JPS6340036B2 - - 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 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 =jZo tan (2πL/λo) (λo is the free space wavelength).

The chi-yoke type radio wave attenuation means is based on the principle that |Z _IN |=Zo tan (π/2)=∞ is achieved by selecting the depth L 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'=L'/4, and in the chi-yoke method, the depth cannot be made substantially smaller than a quarter 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 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 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 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 compared to 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 has the same effect as a chiyoke part of a quarter wavelength. 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
l _p and the wavelength used is λ, the short-circuit impedance (Zc
=0) at the opening B of the chain yoke groove 18, Z _B =jZoc tan2π/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 =-JZop1/tan2π/λl _p . Here l _p =λ/
By choosing 4, it can be converted to |Z _A |=0. The short-circuit condition at the bottom 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 , 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-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 +K tanβ ₂ l ₂ /1−K tanβ ₁ l
₁・tanβ ₂ l ₂ ...(1) (However, K=Zo ₂ /Zo ₁ ) can be derived by simple calculation. In the conventional example, Zo ₂ = Zo ₁ , 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・l total) ...(2), and by setting l total to λ/4, the impedance is inverted. Ta.

On the other hand, according to the configuration of the present invention, the characteristic impedance Zo ₂ >Zo ₁ is satisfied from 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=K
It is sufficient to satisfy tanβ 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. Fourth
In the figure, a groove having an opening B with a short-circuited tip C 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. Excite point A under the same conditions, and measure the depth of the groove.
l When _T 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, the depth of the groove l _T is 1/4
When it is about 80% of the 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 =
l _T /2=λ/10.2, K= _{b 2} /b ₁ =2 to 1≒K
This can be confirmed by substituting tanβl ₁ and 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 applications, a groove cover space (TOP ₁ ) and a bending reinforcement space (l _K1 ) are often provided. 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 TOP ₁ is 2mm and l _X1 is 5~6mm
An example is shown below.

Figure 5 is a top ₁ example of a 915MHz sealing device.
This shows the relationship in which the groove depth l _T changes with the dimensions of .
If the dimension of TOP ₁ is set to 1 to 3 mm, l _T will become 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 with the reinforcement space (l _X1 ) with TOP ₁ = 2 mm fixed. Space l _X1 2~
By setting it to 6 mm, the groove depth l _T 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.

In FIGS. 8 and 9, the radio waves from the heating chamber 26 leak to the outside through the gap between the main body 23 and the door 22.

A recess 27' extending toward the periphery of the main body is formed in the door at a position facing the main body member. The tip of the recess forms a bent portion 28 .

Also, when assembled, the opening side of the groove 27 is B,
The short circuit side will be called C.

One wall surface constituting the groove 27 has a line width of the opening.
Conductor plates 29-a, 29-b in which a ₁ is larger than short-circuit part line width a ₂ and have a bent part at the tip on the opening side
It is composed of a conductor plate wall member 31 in which a plurality of conductor plates arranged at a pitch P in the longitudinal direction of the groove are arranged in a common part 30.

The cover 32 of the groove 27 is made of a microwave-transmissive low-loss dielectric material, and is inserted and fixed into a hole 33 provided in the door 22.

The conductor plate wall member 31 is fixed by means such as welding near the bent portion 28 of the door 22 in an orientation such that the opening groove width b ₁ is smaller than the short circuit groove width b ₂ . It is. Structurally, the present invention has a plurality of conductive plates arranged in the longitudinal direction of the groove, so the impedance changes periodically in terms of radio waves, and the so-called periodic circuit effect prevents radio waves from propagating in the longitudinal direction. It can be done. In other words, for radio waves in the direction of crossing the groove, the new 1/
Utilizing the impedance inversion principle of less than 4 wavelengths,
Since a periodic circuit is used in the longitudinal direction of the groove, leakage of radio waves from inside the refrigerator can be effectively prevented.

Effects of the invention In addition to being able to downsize the radio wave sealing device, the invention has the following effects.

(i) Since the number of grooves is one, it exhibits stable sealing performance even when the gap between the main body and the door changes.
(Multiple grooves often become unstable due to interference between the grooves.) (') The bent part of the door can be used as a surface for attaching the connecting member between the door and the main body.

(ii) Since the folded part of the door is shorter than the depth of the groove, material is saved and the door can be opened and closed smoothly.

(iii) Since one groove is formed in the entire concave portion of the door, the groove width dimension _b2 can be secured to fill the concave portion, making it possible to realize a compact sealing groove with a high Q.

(iv) The overall strength of the door is strong because of the bends.

(v) Since the conductor plate wall member has a common part, its strength is maintained relatively even before it is fixed to the door.

In addition, in the example, a recess is provided in the door member and a conductive plate made of a separate member is fixed to it.
It goes without saying that in the present invention, the recess and the conductor wall member may be made of a common metal sheet metal.

[Brief explanation of the drawing]

Fig. 1, Fig. 2 a, b, and Fig. 3 are cross-sectional views showing conventional radio wave devices, Fig. 4 a, b, and c are electric field analysis diagrams of the groove portion in the present invention, and Fig. 5 a, b, c. teeth
Cross-sectional view, side view, characteristic diagram of the device at 915MHz,
Figures 6a, b, and c are cross-sectional views, side views, and characteristic diagrams of the device at 2450MHz, Figure 7 is a perspective view of a general microwave oven, and Figure 8 is a radio wave sealing device according to an embodiment of the present invention. The sectional view and FIGS. 9a and 9b are enlarged perspective views of the same essential parts. 27...Groove, 27'...Concave portion, 28...Bended portion at the tip of the concave portion, B...Opening portion of the groove, C...Short circuit portion of the groove, 29-a, 29-b...Conductor plate, 30... …
Common part, 31... Conductor plate wall member.

Claims (1)

[Claims] 1. A main body having an opening and to which radio waves are supplied,
A door is provided to cover the opening of the main body so that it can be opened and closed, and a plurality of conductor plates are arranged on the door, the line width of the opening being larger than the line width of the short circuit part, and having a bent part at the tip on the side of the opening. A radio wave sealing device having a structure in which a conductor plate wall member having a common part is bent in a direction in which the groove width of the opening part is smaller than the width of the shorting part groove. 2. The radio wave sealing device according to claim 1, wherein the door is provided with a recessed portion having a bent end portion shorter than the depth of the groove, and the conductor plate wall member is fixed to the recessed portion.