Civil Engineering Reference
In-Depth Information
Generator
Dummy Load
Applicator
Circulator
Directional Coupler
with Power Monitor
Tuner
Figure 6.4
Configuration of a typical power delivery unit used in industrial microwave
heating systems.
6.2.2.1 Isolator (circulator and water load)
As the power travels toward the load, as a result of reflections from surfaces
of the passive microwave components (especially at bends) as well as the
reflections from the surface of the load, a part of this power is reflected
back toward the generation unit. The reflected power should be minimised
through optimising the design of the microwave system and maximising
the impedance matching (explained further in the chapter) to minimise
reflections from the load surface. However, even the most sophisticated
designs cannot completely prevent some amount of backward energy, and a
percentage of the forward energy is usually reflected toward the microwave
generation unit. If not dissipated before reaching the microwave generation
unit, the reflected power could damage the magnetron and other active
microwave components. The isolator is a two-port device made of a fer-
rite material and magnets that does not permit flow of the power in the
reverse direction. For low-power applications, an isolator is normally used
to protect the magnetron from the damaging effects of the reverse power.
However, in high-power applications, instead of using the standard two-
port isolator, an isolator is formed by connecting a circulator to the dummy
waveguide load to absorb the reverse power. The circulator is a three-port
device used to control the direction of power flow (Figure 6.5). Dummy
loads are used to dissipate the microwave power with little or no reflec-
tion (Figure 6.5). The dummy loads used may be generally divided into
wet loads or dry loads. Wet loads absorb the reflected microwave power
by directing it into a high-loss fluid medium, usually water, whereas dry
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