Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE lNv~:N~l~lON
Microwave Oven Including Antenna for Radiating
Microwave
R~ OUND OF THE lNv~N~l~lON
Field of the Invention
The present invention relates to microwave ovens, and
more particularly, to a microwave oven including an
antenna for radiating microwaves which realizes uniform
heating in a cavi~y.
Description of the Background Art
A conventional microwave oven is disclosed in
Japanese Patent Laying-Open No. 62-295386, for example.
In this conventional microwave oven, microwaves generated
~rom a magnetron are propagated via a waveguide into a
cavity in which a food which is the substance-to-be-heated
is accommodated. Fig. 1 is an exploded perspective view
showing a structure of a waveguide of such a conventional
microwave oven.
Referring to Fig. 1, microwaves generated from an
output antenna 101 of a magnetron not shown are propagated
within a rectangular wavaguide 102. At a sidewall of
waveguide 102 in contact with a cavity not shown, a ~
projection portion 103 for coupling having substantially a ~ -
truncated cone configuration is provided projecting inward
the waveguide. A coupling aperture 104 is provided at the
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center of the top portion of projecting portion 103. A
cylindrical radiation antenna 105 is held by a cover 106
of a dielectric material so as to pass through coupling
aperture 104.
Such a conventional microwave oven hals microwaves
emitted into a cavity not shown due to an electric field
generated between cylindrical radiation antenna 105 and
the sidewall of coupling projection portion 103. The
direction of emission is unitary at right angles to the
electric field. Because the cross sectional area of the
cavity is extremely greater than the opening area of
coupling projecting portion 103 at the cavity side,
unidirectional microwaves provided to the cavity do not
spread out widely within the cavity, resulting in
unevenness in heating.
To realize uniform heating within a cavity, a
microwave oven including a flat radiation antenna directly
secured to the antenna of a magnetron with a screw is
disclosed in Japanese Utility Model Publication No. 53-
50122, for example. Fig. 2 is an exploded perspective
view showing structures of a magnetron and a radiation
antenna of such a conventional microwave oven.
Referring to Fig. 2, a flat radiation antenna 113 is
attached to an antenna 112 of a magnetron lll by a screw
114 or the like. A plurality of slits 115 are formed in
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radiation antenna 113 for enabling energy emission in
various modes.
Fig. 3 schematically shows a manner in which
microwaves are propagated according to the structure shown
in Fig. 2. Referring to Fig. 3, microwaves (indicated by
open arrow) are emitted at right angles to an electric
field tindicated by general arrow) generated between a
cavity 116 and a radiation antenna 113. This range of
radiation is limited to that indicated by the broken lines
in F.~g. 3. This means that nonuniform heating occurs
within the cavity.
Furthermoxe, because radiation antenna 113 is secured
to antenna 112 of magnetron 111 by screw 114, a gap may be
formed between antenna 112 and radiation antenna 113 as a ---
result of vibration during usage or by insufficient
tightening of screw 114 when radiation antenna 113 is --~
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reattached after rep~irr~nt or the like to result in a
.-
possibility of a spark occurring in the gap.
SUMM~RY OF THE INVENTION
An object of the present invention is to provide a ---
microwave oven capable of reducing nonuniformity in
microwave radiation in a cavity to suppress unevenness in
heating a food.
Another object of the present invention is to provide
a microwave oven capable of preventing generation of a
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spark between a radiation antenna and an antenna of a
magnetron within a cavity.
A further object of the present invention is to
provide a microwave oven capable of fine-adjusting the
diffusion state of microwaves in a cavity without
modifying the basic design of an antenna.
A microwave oven of the present invention includes a
cavity, a magnetron, a waveguide, and an antenna for
radiation. A substance to be heated is placed in the
cavity. The magnetron includes an output antenna for
generating microwaves. The waveguide provides microwaves
emitted from the output antenna of the magnetron into the
cavity. The waveguide has substantially a truncated cone
configuration in which the cross sectional area at the
lS cavity side is greater than that of the magnetron side.
The output antenna of the magnetron is disposed to project
from the magnetron side bottom of the waveguide into the
inner space of the waveguide. The antenna for radiating
microwaves is fixed within the inner space of waveguide
around the output antenna of the magnetron while
maintaining distances from the waveguide and the output
antenna of the magnetron so that no spark is generated
therebetween.
According to another aspect of the present invention,
a microwave oven includes a fixed pla-te of a dielectric
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material secured to the waveguide, and having an aperture
substantially at the center thereof through which the
output antenna of the magnetron passes. The radiation
antenna is fixed around the aperture of the fixed plate.
According to another aspect of the present invention,
the diameter of the aperture is set substantially equal to
the diameter of the output antenna of the magnetron.
According to a further aspect of the present
invention, the radiation antenna is attached around the
aperture on the magnetron side surface of the fixed pla~e.
According to a still further aspect of the present
invention, a porti~n of the xadiation antenna is bent
towards at least one of the cavity side and the magnetron
side. ~
lS According to yet a further aspect of the present - -
invention, the radiation antenna has an aperture at a - ~
position off the center thereof through which the output ~--
antenna of the magnetron passes. ~
According to yet another aspect of the present --
invention, the radiation antenna has an aperture through
which the output antenna of the magnetron passes, and a
rib all around the perimeter of the aperture and
substantially parallel to the output antennal.
According to yet a still further aspect of the
present invention, the radiation antenna is fixed inclined
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with respect to the sidewall of the cavity.
According to an additional aspect of the present
invention, an opening is formed at an area on the fixed
plate between the outer periphery of the radiation antenna
and the outer periphery of the fixed plate so that a
straight current path is not generated from the radiation - -
antenna to the waveguide. ;
According to a further additional aspect of the
present invention, the radiation antenna has a flat form.
Because microwaves are emitted by an electric field
generated between the output antenna of the magnetron and
the radiation antenna and also by an electric field
generated between the radiation antenna and the sidewall
o~ the waveguide, the emitting area of microwaves is
greater than that of a conventional case where microwaves
are emitted only by an electric field generated between
the output antenna of the magnetron and the sidewall of
the waveguide. Therefore, nonuniformity in the microwave
radiation in the cavity can be suppressed, which in turn
suppresses unevenness in heating a food product.
Because the radiation antenna is attached on a fixed
plate of a dielectric material having an aperture through
which an output antenna of the magnetron passes while
maintaining constant distances from the waveguide and the
output antenna of the magnetron, no spark will occur
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between radiation antenna and the waveguide and between
radiation antenna and the output antenna.
Because the diameter of the aperture of the fixed
plate is set substantially equal to that of the output
antenna of the magnetron, the fixed plate can be securely
fixed after being detached for rep~;rr~nt of the radiation ~-
antenna. Therefore, generation of an unecessary spark can - ~ -
be prevented.
Another advantage of the present invention is that
the stain of a food generated during heating can be ~ ~ -
prevented from adhering to the radiation antenna to -
prevent generation of an unnecessary spark by fixing the
radiation antenna around the aperture on the magnetron
side surface of the fixed plate.
A further advantage of the present invention is that
nonuniformity in heating can be suppressed by bending a -~
portion of radiation antenna towards at least one of the ~-
cavity side and the magnetron side to diffuse microwaves
in a more intricated manner within the cavity.
Still another advantage of the present invention is
to further improve unevenness in heating by providing an
aperture through which the output antenna of the magnetron
passes at a position off the center of the radiation
antenna to intentionally provide emission of microwaves
with directivity.
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Because a rib is provided extending all around the
perimeter of the aperture which the output antenna of the - -
magnetron passes and substantially in parallel to the
output antenna, the microwave coupling between the
radiation antenna and the output antenna of the magnetron
can be intensified to further improve the microwave
emission efficiency of the radiation antenna.
Yet a still further advantage of the present
invention is to suppress nonuniformity in heating by
fixing the radiation antenna inclined with respect to the
sidewall of the cavity to diffuse the microwaves in a more
intricated manner in the cavity.
By providing an opening in an area on the fixed plate
between the outer periphery o~ the radiation antenna and
the outer periphery of the fixed plate so that a straight
current path is not generated from the radiation antenna
to the waveguide, the surface resistivity of the fixed
plate can be increased even if stain from food is attached
to the surface of the fixed plate. Therefore, generation
of an unrequired spark can be prevented.
The foregoing and other objects, features, aspects
and advantages of the present invention will become more
apparent from the following detailed description of the
present invention when taken in conjunction with -the
accompanying drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an exploded perspective view of one example
of a waveguide of a conventional microwave oven.
Fig. 2 is an exploded perspective view of one example
of a radiation antenna of a conventional microwave oven.
Fig. 3 schematically shows propagation of microwaves -
from the conventional radiation antenna of Fig. 2. '~
Fig. 4 is a sectional view of a microwave oven
according to embodiments of the present invention.
Fig. 5 is a sectional view of the main part of the
microwave oven according to the first embodiment of the
present inventlon.
Fig. 6 is a plan view of a radiation antenna
according to the first embodiment of the present
invention. ~ ~ -
Fig. 7 schematically shows propagation of microwaves
by the radiation antenna of the first embodiment of the
present invention.
Figs. 8 and 9 are graphs for describing the effect of
the first embodiment of the present invention. -~
Fig. 10 is a sectional view showing the main part of
a microwave oven according to a second embodiment of the
present invention.
Fig. 11 schematically shows propagation of microwaves
by the radiation antenna according to the second
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embodiment of the present invention.
Fig. 12 i5 a sectional view of the main part showing
a modification of the second embodiment of Fig. 10.
Fig. 13 is a sectional view of the main part of a
microwave oven according to a third embodiment of the
present invention.
Fig. 14 shows the results of a defreezing experiment
using the microwave oven of the third embodiment of the
present invention.
Fig. lS is a sectional view showing the main part of
one modification of the radiation antenna of the third
embodiment of the present invention.
Fig. 16 shows results of a heating experiment by the
microwave oven of Fig. 15.
Fig. 17 is a sectional view showing the main part of
another modification of the radiation antenna of the third
embodiment of the present invention.
Fig. 18 is a sectional view showing the main part of
further modification of the third embodiment of the
present invention.
Fig. 19 is a plan view of a radiation antenna
according to a fourth embodiment of the present invention.
Fig. 20 is a sectional view showing the main part of
one modification of the fourth embodiment of the present
invention.
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: .
Fig. 21 is a sectional view showing the main part of -
a microwave oven according to a fifth embodiment of the
present invention.
Fig. 22 is a plan view of a radiation antenna
according to the fifth embodiment of the pxesent ~-
invention.
Fig. 23 is a table showing a result of heating
experiment by the microwave oven of the fifth embodiment
of the present invention.
Fig. 24 is a sectional view showing the main part of
a microwave oven according to a sixth embodiment of the
present invention.
Fig. 25 is a plan view o~ a radiation antenna
according to the sixth embodiment of the present
invention.
Fig. 26 is a sectional view showing the main part of
one modification of the sixth embodiment of the present
invention.
Fig. 27 is a sectional view showing the main part of
a microwave oven according to a seventh embodiment of -the
present invention. --
Fig. 28 is a plan view of a radiation antenna
according to the seventh embodiment of the present
invention.
Fig. 29 is a plan view of another example of a
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radiation antenna according to the seventh embodiment of
the present invention.
DFscRIprrIoN OF THE PREFERRED EMBODIMENTS
Referring to Fig. 4, a microwave oven according to an
embodiment of the present invention includes an outer
frame 1, a cavity 2 in which a food 3 to be heated is
placed through a front opening (not shown), a turn table 4
rotated during cooking and on which food 3 is placed in
cavity 2, a motor 5 for rotating turn table 4, a rotation
axis 6 passing through a bottom 2a of cavity 2 having one
end attached to turn table 4 and the other end attached to
motor 5, a waveguide 7 having substantially a truncated
cone configuration with an opening at a sidewall 2b of
cavity 2, a magnetron 8 ~ixed to the bottom side plane of
waveguide 7 opposing the opening, and a protection plate 9
of mica covering the opening of waveguide 7 at sidewall 2b
of cavity 2.
Naveguide 7 has substantially a truncated cone
configuration as described above, wherein the cross
sectional area thereof is the largest at the opening at
sidewall 2b of cavity 2 and it becomes smaller as the
distance from sidewall 2b becomes larger. The diameter of
the bottom side plane most distant from the opening is set
to at least 80mm.
Fig. 5 is an enlarged sectional view of a portion A
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surrounded by the broken line in Fig. 4. Referrin~ to
Fig. 5, magnetron 8 includes a vacuum-tube container 19
with an anode and a cathode not shown, radiation fins 10
fixed by brazing or the like to vacuum-tube container 19,
magnets 11, a yoke 12 sandwiching vacuum-tube container 19
and magnets 11 from both sides in the axis direction, and
an output antenna 13 projecting from vacuum-tube container
19. Output antenna 13 includes an antenna cap 14a
provided at one end thereof, and a ceramic insulator
barrel 14b. -
Fixed plate 16 of a dielectric material such as mica
is attached perpendicular to the axi~ direction of output
antenna 13 and in contact with antenna cap 14a. A ~1at
radiation antenna 15 of a metal such as aluminum is
attached on fixed plate 16 with a predetermined distance
from antenna cap 14a.
Fig. 6 is a plan view of radiation antenna 15
attached on fixed plate 16. Fixed plate 16 has an
aperture 17 substantially at the center thereof.
Radiation antenna 15 substantially of a donut shape,
having an outer perimeter smaller than the outer perimeter
of fixed plate 16 and having an inner perimeter greater
than the diameter of aperture 17 is fixed around aperture
17. Radiation antenna 15 is attached by inserting ribs
15a, 15b, and 15c formed at a interval of 120~ at the
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perimeter of radiation antenna 15 into mounting holes 16c,
16d and 16e provided in fixed plate 16, respectively.
Fixed plate 16 is secured within waveguide 7 by having
antenna cap 14a inserted in aperture 17 as shown in Fig.
5, and inserting ribs 16a and 16b formed at the pexiphery
of fixed plate 16 into slits (not shown) formed
appropriately at the sidewall of waveguide 7.
The distance between antenna cap 14a inserted into
aperture 17 and radiation antenna 15 is set to
approximately 2mm so that no spark is not generated
therebetween and so as to be coupled efficiently for
generating microwaves. Radiation antenna 15 is set
sufficiently apart from the sidewall of waveguide 7 at a
distance 50 that a spark does not occur therebetween.
The diameter of aperture 17 of fixed plate 16 is set
substantially equal to the diameter of antenna cap 14a.
Remounting of radiation antenna 15 after dlsmounting
thereof for rep~;rr~nt or the like will be facilitated if
the diameter of aperture 17 is greater than that of
antenna cap 14a. However, this will cause a change in the
distance between radiation antenna 15 and antenna cap 14a
to alter the diffusion state of microwaves in the cavity
2, resulting in nonuniformity in heating, or a possibility
of a spark being generated between radiation antenna 15
and the sidewall of waveguide 7. In order to prevent such
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problems, it is necessary to securely fit antenna cap 14a
into aperture 17 without any gaps.
Because the temperature inside waveguide 7 becomes
high during a heating operation of the microwave oven, a
plurality of vent holes 18 for heat emanation are provided ~ -
at the sidewall of waveguide 7 as shown in Fig. 5. Vent
holes 18 are provided on the sidewall, in parallel to the -
secured radiation antenna 15, and excluding the area
corresponding to the proper attached position of radiation
antenna 15. More specifically, when viewed from the outer
face of waveguide 7 into vent hole 18 with radiation
antenna lS attached to antenna cap 14a, radiation antenna
15 will not be visually identified if located at a proper
position. Radiation antenna 15 will be visually
identified if not attached at a proper position.
Therefore, the attached state of radiation antenna 15 can
be readily confirmed.
According to the structure shown in Figs. 5 and 6, an
electric field (indicated by a general arrow) is generated
between output antenna 13 of magnetron 8 and radiation
antenna 15 in a heating operation. Microwaves (indicated
by open arrow) are emitted at right angles to this
electric field, as schematically shown in Fig. 7. An
electric field is also generated between radiation antenna
15 and the sidewall of waveguide 7. Microwaves are
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similarly emitted from this electric field.
In comparison with a conventional microwave oven in
which microwaves are emitted only from an electric field
between the output antenna of a magnetron and the sidewall
of a waveguide, the microwave oven of the first embodiment
of the present invention has microwaves emitted from the
electric field between output antenna 13 and radiation
antenna 15 and also from the electric field between
radiation antenna 15 and the sidewall of waveguide 7.
Therefore, the emitting area of microwaves is extremely
increased, whereby microwaves are emitted into the
interior of cavity 2 like water sprinkled out from a
shower. Therefore, nonuniformity in radiation of
microwaves within cavity 2 is reduced to mini i ze ;~
unevenness in heating.
Referring to Fig. 5, the distance of (x+y), where x
is the distance from the magnetron 8 side end of antenna ;
cap 14a to radiation antenna 15 and y is the distance from
the center of output antenna 13 to the perimeter of
radiation antenna 15, i.e. the radius of the outer
periphery edge of radiation antenna 15, is set to a value
within the range of 35mm~40mm. -~
This range is obtained from experimental results
shown in Figs. 8 and 9. Fig. 8 is a graph showing the
relationship between microwave power and distance (x+y).
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A peak in power appears between the distance of 35mm to
4Omm. It is appreciated that power decreases as an offset
from this range. Fig. 9 is a graph showing the
relationship between unevenness in heating and distance
(x+y). It is appreciated that unevenness :in heating is
m;ni~l~r when the distance (x+y) is between 35mm to 40mm.
From the foregoing, the distance (x+y) is preferably a
value between 35mm to 4Omm.
The experimental result of Fig. 8 was obtained by an
experiment using a set of a cavity and a magnetron. The
distances of x and y were altered appropriately, and
respective power outputs were meas~lred. The experimental
result of Fig. 9 was obtained as set forth in the
following. seakers containing water were placed at the
four corners and -the center on the floor of the cavity. A
heating operation was carried out for a predetermined time
period. The increass in temperature in each beaker was
measured. Then, an average value of the temperature
increase in each beaker was obtained, and the difference
between the maximum and minimum values of the temperature-
increased value was divided by the average value. This
was represented in a percentage basis for every distance
(x+y), resulting in the graph of Fig. 9.
According to the first embodiment of the present
invention, the emitting area of microwaves from a
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magnetron is increased significantly, whereby
nonunifoxmity in microwave radiation in the cavity can be
suppressed to ~inimi ze unevenness in heating a food item.
Furthermore, even if radiation antenna 15 is
remounted after being removed, the distance between the -
radiation antenna and the output antenna will not change.
Therefore, the possibility of generation of an undesired
spark is eliminated.
Radiation antenna 15 according to the above described
first embodiment is secured on the surface of fixed plate
16 at the cavity 2 side. Therefore, stains such as the
~at, grease, juice evaporated from a food 3 when heating
food 3 in cavity 2 are carried into waveguide 7 through
the gap between cavity 2 and protection plate 9 to adhere
to radiation antenna 15 and the surrounding fixed plate
16. These stains adhexing to radiation antenna 15 and
fixed plate 16 will become the cause of decreasing the
surface resistivity of fixed plate 16. As a result, there
is a possibility of a spark occurring between antenna cap
14a of the magnetron and radiation antenna 15, and between
radiation antenna 15 and waveguide 7.
Fig. lO is a sectional view showing the main part of
a microwave oven according to a second embodiment of the
present invention, preventing generation of such a spark.
Description of components of the second embodiment in Fig.
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lQ corresponding to those of the first embodiment of Fig.
5 will not be repeated.
Referring to Fig. 10, a fixed plate 20 of mica having
an aperture at substantially the center thereof similar to
fixed plate 16 of Fig. 5 is secured within waveguide 7 in
a manner similar to that of the first embodiment of Fig.
5. Fixed plate 20 has a plane 20a facing the cavity and a
plane 20b facing the magnetron side. The microwave oven
of the second embodiment differs from that of the first
embodiment of Fig. 5 in that radiation antenna 21 is
attached to plane 20b of the magnetron side of fixed plate
20. The secured manner of radiation 21 is similar to that
of radiation antenna 15 of Fig. 5.
Fig. 11 schematically shows generation of microwaves
in the second embodiment of Fig. 10. Similar to the first
embodiment of Fig. 7, a wide emitted area of microwaves
can be ensured. The microwave oven of the second
embodiment is advantageous over that of the first
embodiment in the following point.
According to the structure of the microwave oven
shown in Fig. 10, a food 3 is placed in cavity 2, and a
heating operation by microwaves is initiated. As a
heating operation is carried out, moisture from food 3
will float within cavity 2. If food 3 is meat or the
like, fat and grease will be included in the floating
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moisture. Such floating moisture adhered to the inner
wall of cavity 2 will become a stain on the inner surface
when the water moisture is dried up. Such moisture will
be carried as stains into waveguide 7 from the gap between
sidewall 2b of cavity 2 and protection wal.1 9 to adhere to
the surface of fixed plate 20.
The pressure in cavity 2 is Aigher than that in
waveguide 7 due to the high temperature in cavity 2 by
heat emanated from food 3 by microwave heating and the
tight sealing of cavity 2. Therefore, the air in
waveguide 7 will flow towards the magnetron side from the
cavity side, whereby moisture is also carried from the
cavity side to the magnetron side. Thus, moisture will
seldom adhere to face 20b of the magnetron side even if
15moisture will adhere to face 20a of the cavity side of ~.
fixed plate 20.
Thus, generation of a spark due to reduction of a :
surface resistivity of face 20b of fixed plate 20 can be
prevented because stains from the food floating together
with moisture will not adhere to radiation antenna 21
secured to face 20b of the magnetron side of fixed plate
20 and also to the surrounding face 20b of fixed plate 20.
An approach could be taken to improve the heat
maintaining ability within the cavity and prevent heat
from escaping by separating the inner space of waveguide 7
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completely with fixed plate 20 as shown in Fig. 12 so as
to provide a chamber partitioned by protection plate 9 and
fixed plate 20 and a chamber partitioned by fixed plate 20
and the bottom side plane of waveguide 7, in order to
carry out grill cooking by means of a heater (not shown)
in cavity 2. Adhesion of stains onto face 20b of the
magnetron side of fixed plate 20 can be further prevented
by employing such a structure.
According to the above second embodiment of the
prese.nt invention, the attachment of a radiation antenna
to the magnetron side surface of a ~ixed plate pro~ides
the advantage of preventing adhesion of stains such as fat
from heated food onto the radiation antenna and a fixed
plate therearound. As a result, generation of a spark can
be prevented between the radiation antenna and the output
antenna of the magnetron, and between the radiation
antenna and the waveguide.
In the above described first and second embodiments, --~
radiation antenna 15 (or 21) is fixed in a manner parallel
to sidewall 2b of cavity 2. Therefore, microwaves emitted
from the entire circumference of radiation antenna 15 are
reflected similarly in waveguide 7 to be diffused into
cavity 2.
However, there are cases when unevenness in heating
food cannot be improved to a sufficient level with such a
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uniform diffusion. In order to diffuse microwaves
suitable for cooking, it is sometimes necessary to
intentionally induce nonuniformity in microwaves within
the cavity. When the secured position of the radiation
antenna has been eventually determined in a designing
stage, the secured position of the radiation antenna can
no longer be moved in the forward or backward direction.
It was therefore difficult to carry out fine-adjustment of
intentional microwave radiation nonuniformity in the
cavity. ~ ~ -
Fig. 13 is a sectional view of the main part of a
microwave oven according to a third embodiment of the
present invention allowing fine-adjustment of the
diffusion state of microwaves in the cavity. Description
of components in the third embodiment of Fig. 13
correspo~ding to those of the first and second embodiments
shown in Figs. 5 and 10 will not be repeated.
Referring to Fig. 13, a fixed plate 20 having an
aperture approximately at the center is provided within a
waveguide 7 in a manner substantially parallel to the wall
face 2b of cavity 2, similar to fixed plate 16 of Fig~ 5.
A flat antenna 21 for radiating microwaves is fixed on a
face 20a of fixed plate 20 at the cavity side. Radiation
antenna 21 includes an antenna bending portion 22 bent
towards cavity 2. Antenna bending portion 22 is formed as
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an arc of approximately 1/4 the diameter of radiation
antenna 21 in length bent from the outex periphery of
radiation antenna 21 to the center thereof at an angle of
~~ tfor example, approximately 30~) with respect to face
20a.
Similar to the first embodiment of Fig. 5, the
microwave oven of the structure of Fig. 13 has electric -
fields generated respectively between antenna cap 14a of
magnetron 8 and radiation antenna 21, and between
radiation antenna 21 and the sidewall of waveguide 7
whereby microwaves are emitted towards cavity 2. Because
a portion of radiation antenna 21 is bent, the distance
between sidewall 2b of cavity 2 and the outer periphery of
radiation antenna 21 differs in antenna bending portion 22
and the other portion.
Therefore, microwaves propagate into cavity 2 with
the direction of microwaves emitted from bending portion
22 of xadiation antenna 21 closer to cavity 2 being
different from the direction of microwaves emitted from
other portions of radiation antenna 21 distant from cavity
2.
As a result, microwave are propagated in an
intricated manner within cavity 2 to realize a diffused
state of microwaves suitable for cooking. Therefore,
unevenness in heating food can be improved.
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Fig. 14 shows the temperatures measured at various
places when a mass of frozen sliced beef is defreezed for
approximately 10 minutes at a power of 200W. In Fig. 14,
(a) shows temperatures when antenna bending portion 2 was
not provided in radiation antenna 21, i.e. ~=0~, and (b)
shows the case where antenna bending portion 22 is - -
provided, i.e. ~=30~.
By comparing the temperature measured results shown
in (a) and (b), it is appreciated that the temperatures at
the upper right and bottom left corner in (a) are high in
comparison with other places, resulting in excessive
héating. This excessive heating is not seen in (b) having
an antenna bending portion 22 provided. The temperatures
in various places in tb) are substantially equalized in
comparison with the result of (a). It is appreciated by -
the results of these experiments that unevenness in -
heating in the plane direction in the cavity is improved
by bending a portion of radiation antenna towards cavity
2.
The position of antenna bending portion 22 at the
outer periphery edge of radiation antenna 21 is not
limited to that shown in Fig. 15. An-tenna bending portion
22 may be provided in any position at the outer periphery
edge of radiation antenna 21.
As a modification of the embodiment shown in Fig. 13, ~ -~
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an antenna bending portion 22 of radiation antenna 21 may
be bent towards magnetron 8, as shown in Fig. 15.
Referring to Fig. 15, a radiation antenna 21 is fixed to
face 20b of the magnetron side of fixed plate 20. Bending
portion 22 is an arc of approximately 1/4 the diameter of
radiation antenna 21 in length from the outer periphery of
radiation antenna 21 to the center thereof bent towards
magnetron 8 at an angle of ~~ from face 20b.
Fig. 16 is a table showing the relationship between
angle ~~ and the output power when 2 liter of water in a
bottle is heated, and the relationship among angle ~~, the
output power, and the kemperature difference between the
upper portion and the lower portion (temperature
nonuniformity) of a bottle containing 150cc of sake heated
for 76 seconds.
It is appreciated from the table of Fig. 16 that
there is no significant change in the output power in
response to angle ~~ when 2 liter of water is heated. In
contrast, the heat output with respect to 150cc of sake
increases as the bending angle ~~ of antenna bending
portion 22 becomes greater. This means that the provision
of an antenna bending portion 22 as shown in Fig. 15 is
suitable for increasing power with respect to a substance-
to-be-heated of light load. However, unevenness in
heating in the vertical direction is not improved by
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j ' ' .', ' ' ''' ~ ' ,', : ' '
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provision of bending portion 22.
A bending portion may be provided not: only in one -
side of radiation antenna 21, but at both sides of
radiation antenna 21, as shown in Fig. 17. Referring to
Fig. 17, two antenna bending portions 22a and 22b are bent
towards magnetron 8 at an angle of ~~ with respect to face
2Ob of fixed plate 20.
Fig. 18 shows a radiation antenna 21 which has a
configuration of a combination of Figs. 13 and 15. Viewed
from cavity 2 side, an antenna bending portion 22c which
is a left arc of radiation antenna 21 is bent towards
cavity 2 by an angle of ~~ with respect to face 20a of
fixed plat0 20, and an antenna bending portion 22d which
is a right arc of radiation antenna 21 is bent towards
magnetron 8 by an angle of ~~ with respect to face 20b of
fixed plate 20. By including both features, the output
characteristics with respect to light load and unevenness
in heating in the plane direction of the cavity can both
be improved.
Radiation antenna 21 of Fig~ 18 is secured on face
20a of fixed plate 20 at the cavity 2 side by passing the
right antenna bending portion 22d through a slit 20c -
provided at the proximity of the periphery edge of fixed
plate 20.
Even if the structural designing of a microwave oven
2123~
is completed and the fixed position of radiation antenna
21 is eventually determined, the third embodiment of the
present invention allows modification of the heating
unevenness pattern by just adjusting the bending angles of
~ and ~ of antenna bending portion 22 to compensate for
change in the pattern of heat unevenness in cavity 2
caused by a slight modification in the con:Eiguration of
Gavity 2 for reinforcement or the like. Thus, heating
unevenness can be improved without a significant change in
designing.
Fig. 19 is plan view of a radiation antenna of a
mic~owave oven according to a ~ourth embod.iment o~ the
present invention. The radiation antenna of Fig. 19
differs from the radiation antenna of the first embodiment
shown in Fig. 6 as set forth in the following. Radiation
antenna lS of Fig. 19 has an aperture 30 displaced
rightwards from the center thereof. Radiation antenna lS
is secured to fixed plate 16 so that the center of
aperture 30 coincides with the center of apertuxe 17
provided at the center of fixed plate 16.
As already described in conjunction with Fig. 8, the
relationship between distance (x+y), where x is the
distance from the end portion of antenna cap 14a at
magnetron 8 side to radiation antenna lS and y is the
distance from the center of output antenna 13 to the outer
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periphery of radiation antenna 15, and microwave output
changes as shown in Fig. 8. Therefore, by displacing
aperture 30 of radiation antenna 15 from the center
thereof as shown in Fig. 19, output will not be emitted
uniformly from the entire perimeter of radiation antenna
15.
More specifically, microwaves can be emitted with
directivity as a whole by fixing output antenna 13 at a
position displaced from the center of radiation antenna 15
to provide portions differing in microwave emission level.
Even if the configuration o~ the cavity is slightly
modified after the structure designing stage of a
microwave oven is completed, the fourth embodiment of the
present invention allows the microwave diffusion state
within a cavity to be intentionally displaced easily by
adjusting the eccentricity of the center of radiation
antenna 15 with respect to the center of output antenna 18
of the magnetron. As a result, the heating unevenness of
food can be improved. Also the time required for heating -~
can be reduced.
In the embodiment shown in Fig. 19, radiation antenna
15 is secured to fixed plate 16 which is fixedly held by
antenna 13 of magnetron 8 inserted into aperture 17.
Alternatively, a tapped hole may be formed in the center
of fixed plate 16 and a top portion 13a of antenna 13 to
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insert a screw 31 therein to secure fixed plate 16 to
antenna 13 instead of forming aperture 17 in fixed plate
16.
Fig. 21 is a sectional view of the main part of a
microwave oven according to a fifth embodiment of the
present invention. Fig. 22 is a plan view of a radiation
antenna used therein. The microwave oven according to the
fifth embodiment shown in Figs. 21 and 22 i5 similar to
that of the first embodiment shown in Figs. 5 and 6
provided that a rib 31 is formed extending towards cavity
2 and in parallel to antenna cap 14a around the entire
perimeter of aperture 30 formed at the center of radiation
antenna 15. According to the fifth embodiment of the
present invention, an electric field is generated between
output antenna 13 of magnetron 8 and rib 32 of radiation
antenna 15 in addition to the electric field generated in
the microwave oven of the first embodiment in a heating
operaticn.
Fig. 23 is a table showing the relationship among the
length ratio of rib 32 of radiation antenna 15 to antenna
cap 14a of magnetron 8, input/output power, and heating
unevenness.
It is appreciated from the table of Fig. 23 that the
heating efficiency and heating uneveness can be improved
by setting each dimensior so that the ratio of the length
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. ~ : .. . , ~
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of rib 32 of radiation antenna 15 to the length of antenna
cap 14a of magnetron 8 is greater than 1/6.
In the column of "Note~ in the table of Fig. 23, A
implies that rib 32 protrudes from radiation antenna 15
towards cavity 2, and B implies that rib 32 protrudes
towards magnetron 8. It is appreciated from this table
that both heating efficiency and heating unevenness are
more improved in the case of A.
Heating unevenness in the plane direction within the
cavity was measured as follows. Beakers each containing
lOOcc of water were placed in pairs at an interval of 1/4 :
wavelength on the turntable to be sub~ected to heating for -
3 minutes and 30 seconds at 200W. The rise in temperature
in each beaker was measured by which the m; n i mllm value was
substrated from the maximum value to be used as the ;~
measured results.
According to the fifth embodiment of the present
invention, microwaves emitted from output antenna 13 of
magnetron 8 are received by radiation antenna 15. secause : -
rib 32 o~ radiation antenna 15 provided substantially :-~
parralel to and around the entire perimeter of antenna 13
is located in the proximity of the antenna with a large ~ ~;
opposing area, leakage of microwaves is minimized,
resulting in a more intensive microwave coupliny between .
output antenna 13 of magnetron 8 and radiation antenna 15
-30-
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for generating microwaves. Thus, microwaves can be
emitted efficiently from radiation antenna 15 to improve
heating efficiency and heating unevenness.
In the above-described embodiments, radiation antenna
15 is fixed basically parallel to sidewall 2b of cavity 2,
so that microwaves emitted from the entire circumference
of radiation antenna 15 are reflected similarly in
waveguide 7 to be diffused within cavity 2.
Considering heating unevenness in the vertical
direction, merely shifting the attached position of the
radiation antenna in the axis direction of output antenna
13 may just alter the region of heating unevenness, and
may not improve the generation of heating unevenness per
se.
If the mass of the food to be heated is great, the
microwaves absorbed by the food will be increased, so that
heating unevenness in the vertical direction is not so
noticeable. In heating food of a small mass, for example
sake or milk, however, the amount of microwaves not
absorbed by the food and therefore diffused into cavity 2
is great. Accordingly, heating unevenness will become
signif~cant.
Fig. 24 is a cross sectional view showing the main
part of a microwave oven according to a sixth embodiment
of the present invention which is aimed to improve such
-31~
., . ,. , ; ,, , . .; 1 ~, . ,;, , : .
2~23~54
heating unevenness in the vertical direction. The
microwave oven according to the sixth embodiment of the
present invention shown in Fig. 24 differs from that of
the second embodiment shown in Fig. 10 as set forth in the ~-
following. A fixed plate 20 having an aperture 30 is
secured to waveguide 7 in a manner inclined towards cavity -
2 (direction of x in figure) by an angle of approximately ~-
30~ with respect to sidewall 2b of cavity 2, with a face
20a towards cavity 2 and a face 20b towards magnetron 8.
Flat radiation antenna 21 secured to face 20b of fixed
plate 20 at the magnetron 8 side has an opening 34 of
substantially an ellipse shape at the center thereof
greater than aperture 30 of fixed plate 20, as shown in
Fig. 25.
If the shape of opening 34 is identical to that of
the opening of radiation antenna 15 of the first
embodiment of Fig. 6, i.e. a true circle, the distance
between output antenna 13 and radiation antenna 21 will
not be constant around the entire perimeter when radiation ~ :~
antenna 21 with fixed plate 20 is secured in an inclining
manner to output antenna 13 of magnetron 8. For example,
if the most remote distance between output antenna 13 and
radiation antenna 21 is 2mm which provides the optimum
coupling efficiency, there is a possibility of a spark ~ ;
occurring in other regions between output antenna 13 and
21236~4
radiation antenna 21 which are closer in distance. If the
shortest distance between output antenna 13 and radiation
antenna 21 is set to 2mm to avoid occurrence of such a
spark, efficient microwave coupling cannot be achieved at
other regions between output antenna 13 and radiation
antenna 21 due to its greater distance. By providing an
ellipse-shaped opening 34 as shown in Fig. 25, the
distance between output antenna 13 and radiation antenna
21 can be kept constant.
According to the structure of a microwave oven shown
in Figs. 24 and 25, an electric field is generated between
antenna cap 14a and radiation antenna 21 of magnetron 8,
and also between radiation antenna 21 and the sidewall of
waveguide 7, whereby microwaves are emitted towards cavity
2. The distance from sidewall 2b of cavity 2 to the outer
perriphery of radiation antenna 21 is not constant along
the circumference since radiation antenna 21 is fixed in
an inclined manner as shown in Fig. 24. Therefore,
microwaves emitted from radiation antenna 21 close to
cavity 2 are directly propagated to cavity 2, whereas
microwaves emitted from radiation antenna 21 remote from
cavity 2 are reflected at the sidewall of waveguide 7 to
be propagated to cavity 2 in a direction differing from
that of the above described microwaves.
As a result, microwaves are diffused in an intricated
21236~4 ~
manner, whereby heating unevenness with respect to food
will be further improved. ~ -
Fig. 26 shows a modification of the sixth embodiment ~
of Fig. 24. The microwave oven of Fig. 26 differs from ;
that of Fig. 24 in configuration of waveguide 7 and the -~ -
attached manner of radiation antenna 21. Waveguide 7 has
a sidewall in which the cross sectional area thereof - -
becomes greater towards cavity 2 from bottom plane 7a. In ;
other wards, waveguide 7 has a truncated cone
configuration, similar to that of the embodiment of Fig.
24. However, bottom 7a of waveguide 7 is inclined by
approximately 30~ with respect to sidewall 2b of cavity 2.
Radiation antenna 21 is held to be at right angles to
antenna cap 14a, similar to the embodiment of Fig. 10. ~-
Therefore, the microwave oven of Fig. 26 can have
heating unevenness improved within cavity 2 by inclining
radiation antenna 21 by approximately 30~ with respect to
sid0wall 2b of cavity 2. ;~
In the embodiments of Figs. 24 and 26, radiation
antenna 21 is held in a tilted manner so that the distance
between radiation antenna 21 and sidewall 2b of cavity 2
becomes greater downwards. The present invention is not ~;~
limited to this disposition, and radiation antenna 21 may
be inclined so that the distance between sidewall 2b of
cavity 2 and the upper portion of radiation antenna 21 is
-34-
2123~
greater than the distance be~ween sidewall 2b and the
lower portion of radiation antenna 21.
Fig. 27 is a sectional view of the main part of a
microwave oven according to a seventh embodiment of the
present invention. Fig. 28 is a plan view of a fixed
plate and a radiation antenna used therein. The microwave
oven of the seventh embodimen~ is similar to the microwave
oven of the first embodiment shown in Fig. 5, except that
a plurality of punched hole 36 are formed at a region of
fixed plate 16 between the outer periphery of radiation
antenna 15 and the outer periphery of fixed plate 16.
Referring to Fig. 28, a group of punched holes 36a formed
along the outer periphery of radiation antenna 15 and a
group of punched holes 36b formed along the outer
periphery of fixed plate 16 are disposed alternately so
that the center of each punched hole 36a corresponds to
each region between two punched holes 36b.
When stains such as fat and juice from the food are
carried into waveguide 7 from cavity 2 to adhere to fixed
plate 16 to degrade the surface resistivity of the fixed
plate, the provision of groups of punched holes 36a and
36b on fixed plate 16 at the periphery of radiation plate
15 will render the current flow, not through the inner
space of the punched hole, i.e. not through air, but so as
to meander at the surface of fi~ed pla-te 16 of low surface
. . ,
21236~
resistivity avoiding the punched holes, as shown by arrow
g in Fig. 28. This meander of current will result in a
longer path from radiation antenna 15 to waveguide 7. -
Because resistance is generally propagational to the path
length, the surface resistivity of fixed plate 16 is
increased to prevent generation of a spark.
Fig. 29 is a plan view showing a modification of the
embodiment of Fig. 28. Instead of a plurality of punched
holes 36 as shown in Fig. 28, a slit 38 is formed in fixed
plate 16 between the outer periphery of radiation antenna
15 and the outer periphery of fixed plate 16. Similar to
the embodimen~ of Fig. 28, the current path from radiation
antenna 15 to waveguide 7 crosses khe air la~er in slit
38, whereby resistance in the current path is
significantly increased. Therefor0, generation of a spark
can be prevented.
In the case where the center of radiation antenna 15
is displ~ced from the center of fixed plate 16 as in Fig.
29, the provision of a slit 38 in a region of fixed plate
16 where the distance between radiation antenna 15 and the
sidewall of waveguide 7 is short is extremely effective to
prevent generation of a spark between radiation antenna 15
and waveguide 7.
According to the seventh embodiment of the present
invention, the surface resistivity of the path from
-36-
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radiation antenna 15 to waveguide 7 is increased even if
stains of food adheres to a fixed plate. Therefore,
generation of a spark can be prevented.
Although the radiation antenna has been described as
having a flat form in each of the aforementioned
embodiments, the present invention is not limited to the
radiation antenna having such flat form and the radiation
antenna may have other forms such as corrugated form.
Although the present invention has been described and
illustrated in detail, it is clearly understood that the
same is by way of illustration and example only and is not
to be taken by way of limitation, the spirit and scope of
the present invention being limited only by the terms of
the appended claims.
