Biomedical Engineering Reference
In-Depth Information
Equivalent phase plane
-•
(Electric field)
x
E
x
•
z
•
y
H
y
(Magnetic field)
l
(Wavelength)
FIGURE 5.2
Illustration of plane wave.
1888. At the Germany Science Museum (Deutsches Museum), located in the
shoal of the Isar River, Munich, Germany, a relic of his great achievement still
exists. A pair of parallel conductive wires which tested the propagation
characteristics of EM waves, such as the standing-wave distribution, is also
exhibited there.
In many applications, the wave absorber has been designed by taking a
plane wave into consideration. A plane wave is a wave for which the equiphase
surface of the EM field perpendicular to the direction of propagation always
constitutes a plane. In this case, the electric and magnetic fields are orthogo-
nal to each other, as illustrated in Figure 5.2, and the wave does not have com-
ponents of electric and magnetic fields in the direction of propagation, or the
z
axis. The wave with such a field distribution has a TEM mode (
transverse
electromagnetic
mode, see Section 1.3.4). In fact, a TEM mode without elec-
tric and magnetic field components in the direction of propagation can be real-
ized using a transmission line which consists of two conductors (at most, three
conductors), as shown in Figure 5.3. This is obvious from the development of
the coaxial line shown in the same figure. The field distributions in Figures
5.3
a
,
c
and
d
are used for the standing-wave measurement. The reason we
investigate the phenomenon related to the EM plane wave by replacing it by
the transmission line theory is based on the approach mentioned above.
Consider the propagation of a plane wave along the two parallel wires of
the transmission line in Figure 5.4. Based on transmission line theory, the con-
dition of perfectly absorbing the EM wave at the load impedance
Z
R
without
any reflection, which results in thermal energy, is the case where
Z
R
is equal
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