Microwave heating cavities and applicators - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 201

6.2.2.2 Directional coupler and power monitor
The microwave power actually generated and the power delivered to the
load are usually different from the power estimated by the control systems
based on the voltage provided and the designed efficiency of the active
microwave components. This is mainly because of the differences between
the actual efficiency and expected efficiency of the microwave generation
system as well as the losses (because of energy dissipation or reflection) by
the passive microwave components as microwaves travel from the power
generation unit to the load. Measuring the actual forward power generated
and power delivered to the load is important in identifying the energy loss
incurred in the respective power generation and delivery units. In addi-
tion, measuring the reverse power reflected by the load or by the various
passive components is important in the selection of a suitable isolator and
in monitoring its performance to prevent any damage to the components
of the microwave generation unit. The directional coupler is used to take
sample readings of the power propagating in one direction, which are then
used to estimate the total power propagating through the waveguide. The
coupler then sends a signal, which is usually in the form of a DC voltage,
to the meter or some other calibrated signal-measuring device. Similarly,
dual directional couplers are used to take a sample of the power in both
directions. Hence, they can be used to measure both the forward and the
reverse power. Figure 6.6 shows a typical dual directional coupler and
power- monitoring set.
6.2.2.3 Tuner
In microwave heating, the microwave power should be transferred from
the generator to the load with minimal losses. The transfer efficiency of
Figure 6.6 Dual directional coupler and power-monitoring set.