JPH0569277B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an improvement in the door structure of a high frequency heating device.

Conventional technology Grooves with different characteristic impedances are provided in the depth direction on the periphery of the door of a high-frequency heating device, and by making the characteristic impedance of the grooves discontinuous in the depth direction, the effective depth is reduced to a quarter of the wavelength used. There is a proposal in Japanese Patent Laid-Open No. 60-25190 that even if the diameter is smaller than 1, the impedance at the entrance of the groove is maximized, and leakage radio waves can be reduced in the same way as the chiyoke groove. In this conventional example, the shape is quite complicated, such as providing grooves with different widths in the depth direction of the groove and deforming the shape of the peripheral wall of the groove in the depth direction. Furthermore, it is necessary to consider reflection prevention at discontinuous portions of characteristic impedance. Further, as shown in FIG. 7, a cavity resonator 12 for preventing radio wave leakage is bent on the outer periphery of the door 5 to have a rectangle-shaped cross section, and a projecting surface 11 that is one peripheral wall of the cavity resonator 12 is bent. A structure having an inlet 25 in which the end cut and the other wall surface (first wall surface 8) of the cavity resonator 12 are opposed to each other is shown in Japanese Utility Model Application Publication No. 61-795. This conventional example does not describe that the peripheral wall of the cavity resonator 12 is divided into a plurality of conductor pieces. Therefore, higher-order mode radio waves that occur in a direction other than the yz plane as shown in FIG. 8 enter the cavity resonator 12, so the cavity resonator 12 comes out of the resonant state, preventing radio wave leakage. The effect becomes smaller. Assume that the rising surface 23 and the projecting surface 11 of the cavity resonator 12 shown in FIG. 7 are divided into conductor pieces each having a width smaller than 1/2 of the wavelength used in the longitudinal direction (x direction). In this case, the cavity resonator 12 can be regarded as a plurality of parallel resonant elements each having an equivalent capacitance C and an equivalent inductance L arranged in the longitudinal direction (x direction) of the door 5. In each parallel resonant element,
As shown in equation (2) below, the larger the ratio l _M /G of the distance l _M between the entrance 25 of the cavity resonator 12 and the center of area O of the cavity cross section to the entrance dimension G, the greater the equivalent capacitance C. growing. In the cavity resonator 12 of FIG. 7, when l _M /G=1.0, the equivalent capacitance C is smaller than when l _M /G≧1.5 in the present invention, which will be described later. Accordingly, the equivalent inductance L must be increased by that amount according to equation (3) described later, so that it resonates with the frequency of the leaked radio waves. Therefore, as is clear from equation (1) below, it is necessary to increase the cross section AB of the cavity resonator 12, so the conventional cavity resonator 12 is large, resulting in a smaller door and lower cost. It is not suitable for

In addition, FIG. 7 shows the dimensions of each part in the drawing of the specification of Utility Model Application Publication No. 61-795 in the same proportion, and the names and numbers of the components are the same for the parts corresponding to the present invention. There is.

Problems to be Solved by the Invention It is necessary to provide a groove with a complicated shape in the depth direction of the groove, and it takes time and effort to prevent reflections at discontinuous parts of the characteristic impedance, and it is difficult to miniaturize the door. This is an unsuitable point.

Measures to solve the problem A cavity resonator with a rectangle-shaped cross section for preventing leakage radio waves is installed around the door, and three of the four sides of this cavity resonator are installed in the longitudinal direction around the door. The remaining surface and the end cut of the U-shaped conductor piece are made to face each other as an entrance for introducing leakage radio waves into the cavity resonator, and this entrance and the cavity Ratio of the distance l _M between the center of area of the body cross section and the inlet dimension G
l _M /G is set to 1.5 or more, and one end surface of the U-shaped conductor piece is in contact with the second wall surface.

The U-shaped conductor piece is integrally formed from a belt-shaped metal plate separately from the door.

Operation With the above configuration, radio waves that are about to leak through the U-shaped conductor piece are guided into the cavity resonator with a square-shaped cross section as a TEM wave with the shortest wavelength among the transmission modes. This cavity resonator forms a parallel resonant element consisting of an equivalent inductance L proportional to the cross-sectional area of the cavity and an equivalent capacitance C based on the disturbed electric field near the entrance of the cavity as a cylindrical coil with approximately one turn. do. The smaller the entrance of the cavity, the larger C becomes, and L can be made smaller accordingly. In other words, the cross-sectional area of the cavity can be reduced. The radio wave sealing effect is maximized when each side of the square-shaped cross section is smaller than a quarter of the wavelength used.
Further, since the U-shaped metal piece is formed separately from the door, it is possible to select materials, plate thicknesses, processing steps, etc. of the U-shaped conductor piece and the door that are advantageous in terms of cost.

Embodiment The structure and operation of a high-frequency heating device according to an embodiment of the present invention will be explained with reference to the drawings. In Figures 1 and 2, 1 is a heating chamber, and 2 is a heating chamber 1.
3 is the outer box. Reference numeral 4 designates a group of small holes provided in the center of the door 5 as wide as possible to allow viewing into the heating chamber 1. Reference numeral 6 denotes a stepped portion surrounding the small hole group 4, and this stepped portion 6 prevents the end portion of the translucent door inner cover 15 fixed to the inner surface of the small hole group 4 from peeling off during cleaning, etc. This improves the flatness of the sealing surface 7 that makes plane contact with the flange 2 when the door 5 is closed. 8
is a first wall surface bent from the end of the sealing surface 7 at a substantially right angle to the flange 2. Reference numeral 9 denotes a second wall surface extending substantially parallel to the flange 2 from the end of the first wall surface 8. 10 is separate from door 5,
These are a large number of U-shaped conductor pieces welded to the second wall surface 9. This U-shaped conductor piece 10 has a mounting surface 19 that is welded to the second wall surface 9 , a rising surface 23 that faces the first wall surface 8 substantially parallel to it, and an end cut that faces the first wall surface 8 . It consists of three sides, including an overhanging surface 11. The width D (x direction in FIG. 3) of each U-shaped conductor piece 10 in the longitudinal direction of the periphery of the door 5 is made smaller than one-half of the wavelength used. Further, the square-shaped cross section surrounded by the first wall surface 8 and the U-shaped conductor piece 10 forms a cavity resonator 12 having a narrow entrance 25. A projection piece 14 protruding from an opaque dielectric cover 13 that blocks the entrance of the cavity resonator 12 is adapted to be hooked into a mounting hole 18 provided in a rising surface 23 of the U-shaped conductor piece 10. A protruding piece 17 protruding from a dielectric door frame 24 for holding a translucent door outer cover 16 covering the front surface of the door 5 is located at the outermost peripheral edge 2 of the second wall surface 9.
It seems to be stuck at 0.

Next, the effects of the embodiment configured as described above will be explained. Flange 2 surrounding heating chamber 1 opening
The radio wave sealing effect will be qualitatively explained using a simple equivalent circuit as shown in FIG. 21 is a capacity corresponding to the planar contact portion between the flange 2 and the sealing surface 7;
It acts as a kind of bypass capacitor. The planar contact portion is considered to be a parallel plate line, and since the capacity of this line is inversely proportional to the gap of the parallel plates, the capacitance 21 becomes larger as the gap of the planar contact portion is smaller, and the radio wave sealing effect increases. Since the width D (x direction in FIG. 3) of the U-shaped conductor piece 10 is made smaller than one-half of the wavelength used, a The traveling direction of the radio waves entering the cavity resonator 12 having a rectangle-shaped cross section is limited to the yz plane of FIG. 3. If there is no overhanging surface 11, the electric field will be distributed as shown in Figure 6, and the length l of the parallel plate line will cause parallel resonance at about one quarter of the free space wavelength λ, and the impedance will be maximum, preventing radio wave leakage. However, in the case of a 2450 MHz high frequency heating device, l is 30.6 mm, and if you try to mount this on a door, it will be thick, which is disadvantageous both in terms of design and cost.

As in this embodiment, a cavity resonator 1 is provided with an overhanging surface 11, has a square-shaped cross section, and has a narrow entrance 25.
2, the electric field distribution will be as shown in FIG. In this case, most of the electric lines of force are gathered between the vicinity of the end cut of the overhanging surface 11 and the first wall surface 8. Cavity resonator 12 is represented in FIG. 4 as a parallel resonant element consisting of an equivalent inductance L and an equivalent capacitance C. equivalent inductance L
acts as a one-turn cylindrical coil with approximately the same cross section as the cavity resonator 12, and means the equivalent inductance as a constant of the coil, and is the value per unit length in the cylindrical axis direction (x direction). becomes as shown in equation (1). In addition, the equivalent capacitance C is the inlet 2 of the cavity resonator 12.
It is based on the turbulent electric field around 5, and is approximately given by equation (2).

L=μ ₀ AB ...(1) C=(2/πloge√2el _M /G-K)ε ₀ ...(2) where AB: Area of the square-shaped cross section of the cavity resonator 12 μ ₀ : Magnetic permeability of the medium inside the cavity resonator 12 e: 2.72 l _M : Distance between the entrance 25 of the cavity resonator 12 and the center of area O of the cavity cross section ε ₀ : Permittivity of the medium inside the cavity resonator 12 K: Correction term related to the shape of the vicinity of the entrance 25 G: Gap of the entrance 25 (entrance dimension) The resonance frequency ₀ of the cavity resonator 12 can be expressed by equation (3).

₀ = 1/2π√LC ...(3) From equation (2), the smaller the gap G of the inlet 25, the more
Alternatively, it can be seen that as l _M /G increases, the equivalent capacitance C increases. Assuming that the resonance frequency ₀ is constant, it can be seen from equation (3) that the larger the equivalent capacitance C is, the smaller the equivalent inductance L is.
In order to reduce the equivalent inductance L, the area AB of the rectangle-shaped cross section of the cavity resonator 12 can be reduced from equation (1). That is, in order to make the cavity resonator 12 smaller, the gap G between the entrances 25 is narrowed to increase the equivalent capacitance C, and the cavity area AB is correspondingly reduced to reduce the equivalent inductance L, which is constant. Resonant frequency ₀ (heating frequency of high frequency heating device)
Then, by causing parallel resonance, the impedance at the entrance 25 can be maximized to prevent radio wave leakage.

In a high-frequency heating device with a heating frequency of 2450MHz and a high-frequency output of 500W, the flange 2 and the sealing surface 7
The gap between
I heated 275ml and measured the amount of radio wave leakage at a distance of 5cm from around Door 5. As a result, G=5mm
When AB = 15.4 x 15.9 mm, l _M /G = 2.1, the amount of radio wave leakage is 0.1 mW/cm ² or less, and if G = 8 mm, it is necessary to suppress the amount of radio wave leakage to the same level as above. The area of the square cross section is also large, as AB = 20.4 x 18.4 mm and l _M /G = 1.75. Through such experiments, by narrowing the gap G of the inlet 25 to about 4 to 8 mm and increasing l _M /G to 1.5 or more, the dimensions A and B of the cavity resonator 12 having a rectangular cross section can be respectively reduced. It has become clear that it can be made much smaller than 30.6 mm, which is a quarter of the wavelength λ used.

The U-shaped conductor piece 10 that occupies three of the four sides of the cavity resonator 12 is manufactured by making periodic notches in a thin belt-shaped metal plate and then performing a bending process in two places. be done. Since the two locations are bent in the same direction, they can be performed in one step. As described above, the U-shaped conductor piece 10 has a simple shape and good workability. Further, the U-shaped conductor piece 10 is attached to the mounting surface 19.
Since there are notches on the other two sides, the center of gravity is low when the mounting surface 19 is placed at the bottom.
Therefore, it can be stably installed on the second wall surface 9, and a welding rod can be inserted through the notch between adjacent conductor pieces 10 to easily weld to the second wall surface 9. A door 5 integrally formed from a single metal plate,
U-shaped conductor piece 10 formed from a belt-shaped metal plate
The material, plate thickness processing process, etc. can be selected to be cost-effective. For example, a large door that requires increased strength should have a plate thickness of 0.8 mm, and the U-shaped conductor piece 10, which is surrounded by the dielectric cover 13 and door frame 24 and protected from external forces, should have a plate thickness of 0.8 mm. 0.6
Mass production is possible as mm.

Effects of the Invention As described above, according to the present invention, the entrance of a cavity resonator having a rectangle-shaped cross section surrounded by a large number of U-shaped conductor pieces and the first wall surface is connected to the overhanging surface of the U-shaped conductor piece. Since the end cut and the first wall face each other and are narrow, and the dimensions are selected such that l _M /G ≧ 1.5, the cross-sectional dimensions A and B of the cavity are set to the wavelength λ used. The resonant cavity can be made smaller than one-fourth of the original size, and the shape of the resonant cavity is simple.Furthermore, one end surface of the U-shaped conductor piece is in contact with the second wall surface, making it easy to assemble and making the door more compact. , it is possible to provide a thinner and more compact high-frequency heating device, and it also has great economic ripple effects. Further, the U-shaped metal piece has a simple shape, requires fewer processing steps, is advantageous in terms of cost, and is easy to install during welding, so welding to the second wall surface is easy.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a main part showing only the metal part of a door 5 of a high-frequency heating device according to an embodiment of the present invention;
The figure is a sectional view of the main part showing the radio wave seal part around the door, Figure 3 is a diagram showing the xyz direction in Figure 1,
Figure 4 is a simplified equivalent circuit diagram of the radio wave seal part of the door 5, Figure 5 is an electric field distribution diagram of the radio wave seal part, and Figure 6 is a diagram of the electric field distribution of the radio wave seal part of the door 5.
The figure is an electric field distribution diagram of a parallel plate line with the same terminal short-circuited.
FIG. 7 is a configuration explanatory diagram showing a conventional radio wave seal structure, and FIG. 8 is a diagram showing the direction of the electric field. 1... Heating chamber, 2... Flange, 4... Small hole group, 5... Door, 6... Step, 7... Sealing surface, 8
... first wall surface, 9 ... second wall surface, 10 ... U-shaped conductor piece, 11 ... overhanging surface, 12 ... cavity resonator, 19 ... mounting surface, 23 ... rising surface, 25...Inlet, l _M ...Distance between the inlet 25 of the cavity resonator 12 and the center of area O of the cavity cross section, G...Inlet dimension.

Claims (1)

[Claims] 1 In a high-frequency heating device in which a cavity resonator 12 for preventing leakage radio waves is provided at the periphery of a door 5 that opens and closes the opening of the heating chamber 1, the cavity resonator 12 is located at the periphery of the door 5 and when the opening of the heating chamber 1 is closed. a sealing surface 7 that is in planar contact with the flange 2; a first wall surface 8 that is approximately perpendicular to the flange 2 from the end of the sealing surface 7;
A second wall surface 9 that is substantially perpendicular to the wall surface 8 is integrally formed from a single metal plate, and this second wall surface 9 is divided in the longitudinal direction of the periphery of the door 5 into smaller parts than half of the wavelength used. A large number of U-shaped conductor pieces 10 are fixed together, and the first wall surface 8 and the U-shaped conductor pieces 10 form a rectangle-shaped cross section in which each side is smaller than one-fourth of the wavelength to be used. A cavity resonator 12 having an inlet 25 is formed, and the ratio of the distance l _M between the inlet 25 and the center of area O of the cavity cross section to the inlet dimension G (l _M /
G) is 1.5 or more, and the U-shaped conductor piece 10 is connected to the door 5.
Separately from this, it is integrally formed from a belt-shaped metal plate, and includes a mounting surface 19 that is fixed to the second wall surface, and a rising surface 2 that faces the first wall surface 8 substantially in parallel.
3 and an overhanging surface 11 whose end cut faces the first wall surface 8 . A high-frequency heating device characterized in that they are formed facing each other.