Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
In Canadian Application Serial No. 314,131 filed
October 24, 1978; J.B. Weiss assigned to the same assignee as
this application, there is disclosed a combination microwave
and electrically heated oven in which microwave energy is
supplied at the bottom of the oven. However, in such an oven,
the radiating applicator is beneath the food body being heated
so that spillage or other undesirable effects can occur which
reduce the efficiency of the cooling system. In addition, if
high temperature self-cleaning is used, the radiator must be
made of material, such as steel, which will withstand such
temperatures.
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Summary of_the Invention
This invention discloses that a desirable heating pattern
can ~ achieved with a multi-element radiator fed from a common
junction and producing a plurality of substantially directional
patterns of radiation toward a body to be heated with a smaller
spacing between the radiator elemen~s than pTevious radiators
by causing at least one of said elements to radiate a pattern
whose polarization lies in a plane having a substantially different
angle to the plane of radia~ion polarization plane, the radiato~
pattern produced by a different element of said radlator.
More specifically~ the polarization of one of said radiation
patterns lies in the plan substantially parallel to the axis of
roation of the radiator. The radiator el0ments are preferably
ports in a waveguide plenum fed by a common junction such as
a coa~ial line with the microwave energy arriving at different
radiating elements in respectively different phases. The improved
uniformity of the heating pattern in a food body produced by
such independent radiation patterns occurs even when the food
body is positioned directly in the direct radiation patterns
of the radiating elements so that a substantial portion of the
energy is absorbed by the food body as direct radiation from
the radiator rather than reflected from the cavity walls.
This invention discloses that a stationary cover which is
transparent to microwave energy may be positioned over the
radiator in the oven cavity to provide protection of the radiator
from spillage of food or conduction or cooking vapors on the
radiator. More specifically, the transparent cover is supported
on bumps on the bottom wall of the oven cavity providing spaces
through which air passes ~hrough the oven. The air directed
out through the radiator ports past inner surface portions of
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the cover adjacent the radiator to maintain the radiator at a
temperature below that of the oven when electrical resistance
heating is used. More specifically, the cover may be, for
example, a commercially available high temperature material
~$ such as a pie plate sold under the trade names Pyroceram or
Rayceram which will withstand temperatures in excess of 1000F.
The pie plate cover is inverted in a st~ationary position over
the rotating ràdiator and acts as a thermal shield during high
temperature oven operations such as a cleaning cycle to allow
the radiator to remain at temperatures substantially below
temperatures of the oven wall surfaces.
In accordance with this invèntion, the walls of the oven
are preferably made of conventi~nal steel coated with high -
temperature ceramic such as enamel so that in the absence of
large load bodies, microwave energy being radiated into the
oven will be partially absorbed by the oven walls, hence
preventing the build-up of undesirable high intensity field
patterns in the oven. The radiator may be of a high ~conductivity
metal such as aluminum which is protected from the high temperature
of the oven.
This invention further discloses a microwave oven having
a door seal structure incorporated wi~h a conventlonal high-
temperature gas vapor seal for a combina~ion oven and a low
microwave loss microwave choke seal structure between the vapor
seal and the oven interior which allows air to be ~lown
continuously through the oven without substantially reducing the
heating rate of the food body by microwave energy or by resistive
heating. More specifically, the choXe structure may be formed
aither in the door or in the oven wall and provides a high
impedance in series with the input transmission line structure
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at the predominant operating frequency range of the microwave
oven, such as, for example, between 2.4 ko 2.5 KMH.
In accoTdance with this invention, additional resiliency
may be provided for the resilient high-temperature vapor seal
by supporting wire mesh in tubular form on a tubular fiberglass
braid in tu~n supported on a tubular steel mesh structure of
greater diameter wire to provide spring action.
In accordance with ~his invention, sealing action of the
microwave energy in the predominant frequency range may be
enhanced by providing means in an input transmission line section
o a choke-~ype microwave door seal for inhibiting transmission
of such microwa~e energy periphery around the said input trans-
mission line structure. More specifically, such means may
comprise impedance discontinuities such as slots in one of the
walls o said input transmission line structure. Further in
accordance with this invention, such slots preferably extend
~hrough the wall into the choke structure to further assist in
inhibiting periphery mode propagation in the choke structureO
Such a clloke seal is positioned inside the oven between the
high-temperature vapor seal and the oven interior.
In accordance with this invention, a food body may be
positioned on a rack in the radiation patterns from a rotating
radiator, preferably formed of high conductivity, low loss
material such as aluminum, so that a substantial portion of the
microwave energy is absorbed on passing through the food body
prior to reflection from walls of the oven. Therefore, high
efficiency heating may be achieved with microwave energy even
though the walls of the oven are made of inexpensive high-
temperature material such as enameled steel, which slightly
absorbs microwave energy. In accordance with this invention,
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the magnetron may b0 tightly coupled to the oven through a
coupling mechanism such as a waveguide, coaxial transition and
multi-element radiator thereby increasing the efficiency of
the magnetron and hence conversion of input electrical energy
to microwave energy coupled into the body to be heated. More
specifically, where a small food body on a metal dish is placed
in the oven~ substantial microwave energy which is radiated
into the oven, is reflected back to the rotating radiator from
the metal dish or the opposite wall of the oven will arrive at
a common junction~ such as the central conductor of a coaxial
line transition, with substantially different phases so that
relatively low amolmts of energy are reflected back into the
magnetron.
In addition, an electric resistance heater, which may be
positioned directly in the top Of the ovsn while extending through
microwave chokes in the oven wall for connection to an electrical
power source, is used for heating the oven, either separately or
in combination with microwave energy, and is used for treating
the oven walls during the self-cleaning cycle.
When microwave energy alone is being used, deposition of
cooking vapors on the surfaces through which the microwave energy
passes is inhibited by re-heating of such surfaces by the
microwave energy. However, this somewhat reduces the efficiency
of the transfer of microwave energy to the body to be heated,
and accordingly, air directed past the surface o~ the cover is
first heated by passing such air by the generator, such as a
magnetron, to cool the magnetron and to heat the air to a
temperature on the order o 40~ above room temperature, and
hence above 100F, so that substantially no cooking vapors will
deposit on the transparent cover. In accordance with this
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invention, the air heated by the magnetron flows through the
oven without substantially cooling the oven so that clouds of
steam or other cooking vapors are carried away during the cook-
ing process and are not emitted from the oven through the door
when it is opened.
In accordance with the invention, there is provided
in combination: a microwave energy conductive enclosure; means
for simultaneously radiating a plurality of independent differ-
ently polarized primary radiation patterns of microwave energy
in said enclosure, said means comprising a multi-element
rotating radiator positioned in said enclosure; a microwave
transparent concave d;sh covering said radiator in said enclo-
sure, said dish being removably supported by said enclosure in
a stationary position with the opening of said dish facing said
radiator; and an electrical resistance heater positioned in
said enclosure adjacent to the lower wall thereof for heating
the interior of said enclosure; and said heater being spaced
from said dish.
In accordance with another aspect of the invention,
there is provided a combination microwave oven comprising: a
microwave energy conductive snclosure; a rotating radiator
comprising a plurality of spaced ports for simultaneously
radiating a plurality of differently polarized radiation pat
terns into said enclosure; a stationary concave dish which is
substantially transparent to microwave energy supported in
said oven covering said radiator with the opening of said dish
facing said radiator, said dish shielding said radiator from
the interior of said enclosure; and means for passing air into
said oven through said ports to transfer khermal energy to
and/or from said cover.
In accordance with another aspect of the invention,
there is provided the method of heating a body comprising the
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steps of: supporting said body within an enclosure; simulta-
neously radiating a plurality of substantially independent
microwave energy patterns having different polarizations toward
said body Erom a radiator while said radiator rotates about an
axis substan-tially parallel to the plane of polarization of one
oE said patterns, while covering said radiator with a concave
dish which is substantially transparent to microwave energy
and while blowing air past a surface of said dish into said
enclosure to at least partially thermally shield said radiator.
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Brief Description of the Drawings
Other and further objects and advantages of this
invention will be apparent as the description thereof
progresses, reference being had to the accompanying drawings
wherein:
FIG. 1 illustrates a vertical sectional view of a
combination microwave oven embodying the invention with the
door closed;
FIG. 2 illustrates a vertical sectional view of the oven
of FIG~ 1 taken along line 2-2 of FIG. l;
FIG. 3 illustrates a horizontal sectional view of the
oven illustra~ed in FIG. 1 taken along line 3-3 of FIG. 1 but
with the door open;
FIG. 4 illustrates a fragmentary expanded portion of
FIG. 3; and
FIG. S illustrates a vertical sectional view of the,oven
portion of FIG~ 4 taken along line 5-5 of FIG. 4,
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Description of the ~.referred Embodiment
Referring now to FIGS. l, 2 ~ and 3, there is shown a
microwave cavity 10 closed by a door 12 and supplied with
microwave energy from a rotating radia~or 14 in the bottom of
the oven. Radiator 14 is fed with microwave energy from a
magnetron 16 through a waveguide 18 and a coaxial line 20 having
a central conductor 22 rigidly connected to rotating radiator 14
and extending through waveguide 18 to a gear reduction motor 24O
Motor 24 is attached to the bottom of waveguide 18 and rotates
central conductor 22 to rotate radiator 14. Coaxial line 20 has
an outer conductor 26 rigidly connected to the upper wall of
waveguide 18 and extending through the bottom wall of enclosure
lO into a plenum 28 in radiator 14.
As shown more speclfically in FIG. 2 ~ plenum 28 comprises
an upper plate 30 connected to central conductor 22 and having
a plurality of ports 32 therein spaced at diferent distances
at the axis of conductor 22~ Microwave energy is radiated from
plenum. 28 into the oven enclosure 10 through ports 32 and
through a stationary cover 34 which is transparent to microwave
20 energy and which prevents cooking spillage or vapors from
impinging on radiator 14.
A lower plenum cover 38 of radiator 14 prevents radiation
of microwave energy radially outwardly and directs it through
the ports 32, and the lower surface o~ cover 38 is positioned
sufficiently above the bottom wall o enclosure lO for
radiator 14 to rotate freely. An aperture in cover 3S sur-
rounds ~che upper end of outer coaxial conductor 26 which
thus extends slightly into plenum 28~ ~hereby substantially
preventing microwave energy from radiating into enclosure lO
from beneath radiator 14. The length of outer conductor 26
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which extends into plenum 28 may be adjusted to improve impedance
matching conditions.
As shown in FIG. 1, a substantially conical waveguide to
coaxial line transition member 40 is formed by die-stamping the
sheet metal bottom wall of guide 18 upwardly in conical shape
surrounding central conductor 22. A tubular member 42 is
welded to the top of conical member 40 and extends downwardly
surrounding conductor 22 for distanc0s equal to an effective
electrical quarter wavelength at the frequency of magnetron 16
so that it produces a choking action to energy attempting to
escape from waveguide 18 to~ard motor 24. A sleeve bearing 44
of dielectric material is positioned between tubular member 42
and conductor 22 to insure against arcing in the bearing.
Conductor 22 is reduced in diameter just below the lower end of
tubular member 42 producing a land resting on an oil-filled
bronze bearing 46 supported in a plate 48 which attaches motor
24 to the bottom of guide 18. The ends o~ waveguid~3 18 are
closed by shorting members 50 which are positioned to provide a
substantially flat standing wave ratio between the output probe
of magnetron 16 and central conductor 22.
As shown in FIGS. 1 and 2, radiator ports 32 are eaçh fed
with microwave energy through separate plenum waveguide sections
whose axes are at 120 to each other and whose inner ends form
a common junction region containing the central conductor 22.
An impedance matching conical portion 52 formed in plenum plate
30 is welded to conductor 22 to increase its radius as it
approaches upper plate 30 o-f plenum 28. Waveguide section walls
which are.formed by the sides of plenum 28 are made of different
lengths so that energy radiated into plenum 28 from central
conductor 22 arrives at ports 32 in respectively different
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phases. Reference may be had to the aforementioned copending
application ~or an additional description of the details of such
a radiator.
In accordance with this invention, one of the ports 32 has
a vertical aperture 54 forming a radiation polarization pattern
which is parallel to the axis of rotation of radiator 14. Thus, -
.~ the composite of the heating ~c~* produced by the radiationpatterns from ports 32 is different at different distances
above radiator 14, and the waves from aperture 54 are vertically
polari~ed so the waves will arrive at the food body in diferent
phases and polarizations, preferably selected so that reflections
o the waves back to radiator 14 will have substantially cancella-~
tion of the electrical field vectors at the junction of the
plenum waveguides at central conductor 22 to re-reflect such
energy back through ports 32 into the cavity 10. This effect
is preferably chosen to be maximized when the microwave cavity
has no food body positioned therein or when the food body is
positioned in a metal dish. Under these conditions it therefore
is possible to couple magnetron 16 to the oven to operate close
to its maximum efficiency for converting its electrical energy
input to microwave energy output while maintaining low microwave
ene~gy field gradients and hence low wall ~oss~s in the waveguide
18.
In accordance with this invention, oven cavity 10 may be
made of relatlvely lossy or energy absorbing material which may
absorb, for example, a few percent of microwave energy impinging
thereon and re~lecting therefrom. Such ma~erial may be, for
example, conventional sheet s:teel used in conventional ovens and
coated with conventional~en~mel, all in accordance with the
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well-kno~n practice. In addition, conventional broiler and
heating units 60 and 62 may be positioned adjacent the upper and
lower walls of the cavity 10 held by conventional fasteners
in accordance with well-known practice. However, in the case of
the heating unit 62, it preferably is formed in arcuate shape
so that its closest portion is positioned around, and spaced
from, the periphery of cover 34 so as not to overheat the
cover 34.
In accordance with this invention, elements 60 and 62 are
shielded electric resistance heating units connected to power
through the back wall of cavity 10. The outer shields of the
units are grounded to the wall of cavity 10 by tabs 64 whicll are
attached, far example, by welding or crimping to the shields and
which are screwed to the back wall of cavity 10. Tubular
microwave choke elements 66, whose lengths are preferably an
effective quarter wavelength of the microwave frequency in
cavity 10, are attached by welding to the outside of oven cavity
10 and surrounding the shields but spaced therefrom by an
enamel coating on elements 66. Electrical connections to
power and control terminals may be made to the heater and
broiler units in accordance with well-kno~ practice.
Any desired configuration can be used for the radiator 14.
An example providing good results at 2.45 KMH uses waveguides
which are 4 inches wide and 1 inch high fed by a central con-
ductor 22 which is 1/2 inch in diameter and an outer conductor
26 which is 2 inches in diameter. The waveguide 18, which may
be 4 inches wide, is shown as 2 inches high, and the distances
from one shorting member 50 to the center of magnetron output
to the axis of conductor 22 and to the other shorting plate 50
are 3/4 inch, S inches, and 10 1/4 inches, respectively.
.
A food body 70 may be positioned, for example, on a rack
72 above radiator 14 in a dish 74, preferably transparen~ to
microwave energy, and resting on a plat0 76 of materiàl which is
transparent to microwave energy~ such as pyroceram. Rack 72
may be, for example, a welded wire rod having apertures sub-
stantially greater than ~/2 and adjustably supported at differ-
ent levels in cavity 10 by means of grooves 78 in the side walls
of cavity 10 or in any other desired manner.
Air from a blower 80 is blown through the cooling ins o~f
magnetron 16 and then into oven 10, for example, through
apertures in waveguide 18 via duct 82, transmission line 20,
apertures 32 and spaces between the bottom lip of cover 34 and
the bottom o oven 10 where the lip of cover 34 rests on raised
positioning bumps 68 formed in said oven bottom wall, past
heating unit 62 to conduct that alr past ood body 70 during
cooking. The air then exits through a canister 84 at the top
of the oven to the center of a surface burner unit 86
During the oven's self-cleaning cycle, the temperature of
the oven is raised to a temperature between 750F and 1100F
by energi7ing upper heating unît 62 to vaporize deposits on
the wall of oven 10 and to allow the vapor to move by convection
out through canister 84 which may con~ain a catalyst to complete
oxidation of the vapor in accordance with well~known practice.
Air is also drawn by convection into the oven past
radiator 14 and the inner surface of cover 34, which may be,
for example, Pyroceram, thereby maintaining radiator 14 below
the temperature where aluminum would soften while permitting
the upper surface of cover 34 to be heated to self-cleaning
temperatures for Pyroceram surfaces, such as 700F to 900F,
Thermal insulation 88 of, for example, fiberglass is
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provided around oven 10 in a well-known manner surrounded by a
metal skin 90. A light 92 may illuminate oven 10 through an
apertured metal plate 94 covered with a translucent ~yroceram
plate 96.
In accordance w;th this invention, door 12 has a high
temperature vapor seal which is prevented from absorbing large
amounts of microwave energy from the interior of enclosure 10
by a microwave seal positioned between said enclosure interior
and the vapor seal. The seal is described in greater detail
in said copending application.
The dimensions of the apertures 32 and vertical aperture 54
may be of any desired size and are shown here, by way of example,
scaled to produce an improved heating pattern with a reduced
size radiator from that disclosed in the aforementioned copending
appli-ation, however with the distance and size of the slot 32
covered by the lip 56 is closer to the axis of rotation of the
radiator 14 than the farthest slot disclosed in said copending
application, The lip 56 covering this slot 32 causes ~he average
of the radiation pattern radiating from the vertical aperture 54
to be directed at an angle toward the rotational axis of
radiator 14. The shape of patterns of radiation from the
vertical aperture 54 and the other apertùres 32 arè`-governed
by size and shape of the ape,T,tures 32 and 54. Since the,
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radiation pattern from aperture 54 is directed inwardly
toward the axis of rotation 104, additional heatin~ occurs in
the center of the food body. Such a heating is ~dl,fferent for
different positions of the rack 72 in the support grooves 78.,.
when the rack is in its uppermost posi~ion a'large portion of
the microwave energy from aperture 54 is directed across the
microw~ve oven beneath the food body and is reflected -from the
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walls. However, when the rack is in its lowermost position
the energy from the vertical aperture 54 has larger portion
impinging directly on the food body, particulaTly if the food
body is large and hence it is important to couple substantial
amounts of microwave energy il~tO the center of the food body.
The other ports 32 radiate waves that are hoTizontally polarized
in a radiatio~ patte~n whose average direction is upward,
substantially parallel to the rotational axis.
In such a radiating structure provides an improved heating
pattern in which the major portion of the microwave energy may
impinge directly on a food body before reflection from the oven
walls and the ports are further de-coupled from each other for
direct radiation so that radiation from one port does not
affect the radiation pattern from the other ports prior to
impingement af the pattern energy ~n the walls of the oven.
Thls is achieved even though the spacing between the ports is
less than in the structure of the aforementioned c,opending
application so that the cover 34 positioned over the radiation
may be of a lesser size.
Air which is blown through the radiator 14 into the oven
largely exits from the radiator through the por~s 32 and impinges
on the inne~ surface of the cover 34 so that it flows along this
surface and out under the lip of the cover 34 between the'bumps
68 which support the cover lip in a raised position above the
floor of the oven 10.
'This completes the *escription'of the embodiments 9f the
inventio~ ill,ustrated herein. However, many modifications
'thereof will be apparent to persons skilled,in the art without
departimg from the spirit and scope of the invention.
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For example, other sources of auxilliaTy heat such as gas flames
or hot air can be used in place of electrical resistance heaters
and any desired system of timers and/or cont~ols can be used.
Accordingly, it is intended that this invention be not limited
to the terms of the speci~ic embodiment herein except as
defined hereby by the appended claims.
