JPH0136717B2 - - 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, 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 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/ _√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 individualized 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 currently assigned internationally for high-frequency cooking (Iudustial, 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 you use a solid-state oscillator to oscillate at the same frequency of 2450MHz, you will not be able to obtain sufficient output power and the power will be insufficient. Therefore, in order to obtain the desired output power, it is necessary to select a lower frequency, for example, 915M
Hz 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, 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
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=
0) 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 |=∞. When the impedance Z _B of the opening B is viewed from the line starting point A, the impedance Z _A is Z _A =-J2op1/tan2π/λl _p . Here, by choosing l _p =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 dielectric constant ε _r into the leakage path 17 and the chiyoke groove 18, the wavelength λ' becomes the free space wavelength λ.
_However , the same 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, resulting in a shape smaller than the size equivalent to a quarter wavelength. It is something.

DESCRIPTION OF THE EMBODIMENTS The present invention provides, for example, at least two grooves in at least one of the main body or the door of a microwave oven, and the shape of the grooves 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 , ll ₁ , β ₁ . 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 (groove depth) 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・tan
β 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
Z′ is calculated from equation 1 as Z′=Zo ₁・tanβ ₁ l ₁ +tanβ ₂ l ₂ /1−tanβ ₁ l ₁・tan
β 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 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 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 make this principle easier to understand, part of the analysis results are shown in FIG. Figure 4 shows D with terminal A as the excitation source.
A groove having an opening B whose tip C is short-circuited is provided in a part of the transmission line with an open end. 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 lT is changed, the electric field in the transmission line changes as a, b, c, and D
The radio wave does not reach the end in case b, that is, when the groove depth lT is about 80% of a quarter wavelength (less than λ/4 line), and even if it is longer or shorter than that, (In cases a and c), radio waves leak more than in case b. This is l ₁ = l ₂ = lT/2 = λ/10.2,
This can be confirmed by substituting 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 configuration 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 using 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 application, the space of the groove cover
TOP1 is often provided with a bending reinforcement space lX1. 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~
An example is shown when the width is set to 6 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 will be 1 to 6 mm.
It gets deeper.

Figure 6 is an example of a 2450MHz sealing device.
The relationship is shown in which the groove depth lT changes with the reinforcing space Xl1 while fixing TOP1 = 2 mm. By setting the space lX1 to 2 to 6 mm, the groove depth lT becomes deeper by 1 to 3 mm.

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 AA in FIG. 7, and FIG. 9 shows a perspective view of FIG. 8. Figures 8 and 9
In the figure, a sealing plate 26 having a bent portion a faces the first groove, and a first groove 35 and a second groove 36
The conductor plate group 27 that partitions the area consists of parts b, c, d, and e, which are bent into a U-shape as a whole.

The groove cover 28 that covers the first groove 35 and the second groove 36 also consists of f, g, and h portions having the same shape. The opening side grooves of the first and second grooves 35 and 36 are,
The short-circuit groove side groove is indicated by .

The open ends and shorted ends of the first and second grooves 35 and 36 are indicated by 29, 30, 31 and 32, respectively. The punching plate 21 and the door 22 are fastened together with a stopper 33 and screws 34.

The conductor plate 27 is composed of portions c, d, and e having a pitch p and a width a ₁₁ and a portion b having a width a ₁₂ . First groove 3
The gaps between the bent portions a and c of the second groove 36, and the gaps between the bent portions b and the door 22 are b ₁ A and b ₁₂ , respectively, and the gaps between the door wall of the second groove 36 and the e and b portions are b 21 and b ₂₁ , respectively. b ₂₂ . Therefore, the characteristic impedance ratio K ₁ in the first groove 35 is Then, the value of the second groove K ₂ is In both cases, by making K ₁ and K ₂ larger than 1, the groove depths (l ₁₁ + l ₁₂ ) and (l ₂₁ + l ₂₂ )
is configured to be smaller than a quarter wavelength.

FIG. 10 shows measurement examples in FIGS. 8 and 9. Measurements were made at 915MHz with a gap of 2mm between the main body and the door. Dimensions l ₁₁ and l ₂₁ are both 15 mm, b ₁₁ and b ₂₁ are both 5 mm, b ₁₂ and b ₂₂ are both 15 mm, width a ₁₁ and a ₁₂ are 40 mm and 5 mm respectively, pitch p is 50 mm, and conductor plate d is The dimensions are approximately 10 mm, and the dielectric cover is made of ABS resin with a thickness of 2 mm. The vertical axis of the graph is the measured leakage value on a logarithmic scale, and the horizontal axis is the depth of the first and second grooves 35 and 36, scaled as a ratio to the wavelength used.

As is clear from this characteristic diagram, it is shown that the depth of the groove can be further reduced to about 1/4 compared to the conventional λ/4. The important points in the configuration are the dimensions of the C part of the conductor plate, the dimensions of the e part, and the bent part of the sealing plate (a
Part) The reason is that the dimensions are almost the same.

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

(1) Since the dimension b ₁₁ is smaller than b ₁₂ at the bent portion of the sealing plate, the groove portion of the door can be manufactured without undercutting.

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

(3) By making the characteristic impedance ratios of the first groove and the other grooves equal as in the embodiment, the sealing performance can be improved compared to a single groove.

(4) By changing the characteristic impedance ratio of the first groove and other grooves, for example, one can be used for 2450MHz,
The other is for 915MHz, and by setting the impedance inversion frequency of each groove, a high-frequency heater with 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 sufficiently prevent both types of radio wave leakage.

(5) By loading a dielectric material on the opening side of the groove, the phase constant on the opening side can be increased, which can also contribute to reducing the size of the groove.

[Brief explanation of drawings]

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 cross-sectional views and side views of the device at 915MHz;
Characteristic diagrams; Figures 6a, b, and c are cross-sectional views, side views, and characteristic diagrams of the device at 2450 MHz; Figure 7 is a perspective view of a general microwave oven; Figure 8 is a diagram of radio waves in an embodiment of the present invention. A cross-sectional view of the sealing device, FIG.
The perspective view of the figure and FIG. 10 are characteristic diagrams of the device. 21... Door, 26... Sealing plate, 27... Conductor plate, 35... First groove, 36... Second groove, a...
...bending part.

Claims (1)

[Scope of Claims] 1. A main body having an opening and into which radio waves are supplied is provided, and a door is provided to cover the opening of the main body so as to be openable and closable, and at least the portion where the main body and the door face each other is provided. A first groove and a second groove are provided in parallel on one side, and a sealing plate that covers a part of the opening of the first groove has the dimensions of the opening portion and the bent portion approximately equal, and the dimensions of the opening portion and the bent portion are approximately equal. A conductor plate is provided in the groove, has a substantially U-shape, covers a part of the opening of the second groove, and has cut portions periodically in the longitudinal direction of the groove,
A radio wave sealing device in which the bottom of the conductive plate is made thinner so that the characteristic impedance of the opening of the second groove is smaller than the characteristic impedance of the bottom. 2. The radio wave sealing device according to claim 1, wherein the openings of the first groove and the second groove have substantially the same characteristic impedance ratio of the short-circuit portion. 3 Claims in which the ratio of the opening to the short-circuit portion of each groove is determined so that the first groove absorbs radio waves of a first frequency and the second groove absorbs radio waves of a second frequency. The radio wave sealing device according to item 1.