EM Wave Absorbers Protecting Biological and Medical Environment - RF/Microwave Interaction with Biological Tissues - page 198

TABLE 5.1 Reflection Coefficients in EM Wave Absorber

Return Electric Field Electric Field Electric Power

Loss a (dB) Reflection Standing Reflection

-20 0.1 1.2 0.01

-30 0.03 1.06 0.001

a

20 dB means that 99% EM wave energy that was emitted to the absorber is absorbed. Simi-
larly,

-

- 30 dB means that 99.9% EM wave energy is absorbed.

MIT Radiation Laboratory in the United States, an application-type wave
absorber was developed [3]. In the same project, a kind of resonant-type
“Salisbury screen absorber” was developed which was made by putting a resis-
tive sheet with a surface resistance value of 377

one-quarter wavelength
away from a conductive plate. In 1953, a pyramidal wave absorber was
developed by L. K. Neher. This absorber has been broadly used for anechoic
chambers [4].
The EM wave absorber has been defined as an object that can absorb inci-
dent EM waves and convert these into a Joule heat or which can cancel the
phase of the incident wave. The level of absorbing ability of the absorber is
measured quantitatively using the return loss or reflection coefficient in deci-
bels. Though a clear definition of what is a wave absorber has not been deter-
mined, the level of

W

20 dB in the reflection coefficient is considered to be a
standard for a usual wave absorber, and it has been considered that the wave
absorber should be better than this limit indicates. This value of

-

20 dB cor-
responds to a 0.1 value of the reflection coefficient of the electric field and a
0.01 value of the reflection coefficient of electric power, as shown in Table 5.1.
It means that 99% of the total EM wave energy emitted to the wave absorber
is absorbed.

-

5.2 CLASSIFICATION OF WAVE ABSORBERS

Electromagnetic wave absorbers are normally classified from the viewpoint of
constituent material, structural shape, frequency characteristics, and applica-
tion, respectively.

5.2.1 Classification by Constituent Material

The classification from a constituent material viewpoint is as follows:

1. Resistive-Type Absorber . This type of absorber uses resistive materials
such as carbon black, graphite nichrome, and chromium as a basic mate-
rial. These materials are used in a usual plate-type absorber or a film
form. The high-frequency current that flows on the surface of the resis-