JPS6259437B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to a radio wave sealing device for shielding high frequency radio waves. Configuration of Conventional Example and Its Problems A conventional radio wave sealing device of this type will be described using, for example, a microwave oven that cooks food by dielectrically heating it using high frequency waves. A microwave oven is equipped with a heating compartment that stores food and heats it using high-frequency waves, and a door that can be opened and closed to cover the opening of the heating compartment for putting food in and out. At this time, radio wave sealing measures are taken to prevent high-frequency electromagnetic waves inside the heating chamber from leaking outside the chamber and causing harm to the human body. As an example of the conventional technology, U.S. Patent No. 3182164 is the first
As shown in the figure. In FIG. 1, reference numeral 1 denotes a heating chamber of a microwave oven, and a door 4 having a handle 3 that covers an opening 2 of the heating chamber 1 so as to be openable and closable is provided. A hollow wall portion 6 having a gap portion 5 opened toward the heating chamber 1 is formed at the peripheral edge of the door 4 . The depth 7 of this cheese yoke portion 6 is designed to be substantially one-fourth of the wavelength of the high frequency wave used. In this case, the thickness of the door 4 is also a quarter wavelength. In other words, since the frequency of electromagnetic waves conventionally used in microwave ovens is 2450 MHz, a quarter wavelength is approximately 30 mm. In order to face the chiyoke part 6 of this length, the thickness 9 of the peripheral part 8 formed in the opening part 2 of the heating chamber 1 has a value larger than a quarter wavelength. Therefore, the effective size of the opening 2 of the heating chamber 1 is slightly smaller by the peripheral edge 8. Next, as another conventional example, U.S. Patent No.
No. 2500676 is shown in Figures 2a and b. This example also shows the configuration of a microwave oven, in which high frequency waves obtained by oscillation of a magnetron 10 are supplied to a heating chamber 11 to heat and cook food 12 by electromagnetic induction. The opening 13 of the heating warehouse 11 is provided with a door 14 that covers the opening 13 so as to be openable and closable. A groove-shaped yoke portion 15 is also formed at the peripheral edge of the door 14, and this yoke portion 15 prevents high frequency waves from leaking to the outside. The depth 16 of this yoke portion 15 is also one-fourth of the operating frequency.
Designed by wavelength. Therefore, the effective size of the opening 13 is slightly smaller than the heating chamber 11, as in FIG. As mentioned above, the conventional choke section is based on the technical concept of attenuating high frequencies by having a depth of one-quarter wavelength. In other words, the characteristic impedance of the chiyoke section is
Zo, the depth is L, and when the terminal end is short-circuited, the impedance Z _IN at the opening of the chain yoke is Z _IN =jZotan (2πL/λo) (λo is the free space wavelength). The radio wave attenuation means of the chi-yoke method is based on the principle that |Z _IN |=Zotan (π/2)=∞ is achieved by selecting the depth L of the chi-yoke part to be one-quarter wavelength. If there is a dielectric material (relative permittivity ε _r ) in the chiyoke part,
When filling, the radio wave wavelength λ' is compressed to λ'≒λo/ _√r . In this case, the depth L' of the chiyoke portion becomes short as L'≒L/ _√r . However, there is no difference in setting L' = λ'/4, and in the chiyork method, the depth cannot be made substantially smaller than a quarter wavelength, and there is a limit to the miniaturization of the chiyork. It was hot. In recent years, the development of solid-state oscillators has progressed, and the era of practical use has arrived. Microwave ovens are no exception; traditional magnetron oscillators are being replaced by solid-state oscillators. The advantages of solid-state oscillators in microwave ovens are as follows. (1) The drive voltage of a magnetron is approximately 3KV, whereas the drive voltage of a solid-state oscillator using a transistor or the like can be approximately 400V or less, and in reality, approximately 40V is used. Therefore, since the power supply voltage is low, it is safe for the human body, and even if there is a leak, electric shock accidents are unlikely to occur. Therefore, it becomes possible to make it earthless, and it is also possible to develop it into a portable device. (2) While the lifespan of a Gnetron is approximately 5,000 hours, a solid-state oscillator has a long lifespan of approximately 10 times longer. (3) While the oscillation frequency of a magnetron is fixed, the oscillation frequency of a solid-state oscillator can be varied, for example, within a range of 13MHz above and below 915MHz. Therefore, by automatically tracking the frequency based on the size of the load (food to be cooked), the resonance frequency changes and highly efficient operation can be achieved. According to experiment 2450±50M
By automatically tracking the frequency within Hz, it was possible to improve practical load efficiency by approximately 60 to 80% compared to a fixed frequency. (4) Solid-state oscillators may be cheaper than magnetrons in the future due to mass production. In addition, the ISM frequencies (Industrial,
Scientific, Medical) is 5880MHz, 2450MHz,
915MHz, 400MHz, etc., and must not be used outside of this range. Current magnetrons oscillate at 2450MHz as mentioned above, but if you use a solid-state oscillator to oscillate at the same frequency of 2450MHz,
Sufficient output power cannot be obtained, resulting in power shortage. Therefore, in order to obtain the desired output power, it is necessary to select a lower frequency; for example, 915 MHz is suitable. However, this frequency is approximately 2.7
Since it is 1/4 of the wavelength, the wavelength becomes about 2.7 times, and the quarter wavelength becomes about 80 mm. Therefore, if 915MHz is selected as the frequency of the microwave oven, the thickness of the cheese yoke explained in Figures 1 and 2 will exceed approximately 80mm, and the effective size of the opening of the heating chamber will be greater than that of the conventional example. This has the disadvantage that it becomes extremely small, making it extremely difficult to put it into practical use. On the other hand, the advantages of changing the oscillation frequency from 2450MHz to 915MHz are as follows. (1) Because the wavelength has become longer, radio waves can penetrate deep into the food, making it possible to speed up the cooking time. For example, it took more than 50 minutes at 2450MHz and 600W to heat the center of a 12cm diameter meatball to about 50℃, but it took less than 50 minutes at 915MHz and 300W. (2) The cause of uneven burning is standing waves, and the pitch of standing waves is correlated with wavelength. When 915MHz is used, the standing wave pitch is large, making it difficult to notice uneven cooking on the food. Therefore, the frequency used by the microwave oven is 915MHz.
The disadvantage of changing to is that the radio wave sealing means becomes larger. Note that as one means for reducing the thickness of the chiyoke part, there is a structure in which the chiyoke part is filled with a dielectric material. According to this configuration, the dielectric constant of the chiyoke part becomes large, so that the chiyoke part can be made smaller than a quarter wavelength, and the same effect as that of a chiyoke part of a quarter wavelength can be achieved. However, since the dielectric material is expensive, the price of the microwave oven as a whole becomes expensive, and the manufacturing time and cost are high, which hinders its practical use. The principle of the conventional example will be theoretically explained below. The Chi-Yoke method is based on the well-known quarter-wavelength impedance conversion principle. In other words, the characteristic impedance of the chiyoke groove is Zoc, and the depth of the groove is
lc, the characteristic impedance of the leakage path 1 from the heating chamber to the chiyoke groove is Zop, and the length of the leakage path 17 is
When the lp wavelength used is λ, the short-circuit impedance (Zc=
O) is the opening B of the chiyoke groove 18 and Z _B =jZoctan
2π/λlc. 19 is the heating chamber of the microwave oven, and 20 is the door. By choosing lc=λ/4 here, it can be converted to |Z _B |=∞. The impedance Z _A when the impedance Z _B of this opening B is viewed from the line starting point A is

[Formula] becomes. Here, by choosing lp=λ/4, it can be converted to |Z _A |=O. The short-circuit condition at the bottom C of the channel groove 18 appears at the starting point of the line by skillfully utilizing the quarter-wavelength impedance conversion principle, thereby making it practical as a radio wave sealing device. By loading a dielectric material with a permittivity ε _r into the leakage path 17 and the chiyoke groove 18, the wavelength λ' becomes λ/√ _r of the free space wavelength λ, but the wavelength λ' becomes λ/√ r of the free space wavelength λ.
4) A similar effect can be obtained by using the impedance principle. OBJECTS OF THE INVENTION The present invention provides a radio wave sealing device in which the size of the choke portion does not increase even if the oscillation frequency is lowered. Structure of the Invention This invention is a radio wave seal using a new impedance conversion principle, in which each of the leakage path and the groove has a characteristic impedance discontinuity configuration, resulting in a shape smaller than the size equivalent to a quarter wavelength. It is something. DESCRIPTION OF THE EMBODIMENTS The present invention provides a groove in at least one of the main body or the door of a microwave oven, and the shape of the groove is such that the characteristic impedance on the short circuit side is larger than that on the open side, and The groove depth from to the shorted end is 4
It is characterized by being less than one-tenth of a wavelength. The basic idea that makes miniaturization possible is as follows. Let the characteristic impedance and length phase constant of the groove opening be Zo ₁ , l ₁ , and β ₁ . If the characteristic impedance and length phase constant of the groove short-circuited part are Zo ₂ , l ₂ , β ₂ and the distance from the open end of the groove to the short-circuited end (depth of the groove) is l (total), then l (total) = l ₁ + l ₂ . Under the above conditions, the impedance Z at the open end of the groove is Z = Zo ₁ · tanβ ₁ l ₁ +Ktanβ ₂ l ₂ /1−K
tanβ ₁ l ₁ ·tanβ ₂ l ₂ (1) (where K=Zo ₂ /Zo ₁ ) can be derived by simple calculation. In the conventional example, Zo ₂ = Zo ₁ , β ₁ = β ₂ (that is, K=
This corresponds to 1). Therefore _, its impedance Z' _is calculated from _{equation 1 as}
nβ ₁ l ₁・tanβ ₂ l ₂ =Zo ₁ tan(β ₁ l ₁ +β ₂ l ₂ )=Zo ₁ tan(β ₁・
ltotal) ......(2), and by setting ltotal to λ/4, the impedance was inverted. On the other hand, according to the configuration of the present invention, the characteristic impedance satisfies Zo ₂ >Zo ₁ due to the configuration requirements, so the value of the characteristic impedance ratio K in equation 1 is necessarily larger than 1. In order to make impedance Z infinite, the denominator of equation 1 needs to be zero, so 1=
It is sufficient to satisfy Ktanβ ₁ l ₁・tanβ ₂ l ₂ , and the size is equal to the value of characteristic impedance ratio K larger than 1.
Even if l ₁ and l ₂ are made small, the same impedance inversion as in the conventional case can be achieved. The present invention performs impedance inversion using a less than λ/4 line instead of the λ/4 line that has been historically used in the field of radio wave seals. In order to make this principle easier to understand, part of the analysis results are shown in FIG. Fourth
In the figure, a groove having an opening B whose tip C is short-circuited is provided in a part of a transmission line in which the A end is an excitation source and the D end is open. The width of the groove on the short circuit side is twice that on the open hole side. Excite point A under the same conditions, and measure the depth of the groove.
When lT is changed, the electric field in the transmission line is a,
The wave changes as shown in b and c, and the radio wave does not reach the D terminal in case b, that is, when the groove depth lT is about 80% of a quarter wavelength (less than λ/4 line). , even if it is longer or shorter (cases a and c), radio waves leak more than in case b. This is l ₁
=l ₂ =lT/2=λ/10.2, K=b ₂ /b ₁ =2 to 1≒
This can be confirmed by substituting Ktanβl ₁ and tanβl ₂ . The idea of making the characteristic impedance discontinuous is as follows. The present invention has a structure in which the groove portion of the sealing device has one side serving as a ground conductor and a conductor plate having a width dimension a and spaced apart by a gap dimension b. In detail, the width on the groove opening side is a _, the gap is b _{, 1} the effective dielectric is ε _eff , the width on the groove shorting side is a, ₂ the gap is b ₂ , and the characteristic impedance ratio K is calculated by the following formula. Calculate with By setting the value of K to be greater than 1, an attempt is made to make the characteristic impedance discontinuous. In actual applications, a groove cover space (TOP1) and a bending reinforcement space (lX1) are often provided. In these cases, compared to the case where the principle is explained, the radio waves are disturbed and the calculated dimensions are slightly deviated. The details of the deviation are shown below. When the dimension of TOP1 is 2mm and lX1 is 5~6
An example is shown when it is set to mm. Figure 5 is a top 1 example of a 915MHz sealing device.
This shows the relationship in which the groove depth lT changes with the dimensions of .
If the dimension of TOP1 is set to 1 to 3 mm, lT becomes deeper by 1 to 6 mm. Figure 6 is an example of a 2450MHz sealing device.
The relationship is shown in which the groove depth lT changes depending on the reinforcement space (lX1) with TOP1 = 2 mm fixed. By setting the space lX1 to 2 to 6 mm, the groove depth lT becomes 1 to 3 mm deeper. The details of the embodiment will be explained based on the drawings. Figure 7 is a perspective view of the microwave oven, showing the patching plate 2.
A door 22 having a number 1 is attached to a main body covered with a main body cover 23. The main body is provided with an operation panel 24, and a door handle 25 is attached to the door. FIG. 8 shows a sectional view taken along the line A--A in FIG. 7. The groove 27 is constituted by the door 22, and the groove wall d
It consists of a part, an e part, and an f part. A sealing plate 26 having a bent portion c faces the groove 27, and is configured such that the width b ₁ of the opening of the groove is narrower than the width b ₂ of the bottom of the groove. To protect the groove 27 and improve its appearance, a groove cover 28 and a door cover 29 are provided to cover the opening of the groove and the door. FIG. 9 is a perspective view of the door 22 and the sealing plate 26 that constitute the groove 27. The groove wall d section is cut with a pitch P so that the opening part of the groove has a width a ₁ and the bottom part of the groove has a width a ₂ . Therefore, if the gap between the c part of the sealing plate 26 and the h part of the door 22 and the gap between the f part and h part of the door 22 are b ₁ and b ₂ in FIGS. 8 and 9, then the opening The ratio K of the characteristic impedance of the side groove and the bottom side groove is: By making K larger than 1, the groove depth (l ₁ +l ₂ ) is made smaller than a quarter wavelength. FIGS. 10, 11, and 12 show other methods of configuring the sealing plate and door. FIG. 10 shows an example in which the line is divided into two when narrowing the width of the conductor wall on the bottom side of the groove. There may be two or more. In FIG. 11, a part of the wall surface d of the door from the bottom is not cut, but continues, thereby providing a reinforcing effect for the door. In FIG. 12, the characteristic impedance of the groove is changed by making a tapered cut in the wall surface d. In any of these configurations, the depth of the groove can be made one-quarter wavelength or less. Also, Figure 10, Figure 11,
A combination of two or more of the configurations shown in FIG. 12 may be used. FIG. 13 shows an example of how to attach the hinge rotating shaft 31 to the door 22. A hinge mounting plate 31 is provided on the outside of the chiyoke groove 27, and a small groove 32 is provided.
It consists of Needless to say, this radio wave seal configuration can be used not only for 915 MHz microwave ovens, but also for miniaturizing radio wave seal devices for 2450 MHz microwave ovens and other radio wave seal devices. The same effect can be obtained by constructing the sealing device by plating plastic resin instead of by bending a sheet metal. Effects of the Invention According to the present invention, since the chiyoke portion can be made smaller than a quarter wavelength, the radio wave sealing device can be miniaturized, and in addition, the following effects can be obtained. (1) Since the area where the door and the main body face each other is small, the entire main body can be made smaller. (2) The bent part of the sealing plate can also be used as a groove cover holder. (3) Since the dimension b ₁ is smaller than b ₂ at the bent portion of the sealing plate, the groove portion of the door can be manufactured without undercutting. (4) The sealing structure can be achieved by simply attaching a sealing plate to the groove of the door, making it suitable for cost reduction. (5) Since it is a single groove, the number of parts is reduced and costs can be lowered. Also, the chiyoke groove is made up of two parts, the door and the sealing plate, and the four sides around the door can be separated from each other as in the past. Since it is not constructed using several parts, it is also easy to manage dimensions.

[Brief explanation of the drawing]

Figure 1, Figure 2 a, b, and Figure 3 are cross-sectional views of the conventional radio wave seal device, and Figure 4 a, b, and c, respectively.
are electric field analysis diagrams of the groove portion in the present invention, FIG. 5a,
b, c are a cross-sectional view, side view, and characteristic diagram of the device at 915MHz; Figure 6 a, b, and c are a cross-sectional view, side view, and characteristic diagram of the device at 2450MHz; Figure 7 is a perspective view of a typical microwave oven. 8 is a sectional view of a radio wave sealing device according to an embodiment of the present invention, and FIG. 9 is a perspective view of a chiyoke groove portion of the embodiment in FIG.
Figures 10, 11, and 12 are perspective views of the chiyoke groove in other embodiments, Figure 13 is a perspective view of the door hinge installation structure, and Figure 14 is a sectional view taken along the line B-B in Figure 13. be. 22...Door, 23...Body, 27...Groove, 2
6... Sealing plate, d... One wall surface of the groove (conductor wall).

Claims (1)

[Claims] 1. Provide a main body with an opening to which radio waves are supplied,
A door is provided to open and close the opening of the main body, one groove is provided in at least one of the parts where the main body and the door face each other, and a sealing plate covering a part of the opening of the groove is provided at the tip. is bent in an L-shape toward the inside of the groove, one wall of the groove is periodically cut in the longitudinal direction of the groove to divide it into a plurality of conductor walls, and the bottoms of these conductor walls are made thin, A radio wave sealing device in which the characteristic impedance of the opening part of the groove is smaller than the characteristic impedance of the bottom part.