Soil Dielectric Spectra from Vector Network Analyzer Data

Stogryn, 1971). The dielectric relaxation frequency occurs at the peak in the imaginary component, which is For more than two decades, dielectric properties (i.e., permittivity) the midpoint of the change in the real component. Water have been used for soil water content measurements, but unexpected results have been shown for saline soils or soils high in smectite clays. has a sharp dielectric relaxation, which means there is Permittivity is complex with real and imaginary components, and varies a large increase, and then a leveling off of the real permitwith frequency. The frequency dependence is due to dipole rotation tivity at frequencies below the relaxation frequency. In the and charge migration processes under alternating current. Dielectric range of most dielectric measurements in soil (10 MHz to relaxation refers to the reorienting of molecules after an electrical field 1.5 GHz), there is very little change in the real component is removed. The objective of this study was to develop a procedure for of permittivity free water. Also, the permittivity of dry determining complex permittivity spectra for soils. Six soils with a soil and air do not change with frequency. However, when range of mineralogies were preequilibrated at four duplicated water all are mixed together, there is often a large increase in contents. They were packed into truncated coaxial cells, and the reflecreal permittivity as the frequency decreases. The spectra tion-scattering parameter was measured for frequencies between 300 become complicated because of overlapping relaxations kHz to 3 GHz, although the primary calculation procedure was valid to 100 MHz. The calculated complex permittivity was difficult to use and overlapping electrical conductivity contribution to for deriving unique parameters because of overlapping influence of the imaginary component of permittivity. The relaxations electrical conductivity and multiple dielectric relaxation processes are due to interactions between the soil particles and that extended beyond the measured frequency range. The complex water; that is, not all of the water behaves like free water. resistivity was easier to interpret, clearly showing one major relaxation Material scientists have usually measured permittivity process, except for the driest soils. The relaxation frequency determined on very small samples that can be filled, cut, or packed from complex resistivity increased significantly as water content and into airlines (7-mm diam.), but this is much too small electrical conductivity increased. For each soil, the square root of for soil analysis. Material scientists have depended on apparent permittivity, εa 1/2, significantly increased as frequency detime domain measurements (converted to frequency docreased and as water content increased. The εa showed frequency main by lumped capacitance approach; Cole et al., 1989; dependence ranging from 1.1-fold per order of magnitude for Cecil soil to almost 2.5-fold for Ida soil. Developing procedures to extend Berberian and King, 2002) because of the broad meathe measured and calculated frequency range would enhance data surement frequency range of the technique. Hydrologists analysis options. (West et al., 2003; Huisman et al., 2004) have favored the vector network analyzer (VNA) over complicated backcalculations from field TDR equipment (Feng et al., F more than two decades, dielectric properties 1999; Lin, 2003), even though the frequency range is rehave been used for soil water content measurements, stricted for the VNA measurements. The VNA is a tool but unexpected results have been shown for saline soils that determines complex electrical properties of materi(Wyseure et al., 1997; Nadler et al., 1999; Seyfried and als, including the imaginary or loss component or angle. Murdock, 2004; Kelleners et al., 2004a, 2004b; Jones The vector component refers to the vector math used and Or, 2004) or soil high in smectite clays (Bridge et al., for complex numbers (as opposed to a scaler network 1996; Wraith and Or, 1999; Logsdon, 2000). analyzer that only measures the real component). Dielectric spectroscopy (frequency-dependent) is ofThere are considerations for getting reliable and reten used to characterize materials. The spectra are often producible results from VNA data. Will an attachment described by relaxation processes. An applied electrical be used with the VNA that is specifically designed for field causes both the movement of charge carriers and dielectric properties, and what are the attachment limithe alignment of dipolar molecules (Jonscher, 1996; Bakertations for sample size and frequency range? Will homeJarvis, 2000). When the electrical field is removed, the made sample holders be used, and what form of data molecules reorient back to a more stable arrangement, does the VNA generate? Will the measurements be rewith a time lag. The time lag of reorientation or relaxflection or transmission, and which calculation proceation varies as a function of frequency for alternating dures will be used? What calibrations are necessary? electrical fields. Frequency-dependent real permittivity has been meaThe permittivity of free water (Fig. 1) is well defined sured in clay slurries and gels (Raythatha and Sen, 1986; Ishida and Makino, 1999; Ishida et al., 2000; Dudley et al., 2003), for humidified clays at low water contents (CalNational Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011. Manufacvet, 1972, 1975; Logsdon and Laird, 2002, 2003, 2004a, turer’s names are given for information only, and do not constitute endorsement by the USDA. Received 9 Nov. 2004. *Corresponding 2004b), and for soils (Anis and Jonscher, 1993; Arulaauthor ([email protected]). nandan et al., 1973; Campbell, 1990; Arulanandan, 1991; Rinaldi and Fransisca, 1999). The limited soils database Published in Soil Sci. Soc. Am. J. 69:983–989 (2005). has not yet shown what useful information can be gained Soil Physics doi:10.2136/sssaj2004.0352 © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: VNA, vector network analyzer. 983 Published online June 2, 2005