Microwave Absorption and Molecular Structure in Liquids. XXV. Measurements of Dielectric Constant and Loss at 3.1-mm Wavelength by an Interferometric Method

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Abstract
The theory and apparatus for an interferometric method for the measurement of the dielectric constant and loss of a liquid at 3.1‐mm wavelength are described. The 3.1‐mm wave is obtained as the fourth harmonic from a K‐band reflex klystron. The probable error of the dielectric constant measurements is 0.5%, while that of the loss measurements is 2–5% for high loss liquids. For very low losses, only an upper limit can be obtained because of the small amount of harmonic energy obtainable. Four tetrahalogenated methanes, three substituted benzenes, two methylquinolines, three alcohols, and water have been measured and the results examined in the light of measurements at lower frequencies. All of the liquids but the three alcohols give 3‐mm points lying satisfactorily on Cole‐Cole arc plots drawn through points previously obtained at lower frequencies. The losses observed at 3 mm are, however, some‐what higher than the calculated indicating the probable presence of overlapping absorption regions at still higher frequencies. The apparent optical dielectric constant value 6.0 found for water evidences further absorption at much shorter wavelengths. Methyl, ethyl, and n‐octyl alcohols show absorption regions at frequencies too high to be attributable to molecular dipole orientation. The critical or maximum absorption wavelengths found for these additional high‐frequency absorption regions are 0.57 cm for ethyl alcohol and 0.50 cm for n‐octyl alcohol.