US3846667A

US3846667A - Magnetron having external choke structure

Info

Publication number: US3846667A
Application number: US00372903A
Authority: US
Inventors: H Hisada; A Harada
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1972-06-30
Filing date: 1973-06-25
Publication date: 1974-11-05
Anticipated expiration: 1991-11-05
Also published as: JPS5352374Y2; JPS4934666U; DE2333221A1

Abstract

A magnetron comprising an air-tight vacuum envelope including a cylindrical anode having a plurality of vanes, a cathode for thermionic emission arranged on the central axis of the anode and magnets for applying magnetic field to the working space between the anode and cathode. The magnetron is further provided with a choke structure having an opening in the vicinity of the input terminal of the air-tight vacuum envelope, the choke structure being arranged outside of the envelope and with its effective length almost one fourth of the wavelength of microwave energy generated by the magnetron.

Description

O Unlted States Patent [1 1 [111 3,846,667 Hisada et al. Nov. 5, 1974 MAGNETRON HAVING EXTERNAL CHOKE 3,551,735 12/1970 Staats 315/3953 STRUCTURE 3,584,255 6/1971 Staats INS/39.53

[75] Inventors: gi 3 2; 223; @225 Harada Primary Examiner-James W. Lawrence Assistant Examiner-Saxfield Chatmon, Jr. Assigneel Hitachi, -s Tokyo, Japan Attorney, Agent, or Firm-Craig & Antonelli [22] Filed: June 25, 1973 211 Appl. No.: 372,903 [57] ABSTRACT A magnetron comprising an air-tight vacuum envelope [30] Foreign Application Priority Data including a cylindrical anode having a plurality of J 30 1972 J 47 76597 vanes, a cathode for thermionic emission arranged on une apan the central axis of the anode and magnets for applying magnetic field to the working space between the 315/3953 315/ anode and cathode. The magnetron is further pro- 58] Fieid 39 51 vided with a choke structure having an opening in the vicinity of the input terminal of the air-tight vacuum envelope, the choke structure being arranged outside [56] References Cned of the envelope and with its effective length almost UNITED STATES PATENTS one fourth of the wavelength of microwave energy 3,315,121 4/1967 Staats 315/3953 X generated by the magnetron. 3,377,562 4/1968 Staats INS/39.53 3,543,082 11/1970 Boehm 3l5/39.53 X 3 Claims, 3 Drawing Figures PAIENIEIJNUV 51914 $846166!

sum

1 or 3 FIG.

PRIOR ART PATENTEDHBV 51974 3.846667 SHEET 3!!! 3 MAGNETRON HAVING EXTERNAL CHOKE STRUCTURE The present invention relates to a magnetron or more in particular to an improved input structure of the magnetron.

FIG. 1 is a sectional view showing the essential parts of an example of the conventional magnetrons,

FIG. 2 is a sectional view showing the essential parts of an embodiment of the present invention; and

FIG. 3 is a sectional view showing the essential parts of another embodiment of the present invention.

The magnetron which generates microwave energy effectively is used for the microwave oven, defreezer and the like and there are various constructions suggested therefor.

The sectional view of FIG. 1 shows an example of the magnetron of inner-magnet type in general use. In the figure, reference numeral 1 shows an anode cylinder made of oxygen free copper, numeral 2 a plurality of vanes fixed in such a manner as to divide equally the inner wall of the anode cylinder 1, and numeral 3 an anode comprising the anode cylinder 1 and vanes 2. Reference numerals 4 and 5 show conical magnets fixed on the sides of the anode cylinder 1 and numeral 6 a cup-shaped input sealing metal having a bottom portion 6a and a through hole 6b and fixed on the magnet 4. Numeral 7 shows an input insulating material arranged in abutment with the bottom portion 6a of the sealing metal 6 and hermetically fixed on the sealing metal 6 at a connection 60. Reference numeral 8 shows a spiral cathode of thoriated tungsten wire or the like for thermionic emission arranged coaxially with the anode cylinder 1 within the space defined by the edges of the vanes 2. The edges of the vanes 2 are in predetermined spaced relationship with the spiral cathode 8 thereby to form a working space 9. Reference numerals l0 and 11 show end shields arranged on the ends of the spiral cathode 8 for preventing the diffusion of electrons in the direction of the axis of the tube,

numerals

12a and 12b side supports each with an end passed through the through hole 6b of the sealing metal 6 and fixed within the insulating material 7, and numeral 12c a center support similar to the side supports 12a and 12b through which working current is supplied, the other ends of the

supports

12a, 12b and 12c being fixed on the spiral cathode for thermionic emission.

Reference numerals

13a, 13b and 13c show sealing members for hermetically fixing the

supports

12a, 12b and 12c, numeral 14 a radiator for cooling the anode 3, numeral 15 an output sealing member fixed on the magnet 5 numeral 16 a cylindrical output insulator fixed on the sealing member 15, numeral 17 an output terminal fixed on the insulator 16, and numeral 18 a metal rod with the ends thereof connected to one of the vanes 2 of the anode 3 and output terminal 17 respectively, so that microwave output generated in the anode 3 is transmitted through metal rod 18 to the output terminal 17 from which it is emitted outside. Reference numeral 19 shows a magnetic device comprising a permanent magnet,

numerals

20 and 21 yokes for introducing to the working space the magnetic field generated by the magnetic device 19,

numerals

22 and 23 inductors connected to the

supports

12a and 12c,

numerals

25 and 26 connectors for connecting the

supports

12a and 12c to the

inductors

22 and 23, and numeral 24 a capacitor connected to the

inductors

22 and 23, the

inductors

22 and 23 and the capacitor 24 functioning as a filter to erase noise. Numerals 27 and 28 show shield cases for shielding noise waves radiated from the

inductors

22 and 23, and numeral 29 a cylindrical stepped metal member with a flange 29a at one end thereof which is fixed on the shield case with a plurality of screws 30, while the other end of the metal member is forcibly fitted on the input sealing metal member 6 to prevent the noise wave from being radiated outside through the magnetic device. Reference numeral 31 shows an insulating tube for preventing dielectric breakdown between the supports 12a to 12, sealing members to 13c or

connectors

25 and 26 and

shield cases

27 and 28, screws 30 or the cylindrical stepped metal member 29. As will be seen from the above description, the air-tight vacuum envelope comprises various component members including the supports 12a to 12c, input insulator 7, input sealing metal member 6, magnets 4 and 5, anode 3, output sealing member 15, output insulator 16 and output terminal 17.

In the magnetron with the above-described construction, most of the microwave energy is utilized by being taken outside through the output terminal 17, while small part of the microwave energy is coupled to the spiral cathode 8 and adversely affects the

inductors

22 and 23 through the

supports

12a and 12b, with the result that the

inductors

22 and 23 are heated to abnormally high temperatures, thereby breaking the same in an extreme case. Also, in view of the fact that the input insulator 7 positioned between the input sealing metal member 6 and the sealing members 13b and 13c has an impedance Za to high-frequency current and the shield case 28 has a certain impedance Zb, that part of microwave energy which has been transferred to the

supports

12b and 12c is reflected for lack of the matching of impedances and the reflected microwave energy generates an abnormal standing wave in the spiral cathode 8, so that the spiral cathode for thermionic emission is abnormally heated, resulting in unstable electrical characteristics and short life of the magnetron.

These disadvantages depend to a large measure on the shape of the shield case 28 and the shape and positions of the

inductors

22 and 23, and in spite of the amount of time and labor consumed for design work to determine the shape and position of the shield case 28 and

inductors

22 and 23 and the position of the cylindrical stepped metal member 29 of the conventional magnetrons, it is very difficult to meet the requirements for elimination of the above-mentioned disadvantages.

An object of the presentinvention is to provide a magnetron with a long useful life.

Another object of the invention is to provide a magnetron with stable electrical characteristics.

According to the invention, there is provided a magnetron comprising an air-tight vacuum envelope, a cy lindrical anode including a plurality of vanes, a cathode arranged on the central axis of the anode for thermionic emission, an input side insulator, a plurality of magnets for applying magnetic field to the working space between the anode and cathode, a plurality of supports for supporting the cathode through the input side insulator, working current being supplied through the supports, an input side sealing metal member interposed between the anode and the input side insulator, the input side sealing metal member being hermetically attached to the input side insulator, an output side sealing member, an output side insulator, and an output terminal, the air-tight vacuum envelope being defined by the anode, the magnets, the input side insulator, the input side sealing member, the output side sealing member, the outputside insulator and the output terminal, wherein the magnetron further comprises a choke structure having an opening in the vicinity of the hermetic connection between the input side insulator and the input side sealing metal member, the choke structure having an effective length approximately one fourth of the wavelength of microwave energy oscillated by the magnetron and being arranged on the outside of the air-tight vacuum envelope.

The inventors have developed an improved magnetron which obviates the disadvantages of the prior art magnetron on the basis of technical concept that it is possible to increase greatly the transmission attenuation and enlarge considerably the impedance as viewed from the opening to the choke by providing the choke of a wave-guide or a coaxial cable with an inner conductor having the effective length approximately one fourth of the wavelength involved.

The present invention will be explained in detail below with reference to the accompanying drawings.

A sectional view of an embodiment of the invention is shown in FIG. 2, in which the same reference numerals are attached to similar component elements as in FIG. 1. In FIG. 2, reference numeral 32 shows a metal cylinder with a flange which is arranged on the outside of the air-tight vacuum envelope of the magnetron and fixed on the shield case 28 with the screw 30 through the flange 32a and the flange 29a of the cylindrical stepped metal member 29 which is closely attached to the flange 32a. The cylindrical portion 32b is arranged coaxially with the cylindrical stepped metal member 29 and the sealing metal member 6 and extends toward the connection 60 between the input sealing metal member 6 and the input insulator 7 while leaving predetermined space 33. The connection 60, the cylindrical stepped metal member 29 and the flanged cylinder 32 constitute a choke structure with the space 33 as an opening in such a manner that the effective length lof the choke is set at approximately one fourth of the length of the microwave generated by the magnetron, by appropriately determining the length of the cylindrical portion 32b of the flanged cylinder 32 and the length of the large diameter cylinder portion 29b of the metal member 29.

The above-mentioned arrangement permits impedance Zc of the choke as viewed from the opening of the space 33 to be increased greatly in spite of the three supports 12a to 120 being arranged internally, and as a result both the impedance Za due to the input insulator 7 and impedance Zb within the shield case 28 are negligible. For this reason, the standing wave occurring in the cathode 8 depends on the position of the opening of the choke structure and impedance Zc thereof, so that by locating an end of the opening of the choke structure at the connection 6c of the sealing metal member as in the above-described embodiment, it is possible not only to minimize the standing wave in the cathode but also to maximize the tolerance of effective length of the choke. This, together with the employment of such choke structure, contributes to the advantage of greatly increased transmission attenuation, resulting in great attenuation of micro-wave energy transmitted into the shield case 28 thereby to prevent abnormal temperature increase of the

inductors

22 and 23.

Further, the insulating tube 31 is interposed between the flanged cylinder 32 and the insulator 7 in such a manner as to cover the space 33, so that the effect of the dielectric constant of the insulating tube 31 increases the degree of coupling of the choke structure, resulting in an increased efficiency of the choke.

Another embodiment of the invention comprising a magnetron of inner magnet type is illustrated in F IG.3.

In this drawing, reference numeral 42 shows a cupshaped stepped metal member with an end 42a of the large-diameter side wall thereof fixed on an appropriate position on the shield case 48, while the smalldiameter bottom portion 42b is arranged around the input insulator 47 in such a manner as to provide predetermined space 43 between the bottom portion 42b and the connection 46a of the input sealing metal member 46. In this way, the cup-shaped metal member 42, part of the shield case 48 and the connection 46a make up a choke structure with the space 43 as an opening. It is needless to say that the size of the cupshaped metal member 42 as well as the distance from the connection between the shield case 48 and the sealing metal member 46 to the connection of the shield case 48 and the part 42a of the large-diameter portion of the cup-shaped metal member 42 is so selected that the effective length I of the choke is approximately one fourth of the wavelength of the microwave generated by the magnetron. Reference numeral 44 shows a capacitor, numeral 49a magnetic device and numeral'50 a magnetron proper.

As will be noted from the above description, the mag-- netron according to the present invention is characterized by a choke structure provided outside of the airtight vacuum envelope of the magnetron and with its opening situated in the vicinity of the input insulator and the connection of the sealing metal member'so that the effective length of the choke structure is approximately one fourth of the wavelength of the micro-wave energy generated by the megnetron, resulting in a high impedance of the choke structure. For this reason, both the impedance due to the insulator and the impedance within the shield case are negligible and the fact that microwave energy transmitted into the shield case is greatly attenuated permits the prevention of the tem perature of the inductors from being increased to an abnormally high level. Also, since the opening of the choke is situated in the vicinity of the periphery of the input insulator and the sealing metal member, the standing wave generated in the cathode for thermionic emission is minimized while the tolerance of the choke effective length l is maximized. Further, in view of the fact that the choke structure is located outside of the air-tight vacuum envelope of the magnetron, the assembly work of the magnetron is facilitated and an abnormal temperature increase which is often seen in the prior art magnetron containing the choke inside thereof is prevented. Furthermore, the provision of the choke structure outside of the air-tight vacuum envelope makes possible impedance adjustment even after the completion of magnetron in accordance with the characteristics requirements thereof. Still another advantage of the magnetron according to the invention resides in the fact that the shield case, inductors and other equipment are easily designed because it is only their breakdown voltages that require to be taken into consideration because of actually neglecting the effect of microwave energy upon such equipment.

What we claim is:

l. A magnetron comprising an air-tight vacuum envelope, a cylindrical anode including a plurality of vanes, a cathode arranged on the central axis of said anode for thermionic emission, an input side insulator, a plurality of magnets for applying a magnetic field to the working space between said anode and cathode, a plurality of supports for supporting said cathode through said input side insulator, means for supplying working current through said supports to said cathode, an input side sealing metal member interposed between said anode and said input side insulator, said input side sealing metal member being hermetically closely attached to said input side insulator, an output side sealing member, an output side insulator, and an output terminal; said air-tight vacuum envelope being defined by said anode, said magnets, said input side insulator, said input side sealing member, said output side sealing member, said output side insulator and said output ter minal, wherein said magnetron further comprises a choke structure having an opening in the vicinity of the connection between said input side insulator and said input side sealing metal member, said choke structure having an effective length approximately one fourth of the wavelength of microwave energy oscillated by said magnetron and being arranged on the outside of said air-tight vacuum envelope.

2. A magnetron according to claim 1, further comprising an insulating tube fitted around the periphery of said input side insulator and extending from said connection between said input side insulator and said input side sealing metal member, said choke structure comprising a first flanged metal cylinder and a second flanged stepped metal cylinder, said first flanged metal cylinder having a cylindrical portion fitted around the periphery of said insulating tube and extending toward said connection, said cylindrical portion maintaining predetermined spaced relationship with said connection, said second flanged stepped metal cylinder being arranged cocentrically with said first flanged metal cylinder and having a small-diameter cylinder portion forcibly fitted on said input side sealing metal member, said second flanged stepped metal cylinder further having a large-diameter cylinder portion with a diameter larger than the outer diameter of said cylindrical portion of said first metal cylinder, the flange of said second flanged stepped metal cylinder being closely attached to the flange of said first flanged metal cylinder.

3. A magnetron according to claim 1, further comprising a shield case covering the exterior component elements on the input side of said air-tight cacuum envelope, said shield case having part of the bottom thereof comprising part of said input side sealing metal member, said shield case containing a stepped cupshaped metal member concentrically arranged with said input side insulator and input side sealing metal member, said stepped cup-shaped metal member having a bottom with a hole through which said input side insulator is passed, said stepped cup-shaped metal member having a large-diameter portion with its inner bottom surface spaced from the inner bottom surface of said shield case, said stepped cup-shaped metal member having a small-diameter portion with its inner bottom surface spaced from the upper surface of said input side sealing metal member, the wall end of said large-diameter portion of said stepped cup-shaped metal member being fixed on the inner bottom surface of said shield case; said choke structure comprising said stepped cup-shaped metal member, part of said shield case and part of said input side sealing metal member.

Claims

1. A magnetron comprising an air-tight vacuum envelope, a cylindrical anode including a plurality of vanes, a cathode arranged on the central axis of said anode for thermionic emission, an input side insulator, a plurality of magnets for applying a magnetic field to the working space between said anode and cathode, a plurality of supports for supporting said cathode through said input side insulator, means for supplying working current through said supports to said cathode, an input side sealing metal member interposed betweeN said anode and said input side insulator, said input side sealing metal member being hermetically closely attached to said input side insulator, an output side sealing member, an output side insulator, and an output terminal; said air-tight vacuum envelope being defined by said anode, said magnets, said input side insulator, said input side sealing member, said output side sealing member, said output side insulator and said output terminal, wherein said magnetron further comprises a choke structure having an opening in the vicinity of the connection between said input side insulator and said input side sealing metal member, said choke structure having an effective length approximately one fourth of the wavelength of microwave energy oscillated by said magnetron and being arranged on the outside of said air-tight vacuum envelope.

3. A magnetron according to claim 1, further comprising a shield case covering the exterior component elements on the input side of said air-tight cacuum envelope, said shield case having part of the bottom thereof comprising part of said input side sealing metal member, said shield case containing a stepped cup-shaped metal member concentrically arranged with said input side insulator and input side sealing metal member, said stepped cup-shaped metal member having a bottom with a hole through which said input side insulator is passed, said stepped cup-shaped metal member having a large-diameter portion with its inner bottom surface spaced from the inner bottom surface of said shield case, said stepped cup-shaped metal member having a small-diameter portion with its inner bottom surface spaced from the upper surface of said input side sealing metal member, the wall end of said large-diameter portion of said stepped cup-shaped metal member being fixed on the inner bottom surface of said shield case; said choke structure comprising said stepped cup-shaped metal member, part of said shield case and part of said input side sealing metal member.