MICROWAVE MEASUREMENT OF MATERIAL PROPERTIES

Abstract

In recent years, much theoretical and experimental work has been carried out on the determination of dielectric constant and conductivity of material by microwave methods. A main advantage of these methods is that no direct contacts are needed to the sample. Electrical properties or materials can thus be quickly and continuously measured for laboratory and industrial use. In chapter 1, a brief survey of various microwave methods or determining the electrical properties of materials is given. These methods include cavity resonance, reflection and transmission methods. The microwave methods are also compared with other conventional methods. The derivation of dielectric constant and conductivity and their frequency and temperature dependence are discussed. A brief review of some recent work is also presented. The fundamental theory of guided electromagnetic waves is described in chapter 2. Experimental technique and precautions are also discussed. In the experimental work, a short-circuited waveguide method has been used to determine the dielectric constant of some materials. As a tangential function ls involved in this method, an ambiguity occurs in the solution for dielectric constant. In chapter 3, we introduce a new experimental parameter to eliminate the ambiguity. The justification for the new parameter is given and measurements on some standard materials were performed to verify the effectiveness of the parameter. The microwave bridge which was designed and constructed is described in chapter 4. The two arms of the bridge are connected by two l0dB couplers. The phase shift and attenuation due to the insertion of a sample can be read from the calibrated phase shifter and attenuator respectively. The dielectric constant of wax, perspex and silicon can then be deduced experimentally from the measured phase shift of the transmitted microwave. The conductivity of silicon is determined from the phase shift and attenuation of the transmitted microwave. The temperature dependence of the conductivity of silicon was also studied from about 400K to 600K. A thermal control unit including a heat bath was also constructed for this part of the work. The conductivity of silicon was found to decrease with temperature at low temperatures. Above 520K the conductivity increases exponentially with temperature. This ls due to the fact that at low temperatures, the extrinsic carriers contribute more to the conduction current whereas at sufficiently high temperatures the intrinsic carriers contribute more. From the experimental data, the band gap of the silicon sample was found to be 1.110eV, which compares favourably with the standard value of 1.106eV. In chapter 5, the microwave bridge was used to study the moisture content of soil samples. The attenuation and phase shift of the microwaves were assumed to be linearly dependent on the mass of the soil and water. An empirical equation with three independent coefficients was proposed for the dependence of phase shift and attenuation on the moisture content. The coefficients of this equation were obtained from known values of the moisture content of soil. The accuracy of the equation was verified with respect to the thickness and surface condition of the sample. The error of the equation ls less than 0.2% for thicknesses greater than 8mm when the surface of the sample ls cut smoothly. The temperature dependence of the coefficients were also determined by measurements at 283K, 298K, 313K and 328K. A chart from which the value of moisture content may be directly read was also obtained from the empirical equation. Part of our work has been published in the following two papers: 1. K H OH, C K ONG and B T G Tan, 11 Elimination of ambiguity in measurement of permittivity with short-circuited waveguide" Electronics letters 23 ( 1987). 2. K H 01-1, C K ONG and B T G Tan, "A simple microwave method of moisture content determination in soil samples'' published in Journal of Physics E (Sci. lnstrum) (1988).