JPH0138397B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD 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
Z ₀ and the depth is L, and when the terminal end is shortened, the impedance Z _IN at the opening of the chain yoke is Z _IN =jZ ₀ tan (2πL/λ ₀ ) (λ ₀ is the free space wavelength). The radio wave attenuation means of the chi-yoke method is based on the principle that by selecting the depth L of the chi-yoke part to be 1/4 wavelength, |Z _IN | = Z ₀ tan (π/2) = ∞ is achieved. . If a dielectric material (specific dielectric material ε _r ) is filled in the choke part, the wavelength λ' of the radio wave is compressed to λ'≒λ ₀ /√ _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 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. Currently, the ISM frequencies (Industrial,
Scientific, Medical) is 5880MHz, 2450MHz,
915MHz, 400MHz, etc., and must be used outside of this range. As mentioned above, current magnetrons oscillate at 2450MHz, but if a solid-state oscillator were used to oscillate at the same frequency (2450MHz), sufficient output power would not be obtained and the power would be insufficient. 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, Fig. 1,
The thickness of the chiyoke part explained in Figure 2 exceeds approximately 80 mm, and the effective size of the opening of the heating chamber is extremely small compared to the conventional example, which has the disadvantage of making it extremely difficult to put it into practical use. It is. 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 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 frequency used by the microwave oven is 915MHz.
The disadvantage of changing to 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. That is, when the characteristic impedance of the chiyoke groove is Z ₀ and the depth of the groove is l, and the terminal end is short-circuited, the impedance Z _io at the chiyoke groove opening is Z _io = jZ ₀ tan
(2πl/λ ₀ ). However, λ ₀ is the free space wavelength, and in the Chi-Yoke method, by selecting the groove depth l as λ ₀ /4 |
It is based on the principle of achieving Z _IN | = Z ₀ · tan (π/2) = ∞. When the inside of the chiyoke groove is filled with a dielectric material (relative permittivity ε _r ), the radio wave wavelength λ' becomes λ' = λ ₀ /V
is compressed to ε _r . In this case, the groove depth l′ is l′≒
It becomes short as l/V _r . However, there is no difference in setting l'=λ'/4. Therefore, in the Chi-Yoke method, the depth of the Chi-Yoke groove cannot be made substantially smaller than a quarter wavelength, and there is a limit to miniaturization. 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 EMBODIMENTS The characteristic impedance of the radio wave seal device will be described below with reference to FIGS. 3 and 4. FIG. 3 is a perspective view of a parallel line, where the line width is a, the line gap is b, and the dielectric constant of the dielectric medium is ε _r . As is well known, the characteristic impedance Z ₀ in this case is

[Formula] (K: constant of proportionality). Therefore, the characteristic impedance Z ₀ can be made small by widening the line width a, narrowing the line gap b, and increasing the relative dielectric constant ε _r . FIG. 4 shows an example of the structure of the door. In this case, door 1
A groove 21 having a groove width b is formed by wall surfaces 18 and 19 extending in the x direction provided at 7 and a conducting line group 20 having a width a and a peach P. In this case, the conductor line group 20 acts on the wall surface corresponding to the ground plane as a radio wave propagation system, but the characteristic impedance Z ₀ for each line is

[Formula] (K': constant of proportionality) holds almost the same relationship as in the case of parallel lines. The principle of the present invention will be explained using FIGS. 5 to 8. Figures 5a, b, and c show examples in which the impedance of 2, 3, and n small grooves is changed. The section of the characteristic impedance Z ₁₀ has a length l ₁ , the impedance seen from the impedance change point toward the end of the groove is Z _i , and the impedance seen from the groove opening toward the end of the groove is Z _{io o} . Specifically, in the case of Figure 5a where the groove is divided into two, Z ₂ = jZ ₂₀ tanβl ₂ = jX ₂ or less β is β = 2π/λ ₀ Z _io2 = Z ₁₀ Z ₂ +jZ ₁₀ tanβl ₁ /Z ₁₀ + jZ ₂ tanβl _1However , (Z ₁₀ <Z ₂₀ ) In the case of Figure 5 b, Z ₃ = jZ ₃₀ tanβl ₃ Z ₂ = Z ₂₀ Z ₃ +jZ ₂₀ tanβl ₂ /Z ₂₀ +jZ ₃ tanβl ₂ ≡jX ₃ Z _io3 =Z ₁₀ Z ₂ +jZ ₁₀ tanβl ₁ /Z ₁₀ +jZ ₂ tanβl _1However , (Z ₁₀ <Z ₂₀ <Z ₃₀ ) In the case of Fig. 5 c, Z _o = jZ _o0 tanβl _o Z _o-1 = Z _(o-1)0 Z _o +jZ _(o-1)0 tanβl _(o-1) /Z _(o-1)0 +jZ _o
tanβl _(o-1) However, (Z ₁₀ <Z ₂₀ …＜Z ₃₀ ) Z ₂ =Z ₂₀ Z ₃ +jZ ₂₀ tanβl ₂ /Z ₂₀ +jZ ₂ tanβl ₂ ≡jX _o Z _{io o} =Z ₁₀ Z ₂ +jZ ₁₀ tanβl ₁ /Z ₁₀ +jZ ₂ tanβl ₁ . Therefore, the impedance seen from the small groove hole is n
In the case _of discrete characteristic impedances _, Z _{io o} =Z ₀ Z _{2 + jZ 10} tanβl ₁ /Z _{10 +} jZ ₂ tanβl ₁ = _{jZ 10} The above formula is expressed as follows: If Z ₁₀ and X _o tanβl ₁ are equal, |
Z _{io o} ｜ means that it can be made = ∞. That is, Z ₁₀ =
It can be seen that X _o tanβl ₁ is a requirement for increasing the impedance at the groove opening. In the example of λ ₀ = 122.4 mm (f = 2450 MHz) λ ₀ /4 = 30.8 mm, for the case of 2 discontinuities in Figure 5a and 3 discontinuities in Figure 5b, Z ₁₀ ≒X _o tanβl ₁ l ₁ that satisfies the condition,
Combination of l ₂ (l ₃ ), l _tptal with opening characteristic impedance Z ₁₀ and termination characteristic impedance Z ₂₀ or Z ₃₀
If the ratio is 1 to 2, it will be calculated as follows.

【table】

[Table] This result means the following. By setting the characteristic impedance to Z ₁₀ <Z ₂₀ or Z ₁₀ <Z ₂₀ <Z ₃₀ , the groove depth l (total) can be made smaller than a quarter wavelength. The dimensional compression ratio of the depth of the groove is almost determined by the opening characteristic impedance Z ₁₀ and the end characteristic impedance Z _o0 , and is hardly influenced by the number n of changes in the characteristic impedance. The above explanation is for the case where Z ₂₀ /Z ₁₀ = Z ₃₀ /Z ₁₀ = 2, but Fig. 6 shows the characteristics when the ratio of dimensions l ₁ and l ₂ is varied from 1 to 5 in the case of two divisions. It shows the relationship between the impedance ratio and the compression ratio in which the depth of the small groove is reduced in size relative to the depth of the chiyoke groove. This graph shows that by carefully selecting the characteristic impedance, it is possible to reduce the characteristic impedance to less than one tenth of that of the chiyoke groove. Figure 7 is a plot of the absolute value of the characteristic impedance of the opening using the dimension l ₂ as a parameter when the dimension l ₁ is 12 mm, and shows that the maximum value is taken at the dimension l ₂ of 24 mm and 25 mm. There is. Figure 8 shows the measured values of radio wave leakage. This result also
The _l2 dimension shows a minimum value between 23.5 mm and 24.5 mm, which means that: Increasing the absolute value of the aperture impedance of the small groove reduces the amount of radio wave leakage. The depth dimensions (l ₁ , l ₂ ) of the grooves that increase the impedance of the opening of the small grooves should be such that the calculated values and actual measurements agree with each other with high precision.Compared to the depth of the chiyoke groove, it is possible to reliably downsize the groove. . A specific configuration of the embodiment shown in FIG. 9 is shown. In the present invention, in a case where at least one of the wall groups constituting the small groove has a conductor width smaller than the pitch, the conductor width a ₁ of the opening part of each line group is that of the short-circuit termination part.
A is configured to be larger than ₂ . Wall group 22, 2
3 and 24 constitute a small groove 25. In particular, the wall surface 24 is characterized by a group of lines in which the conductor width a ₃ of the opening portion is larger than the conductor width a ₂ of the short circuit portion. FIGS. 10a and 10b show the structure of an embodiment of the present invention. Reference numeral 26 indicates an outer groove, 27 indicates an outer groove wall surface, and 28 indicates a main body, which is particularly characterized by having bent portions 29 at the ends of the conductive wires constituting the wall surface 24. Using the data of the amount of leakage P _L (mM/cm ² ) of the microwave oven (2450 MHz) and the depth of the groove L _T as a parameter, the tip bent part 29 is
Figure 14 shows a comparison between when there is and when there is no. As shown in FIG.
9 (l _T =21 mm), the groove depth is shorter than that without the bent portion (l _T =25 mm). 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. In FIG. 9, a groove having an opening B whose tip C is short-circuited 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. 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 shown in a, b, and c, and in case b the radio wave does not reach end D. , that is, when the groove depth lT is approximately 80% of a quarter wavelength (λ/
(less than 4 lines), and even if it is longer or shorter (cases a and c), radio waves leak more than in case b. In actual application, the space of the groove cover
TOP1 and bending reinforcement space lX1 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 TOP1 is 2mm and lX1 is 5~
An example is shown when the width is set to 6 mm. Figure 10 is an example of a 915MHz sealing device considered at the top.
This shows the relationship in which the groove depth lT changes with dimension 1.
If the dimension of TOP1 is set to 1 to 3 mm, lT will be 1 to 6 mm.
It gets deeper. Figure 11 is an example of a 2450MHz sealing device.
The relationship is shown in which the groove depth lT changes with the reinforcing space lX1 with TOP1 = 2 mm fixed. By setting the space lX1 to 2 to 6 mm, the groove depth lT becomes deeper by 1 to 3 mm. The dimensions of each part shown in FIG. 13 are an example of dimensions when applied to a 2450 MHz microwave oven. In FIG. 13, 30 is a heating chamber, 31 is a door, 32 is a groove opening, and 33 is a short circuit end. Effects of the invention (1) Essentially, the depth of the small groove can be made smaller than a quarter wavelength. (2) By providing a bent portion at the tip, the groove size can be further shortened. (3) Since at least one of the walls constituting the small groove is composed of a group of lines, the radio wave propagation component in the x direction can be reduced and the radio wave sealing performance can be improved. (4) The radio wave seal can be made smaller with a simple configuration of changing the conductor width. (5) The outer groove acts as a second small groove, further improving sealing performance.

[Brief explanation of drawings]

1, 2a and 2b are sectional views of a conventional radio wave sealing device, FIG. 3 is a sectional perspective view of a parallel line, FIG. 4 is a sectional perspective view of a modified parallel line, and 5.
Figures a, b, and c are diagrams explaining the principle of the radio wave sealing device of the present invention, Figures 6, 7, and 8 are characteristic diagrams of the device of the present invention, and Figures 9 a, b, and c are diagrams of the present invention. The electric field analysis diagram of the groove part in the invention, FIG. 10 a, b, and c are
Cross-sectional view, side view, characteristic diagram of the device at 915MHz,
Figures 11a, b, and c are a cross-sectional view, side view, and characteristic diagram of the device at 2450MHz. Figure 12 is a perspective view of a radio wave sealing device according to an embodiment of the present invention. Figures 13a, b
14 is a sectional view and a perspective view of the same device, and FIG. 14 is a characteristic diagram of the same device. 22, 23, 24...Groove wall, 25...Groove, 26
...Outer groove, 27...Outer groove wall surface, 28...Body, 2
9...Tip bending part.

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 a groove is provided in at least one of the parts where the main body and the door face each other, At least one wall forming the conductive wire arranged in this groove is formed by a plurality of periodically continuous wall bodies, the pitch of the plurality of wall bodies is formed to be larger than the width of the groove, and the The conductor is a radio wave seal device with a bent part in the groove opening.