The common household microwave oven (500 to 700 watts average power at 2.45 GHz) is the most widely used (non industrial) microwave applicator. In technical terms it is a rectangular cavity, multimode resonator, used for batch processing. Put simply, it is a metal box whose dimensions are approximately twice the wavelength so a few different spatial distributions of heating are possible over the operating frequency range. The material is loaded via a door, and is all heated for a specific time.

The usual problem with a household oven - and with many industrial applicators - is that the heating pattern is not uniform, and thus the final temperature distribution is not uniform. The reasons for this are as follows, and demonstrate the problems encountered in applicator design.

1. The electric field spatial distribution (ie., the source of the heat ) is inherently sinusoidal(ie., non-uniform) and has peaks at specific locations which change positions as the dielectric constant of the material changes.
2. The strength of the electric field (and thus the heating) is reduced in the interior of a sample because the microwaves are absorbed on the way in.
3. The dielectric constant and the microwave absorption of the material change as the temperature increases, meaning that both of the previously mentioned effects also change with the temperature increase !

For the above reasons, understanding and predicting the temperature distribution in microwave heated material depends upon knowing the temperature dependence of the complex dielectric constant - ie., the real and absorptive parts.

Conventional heating (convective/conductive) is very non-uniform (usually only the surface is heated and the heat must conduct to the interior), and will produce a uniform temperature distribution only if the heating is done very slowly. One of the main advantages of microwave heating is that the heat is deposited in the interior of the sample, avoiding the delay in heat transmission to the interior caused by low thermal conductivity. However, although faster and more uniform, microwave heating is not inherently uniform, and to make use of its high speed in industrial processing usually requires a custom shaped applicator which produces electric field distributions which take into account the material dielectric properties at the processing temperature.

To reduce the electric field non-uniformity problem, it is common practice to move the sample around in the electric field to do some averaging. In batch processing, this is done either by rotating the (solid) material (as in the household oven), or by stirring the (granular or liquid) material during the heating period.

In continuous feed, continuous processing mode, this averaging is usually accomplished by moving the material into the oven , through it , and out at a steady speed , so that each piece sees the same integrated amount of heating - eg., a conveyor belt for solids or a microwave transparent tube for liquids and granular material.