Civil Engineering Reference
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120
100
We t
80
Saturated
Air-dried
60
Oven-dried
40
20
0
0
5
10
15
20
Frequency (GHz)
Figure 1.17
Attenuation factor of concrete.
the following, we first formulate the total energy potential of a microwave
field. Then, we review a number of the methods available for estimating the
portion of this energy that is absorbed and dissipated in a material placed
in a microwave field. The electric
W
e
and magnetic
W
m
energies may be
generally defined as
D
T
⋅
∂
∂
D
t
∫∫
∫∫
WE dD dV
=
⋅
=
E
dt
dV
(1.25)
e
V
0
V
0
B
T
⋅
∂
∂
B
t
∫∫
∫∫
WH dB dV
=
⋅
=
H
dt
dV
(1.26)
m
V
0
V
0
The time derivatives of these expressions are the electric and magnetic
power, respectively:
PE
D
t
=⋅
∂
∂
∫
dV
(1.27)
e
V
PH
B
t
=⋅
∂
∂
∫
dV
(1.28)
m
V
These quantities are related to the resistive and radiative energy, or
energy loss, through Poynting's theorem:
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