EM Wave Absorbers Protecting Biological and Medical Environment - RF/Microwave Interaction with Biological Tissues - page 230

5.32 c , d show the absorbers printed with periodical square frames and double-
layer periodical square frames, respectively. The back of the rubber ferrite is
attached to a conductive plate. These fundamental matching characteristics
have been investigated using FDTD analysis. Matching characteristics corre-
sponding to Figure 5.32 are shown in Figure 5.33. Figure 5.33 a , b show the
matching characteristics in the cases of a thin conductive line lattice and cross-
line patterns, respectively. Figures 5.33 c , d show the case of periodical squares
and double-layer squares, respectively. The matching characteristics shown in
Figure 5.33 a corresponding to Figure 5.32 a shift toward higher frequency
regions as the conductive lattice size b is decreased. This is because these
conductive patterns behave as if inductance is added to an equivalent trans-
mission line circuit of the present absorber, which originally consists of resist-
ance and inductance. However, the matching characteristics in the case of
Figures 5.32 b , c shift toward lower frequency regions as the space between the
adjacent conductive patterns is decreased. This is because these conductive
patterns behave as if capacitance is given at the end of the above-
mentioned equivalent transmission line together with resistance and induc-
tance. If a plate with a double-layer line pattern of squares is attached to the
surface of the ferrite absorber, the matching characteristic starts to exhibit a
twin-peak characteristic as shown in Figure 5.33 d . Figure 5.34 shows a com-
parison of theoretical values with experimental values for a normal incidence
in the case of periodical square patterns. This result shows the validity of the
present analysis based on FDTD analysis.
To summarize the detailed matching characteristics for the present
absorber, we consider the square-line patterns in Figure 5.32 c as an example.
When the size of conductive square b becomes large, considering the other
parameters such as frame width, adjacent space, and absorber thickness as con-
stants, the matching frequency characteristic tends to move toward a lower
frequency. In the case where the frame width a is increased, keeping the other
parameters constants, the matching frequency characteristic also shifts toward
a lower frequency. Further, when the adjacent space c becomes narrow,
keeping the other parameters as constants, the matching frequency character-
istic also moves toward a lower frequency. As for the conductivity of the square
frames, a good matching characteristic is obtained when the conductivity is
larger than 10 4 Sm -1 . Using these characteristics, a slim EM wave absorber can
be designed. For example, when carbonyl iron is used as absorbing material in
place of ferrite, a slim EM wave absorber with a thickness of 2 mm is designed
at the frequency of 2.45 GHz, or the ISM band.