Civil Engineering Reference
In-Depth Information
Table 6.2
Cutoff dimensions of rectangular and cylindrical attenuation
d
ucts at selected frequencies
Cutoff inner diameter of cylindrical
attenuation duct (cm)
Cutoff width of rectangular
attenuation duct (cm)
Frequency
915 MHz
19.23 cm
16.30 cm
2.45 GHz
7.18 cm
6.12 cm
5.8 GHz
3.03 cm
2.58 cm
The openings of continuous-throughput microwave heating systems can
be generally divided into below-cutoff and above-cutoff types. In below-
cutoff openings, the wave amplitude decreases quickly with distance, and
the waves do not propagate. However, in above-cutoff openings, the use of
typical attenuation ducts with metallic walls is not effective unless certain
techniques (introduced later) are applied.
The cutoff frequency of an attenuation duct is the frequency above
which the waves can propagate in the attenuation duct itself. In rectangu-
lar attenuation ducts, the TE
10
mode is the mode with the longest wave-
length. This mode has been shown to have a cutoff frequency of
f
c
= 150/
a
(in MHz), where
a
(in metres) is the width of the opening, as shown in
Figure 6.33. In cylindrical attenuation ducts, however, the longest wave-
length belongs to the TE
11
mode, for which the cutoff frequency can be
estimated as
f
c
= 176/
D
(in MHz), where
D
(in metres) is the inner diam-
eter of the attenuation duct. These equations can be used to estimate the
cutoff dimensions of attenuation ducts at different operating frequencies.
Table 6.2 lists the cutoff dimensions for rectangular and cylindrical attenu-
ation ducts at ISM frequencies commonly used in continuous-throughput
microwave-processing applications, including applications discussed in the
previous chapters. If the cross-sectional dimension is larger than the cutoff
dimension, substantial leakage is likely to occur regardless of the length of
the attenuation duct.
However, if the dimension is smaller than the cutoff dimensions estimated,
the length of the attenuation duct plays an important role in determining the
amount of leakage. As a general rule, as the dimensions of the tunnel reach
that of the cutoff dimensions, a longer attenuation duct is required to mini-
mise the leakage. The required length of the attenuation duct can be esti-
mated by calculating the attenuation constant using the following general
equations for rectangular and cylindrical attenuation ducts, respectively:
22500
2
2
α=
018
.
−
f
(For rectangular attenuation ducts)
(6.22)
a
31100
2
2
α=
018
.
−
f
(For cylindrical attenuation ducts)
(6.23)
D
Search WWH ::
Custom Search