EM Wave Absorbers Protecting Biological and Medical Environment - RF/Microwave Interaction with Biological Tissues - page 222

when the relative permittivity is 14, the matching frequency characteristics
deteriorate in the present frequency region. Common rubber ferrite has a
maximum value between Kf 1

7 GHz with a low K value.
This is one reason why rubber ferrite is presently selected at microwaves.

= 6 GHz and Kf 1 =

Effect of Permittivity. To investigate the effect of permittivity, general design
charts have been established. Figure 5.23 illustrates the cases where the real
part of relative permittivity takes the values 10 and 25 at the frequencies 2.45,
3.0, and 4.0 GHz. These permittivity values are the usual limits for rubber
ferrite. If the rubber ferrite accounts for a large imaginary part of permittiv-
ity, we do not obtain a good matching characteristic. In the present study,
rubber ferrite with a small imaginary part has been assumed from the outset.
These data show an example, with values of Kf 1 =

3 in Eqn.
(5.27), while the ferrite thickness is 6 mm. From these design charts, compar-
ing Figures 5.23 a , d , we find that a larger permittivity value (e r ¢=

4.5 GHz and K =

25) is effec-
tive when the matching frequency is lower. On the contrary, when the
matching frequency is higher, Figures 5.23 c , f show that a smaller permittivity
value is effective for obtaining good matching characteristics.

Effect of Ferrite Thickness. Next let us consider the ferrite thickness effect.
Figures 5.24 and Fig. 5.25 illustrate the behavior of matching characteristics
taking the ferrite thickness as a parameter. Figure 5.24 represents the changes
of matching frequency characteristics when the ferrite thickness is decreased
from the original thickness and hole sizes are increased, keeping hole adjacent
spaces constant. It is found that the matching frequency characteristic shifts
toward a higher frequency region as hole sizes are increased. One can find that
there is an optimum hole size to obtain a good matching characteristic for each
ferrite thickness. Similarly, Figure 5.25 shows the changes of matching fre-
quency characteristics when the ferrite thickness is decreased and adjacent
hole spaces are decreased, keeping hole size constant. Even in this case, it is
found that an optimum adjacent hole space exists for each ferrite thickness.

Method of Perforation. It is obvious that an accurate methodology for con-
trolling the hole diameter for this rubber absorber is important. For this
purpose, the authors investigated a laser perforation method. Figure 5.26
shows an example of holes with a diameter of 2 mm and adjacent hole spaces
of 9 mm. The circular holes are formed using a CO 2 laser with a maximum
power output of 200 W and a CW maximum pulse frequency of 10 kHz (Rofin
Sinar Laser, SC 20). The diameter accuracy is checked using a profile

projector (Nikon, V-12). At present, holes with an error factor of less than
±

¥

0.1 mm can be formed. An error factor of this order does not raise problems
in the present case. Further, smaller diameter perforation is also examined. We
find that 0.1 mm hole diameter can be formed even when sintered ferrite is
used. This accurate perforation method is important when the present
absorber is used in the millimeter frequency region where a small diameter is