Electric heating – Microwave heating – Field modification
Reexamination Certificate
2000-02-17
2001-06-12
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
Field modification
C219S748000, C219S695000
Reexamination Certificate
active
06246039
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high frequency heating apparatus such as a microwave oven, and more specifically to a high frequency heating apparatus for heating an object to be heated inside a heating chamber by a magnetron or the like.
2. Description of the Background Art
A microwave oven is a typical example of a high frequency heating apparatus.
FIG. 12
shows a first prior art example. A microwave oven
100
shown in
FIG. 12
is provided with a magnetron
101
, a wave guide
102
, and a heating chamber
103
. Magnetron
101
is provided with a magnetron antenna
101
A. Magnetron antenna
101
A projects into wave guide
102
. Wave guide
102
connects magnetron
101
and heating chamber
103
. The microwaves generated in magnetron
101
are introduced into heating chamber
103
via wave guide
102
.
Wave guide
102
is conventionally formed as a casing whose dimension does not change in the horizontal direction. As a result, it is difficult to match the impedances of magnetron
101
and heating chamber
103
. Consequently, a high ratio of the microwaves generated in the magnetron is reflected toward the magnetron in a microwave oven as the one shown in
FIG. 12
, which leads to the problem of the heating efficiency of magnetron being poor.
FIG. 13
shows a second prior art example. A microwave oven
200
shown in
FIG. 13
is provided with a magnetron
201
, a wave guide
202
, and a heating chamber
203
. Magnetron
201
is provided with a magnetron antenna
201
A. Magnetron antenna
201
A projects into wave guide
202
. In addition, wave guide
202
is provided with a metal plate
210
for controlling the direction of radiation of the microwaves generated in magnetron
201
to a desired direction. Metal plate
210
is rotatable by a movable member such as a motor, not shown.
A lower light surface of wave guide
202
has a slope. Thus, the area of the vertical cross section of wave guide
202
gradually increases in the direction of the microwave propagation, i. e. from magnetron
201
toward heating chamber
203
. As a result, the impedances between a magnetron and a heating chamber can be matched more easily in the microwave oven shown in
FIG. 13
than in the microwave oven shown in
FIG. 12
, and the heating efficiency can be improved in the microwave oven shown in FIG.
13
.
In the microwave oven shown in
FIG. 13
, however, the cross-sectional area in the above-mentioned direction changes substantially throughout the entire wave guide
202
. The positional relation between magnetron
201
and wave guide
202
would thus greatly affect the above-described impedance matching. In other words, an error in mounting position of magnetron
201
would affect the heating capabilities of the microwave oven, which leads to the problem of unstable heating efficiency in the microwave oven shown in FIG.
13
.
Moreover, metal plate
210
made rotatable by a movable member is provided in the microwave oven shown in FIG.
13
. It may be preferable to match the impedances of the magnetron and the heating chamber by providing a movable member like metal plate
210
and by moving this movable member in an appropriate manner.
A movable member in general, however, has by comparison a more complicated structure than a non-movable member that is simply mounted so that the possibility of malfunctioning is greater in the former. Therefore, the provision of a movable member for the purpose of impedance matching in a microwave oven may, instead, create a new problem of unstable heating capabilities of the microwave oven due to the malfunction of the member.
On the other hand, some conventional microwave ovens provide the microwaves into the heating chamber from one side surface of the heating chamber as shown in
FIGS. 12 and 13
, while other conventional microwave ovens provide the microwaves into the heating chamber from the top surface and the bottom surface of the heating chamber.
FIG. 14
shows a third prior art example.
A microwave oven
300
is provided with a heating chamber
303
, magnetrons
301
and
304
for heating an object to be heated
316
inside heating chamber
303
by generating the microwaves, and wave guides
302
and
305
for introducing into heating chamber
303
the microwaves generated by magnetrons
301
and
304
, respectively. Radiation apertures
313
and
319
are respectively provided on the top and the bottom of heating chamber
303
, and the microwaves guided through wave guides
302
and
305
are provided to heating chamber
303
via the respective radiation apertures
313
and
309
. In addition, a turntable on which the object to be heated is to be placed is denoted by
314
, and a turntable motor for rotating turntable
314
is denoted by
315
in the drawing.
Turntable
314
is preferably made of a microwave-permeable material (such as glass) alone. It, however, is normally difficult to form the turntable only of a material such as glass due to considerations of mechanical strength and mechanical connection to be established with turntable motor
315
. Thus, turntable
314
in a conventional microwave oven is formed by a combination of a metallic receiving base and a plate made of glass or the like. More specifically, turntable
314
is formed by a metallic receiving base connected to turntable motor
315
and a plate made of glass or the like placed on the metallic receiving base.
The microwaves are reflected by metal. In a microwave oven as the one shown in
FIG. 14
, the microwaves irradiated on a portion where a hole width of the metallic receiving base provided at the bottom of turntable
314
is not more than &lgr;/2 (&lgr; is a wavelength of the microwave) are reflected by the receiving base so that the microwaves are not absorbed by object to be heated
316
. In other words, in the microwave oven as the one shown in
FIG. 14
, there exist in the region on turntable
314
a region which absorbs from the lower side the microwaves irradiated from below turntable
314
and a region which does not absorb the microwaves.
As a result, the amount of the microwaves absorbed by the object to be heated greatly varies even with the same heating time, depending on the position at which the object to be heated is placed on turntable
314
in the microwave oven shown in FIG.
14
. In short, the same problem of unstable heating capabilities occurs in the microwave oven shown in FIG.
14
.
SUMMARY OF THE INVENTION
Thus, the present invention was conceived in view of such problems. One object of the present invention is to provide a high frequency heating apparatus having good heating efficiency and stable heating capabilities.
Another object of the present invention is to ensure the impedance matching between a high frequency heating portion and a heating chamber.
According to one aspect of the present invention, a high frequency heating apparatus includes a heating chamber for accommodating an object to be heated, a high frequency heating portion for generating microwaves to heat the object to be heated, and a wave guide for introducing into the heating chamber the microwaves generated by the high frequency heating portion. The wave guide includes a first portion and a second portion. The first portion of the wave guide is a portion that has one end connected to the high frequency heating portion and in which impedance with regard to microwave propagation does not change. Moreover, the second portion of the wave guide is a portion that is connected to the other end of the first portion and to the heating chamber and in which impedance with regard to microwave propagation changes from impedance close to that of the first portion to impedance close to that of the heating chamber.
Therefore, according to the high frequency heating apparatus of the present invention, the high frequency heating portion is connected to the first portion in which the impedance of the wave guide does not change, and the wave guide has the second portion in which the impedance matching can be effected between the high frequenc
Hayami Katsuaki
Omori Yoshiharu
Armstrong Westerman Hattori McLeland & Naughton LLP
Leung Philip H.
Sanyo Electric Co,. Ltd.
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