Microwave Heating
Microwave heating, as distinct from RF heating, is a sub-category of dielectric heating at frequencies above 100 MHz, where an electromagnetic wave can be launched from a small dimension emitter and conveyed through space to the target. Modern microwave ovens make use of electromagnetic waves (microwaves) with electric fields of much higher frequency and shorter wavelength than RF heaters. Typical domestic microwave ovens operate at 2.45 GHz, but 0.915 GHz ovens also exist. This means that the wavelengths employed in microwave heating are 12 or 33 cm. This provides for highly efficient, but less penetrative, dielectric heating.
Although a capacitor-like set of plates can be used at microwave frequencies, they are not necessary, since the microwaves are already present as far field type EM radiation, and their absorption does not require the same proximity to a small antenna as does RF heating. The material to be heated (a non-metal) can therefore simply be placed in the path of the waves, and heating takes place in a non-contact process which does not require capacitative conductive plates.
Read more about this topic: Dielectric Heating
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