Permittivity and Measurements

Propagation of electromagnetic (EM) waves in radio frequency (RF) and microwave systems is described mathematically by Maxwell’s equations with corresponding boundary conditions. Dielectric properties of lossless and lossy materials influence EM field distribution. For a better understanding of the physical processes associated with various RF and microwave devices, it is necessary to know the dielectric properties of media that interact with EM waves. For telecommunication and radar devices, variations of complex dielectric permittivity (referring to the dielectric property) over a wide frequency range are important. For RF and microwave applicators intended for thermal treatments of different materials at ISM (industrial, scientific, medical) frequencies, one needs to study temperature and moisture content dependencies of the permittivity of the treated materials. Many techniques have been developed for the measurement of material permittivity. Some general descriptions of those methods are provided in the literature [1–4]. The measurement results for diverse dielectric media are compiled in the literature [5–7]. The objective of this article is to provide a brief description of basic physical theory for the measurement of permittivity associated with three popular experimental methods and to review the permittivities for different materials. Most of these data are taken from the literature, but some of permittivity data were measured in Biological Systems Engineering Department of Washington State University. The measurements were conducted by means of open-ended coaxial probe method with commercial instrumentation.