Electric heating – Metal heating – Of cylinders
Patent
1988-04-20
1991-08-06
Leung, Philip H.
Electric heating
Metal heating
Of cylinders
219 1055R, 174 35GC, H05B 676
Patent
active
RE0336572
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an apparatus which produces high frequency electromagnetic waves and more particularly it relates to prevention of leakage of said high frequency electromagnetic waves from said apparatus.
BACKGROUND ART
Means for prevention of leakage of high frequency electromagnetic waves will be described herein by taking, as an example, a microwave oven, which cooks food by dielectrically heating it by high frequency electromagnetic waves.
A microwave oven comprises a heating chamber for receiving food for heating it by high frequency waves, and a door adapted to open and close the opening provided in said heating chamber for putting in and taking out food to be cooked, wherein an electric wave sealing measure is taken to prevent high frequency electromagnetic waves in the heating chamber from leaking outside the chamber to hurt the human body when the door is opened to put in and take out food.
As another example, in waveguides utilized in communications systems, an electromagnetic sealing device is provided in the mechanical junction between the waveguides.
As an example of prior art, U.S. Pat. No. 3,182,164 is shown in FIG. 1. In FIG. 1, the numeral 1 denotes a heating chamber for a microwave oven, and a door 4 having a knob 3 is provided for covering the opening 2 in said heating chamber 1 so that the opening can be opened and closed. The peripheral edge of the door 4 is formed with a hollow choke portion having a clearance 5 which opens toward the heating chamber 1. The depth 7 of the choke portion is designed to be substantially 1/4 of the wavelength of the high frequency wave to be used. In this case, the thickness of the door 4 is also 1/4 of the wavelength. This is, since the frequency of the electromagnetic wave heretofore used in microwave ovens is 2450 MHz, 1/4 of the wavelength is about 30 mm. In order to be opposed to the choke portion 6 of this length, the peripheral edge 8 formed around the opening 2 in the heating chamber 1 has a thickness 9 which is greater than 1/4 of the wavelength. Therefore, the effective size of the opening 2 in the heating chamber 1 is smaller by an amount corresponding to the size of the peripheral edge 8.
As another example of prior art, U.S. Pat. No. 2,500,676 is shown in FIGS. 2(a) and (b). This example also shows a construction for microwave ovens, wherein high frequency waves resulting from the oscillation of a magnetron 10 are fed to a heating chamber 11 to cook food 12. The opening 13 in this heating chamber 11 is provided with a door 14 covering the opening 13 so that the latter can be opened and closed. The peripheral portion of this door 14 is formed with a choke portion 15 in the form of a groove to prevent leakage of high frequency waves. The depth of this choke portion 15 is also designed to be 1/4 of the wavelength of the high frequency wave to be used. As a result, the effective size of the opening 13 is correspondingly smaller than that of the heating chamber 11.
As described above, the conventional choke portions are based on the technical concept that 1/4 of the wavelength is essential for attenuating the high frequency waves.
Thus, let Z.sub.0 be the characteristic impedance of the choke portion, and L be its depth. Then, when the terminal end portion is shorted, the impedance Z.sub.IN at the choke opening is given by ##EQU1## (where .lambda..sub.0 is the free space wavelength.)
The electric wave attenuating means of the choke type is based on the principle of selecting the depth L of the choke so that it is 1/4 of the wavelength, thereby achieving. ##EQU2##
If the choke is filled with a dielectric (the specific dielectric constant of the material: .epsilon..sub.r), the wavelength .lambda.' of the electric waves is reduced to ##EQU3## In this case the depth L' of the choke portion is reduced as shown below. ##EQU4## However, the relation L'=.lambda.'/4 is still retained, so that in the choke system it is impossible to make the depth substantially less than 1/4 of the wavelength, imposing limitations on the reduct
REFERENCES:
patent: 3767884 (1973-10-01), Osepchuk et al.
patent: 4254318 (1981-03-01), Ohkawa et al.
patent: 4449025 (1984-05-01), Ikeda et al.
patent: 4471194 (1984-09-01), Hosokawa et al.
patent: 4475023 (1984-10-01), Iwabuchi et al.
Kashimoto Takashi
Kusunoki Shigeru
Nobue Tomotaka
Leung Philip H.
Matsushita Electric Industrial Co.
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