Microwave heating cavities and applicators - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 211

Applicator

Microwave
Source

P u P s

P a

P m

Power
Supply

Coupling

P r

Material

Figure 6.15 A simplified configuration of typical microwave heating systems. (From
Mehdizadeh, M., Microwave/RF Applicators and Probes for Material Heating,
Sensing, and Plasma Generation: A Design Guide. Norwich, NY: William
Andrew, 2009. With permission.)

coupling system. A portion of the supplied power is reflected ( P r ) because of
mismatches; the rest is transmitted to the applicator ( P a ). A portion of the
applicator power is wasted ( P w ); the rest is deposited into the load ( P m ). The
wasted power in most cases is caused by resistive heating in the metallic
parts of the applicator. However, in some cases, other forms of power wast-
age may arise because of radiation or from the heating of other dielectric
bodies present in the applicator.
The system efficiency of the microwave heating system is defined as the
ratio between the power deposited into the load and the utility power. It is
always desirable to maximise efficiency:

P
P

m
u

η s = (6.1)

Similarly, the efficiency of the applicator together with the coupling sys-
tem may be defined as the ratio between the power deposited into the load
and the power supplied by the microwave source:

P
P

m
s

η ac = (6.2)

The applicator efficiency can be similarly defined as the ratio between
the microwave power deposited into the load and the power transmitted to
the applicator:

P
P

m
a

η a = (6.3)

The applicator efficiency is an important quality control measure in the
design of applicators.