Fundamentals of Microwave Technology
Generation of Microwave Energy
Microwave power output for heating processes is produced by a special
tube called a Magnetron. Structurally, a magnetron is a high vacuum
electronic valve consisting of a hollow copper anode incorporating a
series of resonance cavities, at the center of which is an electron
emitting cathode.
To gain a rough idea of how a magnetron generates microwaves, one can
resort to a simple acoustic analogy. You know that if you blow air across
the mouth of a bottle, a tone, or oscillation, will be generated which has
a wavelength proportional to the size of the bottle. If you fill the
bottle partially with water, the frequency of the acoustical tone
generated increases. In the case of a magnetron tube, the electron cloud
generated by the cathode is the 'air' and the resonant cavities of the
anode surrounding the cathode are the 'bottles.' As electrons leave the
central anode they are induced into circular rotation by a magnetic field,
passing many resonator 'bottles' before finally being captured by the
anode. The result is microwave energy generated within a very narrow
frequency bandwidth.
Typical modern magnetron tubes operate at electrical efficiencies
greater than 70% and have lives on the order of 5000 hours.
Interaction of Fluids with Microwave Energy
There are several mechanisms by which matter absorbs microwaves,
however, the most common are dipolar rotation and ionic conduction.
Dipolar rotation accounts for nearly 80% of the heating
contribution at 2450MHz. Dipolar molecules (e.g.:H2O) exist in
nature normally in a random orientation. When imposing an electrical field
(such as microwave energy), dipolar molecules tend to become ordered due
to their asymmetric distribution of unlike charge partners. As the
electrical field dies down, the dipoles return to their random
(disoriented) orientation. The net result is a conversion of energy from
electric field energy to stored potential energy in the material and then
to stored random kinetic or thermal energy in the material. Typical
microwave energy fields are able to pull into alignment only as few as 1
molecule in every 103 - 105 molecules.
Ionic conduction accounts for the bulk of the remaining heating
contribution at 2450MHz. Ions exist in many fluids naturally, however, the
ionic content of a fluid such as water can be increased with addition of a
salt such as sodium chloride (NaCl). The application of an electrical
field to an ion causes an acceleration of that particle towards its like
charge. This acceleration causes an increase in collisions with unionized
molecules. The net result is electric field energy converted into ordered
kinetic energy and then disordered kinetic energy or heat.
The effect of dipolar rotation is strongly temperature and frequency
dependent. Ionic conduction does not depend on temperature or frequency.
Electromagnetic waves with frequencies between 300MHz
and 300GHz are called microwaves, by definition. Their respective wavelengths
are between 1mm and 1m. Only a few frequency bands are allocated for microwave
heating applications, the most common being 2450 MHz
50 MHz or 'S' Band.
©1999 Charm Bioengineering, Inc.