Time domain reflectometry to study the dielectric properties of liquids: Some problems and solutions

After a brief introduction we review the fundamental mathematical relations necessary to discuss the main aspects of the time domain dielectrometry performed with total reflection from an open cell. Experimental problems due to the presence of resonances in the sample cell, spurious reflections, and fringing fields are considered and possible solutions are suggested. An experimental apparatus realized to study liquids with dielectric constant values up to those of water and water solutions from 10 MHz up to 10 GHz in the temperatures range -45 °C+70 °C is also presented. The performances of some different experimental cells for liquids are compared and representative results obtained with different procedures are reported. The use of the numerical solution of a transcendental equation that takes into account for wave propagation allows us to measure the complex dielectric constant without the use of reference liquids, once the electrical parameters of the sample cell are known. In this case an accuracy better than 2% both in the real and in the imaginary parts of the dielectric constant can be obtained up to 7 GHz, as tested with chloroform. Calibration procedures using liquids of known permittivity can remove the contributions of spurious effects, so allowing us to perform measurements with the same accuracy up to 10 GHz and more on highly polar liquids too.