Fundamentals of Electromagnetics - RF/Microwave Interaction with Biological Tissues - page 38

some slow decrease, a third level of about 70-80 from 100 MHz to some giga-
hertz. This last value is that of the dielectric constant of water at microwaves.
One of the main constituents of human tissues is water. Hence, we have about
the same microwave properties as water.
However, as has been said in Section 1.1, microwaves are in the frequency
range in which the wavelength is of the order of the size of objects of common
use—meter, decimeter, centimeter, and millimeter—depending of course on
the material in which it is measured. One may hence wonder whether such
wavelengths can excite resonance in biological tissues and systems. We shall
come back with this question in Chapters 2 and 3.

1.6 RELAXATION, RESONANCE, AND DISPLAY

A good knowledge of the complex permittivity of biological media is neces-
sary for evaluating biological effects as well as in medical applications. Hyper-
thermia is only one example of such an application (treated in great detail in
Chapter 4). A number of measured data are available for characterizing bio-
logical media. It should be mentioned, however, that there are not many meas-
ured data for biological and organic liquids at frequencies above 20 GHz.

1.6.1 Relaxation in Dielectrics

The dielectric constant is the real part of the complex relative permittivity. It
is of primary importance when characterizing dielectrics. It should not be for-
gotten, however, that the permittivity is complex in the frequency domain and
that the dielectric constant gives only partial information.
Up to about 1 GHz, materials respond to relaxation phenomena, already
illustrated in Section 1.3.1. When a rarefied nonpolar gas in which the mole-
cules have no electric dipoles at rest is submitted to an external electric field,
an electric dipole is induced. When a rarefied polar gas which has an electric
dipole at rest is submitted to an external electric field, the dipole orientation
is modified: It essentially rotates. As has been said earlier, when the density
increases, classical physics almost completely fails when trying to establish
quantitative models. It can, however, yield some insight on the phenomena
involved with the dielectric character of materials.
The alignment of the molecule dipolar moment because of an applied field,
called dipolar polarization , is a rather slow phenomenon. It is correctly
described by a first-order equation, called after Debye [4, 5]: The dipolar polar-
ization reaches its saturation value only after some time, called the relaxation
time t (Fig. 1.1).
This looks simple. The process is rather complicated, however. The task of
dielectric theory is difficult, not so much because permanent dipoles cannot
always be identified but mainly because they mutually influence one another:
A dipole not only is subject to the influence of a field but also has a field of