Dielectric Properties of Materials at Microwave Frequencies

KŘIVÁNEK, I.: Dielectric properties of materials at microwave frequencies. Acta univ. agric. et silvic. Mendel. Brun., 2008, LVI, No. 5, pp. 125–132 The paper introduces the review of the present state of art in the measurement of the interaction of electromagnetic waves with diff erent kinds of materials. It is analysis of the possibilities of the measurement of the interaction of high frequencies waves (microwaves) with materials and proposal of the experimental method for the studies mentioned above. The electromagnetic fi eld consists of two components: electric and magnetic fi eld. The infl uence of these components on materials is diff erent. The infl uence of the magnetic fi eld is negligible and it has no impact on practical use. The infl uence of the electric fi eld is strong as the interaction between them results in the creation of electric currents in the material (Křivánek and Buchar, 1993). Experiments focused on the evaluation of the complex dielectric permitivity of diff erent materials have been performed. The permitivity of solid material is also measurable by phasemethod, when the specimen is a part of transmission sub-circuit. Microwave instrument for complex permittivity measurement works in X frequency band (8.2–12.5) GHz, the frequency 10.1 GHz was used for all the measurement in the laboratory of physics, Mendel University in Brno. The extensive number of experimental data have been obtained for diff erent materials. The length of the square side of the aerial open end was 50 mm and internal dimensions of waveguides were 23 mm × 10 mm. The samples have form of the plate shape with dimensions 150 mm × 150 mm × 4 mm. dielectric permitivity, microwaves, phasemethod, waveguides The theory is mainly focused on the microwave transmission and refl ection. High-frequency electromagnetic fi eld analysis simulates the electromagnetic phenomena in a structure using the wavelength of the signal of the same order of magnitude or smaller than the dimensions of the model (Ohlson et al., 1997). It typically examines power fl ow by means of electromagnetic waves, power loss due to electric loss tangent, wave scattering properties (transmission, refl ection and diff raction), and frequency dependent parameters (S-parameters) that are typically measured (Risman, 1991). The underlying problems are defi ned as the high-frequency/ full-wave electromagnetic fi eld problem. The highfrequency band ranges from hundreds of MHz to hundreds of GHz. Typical applications include microwave passive components, antenna radiating structures, radar cross section (RCS) of an aircra , and electromagnetic compatibility assessment of electronic components and products. Analytical expressions describing the fi elds exist only within simple geometries. As soon as complicated structures containing lossy material properties are encountered, equations become diffi cult or impossible to be solved analytically (Ackmann and Seitz, 1984). In these cases numerical modeling provides va luable information on such parameters as the electric and magnetic fi elds, and the power absorbed by the structure (Tinga et al., 1973). Description of the dielectric properties The interaction of the electromagnetic waves with material involves phenomena such as induced electronic, atomic, and space charge polarizations. The extent of this interaction depends on the extent of the wave refl ection or transmission, respectively. The transmission and refl ections are governed by Maxwell equations: