Microwave heating cavities and applicators - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 219

Port
Material load
Distance to
the port
Figure 6.19 Proximity of the load material to the feed point. The surface closer to the
feed point tends to heat faster.
of the major parameters is the penetration depth of fields in the lossy load
material. As discussed in Chapter 1, the penetration depth of a plane wave
in a lossy material is inversely proportional to the attenuation factor of the
material and can be estimated using Equation 1.23. Regardless of the field
distribution, the penetration depth of microwaves in a highly lossy mate-
rial such as concrete can be a major factor affecting the heating uniformity
in multimode cavities. Another factor that could possibly affect tempera-
ture uniformity in multimode cavities is the geometry of the material. The
addition of the load into a multimode cavity considerably changes the field
distribution and sets new boundary conditions for the field. The shape of
the load also considerably affects the configuration of the fields inside the
load material, especially when the relative permittivity is high. The edges of
the objects with complex shapes usually receive more intense field and are
therefore heated faster. For instance, the narrow end of a triangular-shaped
load material is heated faster than its wider end. The third factor affecting
the heating uniformity in multimode cavities is the proximity of load mate-
rial to the feed point (Figure 6.19). The side of the load closer to the feed
point usually receives more energy and is heated faster than sides located
farther away. The nonuniformity of heating caused by this factor increases
when the load material is placed closer to the feed point. Various methods
are usually applied to reduce the effect of load proximity to the feed point.
These include the use of multiple ports and rotation of the load. The latter
method is commonly used in domestic microwave ovens and has proved to
be relatively effective. The first method also has the advantage of exciting
more modes, which could lead to better uniformity.
The microwave field uniformity in multimode cavities can be improved
considerably through basic cavity design. By taking into account the causes
of nonuniformity mentioned on top of the field modal configuration, a num-
ber of approaches may be adopted to improve uniformity of the field in
multimode cavities. The first approach for improving the uniformity of heat-
ing in multimode microwave cavities is by increasing the number of excited