Effect of Exit-Hole Frequency Shift and Wall Loss on the Permittivity Measurements of Seawater at L-Band
[05-Dec-2017] Zhou, Y., Lang, R., Utku, C., and Le Vine, D.
Presented at the 13th Specialist Meeting on Microwave Radiometry and Remote Sensing of the Environment
The accuracy of the George Washington University's (GW) L-band cavity measurements of seawater is examined by measuring the effect of the capillary tube's exit hole on the resonant frequency and by performing a theoretical study of the effect of cavity wall loss on the perturbation equations. The purpose of this study is to confirm the accuracy of past seawater measurements. GW has employed a cavity technique to determine the complex permittivity of seawater at L-band (1.413 GHz). A capillary glass tube is installed in a brass cavity through a center hole in each endplate. Seawater is introduced into the cavity through the glass tube. By applying perturbation theory, the changes in the resonant frequency and in the cavity Q value can be used to determine the real and imaginary parts of the dielectric constant of seawater, respectively. Based on the measurement data, an accurate model function of seawater permittivity has been developed. In this paper, the effect of the exit hole on the cavity resonant frequency is studied by using a specially designed exit hole bushing. The bushing is made so that graphite can be inserted around the tube just after it exits the cavity. This graphite attenuates the fields as they exit the tube. Measurements are being made with and without the graphite to see the effect. Simulations that have been made by  have predicted that a small effect should be observed. In another test of the measurement's accuracy, the effect of cavity wall loss is studied. The naval brass construction of the cavity is more lossy compared to copper, silver or gold. It has been preferred however because it can be machined more precisely. A theoretical study is made to determine the effect of this loss on the accuracy of the perturbation equations. Exact modal equations are developed for the cavity without assuming that the loss in the cavity walls is small. These results are compared with the perturbation theory findings to test their accuracy. Initial results of the exit hole and wall loss effects appear to be small but further results will be reported at the meeting.  Per O. Risman and Birgitta Wäppling-Raaholt (2007), Retro-modelling of a dual resonant applicator and accurate dielectric properties of liquid water from -20°C to +100°C, Meas. Sci. Technol. 18(4), 959-966.