Combined magnetron and resonant stabilizing filter
United States Patent Pickerin A r. l, 1975 [54] COMBINED MAGNETRON AND RESONANT 2.808.538 lO/l957 Cutler INS/39.69 X STABILIZING FILTER 2.976.458 3/l96i Fcinstein.... 315/3917 3.034.014 5/1962 Drcxler 315139.77 5] n nto an ug P e gq he s rd. 3.274.433 9/l966 Pctcrson............ 315/3977 England 3.289.036 llll966 Downing et al INS/39.77 3.334.267 8/l967 Plumrid e ENS/39.55 [73] Asslgnee f l&#39; Elem&#34; valve 3.384.783 5/l968 015011. 315/3977 L1m1ted, Essex. England Filedi 1973 Pn&#39;nmry E.rmni11erSaxfield Chatmon. .lr. [3H App. N0: 424N632 Attorney, Agent. or Firm-Baldwin, Wight &amp; Brown [30] Foreign Application Priority Data [57] ABSTRACT Dec. 2l. i972 Great Britain 59134/72 A Combined magnetron and resonant stabmsing filter 7 structure formed by a multi-cavity magnetron col5r] 315/3951 315/3969 axially surrounded by an outer resonant cavity. in- [Z] ..H0l 25/50 phase cavities being coupled to the outer Cavity 1 0 /337&#39; 315/395. through coupling apertures with a relatively small cou- 3l5/39-69 pling factor and in-phase cavities being electrically locked together by straps for operation exclusively in [56] References Llted the mode.  
  UNITED STATES PATENTS 21111.1 10 9/1952 Powers 315/3977 x 7 1 Draw Fgure COMBINED MAGNETRON AND RESONANT STABILIZING FILTER This invention relates to magnetrons.  
  As is well known. the frequency spectrum of a magnetron, even when it is operated under nearly ideal conditions. is a wide one. Thus, for example, a typical known X-band magnetron. as employed for driving a radar. and operated with a substantially perfectly rectangular pulse l t see. wide, will develop output power at a frequency 50 MHz different from the desired centre frequency, of only about 40 dB below that at the said centre frequency. If, as commonly occurs in practice. the pulse shape is not good and has imperfectly sloping edges, the side band output power may be con siderably larger than this in relation to the output power at the centre frequency.  
  The production of large side band output power is characteristic of magnetrons generally and can be a serious disadvantage in magnetrons for driving radars, for it severely limits the number of radars which can be operated without mutual interference, in a given area and without resorting to wide frequency spacing. Although the production of large side band power can be greatly reduced or even (theoretically) eliminated by specially shaping the modulating pulse employed the adoption of a half sine wave pulse will greatly reduce side band power while the use of a Gaussian pulse will (theoretically eliminate it altogether this expedient is both difficult and expensive to put into practice. Attempts have therefore been made to overcome the problem by providing a sharply frequency selective filter in the output channel fed from the magnetron. In known arrangements of this nature the magnetron is connected through a line of substantial electrical length and which usually incorporates an impedance matching transformer. to a stabilising cavity sharply resonant at the desired centre frequency, and output power to the load (the radar) is taken from this cavity. However in such an arrangement, because the magnetron is capable of operating in any of several modes, severe design diffieulties are encountered.  
  The present invention seeks to overcome or reduce the foregoing defects and difficulties.  
  According to this invention there is provided a combined magnetron and resonant stabilising filter structure comprising a multi-cavity magnetron co-axially surrounded by an outer resonant cavity to which it is coupled, with a relatively small coupling factor, by a plurality of coupling apertures in the anode wall of said magnetron. said apertures providing coupling between said outer cavity and in-phase cavities of the magnetron, and means being provided electrically to lock together in-phase magnetron cavities for operation exclusively in the nmode. Preferably said means are straps.  
  By relatively small coupling factor is meant small in relation to the coupling factor required of the apertures used in a co-axial magnetron in which, as is well known, the co-axial cavity is used to dictate the frequency of the inner anode 1r mode.  
  The coupling apertures are preferably constituted by a ring of holes through the magnetron anode wall which also constitutes the inner wall of the outer cavity.  
  Preferably there is one hole between each of the alternate cavities of the magnetron and the outer cavity.  
  In a preferred embodiment the magnetron has a cylindrical anode wall to the inner surface of which are fixed a plurality of equally spaced radial metal vanes projecting inwards towards an axial cathode to provide a plurality of magnetron cavities, each between two adjacent vanes; alternate vanes are connected together by a metal strap and the remaining vanes are connected together by a second metal strap; the anode wall is coaxially surrounded by an outer cylindrical wall which, with the anode wall, constitute the outer and inner walls of the outer cavity; a plurality of small holes, arranged in a ring in a plane perpendicular to the axis of the whole structure and substantially mid-way along the length thereof, are provided through the magnetron anode wall, there being one hole providing communication between the outer cavity and each of the alternate cavities of the magnetron and situated substantially mid-way between two adjacent vanes; and an output slot is provided in the outer wall of the outer cavity. Preferably this slot extends parallel to the axis of the whole structure and provides communication with the interior of a fitting adapted to receive an output waveguide and mounted on the outside of the outer wall of the outer cavity.  
  The invention is illustrated in the accompanying drawing which shows, so far as is necessary to an understanding thereof, a preferred embodiment in broken array perspective.  
  Referring to the drawing, the structure therein represented provides, in a simple and robust unitary construction, a strapped magnetron and a surrounding cavity which is resonant sharply at the intended centre frequency of operation of the magnetron. The magnetron has an axial cathode (not shown) and an anode structure consisting of a cylindrical anode wall 1 from the inner surface of which project a plurality of radial vanes 2 which are equally spaced round the cathode. Meatl strips 3 and 4 are provided, one connecting together alternate vanes and the other connecting together the remaining vanes. As will be appreciated the magnetron, as so far described, is a strapped multicavity magnetron of well known type with a magnetron cavity between each two adjacent vanes, alternate cavities being in-phase when operation in the 11 mode is occurring.  
  Co-axially surrounding the magnetron anode wall I is a cylindrical wall 5. This is the outer wall of an outer resonant stabilising filter cavity surrounding the magnetron and having the anode wall 1 as its inner wall. A plurality of small holes 6, only one of which appears in the figure, extend through the wall 1 and provide coupling, of relatively low coupling factor, between alternate magnetron cavities, i.e. in-phase cavities, and the outer resonant cavity. These holes lie in a ring in a plane perpendicular to the axis and substantially midway along the length of the wall anode l and each hole is mid-way between two adjacent vanes. A slot 7, parallel with the axis, is provided in the outer wall 5 and leads to a fitting 8, provided on the outside of the said wall 5, for receiving the end of an output waveguide (not shown). The whole structure is closed by end walls (the upper one does not appear in the figure) and is evacuated and made vacuum tight in accordance with well known practice.  
  The illustrated structure as a whole resembles, mechanically, a co-axial magnetron but, despite this superficial resemblance, it is not one but is an ordinary magnetron surrounded by, and coupled with, a stabilising filter constituted by the outer cavity and operating quite differently from a co-axial magnetron. The consequence of the low coupling factor coupling between the outer cavity and the iii-phase magnetron cavities is that the frequency of the ir mode of the magnetron (and also the frequencies of other unwanted modes) is not determined by the frequency ofthe outer resonant cav ity. The straps. or other means known per se for electri cally locking inphase magnetron cavities to secure operation exclusively in the 11 mode. are therefore essential to the invention.  
  The invention has important practical advantages. It provides great reduction of undesired side band power by means of a structure which is simple. very robust. and self-contained. Although the coupling between the magnetron and the outer cavity can not unfairly be regarded as a line. it is a line of almost zero length, so that the difficulties associated with those known arrangements in which external line connection to a cavityfilter is employed. are avoided. The loading of the magnetron anode is symmetrical. Because the coupling between magnetron and outer cavity is by simple small holes. the troublesome resonances which are experienced with co-axial magnetrons having long coupling slots often called slot modes are almost entirely avoided or so modified that the provision of lossy material. usually necessary in co-axial magnetrons to suppress them. is not required. And. finally. acceptably low pushing and &#34;pulling&#34; figures. comparable with those of a co-axial magnetron. are obtainable without attendant disadvantages. notably that of slow starting.  
  Instead of the whole combined structure being evacuated as described the co-axial cavity could be left unevacuated or gas filled as is the case with some co-axial magnetrons. In this case the apertures 6 would be cov ered by dielectric windows or the whole anode cylinder 1 surrounded by a gas tight dielectric (e.g. ceramic) cylinder.  
 I claim:  
  1. A combined magnetron and resonant stabilizing filter structure comprising a magnetron having a cathode. an anode surrounding said cathode said anode constituted by a cylindrical wall. a plurality of members extending towards the cathode from said cylindrical wall to provide a plurality of magnetron cavities each between two adjacent members. and phase locking means electrically to lock together in-phase magnetron cavities for operation exclusively in the &#39;rr-mode; the resonant stabilizing filter structure comprising an outer resonant cavity coaxially surrounding said nagnetron; and a plurality of coupling apertures defined by said cy lindrical wall. said apertures coupling said in-phase magnetron cavities to said outer cavity and being dimensioned to provide a relatively small coupling factor whereby the oscillating frequency of the magnetron is substantially independent of the resonant frequency of the outer resonant cavity.  
  2. A magnetron and filter structure according to claim 1 in which the in-phase anode cavities are electrically locked together by straps which form said phase locking means.  
 Ill  
  3. A magnetron and filter structure according to claim I wherein the coupling apertures are constituted by a ring of holes through the magnetron anode outer wall which also constitutes the inner wall of the outer cavity.  
  4. A magnetron and filter structure according to claim 3 wherein there is one hole between alternate cavities of the magnetron anode and the outer cavity,  
  5. A magnetron and filter structure according to claim I wherein said members are a plurality of equally spaced radial metal vanes projecting inwards towards an axial cathode to provide the plurality of magnetron cavities, each between two adjacent vanes; said phase locking means comprises a first metal strap connecting alternate vanes together and a second metal strap con necting the remaining vanes together; the anode wall is co-axially surrounded by an outer cylindrical wall which. with the anode wall. constitute the outer and inner walls of the outer cavity; said coupling apertures comprising a plurality of small holes. arranged in a ring in a plane perpendicular to the axis of the whole structure and substantially mid-way along the length thereof. there being one hole providing communication between the outer cavity and each of the alternate cavities of the magnetron and situated substantially mid way between two adjacent vanes; and an output slot is provided in the outer wall of the outer cavity.  
  6. A magnetron and filter structure according to claim 5 including a fitting adapted to receive an output waveguide and mounted on the outside of the outer wall of the outer cavity. said output slot extending parallel to the axis of the whole structure and providing communication with the interior of said fitting.  
  7. A combined magnetron and resonant stabilizing filter structure comprising. in combination:  
 a magnetron having a cathode. a cylindrical wall surrounding said cathode and defining an anode. a plurality of members extending towards said cathode from said cylindrical wall to provide a plurality of magnetron cavities each between two adjacent members. and phase locking means electrically locking together those magnetron cavities which are in phase for causing said magnetron to operate exclusively in the Tr-mode at an intended center frequency of operation.  
 said resonant stabilizing filter structure comprising wall means coaxially surrounding said cylindrical wall for defining therewith an outer cavity which is resonant sharply at said intended center frequency of operation of the magnetron. and means coupling said in-phase magnetron cavities to said outer cavity for causing the oscillating frequency of the magnetron to be substantially independent of the resonant frequency of said outer cavity, the means last mentioned comprising a plurality of apertures in said cylindrical wall coupling said in-phase magnetron cavities to said outer cavity; and  
 means for taking output power from said outer cavity.