Biomedical Engineering Reference
In-Depth Information
to the characteristic impedance
Z
c
of the transmission line. This condition also
holds even if the short circuit is connected a quarter wavelength apart from
the load impedance
Z
R
, as shown in Figure 5.4
b
. This is based on the fact that
the input impedance
Z
in
, when looking into the short circuit from the position
a quarter wavelength apart, becomes infinite, as shown in Figure 5.4
c
. In short,
although
Z
in
is connected in parallel with
Z
R
, actually
Z
in
is disconnected from
Z
R
because
Z
in
takes an infinite value. On the basis of the above considera-
tions, let us consider a parallel-plate line equivalent to the present transmis-
sion line in which the TEM mode is propagated. When the medium between
two parallel plates is assumed to be a vacuum and the interval between the
two plates is narrow enough with respect to wavelength, the impedance cor-
responding to
Z
c
in the above-mentioned transmission line is now 120p ohms
(
, which is called
free-space impedance
). Therefore, by mounting a thin
resistive film with this 120p-ohm value of resistance at a position a quarter
wavelength apart from a shorting terminal, the EM wave is absorbed:
Matching is obtained. Since the EM field distribution when the EM wave prop-
agates between this parallel-plate line is identical to the case in which a plane
wave propagates in free space, the EM wave absorber can be constructed, by
placing a resistive film and a shorting board (a conductive plate) in the space,
as shown in Figure 5.5
a
. This is a basic configuration for the resistive-type
absorber.
But now, from this basic structural principle for the resistive-type absorber,
we can suppose a thin
magnetic resistive
film instead of the thin
resistive
film.
The theoretical reason that develops the concept of magnetic resistive film is
based on the duality principle. Actually, duality has been established between
the above-mentioned resistive film and the magnetic resistive film. This duality
=
377
W
Conductive
plate
Conductive
plate
Resistive
sheet
Magnetite
Plane wave
4/
l
d
(a)
(b)
FIGURE 5.5
Principle of EM wave absorber: (
a
) by electric resistive sheet; (
b
) by mag-
netic resistive sheet.
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