Microwave heating cavities and applicators - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 236

Here, α is the attenuation constant (in dB/m), f is the microwave frequency,
a is the width of the rectangular attenuation duct, and D is the inner diam-
eter of the cylindrical attenuation duct. In most practical applications using
ISM frequencies, an attenuation level of 40 dB is satisfactory. Once the
attenuation constant of the attenuation duct (dB/m) is estimated using
Equations 6.22 and 6.23, the length of the attenuation duct required to
reduce the attenuation level to 40 dB can be easily calculated.
As discussed, the use of attenuation ducts alone is only effective when the
dimensions of the openings required are smaller than the cutoff dimensions
for the operating microwave frequency. However, the dimensions of the open-
ings required in applications such as accelerated microwave curing of precast
concrete tend to be larger than the cutoff dimensions at typical microwave
operating frequencies used in such applications (Table 6.2). In such situa-
tions, the attenuation ducts are of the above-cutoff duct type and require the
use of additional measures to minimise power leakage from openings. When
the opening is not excessively large (dimensions close to the operating wave-
length), one common technique adopted is to use absorbing linings inside the
attenuation ducts [17]. Another method used in such cases is to install fer-
ritic materials inside the attenuation ducts. Most common waveguide excited
modes have a strong magnetic field parallel to the walls; therefore, the use of
ferritic materials, which have a high magnetic loss, in attenuation ducts is an
effective method to attenuate the leaked power quickly.
However, when the required opening is considerably larger compared
to the wavelength of the operating microwave frequency (multiwavelength
dimensions), the waveguide modes tend to be in the TEM mode, and wave-
guide walls play an insignificant role in defining the boundary conditions. In
such cases, covering the walls with highly lossy materials or ferritic materi-
als tends to be not as effective. In addition, even when lossy materials are
theoretically effective, their effectiveness may reduce considerably as they get
heated up. Another method to reduce the leakage of microwave power from
continuous-throughput microwave heating systems is the resonant choking
method. Unlike the use of absorbing materials, this method relies on reflecting
the emissions. The effectiveness of the resonant chocking method in reduc-
ing power leakage from above-cutoff attenuation ducts is because leakage
minimisation in above-cutoff ducts is significant mainly at higher frequencies,
which usually have relatively narrow frequency bands. The resonant choking
method takes advantage of the narrow frequency band of such frequencies to
minimise power leakage through the use of band-stop filters installed at the
openings of cavity. In this method, repetitive metallic features are arranged
in the attenuation duct in a way to resonate the operating frequency. A band-
stop filter with such rectangular filter elements is shown in Figure 6.34 [18].