A Fundamental Study on Water Content Comparison in Phenolic Foam Columns by using Microwave Backscattering Measurement

The ability to measure the amount of water dispersed inside a solid object is highly desirable in a number of fields such as agriculture, forestry, and civil engineering. In this study, the authors’ goal was to measure the water content inside phenolic foam columns using only microwave backscattering measurements by a scatterometer developed for airborne synthetic aperture radar (SAR). The experiment was carried out in an anechoic chamber using scatterometers three frequency bands: L, X, and Ku. The column irradiated with microwaves was a cylinder of phenolic foam capable of holding various volumes of water. Four objects with a different volume of water permeating were irradiated by microwaves, and the backscattering was measured. In consideration of the influence of microwave fading, the columns were placed on a turntable and rotated one revolution (i.e., 360°) while taking about 75,000 continuous measurements of the entire surface. The measurements were then evaluated based on variance and median. As a result of measuring the microwave backscattering, it was found that the higher the water content in the column, the higher the radar cross-section (RCS) median, average, and maximum values for that object in all three bands. Regarding the L band, it was clearly shown that it was possible to distinguish when the volume content of water was 25 % and 50 %. Also, when the water content of the column was relatively small, the range of dispersion was large, and when the water content exceeded a certain value, the dispersion widths began to converge. This indicates the possibility that analyzing the variance of the microwave backscattering may be a clue to knowing the dispersion state of the water content of the object. In this experiment, the microwave backscattering was continuously measured while rotating the object one time, and a statistical method was used to analyze the results. This measurement method is new, and it could add a new approach to measuring moisture content noninvasively.