Dielectric measurements on reference liquids using automatic network analysers and calculable geometries

Abstract

Knowledge of the in vivo dielectric properties of human tissues is essential to the development of biomedical techniques which make use of radiofrequency and microwave radiation. The sensors used for measurement of tissue properties can be conveniently calibrated using dielectric reference liquids, but this in turn requires that the dielectric properties of such liquids be accurately known. Many complex permittivity measurements on suitable liquids, such as the primary alcohols and saline solutions, have been reported in the literature, but unfortunately there are in some cases significant discrepancies between reported values. The authors describe a program of work undertaken to resolve some of these anomalies. A number of techniques which make use of 14 mm coaxial cells and sensors coupled to an automatic network analyser are under investigation. Comparison of the different techniques should facilitate evaluation of systematic uncertainties. The liquid measurement geometries and the associated computational methods are described. Details are given of the uncertainty analysis, and examples of mappings from the complex permittivity plane on to the scattering coefficient plane are shown. Some preliminary results are presented, which highlight the value of the comparative approach that has been adopted.