Fundamentals of Electromagnetics - RF/Microwave Interaction with Biological Tissues - page 2

respectively, are of a different nature. It excludes low-frequency (LF) and
extremely low frequency (ELF) effects, which do not involve any radiation. It
also excludes ultraviolet (UV) and X-rays, called ionizing because they can
disrupt molecular or atom structures. The RF/microwave frequency range
covered here may be called nonionizing .
Radiation is a phenomenon characterizing the RF/microwave range. It is
well known that structures radiate poorly when they are small with respect to
the wavelength. For example, the wavelengths at the power distribution fre-
quencies of 50 and 60 Hz are 6.000 and 5.000 km, respectively, which are enor-
mous with respect to the objects we use in our day-to-day life. In fact, to radiate
efficiently, a structure has to be large enough with respect to the wavelength
l. The concepts of radiation, antennas, far field, and near field have to be
investigated.
On the other hand, at RF and microwave frequencies, the electric ( E ) and
magnetic ( H ) fields are simultaneously present: if there is an electric field, then
there is a coupled magnetic field and vice versa. If one is known, the other can
be calculated: They are linked together by the well-known Maxwell's equa-
tions. Later in this topic, we shall be able to separate some biological effects
due to one field from some due to the other field. We need, however, to remem-
ber that we are considering the general case, which is that of the complete
field, called the EM field. Hence, we are not considering direct-current (DC)
and LF electric or magnetic fields into tissue.
Because we limit ourselves to the RF/microwave range, we may refer to our
subject of interaction of electric and magnetic fields with organic matter as
biological effects of nonionizing radiation . It should be well noticed that, by
specifically considering a frequency range, we decide to describe the phenom-
ena in what is called the frequency domain , that is, when the materials and
systems of interest are submitted to a source of sinusoidal fields. To investigate
properties over a frequency range, wide or narrow, we need to change the
frequency of the source. The frequency domain is not “physical” because a
sinusoidal source is not physical: It started to exist an infinite amount of time
ago and it lasts forever. Furthermore, the general description in the frequency
domain implies complex quantities, with a real and an imaginary part, res-
pectively, which are not physical either. The frequency-domain description is,
however, extremely useful because many sources are (almost) monochromatic.
To investigate the actual effect of physical sources, however, one has to
operate in what is called the time domain , where the phenomena are described
as a function of time and hence they are real and physically measurable. Oper-
ating in the time domain may be rather difficult with respect to the frequency
domain. The interaction of RF/microwave fields with biological tissues is inves-
tigated mostly in the frequency domain, with sources considered as sinusoidal.
Today numerical signals, such as for telephony, television, and frequency-
modulated (FM) radio, may, however, necessitate time-domain analyses and
measurements.