Dielectric lossy

The dielectric properties of most foods, at least near 2450 MH2, parallel those of water, the principal lossy constituent of food (Fig. 1). The dielectric properties of free water are well known (30), and presumably serve as the basis for absorption in most foods as the dipole of the water molecule interacts with the microwave electric field. By comparison, ice and water of crystaUi2ation absorb very Httie microwave energy. Adsorbed water, however, can retain its Hquid character below 0°C and absorb microwaves (126). 

Because conduction losses are high for carbon black powder it can be used as lossy impurities or additives to induce losses within solids for which dielectric losses are too small. 

The real part is the magnetic permeability whereas the imaginary part is the magnetic loss. These losses are quite different from hysteresis or eddy current losses, because they are induced by domain wall and electron-spin resonance. These materials should be placed at position of magnetic field maxima for optimum absorption of microwave energy. For transition metal oxides such as iron, nickel, and cobalt magnetic losses are high. These powders can, therefore, be used as lossy impurities or additives to induce losses within solids for which dielectric loss is too small. 

The formation ofC—C bonds between aromatic rings is an important step in many organic syntheses and can be accomplished by chemical, photochemical, or electrochemical means. As was noted earlier, fundamental considerations of the parameters for a dielectric which must be dealt with in designing a thermally stable, low-dielectric-constant polymer naturally lead one to consider rigid-rod, nonconjugated aromatic polymers containing no lossy functional groups. A structure such as poly(naphthalene) is a likely candidate. 

Umashankar, K., Taflove, A., and Rao, S.M., 1986, Electromagnetic scattering by arbitrary shaped three-dimensional homogeneous lossy dielectric objects, IEEE Trans. Antennas. Propagat. 34(6) 758-766. 

Affolter, P., and B. Eliasson, 1973. Electromagnetic resonances and 0-factors of lossy dielectric spheres, IEEE Trans. Microwave Theory Tech., MTT-21, 573-578. 

Lossy Transmission Lines. For lossy transmission lines, the conductor resistance, fl, and dielectric conductance, G, must be considered. The assumption G a)C is usually valid, because the dissipation factor, tan 8 = G/wC, is usually less than 0.01 for most packaging dielectrics (although the dissipation factor may become larger at very high frequencies). For high... 

Figure 1.2. Interaction of microwaves with different materials (a) - electrical conductor, (b) - isulator, (c) - lossy dielectric.

The penetration depth (Dp) of the materials used to denote the depth at which the power density of microwave irradiation is reduced to 37% (i.e., 1/e) of its initial value at the surface of the material. It is proportional to the wavelength of the radiation and depends on the dielectric properties of the material. For lossy dielectrics (e"/e 1) the... 

The waveguide system used to measure the dielectric parameters of water and other lossy liquids has been described previously (3J. Basically it involves the measurement of the power profile of a wave reflected from a movable short circuit as it traverses the liquid under test. 

A more detailed derivation of the dielectrophoretic force in lossy dielectric media has been given by Sher (52) and is based in turn on a derivation of the potential electric energy of a lossy dielectric body given by Schwarz (53). 

The method can be demonstrated by repeating the preceding analysis for a lossy dielectric. The instantaneous charge on a lossless vacuum capacitor C0 is... 

If the capacitor is now filled with a lossy dielectric, its effect can be taken into account by introducing a complex relative permittivity e = e r — je", where s r and s" are respectively the real and imaginary parts of the relative permittivity. It follows that... 

When an a.c. voltage is applied to a perfect capacitor, no energy is dissipated. However, a real capacitor dissipates energy because of lead and electrode resistances, d.c. leakage resistance and, most importantly, dielectric losses. These account for the capacitor s dissipation factor or loss tangent tan 3. It is sometimes convenient to regard the lossy capacitor as an ideal capacitor shunted by a resistance Rp or in series with a resistance rs, as shown in Fig. 5.5. 

In a microwave cavity loaded with a lossy dielectric material (such as a ceramic), the power is absorbed mainly by the casket (specimen enclosure) and the specimen. The losses in the cavity wall can typically be ignored such that Eq. (3) can be written as... 

Complex dielectric constant, with real (storage) and imaginary (lossy) components. 

The dielectric constant has, thus far, been treated as a single number. Such a treatment is equivalent to assuming that e is a static property of a material. Some of the energy of an applied electric field is, however, dissipated. Dissipation occurs when energy is lost to the "internal motions" of the material, which are defined as motions of the atoms from which the material is built [5,6]. This "lossy" component of the response of a material to an electric field is usually expressed in terms of the imaginary component of the complex quantity ... 

FIGURE 4.7 (a) Maximum global error versus distance from two lossy dielectric elliptical scatterers. (b) Convergence of reflection at the rate of the interior scheme... 

J. A. Pereda, O. Garcia, A. Vegas, and A. Prieto, Numerical dispersion and stability analysis of the FDTD technique in lossy dielectrics, IEEE Trans. Microzv. Guided Wave Lett., vol. 8, no. 7, pp. 245-247, July 1998.doi 10.1109/75.701379... 

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