Biological Effects - RF/Microwave Interaction with Biological Tissues - page 90

ing a reduced-scaled antenna for predicting the behavior of a huge antenna to
be designed and built. With this in mind, it is necessary to establish the con-
ditions to be satisfied for ensuring that the EM situation of the reduced-scale
model is identical to that of the normal-scale model. This is accomplished by
transforming the basic equations of the specific technical area, Maxwell's
equations in electrical engineering, to obtain dimensionless equations. This iso-
lates dimensionless factors that have to remain constant when going from one
model to another and indicate how a parameter has to be modified when
another parameter is submitted to a change.
The frequency-scaling application is well known. It is often said that the
conditions of EM similitude are satisfied when a smaller size model is sub-
mitted to a correspondingly smaller wavelength, that is, a correspondingly
higher frequency. From this, it is frequently understood that a two-times
smaller model will yield the same results as the original one if the frequency
is doubled. It should be strongly emphasized that this is true only if there are
no losses [4, 5]. The reason is in the Maxwell's equations: In the presence of
losses there is one more term to take into account. This modifies the factors
that have to remain constant when shifting from one model to another to keep
the equations dimensionless. As a consequence, keeping the ratio size-
wavelength constant is not a sufficient condition anymore. There are circum-
stances where frequency scaling is very inaccurate, if not inadequate. This may
indeed be the case in biological systems because biological losses can be very
important.
The frequency-scaling principle can be used, for instance, to determine the
equivalence between the exposure of a man of a height l m and an animal of a
height l a , with both man and animal having the same orientation with respect
to the exposure field, using the relationship

fl = fl (3.3)

mm aa

where f m and f a are the frequencies at which man and animal are exposed,
respectively [6]. It must be emphasized that the SAR distributions will only
be similar in the two bodies, not equal, and that the equivalence is essentially
valid for low losses, which may not be the case in biological structures.

3.1.3 Thermal Considerations

Thermal effects have been investigated for some time. It should never be for-
gotten that to obtain reliable temperature measurements, only probes suitable
for operation in RF fields should be used. Thermal measurements are impor-
tant, in particular on human beings. A comprehensive database is available on
effects of a thermal nature, but it mainly concerns animal studies and in vitro
studies. Several methods of biological effect determination are based on
thermal measurements: