JPS6359239B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a radio wave seal for a consumer high frequency heating device, commonly called a microwave oven.

Microwave ovens are useful for heating and cooking objects using high-frequency energy. On the other hand, in microwave ovens, electric power leaks to the outside from the gap between the heating chamber wall and the openable door wall (hereinafter referred to as the radio wave passage), so it must be kept below the allowable range from the standpoint of human safety. In order to prevent radio wave leakage as described above, a radio wave seal is applied to the radio wave passage.

As a generally well-known radio wave seal device, there is one called a radio wave seal. The yoke seal is made by setting the dimension from the entrance of the radio wave passage formed by the heating chamber main wall and the door wall to the radio wave resonant groove, and the length of the radio wave resonant groove to be a quarter wavelength of the high frequency generation source being used. This is equivalent to short-circuiting the entrance to the radio wave passage, and provides an ideal radio wave sealing function.

Furthermore, in order to improve heating efficiency, modern microwave ovens use stirrer fans to disturb the electric field, so the chiyoke seal alone cannot perform the radio wave sealing function in the complicated direction of radio wave propagation. Therefore, there is also a radio wave sealing device in which a radio wave passage or a radio wave resonance groove is provided with slits having a periodic structure to restrict the direction of radio wave propagation.

Furthermore, it is well known that a dielectric material is loaded in the radio wave resonant groove to reduce the length of the radio wave resonant groove, thereby achieving compactness.

In the above, the conventional chiyoke seal requires that both the dimension from the entrance of the radio wave path to the radio wave resonant groove and the length of the radio wave resonant groove be one-fourth the wavelength of the high frequency generation source being used. There were naturally limits to how compact microwave sealing devices could be made.

The present invention makes the radio wave sealing device more compact by arranging the radio wave paths periodically with T-branch terminated transmission lines and by changing the line width of the transmission line portion from the T-branch opening to the end of the radio wave path. The present invention provides a radio wave sealing device for a high frequency heating device, which aims to improve the radio wave sealing performance.

The present invention is one of the important characteristics of the T-branch circuit by providing a T-branch in the radio wave path.

Focusing on the fact that "by short-circuiting one of the three ports of a T-branch circuit at an appropriate position, no power is transmitted between the other two ports" and arranging T-branched transmission lines in a periodic manner. By doing so, radio wave propagation in the arrangement direction is restricted.

In addition, in making the radio wave leakage prevention mechanism more compact, the length from the entrance of the radio wave passage to the radio wave resonant groove and the length of the radio wave resonant groove are both reduced to 1/4 wavelength, unlike the conventional chi-yoke method. Instead of reducing the impedance at the entrance of the radio wave path (ideally short-circuited), quantitative analysis of a radio wave path system in which all the radio wave paths with T-branches are placed in impedance and the amount of radio wave leakage itself is reduced is carried out. This is the introduction of a method to do this.

To describe this analysis method in more detail, we assume an impedance Z _L at the end of the transmission line that forms the radio wave path. The reason why we introduce an impedance Z _L (referred to as loss impedance) at the end of the radio wave path is because there is always a gap between the heating chamber wall and the door, that is, a gap in the radio wave path, The idea is that when radio waves leak into the external space from the end of a transmission line, energy is consumed at an impedance of Z _L. When Z _L takes a certain finite value, if the energy passing through the radio wave path is large, the amount of energy consumed by Z _L will be large, so the amount of radio wave leakage will be large, and if the energy is small, conversely it will be less. In the conventional chiyoke method,
The impedance Z _L at the end of the radio wave path was considered to be Z _L = 0, and this was a qualitative idea simply to reduce the impedance at the entrance of the radio wave path.

Therefore, in order to reduce the energy consumed by the radio waves passing through the radio wave path at Z _L (reducing the amount of radio wave leakage), one of the characteristics of the T-branch circuit as mentioned above is necessary.
(i.e., choose a T-branch close to 1/4 wavelength), and the distance from the entrance of the radio wave path to the T-branch aperture is smaller than 1/4 wavelength. We found good results that were not found in the Chiyoke method and confirmed them experimentally.

Furthermore, if this quantitative analysis method is used for the dimension from the T-branch opening to the end of the radio wave path, by changing the line width of the transmission line in this part, it is possible to find a dimension smaller than a quarter wavelength in this part as well. It has been theoretically and experimentally confirmed that the amount of radio wave leakage is reduced.

Based on this result, we have developed a compact radio wave sealing device for high frequency heating equipment with excellent radio wave sealing performance.

This will be explained below using the drawings.

FIG. 1 shows a conventional microwave oven seal. Radio waves propagate to the outside from the radio wave passage 2 and entrance 3 formed by the heating chamber main wall 1 and door 6. In the case of a chiyoke seal, the length from the entrance of the radio wave passage to the radio wave resonance groove entrance 4 and the length of the radio wave resonance groove 5 is By setting both wavelengths to 1/4, an ideal radio wave seal is constructed.

FIG. 2 is a cross-sectional view of a radio wave sealing mechanism for a radio wave passage showing an embodiment of the present invention. Entrance 8 of radio wave passage 7
to the T-branch opening 9 and from the T-branch opening to the transmission line termination 11 are both smaller than a quarter wavelength. The T-branch 10 is selected near a quarter wavelength,
12 is a dielectric material.

FIG. 3 shows transmission lines 13 arranged periodically in a direction perpendicular to the paper plane of FIG. 2 and tapered from the T-branch opening to the end of the transmission line.

FIG. 4 is a diagram in which the radio wave path 7 in FIG. 2 is replaced with an equivalent impedance. The impedance when looking at the heating chamber side from the radio wave path entrance 8 is Z 14, the loss impedance at the end of the radio wave path is Z _L 15, and the characteristic impedance of the transmission line Z _O 16.

FIG. 5 is a graph showing the relationship between the length from the T-branch opening to the end of the transmission line and the characteristic impedance Z _O and loss impedance Z _L of the transmission line according to an embodiment of the present invention.

In FIG. 6, the T branch from the entrance of the radio wave path in FIG. 4 is replaced with Z ₁ 17, and FIG. 7 is an equivalent circuit when Z ₁ is set to infinity in FIG. 6, and l ₃ is the length from the T-branch opening 9 to the transmission line termination 11.

In the above configuration, the radio waves that are about to leak to the outside through the radio wave path 7 are difficult to propagate between 8 and 11 because the T-branch 10 selects a wavelength close to 1/4, and the radio waves also do not propagate in the periodic arrangement direction. It regulates propagation. For quantitative analysis of the amount of radio wave leakage, the radio wave passage entrance 8
The impedance Z of the radio wave path system including loss impedance Z _L is divided into resistance component (Real component) and reactance component (Imagenal component) as Z = R + il, and radio wave leakage is calculated as follows: We discovered the idea that the quantity P _L is proportional to P _L ∝R×Z〓/(R ² +I ² +R×Z〓) ......(1).

The impedance of this radio wave path system is calculated using the following theoretical formula.

Now, when the reference plane is placed at the radio wave passage entrance 8 and the impedance seen from there to the T-branch opening 9 is Z' ₁ , the total impedance of the T-branch opening 9 is Z _T. Z' ₁ =Z ₀ Z _T +jZ ₀ tanβl ₁ /Z ₀ +jZ _T tanβ ₁ (1-1
) where Z ₀ is the characteristic impedance of the transmission line,
β is a phase constant, and l ₁ is a dimension from the radio wave path entrance 8 to the T-branch opening.

(Sanpo Publishing/Electronic Science Series 21, Microwave Circuit Basic Knowledge P.15) Regarding the impedance description of the T-branch opening 9, regarding Z _a to Z _d in Figure 4, refer to the same reference document P. 166, there is the following relational expression Z _a = jB _a Z _b = -jB _b Z _c = -jB _c Z _d = jB _d (1-2) Next, the impedance Z' ₂ of the T-branch section 12 is: If its length is l ₂ , the terminal end is short-circuited, so Z′ ₂ = jZ ₀ tanβl ₂ = jZ″ ₂ (1-3), and Z _b , Z _c , Z _d of T-branch opening 9 Z ₂ = jZ′ ₂ (B _d −B _c )−1/B _b {Z″ ₂ (B
_d −B _c )−1}+B _c (Z″ ₂ B _d −1)=jW ₁ (1−4)
Further, from the T-branch opening 9 to the radio wave path terminal portion 20
The impedance up to Z ₃ is Z ₃ = Z ₀ Z _L + jZ ₀ tanβl ₃ /Z ₀ +jZ _{L tanβl 3} = W ₂ + jW ₃ (1
-5), and the combined impedance with the T-branch 9Z _a is, from equation (1-5), Z ₃ = 1/jB _a (W ₂ + jW ₃ )/1/jB _a +W ₂ + jW ₃ = W ₄ +jW
₅ (1-6) Furthermore, (1-4), (1-6), the combined impedance Z _T with the remaining Z _a of the T-branch section 9 is Z _T = (Z ₂ + Z ₃ )・1/jB _a /1/jB _a +Z ₂ +Z ₃ =
W ₆ +jW ₇ (1-7) Substituting this equation (1-7) into equation (1-1), Z' ₁ = Z ₀ W ₆ + jW ₇ ) + jZ ₀ tanβl ₁ /Z ₀ + j (W ₆ + jW ₇ ) ta
The impedance relational expression nβl ₁ =R+jI (1-8) is obtained.

Therefore, if the impedance Z〓 inside the heating chamber, the impedance Z′ ₁ outside the heating chamber, and the amount of radio wave radiation from the magnetron are P _IN , then the real parallel circuit of Z〓 and Z′ ₁ calculates the amount of radio wave leakage _PL . The results showed that the values and the measured values matched. That is, P _L =1/R/R ² +I ² +I/Z〓P _IN P _L =R×Z〓/R ² +I ² +R·Z〓) P _IN (1-9). The results of the calculated values and actual measured values are shown in FIG.

This shows that from the radio wave passage entrance 8 to the T-branch opening, the radio wave sealing performance is better when the wavelength is shorter than 1/4 wavelength.

Furthermore, in Fig. 6, the length l ₃ from the T-branch opening 9 to the transmission line termination 11, the loss impedance Z _L 15 at the transmission line termination, and the characteristic impedance Z _O 16 of the radio wave path system are shown as a quantitative analysis method for the amount of radio wave leakage. By applying , we get the result shown below.

In FIG. 6, the impedance Z 3 when the loss impedance Z _L 15 at the end of the transmission line is viewed from a distance _{l 3} (from the T-branch opening to the end of the transmission line) is the characteristic impedance of the transmission line Z _O 16
Then, by transforming the above (1-5), Z ₃ = Z _O /Z _O ² + (Z _L tan βl ₃ ) ² {Z _O Z _L (1 + tan ² βl ₃ ) + j (Z _O ² −Z _L ² ) tanβl ₃ } ......(2) Now, if Z ₁ 17 in FIG. 6 is set to infinity (that is, the T-branch 10 is made close to 1/4 wavelength), as shown in FIG. If the circuit is open and a small constant current I flows through the circuit, the larger the impedance of Z ₃ including the loss impedance Z _L 15, the greater the amount of radio wave leakage. P _L = | Z ₃ | | I｜ ² ...... can be easily imagined from the formula (3).

Therefore, the magnitude of the impedance Z _{3 is} determined in equation (2) by determining the amount of radio wave leakage over the _{length of} The shorter the length, the more it increases or decreases.

As an easy-to-understand example, in equation (2), l ₃ = 0, l ₃
When substituting =30.5mm, l ₃ =O→Z ₃ =Z _L ………(4) l ₃ =30.5mm→Z ₃ ¹ =Z _O ² /Z _L ………(5) Therefore, l The condition that the shorter ₃ is, the smaller the amount of radio wave leakage is (Z ₃ < Z ₃ ¹ ) Z ₃ − Z ₃ ¹ = Z _L − (Z _O ² / Z _L ² ) = 1/Z _L (Z _L + Z _O ) Since (Z _L −Z _O )<O, the following condition is required: Z _L <Z _O (6). In other words, if the characteristic impedance Z _O of the transmission line is larger than the loss impedance Z _L at the end of the transmission line, the dimension l ₃ (distance from the T-branch opening to the end of the transmission line) peaks at l ₃ = 1/4 wavelength. The shorter the length, the less radio wave leakage can be achieved.

On the other hand, when Z _L > Z _O , the opposite phenomenon occurs. In other words, unlike the above case, the amount of radio wave leakage becomes minimum near l ₃ = 30.5 mm, and l ₃ = 0, or 1/2
It will have a peak value at the wavelength. In addition, it was actually observed that if the length of _l3 was not made into a slit, the length of _l3 would steadily decrease as the length of l3 increased.

As shown in Fig _. 5, the theoretical curve and the measured value in the case of Z _L > Z _O almost coincide with each other. Although it is effective, it does not contribute to compactness at all.

Therefore, in order to have a radio wave sealing effect and achieve compactness, we are trying to create the condition expressed by equation (6).

Now, the defining formula for the characteristic impedance _ZO of the transmission line is shown as _ZO =120π×(b×a) (7). Here, b18 is the gap of the radio wave path 7, and a19 is the transmission line convergence. In order to create the condition 6, it is sufficient to increase the characteristic impedance Z _O of formula 7, so it is sufficient to increase the gap in the radio wave path or narrow the transmission line convergence.In the present invention, the l ₃ portion 20 Tapered. When taper is applied with l ₃ = 10mm, the amount of radio wave leakage is about 5% compared to before applying taper.
It was less than one-fold. In Figure 5, the point has changed from A to A'. This means that the condition of equation (6) can be reproduced theoretically and experimentally, and it has become possible to achieve a radio wave sealing effect and to achieve compactness.

Note that this is not limited to one embodiment of the present invention, and various other embodiments such as the shape of _l3 as shown in FIG. 8 can be considered.

As described above, the present invention periodically arranges the radio wave path formed by the heating chamber wall surface and the door with a transmission line partially terminated in a T-branch, and changes the line width of the transmission line from the T-branch opening to the transmission line termination. As a result, a compact radio wave sealing device can be realized, and the effects shown in (1) to (4) can be obtained.

(1) A compact radio wave sealing device can be realized, and the space of the main body of the high frequency heating device can be saved.

(2) The amount of radio wave leakage can be quantitatively analyzed based on the magnitude relationship between the impedance Z _O of the transmission line and the loss impedance Z _L at the end of the transmission line, and it is possible to design the shape of the transmission line to improve the radio wave sealing performance.

(3) Due to the compactness of the radio wave seal part, less material is required than before.

(4) The weight of the door can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a diagram of a conventional tea yoke seal mechanism, Fig. 2 is a sectional view of a T-junction showing an embodiment of the present invention, Fig. 3 is a diagram of the shape of a transmission line, Fig. 4 is an impedance equivalent circuit diagram of a radio wave path, and Fig. 4 is a diagram of an impedance equivalent circuit of a radio wave path. FIG. 5 is a graph of the results of one embodiment of the present invention, FIGS. 6 and 7 are equivalent circuit diagrams for quantitatively explaining the present invention, and FIG. 8 is another embodiment of the present invention. 1... Heating chamber, 6... Door, 2, 7... Radio wave passage, 3, 8... Radio wave passage entrance, 10... T branch, 11... Transmission line termination, 12... Dielectric, 1
3...Transmission line, 20...Transmission line tapered portion (l ₃ ).

Claims (1)

[Scope of Claims] 1. A heating chamber has a door that can be opened and closed to take the heated object in and out, and a part of the radio wave path formed by the heating chamber wall surface and the door is a transmission line terminated with a T-branch. The line width and shape of the transmission line arranged periodically from the opening of the T-branch to the end of the radio wave path are changed, and A radio wave sealing device having a configuration in which a dimension and a dimension from the opening of the T-branch to the end of the radio wave path are each smaller than a quarter wavelength. 2. The radio wave sealing device according to claim 1, wherein a dielectric material is loaded in the T-branch portion.