RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
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formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
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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 Bikini
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
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 large range.
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 heating.
Nuclear Bomb Test at the Bikini Atoll
Posted August 8, 2019(original 1/28/2014)
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