EM Wave Absorbers Protecting Biological and Medical Environment - RF/Microwave Interaction with Biological Tissues - page 213

5.5.2 Single-Layer-Type Wave Absorber

Single-layer wave absorbers are composed of materials such as carbon and
magnetic material such as ferrites. These materials are backed with a conduc-
tive plate as shown in Figure 5.1 a . Accordingly, the design of a single-layer
absorber for normal incidence is based on Eqn. (5.10). That is, if two constants
among the dielectric constant e r 2 , relative permeability m r 2 , and thickness d of
the absorber are determined, the remaining constant can be calculated by the
following expression under the condition that the reflection coefficient goes
to zero (matching):

[ ] -

me tanh

tanh

j

(

2 plem

)

d 1

[

( )

]

S =

r 2 r 2 r 2 r 2

Eq. 5.10

[ ] +

( ) me j 2 plem d - 1 1

r 2 r 2 r 2 r 2

When a dielectric material such as carbon is used and its permittivity value is
known, the thickness d can be calculated assuming the relative permeability
value to be 1. When e r 2 and d /l 0 are taken together as parameters, a relation-
ship between thickness d and dielectric constant e r 2 can be found. In magnetic
materials such as ferrite, however, the parameters e r 2 and m r 2 always take values
different from 1 and, moreover, they vary with frequency. Therefore, these
values have to be precisely determined; then the thickness d can be calculated.
An example of a single-layer absorber uses sintering ferrite and rubber
ferrite. Generally, since the sintering ferrite has a large relative permeability
value in comparison with rubber ferrite, it is applied to the absorber in the fre-
quency region from 30 MHz to 1.5 GHz. The thickness is usually 4-10 mm.
Although this absorber is a single-layer absorber, the relative frequency band-
width is comparatively large, ranging from 80 to 150%. For this application,
rubber ferrite material is made by mixing powder of sintering ferrite and
putting ferrite into the rubber. Therefore, this material has special features of
flexibility and can easily be processed. However, the rubber ferrite does not
have a large permeability compared to sintering ferrite because the ferrite
powder has been mixed into the rubber. The frequency band of EM wave
absorption is therefore above 1 GHz. The relative bandwidth is about 30% for
usual rubber ferrite. Features of both materials are shown in Tables 5.3 and 5.4.

5.5.3 Multilayer Wave Absorber

We now introduce the EM absorber that is used for the countermeasure of
TV ghosts as an example of a two-layered absorber. The EM wave absorption
wall for the high-rise building is composed of three layers—granite, ferrite, and
steel bars—and a concrete layer on the surface of the building, as shown in
Figure 5.12. In this construction, the equivalent complex relative permeability
is being adjusted by the change of the magnetic resistance of the ferrite
layer by controlling the slit size between adjacent ferrite tiles. That is, the
good matching characteristic in television frequency bands is realized by