JPS6331917B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high frequency heating device with improved radio wave sealing means.

A microwave oven, which is a typical example of a conventional high-frequency heating device, uses a contact method using a conductive gasket and a method using a radio wave absorbing material to prevent radio wave leakage from the gap created between the heating chamber and the door. , a chiyoke method using radio wave resonance, etc. has been proposed,
It has been put into practical use.

Each of these methods has its own advantages.
Generally, they are mounted on a device in a superimposed manner. One of the difficult issues in constructing such a radio wave sealing means is countermeasures against harmonics.

Although the above-mentioned contact method has sufficient performance in principle even at this high frequency, manufacturing precision is required to maintain contact precision. Further, if fragments of the object to be heated adhere to the contact surface, the performance of this contact method will be significantly reduced, resulting in poor reliability.

On the other hand, most radio wave absorbing materials exhibit sufficient performance within a predetermined band, and do not exhibit sufficient performance for both the fundamental wave and its harmonics.

Furthermore, since the Chi-Yoke method that uses resonance uses resonance, it necessarily requires structural dimensions that are related to the wavelength of the fundamental wave. The conventional Chi-Yoke system configuration will be described below.

Fig. 1 shows a typical conventional radio wave sealing mechanism of the chiyoke type. A is a configuration diagram, and b is its equivalent circuit.

The conventional chi-yoke type radio wave sealing mechanism has a chi-yoke cavity 4 starting from a point B at a length of 1/4 of the fundamental wavelength of the leaked radio wave from the entrance A of the radio wave passage 3 created by the heating chamber opening flange 1 and the access door 2. The first disadvantage is that the dimension in the Z direction must be at least 1/4 of the fundamental wavelength of the leakage radio wave, making it difficult to make it compact.

In addition, the impedance _ZB of the chiyoke cavity 4 viewed from its input end B is ideally configured to be infinite for the fundamental wave, so it becomes zero for the second harmonic. Become. Furthermore, the impedance when looking at the outside space from the terminal C of this radio wave seal mechanism is
If Z _L , the equivalent circuit of this radio wave seal mechanism is b
It can be expressed as shown in the figure, and the impedance Z _A of the radio wave seal mechanism seen from terminal A-A' is Z _A (Z _L + Z _B ) + jtanβl/1+j (Z _L + Z _B ) tanβl;
β=2π/λ _gi where λ _gi is given by the leakage radio wave wavelength and l is given by the radio wave path length. However, each impedance is normalized.

A magnetron, which is a typical example of a high-frequency heating heat source, has an oscillation frequency that varies depending on the load being heated.
Since fluctuations of up to 2450±50MHz occur, the radio wave path length l is equivalently considered to be l=λ _g1 /4±△λ ₁ for the leakage radio wave fundamental wavelength λ _g1 , and for the second harmonic, It is assumed that l=λ _g2 /2±△λ ₂ .

Therefore, for the fundamental wave, Z _B = Joo, tanβloc
Therefore, Z _A O, and for the second harmonic, Z _B
O, tanβl=O, so Z _A Z _L.

For the fundamental wave, the change in Z _L , that is, the change in the gap between the C terminals caused by the assembly of the heating chamber flange 1 and the access door 2, can be sufficiently absorbed, but for the second harmonic, the The second drawback is that it has little strength.

In view of these circumstances, the present invention introduces a parallel transmission line used for radio wave transmission into a radio wave leakage prevention mechanism, connects a branch line to a part of the transmission line, and sets the branch lengths of the branch lines to be different from each other. The main purpose of this invention is to provide a new and compact radio wave sealing mechanism with periodic arrays of closed transmission lines, and to improve radio wave sealing performance.

The present invention will be explained below with reference to the drawings.

FIG. 2 shows the radio wave sealing mechanism of the present invention, a
is a block diagram, and b is its equivalent circuit.

The parts in the figure that are compared with FIG. 1 are indicated by the same numbers. A group of 6 transmission lines having a width W and arranged with a gap S between adjacent transmission lines facing the heating chamber opening flange 1 are formed. This transmission line 6 has a branch line 7 terminating at the short-circuit surface 5 at a length B at a length _l1 from the input end A of the radio wave path 3, and its depth is approximately 1/4 of the leakage radio wave wavelength λ _g . It is. The equivalent circuit of such a transmission line is expressed as shown in diagram b, as is well known in microwave engineering.

Here, the big difference from Figure 1 is that the width of the transmission line is restricted to W, so the radio wave path and branch path can be positioned as a transmission line, so inductive and capacitive parameters can be interposed at the branch point. It becomes possible to express an equivalent circuit. For calculating these parameter values, please refer to the "Waveguide Hand book".
MITRadiation Laboratory P.337 provides a detailed explanation. The impedance _ZA seen from point A toward the radio wave sealing device is related using each parameter of the equivalent circuit. If the impedance Z _A obtained in this way is Z _A = R + jX, and the impedance looking toward the heating chamber from point A is Z _0V , then the microwave power P _io incident on the radio wave sealing device is transmitted from point C to the outside of the main body. Leaked microwave power P _L
The present inventors have experimentally confirmed that the relationship is expressed by the following equation.

P _L ∝R・Z _0V / (Z _0V +R) ² +X ²・P _io Using this relational expression supported by experiment,
As a result _of simulation under certain conditions, the smaller P ₁ is, the more leaked microwave power P _L
obtained fewer results. To demonstrate this l ₁ dimension 30
When we actually measured the attenuation characteristics of the radio wave seal device for mm and 12 mm, the value for l ₁ = 12 mm was 1/2 compared to that for l ₁ = 30 mm.
The result was a ~1/3 lower amount of radio wave leakage. It can be inferred from the above relational expression that the leakage amount can be reduced even if the _l1 dimension is small by designing the value of X to be large. The main structural dimensions at the time of actual measurement are as follows.

The gap between the heating chamber opening flange and the radio wave seal device is 1 mm and 4 mm, and the opposing dimension of the branch path is 8 mm.
Depth of branch path l ₂ = 32 mm, when l ₁ = 12 mm, l ₃ = 10 mm,
When l ₁ = 30 mm, l ₃ = 12 mm, transmission line width W = 10 mm,
Gap S = 10mm.

Calculatedly, the smaller the _l1 dimension, the better the result, but in order to prevent the θ-direction transmission of radio waves incident at an angle θ to the Z direction of the radio wave sealing device, the _l1 dimension faces the heating chamber opening flange. Considerable dimensions should be provided for this purpose. Therefore, in the real configuration l ₁ =
12 mm is considered to be approximately the minimum dimension of l ₁ .
As a result, it is a well-known fact that the dimension from the entrance of the radio wave path to the chiyork cavity in the chiyoke system used is 1/4 wavelength, and as shown in the present invention, a branch path with a gap S is formed. By using a radio wave seal mechanism in which transmission lines are periodically arranged, the length up to the branching point can be
Since l ₁ can be made sufficiently smaller than 1/4 wavelength, it has been shown that the radio wave sealing mechanism in the Z direction can be configured more compactly than the conventional chi-yoke method.

In this radio wave sealing mechanism, the depth of the branch path can be set to approximately 1/4 of the leakage radio wave wavelength, which suggests that countermeasures against fundamental waves and harmonics can be achieved by simply changing the length of the branch path. .

FIG. 3 is a configuration diagram of a high frequency heating device showing an embodiment of the present invention.

Figure A is an overall configuration diagram of the device, Figure B is an enlarged configuration diagram of the radio wave seal mechanism, and Figure C is a sectional view of Figure B.

A book is installed on the entrance/exit door side of the radio wave path 3 formed by the heating chamber opening flange 1 and the entrance/exit door 9 that covers the opening of the heating chamber 8 that accommodates the object to be heated (not shown). Radio wave seal mechanism 10 of the invention,
It is equipped with 11 components.

This radio wave sealing mechanism will be explained below based on figures b and c.

As explained using FIG. 2, the feature of the radio wave sealing mechanism of the present invention is that in a configuration in which transmission lines having branch paths whose terminal ends are short-circuited are periodically arranged with gaps in between, the lengths of the branch paths are made to be different from each other. , the fundamental wave of the leaked radio wave that is to be passed through the radio wave path 3,
It has a high-performance radio wave sealing function against harmonics, and as shown in Figure b, there is a transmission line 10 for the fundamental wave and a transmission line 11 for the second harmonic, which has a particularly large amount of power among the harmonics. and 2-3
They are periodically arranged in a portion of the access door 9 facing the heating chamber opening flange 1 across a gap 12 of mm.

Each cross-sectional view of the transmission line for the fundamental wave and the second harmonic is shown in Figure c.

Branch paths 13 and 14 having predetermined branch path lengths l ₁ and l ₂ located 10 to 15 mm from the input end A of the radio wave path 3
is provided.

The transmission line 10 for the fundamental wave is composed of two metal plates 15 and 16, and 17 is a branch path 1.
This is the short circuit surface of 3. Moreover, this branch path 13 is configured in a U-shape to reduce the thickness of the entrance/exit door. In addition, in order to easily change the length of this U-shaped branch road from a branch road with no bends as shown in Fig. 2, the corner portion of the branch road is
Cut the corners 18 and 19 at approximately 45 degrees to reduce the capacitance generated at the corners, and make the branch path length _l1 correspond to approximately 1/4 wavelength of the fundamental wave by measuring the dimensions as shown in the figure. It consists of

On the other hand, in the transmission line 11 for the second harmonic, the branch path length l ₂ may be 1/2 of the branch path length l ₁ for the fundamental wave, so the branch path 14 having the shorting surface 20 is bent as shown in the figure. It is constructed from a single metal plate.

By adopting the above configuration, leakage radio waves are actively guided to each transmission line due to the effect of the gap 12, but the leakage radio fundamental wave is not influenced by the transmission line 10 distributed around the opening of the heating chamber. On the other hand, for the second harmonic, the transmission line 1
1, the amount of leakage is suppressed.

Further, in principle, the amount of third harmonic leakage is suppressed in the transmission line 10.

Note that the formation of the branch paths is not limited to the one embodiment of the present invention; for example, the radio wave sealing mechanism of the present invention may be provided on the heating chamber opening flange to configure each branch path without bending, or the formation of branch paths may be formed on the entrance/exit door. When installed, the branch path for the fundamental wave may be L-shaped.

As described above, the present invention has a novel radio wave sealing means in which transmission lines having different branch path lengths with short-circuited branch path ends are arranged in a periodic manner on the access door around the heating chamber opening or on the heating chamber opening flange. The present invention provides a high-frequency heating device that (1) has excellent radio wave sealing performance against the fundamental wave and harmonics of leakage radio waves simply by varying the branch path lengths.

(2) A more compact radio wave seal configuration is possible compared to the conventional chiyoke system.

It has the following effects.

[Brief explanation of the drawing]

Figure 1 shows a conventional radio wave sealing mechanism of the Chiyoke method, where a is a configuration diagram, b is its equivalent circuit diagram, and the second
The figure shows the radio wave seal mechanism of the present invention, a is a configuration diagram,
b is an equivalent circuit diagram thereof, FIG. 3 is a block diagram of a high-frequency heating device showing an embodiment of the present invention, a is an overall block diagram, b is an enlarged block diagram of a radio wave seal mechanism, and c is each cross-sectional view of figure B. It is. 2,9... Entrance/exit door, 3...Radio wave passage, 6,1
0, 11... Transmission line (periodically arranged transmission line), 7, 13, 14... Branch path, 8... Heating chamber,
5, 17, 20...short circuit surface, _l1 , _l2 ...branch path length.

Claims (1)

[Claims] 1. A heating chamber for accommodating an object to be heated, an access door for taking the object into and out of the heating chamber, and a radio wave sealing means provided in a radio wave passage formed by the heating chamber and the access door. A high-frequency heating device, wherein the radio wave sealing means has a periodic arrangement of transmission lines having mutually different branch path lengths and short-circuited ends of the branch paths.