JPH0744005B2 - Magnetron for microwave oven - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a magnetron for a microwave oven, and more particularly to a high voltage input terminal fixed to a shield box thereof.

(Prior Art) In a magnetron for a microwave oven, a negative high voltage and heating power for a filament cathode are applied from the cathode input stem side. In a magnetron used for household microwave ovens, this input has an anode voltage of, for example, 4 kV, an anode current of 300 mA, a filament voltage of 3.15 V, and a filament current of 10 A. As a high voltage terminal for introducing such a high voltage and a large current, a feedthrough high voltage capacitor using a ceramic high dielectric element has been generally used conventionally. This is to suppress the noise generated in the magnetron by making the feed-through high voltage capacitor, which is the input terminal, have an electrostatic capacity of several hundreds pF, and further constructing the LC filter together with the choke coil.

By the way, the withstand voltage characteristic of a high-dielectric element to which a high voltage is applied is particularly important. Generally, an insulating resin is coated on the high-dielectric element to protect the interface, but it has a great influence on the manufacturing process. , It is very difficult to control the withstand voltage. For this reason, recently, efforts have been made to simplify the structure of the high-voltage input terminal as much as possible. As long as a high dielectric material is used, there is a limit to its simplification. As a very simplified example, a structure in which a high voltage is supported and a capacitance is formed by a thin layer of insulating resin can be considered.

(Problems to be Solved by the Invention) Molding is used to integrate a plurality of electrodes, which are components of a high voltage input terminal, and an insulating resin. In that case,
Compared with the coefficient of thermal expansion of the electrode metal, that of the resin is considerably large, so that the resin may crack due to heat cycle or the like. Especially when the resin is subjected to tensile stress, the danger is great. Especially in a magnetron for microwave oven, since the choke coil and the shield box, which are the input conductors, become hot during operation, the input terminals attached to this are 100
The temperature may rise above ℃. Therefore, the resin used for it is
Requires sufficient heat resistance. When a mold is used for the structure of the insulating resin, it is necessary to make the high-voltage input terminal have a necessary and sufficient capacitance function. It will be difficult to keep. In addition to the mechanical stress, it becomes relatively difficult to suppress the occurrence of voids and the like and sufficiently exhibit the withstand voltage performance of the insulating resin material itself.

Conventionally known high-voltage input terminals include, for example,
No. 5652, No. 50-58634, No. 60-12696
There is a configuration disclosed in Japanese Patent No. 3 or Japanese Utility Model Laid-Open No. 60-129058. However, in order to provide the electrostatic capacity and the high withstand voltage function only with the insulating resin, the arrangement of each electrode is complicated, the flow of the resin at the time of resin molding becomes complicated, and a resin layer that is homogeneous when viewed microscopically is obtained. I found that I could not get it.

The present invention eliminates the above-mentioned inconveniences and provides a high-voltage input terminal with high reliability, which has a particularly high withstand voltage characteristic while allowing a single mold insulating resin to have an electrostatic capacity and a high withstand voltage function. The purpose is to provide a magnetron.

[Structure of the Invention] (Means for Solving the Problems) The present invention has a coaxial structure as a structure capable of smoothing the resin flow at the time of a mould, by devising the electrode shape of the high-voltage input terminal and the shape of the insulating resin portion. The insulating resin layer of the capacitor formed between the inner cylindrical high-potential electrode and the outer cylindrical ground potential electrode that face each other in close proximity to each other, and the cylindrical insulation protruding around the center conductor on both ends of the insulating resin layer. This is a magnetron for a microwave oven in which a sheath portion has a high-voltage input terminal formed substantially continuously in a straight line and fixed to a shield box wall.

As a result, the relationship between the inner cylindrical high potential electrode and the outer cylindrical ground potential electrode, or the electrical insulation of the cylindrical insulating resin sheath that extends the creepage distance by separating the input center conductor and enlarging the creepage distance The insulating resin in the part to be filled becomes a basic flow in one direction vectorally at the time of molding, has a high voltage input terminal without generation of voids, etc., and a highly reliable magnetron can be obtained.

(Operation) According to the present invention, necessary and sufficient withstand voltage characteristics can be obtained even if the insulating resin filled between the electrodes is made a relatively thin layer. That is, according to the consideration of the present inventors, in the case of resin filling of this kind of high voltage input terminal, the basic resin flow at the time of molding is in the same vector direction, and the abrupt change of the flow direction and the flow cross-sectional area are made. It has been found that it is important and effective to have little change. A high-voltage input terminal has a thin layer of insulating resin that has a capacitance function and a withstand voltage function, and the basic resin flow during molding is in the same vector direction. The structure of the insulating resin can be made extremely good, and not only can it withstand mechanical stress, but also the occurrence of voids can be suppressed and the withstand voltage performance of the insulating resin material itself can be fully exhibited.

(Example) Hereinafter, an example will be described with reference to the drawings. The same parts are represented by the same symbols.

The magnetron is oscillating body 21 as shown in Figure 1
A radiator 22 is fixed to the outer circumference of the anode cylinder, and a ring-shaped permanent magnet 23 is stacked on the anode cylinder.
The oscillator body 21 includes an anode containing a resonance cavity (not shown), a cathode arranged on the central axis of the anode, a pair of pole pieces provided at both openings of the anode, and an electrically connected anode. An output section provided at the tip of an antenna lead (not shown) coupled to the output section, and an input stem section having a cathode lead terminal that supports the cathode and guides heating power thereto are provided. A ferromagnetic yoke 24 is provided so as to surround the permanent magnet 23, and a shield box 26 that covers the input stem portion 25 of the oscillator main body is fixed thereon. In the shield box 26, one end 29a of a choke coil, that is, an inductor 28 is connected to the cathode input terminal 27 of the stem portion, and the other end portion 29b is a U-shaped end portion 31a of the central conductor 31 of the high voltage input terminal 30. Connected by welding.

The high-voltage input terminal 30 includes an inner high potential electrode 32 formed of a bottomed metal cylinder connected to the center conductor, an outer ground potential electrode 33 formed of a metal cylinder coaxially arranged at a predetermined distance outside thereof, and those The insulating resin portion 34 covers the space and the outer circumference of the ground potential electrode.

This high voltage input terminal 30 has two terminals as shown in FIG.
Mounting flange that extends laterally for each of the 30 ground potential electrodes
35, 35 are integrally connected, and the through hole of the shield box
It is attached to part 26a and is electrically and mechanically connected and fixed. Reference numeral 35a indicates a portion where both flange portions 35, 35 are joined by a method described later, and reference numerals 36, 36 ... Represent through holes for attachment. The outer end of the center conductor 31 is
The plate-shaped Faston-type external connection terminal 31b is used as a terminal for electrically connecting the magnetron to an external power supply circuit.

Therefore, the high-voltage input terminal 30 is filled with the insulating resin portion 34 between the inner cylindrical high-potential electrode 32 and the outer cylindrical ground-potential electrode 33 coaxially arranged on the outer side of the inner high-potential electrode 32, and static electricity is kept between both electrodes. A resin portion 34a that constitutes the capacitor portion C that produces a capacitance, and a cylindrical sheath portion 37, 38 that extends to both sides of the capacitor portion C and surrounds both ends of the center conductor at intervals to secure a creeping distance. 34b covering the outer electrode and the outer electrode
Are continuously and integrally formed of a single resin, and are substantially linearly formed in the axial direction of the input terminal.

This high-voltage input terminal 30 has a capacitance of several tens of pF,
Together with the inductor 28, an LC filter that suppresses undesired noise leaking from the input side is configured. In addition, in order to obtain sufficient creepage withstand voltage characteristics, the insulating resin part 34
The outer cylindrical sheath 2 so that it surrounds the external connection terminals.
7 and an inner cylindrical sheath 38 surrounding the central conductor extension within the box.

Next, the configuration of the high-voltage input terminal 30 will be described according to a preferred order of assembly.

First, each electrode component as shown in FIG. 3 is prepared. The center conductor 31 is a faston-type external connection terminal in which a main part of an iron plate having a predetermined length is bent in a U-shaped cross section, and one end is a flat plate.
31b. A through hole 31c for inserting the end 29b of the inductor is formed in a portion near the U-shaped end 31a.
Since the central conductor has a U-shape and a non-rotationally symmetric shape in this way, it is possible to suppress positional changes in the rotational direction of the external connection terminal 31b and the ground potential electrode 33 during resin molding, which will be described later, for stable positioning. Serve as a positive to do. Further, the U-shaped configuration also improves the mechanical strength of the central conductor.

Further, the iron plate is deep-drawn by a press to prepare a cylindrical inner high potential electrode 32 having a bottom. The high potential electrode 32 has a square through hole 32a formed at the bottom thereof, and the center conductor 31 is inserted therein. Then, they are positioned coaxially with each other, and the bottom through hole 32a of the high-potential electrode 32 and the external connection terminal side U-shaped end 31d of the central conductor are electrically and mechanically coupled by welding. Then, these tin platings are applied.
The burrs at the ends of the electrodes are rounded by removing them by tumbling before plating in order to enhance the discharge prevention effect.

On the other hand, the iron plate is deep-drawn by a press to prepare an outer cylindrical ground potential electrode 33 having a flange portion 35 for attachment to the shield box. The flange portion 35 has through holes 36, 36 ... At three corners. This outer electrode 33
Is plated with nickel.

Next, as shown in FIG. 4, each electrode component is positioned using a mold not shown. Outer cylindrical ground potential electrode 33
Are arranged on the outer side of the inner cylindrical high-potential electrode 32 coaxially with a distance of, for example, about 1 mm. The mold not shown has a shape such that the insulating resin portion forms the outer surface shown in each drawing. When molding the insulating resin, resin is injected as shown by arrow S1 from pin point gates P having a diameter of 0.9 mm of the mold provided at three locations on the circumference surrounding the external connection terminal 31b. The insulating resin has, for example, an average diameter of 30 to 70 μm and an average length of 300 to 5 μm.
Flame-retardant engineering plastics such as polybutylene terephthalate (PBT) resin containing about 30% by weight of glass fibers in the range of 00 μm are preferred.

The resin, which has been injected with equal pressure in the direction of gravity, advances in the axial direction as indicated by arrow S2 and simultaneously in the circumferential direction as indicated by arrow S3 in the region which will be the external connection terminal. As a result, the outer cylindrical sheath portion 37 surrounding the external connection terminal 31b is formed. The space for forming the outer cylindrical sheath portion 37 also serves as a runner for running the resin in the circumferential direction during injection molding. That is, as a result, the resin can be made to flow to each part related to the holding of the dielectric withstanding voltage, in a state close to the simultaneous injection from the ring-shaped gate. The outer cylindrical sheath portion 37 is formed to have a height of 5 to 6 mm and an end width of 1.3 mm, for example. The further injected resin flows to the root portion of the external connection terminal 31b, and the inner and outer cylindrical electrodes 32, as indicated by arrows S4 and S5 in the figure.
It flows into the gaps between the 33 and the outer peripheral space of the outer electrode 33 at a substantially uniform pressure. Further, the resin that has passed through the gap between the inner and outer cylindrical electrodes 32, 33 is the inner cylindrical sheath 38 as shown by the arrow S6.
Flow into the area of. As described above, the shapes and arrangements of the electrodes are such that the basic flow when the insulating resin is filled is unidirectionally vectorial. As a result, the insulating resin layer 34a of the capacitor C formed between the cylindrical high-potential electrode and the outer ground potential electrode that are coaxially closely opposed to each other, and the center conductor are provided so as to protrude from both ends thereof. The tubular insulating sheath portions 37 and 38 are continuously formed in a substantially straight line. And by this, the generation of voids is particularly suppressed, and the withstand voltage performance of the insulating resin material itself can be sufficiently exhibited. The rectangular flange of the ground potential electrode 33
The reference numeral 35 extends laterally from the insulating resin portion and is exposed to the outside.

The outer surface of the insulating resin 34 of the high voltage input terminal is 3 μm.
m or less, and particularly preferably 1 μm or less on average with a surface irregularity grade and formed smoothly. As a result, adhesion of moisture or the like to the surface of the insulating resin is suppressed, and deterioration of the withstand voltage performance of the surface is prevented even when the magnetron is used for a long time.

Thus, the high voltage input terminal as shown in FIG. 6 and FIG.
Get 30 For example, the diameter of the inner high-potential electrode 32 is 10 mm, the distance between the inner and outer electrodes forming the capacitor section C is 1 mm as described above, and the overlapping axial length of both electrodes 32, 33 is 13 m.
When the outer cylindrical sheath 37 has a length of 6 mm, the inner cylindrical sheath 38 has an outer diameter of 16 mm, and its length is 15 mm, the relative permittivity of the PBT is about 4. Has a capacitance of about 20 pF. The withstand voltage was 20 kV or more in AC voltage. In particular, the insulating resin is different in type because the portion 34a that forms the capacitance, the sheath portions 37 and 38 that secure the creepage distance, and the outer sheath portion 34b that covers the outer electrode are integrally formed of a single resin. A phenomenon such as deterioration of withstand voltage due to peeling at the material boundary does not occur at all, and a stable high-voltage input terminal can be obtained. Furthermore, by using PBT or the like as the insulating resin, stable performance can be maintained up to about 120 ° C. As a result of various attempts, it was possible to obtain withstand voltage characteristics close to the limit withstand voltage of the material by appropriately adjusting the resin molding conditions. Ring-shaped grooves 39 and 40 are formed in each sheath to further increase the creepage distance.

As shown in FIG. 6, both ends of the inner and outer electrodes 32, 33 are displaced by a predetermined distance L1, L2 along the axial direction,
One of them is formed into a curved surface. Thereby, even if the facing interval between the end portions is relatively small, the concentration of the electric field there can be relaxed and the withstand voltage characteristic can be improved.

Next, as shown in FIG. 8, two identical high voltage input terminals 30 are partially overlapped with each other in which one through hole 36 of each flange portion 35, 35 is formed. It is sandwiched between 41 and 42 and energized while pressure is applied as indicated by arrow F to perform pressure welding. In this way, two high voltage input terminals in which the respective flange portions 35 as shown in FIGS. 2 and 9 are aligned and integrated on the same surface are completed. By welding both flanges after resin molding, the positioning accuracy of the high-potential electrode and the ground-potential electrode related to the dielectric strength can be assembled independently, and the resin can be filled and the high-voltage input terminal is highly reliable. Is obtained. Furthermore, it becomes easy to press each ground potential electrode alone. This pair of high voltage input terminals has 6
Since the individual mounting through holes 36 are provided, they are fixed to the shield box by caulking or the like. As a result, the shield box and the flange portion of the high-voltage input terminal are more closely contacted with each other, and the risk of radio wave leakage is eliminated.

In the embodiment shown in FIG. 10, the sides 35b, 35b of the ground potential electrode flange portions 35, 35 of the two high voltage input terminals 30 are formed in a crank shape, and they are butted and welded together. It is a thing. As a result, the sides to be joined of the respective flanges are clear, and automatic mechanization of assembly is facilitated.

In the embodiment shown in FIGS. 11 to 13, the ring-shaped groove formed in each of the sheath portions 37 and 38, and a part of the outer periphery of the sheath portion, a material having tracking resistance better than the insulating resin 34 , Ring-shaped barriers 43, 44, 45 made of resin such as epoxy resin or silicon rubber are filled. Thus, even if a discharge occurs on the surface, the carbonized path on the insulating resin surface due to the discharge is surely blocked and prevented by this barrier portion, and the continuation of the discharge is prevented. Further, even if the surface of the resin portion is condensed, the withstand voltage characteristic is prevented from being deteriorated.

In the embodiment shown in FIG. 14, ring-shaped barriers 45a and 45b provided on the outer periphery of the insulating resin are formed so as to slightly project from the insulating resin surface. Thereby, the effect of barrier formation can be further enhanced.

In the embodiment shown in FIG. 15, the end portion of the cylindrical outer ground electrode 33 is folded outward to form a folded portion 46, which is further expanded from the middle to the outside to form a flange portion 35 for attachment to the shield box.
Is formed. And the length of the inner cylindrical high potential electrode 32
L3 is set to about 1/4 of the third harmonic wavelength, and the opening end portion 32b thereof is located behind the folded-back portion 46 of the outer ground electrode 33 by a predetermined distance L4. Thereby, the inner high potential electrode
A choke groove 32c, which has a high impedance with respect to the third harmonic when viewed from the oscillator body side, is formed between 32 and the central conductor 31 passing therethrough, and the effect of further suppressing the external leakage of this harmonic component is provided. can get. The inner high potential electrode 32
The length L3, that is, the depth of the choke groove, is not limited to the third harmonic, and can be set to a dimension that exhibits a choke effect on other harmonics. Alternatively, this inner high potential electrode 32
One or a plurality of conductor cylinders having different lengths may be coaxially connected to each other inside, and a plurality of choke grooves may be formed so as to obtain a choke effect with respect to arbitrary harmonics. Also in these cases, the capacitor part C made of insulating resin is used.
The resin 34a and the cylindrical sheath portions 37, 38 on both sides are single and continuous, and are substantially aligned.

The embodiment shown in FIG. 16 to FIG. 18 is a disk-shaped part near the end of the capacitor part C of the high-voltage input terminal 30 on the external circuit side, that is, at the root of the faston-type external connection terminal 31b of the central conductor 31. The high-frequency absorber 47 is fitted, and the central portion through hole 48a and the short cylindrical portion 48b are formed on the outer surface and outer peripheral surface thereof.
And a dish-shaped high-frequency reflection conductor 48 having the above is closely arranged. The high-frequency absorber 47 is a material that absorbs microwaves such as ferrite, ceramics such as silicon nitride (SiC), carbon, or a material in which ferromagnetic powder generally called polyiron is molded with an organic insulator. And a through hole 47a for allowing the external connection terminal 31b to pass therethrough is formed at a predetermined position. The high frequency reflecting conductor 48 is made of a suitable thin metal plate such as aluminum or stainless steel. The reflective conductor may be a conductive thin film deposited on the surface of the absorber. The outer diameter of the high-frequency reflection conductor 48 that is in close contact with the high-frequency absorber 47 and the surface on the side of the external connection terminal thereof is larger than the outer diameter of the inner cylindrical high-potential electrode 32, and the cylindrical sheath portion 37. It is fitted inside and is embedded with a PBT resin cover 49.

According to this embodiment, a part of the high frequency wave that leaks from the input terminal to the outside as shown by the arrow Rf in FIG.
While being absorbed by the high-frequency absorber 47, a part of it is reflected by the high-frequency reflection conductor 48 and again absorbed by the absorber,
External leakage is suppressed. This high-frequency absorption effect is particularly effective in absorbing the second harmonic, the third harmonic, and spurious components having frequencies close to those.

That is, in the structure shown in FIG. 17, when the outer diameter of the inner cylindrical high potential electrode 32 is 11 mm, the outer diameter d of the high frequency absorber 47 is 12 mm.
FIG. 19 shows the measurement results of the high frequency leakage suppression effect when the thickness is set to mm and the thickness t is set to 1.5 mm. This is when the frequency f0 of the oscillation fundamental wave is 2450MHz, and its second harmonic (2f
The external leakage levels of the components of 0), the third harmonic (3f0), and the fourth harmonic (4f0) were measured. Curve A shows the harmonic leakage level of the five magnetrons leaking to the external power supply side when there is no high-frequency absorber or high-frequency reflecting conductor. On the other hand, the curve B is the measurement result of five pieces with the high-frequency absorber and the high-frequency reflection conductor attached. As is clear from this comparison, the one in which the high-frequency absorber and the high-frequency reflecting conductor are mounted has a remarkable effect of suppressing the leakage of the second and third harmonic components. Therefore, a remarkable leakage suppression effect of spurious components having frequencies around this is also obtained.

The outer diameter of the high-frequency reflection conductor 48 that is in close contact with the surface of the high-frequency absorber 47 or its external connection terminal side is larger than the outer diameter of the inner cylindrical high-potential electrode 32 as described above, and particularly preferably. The size is set to be equal to the inner diameter of the outer cylindrical ground potential electrode 33 or slightly larger than the inner diameter of the outer cylindrical ground potential electrode 33 within a range that can be fitted inside the cylindrical sheath portion 37. As a result, the effect of absorbing the high frequency component Rf, which particularly passes through the resin portion of the capacitor portion C, by the absorber can be obtained more reliably.

Furthermore, the high-frequency absorber and the high-frequency reflection conductor are
It may be provided inside the capacitor portion C, that is, at the inductor 28, or both.

The embodiment shown in FIGS. 20a and 20b is a high voltage input terminal 30 in which two capacitor parts are integrated. The inner high potential electrode has two metal cylinders 32m and 32n arranged side by side. The inner high-potential electrode may be formed by dividing a metal cylinder having an elliptical cross section into two equal parts, electrically separating them, and arranging them in parallel with each other. The outer ground potential electrode surrounding them is composed of a single metal cylinder having an elliptical cross section. Further, a high-frequency absorber 47 having two through holes 47a and a dish-shaped high-frequency reflecting conductor 48 having two through holes 48a in close contact with the high-frequency absorber 47 at the base of the external connection terminal 31b. It is fitted and provided in close contact with the surface on the external connection terminal 31b side, and is embedded with an insulating resin (not shown). As a result, it is possible to obtain a high voltage input terminal that is compact and has a high frequency leakage suppression effect.

[Effects of the Invention] As described above, according to the present invention, even if a thin layer of an insulating resin having a capacitance function and a withstand voltage function is provided for a high voltage input terminal, a basic resin flow at the time of molding is achieved. Since they are in the same vector direction and are not accompanied by sudden changes in the flow direction and changes in the flow cross-sectional area, the microstructure of the insulating resin after molding can be made extremely good and mechanical stress can be reduced. Not only does it cease, but the occurrence of voids and the like can be suppressed and the withstand voltage performance of the insulating resin material itself can be fully exerted.

Thus, even if a high voltage input terminal having a relatively simple structure is used, it is possible to obtain a magnetron for a microwave oven, which has an excellent withstand voltage characteristic and has little high frequency leakage to an external power supply circuit.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view of an essential part showing an embodiment of the present invention.
FIG. 4 is a side view of a main part thereof, FIG. 3 is a perspective view of each electrode before assembly, FIG. 4 is a cross-sectional view schematically showing the flow of resin at the time of resin filling, FIG. FIG. 7 is a longitudinal sectional view after resin filling, FIG. 7 is a side view thereof, FIG. 8 is a top view showing a state in which a plurality of pieces are integrated, and FIG. 9 is a top view showing a completed state.
FIG. 10 is a side view of an essential part showing another embodiment of the present invention, and FIG.
The figure is a longitudinal sectional view of a main part showing still another embodiment of the present invention,
FIG. 12 is a cross-sectional view taken along line 12-12 of FIG. 11, FIG. 13 is a top view of the integrated body, and FIG. 14 is a top view of essential parts showing still another embodiment of the present invention. FIG. 16 is a longitudinal sectional view of an essential part showing still another embodiment of the present invention, FIG. 16 is a longitudinal sectional view of an essential part showing still another embodiment of the present invention, FIG. 17 is an enlarged view of the essential part, and FIG. FIG. 19 is a perspective view of a main part thereof, FIG. 19 is a characteristic diagram showing a comparison of harmonic leakage levels, FIG. 20a is a side view of a main part of still another embodiment of the present invention, and FIG. It is a principal part perspective view. 21 …… Oscillator main unit, 25 …… Stem part, 26 ・ ・ ・ Shield box, 28 …… Inductor, 30 …… High voltage input terminal, C ……
Capacitor part, 31 ... Central conductor, 31b ... External connection terminal, 32 ... Cylindrical inner high potential electrode, 32c ... Choke groove, 3
2 …… Cylinder outer ground potential electrode, 35 …… Flange part, 35a…
… Joining parts, 34, 34a, 34b …… Insulating resin parts, 37, 38 …… Cylindrical sheath part, 43, 44, 45, 45a, 45b …… Barriers for tracking prevention, 46 …… Folding part, 47 …… High frequency absorber, 48 ...
… High frequency reflection conductor, 48a …… Short tube.

Claims (8)

[Claims] 1. An anode containing a resonant cavity, a cathode arranged on the central axis of the anode, a pair of pole pieces provided in both openings of the anode, and an antenna electrically coupled to the anode. An oscillator main body having an output part provided at the tip of the lead and an input stem part having a cathode lead terminal that supports the cathode and guides heating power to the cathode, and a permanent magnet magnetically connected to the oscillator main body. And a shield box surrounding the input stem of the oscillator body, and a part of the outer cylindrical ground potential electrode is fixed to the shield box wall, and the cylindrical high potential is inside the ground potential electrode. The electrode is arranged and the center conductor is penetrated inside it,
An insulating resin is filled between these two electrodes to form a capacitor part, and one end of the center conductor is connected to the cathode lead of the input stem part through an inductor and the other end is connected to an external circuit for external connection. In a magnetron for a microwave oven, which comprises a high-voltage input terminal used as a connection terminal, the high-voltage input terminal is a capacitor formed between an inner cylindrical high-potential electrode and an outer ground-potential electrode coaxially closely facing each other. A magnetron for a microwave oven, characterized in that an insulating resin layer of a portion and a cylindrical insulating sheath portion projecting from both end sides of a central conductor are continuously formed in a straight line. 2. The magnetron for a microwave oven according to claim 1, wherein the insulating resin of the high voltage input terminal is a polybutylene terephthalate resin containing glass fiber. 3. The high voltage input terminal is provided with a tracking barrier made of an insulating material having a tracking resistance higher than that of the insulating resin on a part of an outer peripheral surface of the insulating resin. A magnetron for a microwave oven according to the item. 4. A high-voltage input terminal is provided with a high-frequency absorber near the end of the capacitor portion on the side of the external connection terminal, surrounding the central conductor, and provided inside the cylindrical insulating sheath portion with a high-frequency absorber. The magnetron for a microwave oven according to claim 1, wherein the magnet is covered with a high-frequency reflection conductor. 5. The high voltage input terminal according to claim 4, wherein an outer diameter of the high frequency absorber or the high frequency reflection conductor is larger than an outer diameter of the inner cylindrical high potential electrode. The described magnetron for a microwave oven. 6. The high-voltage input terminal according to claim 4, wherein the outer diameter of the high-frequency absorber or the high-frequency reflection conductor is equal to or larger than the inner diameter of the outer ground potential electrode. A magnetron for a microwave oven according to the item. 7. The high voltage input terminal according to claim 1, wherein the inner cylindrical high potential electrode and the outer ground potential electrode are arranged such that ends thereof are displaced by a predetermined dimension along the axial direction. Magnetron for microwave oven. 8. The magnetron for a microwave oven according to claim 1, wherein the outer surface of the insulating resin of the high-voltage input terminal is smooth with an uneven grade of 3 μm or less on average.