Civil Engineering Reference
In-Depth Information
used for microwave heating generally have dimensions such that only the
dominant mode propagates along it.
The most fundamental modes in the TE and TM categories that are
usually excited by the microwave heating systems are the TE
10
and TM
11
modes. The TE
10
mode is the usual choice for single-mode commercial
waveguides. It does not vary in one of the transverse directions and has a
sinusoidal distribution in the other. Hence, it represents a heating problem
similar to that of heating a two-dimensional slab. The form of the incident
TE
10
mode may be represented as
=
(
)
=
{
[
]
}
E
00
,,
E
00
,,sin(
π
ay a
−
)
2
(1.22)
z
where 2
a
is the width of the waveguide. The concepts of wave propagation
in waveguides as well as the multimode cavities used for the heating of con-
struction materials are presented in Chapter 6.
1.7 PENETRATION DEPTH AND
AT TE N UATIO N FACTO R
As a result of the energy losses, the microwave energy attenuates as it prop-
agates in the dielectric material. Figure 1.15 shows the microwave electric
field distribution inside a 10-mm thick saturated concrete block when sub-
jected to 1 W of microwave power through direct contact with a standard
WR340 rectangular waveguide (Figure 1.16). It can be seen that, as a result
4000
2.45 GHz
10.6 GHz
18 GHz
3000
2000
1000
0
-1000
-2000
-3000
0
2
4
6
8
10
Distance from the Microwave Exposed Surface (cm)
Figure 1.15
Distribution of electric field in a saturated concrete at various microwave
frequencies.
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