Customized systems for complex permittivity measurements on liquid samples at microwave frequencies: A comparative analysis

In this paper, two customized systems for microwave permittivity measurements on liquid samples, based on reflectometric measurements, are presented and analyzed. Their performance is compared against the one deriving from the widely adopted commercial measurement setup, based on an open-ended coaxial line. The first custom system is built by properly modifying an N-type coaxial connector and relies on vector reflection coefficient measurements, while the second one uses a portion of liquid-filled coaxial line in conjunction with a time-domain reflectometry equipment. The systems are built for quality control and diagnostic purposes and their final aim is to extract the Cole & Cole dielectric model of the liquid under test. In order to metrologically characterize the systems, a series of repeated measurements is performed on a set of reference liquids. After the Cole & Cole parameters are extracted for each considered measurement method, the resulting type A uncertainty is evaluated. Comparison with literature reference data for the liquids also allows estimation of measurement bias. The analysis evidences that custom probes generally exhibit an acceptable measurement bias, but with a relevant loss of accuracy in the estimation of some Cole & Cole parameters for lossy liquids. Possible improvements are under investigation for enhancing their performance and make them appealing substitutes for the commercial setup.