Biomedical Engineering Reference
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λ/
4
Spacer (losing no dielectric)
Z
0
Resistive sheet
R
s
Conductive plate
(a)
Conductive plate
λ/
4
Z
in
Z
c
Z
0
; Normalized input impedance
; Intrinsic impedance
; Free space impedance
Z
in
=
∞
(b)
R
s
∞
(c)
FIGURE 5.9
Explanation of a quarter-wavelength-type absorber.
By imposing the condition that the reflection coefficient
S
becomes zero, we
obtain the relation
R
s
=
). This is obviously the match-
ing condition for the present absorber. This way, if the space between the con-
ductive plate and the coated resistive plane is equal to a quarter wavelength
and if, moreover, the input impedance
R
s
is equal to 120p ohms, a l/4 absorber
can be realized. Generally, as is clear from this constitution method, a l/4
absorber is narrow band. Actually, since the spacer also has to some extent a
dielectric constant, this dielectric constant should be taken into consideration
when we determine the space between the conductive plate and the coated
resistive plane. Because the constitution method of a l/4 absorber is simple
and it can be thinned at high frequency, it is now used abundantly as a
millimeter-wave absorber.
As an example of l/4-wave absorber, an EM wave absorber for a ship radar
using resistance cloth woven with electroconductive fiber is shown in Figure
5.10. This absorber, composed of resistive cloth woven with a fiber at 3-mm
intervals and with a resistance of 114 k
Z
0
=
120p ohms (
=
377
W
/m, is used as EM wave absorbing
material while foam polyethylene is used as the spacer. The weight is only
500 g/m
2
. Normal incident characteristics for this ship radar absorber are
shown on Figure 5.11. It is shown that each sample, A, B, and C, leads to a
W
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