As described in Chapter 2, the two general classes of microwave applicators are multimode and single mode cavities. Key features of multimode ovens include:
suitability for bulk processing applications;
oven dimensions that are often determined by product dimensions;
moderate to high efficiency;
adaptability to batch or continuous product flow;
performance that is less sensitive to product position or geometry; and
good uniformity that may require motion of product or hybrid heating.
Multimode applicators are often used for processing bulk materials or arrays of discrete material, whose overall dimensions are too large (larger than the wavelength of the operating frequency) to permit consideration for use in a single-mode oven. These applicators, in their simplest configuration, take the form of a metal box that is excited (driven) at a frequency well above its fundamental cutoff frequency. For example, the common home microwave oven typically has internal dimensions on the order of 12 to 16 in., while the wavelength is 4.8 in. The larger dimension corresponds to a cutoff frequency of about 400 MHz as compared with the operating frequency of 2.450 GHz.
Because the dimensions of the enclosure are very large when expressed in terms of the free-space wavelength of the operating frequency, a large number of standing-wave modes can exist at or very near the operating frequency inside the cavity. To establish a reasonably uniform electric field strength throughout the cavity, it is desirable to excite as many of these modes as possible. When multiple modes are excited, heating nonuniformity is minimized even when the field perturbing effects of the materials being processed are present.
Multimode applicator design involves a number of basic design parameters. They include uniformity of heating, required microwave power, applicator size, leakage suppression, and required performance characteristics.
Uniform heating is difficult to obtain in a multimode oven. This difficulty arises from the unpredictable way in which the parameters affecting uniformity change with time. As a result, a number of techniques, in addition to excitation of multiple standing-wave modes, are used to promote uniform heating. They include metallic mode stirrers to ensure that all the possible modes are excited; surface scanning to direct the energy at regions of interest; product motion; and, in some cases, hybrid heating using conventional heating to replace surface losses.