EM Wave Absorbers Protecting Biological and Medical Environment - RF/Microwave Interaction with Biological Tissues - page 209

layer and if the input impedance Z 1 which looks into the absorber plane in the
first layer is obtained.

5.4.3 Taper-Type Wave Absorber

Among the many wave absorbers used in anechoic chambers are the sawtooth
and pyramidal wave absorbers. They are composed of a taper structure in the
plane of incidence of the EM wave. Generally, in this case, a rigorous analysis
of the reflection coefficient becomes very complex. Accordingly, in this text-
book, we introduce an approximate method for calculating the reflection coef-
ficient in the present case. This method is based on the idea that the dielectric
constant at any taper portion is proportional to the area of the cross section
at any taper portion, as shown in Figure 5.8. As a result, as shown in Figure
5.8, the equivalent dielectric constant at any taper portion is given by the
expression:

cross section of taper portion
whole cross section of absorber

= Ê
Ë

ˆ
¯

¥-

(

e 11

) + (5.24)

e

r

re

According to this, the dielectric constant e r at any taper portion is approxi-
mated by the equivalent dielectric constant e re under the condition of dividing
the taper portion as thin as possible. As a result, the analytical method of the
multilayer absorber mentioned above is applied to the analysis of the reflec-

FIGURE 5.8 Approximation method for equivalent dielectric constant.