Electromagnetic wave energy seal arrangement

An electromagnetic wave energy seal for a microwave oven comprises a .lambda.g/4 choke seal including an internal plate for forming a first capacitive seal, an external plate for forming a recessed chamber with an external plate, a stepped portion having a vertical portion, slots formed with predetermined periodical intervals, a bent portion for forming a second capacitive seal, and a choke cover for blocking the opening of the recessed chamber, whereby the radio frequency leakage can be perfectly prevented from a heat cavity of the microwave oven and the thickness of a door of the microwave oven can be reduced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electromagnetic wave energy seal 
arrangement for a microwave oven and particularly, to a choke seal member 
for a door of a microwave oven in which the leakage of electromagnetic 
wave energy through a gap between the door and a front panel of a heating 
cavity of the microwave oven can be effectively prevented from the heating 
cavity and also the thickness of the door of the microwave oven can be 
reduced. 
2. Description of the Prior Art 
Various types of microwave ovens are well known in the art as a useful 
kitchen utensil which cooks foods by producing heat from food itself by 
radiating high frequency electromagnetic wave generated from a magnetron 
to foods within a hating cavity. However, when the electromagnetic wave 
generated from the magnetron in such microwave ovens is radiated to the 
human body over a predetermined quantity, fatal harm may be inflicted to 
the human body. Therefore, the leakage of the electromagnetic wave to 
exterior from the heating cavity of such microwave ovens should be 
prevented. 
It is inevitable that such microwave ovens used in general are made to have 
a predetermined gap between the door and the front panel of a heating 
sanity in their structure. Therefore, in order to prevent the leakage of 
electromagnetic wave energy through the gap, generally a capacitive seal, 
a .lambda.g/4 choke seal (.lambda.g is the wavelength of electromagnetic 
wave within the heating cavity), and a ferrite rubber which is a high 
frequency absorption material, are widely known and having been used. 
Such a conventional electromagnetic wave energy leakage preventing device 
is described in FIG. 2. As shown in FIG. 1, the microwave oven includes a 
main body 1, a door 2 for opening and closing a heating cavity 8, a glass 
3 for viewing through the interior attached to the door 2 so as to be able 
to see the interior of the heating cavity 8, which is mounted with keeping 
a predetermined distance with an external glass 3' (FIG. 2). FIG. 2 shows 
the structure of a choke seal for preventing electromagnetic wave energy 
leakage formed along with a circumferential edge of the door 2, wherein as 
shown in the drawing, the substantially rectangular .lambda.g/4 recessed 
chamber 6 having a predetermined length L and width W is formed by 
internal and external plate members 5 and 4. The internal plate member 5 
is bent in its bottom end portion toward the recessed chamber 6 about the 
intermediate portion of the choke channel 6. An opening 6' is formed at 
its lower side and also, the opening 6' is closed with a choke cover 7 
made of plastic material which is attached with a ferrite rubber 7a 
disposed at inside thereof. 
And, a capacitive seal is formed between a front panel 1' of the heating 
cavity 8 and the internal plate member 5 of the recessed chamber 6. A 
transmission line of the portion where a capacitive seal is formed has 
wave number of: 
##EQU1## 
wherein, K.sub.0 =2.pi./.lambda., b represents a distance between the 
front panel 1' and the internal plate member 5. And, wherein when value b 
is very little, TE mode and TM mode are cutoff and only the transmission 
is carried out only in the TEM mode. That is, since the characteristic 
impedance of a parallel transmission line in case of TEM mode is 
proportional to 
##EQU2## 
electromagnetic wave energy leaked from the heating cavity 8 can be 
reflected by making either the value b to be very little or the value 
.epsilon. to be great, wherein .mu. is permeability and .epsilon. is 
permittivity. 
And, an upper horizontal portion 4a of the external plate member 4 for 
forming the recessed chamber 6 is a plane that impedance is zero, and if 
seeing the recessed chamber 6 as a transmission line, as shown in FIG. 3, 
impedance at a point distanced apart as much as a length l from a load Zl 
is represented by 
##EQU3## 
wherein zo is the characteristic impedance of transmission line, .beta. is 
a propagation constant, both of which are values determined by the 
structure of transmission line. 
In the above expression, assuming that zl=o, then it becomes Zin=jzo tan 
.beta.l, when l becomes .lambda.g/4, Zin becomes infinity; when 
l=.lambda.g/2, then Zin=o. Therefore, as shown in FIG. 2, in case when the 
electromagnetic wave energy within the heating cavity 8 is leaked from a 
point A to a point C, and flowed into the interior of the recessed chamber 
6 with a path of B from a point D, the point D is located at a point 
distanced away as much as .lambda.g/4 from a plane 4a where the impedance 
is zero, so that the impedance of point D becomes infinity. Thus, the 
recessed chamber 6 produces resonance reflect the electromagnetic wave 
energy toward the heating cavity 8 and thereby becoming possible to 
prevent the leakage toward exterior. 
However, according to the choke seal of the conventional structure as 
aforementioned, when the gap between the internal plate 5 of the recessed 
chamber 6 and the front panel 1' of the heating cavity 8 becomes great, 
the electromagnetic wave energy is transmitted not only in the TEM mode 
but also in the TE mode or TM mode, thus .lambda.g/4 choke seal becomes 
impossible to exert its own function. Therefore, the leakage of 
electromagnetic wave energy is outstandingly increased. 
Furthermore, since the .lambda.g/4 choke seal and the expensive ferrite 
rubber are used together, the cost of product is increased and since the 
length and width of the recessed chamber 6 are relatively long and wide, 
the thickness of the door 2 becomes thicker, and therefore, there has been 
a problem that it is difficult to make the product in compact. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved electromagnetic waver energy seal for a microwave oven wherein a 
stepped portion having periodical slots is formed at the bottom end 
portion of the internal plate member of a door of the microwave oven for 
forming the choke seal, and an auxiliary capacitive seal is formed between 
the bottom end portion of external plate member and the front panel of the 
heating cavity so that the leakage of the electromagnetic wave energy can 
be effectively prevented without using the expensive ferrite rubber, as 
well as the thickness of the door can be reduced. 
Another object of the present invention is to provide an electromagnetic 
wave energy seal including slots with predetermined periodical intervals 
at a vertical portion of the a stepped portion formed by bending in two 
steps the bottom end portion of internal plate member of the door of the 
microwave oven and horizontal portion of lower side bent inwardly toward a 
recessed chamber extended from the vertical portion, and a C-shaped bent 
member is formed at the bottom end portion of the external plate member 
for forming the recessed chamber together with the internal plate member 
so that an auxiliary capacitive seal is formed between the front panel of 
the heating cavity and the bent member. 
Other objects and further scope of applicability of the present invention 
will become apparent from the detailed description given hereinafter. It 
should be understood, however, that the detailed description and specific 
examples, while indicating preferred embodiments of the invention, are 
given by way of illustration only, since various changes and modifications 
within the spirit and scope of the invention will become apparent to those 
skilled in the art from this detailed description. 
Briefly described, the present invention relates to an electromagnetic wave 
energy seal for a microwave oven comprising a .lambda.g/4 choke seal 
including an internal plate for forming a first capacitive seal, an 
external plate for forming a recessed chamber with an external plate, a 
stepped portion having a vertical portion, slots formed with predetermined 
periodical intervals, a bent portion for forming a second capacitive seal, 
and a choke cover for blocking the opening of the recessed chamber, 
whereby the radio frequency leakage can be perfectly prevented from a heat 
cavity and the thickness of a door of the microwave oven can be reduced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, a preferred embodiment of the present invention will be 
described in detail with reference to FIGS. 4 to 9 as follows: 
Referring to FIGS. 4 and 5, in which parts similar to those previously 
described with reference to FIGS. 1 and 2 are denoted by same reference 
numerals, the electromagnetic wave energy leakage preventing device 
according to the present invention is same as conventional structure in a 
manner that an internal plate member 5 and an external plate member 4 of a 
door 2 are extended and formed, whereby a recessed chamber 6 has a 
substantially rectangular configuration. A capacitive seal 9 is formed 
between the internal plate member 5 forming the recessed chamber and an 
upper portion of a front panel 1'. A stepped portion 5' bent in two step 
toward the recessed chamber 6 is provided at the bottom end portion of the 
internal plate member 5. As shown in FIG. 5, slots 5'd are formed with a 
predetermined periodical interval at a lower horizontal portion 5'b 
inwardly bent toward the recessed chamber 6 and a vertical portion 5'c of 
the stepped portion 5'. 
Furthermore, the bottom end of the external plate member 4 is downwardly 
bent in parallel with the front panel 1', whereby a C-shaped bent portion 
4b for forming an auxiliary capacitive seal 10 between the front panel 1' 
is formed and an opening 6' of the recessed chamber 6 formed between the 
lower horizontal portion 5'b of the stepped portion 5' and the upper 
surface of the bent member 4b is made to close with a choke cover 7 made 
of plastic material. 
According to the present invention, the microwave oven operates as follows: 
Flow of the electromagnetic wave energy leaked to the exterior from the 
heating cavity 8 can be expressed by Poynting's vector S=E.times.M 
[mW/cm.sup.2 ] as shown in FIG. 6, wherein S represents a component of Z 
direction, E represents a component of x direction, and M represents a 
component of y direction. In order to prevent such vector components, a 
choke for blocking the proceeding of electromagnetic wave energy toward x 
direction and a choke for blocking the proceeding toward z direction are 
respectively required. The slots 5'd formed periodically at the stepped 
portion 5' are made to operate as a band stop filter against the 
electromagnetic wave energy proceeding to x direction so that the leakage 
of x direction can be prevented. The fringing capacitance formed by such 
periodical slot structure is shown in FIG. 8. That is, fringing of C1 and 
C2 is produced between the front panel 1' and slots 5'd, and fringing of 
C3 and C4 is produced between slots 5'c and the vertical portion 4d of 
the external plate member 4 of the recessed chamber 6 (FIG. 8). 
A matter that such periodical slot structure can be operated as a filter is 
seen in Floquet's Theorem. That is, in the structure, when w-.beta. 
relation (Dispersion relation) is induced, .beta. becomes an imaginary 
number at a certain band. This region is that the electromagnetic wave 
energy is attenuated, wherein w-.beta. relation becomes a function of the 
width D1 of the vertical portion 5'c of the stepped portion 5' and width 
D2 of the slot 5'd as shown in FIG. 5. It is also a function of the gap 
between the front panel 1' and the vertical portion 5'c. Therefore, when 
the electromagnetic wave energy is made to be attenuated in the vicinity 
of 2450 MHz of the radio frequency used for the microwave oven, since the 
periodical structure operates as the band stop filter, the leakage of 
electromagnetic wave energy proceeding to x direction can be prevented. 
Furthermore, for the choke of z direction, the .lambda.g/4 choke seal 
serves main role and the flow of electromagnetic wave energy from the 
heating cavity 8 can be expressed as shown in FIG. 9. That is, when the 
electromagnetic wave flows from point A of the heating cavity 8 to point c 
of exterior, the impedance relation at the heating cavity 8 can be 
expressed by jX. When a point that jX becoming infinity is positioned on 
the A.fwdarw.C path, the AC transmission line is opened and the leakage of 
radio frequency energy is prevented. 
The .lambda.g/4 choke seal includes two capacitive seals 9 and 10 as 
described above and the stepped portion 5' formed with the slots 5'd 
having a predetermined periodical interval. When the choke seal of such 
structure is formed, the length L and width W of the recessed chamber 6 
and the height H between the lower horizontal portion 5'b of the stepped 
portion 5, and the lower horizontal surface 4c of the external plate 
member 4 are important factors and those of important factors in the 
formation of the periodical slots 5'd and the first and second capacitive 
seals 9 and 10 are D1, D2, E, and the upper vertical portion of the 
internal plate body 5 as shown in FIG. 5. These respective factors are 
suitably controlled, whereby optimum state is observed so that the 
examination for obtaining optimum structure of the choke seal is carried 
out by the equipment as shown in FIG. 6, wherein reference numeral 11 
denotes a radio frequency detector and 12 is a spacer. 
FIG. 7 shows the change of leakage according to the length L at a state 
that various factors are set, wherein it can be seen that the leakage is 
minimum when the length L is .lambda.g/4. 
As described above in detail, the electromagnetic wave energy leakage 
preventing device according to the present invention has an effect that 
slot structure having predetermined periodical intervals and the auxiliary 
capacitive seal are formed, whereby the leakage of electromagnetic wave 
energy to x direction and z direction can be simultaneously prevented 
therefrom so that the perfect radio frequency leakage preventing effect 
can be obtained without using the expensive ferrite rubber. Furthermore, 
an effect also can be obtained that the stepped portion bent in two steps 
to the internal plate member of the recessed chamber, thereby rendering to 
reduce the width of the recessed chamber so that the thickness of the door 
of the microwave oven can be made thinner than the conventional device. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included in 
the scope of the following claims.