and
40.68 ± 0.02034 MHz) are permitted for industrial, scientific and
medical applications. In Fig. 2 a schematic arrangement for MW
heating is compared with a typical RF heating parallel plate electrode configuration. In MW heating (Fig. 2a), special oscillator
tubes known as magnetrons or klystrons emit microwaves which
are transferred by a waveguide into a metal chamber or cavity
where the target material to be heated is placed. Resonant electromagnetic standing wave modes are then established within the
cavity (Piyasena et al., 2003a) although turntable trays and/or stirrers can be used to improve the uniformity of the electromagnetic
field within the chamber and around the target material. In contrast RF energy is generated by a triode valve and is applied to
material via a pair of electrodes (Rowley, 2001). In the parallel
plate RF system described in Fig. 2b, one of these electrodes is
grounded which sets up a capacitor to store electric energy. The
target material to be heated is placed between but not touching
the parallel electrodes. It must be noted that while the use of parallel plate electrodes (or ‘‘through-field” applicators) is the most
commonly used electrode configuration for heating thicker materials, two other configuration types are included in Jones and Rowley