A phase shift method of measuring microwave absorption in liquids with experimental results

Abstract

The method evolved from an Investigation of transmission line methods of measuring the absorption of electromagnetic radiations in liquids. An equation was derived relating the phase angle of the voltage reflection coefficient to the length of the liquid filled line beyond a short circuit. The experimental results show that this equation describes exactly the behaviour of the phase angle as the depth of liquid filling the transmission line is varied. The equation has been shown to be true for a wide range of liquids (a) Non-absorbing liquids, i.e. Benzole. (b) Liquids with a low absorption, i.e. Chlorobenzene and Cyclohexanol. (c) Dilute solutions of polar substances in non-polar solvents, i.e. Chlorobenzene in Benzole. (d) Liquids with high absorptions, i.e. Water and Alcohol. (e) Electrolytic solutions, i.e. Sodium Chloride in water. Values of the phase constants and attenuation coefficients for these liquids have been deduced from the graphs showing phase angle plotted against liquid depth, with an accuracy that compares favourably with other established methods. The method has been applied to an investigation of the absorption of microwaves (lambda = 9.1 sms) by ox blood and serum at room temperature and the inner body temperature of 37°C, and these results have been compared with those obtained for distilled water at these two temperatures. The variation of the phase constant and attenuation coefficient of sodium chloride solutions with concentration has also been investigated, from 0.15N (physiological saline) up to 2N. This method, which depends upon measurement of phase shift alone for the determination of alpha and beta, requires a very much simpler form of apparatus than other methods. The method is best suited to liquids with low values of alpha and high values of beta, and a modification to the apparatus and experimental method has been suggested which should enable values to be obtained more rapidly. This modification has not so far been made, since the accuracy obtainable with this phase shift method is inferior to that of the null techniques recently introduced into dielectric measurements.