JPS6259438B2 - - 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, the frequency of electromagnetic waves conventionally used in microwave ovens is 2450MHz, so 1/4
The wavelength is approximately 30mm. 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. That is, when the characteristic impedance of the chain yoke is Z _p and the depth is L, and 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 ) becomes. 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 (relative dielectric constant εr) in the chiyoke part,
When filled, the wavelength λ' of the radio wave is compressed to λ'=λo/√. In this case, the depth L' of the chiyoke portion becomes short as L'=L/√. 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, solid-state oscillators have advanced 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 driving voltage of a magnetron is approximately 3KV, whereas the driving 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 magnetron 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 the experiment 2450±
By automatically tracking the frequency within 50MHz, we were able to improve the practical load efficiency by approximately 60 to 80% compared to a fixed frequency. (4) Solid-state oscillators could become cheaper than magnetrons in the future due to mass production. In addition, the ISM frequencies (Industrial, Scientific,
Medical) is 5880MHz, 2450MHz, 915MHz,
400MHz and must not be used outside this range. The current magnetron is as described above.
It is oscillating at 2450MHz, but it is a solid-state oscillator.
If you oscillate at the same frequency of 2450MHz, you will not be able to obtain sufficient output power, resulting in a power shortage. Therefore, in order to obtain the desired output power, it is necessary to select a lower frequency, e.g.
915MHz is appropriate. However, since this frequency is about 1/2.7 of the conventional frequency, the wavelength is, on the contrary, about 2.7 times, and the quarter wavelength is about 80 mm. Therefore, if 915MHz is selected as the frequency of the microwave oven, Figures 1 and 2
The thickness of the chiyoke part explained in the figure exceeds approximately 80 mm, and the effective size of the opening of the heating chamber is extremely small compared to the conventional example, which has the disadvantage of making it extremely difficult to put it into practical use. be. 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 process. For example, it took more than 50 minutes at 2450MHz and 600W to heat the center of a 12cm diameter lump of meat 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 standing wave pitch 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 disadvantage of changing the operating frequency of the microwave oven to 915MHz 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. That is, the characteristic impedance of the chiyoke groove is Zoc, the depth of the groove is l _c , and the leakage path 1 from the heating chamber to the chiyoke groove is
When the characteristic impedance of the channel 17 is Z _pp , the length of the leakage path 17 is l _p and the wavelength used is λ, the short circuit impedance (Z
_c = O) is the opening B of the chiyoke groove 18 and Z _B =
jZoctan2π/λl _c . 19 is a heating chamber of the microwave oven, and 20 is a door. Here, by choosing l _c =λ/4, 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 l _p =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/√ of the free space wavelength λ, but the quarter wavelength (λ'/4) impedance principle A similar effect can be obtained by using it. 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 EMBODIMENTS The present invention provides, for example, at least one 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. , is characterized in that the groove depth from the open end to the shorted end is less than a quarter 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=jZo 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 It was reversed. 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 increased by the larger value of characteristic impedance ratio K.
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 facilitate understanding of this principle, some of the analysis results are shown in Figures 4a, b, and c. In FIG. 4, a groove having an opening B whose tip C is short-circuited is provided in a part of the transmission line with the A end used as an excitation source and the D end open. The width of the groove on the short circuit side is twice that on the open hole side. When point A is excited under the same conditions and the groove depth l _T is changed, the electric field in the transmission line changes as shown in Figure 4 a, b, and c. In the case of Figure 4 b, the groove depth l _T is approximately 80°, which is one-quarter wavelength.
% (for lines less than λ/4), and even if it is longer or shorter (in the case of Figure 4 a and c), the fourth
Radio waves leak more easily than in Figure b. This is l ₁ = l ₂
This can be confirmed by substituting =lT/2=λ/10.2, K=b ₂ /b ₁ =2 into 1≒Ktanβ l ₁・tanβl ₂ . The idea of making the characteristic impedance discontinuous is as follows. In the present invention, one side of the groove part of the sealing device is used as a ground conductor, and a conductor plate having a width dimension _a is arranged with _a gap dimension b separated from the other. Calculate the characteristic impedance ratio K using the following formula in a configuration where the width on the groove short-circuit side is a ₂ and the gap is b ₂ . By setting the value of K to be greater than 1, an attempt is made to make the characteristic impedance discontinuous. The details of the embodiment will be explained based on the drawings. Figure 5 is a perspective view of the microwave oven with 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. 6 shows a sectional view taken along the line A--A in FIG. 5, and FIG. 7 shows a perspective view of the conductor portion forming the groove 26 in FIG. In FIGS. 6 and 7, the groove 26
A sealing plate 27 having a bent portion a faces the door plate 27, and a door plate 28 constituting the wall surface of the groove is bent concavely.
Consists of parts b, c, d, e, and f. A groove cover 29 that covers the opening of the groove 26 has a claw g portion that serves as a pull-out preventer. The entire outer surface of the door is covered with a door cover 30. The opening side groove of the groove 26 is indicated by 26 and has a depth of l ₁ , and the short circuit side groove is indicated by 26 and has a depth of l 1 .
_l2 . The open end and short-circuit end of the groove are 31, respectively.
32. The outer peripheral side wall surface of the door plate 28 is provided with cut portions 33 at pitches P, and the conductor width is f of a ₁ at each pitch P.
The sum of the conductor widths is _a2 , the e part, and the reinforcement space d part. The gaps between the bent portions a and f of the sealing plate 27 and between the portions b and e are defined as b ₁ and b ₂ , respectively. Therefore, the characteristic impedance ratio K in the groove 26 is By making the value of K larger than 1, the groove depth (l ₁ +l ₂ ) is configured to be smaller than a quarter wavelength. When applying the present invention to actual products, it is not uncommon to provide a groove cover space (Top 1) and a bending reinforcement space (lx1). Compared to when the principle is explained, the radio waves are disturbed and the dimensions are slightly deviated from the dimensions calculated according to the principle. The details of the deviation are shown below. When the dimension of TOP1 is 2mm and lx1 is 5~
An example is shown when the width is set to 6 mm. Figure 8 is a top 1 example of a 915MHz sealing device.
This shows the relationship in which the groove depth l _T changes with the dimensions of .
If the dimension of TOP1 is 1 to 3 mm, l _T is 1 to 6 mm.
It gets deeper. Figure 9 is an example of a 2450MHz sealing device.
The relationship is shown in which the groove depth l _T changes depending on the reinforcement space (lx1) with TOP1 = 2 mm fixed. space lx
By setting 1 to 2 to 6 mm, the groove depth l _T is 1 to 3
mm deeper. FIG. 10 shows another embodiment. Portions corresponding to those in FIG. 7 are designated by the same reference numerals. The structure is the same as that shown in FIG. 7 except that the shape of the notch is different. The key point in the structure is that the dimensions of the f section of the door plate and the dimensions of the a section of the sealing plate are made almost equal. Needless to say, the miniaturization of the sealing device described above can be applied not only to radio frequency seals of 915 MHz but also to radio wave seals of 2450 MHz and other frequencies.
In addition to the construction of the sealing device by bending a sheet metal, it is also possible to use a method of plating plastic with metal. Effects of the Invention As described above, in addition to the effect of achieving miniaturization, which is the purpose of the invention, the following effects are obtained. (1) 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. (2) The bent part of the sealing plate can also be used as a groove cover holder. (3) The reinforced space makes the door stronger. (4) Since a portion of the groove is covered by the sealing plate, the opposing area between the main body and the door can be reduced. (5) Since the depth of the groove can be changed by changing the characteristic impedance ratio K, it is easy to design with consideration to design and strength.

[Brief explanation of the drawing]

Fig. 1, Fig. 2 a, b, and Fig. 3 are sectional views of conventional radio wave sealing devices, and Fig. 4 a, b, c, respectively.
is an analysis diagram of the electric field in the groove of the radio wave sealing device according to the present invention, and FIG. 5 is a perspective view of a general microwave oven.
FIG. 6 is a sectional view of a radio wave sealing device according to an embodiment of the present invention, FIG. 7 is a perspective view of the groove in FIG. 6, and FIGS. 8a, b, and c are sectional views and front views of a 915MHz radio wave sealing device Figures, characteristic diagrams, Figure 9 a, b, c are
Cross-sectional view, front view of 2450MHz radio wave seal device,
The characteristic diagram, FIG. 10, is a perspective view of another embodiment of the present invention. 22... Door, 26... Groove, 27... Sealing plate,
31... Opening end, 32... Short-circuited end, 33... Notch, a... Bent part, P... Pitch.

Claims (1)

[Claims] 1. A main body having an opening and into which radio waves are supplied is provided, a door is provided to cover the opening of the main body so as to be openable and closable, and at least one of the parts where the main body and the door face each other is provided with at least one A sealing plate having a groove and an L-shaped bent portion at the tip covers a part of the opening of the groove, and a notch is provided from the opening end of the groove at a pitch P in at least one wall of the groove. Conductor strips are arranged continuously in the longitudinal direction of the groove, the cut ends before reaching the shorted end of the groove, and each conductor strip is arranged so that the conductor width at the open end of the groove is on the side closer to the shorted portion. A radio wave seal device configured to be wider than the conductor width.