Marsland and Evans, and Marcuvitz. The frequency range considered is from 500 MHz to 40 GHz and results are verified by tests on methanol - water mixtures. The results show that, for the purpose of online implementation, Cole's simple bilinear model does not give a significantly reduced precision compared to the more sophisticated models and hence can be advantageously used due to its simplicity and robustness. (Original abstract - amended)