Study On Measurement With Swept-frequency For Complex Permittivity Of Liquid Materials

With the development of materials science and technology, liquid materials whichwere not put in use have been increasingly arousing attention. Dielectric constant is themost important index for electric materials, of which different indexes have differentapplication ranges. Thus, in this paper, an integrated automatic test system which wasdesigned with the transmission line method was adopted to measure complexpermittivity of liquid materials.Considering the radiation effects of an open-ended coaxial probe which is in liquid,on the basis of capacitive model a new equivalent circuit model (antenna model) wasestablished between the dielectric constants of liquids and the end face reflectioncoefficient and the functional relation between the two was deduced theoretically.Different from the traditional method of measurement known liquid, it was proposedcreatively that by basing on any permittivity of given materials and employing anaccurate simulation software, the parameter values in functions could be preciselydetermined. By employing this method, not only can errors of definitive parametervalues led by different temperatures, fixtures, and frequencies be avoid, but also theprocedure becomes more convenient and efficient because the measurements of plentyof liquids to determine the functional relations undetermined parameters has becomeunnecessary. In this paper, the undetermined parameters “K” have been determinedprecisely, while accurate measurement of antenna models has been designed. When thefrequency is below20GHz, the computing result of equivalent model is identical withthe simulative result of3D electromagnetic field, which demonstrates that the accuracyof antenna model is great and the parameter value “K” has been determined accurately.Being based on that, a piece of automatic test software has been designed, alongside anautomatic test system having been organized.In the paper, minute inspections and analyses of substantial types of liquids at distincttemperatures were conducted, and the test results of high dielectric constant materials,such as deionized water, at different temperatures (20℃,25℃,35℃) together withCalculation results by using the Cole-Cole formula were comparatively analyzed. The test result shows that the test frequency was up to1~20GHz. The test errors of both realpart test and imaginary part test of complex dielectric constant were below5%, whichindicated that the precision of the test was extremely high. The result of testing themedium and low dielectric constant materials, such as Industrial ethanol shows that thetest errors of both real part test and imaginary part test of complex dielectric constantwere below3%, which indicated that the precision of the test was high equally. Therepeatability of test system and the repeatability of test results met the requirements ofindicators. Thanks to the consideration of the existence of radiation in equivalent modelas well as the accurate simulation to determine undetermined parameter being adopted,the automatic test system performed well when measuring the dielectric constants ofliquids. Test results in every aspect of this system excelled the anticipative designguideline and the results of transmission method for the measurement of complexdielectric constants of liquids. The method of measurement and the method ofdetermining undetermined parameter positively propelled the measurement of dielectricconstants of materials in transmission method.