Patent Publication Number: US-9852872-B2

Title: Magnetron

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. Bypass Continuation Application of International Application No. PCT/JP2014/004408, filed Aug. 27, 2014, which claims priority to Japanese Patent Application No. 2013-178055 filed Aug. 29, 2013 the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a magnetron, and is suitably applied to a continuous wave magnetron used in microwave heating equipment such as microwave ovens. 
     BACKGROUND ART 
     As shown in  FIG. 14 , a conventional anode structure  100  of a typical magnetron, such as those for microwave ovens, which oscillates to generate 2,450 MHz-band microwaves, includes an anode cylinder  101 ; and vanes  102 , which are radially disposed inside the anode cylinder  101 . 
     The vanes  102  are connected together through a pair of large and small strap rings  103 , which each are brazed to both upper and lower ends of every other vane  102  in the circumferential direction. 
     In an electron interaction space surrounded by free ends of a plurality of vanes  102 , a spiral cathode  104  is disposed along an axis of the anode cylinder  101 . Both ends of the cathode  104  are fixed to an output side end hat  105  and an input side end hat  106 . 
     To both ends of the anode cylinder  101 , pole pieces  107  and  108 , which are almost funnel-shaped, are fixed. 
     The strap rings  103  are designed to alternately keep the vanes  102  at the same potential. As described above, the structure in which a pair of large and small strap rings  103  are provided at both upper and lower ends of the vanes  102  is currently popular. There are other structures, such as a structure in which the upper and lower ends are each provided with one strap ring, or a structure in which one of the upper and lower ends is provided with two or more strap rings, or a structure in which two strap rings are provided in an up-down direction central portion of vanes. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     [Patent Document 1] Japanese Patent Application Laid-open Publication No. 2013-73730 
     [Patent Document 2] Japanese Patent Application Laid-open Publication No. H07-302548 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     A cavity resonator with the above-described configuration, which is separated by vanes  102  of the magnetron, has a specific frequency. However, in the case of the typical strap ring type, the frequency is significantly affected by the capacitance between vanes and strap rings and the capacitance between the strap rings. 
     For example, for the sake of an improvement in productivity or a reduction in costs, strap rings may not be provided on both upper and lower ends of a vane, and instead two strap rings may be provided on only one end. In such a case, the capacitance of the cavity resonator becomes smaller than cases where the upper and lower ends are each provided with two strap rings. 
     As a result, in some cases, the frequency of the cavity resonator becomes several hundreds of MHz higher than cases where the upper and lower ends are each provided with two strap rings. It is necessary to regulate the frequency. 
     In this case, for example, possible measures to be taken include: narrowing the distance between the strap rings and the vanes; and increasing the cross-section of the strap rings. 
     However, if such measures are taken, a short circuit may occur during the brazing between the strap rings, or between the strap rings and the vanes, due to brazing material, or the volume of the strap rings would increase. This leads to a reduction in productivity or an increase in costs. 
     If only one end of the vane is provided with strap rings, an imbalance in electric field distribution between the upper and lower ends of the vane becomes larger than the structure in which strap rings are vertically symmetrically disposed at the upper and lower ends. This leads to a decrease in load stability, electronic reverse shock, and efficiency, or is prone to undesired noise. 
     In particular, the load stability and the reverse impact by electrons may be a major problem when the magnetron is used in microwave heating equipment such as microwave ovens where reflected waves come back. Accordingly, the structure in which only one end of the vane is provided with strap rings has not been put into practical use so far for the magnetrons of microwave ovens. The structure is therefore not being used except for a pulse magnetron or the like that is substantially free of such worries. 
     Incidentally, to improve the stability of oscillation, another proposal is to provide one end of the vane with three or more strap rings. According to this configuration, the cross section of the strap rings is relatively small compared with the structure in which one end is provided with two strap rings, and the stability of oscillation increases. However, in the case of this configuration, the diameter of an outermost strap ring is greater than that of the structure in which two strap rings are provided. If the strap rings are punched from plate-like material, an even larger material is required, and an amount of scraps would increase, resulting in a decrease in material efficiency and diminishing the effects of cost reduction. 
     Regardless of how many strap rings are provided, making adjustments to the frequency would be difficult particularly when only the output side is provided with strap rings. Usually, in consideration of variations associated with the accuracy of components and assembling, the resonance frequency of the anode structure is designed in such a way as to be slightly higher than a predetermined frequency, and the frequency is adjusted after the assembling. 
     In this case, for example, various adjustment methods may be available, such as partially removing the vanes or deforming the strap rings. However, in terms of productivity, side effects on characteristics, and easiness of the adjustments, what is frequently used is a method of adjusting the frequency to a desired frequency by inserting an antenna coming from an anode structure assembly into a waveguide of the measurement use, deforming an input side strap ring in an axis direction while monitoring the resonance frequency, and thereby narrowing the distance between the strap ring and a vain and increasing the capacitance. 
     However, according to this adjustment method, the strap ring needs to be provided at the input side. If strap rings are provided only at the output side, this adjustment method cannot be used. Moreover, if the cross section of the strap ring is large, it is difficult to deform the strap ring itself, and the adjustment method cannot be used. 
     If the upper and lower ends of a vane are each provided with one strap ring, then the capacitance between the strap rings comes to zero. Therefore, the cross section (volume) of the strap ring needs to be significantly larger compared with cases where each is provided with two strap rings. As a result, it is difficult to deform the strap ring itself, and the above-described adjustment method cannot be used. 
     Furthermore, it is known that the structure in which strap rings are provided in the central portion of the vane is highly unfavorable in terms of productivity. 
     The present invention has been made to solve the above problems. The object of the present invention is to provide a magnetron that is low in costs and excellent in productivity without any adverse effects on the characteristics. 
     Means for Solving the Problems 
     To achieve the above object, a magnetron of the present invention is characterized by including: an anode cylinder that cylindrically extends along a tube axis; a plurality of vanes that extend from an inner surface of the anode cylinder toward the tube axis in such a way that free ends form a vane inscribed circle; two large and small strap rings that are different in diameter and which alternately short-circuit the plurality of vanes; a cathode that is disposed along the tube axis in the vane inscribed circle formed by the free ends of the plurality of vanes; pole pieces that are disposed at both ends of the anode cylinder in a tube axis direction and which lead magnetic flux into an interaction space between the free ends of the plurality of vanes and the cathode; and an antenna that is pulled out from at least one of the vanes, wherein the strap rings are only disposed on a cathode input side one of two ends of the vane in the tube axis direction, the shape of the pole piece that is disposed at one end of the anode cylinder in the tube axis direction and the shape of the pole piece that is disposed at the other end are asymmetrical, and the pole pieces that are disposed at both ends of the anode cylinder in the tube axis direction include protruding flat surfaces, and a diameter of the protruding flat surface of the pole piece that is disposed at one end or input side is larger than a diameter of the protruding flat surface of the pole piece that is disposed at the other end or output side. 
     Advantages of the Invention 
     According to the present invention, it is possible to provide a practical magnetron without a significant decrease in productivity or characteristics from a conventional one, while cutting costs by reducing the number of parts with the use of two strap rings on one side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross-sectional view of an entire magnetron according to one embodiment of the present invention. 
         FIG. 2  is a longitudinal cross-sectional view of major portions of a magnetron according to one embodiment of the present invention. 
         FIG. 3  is a lateral cross-sectional view of major portions of a magnetron according to one embodiment of the present invention. 
         FIG. 4  is a longitudinal cross-sectional view showing dimensions of major portions of a magnetron according to one embodiment of the present invention. 
         FIG. 5  is a diagram and table showing relation between dimensions of a pole piece and efficiency illustrating a magnetron according to one embodiment of the present invention. 
         FIG. 6  is a diagram and table showing relation between dimensions of a pole piece and higher harmonic waves illustrating a magnetron according to one embodiment of the present invention. 
         FIG. 7  is a diagram and table showing relation between dimensions of a vane inscribed circle and efficiency illustrating a magnetron according to one embodiment of the present invention. 
         FIG. 8  is a diagram and table showing relation between dimensions of a vane inscribed circle and load stability illustrating a magnetron according to one embodiment of the present invention. 
         FIG. 9  is a diagram and table showing relation between dimensions of a pole piece and load stability illustrating a magnetron according to one embodiment of the present invention. 
         FIG. 10  is a diagram and table showing relation between the reverse impact by electrons and a ratio in dimensions of a pole piece to a vane inscribed circle illustrating a magnetron according to one embodiment of the present invention. 
         FIG. 11  is a diagram and table showing relation between magnetic flux density and a ratio in dimensions of a pole piece to a vane inscribed circle illustrating a magnetron according to one embodiment of the present invention. 
         FIG. 12  is a lateral cross-sectional view of major portions of a conventional magnetron, showing the direction of shear droop. 
         FIG. 13  is a diagram showing fundamental-wave spectrums of a magnetron of the present invention and a conventional magnetron. 
         FIG. 14  is a longitudinal cross-sectional view of major portions of a conventional magnetron. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     One embodiment of a magnetron of the present invention will be described with reference to the accompanying drawings. Incidentally, embodiments described below are given for illustrative purposes only, and the present invention is not limited to these embodiments. 
       FIG. 1  is a longitudinal cross-sectional view schematically showing a magnetron  1  according to the present embodiment. The magnetron  1  is a magnetron for microwave ovens that generate a 2,450 MHz-band fundamental wave. 
     The magnetron  1  includes, as a main component, an anode structure  2  that generates a 2, 450 MHz-band fundamental wave. Below the anode structure  2 , an input unit  4 , which supplies power to a cathode  3  located at the center of the anode structure  2 , is disposed. Above the anode structure  2 , an output unit  5 , which leads microwaves generated from the anode structure  2  out of a tube (or magnetron  1 ), is disposed. 
     The input unit  4  and the output unit  5  are joined to an anode cylinder  6  of the anode structure  2  in a vacuum-tight manner by an input side metal sealing member  7  and an output side metal sealing member  8 . 
     The anode structure  2  includes the anode cylinder  6 , a plurality of vanes  10  (e.g. 10 vanes), and two large and small strap rings  11 . 
     The anode cylinder  6  is made of copper, for example, and is formed into a cylindrical shape. The anode cylinder  6  is disposed in such a way that the central axis thereof passes through a tube axis m, or the central axis of the magnetron  1 . 
     Each of the vanes  10  is made of copper, for example, and is formed into a plate shape. Inside the anode cylinder  6 , the vanes  10  are radially disposed around the tube axis m. An outer end of each vane  10  is joined to an inner peripheral surface of the anode cylinder  6 ; an inner end of each vane  10  is a free end. A cylindrical space surrounded by the free ends of the plurality of vanes  10  serves as an electron interaction space. 
     Among both upper and lower ends in the direction of the tube axis m of the plurality of vanes  10 , the two large and small strap rings  11  are fixed to the lower end positioned at an input side. 
     In the electron interaction space surrounded by the free ends of the plurality of vanes  10 , the spiral cathode  3  is provided along the tube axis m. The cathode  3  is disposed away from the free ends of the plurality of vanes  10 . The anode structure  2  and the cathode  3  work as a resonance portion of the magnetron  1 . 
     On an upper and a lower end of the cathode  3 , end hats  12  and  13  are fixed in order to prevent electrons from leakage. The end hat  12  located at upper end positioned at an output side is formed into a disc shape. The end hat  13  located at the input side lower end is formed into a ring shape. 
     The input unit  4  located below the anode cylinder  6  includes a ceramic stem  14 ; a center support rod  15  and a side support rod  16  planted in the ceramic stem. 
     The center support rod  15  passes through a central hole of the input side end hat  13  of the cathode  3  and then through the center of the cathode  3  in the direction of the tube axis m, and is joined to the output side end hat  12  of the cathode  3 . The center support rod  15  is electrically connected to the cathode  3  via the end hat  12 . 
     The side support rod  16  is joined to the input side end hat  13  of the cathode  3 . The side support rod  16  is electrically connected to the cathode  3  via the end hat  13 . The center support rod  15  and the side support rod  16  are designed to support the cathode  3  and supply current to the cathode  3 . 
     On an inner side of the upper end (output side end) of the anode cylinder  6  and on an inner side of the lower end (input side end), a pair of pole pieces  17  and  18  are provided in such a way that the space between the end hats  12  and  13  is sandwiched and that the pole pieces  17  and  18  face each other. 
     A central portion of the output side pole piece  17  has a through-hole  17 A whose diameter is slightly larger than the output side end hat  12 . The output side pole piece  17  is substantially formed into a shape of funnel that spreads around the through-hole  17 A toward the output side (upper side). The output side pole piece  17  is disposed in such a way that the tube axis m passes through the center of the through-hole  17 A. 
     A central portion of the input side pole piece  18  has a through-hole  18 A whose diameter is slightly larger than the input side end hat  13 . The input side pole piece  18  is substantially formed into a shape of funnel that spreads around the through-hole  18 A toward the input side (lower side). The input side pole piece  18  is disposed in such a way that the tube axis m passes through the center of the through-hole  18 A. 
     To the upper end of the output side pole piece  17 , a lower end of the substantially cylindrical metal sealing member  8 , which extends in the direction of the tube axis m, is fixed. The metal sealing member  8  is also in contact with the upper end of the anode cylinder  6 . To the lower end of the input side pole piece  18 , an upper end of the substantially cylindrical metal sealing member  7 , which extends in the direction of the tube axis m, is fixed. The metal sealing member  7  is also in contact with the lower end of the anode cylinder  6 . 
     To the upper end of the output side metal sealing member  8 , an insulating cylinder  19 , which is part of the output unit  5 , is joined. To an upper end of the insulating cylinder  19 , an exhaust tube  20  is joined. 
     An antenna  21  that is lead out from one of the plurality of vanes  10  passes through the output side pole piece  17  and extends inside the metal sealing member  8  toward the upper end thereof; the tip of the antenna  21  is held by the exhaust tube  20  and thereby fixed. 
     To the lower end of the input side metal sealing member  7 , the ceramic stem  14 , which is part of the input unit  4 , is joined. That is, the center support rod  15  and side support rod  16 , which are planted in the ceramic stem  14 , go inside the metal sealing member  7  to be connected to the cathode  3 . 
     Outside the metal sealing members  7  and  8 , a pair of ring-shaped magnets  22  and  23  are provided in such a way that the anode cylinder  6  is sandwiched in the direction of the tube axis m and that the magnets  22  and  23  face each other. The pair of magnets  22  and  23  generate a magnetic field in the direction of the tube axis m. 
     The anode cylinder  6  and the magnets  22  and  23  are covered with a yoke  24 ; the pair of magnets  22  and  23  and the yoke  24  constitute a magnetic circuit. A magnetic flux coming from the magnets  22  and  23  of the magnetic circuit is led by the pair of pole pieces  17  and  18  to the electron interaction space between the free ends of the vanes  10  and the cathode  3 . 
     Between the anode cylinder  6  and the yoke  24 , a radiator  25  is provided. The radiator  25  releases the heat generated by the oscillation of the anode structure  2  out of the magnetron  1 . 
     The configuration of the magnetron  1  has been outlined above. 
     With the use of  FIGS. 2 to 4 , the configuration of the vanes  10 , strap rings  11 , and pole pieces  17  and  18  will be described in more detail.  FIG. 2  is a longitudinal cross-sectional view of the anode structure  2  and  FIG. 3  is a lateral schematic view of the anode structure  2  when seeing from the output unit&#39;s side. Incidentally, in  FIG. 3 , in order to make the configuration of the vanes  10  and strap rings  11  to explain easily, portions other than the anode cylinder  6 , vanes  10 , and strap rings  11  are omitted.  FIG. 4  is a longitudinal cross-sectional view showing dimensions of each portion of the anode structure  2 . 
     As described above, inside the anode cylinder  6  of the anode structure  2 , the plurality of vanes  10  are radially disposed around the tube axis m. To the input side ends of the plurality of vanes  10 , two large and small strap rings  11  are fixed. 
     Incidentally, among the two large and small strap rings  11 , the strap ring  11  that is larger in diameter is referred to as a large-diameter strap ring  11 A, and the strap ring  11  that is smaller in diameter is referred to as a small-diameter strap ring  11 B. 
     According to the present embodiment, inside the anode cylinder  6 , ten vanes  10  are disposed. The ten vanes  10  consist of five vanes  10 A and five vanes  10 B. Inside the anode cylinder  6 , the vanes  10 A and the vanes  10 B are alternately disposed in such a way that the vanes  10 A are adjacent to the vanes  10 B. Incidentally, as shown in  FIG. 3 , a circle Cr that is inscribed to the free ends of the vanes  10 A and  10 B will be referred to as a vane inscribed circle Cr. 
     In the input side ends (lower ends) of the vanes  10 A, there are respectively a stepped notch  30  formed to be deeper than the thickness of the large-diameter strap ring  11 A and small-diameter strap ring  11 B. In the input side ends (lower ends) of the vanes  10 B, there are respectively a stepped notch  31  formed to be deeper than the thickness of the large-diameter strap ring  11 A and small-diameter strap ring  11 B. 
     The large-diameter strap ring  11 A is inserted into the inner portions of the notches  30  of the vanes  10 A and the inner portions of the notches  31  of the vanes  10 B. In this manner, the large-diameter strap ring  11 A is embedded in the lower ends of the vanes  10 A and  10 B close to the center of the tube axis m. 
     Incidentally, the large-diameter strap ring  11 A is joined by brazing to inner edges of the notches  30  of the vanes  10 A while not being in contact with the notches  31  of the vanes  10 B. 
     That is, the large-diameter strap ring  11 A is joined only to the vanes  10 A, thereby connecting the five vanes  10 A together. To the output side end (upper end) of one of the vanes  10 A that are joined to the large-diameter strap ring  11 A, the antenna  21  is connected. 
     The small-diameter strap ring  11 B is inserted into the inner portions of the notches  30  of the vanes  10 A and the inner portions of the notches  31  of the vanes  10 B. In this manner, the small-diameter strap ring  11 B is embedded in the lower ends of the vanes  10 A and  10 B close to the center of the tube axis m. 
     Incidentally, the small-diameter strap ring  11 B is joined by brazing to inner edges of the notches  31  of the vanes  10 B while not being in contact with the notches  30  of the vanes  10 A. 
     That is, the small-diameter strap ring  11 B is joined only to the vanes  10 B, thereby connecting the five vanes  10 B together. 
     Inside the anode cylinder  6 , the cathode  3  is provided in the electron interaction space surrounded by the free ends of the vanes  10 A and vanes  10 B. To the upper and lower ends of the cathode  3 , the end hats  12  and  13  are respectively fixed. 
     Inside the anode cylinder  6 , there are provided a pair of pole pieces  17  and  18  facing each other which sandwich the space between the end hats  12  and  13 . 
     Both the output side pole piece  17  and the input side pole piece  18  are substantially funnel-shaped as a whole. However, the output side pole piece  17  and the input side pole piece  18  are partially different in shape. 
     The output side pole piece  17  includes a lower end portion  17 B, which is at right angles to the tube axis m and at the center of which the through-hole  17 A is formed; an intermediate portion  17 C, which is located outside the lower end portion  17 B and conically extends from the outer edge of the lower end portion  17 B toward the output side (upper side); and an upper end portion  17 D, which is located outside the intermediate portion  17 C and parallel to the lower end portion  17 B. The output side pole piece  17  is substantially funnel-shaped as a whole. 
     In that manner, the output side pole piece  17  is shaped in such a way that the center portion (lower end portion  17 B) protrudes toward the lower side (or the input side). A flat surface  40  of a lower end of the lower end portion  17 B will be referred to as a protruding flat surface  40 . 
     The input side pole piece  18  includes an upper end portion  18 B, which is at right angles to the tube axis m and at the center of which the through-hole  18 A is formed; an intermediate portion  18 C, which is located outside the upper end portion  18 B and conically extends from the outer edge of the upper end portion  18 B toward the input side (lower side); and a lower end portion  18 D, which is located outside the intermediate portion  18 C and parallel to the upper end portion  18 B. The input side pole piece  18  is substantially funnel-shaped as a whole. 
     In that manner, the input side pole piece  18  is shaped in such a way that the center portion (upper end portion  18 B) protrudes toward the upper side (or the output side). A flat surface  41  of an upper end of the upper end portion  18 B will be referred to as a protruding flat surface  41 . 
     The protruding flat surfaces  40  and  41  of the output side pole piece  17  and input side pole piece  18  are different in diameter each other. 
     Incidentally, in this case, as shown in  FIG. 4 , the diameter of the protruding flat surface  40  of the output side pole piece  17  is defined as a diameter of a circumference containing an intersection point where an extension of the protruding flat surface  40  crosses an extension of a tapered surface of the intermediate portion  17 C. The diameter of the protruding flat surface  41  of the input side pole piece  18  is defined as a diameter of a circumference containing an intersection point where an extension of the protruding flat surface  41  crosses an extension of a tapered surface of the intermediate portion  18 C. 
     The dimensions of major portions will be described below. The outer diameter Rlo of the large-diameter strap ring  11 A is 20.3 mmφ; the inner diameter thereof is 18.05 mmφ; the thickness thereof is 1.3 mm. 
     The outer diameter of the small-diameter strap ring  11 B is 16.75 mmφ; the inner diameter Rsi thereof is 14.5 mmφ and the thickness thereof is 1.3 mm. 
     The diameter Rop of the protruding flat surface  40  of the output side pole piece  17  is 12 mmφ. The diameter Rip of the protruding flat surface  41  of the input side pole piece  18  is 18 mmφ. 
     The dimensions are set in such a way as to satisfy the following formula (1).
 
 Rop &lt;( Rsi+Rlo )/2≦ Rip   (1)
 
     Actually, in the case of the present embodiment, (Rsi+Rlo)/2 is 17.4; the diameter Rop of the protruding flat surface  40  of the output side pole piece  17  is 12; and the diameter Rip of the protruding flat surface  41  of the input side pole piece  18  is 18. Therefore, the above formula (1) is satisfied. 
     The dimensions of other parts will be described below. The inner diameter of the anode cylinder  6  is 36.7 mmφ. The vanes  10 A and  10 B are 1.85 mm in thickness, and 8.0 mm in height in the direction of the tube axis m. The vane inscribed circle Cr is 8.7 mmφ in diameter. The outer diameter of the cathode  3  is 3.9 mmφ. 
     The outer diameter of the end hats  12  and  13  is 7.2 mmφ. The inner diameter of the output side pole piece  17 , i.e. the diameter of the through-hole  17 A is 9.2 mmφ; the inner diameter of the input side pole piece  18 , i.e. the diameter of the through-hole  18 A is 9.4 mmφ. 
     As described above, according to the present embodiment, the two large and small strap rings  11  ( 11 A and  11 B) are disposed only at the lower end sides, i.e. the input sides in the direction of the tube axis m of the plurality of vanes  10  ( 10 A and  10 B). Moreover, the diameter Rip of the protruding flat surface  41  of the input side pole piece  18  is larger than the diameter Rop of the protruding flat surface  40  of the output side pole piece  17 . 
     Then, the diameter Rop of the protruding flat surface  40  of the output side pole piece  17 , the diameter Rip of the protruding flat surface  41  of the input side pole piece  18 , the outer diameter Rlo of the large-diameter strap ring  11 A, and the inner diameter Rsi of the small-diameter strap ring  11 B are set in such a way as to satisfy the above formula (1). 
     Although the details will be given later, this magnetron  1  is more practical than the conventional one without a significant decrease in productivity or characteristics, while achieving a reduction in costs by reducing the number of parts, i.e. the number of strap rings  11  ( 11 A and  11 B), only two of which are provided on one side. 
     In order to prove that the above-mentioned advantageous effects can be actually achieved, several verification experiments are carried out. The results will be described below. 
     In order to compare with the magnetron  1  of the present embodiment, prototype tubes were made in such a way as to have different dimensions of the output side pole piece and input side pole piece.  FIGS. 5 and 6  show the results of verifying these prototype tubes, with a focus on efficiency and higher harmonic waves, which would become unnecessary radiation. 
     As shown in  FIG. 5 , in all of both the output side pole piece and the input side pole piece, lowering of output efficiency occurs, so that the diameters Rop and Rip of the protruding flat surfaces become larger. Particularly diameter Rop of the protruding flat surface of the output side pole piece has a greater influence on the efficiency. 
     Furthermore, based on the results of verification, in order to secure the same level of efficiency (70%) as the conventional magnetron in which a pair of large and small strap rings is provided at both upper and lower ends of the vane, the diameter Rop of the protruding flat surface of the output side pole piece is preferred to be at between about 12 mmφ and 14 mmφ. In such a case, the allowable range of the diameter Rip of the protruding flat surface of the input side pole piece is expected to be up to 20 mmφ. 
     As for higher harmonic waves, as shown in  FIG. 6 , when the diameter Rip of the protruding flat surface of the input side pole piece is 18 mmφ, the levels of the second and seventh higher harmonic waves become slightly higher. However, the levels of the fourth, fifth, and sixth higher harmonic waves decrease. 
     Incidentally, the data shown in  FIG. 6  are the results of verification on prototype tubes in which, in view of the efficiency, the diameter Rop of the protruding flat surface of the output side pole piece was fixed at 12 mmφ, and the configuration of components remained unchanged except for that of the input side pole piece, and only the diameter Rip of the protruding flat surface of the input side pole piece was changed. 
     It is clear from the above verification results that the magnetron  1  of the present embodiment has well-balanced excellent characteristics by achieving 70% or more of efficiency and curbing unnecessary radiation, because the diameter Rop of the protruding flat surface  40  of the output side pole piece  17  is 12 mmφ and the diameter Rip of the protruding flat surface  41  of the input side pole piece  18  is 18 mmφ. 
     In the case of the magnetron  1  of the present embodiment, the load stability is 1.6 A, and the reverse impact by electrons is 88%. In the case of the conventional magnetron in which a pair of large and small strap rings is provided at both upper and lower ends of the vane, the load stability is 1.8 A, and the reverse impact by electrons is 90%. 
     In that manner, the load stability and the reverse impact by electrons of the magnetron  1  of the present embodiment are lower than those of the conventional magnetron. However, the load stability and the reverse impact by electrons of the magnetron  1  of the present embodiment are within a range where no practical problems occur. The reason is considered to be that the output side pole piece  17  and the input side pole piece  18  have the above-described shapes and dimensions, and that the large-diameter strap ring  11 A and the small-diameter strap ring  11 B are embedded in the lower end portions of the vanes  10 A and  10 B. 
     As for the reverse impact by electrons, the antenna  21  connected to a vane  10 B that is joined to the small-diameter strap ring  11 B is known to achieve better results than the antenna  21  connected to a vane  10 A that is joined to the large-diameter strap ring  11 A as in the case of the magnetron  1 . 
     However, such antenna  21  being connected to the vane  10 B comes with a side effect, the level of the third higher harmonic wave becomes significantly higher. Therefore, the antenna  21  being connected to the vane  10 B is not appropriate for the magnetron  1 . 
     Furthermore, in general, it is known that the vanes that are higher in the direction of the tube axis work better in terms of the load stability and efficiency and the like. However, in the case of the magnetron  1 , if the height of the vanes  10 A and  10 B in the direction of the tube axis m is greater than 8.0 mm, a difference in electric field distribution between upper and lower portions of the anode structure  2  becomes larger. This configuration is therefore likely to cause a worsening of characteristics such as higher harmonic waves and runs counter to efforts to reduce the costs. 
     In terms of the load stability and output and the like, it is difficult to set the height of the vanes  10 A and  10 B in the direction of the tube axis mat less than 8.0 mm. Accordingly, given manufacturing tolerances and the like, it is preferred that the height of the vanes  10 A and  10 B in the direction of the tube axis m should practically be between 7.8 mm and 8.2 mm. 
     Moreover, a significant increase in the cross section of the strap rings  11  ( 11 A and  11 B) and in the thickness of the vanes  10  ( 10 A and  10 B) from the conventional dimensions is not a practical option in terms of costs and productivity. There is also a limit on attempts to significantly reduce the dimensions, because problems could arise in terms of durability and heat resistance. 
     Therefore, if the height of the strap rings  11  in the direction of the tube axis m is represented by HS, the thickness in the radial direction thereof by WS, the height of the vanes  10  in the direction of the tube axis m by HV, the thickness thereof by TV, and the distance between the free ends of adjoining vanes  10  by GV, it is desirable that these dimensions be within the ranges expressed by the following formulae (2) to (4). 
     Incidentally, no distinction is made between the large-diameter strap ring  11 A and the small-diameter strap ring  11 B because HS is equal to WS. No distinction is made between the vanes  10 A and  10 B because the vanes  10 A and  10 B are equal in size.
 
0.1≦ HS/HV≦ 0.19  (2)
 
0.06≦ WS/WV≦ 0.09  (3)
 
 GV /( GV+TV )≦0.375  (4)
 
     That is, in the case of the magnetron  1 , it is preferred that HV be in a range of 7.8 mm to 8.2 mm; that HS be in a range of 0.8 mm to 1.5 mm; that WS be in a range of 0.9 mm to 1.3 mm; that WV be in a range of 13.7 mm to 14.1 mm; that TV being a range of 1.70 mm to 1.85 mm; and that GV be in a range of 0.929 mm to 0.929 mm+10%. 
     As described above, in the case of the present embodiment, the inner diameter of the output side pole piece  17  is 9.2 mm; the inner diameter of the input side pole piece  18  is 9.4 mm; and the diameter of the vane inscribed circle Cr is 8.7 mmφ. 
     As shown in  FIGS. 7 and 8 , a larger diameter (represented as Ra) of the vane inscribed circle Cr leads to an increase in efficiency but a reduction in load stability. Accordingly, in the case of the present embodiment, the diameter Ra of the vane inscribed circle Cr is set at 8.7 mmφ. Therefore, it is possible to achieve a load stability of 1.5 A or more, which does not cause any practical problem, while obtaining 70 percent or more of efficiency. 
     A larger inner diameter (represented as Rpp) of the input side pole piece  17  is better in terms of the reverse impact by electrons. However, if the inner diameter is significantly different from the size of the electron interaction space, a sufficient amount of magnetic flux is unlikely to enter the electron interaction space. As a result, as shown in  FIG. 9 , the load stability would decrease. Therefore, the inner diameter Rpp of the input side pole piece  17  needs to be appropriately designed relative to the diameter Ra of the vane inscribed circle Cr. 
     Accordingly, the inner diameter Rpp of the input side pole piece  17  is preferably set so that the ratio of the inner diameter Rpp to the diameter Ra of the vane inscribed circle Cr comes within the range of 0.95 to 1.13. 
     The findings are based on the results of verification which focused on the reverse impact by electrons and the magnetic flux density inside the electron interaction space when the diameter Ra of the vane inscribed circle Cr remained unchanged and when the inner diameter Rpp of the input side pole piece  17  was changed.  FIGS. 10 and 11  show data of the results of verification. 
     It is clear from the results of verification that, when the ratio of the inner diameter Rpp of the input side pole piece  17  to the diameter Ra of the vane inscribed circle Cr is within the range of 0.95 to 1.13, the reverse impact by electrons is 87% or more and the magnetic flux density inside the electron interaction space is 200 mT or more. In this manner, practically sufficient characteristics are obtained. 
     Further, it is similarly preferred that the inner diameter of the output side pole piece  17  be set so that the ratio of the inner diameter of the output side pole piece  17  to the diameter Ra of the vane inscribed circle Cr is included in the range of 0.95 to 1.13. 
     Besides, as shown in  FIG. 14 , in the case of the conventional magnetron, one type of vanes  102  having the same shape is disposed in such a way as to be alternately turned upside-down. In the magnetron  1  of the present embodiment, as shown in  FIGS. 2 and 3 , two types of vanes  10 A and  10 B having notches  30  and  31  that are different in shape are alternately disposed. 
     In this manner, in the case of the magnetron  1  of the present embodiment, the number of types of vanes is increased to two. However, press dies used to produce the vanes can punch out multiple rows of components at once on a metal plate. Therefore, there is no extra cost for the dies, even when compared with cases where only one type of vanes is used as in the conventional case. 
     At a time when the vanes are formed by press working, a shear droop would be formed on the free-end side of one surface in the thickness direction. 
     In the case of the conventional magnetron, one type of vanes  102  is disposed in such away as to be alternately turned upside-down. Therefore, as shown in  FIG. 12 , the vanes  102  are alternately disposed so that the surfaces where the shear droop PD is formed face each other. Accordingly, in the case of the conventional magnetron, one surface in the thickness direction of each vane  102  cannot be turned in the same direction around the axis, i.e. the clockwise direction in the diagram, and the shear droop PD cannot be aligned in the same direction. 
     In the case of the magnetron  1  of the present embodiment, two types of vanes  10 A and  10 B are alternately disposed. Therefore as shown in  FIG. 3 , the two types of vanes  10 A and  10 B can be alternately disposed in such a way that a surface where the shear droop PD is formed faces another surface where no shear droop PD is formed. 
     Still, the press stamping directions of the two types of vanes  10 A and  10 B are the same. Accordingly, the shear droop PD is formed on the free-end side of one surface in the thickness direction of each vane. 
     Therefore, in the magnetron  1 , one surface in the thickness direction of each vane  10 A,  10 B can be turned in the same direction around the axis, i.e. the clockwise direction in the diagram, and the shear droop PD can be aligned in the same direction. 
     Thus, in the case of the magnetron  1 , compared with the conventional magnetron, the variation in shape of each cavity resonator that is divided into 10 by each vane  10 A,  10 B, can be reduced, resulting in a decrease in the variation of the frequency. Consequently, it is possible to make smaller the spread of a fundamental-wave spectrum. 
       FIGS. 13(A) and 13(B)  show the fundamental-wave spectrum of the magnetron  1  of the present embodiment ( FIG. 13(A) ), and the fundamental-wave spectrum of the conventional magnetron ( FIG. 13(B) ). As can be seen in  FIG. 13 , the fundamental-wave spectrum of the magnetron  1  of the present embodiment favorably compares with the fundamental-wave spectrum of the conventional magnetron. 
     As described above, in the case of the magnetron  1  of the present embodiment, the two large and small strap rings  11  ( 11 A and  11 B) are only disposed on the lower end sides, i.e. input sides, in the direction of the tube axis m of the plurality of vanes  10  ( 10 A and  10 B). The diameter Rip of the protruding flat surface  41  of the input side pole piece  18  is made larger than the diameter Rop of the protruding flat surface  40  of the output side pole piece  17 . 
     In that manner, it is possible to provide a practical magnetron without greatly reducing productivity or characteristics from a conventional one, while cutting costs by decreasing the number of parts with the use of two strap rings on one side. 
     Furthermore, according to the present embodiment, the diameter Rop of the protruding flat surface  40  of the output side pole piece  17 , the diameter Rip of the protruding flat surface  41  of the input side pole piece  18 , the outer diameter Rlo of the large-diameter strap ring  11 A, and the inner diameter Rsi of the small-diameter strap ring  11 B are set in such a way as to satisfy the above formula (1). 
     Furthermore, according to the present embodiment, the height HV in the direction of the tube axis m of the vanes  10  is set in such a way as to be within the range of 7.8 mm to 8.2 mm. Moreover, the height HS in the direction of the tube axis m of the strap rings  11 , the radial-direction thickness WS, the height HV in the direction of the tube axis m of the vanes  10 , the thickness TV, and the distance GV between the free ends of adjacent vanes  10  are set in such a way as to be in the ranges expressed by the above formulae (2) to (4). 
     Furthermore, according to the present embodiment, the inner diameter Rpp of the input side pole piece  17  is set in such a way that the ratio of the inner diameter Rpp to the diameter Ra of the vane inscribed circle Cr is between 0.95 and 1.13. 
     Furthermore, according to the present embodiment, two types of vanes  10 A and  10 B are alternately disposed. In this manner, the shear droop PD that is formed on each vane  10 A,  10 B is aligned in the same direction. 
     As a result, it is possible to provide a magnetron with well-balanced excellent characteristics in terms of efficiency, higher harmonic waves, which would become unnecessary radiation, load stability, the reverse impact by electrons, magnetic flux density in the electron interaction space, variation in the frequency, and the like. 
     Incidentally, in the case of the above-described embodiment, the dimensions of each portion of the magnetron  1  are expressed in mm (millimeter). This is one example when the magnetron is used in microwave ovens and the like. For example, in the case of an even larger magnetron, the dimensions of each portion could be much larger. However, even in such a case, the relative dimensions of each portion should remain the same as in the magnetron  1 . 
     EXPLANATION OF REFERENCE SYMBOLS 
     
         
           1 : Magnetron 
           2 ,  100 : Anode structure 
           3 ,  104 : Cathode 
           6 ,  101 : Anode cylinder 
           10 ,  102 : Vane 
           11 ,  103 : Strap ring 
           17 ,  18 ,  107 ,  108 : Pole piece 
           21 : Antenna 
           40 ,  41 : Protruding flat surface 
         PD: Shear droop