Transformer having shielding wall for driving a magnetron

A stationary induction apparatus for supplying and receiving high frequency power of several tens KHz, wherein a partition wall made of a non-magnetic metallic foil in a thickness less than 1/10 the depth of penetration in the using freqency for electromagnetic induction is provided so as to separate the supply side (primary side) from the receiver side (secondary side), so that the stationary induction apparatus is not hurt in its function as an induction apparatus, but can electromagnetically shield the primary side from the secondary side, and the secondary side of the boosting transformer of the magnetron is able to be placed within the shield casing, thus realizing a light, compact and inexpensive microwave generating device for use in an electronic oven.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a stationary induction apparatus 
for transmitting or supplying electromagnetic energy through 
electromagnetic induction, and more particularly, to a high-frequency 
heating apparatus to which is applied a technique to electromagnetically 
shield the power supply side of the stationary induction apparatus 
(primary side) from the power receiver side (secondary side), without 
detracting from the electromagnetic induction function therebetween. 
2. Description of the Prior Art 
Due to the recent progress in inverter technology, an inverter source 
system is now being put into practical use. According to this inverter 
source system, commercial frequencies are, before being boosted by a 
boosting transformer and added to a magnetron, converted into high 
frequencies. Therefore, if the inverter source system is applied to an 
electronic oven, a power source of the magnetron of the electronic oven 
can be rendered compact in size, light in weight and low in manufacturing 
cost. 
Attempts have been made to construct such a microwave generating device for 
use in an electronic oven (a high frequency heating apparatus) that is 
light in weight and compact, wherein the primary side coil (power supply 
side induction element) of the boosting transformer (stationary induction 
apparatus) is positioned outside a shield casing, while the secondary side 
coil (power receiver side induction element) is placed inside the shield 
casing of the magnetron, so that the primary side coil and the secondary 
side coil are inductively coupled to each other to function as a 
transformer, and wherein the primary side coil and the secondary side coil 
are separated from each other by the shield casing, thereby to maintain 
the shielding function of the shield casing. 
In the electronic oven using the above-described system of the inverter 
power source, the high frequency waves of several tens of KHz produced in 
the inverter circuit can be advantageously utilized as they are for 
induction heating. Therefore, if a heating coil for induction heating is 
added to the electronic oven employing the inverter power source, and the 
power is arranged to be supplied from the common inverter power source 
either to the heating coil or to the microwave generating device, an 
electronic oven equipped with the function of an electromagnetic cooking 
apparatus which can serve both as an electronic oven and as an 
electromagnetic cooking apparatus will be brought into the market. In this 
case, although it may be possible to install the heating coil for 
electromagnetic cooking on the upper surface of the body of the electronic 
oven, it will be more convenient to place the heating coil at the bottom, 
inside the heating chamber, of the electronic oven, so that the room of 
the heating chamber can be a common space for an object to le heated by 
the electronic oven and by the electromagnetic cooking apparatus. However, 
if the heating coil is placed within the heating chamber where the 
microwave is irradiated, the exciting coil should be protected from 
damages caused by the microwave, and for this purpose the exciting coil is 
necessary to be shielded from the microwave, while the exciting coil which 
is an induction element at the power supply side is necessary to be 
inductively coupled to a metallic pan or the like (an object to be heated) 
which corresponds to an induction element at the power receiver side. 
Both in the above-described microwave generating device and in the 
electronic oven functioning also as an electromagnetic cooking apparatus, 
the electromagnetic waves should be shielded between the power supply side 
and the power receiver side, but, the high-frequency power of several tens 
of KHz should be transmitted through electromagnetic induction between the 
power supply side and the power receiver side. The above-described 
shielding and transmission of electromagnetic energy are contradictory 
from a physical viewpoint, and accordingly conventional techniques could 
not realize such ideas as the above microwave generating device or the 
electronic oven with the electromagnetic cooking apparatus. 
SUMMARY OF THE INVENTION 
An essential object of the present invention is to provide a stationary 
induction apparatus wherein an induction element at the power supply side 
(the primary side circuit) and an induction element at the power receiver 
side (the secondary side circuit) are electromagnetically shielded from 
each other by a partition wall made of a metallic foil having a thickness 
less than 1/10 the depth of penetration in the operating frequency 
(several tens of KHz), making use of the fact that the frequency of the 
electromagnetic wave necessary to be shielded is generally higher than the 
several tens of KHz which is used for transmission of the energy through 
electromagnetic induction. 
The stationary induction apparatus according to the present invention is 
applicable to those described in the item of "Description of the Prior 
Art". In other words, a microwave generating device in the high frequency 
heating apparatus is one example. More specifically, in the microwave 
generating device, the secondary side of the boosting transformer 
(stationary induction apparatus) for supplying positive voltage (high 
voltage) and heater voltage (low voltage) to the magnetron is 
electromagnetically shielded from the primary side of the boosting 
transformer, through combination of the inverter power source, so that the 
leakage of unnecessary electromagnetic waves from the cathode stem 
connected to the secondary side is prevented. Accordingly, the stationary 
induction apparatus of the present invention can achieve a highly 
efficient, light in weight and compact microwave generating device. 
A second application example is an electronic oven which can serve also as 
an electromagnetic cooking apparatus, which has the heating coil placed in 
the bottom inside portion of the heating chamber. According to the 
arrangement of this case, the heating coil which is an induction element 
at the power supply side is electromagnetically inductively coupled to a 
metallic pan (an object to be heated) which corresponds to an induction 
element at the power receiver side, while the heating coil including the 
plane facing the pan and he heating coil is electromagnetically shielded 
from the microwaves, so that the heating coil is not damage by the 
microwaves.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before the description of the present invention proceeds, it is noted here 
that like parts are designated by like reference numerals throughout the 
accompanying drawings. 
Also, before beginning the description of the present invention, a 
description is provided first with reference to the disclosure in Japanese 
Utility Model Laid-Open Publication No. 61-107190 (107190/1986) as to how 
the shielding of the microwaves and the transmission of the power through 
electromagnetic induction could not be technically achieved all together 
heretofore according to the conventional methods. 
Generally, a filter circuit is formed in the magnetron by a choke coil 
provided within a shielding casing covering a cathode stem of the 
magnetron and a through capacitor, so as to prevent leakage of the 
electromagnetic waves from an input terminal side, namely, the cathode 
stem side. However, when an inverter power source using, for example, a 
frequency of over 20 KHz, the power which is inverted into high-frequency 
waves by the inverter is unfavorably controlled by the filter circuit and 
therefore, becomes difficult to be supplied to the magnetron. FIG. 1 shows 
a cross sectional view of an essential portion of a conventional microwave 
generating device using the inverter power source disclosed in the above 
Japanese Utility Model Laid-Open Publication No. 61-107190, which is 
proposed for the purpose of solving the above-described disadvantage. In 
the microwave generating device of FIG. 1, an oscillator tube 14 has an 
antenna 19 at one end thereof, and a cathode stem 20 at the other end 
which has cathode terminals 21 and 22. The cathode stem 20 is surrounded 
by a shield casing 23 which is electrically connected to the oscillator 
tube 14 and formed by a non-magnetic conductive material. The shield 
casing 23 is completely shielded between the inside and the outside of the 
casing electromagnetically. A primary side core 24 wound by a primary side 
winding 6a of a boosting transformer 6 is positioned outside the shield 
casing 23 opposite to a secondary side winding 6b and a low voltage 
secondary side winding 6c inside the shield casing 23, via wall surface 
23a. In the instant reference, the primary side core 24 and the secondary 
side core 25 are so positioned via the wall surface 23a as to be 
electromagnetically coupled to each other through mutual induction. 
Accordingly, the high frequency waves can be prevented from leaking from 
the side of the input stem of the magnetron, and simultaneously the power 
converted into high frequency waves can be supplied to the magnetron. 
However, although it appears that the above-described arrangement 
functions as explained, practically, the construction is merely a desk 
theory that is impossible to be achieved, the reason for which will be 
described hereinbelow. 
According to the description of the abovementioned Japanese Utility Model 
Laid-Open Publication, the shield casing 23 surrounding the cathode stem 
20 provided at one end of the oscillator tube 14 is formed by a 
nonmagnetic conductive material, and the primary side and the secondary 
side of the boosting transformer are placed outside and inside the shield 
casing 23 via the wall surface 23a. In this case, it is true that the 
leakage of the high frequency waves from the cathode stem 20 can be 
prevented, but the primary side and the secondary side of the boosting 
transformer are shielded at the same time and therefore, the 
electromagnetic energy cannot be transmitted to the magnetron. Therefore, 
such a position or arrangement of the primary side and the secondary side 
of the boosting transformer cannot be present that is able to achieve 
electromagnetic coupling therebetween through mutual induction. This is 
because, since the wall surface 23a of the shield casing 23 is formed by a 
conductive material, the magnetic field produced by the high frequency 
current running in the primary winding 6a of the boosting transformer 6 
generates an induced current in the wall surface 23a, that is, an eddy 
current, and the electric energy of the magnetic field is consumed as 
Joule heat by this eddy current. As a result, the electric energy is never 
transmitted to the secondary side windings 6b and 6c of the boosting 
transformer 6, or is attenuated on a large scale. From another viewpoint, 
the shield casing 23 made of a conductive material is equivalent to that a 
third secondary winding which is short-circuited and which is added to the 
boosting transformer 6. Therefore, it may be so considered that the 
electric energy of the magnetic field is almost entirely consumed by the 
short-circuited current flowing in the short-circuited secondary winding. 
In any case, the conventional arrangement is merely non-workable idea, 
since the shield casing 23 has a function which is impossible to achieve 
by conventional means or measures. 
In order to eliminate or solve the above-described disadvantages inherent 
in the prior art, the present invention provides a shielding technique to 
be applied to a high-frequency heating apparatus installed with a 
stationary induction apparatus, whereby the power supply side (the primary 
side) of the stationary induction apparatus can be shielded from the power 
receiver side (the secondary side) electromagnetically, without detracting 
from the electromagnetic induction function therebetween. 
Although various modes can be considered for practicing the present 
invention, some will be described one by one with reference to their 
embodiments. 
A first embodiment is related to a magnetron and its boosting transformer, 
the construction and the operation of which will be described with 
reference to FIGS. 2 to 5. 
In FIG. 2, there is shown a cross sectional view of an essential portion of 
a high frequency heating apparatus which includes a stationary induction 
apparatus according to the first embodiment of the present invention. FIG. 
3(a) is a circuit diagram schematically showing the circuit construction 
of the high frequency heating apparatus including the essential portion 
shown in FIG. 2. 
Referring to FIG. 3(a), an electric power at commercial frequencies 
supplied by a power source 1 is rectified by a rectifier circuit 3, and 
then supplied to an inverter circuit comprising a capacitor 8, a diode 9 
and a transistor 10 which is a switching element, through a low-pass 
filter composed of an inductance 5 and a capacitor 4. The transistor 10 is 
turned on and off by a high frequency pulse generated from a control 
circuit 11. 
After the power is converted into a high frequency wave of approximately 20 
KHz by the inverter circuit and supplied to a primary winding 7a of a 
boosting transformer 7, it is excited at a secondary winding 7b as the 
anode voltage of the magnetron and at a secondary winding 7c as the 
cathode voltage of the magnetron, respectively, to inputted to the 
magnetron 13. In FIG. 2, an oscillator tube 14 is a main body of the 
magnetron, which has an antenna 19 at one end and a cathode stem 20 at the 
other end. A shield casing 28 enclosing the cathode stem 20 has a lid 28a 
which is a part of the wall surface of the casing and composed of a shield 
film 28b of a non-magnetic metallic thin film or a metallic evaporated 
film, and a support member 28c made of a dielectric material which 
supports the shield film 28b. 
The boosting transformer 7 has its core divided by the lid 28a of the 
shield casing 28 to be positioned inside and outside of the casing 28. A 
primary side core 7p outside the shield casing 28 is wound with a primary 
winding 7a, and a secondary side core 7s inside the shield casing 28 is 
wound with secondary windings 7b and 7c. Generally the core is made of 
ferromagnetic material. As has been already mentioned above, the primary 
winding 7a is supplied with the voltage converted into high frequency by 
the inverter circuit, while the high voltage and the low voltage generated 
respectively in the secondary windings 7b and 7c are, through cathode 
terminals 20 and 21, inputted as the anode voltage and the cathode voltage 
to the magnetron, respectively. 
The shielding effect of the shield casing 28 and the operation of the 
boosting transformer 7 will be explained hereinbelow. 
Generally, when a conductive member which is represented by metal is 
applied with an alternating magnetic field, whether it is magnetic or 
non-magnetic, an induced current is produced. The depth by which the 
induced current penetrates from the surface is limited, and the depth when 
the current density I becomes 1/e of the surface current density Io is 
called as the depth of penetration .rho.(m) which establishes an equation 
.delta.=.sqroot..rho./.pi.f.mu., wherein .rho. is a specific resistance of 
the conductor (ohm.m), f(Hz) is frequency and .mu.r is a specific magnetic 
permeability. 
In general, the current density I in the conductor at a position spaced the 
surface is expressed by an equation, I=IO.multidot.e.sup.-x/.delta. 
wherein Io represents the current density on the surface. Attenuation of 
the magnetic field is proportional to the attenuation of the current, and 
the attenuation ratio A (dB) is expressed as follows, 
##EQU1## 
Accordingly, for example, in the case of an aluminum plate, the depth of 
penetration .delta. is 0.6 mm or so in the frequency of 20 KHz. Even if 
the aluminum plate is as thin as about 0.6 mm, the attenuation ratio A is 
8.686 dB. Thus, the alternating magnetic field applied to one surface of 
the aluminum plate hardly penetrates to the opposite surface, and almost 
all of which is consumed as an ohmic loss by the induced current in the 
aluminum plate. 
However, if an aluminum leaf or foil, rather than a plate, having thickness 
being 20 .mu.m is employed, this thickness of 1/30 of .delta.=0.6 mm (600 
.mu.m) which is the depth of penetration at 20 KHz frequency the 
attenuation ratio A becomes 0.3 dB. Accordingly, contrary to the above 
case of the aluminum plate, the energy can be transmitted to the opposite 
surface to nearly a full scale (97%). 
FIG. 4 indicates how the shielding rate with respect to the alternating 
magnetic field of the metallic foil or the conductive coating film is 
changed in accordance with the frequency. As is clear from the graph of 
FIG. 4 and also from the equation .delta.=.sqroot..rho./.pi.f.mu., the 
depth of penetration .delta. becomes smaller in accordance with the 
increment of the frequency f, thereby increasing the shielding effect of 
the metallic foil or the conductive coating film. 
In the case where a metallic thin film, that is, non-magnetic stainless 
steel L(SUS) having a thickness of 50 .mu.m i employed for the shielding 
film 28b, it can be said that the metallic thin film displays no shielding 
effect at all with respect to the alternating magnetic field having a 
frequency of less than 100 KHz. Consequently, the boosting transformer 7 
perfectly serves as a transformer even though the primary side and the 
secondary side thereof are divided by the lid 28a. In the meantime, noises 
generated by the magnetron are spread widely among the low frequency 
region as well as to the basic frequency region of 2450 MHz. It is the 
high frequency region of over 10 MHz that is essentially necessary to be 
shielded, even if the shield film 28b is a metallic foil of non-magnetic 
stainless steel (SUS) 50 .mu.m, a shielding effect is achieved of over 25 
dB (magnetic field mode) and moreover, the shielding efficiency increases 
as the frequency becomes higher. It is to be noted here that the windings 
of the transformer are coupled to each other by the alternating magnetic 
field, and the data indicated in FIG. 4 is obtained in the magnetic field 
mode. However, it is made clear by experiments that, when the same 50 
.mu.m non-magnetic stainless steel (SUS) in the shape of a foil is 
employed in the electric field mode, the shielding effect having the 
attenuation ratio over 40 dB is achieved, including in the frequency 
region less than 10 MHz. Therefore, with respect to the electromagnetic 
waves leaking from the cathode stem 20 which should be prevented, actual 
effective shielding is exercised in the middle of the magnetic field mode 
and the electric field mode, and accordingly the non-magnetic stainless 
steel is sufficiently effective as the shield film for prevention of 
leakage of the high frequency waves. 
Although the thickness of the shield film 28b is determined with 
consideration of the boosting transformer 7 and the shielding 
characteristic, etc., the loss in the shield film 28b should be also 
considered, and therefore a standard thickness of the shield film 28b is 
less than 1/10 the depth of penetration .delta.. 
In the case where the shield film 28b has the thickness 1/10 the depth of 
penetration .delta., the attenuation ration A is approximately 10%, in 
accordance with the earlier-described equation 
EQU A=8.686.times.x/.delta. (dB), 
that is, A=0.8686 dB. 
On the, contrary, the minimum thickness of the shield film is determined by 
the lowest frequency to be shielded and the shielding characteristic 
(attenuation ratio) to be required. Although the unnecessary radiation 
leaking from the cathode stem 20 of the magnetron 13 includes not only the 
high frequency region around 2450 MHz, but also the low frequency region, 
the lowest frequency for which the high shielding effect of the shield 
casing is approximately 20 MHz. Supposing that the inverter frequency is 
20 KHz, the above-mentioned lowest frequency 20 MHz is 1000 times the 
inverter frequency, and the depth of penetration is approximately 1/30. 
Accordingly, if the shield film is made of aluminum, the depth of 
penetration is 0.02 mm (20 .mu.m) in the 20 MHz frequency which is 1/30 
the depth of penetration 0.6 mm in the 20 KHz frequency, which will be the 
standard thickness of the shield film. In practice, however, for 
maintaining the transmission loss as low as possible in the stationary 
induction apparatus, the shielding characteristic may be sacrificed a 
little, and a thinner shielding film, namely, a shielding film having 
1/100 or 1/200 the depth of penetration in the inverter frequency is 
employed in many cases. 
As will be understood from the description above, according to the first 
embodiment of the present invention, the shield casing 28 is able to 
prevent leakage of the high frequency waves from the cathode stem of the 
magnetron, and at the same time, the boosting transformer 7, even though 
it is divided by the wall surface of the shielding casing 28, is able to 
perform its function as a transformer. 
If the conventional boosting transformer which steps up the commercial 
frequency as it is employed for treating general power, for example, of 
about 1 KW or so, since the conventional boosting transformer is almost an 
iron block having 4 Kg in weight and 110.times.110.times.70 mm in outer 
dimension which is considerably heavier and bulkier as compared with the 
magnetron having 1 Kg in weight and 100.times.100.times.50 mm in outer 
dimension, not only the whole power source circuit becomes large in weight 
and size, but the boosting transformer is impossible to be integrally 
formed with the shield casing of the magnetron. On the contrary, according 
to the present invention, because the power is converted into high 
frequency waves of several tens of KHz by the high frequency inverter 
circuit, and moreover, the material of the magnetic core is changed from a 
plate-like silicon steel to ferrite, a light and compact structure of the 
boosting transformer, i.e., 400 g in weight and 70.times.70.times.40 mm in 
outer dimension, can be realized, so that the boosting transformer is able 
to be integrated with the shield casing of the magnetron. 
If the shield film is made of a magnetic metal, the shield film unfavorably 
forms a branching circuit (shunt circuit) in the magnetic circuit of the 
boosting transformer 7, resulting in a reduction of the magnetic property. 
Therefore, it is preferable to use a non-magnetic metal such as aluminum, 
etc. for the shield film as described above. However, if a non-magnetic 
material such as represented by copper (Cu) or aluminum (Al) is employed, 
the depth of penetration .delta. becomes small because of the small 
specific resistance (ohm.m) thereof, giving rise to a trouble in 
processing. Therefore, the material of the shield film should be decided 
carefully with the above-described facts taken into consideration. For 
example, Monel (an alloy mainly composed of nickel and copper) can be said 
to be suitable for the material of the shield film since it is 
non-magnetic, but has relatively large specific resistance. 
Further, in order to lessen the eddy current loss by the magnetism leaking 
from the boosting transformer, it is preferable that the other shield wall 
surface of the shield casing 28, except the shield film 28b, is made of, 
for example, aluminum which shows good electric conductivity, and is 
non-magnetic metal. The thickness of the wall surface is desirably over 
0.6 mm which is the depth of penetration in the inverter frequency (20 
KHz) of aluminum. 
A ventilation hole 28d is formed in the surface of the shield casing 28 so 
as to cool the secondary side of the boosting transformer 7. The secondary 
side is forcibly cooled by a cooling fan 2 from outside of the shield 
casing 28. 
Although it is so arranged in the present embodiment that the voltage at 
the secondary side of the boosting transformer is added to the magnetron 
as it is as an anode voltage, the voltage at the secondary side may be 
doubled and rectified by a diode and a capacitor, or subjected to the full 
wave rectification by many diodes, and then added to the anode electrode. 
Of course, various types of circuit constructions may be possible. In the 
circuit constructions described above, such circuit elements as the diode, 
the capacitor, etc. are naturally placed inside the shield casing. FIG. 
3(b) shows one example of such circuit construction as above, in which a 
diode 29 and a capacitor 30 constitute a voltage doubler circuit. 
As is described hereinabove, the following effects can be achieved by the 
microwave generating device installed with the stationary induction 
apparatus of the present invention. 
(1) The microwaves leaked from the cathode stem of the magnetron and the 
high frequency wave noises accompanied with the leakage of the microwaves 
are shielded within the shield casing, whereas the secondary side circuit 
of the boosting transformer inside the shield casing is able to be 
magnetically coupled to the primary side circuit outside the shield 
casing, thereby effecting the function as a transformer. As a result, the 
power which is converted into high frequency waves by the inverter can be 
effectively supplied by the magnetron. 
(2) When the conventional boosting transformer which steps up the 
commercial frequency as it is employed to handle quite general power, for 
example, about 1 KW, the boosting transformer is impossible to be 
integrally formed with the magnetron since the boosting transformer is as 
heavy as over 4 Kg, that is, four times the weight of the magnetron and, 
the outer dimension of the boosting transformer is very large. However, 
due to the high frequency inverter employed, since the power, is converted 
into high frequency waves of several tens of KHz according to the present 
embodiment, and moreover, the magnetic core is changed from silicon steel 
plate to ferrite according to the present embodiment, the microwave 
generating device is realized in a substantially compact structure, 400 g 
in weight and small, wherein the boosting transformer and the magnetron 
are integrally formed. 
(3) A filter circuit composed of an inductance and a capacitance (a through 
capacitor) which is provided in the prior art shield casing for avoiding 
leakage of high frequency wave noises is not necessary in the present 
embodiment. Accordingly, the voltage decrease and the power loss caused by 
the filter circuit are solved. At the same time, since the capacitance 
(through capacitor) and the inductance which are necessary to be high in 
pressure resisting property and accordingly expensive because the anode 
voltage is as high as about 4KV can be solved, the manufacturing cost of 
the apparatus can be substantially reduced. 
Similarly to the first embodiment, a second embodiment is related to the 
magnetron and the boosting transformer. The difference resides in the 
structure of a shielding wall provided between the primary side (power 
supply side induction element) and the secondary side (power receiver side 
induction element) of the boosting transformer. With reference to FIGS. 5 
through 7, the difference of the second embodiment from the first 
embodiment will be described. 
FIG. 5 is a cross sectional view of an essential portion of a microwave 
generating device utilizing the inverter power source and installed with a 
stationary induction apparatus (boosting transformer) according to the 
second embodiment. 
In the construction of FIG. 5, the lid 28a which is a part of the wall 
surface of the shield casing 28 enclosing the cathode stem 20 is 
constructed by a honeycomb core 28f made of aluminum foil and a supporter 
28c made of a dielectric material for supporting the honeycomb core 28f. 
The construction of the lid 28a of the second embodiment is different from 
that of the first embodiment shown in FIG. 2. 
With reference to FIG. 6, the structure of the aluminum honeycomb core 28f 
will be described in detail. 
FIG. 6(a) is a perspective view of a part of the aluminum honeycomb core 
28f cut out in general rectangular shape, and FIG. 6(b) is an enlarged 
view of one (unit structure) of the cells composing the aluminum honeycomb 
core 28f. As seen from FIG. 6(b), the core 28f is formed by many hexagonal 
cells by attaching a plurality of aluminum foils as shown. The honeycomb 
core 28f has a relatively large mechanical strength in spite of its small 
weight. Moreover, the honeycomb core 28f exhibits shielding effects 
against electromagnetic waves while the gas permeability, thereof is 
maintained. Accordingly, the honeycomb core has been put into wide use, 
for example, in a ventilation window of a shield room for shielding of 
electromagnetic waves, etc. According to the second embodiment, the 
aluminum foil thickness S of the honeycomb core 28f is, in the case where 
the inverter frequency is 20 KHz, below 1/10 of the depth of penetration 
0.6 mm of the aluminum with respect to the above frequency. The cell size 
D and the core height T of the honeycomb core 28f are set on the basis of 
the largest frequency of the unnecessary radiation to be shielded, and in 
the second embodiment, the core height T is 1.5 mm and the cell size D is 
approximately 3 mm. 
Hereinafter, the operation of the microwave generating device having the 
above-described construction according to the second embodiment of the 
present invention will be described. First, the function of the boosting 
transformer will be explained with reference to FIG. 7. 
FIG. 7(a) indicates the relation of the honeycomb core 28f made of the 
aluminum foil 32, the boosting transformer 6 which is separately arranged 
by the honeycomb core 28f outside and inside the shield casing 28, and the 
line 31 of magnetic flux in the high frequency magnetic field of 20 KHz 
which is generated when the boosting transformer 7 is driven. FIG. 7(b) is 
an enlarged view showing the relation between the honeycomb core 28f and 
the line of magnetic flux 31. The line of magnetic flux 31 is generally 
parallel to the axial direction of each cell (unit structure) forming the 
honeycomb core 28f. 
As has been described in the first embodiment, generally, an electric 
conductor which is represented by metal, when applied with an alternating 
field, generates an induced current, whether the conductor is magnetic or 
non-magnetic. The induced current penetrates from the surface of a limited 
depth. The depth when the current density I becomes 1/e of the surface 
current density Io is called as the depth of penetration .delta.(m). And 
the depth of penetration .delta.(m) is expressed by an equation: 
EQU .delta.=.sqroot..rho./.pi.f.mu. 
wherein .rho.(ohm.m) is a specific resistance of the conductor, f(Hz) is a 
frequency and .mu.r is a specific permeability. The current density I in 
the conductor at a position x from the surface is expressed: 
EQU I=Io.multidot.e.sup.-x/.delta. (1) 
wherein Io is the current density at the surface. By way of example, when 
the conductor is aluminum, the depth of penetration n.delta. is 
approximately 0.6 mm when the frequency is 20 KHz. The direction of the 
induced current is vertical to the direction of the magnetic field (the 
line of magnetic flux 31). 
FIG. 7(c) shows the relation of the aluminum foil 32 having the thickness 
S, the line of magnetic flux 31 in the alternating field added parallel to 
the aluminum foil 32, and the induced current produced in the aluminum 
foil 32. The current induced into the aluminum foil is exponentially and 
functionally decreased from the surface, as indicated by equation (1). The 
direction of the current induced in the aluminum foil 32 is vertical to 
the magnetic field, and moreover, the current density is Ia and Ib, 
opposite to each other, on the reverse and front surfaces of the aluminum 
foil, respectively. Meanwhile, if the thickness S of the aluminum foil 
composing the honeycomb core 28f is sufficiently smaller than the depth of 
penetration .delta.=0.6 mm, for example, if the thickness S is set to be 
1/10 of the depth of penetration, the current densities Ia and Ib induced 
in the opposite directions on the reverse and front surfaces of the 
aluminum foil are negated with each other, and accordingly, the actually 
running current density I is the difference between the current densities 
Ia and Ib. Therefore, the current induced in the aluminum foil placed 
parallel to the magnetic field is nearly neglectable if the thickness of 
the aluminum foil is less than 1/10 the depth of penetration .delta., 
e.g., as small as 0.02 mm. 
However, in the case where the honeycomb structure is created by attaching 
foils as in the present embodiment, the foils are doubled at two of the 
six sides of the hexagonal cell (unit structure). Therefore, the actual 
thickness of the aluminum foil at those two sides doubles. This fact 
should be taken into consideration when the thickness of the aluminum foil 
is determined. 
Thus, as described above, although the aluminum honeycomb core 28f is 
interposed between the primary side core 7p and the secondary side core 7s 
of the boosting transformer 7, energy loss is hardly produced in the high 
frequency magnetic field at 20 KHz, and accordingly the boosting 
transformer 7 can fully operate. 
The operation and function of the shield casing 28 of the magnetron 13 will 
now be described. 
The unnecessary radiation leaking from the cathode stem 20 of the magnetron 
13 is spread along the low frequency region as well as the 2450 MHz high 
frequency region. However, the shield casing 28 should do with the 
shielding effect only in the range of about 20 MHz through 20 GHz. 
In the case of the 20 MHz frequency, the depth of the penetration .delta. 
of the aluminum foil composing the honeycomb core 28f is 1/.sqroot.1000 
the value in the case of 20 KHz, that is, about 0.02 mm, which is equal to 
the foil thickness of the honeycomb core 28f according to the present 
embodiment In other words, when the frequency is over 20 MHz, the 
thickness of the aluminum foil composing the honeycomb core 28f becomes 
equal to or larger than the depth of penetration. Therefore, when the 
frequency is over 20 MHz, the aluminum foil having the thickness 0.02 mm 
shows the characteristic of a usual aluminum plate. Thus, if each of the 
cells constituting the honeycomb core 28f is regarded as a wave guide 
which displays a cut-off phenomenon to the leaked electromagnetic waves, 
the cell, showing the cut-off phenomenon, is said to have the shielding 
characteristic. On the other hand, in a higher frequency, according to the 
present embodiment, even when the largest frequency of the unnecessary 
radiation extends to 20 GHz, the cell size D is 1/5 the wavelength 
.lambda.=15 mm since the cell size D is set to be 3 mm, which is 
completely within the, .shielding region. Therefore, the honeycomb core 
28f can exhibit high shielding effects. 
As can be understood from the foregoing description, in the construction of 
the microwave generating device, not only the shield casing 28 is able to 
prevent the unnecessary radiation from leaking from the cathode stem 20 of 
the magnetron 13, but the boosting transformer 7 can fulfill its function 
as a transformer although it is separated by the wall surface of the 
shield casing 28. 
The thickness of a metallic foil composing the honeycomb core 28f is 
decided, as is clear from the above description, depending on electric 
characteristics such as the inverter frequency (20 KHz in the present 
embodiment), the lowest frequency of the unnecessary radiation to be 
shielded, and the efficiency of the boosting transformer, etc. However, 
considering the mechanical strength of the honeycomb core 28f, the optimum 
thickness is about 1/10-1/100 the depth of penetration at the inverter 
frequency. 
In the meantime, when a magnetic metal is employed for the honeycomb core 
28f, the honeycomb core 28f generates a branching circuit (shunt circuit) 
in the magnetic circuit of the boosting transformer 7, resulting a 
decrease of magnetic efficiency. Therefore, the honeycomb core 28f is 
preferably formed by a non-magnetic metal such as aluminum employed in the 
present embodiment. However, if nonmagnetic copper (Cu) or aluminum (Al) 
is used for the honeycomb core 28f, since the specific resistance (ohm.m) 
is small, the depth of penetration .delta. is small, giving rise to a 
processing problem. Therefore, the material of the honeycomb core 28f 
should be selected from the total viewpoint, with the above-described 
facts noted. For example, Monel metal, which is an alloy mainly composed 
of nickel and copper, is non-magnetic and, at the same time, relatively 
large in specific resistance, and accordingly, it can be said to be used 
suitably for the honeycomb core 28f. 
In the above-described embodiment, the shielding wall is formed in the 
honeycomb structure, namely, by sticking metallic foils into hexagonal 
cells. However, the shielding wall for shielding electromagnetic waves is 
not restricted to be in the honeycomb structure. 
In other words, the shielding wall may have a structure in which many 
through holes (cells) each having a polygonal cross section, for example, 
a triangular cross section, a square cross section or a hexagonal cross 
section pass through the shielding wall and are placed adjacent to each 
other having a wall surface in common. Of course such shielding wall as 
constructed in the above-described manner will function similarly as the 
shielding wall in the structure of the honeycomb. 
Ventilation openings 28d and 28e are formed in the wall surface of the 
shielding casing 28 so as to cool the secondary side of the boosting 
transformer 7. It is so designed that the secondary side of the boosting 
transformer 7 is forcibly cooled by a cooling fan 2 from outside of the 
shield casing 28. 
It goes without saying that the microwave generating apparatus according to 
the second embodiment operates exhibiting the same function and the same 
effects as the apparatus according to the first embodiment. 
An electronic oven which is designed to act as an induction heating range 
will be described with reference to FIGS. 8 through 11. The construction 
of a stationary induction apparatus according to a third embodiment of the 
present invention is applied to an exciting coil of the electronic oven 
referred to above. 
FIG. 8 shows a cross sectional view of an electronic oven functioned with 
an induction heating range to a heating coil portion of which is applied 
the structure of the stationary induction apparatus of the present 
invention. FIG. 9 shows an essential portion of FIG. 8, namely, the 
heating coil portion of FIG. 8, in detailed manner. 
Referring to FIG. 8, microwaves (high frequency electromagnetic field) 
generated by a magnetron 71 are irradiated into a heating chamber 35 
through a wave guide 72 and a rotary antenna 33. The rotary antenna 33, 
generally in a fan-like configuration, is rotated by a motor 34, so that 
the induction heating is uniformly carried out. 
On the other hand, an exciting coil 38 is provided right over the rotary 
antenna 33, and a partition plate 39 made of a dielectric material such as 
a heat resisting ceramic plate or the like is placed so as to be almost in 
contact with the exciting coil 38. The partition plate 39 serves not only 
to protect the rotary antenna 33 and the exciting coil 38 in the bottom 
within the heating chamber 35, but to place an object 37 to be heated 
thereon. 
The high frequency power is supplied to the boosting transformer for 
driving the magnetron 71 and to the exciting coil 38 by a circuit, e.g., 
shown in FIG. 11. The circuit of FIG. 11 includes a power source 42 of 
commercial frequencies, a smoothing circuit 48 comprised of a rectifier 
47, an inductance 43 and a capacitor 44, an inverter circuit 49 comprised 
of a switching element 46 and a capacitor 45, and a controlling-driving 
circuit 50 which turns the switching element 46 on or off. The high 
frequency voltage at several tens of KHz produced by the 
controlling-driving circuit 50 is added either to the exciting coil 38 for 
induction heating or to the boosting transformer 40 of the magnetron 71 
which is a source of microwaves of the dielectric heating, in accordance 
with switching of a switch 40. 
FIG. 9 is a detailed view showing the construction of the exciting coil 38 
of FIG. 8; FIG. 9(a) being a broken view, and FIG. 9(b) being a plan view 
of FIG. 9(a). The exciting coil 38 in FIG. 9 is comprised of an exciting 
coil winding 38a having a winding wound into a flat board shape, a ferrite 
core 53 provided on the under surface of the exciting coil winding 38a for 
shielding of magnetism, aluminum honeycomb cores 51a and 51b in a 
perforated disc shape provided so as to hold both the exciting coil 
winding 38a and the ferrite core 53 from up and down and, shield rings 54a 
and 54b respectively in the inner peripheral side and the outer peripheral 
side of the aluminum honeycomb 25 cores 51a and 51b. The shield rings 54a 
and 54b are made of non-magnetic metallic plates. Moreover, the shield 
rings 54a and 54b have many slit holes 55a and 55b formed in a radial 
direction, thereby to hinder the flow of the induced current in a 
circumferential direction generated on the shield rings 54a and 54b and to 
prevent the loss and the temperature rise resulting from the heat 
generation. 
Although the structure of the aluminum honeycomb cores 51a and 51b is the 
same as already described with reference to the second embodiment, the 
cell size D and the core height T of the cores 51a and 51b should be 
determined so that the 2450 MHz microwaves employed for dielectric heating 
can be sufficiently shielded, and also the distance between the exciting 
coil winding 38a and the object 37 to be heated is within the range 
possible for induction heating. Accordingly, in the third embodiment of 
the present invention, the core height T for both the cores 51a and 51b is 
1.5 mm, with the cell size D is set to be approximately 3 mm. 
The operation of the electronic oven equipped with the function of the 
induction heating range as configured above will be described below with 
reference to FIG. 10. 
In FIG. 10(a), there is indicated the mutual relationship among the 
exciting coil winding 38a, the line of magnetic flux 61 showing the high 
frequency magnetic field generated by the exciting coil winding 38a, the 
metallic object 37 to be heated which is heated by the current induced in 
the high frequency magnetic field, the ferrite core 53 for shielding the 
leakage of the induced current below the high frequency magnetic field, 
and the aluminum honeycomb core 51a for preventing the exciting coil 
winding 38a from being heated by the high frequency waves, that is, 2450 
MHz microwaves for dielectric heating, or from discharging. Since the 
honeycomb core 51a interposed between the exciting coil winding 38a and 
the object 37 to be heated has the cell size D small enough with respect 
to the wavelength of the high frequency electromagnetic field microwaves) 
at 2450 MHz, each of the cells of the honeycomb core 51a can be regarded 
as a wave guide, and therefore the honeycomb core 51a can display high 
shielding effects (with respect to 2450 MHz) due to the similar effects as 
obtained of the cut-off phenomenon by the wave guide. Meanwhile, the 
relation between the high frequency magnetic field 61 in the 20 KHz 
frequency used for induction heating and the honeycomb core 51a shown in 
FIG. 10(a) is enlarged in FIG. 10(b). The line 61 of magnetic force of the 
high frequency magnetic field is generally parallel to the axis direction 
of the cells composing the honeycomb core 51a. 
Consequently, for the same reason as in the second embodiment, the 
honeycomb core 51a hardly produces any loss with respect to the 20 KHz 
high frequency magnetic field, although the honeycomb core 51a is 
intervened between the object 37 to be heated and the exciting coil 
winding 38a. The honeycomb core 51a works to shield (protect) the exciting 
coil winding 38a from the 2450 MHz high frequency electromagnetic field, 
i.e., microwaves. 
Since the aluminum honeycomb core 51b completely covers the ferrite core 
53, such disadvantage is solved that the ferrite core 53 absorbs the 
microwaves and generates heat. 
Although the above description is made on the assumption that the honeycomb 
core is made of an aluminum foil, naturally, it is not restricted to the 
aluminum foil. However, a non-magnetic metal is better since a branching 
circuit is not formed in the non-magnetic metal in the high frequency 
magnetic field. 
In addition, the aluminum honeycomb core is employed in the upper and lower 
surfaces of the exciting coil winding, and the inner and outer peripheral 
surfaces of the honeycomb core are constituted by a non-magnetic metal 
such as aluminum, but, the employment of the honeycomb core is not limited 
to the range of the present embodiment. 
Moreover, such advantageous characteristics of the honeycomb structure as 
good gas permeability and light weight can be fully utilized. 
Particularly, the good gas permeability of the honeycomb structure is 
effective to control the temperature rise of the exciting coil winding 
38a. 
Accordingly, the application of the structure of the stationary induction 
apparatus of the present invention to the exciting coil will bring about 
the following effects. 
(1) Since the honeycomb coil winding is covered with the honeycomb core 
made of a metallic foil, the exciting coil winding can be prevented from 
generating heat or discharging by the influences of the microwaves, and at 
the same time, the high frequency magnetic field for induction heating 
generated by the exciting coil winding can reach the object to be heated 
without being attenuated, so that the generation of heat in the exciting 
coil at the induction heating time can be inhibited, while maintaining 
high heating efficiency. 
(2) Furthermore, although it is natural, the honeycomb core has gas 
permeability, and accordingly, is effective to cool the exciting coil, so 
that the discharging of the heat can be effectively performed. 
(3) The honeycomb core also covers the ferrite core below the exciting coil 
winding for shielding of the magnetism, and therefore, the ferrite core is 
prevented from absorbing the microwaves and generating heat. 
Due to the above-described advantages, a high frequency heating apparatus 
capable of induction heating and dielectric heating, which has not been 
believed previously to be practical, can be realized, with the exciting 
coil for induction heating provided within the heating chamber for 
dielectric heating, through application of the structure of the stationary 
induction apparatus according to the present invention to the induction 
heating coil of the apparatus. 
Although the present invention has been fully described in connection with 
the preferred embodiments thereof with reference to the accompanying 
drawings, it is to be noted that various changes and modifications are 
apparent to those skilled in the art. Such changes and modifications are 
to be understood as included within the scope of the present invention as 
defined by the appended claims unless they depart therefrom.