Title:
HIGH FREQUENCY HEATING APPARATUS
United States Patent 3678238


Abstract:
A high frequency heating apparatus, such as an oven, using microwave energy having an enclosure with an opening into the heating cavity which is to be sealed by the door. Peripheral metallic surfaces of the enclosure surrounding the opening and the door are spaced apart to leave a gap when the door is closed, the surfaces defining a parallel plate transmission line. The gap includes a first narrow portion of a quarter wavelength of the frequency of the energy from the heating source originating from an origin, which is the closest point to the entry into the heating cavity, which origin point serves as the input to the transmission line. A wide gap portion of a quarter wavelength has one end located adjacent the first narrow gap and a second narrow gap portion is located adjacent the other end of the wide gap portion. The second narrow gap portion is of a length less than a quarter wavelength long and is the output point of the transmission line. Due to the step down ratio of the transmission line the input point at the first narrow gap portion has a relatively low impedance which prevents energy from leaking out of the cavity.



Inventors:
Yasuoka, Yoshio (Osaka, JA)
Shirakami, Yoshiaki (Otsu, JA)
Application Number:
05/110009
Publication Date:
07/18/1972
Filing Date:
01/27/1971
Assignee:
SANYO ELECTRIC CO. LTD.
Primary Class:
International Classes:
F24C7/02; H05B6/76; (IPC1-7): H05B9/06
Field of Search:
219/10.55 126
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Primary Examiner:
Staubly I, R.
Assistant Examiner:
Jaeger, Hugh D.
Claims:
What is claimed is

1. High frequency heating apparatus comprising:

2. Apparatus as in claim 1 wherein the electrical length of said first and second transmission line sections is an odd number times substantially a quarter wavelength of the electromagnetic wave energy.

3. Apparatus as in claim 1 wherein said opposing surface portions of said enclosure and said door are formed to provide a relatively narrow gap of first width therebetween at the portions defining said first and third transmission line sections and a relatively wide gap of said second width at the portion defining said second transmission line section.

4. Apparatus as in claim 2 wherein the length of said third transmission line section is less than one quarter wavelength of the electromagnetic wave energy.

5. Apparatus as in claim 3 wherein a groove is provided in the surface of the door to form the wide gap portion of said second width.

6. Apparatus as in claim 3 wherein a groove is provided in the surface of the enclosure to form the wide gap portion of said second width.

7. Apparatus as in claim 3 wherein grooves are provided in the opposing surfaces of the door and the enclosure to form a common wide gap portion of said second width.

8. Apparatus as in claim 1 wherein a dielectric material is located in at least a portion of the gap.

9. Apparatus as in claim 1 wherein a dielectric material with a high dielectric constant is located in the gap at the high characteristic impedance second transmission line section and a dielectric material with a low dielectric constant is provided at at least one of the low characteristic impedance first and third transmission line section.

10. Apparatus as in claim 1 wherein said means for holding the door and enclosure in spaced relationship comprises means made of a material for lowering the characteristic impedance located in the gap between the opposing surfaces of the door and enclosure forming the low characteristic impedance third transmission line section.

11. Apparatus as in claim 10 wherein the spacer means is of a material which includes ferrite magnetic rubber.

12. Apparatus as in claim 1, wherein each of the opposing surfaces of the door means and enclosure has a first surface element of a quarter wavelength in length next to the origin of the gap which is generally transverse to the plane of the door and a second surface element generally parallel to the plane of the door, the wide gap portion being provided in the second surface element of at least one of the door and enclosure surfaces.

13. Apparatus as in claim 1, wherein said opposing surfaces and the gap therebetween define a plurality of high characteristic impedance portions and low characteristic impedance portions.

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to high frequency heating apparatus and more specifically to an improvement in means for preventing leakage of high frequency electromagnetic energy through a gap between an oven enclosure and door of such apparatus during its heating operation.

2. Description of Prior Art

As well known, one of the greatest problems in high frequency heating apparatus is the leakage of high frequency electromagnetic energy radiated through a gap between an oven enclosure and door of such apparatus. A typical conventional seal means for preventing leakage of radiation is a metallic spring contact means provided either on the door of the heating apparatus or on the front of the enclosure. In such a sealing arrangement, the contact is likely to become poor, unless the spring is kept clean with special care, resulting in leakage of electromagnetic energy. It is also known that, whenever a metal-to-metal contact occurs, it inevitably gives rise to discontinuity between the contact surfaces and the electric arc generated as a result of this as the contact further deteriorates the contact seal means rapidly. This invention eliminates the above disadvantages.

SUMMARY OF THE INVENTION

In accordance with the invention, a gap is provided between the metal peripheral wall surface surrounding the opening of the oven enclosure of a high frequency heating apparatus and the metal peripheral surface of the door for closing the said opening in order to dispense with a conventional metal-to-metal contacts and to avoid generation of an electric arc between the door and enclosure. The peripheral wall surface encompassing the opening of the enclosure and the peripheral surface of the door are so constructed that they are opposite to each other, but not in contact, when the door is closed, to form a gap therebetween. The portions of the surfaces in opposition which form the gap are somewhat longer than a half wavelength of the high frequency energy of the oven and serve as a parallel-plate transmission line.

A groove of a quarter wavelength in width is provided in the peripheral surface of the door and/or enclosure spaced a quarter wavelength away from the origin of the gap defined by the door and the enclosure at the entrance into the cavity. The origin of the gap is effectively the input to the transmission line. The gap from the origin to the groove is a quarter wavelength in length. The width of the gap in the first narrow groove portion is substantially smaller than that formed by the groove. A second narrow output gap portion is located adjacent the groove at a point remote from the origin and serves as the output point of the transmission line. This second narrow gap is substantially small in length with respect to a quarter wavelength of the operating frequency. Though a quarter wavelength is preferred for the length of each of the wide and narrow (origin) gap portions they may be an odd number times the quarter wavelength of the electromagnetic wave in question. The characteristic impedance of each narrow gap portion is relatively small, while that of the wide gap portion is relatively large.

It has been found that arrangement of the wide gap portion and narrow gap portions in the above manner makes the transmission line serve as an impedance transformer with respect to the high frequency energy. The impedance at the output point, which is comparatively small due to narrowness of the gap at this point, but not zero due to non-metallic contact of the two opposing surfaces, is transformed step-down-wise at the input point into an impedance much smaller than the impedance at the output point. The extremely small impedance at the gap origin, or the input point, effectively prevents leakage of the high frequency energy through the gap. The door in accordance with the invention may be opened and closed just in the same conventional way, while the heating apparatus is not operating.

A dielectric material may be inserted into the gap between all or a portion of the two opposing surfaces so that the gap can be made small enough to avoid generation of an arc and at the same time the effective wavelength of the energy from the apparatus in connection with the transmission line is shortened. The word "wavelength" used in connection with this invention means the wavelength of the high frequency electromagnetic energy generated in the high frequency heating apparatus and transmitted through the transmission line formed between the peripheral surfaces of the door and the enclosure of the said heating apparatus. For instance, therefore, if a high frequency energy supply apparatus generating such energy at the frequency of about 2,450 magahertz is used, one wavelength in this case corresponds to ca. 122.4 mm in the air. However, if it is desired to make the seal means even smaller, the space between the surfaces may be filled with a dielectric material, in which case the wavelength of radiated energy may be made even shorter. Thus, the word "wavelength" should not be construed to means only the wavelength of the high frequency energy propagated in the air but also in a dielectric medium.

Therefore, the principal object of the invention is to provide a seal means for a microwave type heating device having a comparatively simplified structure, which is easy to manufacture, for effectively preventing leakage of high frequency energy through a gap formed between opposing surfaces of the periphery of the door and the periphery of the opening of the oven enclosure of the high frequency heating apparatus, which surfaces are not in metallic contact with each other.

Other objects and advantages of the invention will become more apparent upon reference to the following description made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a high frequency heating apparatus in accordance with the invention,

FIG. 2 shows an enlarged sectional view of the seal means of the upper left portion of the FIG. 1,

FIG. 3 shows an enlarged sectional view of the seal means of another embodiment of the invention,

FIG. 4 shows an enlarged sectional view of the seal means of a further embodiment of the invention,

FIG. 5 shows an enlarged sectional view of the seal means of a still further embodiment of the invention, and

FIG. 6 shows a sectional view of a parallel-plate transmission line for explanation of a principle of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, in a housing 1 is provided a hollow rectangular enclosure 3, made from a thin metal material, having an opening or apperture 2 in front of it, this enclosure 3 being adapted to be a cooking oven. The opening 2 of this enclosure 3 is provided with a door 4, so that it can be opened or closed, for insertion or removal of a material being heated or cooked, in such a way that the door is hinged at the lower part of the said housing 1. The door 4 is provided with a number of small holes 5 for inspection of the state of the material being heated in the enclosure 3 and also for discharge of the vapor generated inside the enclosure 3. The enclosure 3 is provided at its top wall with a hole 6 for supplying high frequency energy and is also provided at its back wall with a number of small holes 7 for ventilation. Positioned between the back wall of the enclosure 3 and the housing 1 is provided a high frequency generating apparatus 8, comprised of a conventional magnetron tube and the high frequency energy generated therein is transmitted through a waveguide 9 into the enclosure 3 from the said hole 6.

The magnetron tube 8 is an air-cooled type in the embodiment shown and is cooled by means of a blower 10 provided in the housing 1. After cooling the tube 8, the air blown is sent through the holes 7 into the enclosure 3 and by this air the vapor generated from the material is discharged through the small holes 5 of the door 4. Inside the enclosure 3 is provided a rotatable stirrer 11 driven by any suitable prime mover for stirring the high frequency energy radiated from the hole 6 into the enclosure 3 so that uneven heating of the material in the enclosure 3 may be avoided.

The door 4 comprises a frame 12, an outer moulding type cover 13 which is preferably made of aluminum, covering frame 12 and a knob 14 for opening and closing the door 4 manually. The peripheral surface of the door 4 and the peripheral wall surface of the enclosure 3 encompassing the opening 2 are so constructed that both surfaces lie opposite to each other, but not in metallic contact, with the door 4 closed, to form a gap 16 therebetween. The gap extends from an origin point a which is effectively the entrance to the heating cavity, and the outer wall of the enclosure and the door. Both of these surfaces defining the gap serve as a parallel-plate transmission line for the high frequency energy of magnetron 8.

The spacing of both surfaces and the gap formed thereby is shown in greater detail in FIG. 2. Referring to FIG. 2, the door 4 is indented at its periphery so that the periphery of the enclosure 3 encompassing the opening 2 may fit in the door indent. It is seen that there is a peripheral non-metallic contact, defining the gap, between two sets of right angled surface elements. The first set of elements is on the door and comprises a first surface element 28 substantially perpendicular to the plane of the frame 12 and the other is a second surface 29 substantially parallel to but stepped outwardly with respect to the plane of the frame 12. The second set of similar right angled surface elements 18 and 19 are on the enclosure 3. As can be seen there is a peripheral opposition but no metallic between the wall surface encompassing the opening 2 of the enclosure 3 and that of the door 4. That is, the metallic surface elements 18 and 19 are opposite to the surface elements 28 and 29, respectively, in case of the door 4 being closed, but are not in contact with each other.

In the embodiment under discussion, as shown in FIG. 2, the length of each of the surface elements 18 and 28 is chosen to be a quarter wavelength of the electromagnetic wave radiated into the enclosure 3. In addition, a groove 15, a quarter wavelength in length, is provided in the surface of element 29 of the peripheral surface of the door 4 in a position next to the surface of element 28. As seen from FIG. 2, the groove 15 constitutes a wide gap portion of a quarter wavelength in length between the surfaces of elements 29 and 19. The opposing pair of surface elements 28 and 18 form a narrow gap portion 16 a quarter wavelength in length. As seen from FIG. 2, the beginning of the groove, or wide gap portion, 15 begins a quarter wavelength away from the origin a of the gap 16 defined by surfaces of elements 18 and 28. In the case of the door 4 being closed, origin a constitutes an input point of a transmission line.

Above the end of the wide gap portion is formed a second narrow output gap portion (between points 30 and 31 in FIG. 2) which is substantially small in length as compared with a quarter wavelength. This second narrow output gap portion constitutes an output point of the transmission line. In FIG. 2, there is also shown a dielectric spacer 21 attached to the surface element 19 in a position just outside of the groove 15,in the second narrow gap portion. Spacer 21 serves to keep both surfaces of the door 4 and enclosure 3 very close to each other, but spaced to prevent metallic contact when the door 4 is closed. As a result, the impedance at the output point with respect to the high frequency is kept very low or small, but not zero, when the door 4 is closed.

The arrangement of the wide gap portion and the two narrow gap portions in the above manner makes the gap 16, or transmission line, act as an impedance transformer. As a result, the small impedance at the output point is transformed step-down-wise at the gap origin a, or the input point, into a much smaller impedance. By virtue of the extremely low or small impedance at the gap origin a, the gap 16 serves to prevent the leakage of the high frequency energy therethrough. A more detailed description as to the principle of the invention is given below.

Referring to FIG. 3, another embodiment of the invention is shown, in which both the peripheral surface of the door 4 and the peripheral wall surface of the enclosure 3 encompassing the opening 2 are made substantially parallel to the plane of door frame 12. In the same way, as described above, a groove or wide gap portion 17 of a quarter wavelength in length is provided on the peripheral surface of the enclosure 3 starting a quarter wavelength away from the origin a of the gap 16 and located between the two narrow gap portions. In the embodiment shown, both the narrow and wide gap portions are filled with a dielectric material 20 of synthetic resin such as polypropylene, which can be attached to either the door or the enclosure. One of the purposes of using such material, as described in detail in the section above entitled SUMMARY OF THE INVENTION, is that the geometry of a quarter wavelength in the dielectric material is substantially made smaller as compared to air. Another purpose is to keep the gap spacing at the output point as well as the narrow gap portion at the origin as small as possible, but in a non-metal contact manner, so that the smallest possible, but not zero, impedance is obtainable at the output point and the narrow gap portion. Thus, a spacer 21 in FIG. 2 is omitted. If desired, the dielectric material can have different constants along its length. For example, the material in the narrow gap portions of low characteristic impedance can have a relatively low dielectric constant and the material in the higher characteristic impedance gap portion can have a higher dielectric constant.

FIG. 4 illustrates a further embodiment of the invention, in which opposing grooves 15 and 17 are respectively provided on the peripheral surface of the door 4 and the peripheral wall surface of the enclosure 3, both being substantially parallel to the plane of the frame 12. Both grooves 15 and 17 are located a quarter wavelength away from the origin a, when the door 4 is closed, so that both grooves 15 and 17 constitute a single common wide gap portion. The two narrow gap portions are provided on each side of the wide gap portion in the same manner as previously described. The embodiment is also shown employing the dielectric material 20 for the same purpose as mentioned previously.

FIG. 5 shows a still further embodiment of the invention. The embodiment shown is substantially the same as that shown in FIG. 2, except that FIG. 5 embodiment dispenses with the spacer 21 and instead employs the dielectric material 20 for the purposes previously mentioned.

To facilitate a understanding of the principle of the seal of the present invention, an explanation is made by referring to FIG. 6. Let it be assumed that the enclosure 3 and door 4 are replaced with the high frequency energy transmission line of upper and lower parallel plates 22 and 23, respectively, in FIG. 6. The gap spacing of the first quarter wavelength portion a-b from the origin a of the gap 16 is small and the characteristic impedance of this portion is denoted as Zo1. The gap spacing of the next quarter wavelength portion b-c is wide, and the characteristic impedance of this portion is denoted as Zo2. Likewise, the quarter wavelength portions c-d and e-i of narrower gap spacing and the quarter wavelength portions d-e and f-g of wider gap spacing are alternately provided to form a multi-stage standing wave circuit.

A calculation can be made with respect to the multi-stage circuit of the impedance Zo1 and Zo2 mentioned above. Let it be assumed that there is a load impedance ZA at the terminal of the line in FIG. 6. Each characteristic impedance in the portions a-b, c-d and e-f of narrower gap spacing is Zo1 and each characteristic impedance in the portions b-c, d-e and f-g of wider gap spacing is Zo2, as mentioned previously.

Therefore, the input impedance Zf seen from the point f is:

where l is the length of line and β is the wavelength constant.

If the length of line is chosen to be odd number times the wavelength, an equation from which the tangent function is eliminated may be obtained. From the Equation (1) the following equation is obtained:

Zf = Zo22 /ZA

The input impedance at the point e is: ##SPC1##

It is seen from the above that arrangement of the wide gap, or high characteristic impedance portion, of an odd number times the quarter wavelength in length and the narrow gap, or low characteristic impedance portion, of an odd number times the quarter wavelength in length of the transmission line makes the line act as an impedance transformer. In addition, it is clear that the greater the number of stages of the narrower gap portion (the characteristic impedance of which is Z0 1 ) and of the wider gap portion (the characteristic impedance of which is Z0 2) and the smaller the value of Z0 1 is as compared to that of Z0 2, the more possible it is to make the input impedance at the origin a of the gap closer to zero meaning that less of the high frequency energy can escape from the open end of the gap. It is well known, though detailed explanation is omitted herein, that the narrower the gap spacing is in a high frequency transmission line, the smaller is the value of the characteristic impedance.

Leakage of electromagnetic wave can be effectively prevented by making the value of Z0 1 /Z0Zo2 as small as possible and nearer to zero, where Z0 1 is the characteristic impedance of the narrow gap portion and Z0 2 is the characteristic impedance of the wide gap portion, in the case of the door 4 closing the opening of the enclosure. Each of the embodiments of high frequency heating apparatus shown in FIGS. 1 through 5, has only one narrow (from the origin) and one wide gap portion. The second narrow gap portion between points 30-31, in FIG. 2, correspond to the output impedance ZA of FIG. 6. However, by repeating such portions, as shown in FIG. 6 it is possible to provide a more effective seal to the high frequency energy.

In the embodiments shown and described, the gap comprises the wide gap portion and the narrow gap portion in order to implement a high characteristic impedance portion and low characteristic impedance portion, respectively. Alternatively or concurrently with such implementation, the high characteristic impedance portion may be filled with a dielectric material with a high dielectric constant and vice versa to perform the same purpose. Further, it is to be pointed out that the wide and narrow gap portions may be any odd number times a quarter wavelength of the frequency used in length, though a quarter wavelength is preferred in view of the small goemetry of the seal means which is preferably obtained.

FIG. 2 shows an embodiment in which a dielectric spacer 21 is provided at the output point to keep both surfaces as close as possible, but out of metallic contact. This spacer, however, may be any materials, such as a ferrite magnetic rubber, which serve to decrease the impedance of the transmission line, in order to make the impedance at the output point even lower.