Mode stirrer

In multimode cavities (conceptually similar to a domestic oven) the microwaves, which enter the cavity, are reflected by the walls and the load over the typically large cavity. A mode stirrer ensures that the field distribution is as homogeneous as possible. 

Microwave radiation is generated by means of a magnetron (microwave diode). The radiation is then directed through a waveguide into the interior of the oven. Even distribution of the radiation is facilitated by a mode stirrer that prevents the development of standing waves, as well as a turntable and perhaps a rotating antenna located beneath the oven floor. 

The mode stirrer is a periodieally moving metal vane (flat blades) that continuously changes the instantaneous field pattern inside the cavity. The shape of the vane as well as its movement is such that the microwave field is continuously stirred. Hence, the field intensity becomes homogeneous in all directions and all locations throughout the cavity. Now, the microwave absorbing materials can be placed aitywhere inside the cavity, because the field is homogeneous throughout the cavity. 

To improve the uniformity of heating two basic methods are used. A turntable which rotates at a constant speed can be used to move the load successively through the nodes and antinodes. Alternatively, a mode stirrer can be used. This is a structure such as a metallic multiblade fan which perturbs the electromagnetic field continuously by its rotation inside the multimode oven. It is quite common to find both of these techniques in operation. 

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