EM Wave Absorbers Protecting Biological and Medical Environment - RF/Microwave Interaction with Biological Tissues - page 220

High

Adjacent hole space

FIGURE 5.20 Relation of both hole sizes and adjacent hole spaces versus frequency.

Figure 5.20. Hatched-line regions represent the values of the reflection coef-
ficient under 20 dB. This figure means that both the hole size and the adja-

cent hole distance need large sizes to obtain matching at a higher frequency.

-

Effect of Permeability. To evaluate the matching characteristics, the following
frequency dispersion equation of permeability is introduced [9]:

Kf

m ¢= + 1

1

=¢- ¢ m j m (5.27)

r r r

f + jf

1

where K is the static magnetic susceptibility when f is zero, f is the operating
frequency, and the limit of Kf 1 is defined as the following expression derived
from experimental investigation:

Kf 1 £ 10 GHz (5.28)

In this expression, if Kf 1 takes on a larger value, the imaginary part of the
relative permeability tends to increase. This means that the effect of changing
Kf 1 is mainly reflected in the value of the imaginary part of permeability. On
the other hand, the change in value of K primarily affects the real part of the
permeability: If K takes on a larger value, the real part of permeability is
almost 1.0 in the actual frequency region. When K takes on a lower value, the
real part of permeability becomes greater than 1.0. Figure 5.21 shows cases in
which Kf 1 is taken as a parameter and K is assumed to take extreme values
between 10 and 10 3 . Figure 5.22 shows the matching characteristics with holes
when Kf 1 is taken as parameter, the values of K being 10 and 10 3 . It becomes
clear that we obtain good matching characteristics when the value of K is low,
even if Kf 1 is modified. Further, as the value of Kf 1 increases above 7 GHz