JPS6313319B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a radio wave sealing device for shielding high frequencies.

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. When opening the heating cabinet, radio wave sealing measures are taken to prevent high-frequency electromagnetic waves inside the heating cabinet from leaking outside and harming 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, US Patent No. 2500676
The numbers are shown in Figure 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 cheese yoke portion 15 is also designed to be a quarter wavelength of the operating frequency. 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 1/4 wavelength.

If a dielectric material (relative dielectric constant εr) is filled in the cheese yoke, 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 town yoke method, the depth cannot be made substantially smaller than a quarter wavelength, and there is a limit to the miniaturization of the town yoke. It was warm.

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 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 experiment 2450±50M
By automatically tracking the frequency within Hz, we were able to improve 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, 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 1/2, the wavelength will be about 2.7 times,
A quarter wavelength is approximately 80mm. 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 process. 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 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. 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=
0) is the opening B of the chiyoke groove 18, and ZB=jZoctan2π/λlc. 19 is a heating chamber of the microwave oven, and 20 is a door. Here, by choosing lc=λ/4, it can be converted to |Z _B |=∞. When the impedance Z _B of the opening B is viewed at the line starting point A, the impedance Z _A is Z _A =-jZop1/tan2π/λlp. Here, by choosing lp=λ/4, it can be converted to |Z _A |=0. 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 εr into the leakage path 17 and the chiyoke groove 18, the wavelength λ' becomes the free space wavelength λ.
A similar effect can be obtained by using the quarter wavelength (λ'/4) 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 The present invention is a radio wave seal that uses a new impedance conversion principle, and has a shape smaller than the size equivalent to a quarter wavelength by having each of the leakage path and the groove have a characteristic impedance discontinuous configuration. That is.

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. , 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, length, and phase constant of the groove opening be Zo ₁ , l ₁ , and β ₁ . If the characteristic impedance, length, and phase constant of the groove short-circuited part are Zo ₂ , l ₂ , and β ₂ , and the distance (groove depth) from the open end of the groove to the short-circuited end 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−Ktanβ ₁ l ₁・ta
nβ ₂ l ₂ ...(1) (However, K=Zo ₂ /Zo ₁ ) can be derived by simple calculation.

In the conventional example, this corresponds to Zo ₂ =Zo ₁ and β ₁ =β ₂ (that is, K=1). Therefore its impedance
From equation 1, Z′ is Z′=Zo ₁・tanβ ₁ l ₁ +tanβ ₂ l ₂ /1−tanβ 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 the impedance was inverted by setting ltotal to λ/4.

On the other hand, according to the configuration of the present invention, the characteristic impedance is Zo ₂ > Zo ₁ according to the configuration requirements, so the value of the characteristic impedance ratio K in equation 1 is necessarily 1.
Become bigger. In order to make impedance Z infinite, the denominator of equation 1 needs to be zero, so 1=
It is sufficient to satisfy Ktnβ 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 reversal 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 FIGS. 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 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. 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, and the reason why the radio wave does not reach end D is In the case of Figure 4b, the groove depth l _T is approximately 80% of a quarter wavelength.
(full track at the end of λ/4), and even if it is longer or shorter (in the case of Figure 4 a and c), the 4th
Radio waves leak more than in Figure b. This is l ₁ = l ₂
This can be confirmed by substituting =l _T /2=λ/10.2, K _{=b 2} /b ₁ =2 into 1≒ Ktanβl ₁・tanβl ₂ .

The idea of making the characteristic impedance discontinuous is as follows.

The present invention has a structure in which conductor plates having a width dimension a are arranged in a groove portion of a sealing device with one side serving as a ground conductor and spaced apart by a distance b.

In detail, the width on the groove opening side is a ₁ = b ₁ The effective permittivity is εeff, and the width on the groove short circuit side is a ₂ = b ₂ The characteristic impedance ratio K is as follows. Calculate with the formula, 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, showing the patching plate 2.
A door 22 having a number 1 is attached to the 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 line AA in FIG. 5, and FIG. 7 shows a perspective view of FIG. 6.

In FIGS. 6 and 7, the door plate 27 forming the groove 26 has a wall surface on the outer peripheral edge side of the groove bent into a substantially U-shape, and consists of portions a, b, c, e, and f. The open end of the groove 26 is indicated by 28, the short-circuited end by 29, and the open-hole side groove and the short-circuited side groove are respectively indicated by . The side groove of the opening is filled with a dielectric material 30, the dielectric constant of which is εr. A door cover 31 covers the outer surface of the door. The d, e, and f portions of the door plate 27 that constitute the outer peripheral side wall surface of the groove have a width a ₁ , and the c portion has a width
In a ₂ , the pitch is p. The distance between part a and f is b ₁ , and the distance between part a and c is b ₂ . 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.

FIG. 8 shows how the groove depth changes when impedance reversal occurs due to the difference in the ratio of the depth l ₁ of the open hole side groove and l ₂ of the short circuit side groove in the embodiment of the present invention. This is what is shown. The horizontal axis shows the percentage of the depth l ₁ of the hole side groove in the total groove depth (l ₁ + l ₂ ), and the vertical axis shows the groove depth where impedance reversal occurs. In other words, the value of (l ₁ +l ₂ ) that satisfies the above-mentioned equation 1=Ktanβ ₁ l ₁ ·tanβ ₂ l ₂ is shown normalized by the lowest value. Here, the value of K is 6, and εr is the dielectric constant of the dielectric filled in the side groove of the opening. As is clear from this graph, when the ratio of l ₁ to l ₂ is approximately 1:1, the depth of the groove is minimized, and the size of the seal structure can be optimized.

FIGS. 9a and 9b show an example in which the opening side of the groove is filled with a dielectric material when the value of K is 1,
FIG. 10 shows the substantial groove depth reduction effect at that time. It has been known for some time that the cavity can be reduced by filling the groove with dielectric material, but
From the graph in FIG. 10, it can be seen that filling the dielectric material to about half of the opening of the groove produces almost the same effect (within 1.1 times the minimum value). Therefore, if the opening side of the groove is filled with dielectric material,
A large reduction effect can be achieved with a small amount of material.

It goes without saying that what has been described above applies not only to the high frequency of 915MHz but also to other frequencies such as 2450MHz. The sealing device may be formed by bending a sheet metal or may be formed by plating a plastic resin with metal.

Essentially, error factors in the impedance inversion method that satisfy 1=Ktanβ ₁ l ₁ tanβ ₂ l ₂ will be described.

When applying the present invention to actual products, it is not uncommon to provide a space TOP1 for a groove cover and a space _lx1 for bending reinforcement. Compared to the case where the principle is explained, radio waves are disturbed and the dimensions are slightly deviated from those calculated according to the above formula.
The details of the deviation are shown below.

When the dimension of TOP1 is 2mm and lx ₁ is 5~6
An example is shown when it is set to mm.

Figure 11 is an example of a 915MHz sealing device considered at the top.
This shows the relationship in which the groove depth l _T changes with dimension 1.
If the dimension of TOP1 is set to 1 to 3 mm, l _T becomes deeper by 1 to 6 mm.

Figure 12 is an example of a 2450MHz sealing device.
The relationship is shown in which the groove depth l _T changes with the reinforcing space lx ₁ while fixing TOP1 = 2 mm. Space lx ₁ to 2
By setting it to 6 mm, the groove depth l _T becomes deeper by 1 to 3 mm.

Effects of the Invention As is clear from the examples, in addition to the effect of achieving miniaturization, which is the object of the invention, the following effects are obtained.

(1) It has a simple structure with a folded door plate and is small, making it suitable for cost reduction.

(2) Since the outside is covered with a door cover, the exterior is finished neatly and at the same time the seal structure is reinforced.

(3) Since the depth of the groove is not limited to 1/4 of the frequency used, it becomes easier to design with consideration to design and strength.

(4) The bent part can also be used as a dielectric cover holder.

[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.
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, and FIG. 8 is a characteristic diagram of the structure of the groove depth, FIGS. 9a and b are sectional views of a radio wave sealing device partially filled with dielectric material, and
The figure is a characteristic diagram of groove depth and dielectric filling amount of the radio wave sealing device in Figure 9, and Figure 11 a, b, and c are 915MHz.
Sectional view, front view, and characteristic diagram of the sealing device, No. 12
Figures a, b, and c are a cross-sectional view, a front view, and a characteristic diagram of a 2450MHz sealing device. 22...door, 26...groove, 28...opening, 29...
Short circuit part, P... pitch, a, b, c, e... wall group,
... Opening part gutter, ... Short circuit part gutter, 30... Dielectric material.

Claims (1)

[Scope of 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 a portion of the main body and the door facing each other is provided. A small groove is provided on one side, and the groove has an opening and a short circuit, and at least one wall surface of the groove is a line group consisting of a group of walls such that the conductor width is smaller than the pitch in the longitudinal direction of the groove. Each line of the line group is composed of an opening side groove and a short circuit side groove, the groove width of the groove opening part is smaller than the groove width of the short circuit part, and the conductor width of the groove opening part is made smaller than that of the short circuit part. is larger than the conductor width of the short-circuited part, and the effective permittivity of the groove opening part is made larger than the effective permittivity of the short-circuited part. A radio wave seal having a characteristic impedance smaller than that of the short-circuit groove, a groove depth substantially shorter than a quarter wavelength, and the depths of the open-hole groove and the short-circuit groove being approximately equal. Device. 2. The radio wave sealing device according to claim 1, wherein a dielectric material is filled on the opening side of the groove, and a gap is provided on the short circuit side.