JPS6316872B2 - - 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 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 correspond to 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 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 driving voltage of a magnetron is approximately 3KV, whereas the driving voltage of a solid-state oscillator using a transistor etc. may be approximately 400V or less, and in reality it is 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, etc., 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, this frequency is about 1/2.7 compared to the conventional frequency, so
Conversely, the wavelength is approximately 2.7 times, and a quarter wavelength is approximately 80
It becomes 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 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. 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 ZB=jZoctan
2π/λlc. 19 is the heating chamber of the microwave oven, and 20 is the 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 |=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 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, which has a shape smaller than the size equivalent to a quarter wavelength by adopting a characteristic impedance discontinuous configuration of the leakage path and the groove. It 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. , 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, 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・tan
β ₂ l ₂ …(1) (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 (ie, 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≠1. In order to make the impedance Z infinite, the denominator of equation 1 needs to be zero, so 1=Ktanβ ₁ l ₁・
It is sufficient to satisfy tanβ ₂ l ₂ , and when the value of the characteristic impedance ratio K is made larger than 1, impedance inversion similar to the conventional one can be achieved even if the dimensions l ₁ and l ₂ are made smaller.

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 with a short-circuited tip is provided in a part of a transmission line whose end is an excitation source and whose end is open. The width of the groove on the short circuit side is twice that on the open hole side. When the point is excited under the same conditions and the _depth of the groove is changed, the electric field within the transmission line changes as shown in Figure 4 a, b, and c. The reason why the radio wave does not reach the end is In case 2, the groove depth l _T is approximately 80% of a quarter wavelength.
(line less than λ/4), and whether 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.

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 with 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.

In actual applications, a groove cover space (Top 1) and a bending reinforcement space (lx ₁ ) are often provided. 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 5~6mm
An example is shown below.

Figure 5 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 6 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 (lx ₁ ) with Top1 = 2 mm fixed. Space lx ₁ to 2
By setting it to ~6 mm, the groove depth l _T becomes 1 to 3 mm deeper.

The details of the embodiment will be explained based on the drawings. FIG. 7 is a perspective view of a microwave oven in which a door 22 having a patching plate 21 is attached to a main body covered with a main body cover 23. FIG. The main body is provided with an operation panel 24, and a door handle 25 is attached to the door. 8a and 8b respectively show a sectional view along the line AA and BB in FIG. 7, and FIG. 9 shows a perspective view of the conductor portion constituting the groove. In FIG. 8a, the tip of the conductor piece 22a forming the groove 26 is bent into a U-shape, forming the wall surface of the opening side groove having a groove width _b1 and the shorting side groove having a groove width _b2 . A groove cover 27 covers the open end of the groove 26. an open end of the groove 26;
The shorted ends are indicated at 28 and 29, respectively. The outside of the door is covered with a door cover 30. In FIG. 8b, the conductor piece 22b has the same shape as the conductor piece 22a, and forms the wall surfaces of the opening side groove and the short-circuit end side groove with a groove width _b3 . As shown in FIG. 9, the conductor pieces 22a and 22b have conductor widths a ₁ and a ₃ at the open end groove portion, respectively, and conductor widths a ₂ and a ₄ at the short-circuit end groove portion, respectively.

Therefore, the characteristic impedance ratio K ₁ in the groove shown in cross section A-A is The characteristic impedance ratio K ₂ in the groove shown in cross section B-B is In both cases, by making K ₁ and K ₂ larger than 1, the groove depths (l ₁ + l ₂ ) and (l ₃ + l ₄ ) can be reduced to 4.
The wavelength is smaller than 1/1 wavelength.

10a and 10b show other embodiments of the present invention, and parts corresponding to those in FIG. 9 are designated by the same reference numerals. In the embodiment shown in FIG. 10a, a sealing plate 31 faces the opening of the groove. In FIG. 10b, the groove width is changed by changing the shapes of the conductor pieces 22a and 22b.

It goes without saying that the configuration of the radio wave seal of the present invention 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.

The values of K ₁ and K ₂ are a ₁ , a ₂ , a ₃ , a ₄ and b ₁ , b ₂ ,
It is obvious that it can be set arbitrarily by adjusting b ₃ and b ₄ . Therefore, by appropriately selecting K ₁ and K ₂ , one groove can have a sealing effect for two or more types of frequencies. For example, if K ₁ ≒ 1, K ₂ ≒ 6, and l ₁ = l ₂ = l ₃ = l ₄ ≒ 1/16 wavelength, a seal structure for two frequencies, the fundamental wave and the double frequency, can be achieved. , 1=
It can be easily derived from the relational expression Ktanβ ₁ l ₁・tanβ ₂ l ₂ .

Effects of the Invention According to the present invention, as described above, impedance inversion can be performed with a line of 1/4 wavelength or less, so it is possible to realize the miniaturization of the radio wave sealing device, which is the purpose of the invention.
In addition, the following effects occur.

(1) A compact and high-performance sealing device can be provided by configuring the fundamental wave and double frequency wave to be prevented from leaking in one groove.

(2) 915MHz by changing the characteristic impedance ratio of the groove
By setting the impedance inversion frequency to 2450MHz, 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.

(3) Since the depth of the groove of the radio wave seal device is not limited to 1/4 wavelength, it becomes easy to design with consideration to strength and design.

(4) The structure of the sealing device is simple and compact, making it suitable for cost reduction.

[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.
5A, B, C and 6A, B, C are cross-sectional views and front views illustrating dimensional calculation error factors, respectively. Characteristic diagram, FIG. 7 is a perspective view of a general microwave oven, FIGS. 8 a and b are sectional views of a radio wave sealing device in an embodiment of the present invention, and FIG. 9 is a groove portion in FIG. 8. Perspective view of a conductor,
FIGS. 10a and 10b are perspective views of other embodiments of the invention, respectively. 22...Door, 22a, 22b...Conductor piece, 26...
Groove, a ₁ , a ₂ , a ₃ , a ₄ ... conductor width, b ₁ , b ₂ , b ₃ , b ₄
…Groove width.

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 one or more A groove is provided, and at least one wall surface of the groove is constituted by a group of conductor pieces that are arranged continuously at intervals in the longitudinal direction of the groove and parallel to the wall surface of the groove, and the conductor pieces are arranged so that the groove width is periodic. The conductor line is constructed by arranging the conductor so that it changes to
A radio wave seal in which at least one of the permittivity, the conductor width, and the groove width is changed in the groove to periodically change the ratio of the characteristic impedance of the opening part of the groove to the characteristic impedance of the short-circuited end of the groove. Device.