Microwave heating cavities and applicators - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 221

cavity shown in Figure 6.20 through the use of a hexagonal shape rather
than a simple rectangular shape.
The second approach for improving heating uniformity in multimode
microwave cavities is by mechanically moving the load in the cavity. For
example, the use of a turntable is an inexpensive and relatively effective
method commonly used in domestic microwave ovens to improve heating
uniformity. The load placed on the turntable continuously stirs the field by
modifying the boundary conditions as it revolves within the cavity. This
effect is more prominent for loads with less-regular shapes. However, it
should be noted that, although the use of a turntable improves heating uni-
formity compared to the static case, it does not lead to overall uniformity.
Circular symmetrical nonuniform heating is commonly observed in most
cavities with a turntable. The pattern of this nonuniformity depends on the
field pattern of the material-loaded cavity.
Another mechanical tool with a relatively similar function as the turn-
table that is commonly used to improve heating uniformity in microwave
ovens is the mode stirrer (Figure 6.21). A mode stirrer is a metallic device
that disturbs the field by moving continuously in the multimode cavity. The
periodic changes in the field boundary conditions as a result of the distur-
bance caused by the stirrer tend to change the location of the true nodes
and nulls in the cavity and thereby improve heating uniformity.
The effectiveness of mode stirrers varies with the type of load material as
well as the design and location of the mode stirrer. It has been shown that
when thick and highly lossy loads are placed in the cavity, the impact of
Mode Stirrer
Waveguide Ports
Figure 6. 21 A mode stirrer used to improve heating uniformity in a domestic microwave
oven.