Precision Measurement of the Microwave Dielectric Constant of Liquids

A method utilizing reflection measurements on a liquid-filled line section terminated in a short circuit is described. Its chief feature is the use of line lengths such that the impedance at the liquid surface is real, which simplifies the calculation ofε* from the measurements. Additionally it reduces systematic errors due to the window used to constrain the liquid surface, and the nonzero impedance of the sliding plunger used to adjust the length. These factors give errors inε′ about an order of magnitude below those inε″. This makes the method particularly suitable for determination of the dielectric relaxation time of solutions of polar in nonpolar liquids. Limits on the window and plunger parameters to give an accuracy of about 5% inε″ and 0.5% inε′ are derived. Results of measurements with equipment satisfying these requirements are given, and it is seen that relaxation times can be found to about 5% accuracy for solutions 1–10 Molar % nitro-benzene in benzene.