JPS6316868B2 - - 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 cheese 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 of the chi-yoke part to be one-quarter 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 becomes short as L'≒L/ _√r . However, there is no difference in setting L' = λ'/4, and in the chi-yoke method, the depth cannot be made substantially smaller than 1/4 wavelength, and there is a limit to the miniaturization of the chi-yoke part. 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 practice, 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 (Industrial, Scientifia,
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 time. 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 pitch of standing waves is correlated with wavelength. When using 915MHz, 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 Z _B =jZoctan2π/λ. 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 | =∞. The impedance when looking at the impedance Z _B of this opening B at the line starting point A
Z _A becomes 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 , 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 , l ₁ , and β ₁ . The characteristic impedance and length phase constant of the groove short-circuit part are Zo ₂ , l ₂ , β ₂ The distance from the open end of the groove to the short-circuit 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 · 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 (ie, 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.

According to the configuration of the present invention, the characteristic impedance satisfies Zo ₂ >Zo ₁ according 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 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 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 one of the transmission lines with the A end as the excitation source and the D end open.
A groove having an opening B whose tip C is short-circuited is provided in the section. The width of the groove on the short circuit side is twice that on the open hole side. Excite point A under the same conditions, and measure the depth of the groove.
When lT is changed, the electric field in the transmission line is a,
It changes as shown in b and c, and the radio wave does not reach the D end in case b, that is, when the groove depth lT is about 80% of a quarter wavelength (less than λ/4 line). ,
Even if it is longer or shorter than that (in the case of a, c),
Radio waves leak more easily than in b. This is l ₁ = l ₂ =
lT/2λ/10.2, K=b ₂ /b ₁ =2, 1≒Ktanβl ₁・
This can be confirmed by substituting 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 short side of the groove is a, ₂ the gap is b ₂ , and the characteristic impedance ratio K is calculated using the following formula. calculate, By setting the K value 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. FIG. 5 is a perspective view of a microwave oven, in which a door 22 having a batching plate 21 is attached to a main body covered with a main body cover 23. FIG. An operation panel 24 is provided on the main body, 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 view taken along line BB. The main body 27 surrounding the heating chamber 26 has an open front surface. A door 29 having a protrusion 28 into the heating chamber is provided. The door 29 is the base plate 30
It consists of a sealing plate 31 and the like, and forms a groove 32.
The trench has a dielectric material 33 filling the opening.

The conductive lines 34 have an opening line width a and a short side line width _a2 and are periodically arranged at a pitch P.

In this case, the characteristic impedance ratio K of the groove is K=a ₁ /a ₂ ×√ _r (>1), and the impedance can be inverted when the groove depth l is shorter than λ/4.

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

(1) Since one of the groove walls is composed of a group of conducting lines, it is possible to prevent radio waves from propagating in the direction of the conducting line, improving sealing performance.

(2) The configuration in which the groove is installed in the heating chamber rather than the opening plane of the heating chamber keeps the leakage characteristics relatively low when the door is opened. However, the only drawback was that the effective volume of the heating chamber became small, but the present invention can reduce this drawback.

(3) The door can be opened easily because the heating chamber has a small protruding dimension, and the clearance between the main body wall and the door side, which was required for conventional doors of this type, can be reduced. Therefore, the amount of leakage can be further reduced.

[Brief explanation of the drawing]

Figures 1, 2a, b, and 3 are cross-sectional views of conventional radio wave sealing devices, Figures 4a, b, and c are electric field analysis diagrams of the groove in the present invention, and Figure 5 is a typical electronic A perspective view of the microwave oven, and FIGS. 6 and 7 are cross-sectional views of a radio wave sealing device according to an embodiment of the present invention. 28...Protrusion, 29...Door, 32...Groove, 34...Conducting path, l...Depth of groove.

Claims (1)

[Claims] 1. A main body having an opening and a heating chamber 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 when the door is closed, the heating chamber is heated from the plane of the opening. It has a protruding part that projects into the room, a groove is provided in at least one of the main body or the door and has an opening on the surface where the main body and the protruding part face each other, and one of the groove walls is composed of conductive paths arranged in the direction around the door. , by changing at least one of the dielectric constant conductor width and the groove width within the groove so that the characteristic impedance of the shorted part is larger than the characteristic impedance of the open part, the depth of the groove is adjusted to 4 A radio wave seal device that reverses impedance by less than 1/2.