Courtesy of Raytheon Ca. FIG.101.
EVOLUTION OF MICROWAVE COOKING
The development effort undertaken by Raytheon and centered on the
"Radarange" remains the most intensive program to date. The
array of microwave ovens in Fig. 101 illustrates the progression of models
up to the year 1954. Under date 1945 is pictured a lightweight device for
use in aircraft. It was capable of defrosting and heating eight ounce
meals in aircraft from 10° to 170°F. (-12° to 77°C.) in one minute. In
this model, the cavity is shaped like a drawer for which a pull handle is
visible near the top of the cabinet. Aircraft meals have tended to depend
upon the high quality and variety possible with ground catering
facilities, supplemented by cabinets which keep the food hot aloft.
However some version of this microwave heater may appear at another time as a part
of a high speed, high capacity aircraft meal system.
The 1946 model is a sandwich heater and the cavity is a
removable box shown at the side. When this "cavity" was moved
into position, the sandwich could be heated in a few seconds. This machine
anticipates the fixed-load, short duty cycle principle which is seen today
in microwave equipped vending machines.
First Cooking Model
In 1947, the 1132 model or the "white range"
appeared. It had an output of about 1.6 kilowatts with one water-cooled,
permanent magnet magnetron. This was an extremely concentrated and heavy
box of electronic hardware compared with the versions which appeared
later. The microwaves entered from a U-shaped transmission line waveguide
through an "iris" window of Pyrex glass on the rear wall of the
oven just above the mid-point. This range introduced a "stirrer"
or mode agitator which was designed to improve the distribution of heat.
It consisted of two chromium plated back-to-back hollow hemispheres in the
top of the oven. These rotated at about 2 to 3 revolutions per second with
power coming from a motor and turn-table in the upper compartment. The
magnetron was mounted with the antenna positioned vertically in the wave
guide under the oven. It operated on direct-current power from a power
supply that was designed to be highly protective for the magnetron.
This model had a small oven and a high power output
which tended to give great microwave field densities and to exaggerate any
shortcomings in the distribution of heat. Accordingly, the first real
acquaintance that many persons had with microwave cooking was marred by
the uneven heating results.
Range with Adjustable Power
Next, chronologically, comes the 1150 model which
utilized two air-cooled QK-312 magnetrons in an oven which is huge by
comparison. It measures 21 X 29 X 26 inches in height, width, and depth,
and the two magnetrons are directly inserted into the roof of this oven.
The oven has an opening which measures 14 X 29 inches,
The range is shown under the year 1951. It has been in
use since then aboard the liner "United States." The
distinctions of this model included, first of all, the unusual power
output -- theoretically over two kilowatts could be obtained. However, it
recommended that the cooking be done within a setting
of 200 or 300 milliamperes of magnetron current. This meant that useful
power between 1.1 and 1.7 kilowatts was available. These two magnetrons
were cautiously under-rated, and the operator (who was most likely to be a
chef in this case) was provided with a variac on the control panel. The
recommended range of cooking speed on the dial for the variac corresponded
with the range of 400 to 600 milliamperes, the total for the two
magnetrons. Therefore, the chef was capable of changing the magnetron
operation to suit his cooking needs and he could actually go lower than
200 milliamperes and higher than 300 milliamperes for each magnetron if he
so desired, since the control was fully available.
The energy supplied to the oven is dependent on the
magnetron current which is, in turn, determined by the magnetic field and
the voltage applied to the magnetron. The magnetic field is a permanent
one, and for this case, the current is controlled by varying the voltage
applied to the magnetron. This is accomplished by a line corrector
transformer which operates the variac. The end result is an unusual
flexibility in the cooking which gives a beginner a slow microwave heating
process that minimizes any errors in estimation of the process time.
Later, when he is skilled, he would operate closer to the "red"
portion of the variac dial, obtain very fast cooking,
and be able to process more food. Consequently, this model introduced good
power, adequate volume, and very flexible control.
However, there is still another feature which gives an
insight into the practices in microwave cooking. The 1150 range was
provided with a drainage hole located at the low point in a sloping oven
floor. When this range was being developed, it had been observed that fat
which melted downward from the roasts collected in the utensil. This fat
was at least 100°F. (56°C.) cooler than drained fat in ordinary roasting
and it had a tendency to keep the bottom of the roast cooled, shielded it
from microwaves, and in general interfered with results. Thus, the
stainless steel oven floor with its sloping surface could be used as a
substitute for the utensil. The hole permitted fat to drain through a tube
into a receptacle (accessible from the front of the range for emptying).
The surface was easily wiped clean after use since this was a cool oven
with no infrared heating. It was easily possible to roast 50 pounds of
meat or more in one load in about two hours. The contribution of oven size
was more in the direction of convenience and microwave performance than in
capacity. However, some improvement in rate of cooking was obtained
because the magnetron efficiency curve rises in this region of loading and
In 1954, an 1161 mode] appeared which was designed for
commercial use in restaurants, hospitals, hotels, and institutions of this
type. Microwaves were generated in this model by two QK-390 air-coo
magnetrons adjusted to provide about 1.5 kilowatts for cooking heating.
The oven measured 20 X 22 X 12 inches in width, depth and height and the
magnetrons were again directly inserted into the top.
The uniformity of heating in this oven reached the
highest standand! of development up to that time. It was achieved by means
of t dimensions, an improved stirring device with fan-type blades, and t
use of two magnetrons and their locations. A choice for power input was
provided by a switch for high, medium, and low setting's, , later, high,
low, or off. At the low setting, only one magnetron was energized and the
pattern of heating was much stronger beneath the tube than in the rest of
the oven. The skills needed for actually cooking with this range rather
than heating food only, tended to limit its use to carefully designed
systems for food service or to simple standard-quantity heating
operations. The size of the oven contributed to the performance even if it
was not fully utilized with heavy loads.
A smaller oven would not have heated small,
randomly-placed loads as reliably. At first a ten-minute recycling timer
was used but it w later changed to a 21-minute non-cycling one. In all
cases, the cooling began when the "cook" button was pressed.
However, with the ten-minute timer, the button had to be repressed after
each ten minutes. There was an element of security in the shorter period,
but was generally inadequate at 1.5 kilowatts.
A companion oven with a single magnetron of the same
designation was also available in 1954. It provided a smaller oven which
measured 14 X 18 X 10 inches in width, depth, and height. The height in
these cases was measured to the top of the door opening, and the were
about two more inches available until the rotation circle of the stirrer
terminated the useful height. The performance of the 1170 as this oven was
called was clearly overshadowed by its bigger brother the 1161.
The commercial users were often interested in the heating of
eight-ounce prepared, refrigerated casserole or some equivalent load. In
quick-service food establishments, the one minute needed for this heating
in the large range was satisfactory, but twice this time (a required in
the 1170) apparently seemed disproportionately longer. There were other
benefits, however, in the 1161 including a superior pattern of heating
with two magnetrons working into the larger space.
Further development led to the domestic microwave ovens
which were introduced in 1955 and 1956. With these models Raytheon became
an original equipment manufacturer for a group of domestic range companies
including Hotpoint, Westinghouse, Tappan, Kelvinator (American Motors),
and Whirlpool. Power supplies and magnetrons were furnished to each
company which designed its own domestic range around these components.
Tappan, meanwhile, had pursued the development continuously and produced a
series of designs which were called the 1180 models. In one 1180 model, a
major change was made in the introduction of microwaves through a
transparent oven bottom. This left the top of the oven free for other
possibilities, such as the introduction of a broiler element for browning
the top surface and sides of food.
The transparent bottom had to be designed so that
spilled fluids did not penetrate into the stirrer compartment and
magnetron section which were just beneath it. At about the same time,
attempts to introduce microwaves into an oven directly through the broiler
indicated the feasibility of such a design. Simultaneously, a built-in
version of the oven was visualized which was essentially a match for the
conventional built-in oven. Behind the panel, on which controls were
mounted, there was space for some of the electronic components, while the
rest could be installed behind the oven.
Microwave cooking was introduced into many homes with
ovens of this design and considerable experience was accumulated.
Improvements in the magnetron were long overdue, however, since the value
of the power supply and magnetron alone was greater than some
complete conventional ranges which were readily available. When the range
manufacturers had completed the cabinet and put the device in position for
sale, its price was too great for wide appeal. Similarly, the magnetron
performance was a critical factor in marketing these ovens because the
tube was the heart of the oven. When it failed, there was no further
There was a demand for an inexpensive generator and
possibly one which could be replaced in part when it failed retaining, for
example, the magnet. Also some components could be eliminated from the
power supply by delivering alternating current to the generator: in a
sense this would use the diode magnetron as the rectifier. It was also
desirable to reduce, if possible, the warm-up time delay which was
currently about 75 seconds.
A new magnetron became available in 1960 which met many
of these requirements and increased the power by 20 per cent. It was
produced by Litton Industries, San Carlos, California and called the
"Microtron"; one type, the L-3510, is a microwave heating unit
consisting of a cw magnetron, a high voltage transformer, a filament an
isolation transformer, and an electromagnet and filter assembly, These
units are shown in Fig. 102 with a pencil included to suggest
relative sizes. When assembled, the magnetron fits inside the
electromagnet with the larger glass envelope passing further into a wave
guide on which the whole assembly is mounted.
The lines for the cooling water encircle the midsection
of the magnetron and connect with the closed cooling system. Above the
plate transformer is a terminal strip which is used by the service man
when the range is installed. After he determines the line voltage
available, he can compensate for variations in it by choosing the correct
tap, The taps on the plate side of the strip compensate for variations
from 208 to 240 volts while those on the filament side handle variations
from 104 to 120 volts. The high voltage transformer is also connected to
the cooling system as indicated by the connecting tubes at the base.
FIG. 102. MICROTRON, MICROWAVE POWER UNIT
Also shown are the magnetron circuit connections, filter assembly, and
These components are installed in position in the microwave oven power
supply compartment as illustrated in Fig. 103.
Courtesy 0f Litton Industries FIG.103. MICROWAVE
OVEN POWER SUPPLY COMPARTMENT
The oven dimensions are 18 X 14 X 12 inches for width,
depth, and height. This size permits top placement of such Units as: the
wave guide; stirrer motor, drive wheel, and belt; direct-current power
supply, test jack panel, and magnetron output power adjustment. In the
rear of the
oven, the water hoses connect the reservoir, pump, and
cooled components with the radiator which is underneath the oven cavity.
Forced circulation of air is provided over the radiator and out through
the top compartment. Air may also be exhausted from the oven to prevent
condensation during cooking. This would apply especially in an oven when
only microwaves are used.
Two new magnetrons were developed by the Raytheon
Company for microwave heating; the QK707 and the QK904. The QK707,
FIG. 104. MAGNETRON QK707 WITH CLOSED
LIQUID COOLING SYSTEM Courtesy of Raytheon Co.
Fig. 104, was used in place of the QK390 in microwave
ovens such as the Mark III, IV, and V. It has an output of 0.8 kilowatt.
development produced in 1962 the QK904 which is shown
in Fig. 1,05. It incorporates many of the features deemed desirable for a
heating type of magnetron. For example, it operates on unregulated
alternating current as well as direct current for its 3.2 kilowatts anode
voltage. It has a higher output which will bring single generator ovens up
to 1.25 kilowatts.
A microwave oven may be regarded as consisting of eight
major components: (1) the power supply which draws electrical power from
the line and converts it to the form required by the microwave generator;
(2) the generator or power tube which is an oscillator capable of
converting the power supplied into microwave energy; (3) the transmission
section which propagates, radiates, or transfers the generated energy to
the oven; (4) coupling devices which permit the
FIG. 105. MAGNETRON QK904
Courtesy of Raytheon Co.
transfer of energy to the oven; (5) the distributor
which rotates and scatters the transmitted energy through the oven,
disturbing the standing wave patterns; (6) the cavity or oven which
encloses the cooking food within metallic walls so that the distributed
energy is reflected and intercepts the food from many directions with more
or .less uniform energy density; (7) energy sealing or trapping structures
which serve to retain the microwave energy within the oven confines during
operation; and (8) operating and safety controls which allow selection of
cooking conditions and interrupt the flow of power if necessary.
In addition, auxiliary heat may be incorporated in the
form of a broiler element or complete hot oven components. There are also
subsidiary requirements for cooling the generator, the transformers, and
other electrical components which dissipate heat. Provision is usually
made too, for exhausting the oven of cooking vapors, and for visual and
audible indicators of the end of the cooking cycle.
The character of the eight components has already been
considered along with various topics; for example, the dimensions of the
cavity compared to the wavelength and cavity design were taken up in Chap
ter 5. It was shown that the length of a cavity wan
should be greater than a half wavelength and any multiple of a half-wave
in the direction of propagation win satisfy the requirement (Ramo and
Whinnery 1944). The cavity can then resonate and microwave energy can pass
from the wave configuration established in the cavity to the food.
The rays then attenuate since they are partially absorbed in their
transmission through the food; they impinge on metal cavity walls and
' reflect again and again until the energy is used up in the heating
process. The number of modes which can be supported in the cavity II
was given as a function of the volume of the cavity. Skin depth,
current at the walls, impedance concepts, cut-off frequency, and the Q,
of the cavity were defined.
WAVES, FIELDS, AND FOOD
The disappearance of energy from the waves in the
cavity depends on the amount of energy present and on the amount of
absorbing material present. The direction of the microwave alternates from
and to the generator at the rate defined by the frequency. Therefore, the
system is oscillating and oscillations are usually spoken of as damped if
something absorhs some of the energy. The change in energy with time would
be described in an exponential manner (familiar to biologists in
deactivation processes) where the change at any time depends on the amount
of the variable, in this case, energy left in the systeml
For resonant cavities, Q is the quality factor
Q = Wo (energy stored in the circuit)
average power loss
where Wo = l/VLC and L is the inductance
and C is the capacitance. Also,
Q = TT (energy stored in the circuit)
energy lost per half cycle
From these ratios, the meaning of a "high" Q or
"low" Q cavi,tYmay be understood. In the high Q cavity,
there will be considerabIere active and circling power, and a high SWR in
the oven which Could then be considered as efficient for heating.
.The rigorous definition of cavity dimensions in terms of wavelengths
V (mlax)2 + (n/ay)2 + (plaz)2
w AD is the free space wavelength, , y, are
ats for the box dimensions, and , n, are
integers. Usually the
Iheight, width, and depth will differ considerably.
This analysis is for the high Q cavity with no damping. The curves
for resonance would have sharp peaks (in the lossless cavity) but in the
highly damped one as when food is heated, the resonance maxima are not
sharp. The cavity will have broad resonance peaks with heavy loads and
more narrow ones with light loads. The effect of broadening these peaks is
to stabilize the field distribution and thus the load impedance of the
generator (Schmidt 1961). Loading the oven with food ,is favorable for
generators but now there is no longer a high Q cavity. Itdevelops
that in such cases, one is working with a high quality cavity in which the
Q has been lowered (by the food).
Broadening of Resonant Peaks
If the energy is carried by the modes and distributed
to the food more or Jess by rays, this broadening of resonance peaks
begins to ap'pear as a phenomenon of expanding resonance points in the
frequency space until they more or less meet, so that any initial ray
direction is permitted (Rapuano and Smith 1955). A mode stirrer meets the
need for distributing the incoming energy in these various directions. For
the food, this means that it is being irradiated from many directions. The
stirrer is effective if it helps the cavity to fill with energy ,so that
the heating of the food will be independent of position. It also will
increase the number of resonances.
The idea is to use dimensions, reflecting sides, and a
stirrer to pro
duce an oven in which a great many different modes with
overlapping resonance bands may be excited.
If the cavity is filled with food, the reflections are
effect depending upon the frequency and associated
penetration. Therefore, if the stirrer is to be most effective, it should
be located between the generator and the load (Smith 1957).
The stirrer is an important feature of the oven but
,remains to be done before its action is fully
understood. It is best to I'ook at the stirrer as a tuning device and to
consider ways and means oftuning the cavity for maximum energy
distribution and utilization
under the conditions to be met in loading.
The stirrer, as one of a number of tuning devices, such
as rotating antennas, food turntables, "paddle-wheels" and
vibrating cavity walls, can be evaluated by the effect it has on the
distribution of energy. Generally, this mode stirrer will look like an
air-circulating fan but closer inspection shows that the blades are
designed for reflected
energy rather than moving a column of air. The blades
may be pitched at an angle of about 450, the angle depending upon the
relative position of the magnetron probe. That is, if you assume that ,the
energy is to be directed vertically when it: strikes the stirrer blade,
and the main flow of microwave power comes at the blade at an angle of
60° with respect to the vertical axis of rotation, a solution of the
optical angles involved gives a blade which makes an angle of 30° with
the vertical axis of rotation (Smith 1957).
Two fans were used to distribute the energy in the
cavity by Becker and Autler (1946). They were showing that water vapor has
a max. imum absorption of energy at a wavelength of 1.34 centimeters. They
were not primarily interested in stirrers. A large copper fan was used at
the top and a smaller one at one side wall. The energy distribution at
different walls was very uniform.
The energy will a]so be caused to reflect from an
inclined plate at the end of the wave guide according to Blass (1952). The
angle of inc]ination is 45°. With a stirrer in front of such a sloping
blades may be at an angle of 45° with the rotation plane
of the fan.
Fig. 106 shows an oven with the stirrer and an interna]
reflector in use with a cylindrical wave guide and a broiler unit. The
broiler can be considered as an oven structure which absorbs a slight
amount of energy. The mounting is necessarily smooth where the wall is
penetrated to avoid arcing there. There will be oven situations with wave
guides propagating energy into the cavity; another type has a dipole
radiation pattern from an internal antenna; and there is also the type
which gives propagation from the AI4 radiating probe of the generator when
it is directly inserted.
Even without the stirrer, the beam from a wave guide
can be sent out into the cavity from metal reflectors (one wavelength
long) so that the energy will have several reflections before it returns
toward the source. Instead of using reflectors, the shape of the oven
itse]f can be designed for this effect. When the generator probe is
directly inserted, propagation toward the load is improved by placing it
at the intersection of two lines which are parallel to the center lines of
the oven but moved from oven center by an eighth wavelength at the
operating frequency (Blass 1952). This combination of sloping even sides
and displaced magnetron probes may be seen in the construc. tion of the
large 1150 model which is discussed on page 264.
HE Horn and Cavity
When a horn attachment can be used at the waveguide termination, a
broad energy beam in the H-plane is obtained which can be
Courtesy of Philips Tech. Rev.
FIG. 106. POSITION FOR MODE STIRRER IN
MICROWAVE OVEN AT 2,450 Mc
Dimensions 440 X 400 X 360 mm. (17.3 X 15.7 X 14.2 in.).
'distributed as it reflects on the sloping oven sides
or strikes the rotating reflectors. Then if the horn is designed as are HE
'both magnetic and electric flares, it becomes possible
to consider a stirring device of the paddlewheel or fan type in the
horn itself. In this way, the energy is certain to be disturbed
before it gets to the oven, and for a small load and small oven, this
might be the method of choice.
Fig. 107 shows the method used by Haagensen (1958) for
this design. His concept also included a provision for matching energy
smoothly into a container of food. He used a Pyrex glass floor in the
cavity: this in itself was transparent to microwaves, but had a quite
different impedance from the horn feed. When a container such as a
ceramic or glass casserole dish is placed on the g]ass
floor, the Gombined thickness produces a good impedance match into the
foodin the casserole. In optical terms this wou]d give a coupling iris or
a sharp discontinuity in the impedance at the container location. Then the
food would draw most of the energy and heat effective],y.
The construction also protected the magnetron from
damage due to overheating or mode shifting by the use of a four spoke radial
aperture in the top of the cavity. When the generator was accidentally
energized with no load, the microwaves passed harmlessly,
FIG. 107. HE HORN AND PADDLEWHEEL STIRRER IN
SPACE PROVIDED BY THE DOUBLE FLARE (Two VIEWS)
A. Transparent glass floor.
B. Four spoke radial aperture covered by absorbing
C. Paddlewheel stirrer.
out these coupling slots. A functional and attractive
design' was provided for the door (not shown) in which energy containment
was assured by a grill with openings less than a half wavelength.
In this oven, there were a number of innovations in the
use of materials which combined special mechanical and electrical
requirements. For example, the Pyrex glass tray has a thickness much less
than a quarter wavelength so that with its dielectric
constant quite removed in value from the air in the horn, it presents an
impedance discontinuity to the energy propagating from the transmission
line. The safety slots in the top are about one-half wavelength long. The
combined thickness of containers and Pyrex tray is a quarter wavelength
for the radiated and transmitted energy. Both have a dielectric constant
of about six (or about equal to the square root, of the dielectric
constant of the food multiplied by the dielectric Constant of free space).
At 2,200 megacycles, the thickness for the combined
dielectrics (tray and container bottom) would be slightly less than
'The stirrer rod being in the microwave field is constructed of an
insulator (nylon) for non-arcing, and nylon is also chosen for some of
lUhe studs and guides for the door. The stirrer blade contains a slot and
'resembles an asymmetrical paddlewheel in the "flowing energy
stream." The asymmetry and slot combine to reduce the cyclical -rises
in the standing wave ratio and also prevent self-heating of the mode
FIG. 108. POLARIZATION STIRRER
Other Tuning Procedures
Hall (1952) has given six objectives in the design of
an effective microwave oven: (1) uniformity of distribution of microwaves
in the device; (2) uniformly integrated microwave heating pattern; (3)
maximum use of the input power in heating food; (4) uniformity of heating
obtained from periodic change in the field distribution; (5) continuous
changes in the mode of the waves in the cavity and obtaining a uniform
heating from changing complex modes; and (6) heating food masses uniformly
where the wavelengths are those of microwave frequencies.
He recommended that the oven dimensions should differ, for
example, by a quarter wavelength in magnitude. The favorable effect of
this asymmetry on the number of modes that will exist in a given range of
frequencies is enhanced by introducing the power at a point displaced from
the center of one wall. Most of the ovens which have been described have
illustrated this feature.
The maxima and minima in the fields are therefore moved
around in the cavity but they persist. The Hall oven is visualized in the
1132 model which employed the wave guide feed through an iris in the rear
wall. The insertion was asymmetrical and high with respect to the rear
wall but on the mid line.
As a means of disturbing the modes, he a]so described a
piston with a flat, square surface reciprocating in the top of the cavity.
The piston area was such that a side was three-fourths of the comparable
oven dimension. A slow rate of oscillation was prescribed for this member
(about 75 per minute).
Hall also described a type of mode shifter which
includes a provision for polarizing the energy. Two grids of rods were
used on different levels. This stirrer may be visualized as replacing one
of the others which have been described and it is shown in Fig. 108. Each
quadrant could be expected to yield a different effect
on waves approaching it.
In the third quadrant (clockwise), for example, the
conducting rods will reflect both horizontal polarization components from
the stirrer-that is, both the component of the waves which is parallel to
dimension Wand that parallel to dimension D. Therefore there is a dif
ferent polarization, reflection action in each of 4
quadrants. First there are no reflections, then one horizontal
polarization, then both horizontal polarizations, and lastly, on]y one
again. In addition, one would have the action of waves passing on for a
wall reflection beyond the stirrer. The disc]osure (Hall 1952) explains
the effect in terms of the distances travelled by the waves and the
perturbation is of the distances.
At high microwave frequencies which result in few transmissions
through the food, quick absorption, and short penetration, this stirrer
participates to a lesser extent in the perturbation. When the stirrer is
interposed between food and microwave source however, as in the HE horn,
there would always be some perturbation.
BECKER, G. E., and AUTLER, S. H. 1946. Water vapor
absorption of electro
magnetic radiation in the centimeter wavelength range.
Physical Rev. 70,
5 and 6, 304-307.
BLAss, J. 1952. Heating apparatus. U. S. Pat. 2,618,735
HAAGENSEN, D. B. 1958. Electronic heating apparatus. U.
S- Pat. 2,827,537
HALL, W. M. 1952. Heating apparatus. U. S. Pat.
2,618,735 Nov. 18.
KINN, T. P., and MARCUM, J. 1947. Microwaves and their
possible use in high
frequency heating. Electronics 20, No.3, 82-90.
MORSE, P. W., and RIVERCOMB, H. E. 1947. UFH heating of
frozen foods. Electronics, No. 10,85-89.
RAMO, S., and WHINNERY, J. R. 1944. Fields and Waves in
John Wiley and Sons, Inc., New York.
RAPUANO, R. A., and SMITH, R. V. 1955. Design considerations of
microwave ovens. IRE Convention Record, Part 9, 3-9.
SCHMIDT, W. 1961. The heating of food in a microwave cooker. Philips
Rev. 22, 3, 1960-61. SMITH, R. V. 1957. Heating devices. U. S. Pat.
2,813,185. Nov. 12. SPENCER, P. L. 1949,1950,1952. U. S. Pats. 2,480,679;
SUSSMAN, L. 1947. Evaluation of electronic cooking device for
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