JPS6316873B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application 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, and food 12 is heated 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 with 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 1/4 wavelength.

If a dielectric material (relative permittivity ε _r ) is filled in the cheese yoke, the wavelength λ' of the radio wave 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 magnetron is 5,000 hours, a solid-state oscillator has a long lifespan of about 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 about 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 this range. The current magnetron is as described above.
It is oscillating at 2450MHz, but if a solid-state oscillator were to oscillate at the same frequency of 2450MHz, it would 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; for example, 915 MHz is suitable. 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, 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 deeper into the frequency range, 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 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 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 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 entire microwave oven becomes expensive. Moreover, the manufacturing time and cost were high, which hindered its practical application.

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
l _c , 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
_lpWhen the used wavelength is λ, the short circuit impedance (Zc=
O) is the opening B of the chiyoke groove 18 and Z _B = jZoctan
2π/λl _c . 19 is the heating chamber of the microwave oven, and 20 is the door. Here, by choosing l _c =λ/4, it can be converted to |Z _B |=∞. When the impedance Z _B of the opening B is viewed from the line starting point A, the impedance Z _A is Z _A =-jZop1/tan2π/λl _p . Here, l _p =λ/
By choosing 4, it can be converted to |Z _A |=O. The short-circuit condition at the lower part C of the chain yoke 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 dielectric constant ε _r into the leakage path 17 and the chiyoke groove 18, the wavelength λ' becomes the free space wavelength λ.
However, 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 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, so that the size and shape are different from each other, which is equivalent to a quarter wavelength. It is something.

DESCRIPTION OF THE EMBODIMENTS The present invention provides 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 changes the characteristic impedance on the short circuit side from that on the open side. The depth of the groove from the to the short-circuit end is not limited to a quarter wavelength of the operating frequency, and grooves with different characteristic impedance ratios of the opening part and the short-circuit part are arranged periodically in the longitudinal direction of the groove. It has particular characteristics.

The basic idea that the depth of the groove is not limited to 1/4 wavelength is as follows.

Let the characteristic impedance and length phase constant of the groove opening be Zo ₁ , l ₁ , and β ₁ . Let the characteristic impedance and length phase constant of the groove short-circuit portion be Zo ₂ , l ₂ , and β ₂ . The distance from the open end of the groove to the shorted 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−Ktanβ ₁ l ₁ · tanβ
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 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 is Zo ₂ ≠ Zo ₁ due to the configuration requirements, so the value of the characteristic impedance ratio K in equation (1) is K≠
It is 1. To make impedance Z infinite, the denominator of equation (1) should be zero, so 1=
It is sufficient to satisfy Ktanβ ₁ l ₁・tanβ ₂ l _2. For example, when the value of the characteristic impedance ratio K is made larger than 1, even if the dimensions l ₁ and l ₂ are made smaller, the same impedance inversion as before is achieved, and ltotal does not necessarily require a length equivalent to λ/4. In other words, if the characteristic impedance ratio K is changed while keeping the dimension ltotal constant, the wavelength at which impedance inversion occurs will change.

In the present invention, taking advantage of this fact, grooves having different characteristic impedance ratios K are periodically arranged in the longitudinal direction of the chiyoke grooves to prevent radio wave leakage of two or more types of frequencies.

The present invention performs impedance inversion on a line other than the λ/4 line historically used in the field of radio wave seals. In order to make this principle easier to understand, some of the analysis results are shown in Figure 4 a, b,
Shown in 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, and the radio wave does not reach end D. is 2, that is, when the groove depth l _T is about 80% of a quarter wavelength (less than λ/4 line), and even if it is longer or shorter than that (Fig. 4a, (c), the radio waves leak more than in Fig. 4b. This can be confirmed by substituting l ₁ =l ₂ =l _T /2=λ/10.2 and K =b ₂ /b ₁ =2 to 1≈Ktanβl ₁ ·tanβl ₂ .

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

In the present invention, the structure of the sealing device has a configuration in which conductor pieces having a width dimension a are arranged with one side serving as a ground conductor and separated by a gap dimension b.

In detail, the width on the groove opening side is _a , the gap is b _{, 1} the effective permittivity is ε _l ff, the width on the groove shorting side is a, ₂ the gap is b ₂ , and the characteristic impedance ratio K is as follows. Calculate with the formula, By setting the value of K to K≠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 has an operation panel 24
A door handle 25 is attached to the door. Figure 6 shows a sectional view taken along line A-A in Figure 5, and Figure 7.
The figure shows a sectional view taken along line B-B. There are two types of wall surfaces forming the groove 26, which are arranged alternately in the longitudinal direction of the groove, as shown in FIG. Figure 6,
In FIG. 8, the conductor piece 22a, which is one wall surface forming the groove, is bent into a substantially U-shape, and
Consists of e and f parts. The open end of the groove is indicated at 27 and the shorted end at 28. Groove cover 2 that covers the opening side of the groove
9 is provided with claws 30 for preventing slipping off at certain intervals. The outside of the door 22 is covered with a door cover 31 made of synthetic resin. The conductor piece 22a has a width _a1c ,
It consists of parts d and e and part f with a width a ₂ , and the gap between the bent part C of the conductor piece and the door 22 is b ₁ and the part f and the door 2.
The gap between 2 and 2 is b ₂ . Therefore, the characteristic impedance ratio K _a of the open hole side groove ( ) and the short circuit side groove ( ) in the groove 26 is becomes.

When the value of K _a is 1, this groove acts as a chiyoke portion for radio waves having a wavelength equivalent to approximately four times the depth of the groove (l ₁ +l ₂ ). When K _a is larger than 1,
It corresponds to radio waves with longer wavelengths, and when _Ka is made smaller than 1, it works as a chiyoke groove that corresponds to radio waves with shorter wavelengths. In this example, we set K _a to be greater than 1,
It is configured to be effective against radio waves with a wavelength longer than four times the depth of the groove (l ₁ +l ₂ ).

In FIGS. 7 and 8, the conductor piece 22b on one wall of the groove 26 has a bent tip.
In this example, the open end side of the groove () and the shorted end side ()
The ratio of characteristic impedances K _b is approximately 1. Therefore, the structure is such that it functions as a chiyoke groove for a wavelength equivalent to approximately four times the groove depth (l ₁ +l ₂ ).

In this way, the structure is such that a single groove with a constant depth can prevent leakage of radio waves of different wavelengths.

FIGS. 9a and 9b show examples of other shapes of the conductive plate constituting the wall surface of the groove. In FIG. 9b, a sealing plate is provided in the groove, and the formation of the two wall surfaces of the groove is periodically changed.

In the above example, the grooves are formed by bending a sheet metal, but it is also possible to form the grooves by plating plastic with metal. In addition, this seal structure
It goes without saying that not only the 915MHz radio wave seal but also 2450MHz and other high frequencies have the same effect.

As mentioned above, conventionally, in order to perform radio wave sealing of two or more wavelengths, for example, the fundamental wave and the second harmonic,
Items that require two or more grooves can be combined into one, and by increasing the value of K, the depth of the grooves can be reduced.

We will discuss the error factors of the impedance inversion method that essentially satisfies 1=Ktanβ ₁ l ₁ · tanβ ₂ l ₂ .

When applying the present invention to actual products, it is not uncommon to provide a groove cover space (TOP1) and a bending reinforcement space (l _x1 ). 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 discrepancy are shown below.

When the dimension of TOP1 is 2mm and I _x1 is 5~6mm
An example is shown below.

Figure 10 is an example of a 915MHz sealing device.
This shows the relationship in which the groove depth l _T changes with the dimensions of TOP1.
If the dimension of TOP1 is set to 1 to 3 mm, l _T becomes deeper by 1 to 6 mm.

Figure 11 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 (l _x1 ) with TOP1 = 2 mm fixed. Space l _x1 to 2
By setting it to 6 mm, the groove depth l _T becomes deeper by 1 to 3 mm.

Effects of the invention In addition to the above-mentioned effect of achieving miniaturization, which is the purpose of the invention, the following effects are produced.

(1) A radio wave sealing device can be provided with a simple configuration of changing the shape of the groove wall, contributing to cost reduction.

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

(3) The characteristic impedance ratio of the grooves, for example,
By setting the impedance inversion frequency to 2450MHz and the other to 915MHz, a high-frequency heater having two oscillation sources in the heating chamber can be realized.

Therefore, in a microwave oven, cooking can be done at a frequency of 915MHz when low power is required, such as when defrosting frozen food, and at a frequency of 2450MHz when high power is required, such as for high-speed cooking. Moreover, it is possible to provide a radio wave sealing body that can prevent both types of radio wave leakage.

(4) Since the depth of the groove is not limited, it is easy to design with consideration to design and strength.

(5) By installing a groove cover, the appearance can be finished beautifully.

[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 analytical diagram of the electric field in the groove according to the present invention, FIG. 5 is a perspective view of a general microwave oven, FIGS. 6 and 7 are cross-sectional views of a radio wave sealing device according to an embodiment of the present invention, and FIG. 6 and 7, FIGS. 9a and 9b are perspective views of a sealing groove in another embodiment of the present invention, and FIGS. 10a, b, and c are views of a 915MHz sealing device. Cross-sectional view, front view, characteristic diagram, Figure 11a, b, and c are cross-sectional views of the 2450MHz sealing device,
They are a front view and a characteristic diagram. 22...Door, 22a, 22b...Conductor piece, 26...
Groove, 27...Opening part of the groove, 28...Short circuit part of the groove.

Claims (1)

[Scope of Claims] 1. A main body having an opening and into which two types of radio waves having different frequencies are supplied, a door covering the opening of the main body so as to be openable and closable, and a door connecting the main body and the door. one or more grooves are provided in at least one of the opposing portions, at least one wall surface of the groove is constituted by a group of conductor pieces continuously installed at intervals in the longitudinal direction of the groove, and the groove is , for each conductor piece, on the open side of the groove and on the short circuit side of the groove,
At least one of the groove width, the conductor single width, and the effective dielectric constant is different from each other, and the conductor piece group includes conductor pieces having two or more different shapes arranged periodically in the longitudinal direction of the groove, and at least One type is a radio wave seal device in which the characteristic impedance of the opening of the groove is lower than that of the bottom, and has sealing characteristics against radio waves of periodically different frequencies. 2 Among groups of conductor pieces of two or more different shapes,
2. The radio wave sealing device according to claim 1, wherein the bottom of the conductor piece is narrowed so that the depth of the groove is shorter than one quarter of the wavelength used. 3 Two types of conductor pieces are arranged alternately in the longitudinal direction of the groove, and the ratio of the characteristic impedance on the opening side of the groove formed by each conductor piece to the characteristic impedance on the short-circuit side is changed to The radio wave sealing device according to claim 1, wherein the groove absorbs the fundamental frequency and the second harmonic.