Magnetron and microwave heating device

Details Magnetron and microwave heating device Magnetron and microwave heating device

2002-05-21

2003-11-18

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Combined load device or load device temperature modifying...

Distributed parameter resonator-type magnetron

C315S005130, C333S182000

Reexamination Certificate

active

06650057

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
This application is related to Japanese Patent Application No. 2001-152195 filed on May 22, 2001 whose priority is claimed under 35 USC §119, the disclosure of which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetron. More specifically, it relates to a highly reliable magnetron used in microwave heating devices such as microwave ovens or radars. The magnetron inhibits heat generation by an inductor which constitutes a noise-suppressing filter circuit provided on the input side of the magnetron and prevents reverse heating of a cathode of the magnetron.
2. Description of Related Art
Microwave ovens, which are one of microwave devices, are widely used throughout the world. When noise is caused by the microwave oven, it generates noise in radios, television sets and other telecommunication equipment and their normal operation may be hindered. Therefore, the noise from the microwave oven needs to be prevented. The noise from the microwave oven is mainly generated by a magnetron used as a source of microwave oscillation. The noise caused by the magnetron widely ranges from a low-frequency band of several hundred kHz to a high-frequency band of several dozen GHz.
In a conventional magnetron in which a cathode is arranged in the center of an anode tube and an anode cavity is formed around the cathode, a low-pass filter is utilized as one of means of preventing the noise. As shown in
FIG. 7
to
FIG. 9
, the low-pass filter is comprised of an inductor
54
which is connected to cathode terminals
52
and
53
of a magnetron main body
51
to constitute a noise-suppressing filter circuit on the input side (hereinafter referred to as a cored inductor), and a feed-through capacitor
56
. The cathode terminals
52
and
53
, the cored inductor
54
and the feed-through capacitor
56
are shielded in a filter box
57
. The technique of preventing the noise by using the low-pass filter is most commonly adopted to prevent the noise caused by the magnetron in the microwave oven.
The cored inductor
54
is a radio wave absorber and comprised of a core
54
b
made of ferrite having high relative permeability and a coil
54
a
made of a cupper wire coated with an insulating material such as polyamide imide and wound about the outer circumference of the core
54
b
so that turns thereof are in close contact. The cored inductor
54
is connected to the cathode terminals
52
and
53
with the intervention of an electrical linear portion
54
c.
The length of the linear portion
54
c
is adjusted so that impedance on the cathode terminals
52
and
53
as observed from the cathode will be infinitely large. From the viewpoint of designing the magnetron, the length of the linear portion
54
c
is one of important factors in order to prevent a fundamental frequency of the microwave induced by the cathode (an oscillated frequency, e.g., a microwave of 2,450 MHz) from leaking out of the cathode terminals
52
and
53
. Therefore, the length of the linear portion
54
c
is optimally determined in accordance with the design of the magnetron main body
51
.
If the fundamental oscillation frequency generated in the magnetron main body
51
, for example, part of a microwave output of 2,450 MHz, is leaked into the cathode terminals
52
and
53
together with the noise, the oscillated microwave is wasted and the core
54
b
absorbs the microwave energy. As a result, the oscillation efficiency decreases. Further, if large microwave energy is leaked out, the core
54
b
generates heat and an insulative coating on the coil
54
a
is burned out to cause dielectric breakdown, or the temperature of the feed-through capacitor
56
connected in series is raised to cause dielectric breakdown. Therefore, the length of the linear portion
54
c
is adjusted so that the impedance on the cathode terminals
52
and
53
as observed from the cathode will be the maximum to reduce the leakage of the microwave energy.
Japanese Patent Publication No. 57(1982)-17344 describes a technique of reflecting or attenuating the leaked microwave energy by connecting between the cored inductor and the cathode terminals an air-core inductor which does not have the ferrite core.
However, in view of characteristics of the magnetron, a phenomenon called reverse heating of the cathode should be considered. Among electrons rotating between the anode tube and the cathode of the magnetron, generated are electrons which are accelerated by a high-frequency electric field and then collide against the cathode. The reverse heating of the cathode is a phenomenon in which a filament to constitute the cathode is heated and damaged by the collision of the electrons. If load impedance increases due to the reverse heating of the cathode, life of the filament may be extremely shortened.
The reverse heating of the cathode can also be optimized by adjusting the length of the linear portion
54
c
extending from the cathode terminals
52
and
53
to the cored inductor. However, as shown in
FIG. 10
, the optimum length with respect to the reverse heating of the cathode and that with respect to the temperature variation of the cored inductor do not agree with each other.
FIG. 10
shows the temperature variation of the cored inductor
54
(solid line R
1
) and the reverse heating of the cathode of the magnetron (broken line R
2
) with respect to the length of the linear portion
54
c
of the conventional magnetron, respectively. The reverse heating of the cathode of the magnetron is shown by the ratio of filament currents before and after the oscillation expressed as a percentage. The smaller the value is, the greater influence is caused by the reverse heating of the cathode.
Even in the case where the air-core inductor without the core is connected between the cored inductor and the cathode terminals as described in Japanese Patent Publication No. 57(1982)-17344, the length of the linear portion extending from the air-core inductor to the cathode terminals needs to be optimized. Therefore, the design of the inductor is complicated and the size of the whole inductor may increase. Accordingly, an insulating distance between the inductor and the filter box needs to be ensured, which inevitably increases the size of the filter box as well.
SUMMARY OF THE INVENTION
The present invention has been in view of the above-described circumstances. An object of the present invention is to provide a highly reliable magnetron capable of inhibiting heat generation by the inductor which constitutes the noise-suppressing filter circuit provided on the input side of the magnetron and preventing the reverse heating of the cathode of the magnetron, without increasing the size of the filter box.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter.
In order to achieve the above object, according to the present invention, there is provided a magnetron comprising a cathode terminal of a magnetron main body and an inductor connected to the cathode terminal to constitute a filter, wherein the inductor includes an air-core coarse inductor and a cored inductor connected in series, the air-core coarse inductor being connected to the cathode terminal side and the air-core coarse inductor includes a large pitch winding (hereinafter referred to as a large pitch winding) provided on the cathode terminal side and a small pitch winding (hereinafter referred to as a small pitch winding) provided on the opposite side.
In the invention, it is preferred that an interval between the air-core coarse inductor and the cored inductor connected in series is 3.0 mm or more. It is also preferred that an interval between turns of the small pitch winding is 1.0 mm or less. Further, it is preferred that an interval between turns of the large pitch winding is 1.5 mm or more. Still Further, it is preferred that the number of turns of the small pitch winding is 1.5 or more.
However, it should be unders

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