May 1945 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Chances are when you think about
high power RF frequency sources, it pertains to transmitters for analog or digital
data, or maybe for imaging applications like x-ray machines. There is, however,
another very large industry that uses high frequency electromagnetic energy for
heating of materials in product processing. Frequencies range from tens of kilohertz
to tens of megahertz, with power levels into the megawatt realm. This article from
a 1945 edition of Radio-Craft points out a few applications used in the day, and
now there are many more areas where high power RF is used. Adhesive curing is a
major area, especially for laminated materials like plywood and composite structures.
Decontamination of medical and food products is another. Both inductive and capacitative
heating methods are used in metals processing.
Comdel, an RF Cafe advertiser, is a major supplier of high power
RF amplifiers for these purposes.
High Frequency Heating at a Glance
Millions of dollars invested in H-F Heating
give ample testimony for the future of the art. Industry and the home offer expanding
postwar markets for the apparatus.
By William Lyon
The art of Radiothermics is divided into two branches: - Induction and Capacitative
Induction Heating works at a much lower frequency than that usually employed
in a capacitative rig: - a wide range usually between 20 and 500 kc. It has reached
a point of successful application during years of development the metal industries.
Highly skilled labor isn't required to operate the equipment. Workers can be quite
In the accompanying illustrations stippled areas within the field of the coils
show the sections being heat-treated. Hardening metals is accomplished with speed
and uniformity. Gears and modern cylinder walls are two of the most glowing examples.
Annealing and normalizing is accomplished in a fraction of the previously required
time. All such applications as well as bracing and soldering have been put on the
production line. They pass through the magic coil and, presto, the job is done!
And melting metal is almost as simple as flowing water.
Bombarding radio tubes is one of the oldest
Induction Heating accomplishments. Intricate glass products reached the production
line through the same process. Imagine an internally water-cooled radio tube being
turned out by the older methods! The cost would be prohibitive.
Fairly small portable rigs are used for metallurgical analysis and laboratory
Capacitative Heating is also called Dielectric Heating because, unlike Induction
Heating, its application is to non-conductive material. The frequency range is higher
and greater than that employed in Induction Heating.
The subject to be heated is placed between the two plates of a condenser. This
is admirably illustrated in the accompanying sketch heating plastic preforms. While
on the subject of plastics it is worthy of note that High Frequency Heating will
"iron the bugs out" of future Bakelite radio cabinets. These housings have always
been held to a wall thickness of one-eighth of an inch because of curing difficulties.
Strengthening ribs are rarely over one quarter of an inch thick and at these particular
points you will invariably discover distortion in the walls.
Future plastic cabinets will have wails one quarter of an inch thick with ribs
and bosses from fifty percent to double this thickness. Capacitative Heating will
be responsible for the uniformity and strength.
Almost any amount of heat at very high frequency can be applied when necessary
for high speed of production, or necessary long, or short, curing time.
The illustrations showing package sealing and the dehydrating of chemicals. will
be understandable from the foregoing. Bonding wood and pressure gluing are especially
important in the aviation industry. Further examples could be given in catalogue
form. The field is enormous!
Not the least engaging of future developments will be the application of Capacitative
Heating to the home kitchen. Meals will be cooked in a matter of seconds with all
of the major food values retained.
In tomorrow's world the Electronics and Radio Serviceman will have an ever expanding
field in both the home and industry. Radio-Craft intends to run an article on an
experimental 1-Kw Capacitative Heater, with constants and constructional information,
in an early issue.
Posted September 9, 2021
(updated from original post on 10/2/2014)