Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
Microwave ovens have been produced in which a radiator
provides a multi-mode pattern in the oven and such patterns have
been stirred by reflective mode stirrers. However, heating
patterns in such ovens have generally been non-uniform so that
food bodies placed in the oven have to be periodically rotated
or moved in the oven to produce uniform heating thereof.
In addition while separate radiator patterns from different
radiating apertures supplied from a rotating plenum have been
used, such radiating systems are relatively expensive.
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Summary of the Invention
In accordance with this invention there is disclosed a micro-
wave oven having a plurality of separate radiating elements supplied
from a common region by transmission lines comprising conductive
members spaced from a common ground plane such as adjacent regions
of a wall of the oven. More specifically radiating elements
are fed by separate micro-strip transmission lines formed by
conductive strips spaced from the oven wall and using the oven
wall as the ground plane. Preferably the conductive strips are
electrically coupled to a conductor extending through a ground
plane aperture formed by the wall of the oven. The electrical
conductors all move with respect to the oven wall. The oven
wall thus acts as a common ground plane.
The radiating elements are preferably for~ed as extensions
of the conductors and the extension, extend at an angle with
respect to the ground plane region of the oven wall. While a
large number of different structures may be used for the radiating
elements including elements having portions extending parallel
to the ground plane and elements extending at different directions
to the ground plane, the radiating elements preferably comprise
portions spaced at different distances and directions from the
axis of rotation of the radiating structure. In addition, while
the axis of rotation of the structure is preferably perpendicular
to the wall of the oven it may, if desired, be at an angle with
respect thereto which is other than perpendicular.
In accordance with this invention the microwave energy may
be supplied to the radiator from a source outside the oven cavity.
More specifically, a waveguide fed by a magnetron may have a
conductor extend from the radiator through the oven wall into
the waveguide. Means such as a motor positioned outside the
waveguide may be used for rotating the conductor and the
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radiator may be supported from the conductor. Alternatively
the conductor supporting the radiator may be the output structure
of a magnetron extending into the oven through an aperture in
the oven wall.
While the lengths of the radiating structure strip transmission
lines and the radiating element fed thereby may be of any
desired valve, the portions thereof extending in any particular
direction are preferably less than the free space wavelength of
the microwave energy in the cavity.
In accordance with another aspect of the invention the
radiating structure may be incorporated in a conventional micro-
wave oven having electrical heating elements positioned therein.
More specifically, heating elements such as calrod units may be
positioned, for example, adjacent the bottom wall of the oven
surrounding a radiating structure fed with microwave energy
from a waveguide through a coaxial line. Alternatively, heating
may be supplied by convection hot air in which a heating element
or a gas burner external to the oven may heat vapor which is
recirculated through the oven.
In accordance with this invention the radiating structure
may be supported by a solid dielectric such as plastic if no
additional conventional heating is used, or may be supported
by ceramic if additional heating by calrod units or gas heaters
positioned outside the oven are used.
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Brief Description of the Drawings
Other and further objects and advantages of the 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 microwave
oven embodying the invention taken along line 1-1 of FIG. 2;
FIG. 2 illustrates a transverse sectional view of the oven
of FIG. 1 taken along line 2-2 of FIG. l;
FIG. 3 illustrates embodiment of the invention wherein auxil-
liary heating elements such as bake and/or broiler units are in-
corporated in the oven;
FIG. 4 is a partial sectiona~ view of the oven of FIG. 3
taken along line 4-4 of FIG. 3;
FIG. 5 illustrates a transverse sectional view of the
oven with door open taken along line 5-5 of FIG. 3;
FIG. 6 illustrates a vertical sectional view taken along
line 6-6 of FIG. 1 of an alternative embodiment of the invention
wherein the radiating structure is supported directly from a
magnetron output structure in the oven and is rotated by air
directed against portions of the radiating structure; and
FIG. 7 illustrates a transverse sectional view of the
embodiment of FIG. 6 taken along line 7-7 of FIG. 6
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Descri~tion of the Preferred Embodi~ent
Referring now to FIG.'s 1 and 2 there is shown a microwave
oven 10 comprising an enclosure 12 made, for example, of conductive
material such as stainless steel or steel coated with porcelain.
Enclosure 12 has an access opening closed by a door 14 which is
sealed, to the walls of the enclosure 12, by a microwave choke
type seal shown, for example, in Patent No. 3,767,884. Microwave
energy, supplied by a source such as a magnetron 16, is fed
through a waveguide 18 to a conductive element 20 which extends
through an aperture 22 in the lower wall of waveguide 18 and
the upper wall 24 of enclosure 12. The upper end of conductor
20, which is in waveguide 18 is attached to a motor 26 by a
dielectric coupling member 28 which extends through an aperture
in the upper surface of waveguide 18 to the motor 26. The size
of the aperture is substantially less than one-half wavelength
of the microwave energy in dielectric coupling me~ber 28 and a
quarter wavelength choke (not shown) may surround member 28
between waveguide 18 and motor 26. Dielectric coupling member
28 is rotated by a motor 26 and thereby rotates conductor 20.
Energy from magnetron 16 having a frequency of, for example,
2.45 KmH is propagated from magnetron output probe 30 through
waveguide 18 and conductor 20 into enclosure 12 with impedance
matching of waveguide 18 to magnetron 16 being selected by the
positioning waveguide shorting end plate 32 to permit magnetron
16 to operate at maximum efficiency in accordance with well-known
practice. The other end of waveguide 18 which ad~acent to which
conductor 20 extends into waveguide 18 is impedance matched to
conductor 20 by selecting the position of waveguide shorting
plate 34. The position of plate 34 is selected.to match the impedance
presented in waveguide 18 through conductor 20 from a radiating
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structure 36 supported in enclosure 12 by conductor 20. While
any desired impedance matching structures may be selected by
selecting the position of plates 32 and 34 they are preferably
chosen so that the magnetron, operating for example, with 4,000
volts anode to cathode potential will provide microwave power
at high efficiency to the radiating structure 36 in enclosure
12.
Radiating structure 36 comprises a dielectric support plate
38 which supports flat strips of metal 40 which may be, for
example, between one quarter inch and one inch in width and which
extend radially outwardly from conductor 20 and electrically
connected thereto. While in this embodiment of the invention
three strips 40 are illustrated, any desired number can be used.
Preferably, while, the lengths of strips 40 are different and
the angular separation between adjacent strips 40 is the same,
different seperation angles could be used.
The outer end of strips 40 are bent downwardly into radiating
elements 42 of conductive material which act as separate radiators.
While the length of such radiating elements 42 is preferably
substantially a quarter wavelength at the frequency of magnetron 16;
however, radiators which are substantially multiples of one
quarter wavelength long could be used.
The spacing of strips 40 from the top wall 24 of enclosure 12 is
preferably selected to produce micro-strip transmission lines
wherein the impedance of each line is between 75 and 300 ohms so
that their junction at conductor 20, an impedance of between
25 and 100 ohms, is presented. Preferably the widths of strips
40 are in the range of .25" to 1" and their spacing from upper
wall 24 is less than their average width. Thus, when conductor
20 is rotated by motor 26, the individual strips 40 and the
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radiating elements 42 fed thereby rotate about an axis through
conductor 20, with dielectric member 28 providing a bearing
against the upper wall of waveguide 18 to prevent sidewise
motion of the radiator 36.
The portions of strips 40 together with the adjacent
portion of the upper wall 24 act as individual micro-strip
transmission lines and the aperture 22 together with the conductor
20 act as a coaxial transition fed from the waveguide 18 into
the enclosure 12 with microwave energy propagating radially
outwardly along the micro-strip transmission lines while the
wall 24 acts as the ground plane for the micro-strip transmission
lines and with the individual strips 40 acting as movable
conductive members of the micro-strip transmission lines acting
in connection with the fixed ground plane 24. This invention
discloses the discovery that the major portions of the microwave
energy from magnetron 16 may be thus radiated from the radiating
elements 42 as separated radiation patterns with each pattern
having at all times a substantial component directed downwardly
toward a load such as a food body 44 positioned in enclosure 12.
Rotation of structure 36 can thus be made to produce many addition
modes in enclosure 12 and hence a heating pattern, substantially
without cold spots, so that food body 44 need not be turned or
changed in position during a cooking cycle.
In addition, since a substantial portion of the microwave
energy upon being radiated from the elements 42 first impinges
on the food body 44 prior to reflection from the walls of enclosure
12 efficient coupling of microwave energy from the magnetron 16
into the food body 44 can occur.
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The radiating structure 36 simultaneously radiates three
separate directing patterns into the oven having different polar-
izations positioned at different distances from the axis of
rotation of the radiating structure 36 so that different toroidal
radiation patterns are formed in the oven with preferably larger
amounts of microwave energy being directed toward the central
regions. While some radiation directly from the members 40 is
encountered, by selecting the spacing between members 40 and
wall 24 to have an effective electrical distance which is
substantially less than one quarter of a wavelength of the
frequency of magnetron 16, the major portion of the microp~Jave
energy fed to enclosure 12 from magnetron 16 will be radiated
from radiating elements 42.
Thus, it may be seen that a relatively inexpensive
radiating structure may be provided in accordance with this
invention which will radiate peak powers in excess of one kilowatt
into an oven while providing an improved heating pattern within
the oven.
Conductive radiating elements 42 are protected by dielectric
shield member 46, which is positioned in enclosure 12 below
radiating structure 36 to prevent accidental contact of the
radiating structure 36 by dishes or food bodies as they are
being positioned in the oven.
A blower structure 48 may blow air across the magnetron 16
to cool the magnetron and may blow a portion of the air in through
apertures in end plate 32 and thence into the oven via aperture
22 and through apertures in dielectric shield 46 so that cooking
vapors do not condense in the waveguide or on the radiating
structure. Air thus blown into the oven may be extracted there-
from through an aperture 50 covered by a screen 52 which prevents
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microwave energy from leaking out of the oven in accordance with
well-known practice.
While an energy load such as a glass plate 54 may be positioned
in the bottom of the oven to support the food body 44, and to
provide a minimum load if the oven is energi~ed when no food
body is positioned therein, such a plate 54 need not necessarily
be used to prevent excess reflections of microwave energy back
to magnetron 16. Rather the micro-strip members 40 may have
different lengths chosen such ~hat the microwave energy reflected
by enclosure 12 to elements 42 is transmitted to conductor 20 in
different phases which produces substantial cancellation at
conductor 20 thereby reducing the amount of microwave energy
reflected back to magnetron 16 from the enclosure 12 when light
loads are being heated.
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Description of an Alternate Embodiment
Referring now to FIG.'s 3 and 4 there is shown an alternate
embodiment of the invention wherein a stove 60 has a conductive
enclosure 62 with a door 64 through which a food body 66 may be
positioned in the enclosure 62. Body 66 may be supported, for
example, on a metal rack 68 whose position may be adjusted by
selecting the level of different protuberances 70 on the side
walls of the enclosure 62 supporting the rack 68. A ceramic
plate 72 is positioned in the middle of the rack and a dish 74,
which is preferably transparent to microwave energy, rests on
plate 72 and supports the food body 66.
A radiating structure 76 positioned adjacent the bottom of
the oven surface comprises a plurality of metal conductive members
78 connected respectively to a plurality of metal radiating
members 80 which may be extensions of conductive member 78.
A central conductor 82 of a coaxial line 84 is connected to
conductor members 78 which extend radially outwardly to radiating
members 80. Coaxial line 84 extends through the bottom wall
of the enclosure 62 to a waveguide 88. Conducto.r 82 extends
through waveguide 88 where it is supported by a dielectric bearing
and conductive choke assembly 90. An extension of conductor 82
which extends below waveguide 88, is rotated by a motor 92 to
rotate radiator 76.
Microwave energy at a frequncy of 2.45 KMhz is generated
by a magnetron 94 and is propagated through waveguide 88 and
coaxial line 84, to stripline conductive members 78 which extend
radially outwardly from conductor 82 at different radial angles
parallel to the bottom wall of enclosure 62 to radiating elements
80. Conductive members 78 are preferably in a plane spaced from
the conductive stationary bottom wall of enclosure 62 by a
distance which is less than an effective quarter wavelength
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of the frequency of the microwave energy in enclosure 62. Conductive
members 78 are preferably flat strips whose width is from .2
inches to 1 inch forming separate transmission lines whose impedances
are determined by their effective electrical spacing from said
bottom wall of enclosure 62. The outer ends of the conductive
members 78 may be bent upwardly at obtuse angles of, for example,
45 degrees to form the radiating elements 80, elements 80 may
be, for example, substantially one quarter wavelength long and
directively radiate separate patterns toward the food body 66.
As the radiator 76 is rotated by motor 92 the radiation patterns
from radiating elements 80 form toroidal patterns whose axes
lie along the axis of rotation of the structure 76. Due to the
different distances of the radiating elements from said axis of
rotation and the angles at which the radiating elements lie with
respect to the axis of rotation, the toroidal radiation patterns
are of different sizes and/or shapes. By selecting the distances
of elements 80 favor said axis of rotation, and their angles
with respect thereto, the average intensity of the heating pattern
in the centers of the toroids may be selected. Preferably the
average heating pattern intensity is somewhat greater in the
center than toward the oven side walls.
A ceramic cover 96 is positioned over structure 76 spaced
therefrom and accurately positioned by being supported on bumps 98
formed in the bottom wall of enclosure 62 so that the radiator
76 is protected from spills of food or damage from dishes which
might otherwise be dropped on the radiator 76. Air from a blower
100 is blown through the fins of magnetron 94 to cool the anode
and then through apertures 102 in the side wall of waveguide 88
into waveguide 88 and into the enclosure 62 through coaxial
line 84 and under the edges of cover 96 between bumps 98. Air
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entering the oven is allowed to escape through a screened exhaust
vent 104 in the upper wall of the oven connected to a pyrolitic
cleaning element 106 so that oven vapors are pyrolized before
being exhausted into the room.
Auxilliary heating is provided for the oven by broiler
calrod unit 108 supported adjacent the upper wall of the oven
and by a baking calrod unit 110 surrounding the dish 74 spaced
therefrom and supported from the floor of the oven. Thermal insu-
lation 112 is positioned around the oven to prevent loss of thermal
energy. Preferably, blower 100 operates whenever either the
conventional heat is energized or the microwave heat is energized
so that air is blowing by cover 96-to maintain the cover below
its softening temperature of 700 F or 800 F even though the calrod
unit 110 spaced therefrom by a distance of an inch or so may be
heated to temperatures above 1,000 F.
In accordance with this invention, door 64 has a microwave
choke 114 formed thereon which may be, for example, of the type
illustrated in Patent No. 3,767,884. A high temperature vapor
seal 116 is positioned outside the choke region so that cooking
vapors are exhausted to the room only through vent 104. A latch
118 prevents opening of door 64 during operation in accordance
with well-known practice.
Auxilliary equipment such as a light 120 above the top wall
of the oven may be positioned to illuminate the oven through a
translucent ceramic 122 and a microwave impervious screen 124.
Top burners 126 and associated controls may also be provided.
In accordance with this invention there is disclosed the
discovery that the directive heating pattern of applicant's
radiator 76 may be used in combination with conventional heating
provided by the calrod units to reduce the cooking time and to
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produce any desired degree of cooking and/or surface browning of a
a food body.
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Descr ption of a ~urther Embodiment
Referring now to FIG.'s 6 and 7 there is shown a further embodi-
ment of the invention wherein an oven 130 comprises a conductive en-
closure 132 having a door 136 by means of which a food body 138 may
be positioned within the enclosure 132. A source of microwave energy
having a frequency of 2.45 KMhz is provided by magnetron 140
whose output 142 is inserted through the upper wall 144 of enclosure
132. Output 142 comprises a conductor 146 inside a ceramic
cylinder 148 and connected to an output end cap 150 in accordance
with well-known practice.
In accordance with this invention a radiating structure 152
is supported in enclosure 132 from-output cap 150. Radiating
structure 152 comprises a dielectric plate 154 supporting a
plurality of conductive members 156 on its lower surface which feed
metal radiating elements 158. Conductive members 156 are connected
together at their inner ends and rest on a thin dielectric washer
160 which in turn rests on a metalic washer 162 which is attached
to a metal conductor 164. Conductor 164 extends through dielectric
plate 154 and is attached by a spring clip 166 to a conductive
cap 150 of the output structure 142. As a result, disc 154
carrying conductors 156 and radiating elements 158 is free to
rotate about conductor 164. Effective electrical conduction
between output cap 150 and metal conductive members 156 occurs since
the dielectric washer 160 acts as a low impedance capacitive coupler
at microwave frequencies. If desired, a metal to metal contact
may be used between the support washer 162 and the members 156;
however, better wear characteristics can be achieved if dielectric
material such as mylar or nylon is used for thin bearing washer 160.
Air from a blower 168 is blown through fins attached to the
anode of magnetron 140 and thence via a duct 170 and apertures
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172 in the upper wall 144 of enclosure 132 to a plenum 174 having
a nozzle 176 which directs the air tangentially against the
periphery of dielectric disc 154. The air impinges on a plurality
of dielectric paddles 178 attached to the rim of disc 154 to
rotate disc 154 about conductive member 164. Plenum 174 is
shown as attached to upper wall 144, for example, by screws
180. However, if desired, it could be formed integral with a
dielectric cover member 182 which is supported on projections 184
below radiating structure 152 and protects the radiating structure.
The air passes through apertures 186 in cover 182 into the oven
enclosure 132 and thence out via a screen vent (not shown) so
that oven cooking vapors may be vented from the oven. The
conductive members 156 are flat strips lying in a plane spaced
parallel to the upper wall 144 by a distance which is less than
one quarter of an effective electrical wavelength of the microwave
energy and the width of said members 156 are selected to provide
an impedance match to the output structure 142 of magnetron
140 which will cause the magnetron to operate with optimum effic-
iency. For example, said spacing is preferably less than their
spacing from wall 144.
The food body 138, as illustrated herein, rests on a glass tray
188. However, if desired, it may be supported on a metal rack simi-
lar to the embodiments of FIG. 4 and additional heat from hot air,
or from a browning calrod unit positioned in the oven, could be
used.
This completes the description of the embodiments of the in-
vention illustrated herein; however, many modifications thereof
will occur to persons skilled in the art without departing from
the spirit and scope of this invention. For example, a wide
variety of sizes, shapes, and orientations of the radiating
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elements could be used, more than three micro-strip lines
and radiating elements could be used. Accordingly it is contemplated
that this invention be not limited to the particular embodiments
illustrated herein except as defined be the appended claims.
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