Fundamentals of Electromagnetics - RF/Microwave Interaction with Biological Tissues - page 28

is perfectly correlated with the incident signal. The perfect reflector may be
called a white body .
Physically, blackbody radiation is produced by a body that is considered as
a closed volume whose walls are in thermal equilibrium at a given tempera-
ture. An outside source maintains the whole wall at a constant temperature.
It the body is a blackbody, there are no losses and the energy balance is zero.
The incident radiation is transformed into thermal agitation, which in turn is
transformed into emitted radiation. The emitted signal is perfectly noncorre-
lated with the incident signal [17]. As a consequence, a blackbody radiates at
least as much energy as any other body at the same temperature. At microwave
frequencies, good approximations to blackbodies are the highly absorbing
materials used in the construction of anechoic chambers. Real materials,
such as the earth for instance, usually referred to as grey bodies , emit less than
blackbodies and do not necessarily absorb all the incident energy. As an
example, part of the solar energy incident upon the earth is directly reflected
back, so that only part of the incident energy is transformed by a blackbody
process and emitted back. As a consequence, the “blackbody temperature” of
the earth, that is, the temperature at which a blackbody would emit the same
energy as that emitted by the earth, is lower than the physical temperature of
the earth; at microwaves, this is about 254 K.
It must be noted that blackbody radiation is usually at a very low level with
respect to other radiation sources, in particular power transmitted by com-
munications systems and, especially, by radars. A few examples are worth men-
tioning. The sun's total power output is approximately 3.9 ¥ 10 26 W, little of
which appears at wavelengths below 0.3 nm or above 3 mm. The solar energy
per minute falling at right angles on an area of 1 cm 2 at a solar distance of 1
astronomical distance (equal to the mean earth's solar distance) is called the
solar constant and at a point just outside the earth's atmosphere has a value
of 0.14 W cm -2

= 1.4 kW m -2 . The planet earth reradiates a part of the received
solar energy, equal to the difference between the energy received and the
earth's albedo (relative energy absorbed is 0.34 for the earth), equal to
0.023 W cm -2 = 230Wm -2 . Cosmic noise extends from about 20 MHz to about

4 GHz. Man-made noise is an unwelcome by-product of electrical machinery
and equipment operation and exists from frequencies of about 1 MHz to about
1 GHz. The peak field intensity in industrial areas exceeds the value of cosmic
noise by several orders of magnitude, which draws attention to the need for
judicious ground station site selection. Blackbody radiation is evaluated later
in this section.
Blackbodies will be reevaluated in Chapter 2, devoted to an introduction
to biological effects, more precisely hazards due to RF and microwave fields.
In that chapter, energy considerations will be compared with entropy consid-
erations while evaluating the possibility of isothermal biological effects.

Planck's Radiation Law Planck established the mathematical expression of
the energy emitted by a blackbody in 1901, in his blackbody radiation law, gen-
erally termed Planck's law. It yields the spectral energy density per unit