Fundamentals of Electromagnetics - RF/Microwave Interaction with Biological Tissues - page 24

topic. In such cases, near-field calculations have to be done, which may require
much more attention.
As has been said before, antennas are reciprocal, except for very special
devices. Hence, near-field situations have to be taken into account not only for
transmitting antennas such as TV and FM radio transmitters but also for
receiving antennas. Antennas can indeed be implanted in living tissues and
organisms for medical applications.
When the antenna is used to deliver microwave power to heat tissue, the
size and location of the microwave field have to be carefully located to control
the affected tissue. Hence, the type and shape of the antenna are very much
dependent upon the specific application, and there are a variety of applicators.
A main problem is of course that of matching the applicator to the tissue.
However, EM energy transfer depends, to a great extent, on the absorption
properties of the tissue. It also depends on the frequency. As an example,
sources at millimeter waves yield results similar to IR frequencies. These
aspects will be investigated in detail in Section 1.5.

1.4 RF AND MICROWAVE ENERGY

1.4.1 Power and Energy

Adequately combining Maxwell's equations yields what is called Poynting's
theorem . In the time domain, it expresses equality between the spatial variation
of EM power and the time variation of EM energy , the sum of the electric and
magnetic energies [3, 6, 7, 11]. The cross product of the electric field and the
magnetic field is called the Poynting vector, with units of volt-amperes per
square meter (watts per square meter). Being a cross product, it is perpendi-
cular to the plane of the two vectors. In Section 1.3.4, we have seen that the
TEM wave is the simplest wave structure, with the electric field perpendicu-
lar to the magnetic field, both fields being perpendicular to the direction of
propagation. Hence, the Poynting vector of a TEM wave is in the direction of
propagation.
Power and energy have not to be confused. Electromagnetic power is rep-
resented by the Poynting vector, just described. The integration of this vector
over an open surface yields the power flow through the surface, in watts .The
integration over a closed surface, with the normal to the surface considered
as positive when extending outside the surface, also yields the power flow
through the surface, that is, the total power coming out of the volume bounded
by the closed surface. If this power is negative, it means that the net power is
entering the volume, which also means that the medium inside the volume has
absorbing losses—electric, magnetic, or conductive. The total power absorp-
tion is obtained by integrating the losses over the whole volume: Absorption
is associated with power .
On the other hand, in the domestic sense of the word, energy means power
absorption for some time in watt-seconds . In electromagnetics, however,