Dielectric metrology with coaxial sensors

Coaxial sensors are used in applications that require accurate and traceable measurements of complex permittivity. For example, coaxial sensors are often used for measurement of the complex permittivity of tissue equivalent materials (TEMs) used in specific absorption rate (SAR) measurements of exposure to RF fields. It is therefore important that well-founded metrological techniques for their use are developed and published. Although there are many published papers on coaxial sensors, few discuss the experimental techniques required to obtain the most accurate results. In this paper experimental approaches for obtaining the most accurate measurements are described. Common pitfalls with the technique are discussed. A Monte Carlo modelling (MCM) technique is used to provide estimates of uncertainty which are compared to those of measurements on reference liquids made with 3.5 mm, 7 mm and 15.1 mm diameter sensors. The MCM technique allows uncertainties to be estimated when measuring dielectrics for which there are no reference materials that have comparable properties, for example for TEMs at frequencies below 100 MHz.