Microwave heating cavities and applicators - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 215

6.4.1 Multimode microwave cavities
Multimode resonant cavities are the most commonly used type of applica-
tor in microwave heating systems. Because of their robustness and simplic-
ity of design and manufacture, multimode resonant cavities are used as
applicators for domestic microwave ovens. In principle, any metallic enclo-
sure with dimensions a few times larger than the microwave wavelength
may serve as a basic multimode applicator for microwave heating purposes.
In such cases, the microwave energy is transmitted using a transmission
line or waveguide to the cavity, which contains a lossy material. The ease
of operation and elimination of the need for regular tuning are among the
main advantages of such basic multimode applicators. Unlike other types
of applicators available that can be used in a variety of applications, multi-
mode cavities are used only in microwave heating and plasma activation
applications. The multimode applicators are considered the most suitable
type of applicator for microwave-assisted separation of mortar from RCA
and microwave-assisted accelerated curing of precast concrete. Therefore,
the basics for the design of such applicators are discussed in detail in the
following section.
Multimode microwave cavities can be made in different three-dimensional
(3D) configurations with dimensions that are at least two times larger than
the operating microwave wavelength. Rectangular microwave cavities are
preferred because they are both simple to analyse and manufacture. The
exact analytical solutions for various basic shape multimode cavities are
available in the literature [2-5]. A typical microwave cavity with an arbi-
trarily selected set of dimensions is schematically shown in Figure 6.17a [6].

E v

0.1
0.075
0.05
0.025 0

25

E

0.2

0

25

0.1

0.1

35 cm

0.2

0.3

0

(a) (b)

Figure 6 .17 Power density distribution under a TE 324 mode in a rectangular micro-
wave cavity. (From Mehdizadeh, M., Microwave/RF Applicators and Probes for
Material Heating, Sensing, and Plasma Generation: A Design Guide. Norwich,
NY: William Andrew, 2009. With permission.)