As legend goes, the use of microwaves for preparing food was pursued
after a serendipitous discovery by Raytheon engineer
Percy Spencer whereby he noticed the chocolate bar in his pocket
had melted while he was working near a radar transmitter magnetron.
Being a newly discovered phenomenon in 1945, Mr. Spencer was probably
not aware that his own body parts were being likewise cooked, but
he did recognize the commercial potential of an oven that used microwaves
to cook food. It only took Raytheon (Amana) to have the first Radarange
available for sale to professional kitchens. This article was printed
a full decade after the discovery and even then the size and power
consumption was too great for grandma's countertop. Of course grandma
would never even have considered using such a newfangled device.
In fact, many people equated the microwave's cooking action with
using a nuclear reaction to do the job, hence the term "nuking"
food in the microwave. Keep in mind that it was the era of the first
atomic bomb explosions where gruesome films were shown burning the
flesh off of test animals on ships at the
Atoll. The crucial difference between microwave radiation and
nuclear radiation is that microwaves are non-ionizing, meaning that
they do not contain enough or the right wavelength energy to knock
electrons out of their orbits (yeah, an outmoded term but it sounds
cooler than probability density), whereas decaying nuclear products
do. I remember my mother's first microwave oven (c.1975) was huge
and weighted probably 30 pounds; it occupied the full depth of a
standard countertop (25"). For many years there was a concern about
microwave radiation leaks around the door gaskets and even the chassis,
but that issue has been resolved in modern appliances. In fact,
with microwave ovens operating in the same 2.4 GHz frequency
band as Bluetooth and WiFi, there was also a big discussion in the
mid-1990s about whether microwave ovens would interfere with and
even prevent those technologies from being usable. Just as Y2K (remember
that?) turned out to be a non-event, so did microwave oven interference.
BTW, I took a picture of one of the original
Radaranges on display in the
National Electronics Museum at the 2009 MTT-S shown in Boston.
Only the food gets hot in the new "Radarange". Cooking utensils
and interior of oven remain cool since these materials do
not absorb the microwaves that cook the food.
Shown at left-top, floor model 1161 has completed two years
of field tests to prove its worth. This unit uses two magnetrons
to generate the microwave cooking energy. Maintenance of
new unit has been simplified. Parts are readily accessible.
Note air-cooling unit at left.
Model 1170, above, is smaller of two models of "Radaranqe"
now commercially available.
Food can be cooked in a fraction of the time it usually requires,
by the new "Radarange" which utilizes microwave energy to heat nothing
but the food itself. Made by the Raytheon Manufacturing Company
of Waltham, Mass., the new oven can prepare an 18-pound roast of
beef in 40 minutes. At a recent demonstration, a chicken was roasted
in 9 minutes; an apple pie baked in 6 minutes; and steaks done in
1 minute. The walls of the oven, as well as the utensils holding
the food, remained cold and could be touched with bare hands.
Designed for primary cooking, defrosting, and reheating, the
"Radarange" heats food by microwave energy generated at 2450 megacycles,
produced by QK-390 continuous wave, air-cooled magnetrons. In model
1161, two magnetrons produce a maximum of 1600 watts; in smaller
model 1170, one magnetron produces a maximum of 800 watts. In both
cases the microwave energy is directly coupled to the oven cavity,
where it is confined by the metal walls and a door designed with
appropriate chokes. Thus, instead of the food being cooked by the
conventional method of applying heat to its surface and then waiting
for the heat to be conducted through the food, the food in the "Radarange"
oven is penetrated by the microwaves to a depth of about 2 1/2 inches.
As it penetrates, the microwave energy sets up molecular friction
deep within the food which in turn creates the heat that cooks it.
This process is the key to the tremendous reduction in time needed
for cooking the food.
No physical change takes place in the food, except the normal
changes caused by the heat. And the only heat present is within
the food itself. Since the stainless steel of the oven, and the
material of which cooking utensils and plates are made resist microwave
penetration, they do not get hot.
The "Radarange" oven's tremendous speed helps reduce food wastage.
For example, in estimating a restaurant's daily needs for roasts,
only enough meat for the smallest expected amount of business is
cooked by conventional methods. Should extra business develop later
in the day, extra roasts may be prepared in the "Radarange" oven
in ample time to meet the orders. In most establishments, this procedure
may be used repeatedly, keeping just "one roast ahead" of business.
Speed of "Radaranqe" is shown by cooking
times required for various dishes.
Except for the magnetrons, all the electrical equipment in the
"Radarange" runs at power frequencies and consists of power supply
and control equipment. The only tubes besides the magnetrons are
the rectifier tubes in the power supply, which furnishes 320 ma
at 5000 volts to each magnetron.
Magnetrons are essentially constant voltage devices like gas
voltage regulator tubes and will draw widely fluctuating amounts
of current with very small changes in voltage. Therefore, some means
of current control is required. This control is accomplished very
simply with a saturable reactor circuit in series with the primary
of the high-voltage transformer. Using this reactor, the magnetron
circuit, and thus the cooking speed, is held virtually constant
for changes in line voltage of plus 10 or minus 5 percent from the
design voltage. The reactor is also used to provide lower oven heats
by reducing the magnetron current. This method is a particularly
convenient way to control heats, as all switching can be done in
the low current control circuit rather than in the power circuit.
Power is provided for exhausting steam from the oven and cooling
magnetrons and other components. Considerable care has been taken
to insure long life by running the electrical components cool. The
air in commercial kitchens is usually hot, grease laden, and frequently
full of lint. The electronic components in the range operate at
relatively high power and must be kept cool if long and trouble-free
operation is to be achieved. Cooling air, therefore, is drawn in
at the front, where the air is most apt to be cool and free of grease
when the range is placed in a row of other cooking equipment. The
air is then filtered and forced over the electrical components and
out through openings in the back of the cabinet.
The microwave ovens are designed to operate on 208 to 230 volts,
single phase, 60 cycles. The neutral wire carries no current and
can be used for grounding. The conversion from 208 to 230 volts
is made by changing a transformer tap within the range. The large
model consumes about 1.02 kw on standby and about 5.3 kw on high
heat. The power demand for the small model is half of that of the
The efficiency of the magnetron itself is about 50 percent; that
is, about half of the power supplied to the magnetron is fed into
the oven as microwave power. Over 90 percent of this power is converted
to heat in the food. The remaining 10 percent is dissipated in random
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas
and reference material while performing my work as an RF system and circuit design engineer.
The Internet was still largely an unknown entity at the time and not much was available
in the form of WYSIWYG
All trademarks, copyrights, patents, and other rights of ownership to images and text
used on the RF Cafe website are hereby acknowledged.