US2450619A - Tunable magnetron - Google Patents

Description

8- s. SONKIN 2, 1

TUNABLE MAGNETRON Filed. Oct. 19, 1945 2 Sheets-Sheet 1 I FIG. 1.

INVENTOR. SIMON SONKIN ATTO R N EY Oct. 5, 1948. I s. SONKIN 2,450,619

I TUNABLE MAGNETRON Filed Oct. 19, 1945 2 Sheets-Sheet 2 FIG. 2.

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WAVE LENGTH (CM) v I INVENTOR. FIG.3. SIMON SONKIN ZMMQ ATTOR N EY Patented Oct. 5, 1948 2,450,619- TTJNABEE MAGNETRON' Simon Sonkin, New York, N. Y., assignor to the United States of America as represented by the Secretary of War Application October 19, 1945, Serial No. 623,424 '1 Claim. (01. nip-27.5)

The invention described herein may be manufa'ctured and'used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to ultra-high frequency generators of magnetron type in which ultrahigh frequency oscillations are generated by a plurality of resonators set into oscillations by high velocity electrons moving along curvilinear orbital" paths, these paths being followed by the electrons because of the joint action of the electrostatic and electromagnetic fields.

It is an object of this invention to provide positive means for adjusting the frequency of oscillation of a magnetron.

An additional object of this invention is to provide a tunable magnetron in which tuning is accomplished by means of a plurality of metallic tuning pins which protrude into one of the end spaces of the magnetron through the adjacent pole piece, the mechanism for inserting the pins into the cavities of the anode being mounted in a well within the pole piece.

Still another object of this invention is to provide a tunable magnetron having a reluctance of its magnetic circuit equal to, or if so desired smaller than an identical untunable magnetron.

Yet another object of this invention is to provide a tunable magnetron capable of covering a reasonably large tuning range.

The additional objects of this invention are the provision of tuning instr'umentalities for inagnetrons which have reasonably long life of 'all movable parts, reproducibility and stability of the desired frequency, and reasonable weight of the entire combination so that a tunable magnetron does not weigh appreciably more than the fixed frequency magnetron's.

The invention is an improvement of the tuning systems disclosed in my co-pendingapplications on tunable magnetron, Serial No. 623,422, filed Ucto'ber I9, 1945, now abandoned, and Serial No; 623,423, filed October 19, 1945, and especially on the tuning system disclosed in the first application Where, in one embodiment of the invention, tuningof amagnet'ron' is accomplished by means of metallic pins insert'able into, the resonating cavities of an anode with the pin holding structureas well as the pins being all mounted in one of the end spaces of the magnetron. Because of this type of mounting of the" tuning pins it becomes necessary to increase the end space of the magnetron with the concomitant increase in the reluctance of the magnetic circuit ofxth'e, tube s'oa that it the magnetic field is to remain conf Fig. 2.

2 stant with said structures as compared magnetic field of must be a very large increase in the coercive force to the supplied by the permanent magnet. This can be accomplished only by increasing the size of the permanent magnets. When the weight requirements are critical, such increase in the weight of the permanent magnets increases the total weight and bulk of the oscillator to undesirable proportions. The additional electrical disad-v vantage of such structure is in the increased loading of the anode with the resulting lowering of its eificienoy. The mechanical disadvantage of the end space-mounted tuning system resides in the fact that it cannot be removed or inserted very readily into the pole piece since it represents an integrated structure with-the pole piece. The invention avoids all of these difiiculties by placing the tuning instrumentalities into one pole. piece with the pins moving through small holes in the pole piece and the supporting mechanism for the pins being mounted in a well within the pole piece. Using this superior relationship of the elements the use of the tuning pins does not produce an increase in the length of the magnetic gap so that the reluctance of the magnetic circuit remains the same as in the non-tunable magnetrons. g g ,7

These and other features of the invention will be more clearly understood from the following detailed description and the accompanying drawings in which: l

Figure 1 is a vertical cross-sectional view of a tunable magnetron;

V Figure 2 is the cross-sectional view of the anode structure of the magnetron taken along line 22 illustrated in Fig. 1, and,

Figure 3 illustrates a series of typical performance curves obtainable with the tuning arrangement disclosed in Figs. 1 and 2.

Referring to Figs l and 2-, they illustrate, by the way of an example, a tunable, strapped, hole and-slot, planetary type magnetron having twelve resonating circuits, as illustrated more clearly in A mounting plate It, made of nonmagnetic metal such as bronze; supports the entire magnetron structure. A glass boot I2 is ate tached to the mounting plate by studs. "and I6, a fibre washer l8 being insertedbetween the glass boot and the mounting plate. The lower portion of the glass boot is not illustrated in the dr-awing; it is constructed'in the known manner, terminatingin the connections for the cathode and the heater circuits including a heaterbatteryil and a cathode-anodesource 15. A cathode pole the untuned magnetron therepiece 20 is mounted on top of the mounting plate, the pole piece being spot-welded to the plate at i9 and 2!. The cathode structure of the magnetron is axially mounted in a well 23 of the cathode pole piece, this structure including a (Kovar) eyelet 22 Welded with its flange 24 to the pole piece soas to form a hermetic seal between the eyelet and the pole-piece. The lower portion of the eyelet is connected to a re-entrant glass seal 26 which forms a gas seal between the (Kovar) tube 28 and the (Kovar) eyelet. The upper portion of (Kovar) tube 28 terminates in a cathode 30 which is provided with the cathode end-disks 32 and 34 which maintain the spacecharge condition within the discharge space of the magnetron at desired values and electrically seal off the discharge space from the .end zones of the magnetron. Within the (Kovar) tube 28 is mounted an insulated conductor 36 the upper portion of which is connected to a heater coil 38 which supplies the necessary heat to the cathode. The outer surface of the cathode cylinder 39 is coated in well known manner with the electronemissive oxides such as barium and strontium oxides. The cathode is mounted centrally in the discharge space 40 of the magnetron and is sup ported in this position by pole-piece 2D, the (Kovar) tube 28, eyelet 22 and glass seal 25. The anode 42 is made of a block of conductive material such as copper shaped into a plurality of res onators 4| whose boundaries are defined by the cylindrical cavities 44 and slots 45. The resonators terminate at the anode surfaces 68 which surround the central discharge space 49 of the magnetron. The invention is disclosed with a l2-cavity anode but it is to be understood that the teachings of this invention are applicable to other types of anodes having a'smaller or larger number of cavities, and the cavities may have either a cylindrical shap illustrated in the figures or any other desired shape. When the shape of the cavities is modified the shape of the tuning pins should be modified also to follow the inductive surfaces of the cavities.

To separate the modes of oscillations of the resonators and to facilitate the oscillations in the pi mode, the anode is preferably strapped by means of straps 50 and ring type of strapping being illustrated in the drawing. The anode of the magnetron is mounted in a non-magnetic metallic shell 54, this shell being welded to the upper tuning pole-piece 56 as well as the lower cathode pole-piece 20, as illustrated at 68, 69, 10, and II, respectively. The left portion of shell 54 is provided with the cooling fins 55 while the right portion is provided with an opening for connecting a coaxial line 58 which terminates in a coupling loop 53 positioned inthe lower end zone of the magnetron directly under one of the holes in the anode. Only a portion of a flange 6B of the coaxial line is illustrated in the figure; the flange is connected to a wave guide by means of bolts which fit into the holes 62 of the flange. The coaxial line 58 is welded to shell 54 thus forming an air-tight joint between the non-magnetic copper stud 64 of the coaxial line and the shell. Only a portion of a glass seal 66 of the coaxial line is visible in the drawing. The magnetic flux is provided by two permanent magnets fitting against the cathode and the tuning pole- pieces 20 and 56, only one magnet-magnet 51being visible in the figure. The lowerportions of the magnets are fastened to plate [0, and the'upper portions are inter-connected through flanges 59, BI and bolts 63 and 65. The adjusting elements 4 of the tuning mechanism are mounted in wells 13 and 88 provided for this purpose in the upper pole-piece, which is called for this reason a tuning pole-piece.

The tuning mechanism includes twelve metallic tuning pins 12 protruding through the pole-piece, an adjustable pin holder 14 made of magnetic material, a non-magnetic driving screw '16, Monel metal bellows 18, a non-magnetic tuning knob 80, a non-magnetic ball racer 82, and non-magnetic balls 84. Wells 13 and 86 have their dimensions proportioned so as to allow the desired degree of vertical travel of the pin holder and the concomitant vertical adjustment of the pins within the cylindricalcavities of the anode. The tuning elements are constructed so as to permit complete withdrawal of the pins from the anode for maximum wave length, and lowering of the pins into the anode until there is an evidence of excessive degree of coupling between the anode and the pins. This will be discussed more fully later in the specification. The Monel metal bellows 1B are welded around their lower circumference to the pole piece as illustrated at 88 and 89, and around their upper circumference to the upper flange of the pin'holder 14, at 90 and 9|, thus completing the gas-tight'ioint between the pole piece and the pin holder. The tuning pole piece is provided with twelve holes through which the tuning pins slide up and down during the adjustment of the frequency of the magnetron. The non-magnetic tuning knob is calibrated in terms of wave length or frequency, the angular position of this knob'against a pointer 92, fastened to pole piece 56, indicating the frequency generated by themagnetron. Evacuation of the magnetron is eifected through a metallic tube 93 which is sealed upon the completion of the evacuation and degassing process of the magnetron.

Since the alignment of the tuning pins 72 with respect to the holes 44 is accomplished by the ball bearing surfaces, it becomes necessary to construct the ball bearing so that there is no lateral play between the balls and the racers of the bearing. This is accomplished by providing split racers TI and 19, the lower racer 19 being held tightly against the balls 84 by a spring 8|. The spring itself is compressed against the lower, fiat surface of the racers" and 19 by means of a race holder 83. The race holder is fastened to the tuning knob 80 by aplurality of set screws 85. The pressure exerted by spring 8| holds the balls of the ball bearing in a continuous engagement with all the racing surfaces of the racers so that any possibility of the lateral play in the ball bearing is thus eliminated. This construction of the ball bearing holds the driving screw 16 in strict alignment with the common longitudinal center line of the pole piece and the anode, and since screw 16 has a threaded engagement 6'! with pin holder '14, the pin holder itself, together with the pins, is kept in the equally strict alignment with the holes in the tuning pole piece and the anode.

The tuning of the magnetron is accomplished by turning the tuning knob 80. Since the nonmagnetic driving screw 16 is fastened to the tuning knob, it either raises or lowers the pin holder 14 because of the threaded engagement 63 between the pin holder and the screw. The torque exerted by screw 16 on the pin holder is resisted by the bellows 18 which, as it will be remembered, are soldered on top to the pin holder and on the bottom to the pole piece. The tuning pins thus remain centered in the holes of the pole piece ana mi and there isrnc twistin icrceexcrtednmn many-time. This isan importantieaturc when the, pins are made .of pure copp 91s it Mm appear later in the specification. fltlhus-z turningi of knob 380 changes :the degreemf :pin :penetration into the cavities 44 wei the magnetron.

- Ehe nupper end :zone "1.5 of sthe .;;magnetr on is provided with: a copper: ring 29.4 which is proyided with twelve holes, the tuning pins .;passin through :the-:-ring. Ring .94 "performsa :very im- .-portant junction --of shortening the electrical length of the pins; :the functioning ofiring Mcwill bedescribed later i-n thisispeciflcation.

The effect of the tuning pins on the resonators may lie-analyzed as follows:

'The fundamentalmethod of varying the frequency of --any--resonating circuit is to change either the inductance -or 1 the capacity or both. Because the circuit constants inany planetary hole-and-slot magnetron -are-not lumped, it is of courseimpossible to-vary strict-1y only the induc tanoeor only the capacity :of-any single resonating circuit. However, it is possibleto provide a tuning-meanswhich has a predominantly inductiveover-all eflect. In a magnetron oscillater oi the strapped ho'le-and-slot type, it is diflicult'to introduce large changes in resonator capacity essentially because large capacity changes would involve the use of :very 'small clearances between the elementsat high radio frequency potential. Moreovenby "far the larger part of the capacitive changes would take place at-the end portion of'the'mec'hanical movement,

that is,- when the clearances-between the tuning surfaces would be approaching-zerovalues. Accordingly, it is a theoretical andcpractical impossibility to achieve any linear relationship'between the :Wave length and-the movement of the tuning surfaces in'any capacitive tuning of the magnetron. The inductance of the resonating circuits may, however, be altered simply iby-the insertion of-any conductor into the inductive portion of the circuit, the conductoracting in effect as a short-circuiting means for some portion of the inductance-present in theresonating circuit. Thus a large change in inductance may easily be introduced into =amagnetron-of the hole-sandslot type by the simple insertion "of a cylindrical metallic pin into the cylindrical portion of the resonator with the pinsbeing nearest to the 'inductive portions'and farthest from the capacitative portions of the resonators. For thispurpose, each pin '12 is positioned-Withinthe hole of each resonator along the radial -axis195yFig. 2, thereof, 'but'farther from the slot entrance and nearer to the opposite surface of thehole. In the case'of circular'holes and pins, the-pins are eccentrically disposed within the holesas illustrated in Fig. '2 with the-pitch circle 96 of the holes being smaller than the pitch circle 91 of the-pins; stated-differently, the locus=91 oi the pin axesis a cylinder of greaterradius than the locus 96 of the hole axes. Besides'obtaining maximum changes in I the inductance, such positioning of the tuning pins'also removes them from thestamping ele ments 5!! and'5l thus avoiding any possibility of accidental-shorting of the resonating elements which'may take place "if the tuning pins happen totouch the strapping elements. This is anim portant factor in the ultram-igh frequency =magnetrons having small physical dimensions.

"The number of tuning pins, as disclosed, is

equal to the number of cavities since it is desirableto obtain a maximumtuning-range with a givenfixed anode structure. There -=is also a sci-meta thought.advibcatinewitneadcs rabilitacf altering-the inductance ,of. ,all resonators gin -=a similar-way. r'nccordingto thisrl'fidSOfliIlg :thensc flf' i the tuningtpinfin' each-cylindrical jhcle would he' preferable since cltmiWOllld :insure 1 the \rmaintenance of :an :angularly symmetrical fieldv pattern within the interaction vspace for all .'rinse .ntion depths .of the rzpins. :Gzeneral experience, with mag-netrons indicates an :opti-mum rtuhe ,zoperam tion when such. symmetry -zexists.

fit lis W :possiblerto: analyze .theeiunction 11pm. termed r-rthe resonance suppressin ring :5. Gonsider 1a magnetron :with :the .geometry iridicatedin'PFigs; 1 iand?2. suchgeometry :thepins themselves, together with the annular region between the pins andthee-anode shell, constitute an osciil-atorycircuit. 1 In :a general way the circuit-capacit-y occurs between l the ,walls .-:andthe pins of the cylindrical resonator and thetindum tanoe in the-iannular; region. fI'hisisy-stem'of pins will :be characterized. by-a= series of modes of cs1 .cillatiomeachcf a-characteristic wavelength. Whilethecharacteristic. wave length of the: anode decreases with -increasing .pin penetration, that of mach oi the modestof the pin system I will increase-with the increasing -.-.pin penetration. 1 If for: some pin :penetration the .characteristicwave lengthsof r the anode :and of the pins structure are; coincident, and if there: is :anytcoupling between the two; large-deviation fromv normalctube operation-may be expected. ZFirst; the resonant pinsystem niaypull the magnetron .wave length over :and above-Many alteration .in wave length lay-the lrnown resonant change in inductance; second, it may absorb significant fractio-n of the-electronically generated power; third, by-ithe distortion otthe radio frequency pattcrnsin the magnetron, it may impair the electronic cfii' c-iency-andgf-ourth, it may greatly alter the pulling figure. Moreoven ther-emay be-a -pr-esencecf a very dominant auxiliary .effect in which the pin system 12-apparently acts as 3, coupling device between ithe magnetron anode 'dil'and the coaxial line 28-nwhich-supports the --cathode-which :di warts-"a large -quantity of the ultra-high-irequency power lnto the-coaxial-line of the oathcda the latter phenomenon in an extreme case completely blocking the normal operation of' the magnetron.

In order that the tube may operate satisfactorily over-the entire tuning :range, it is therefore -neoessary*-that the -spectrum oithe pin structure be -outside the range "of the operating wave lengths of the -magnet-ron. In general the -f-ree length of the pi-ns whic'h electrically eng-ages tlsie anode mustbe considerably less than M l-since the magnetron end space contributes a -'large amount ofloading. It-is this function, that shorting of the --electrical "length of the tuning pins, that is performed by the resonance sup =pressing-ring 94r It reduces to-aslargean extent as pessible the-luniped--inductance in the end space and effectively shortens "the electrical length --of"the pins --en hancing-- the tuning properties of, the pins --and--the efflciency -of the Thus the -resonance-suppressing ring reduces to a -;minimum the-undesirable degree of coupling between the anodefresonators and the pins, the pirrresonanees beingmade by the-resonance suppressing ring completely-beyond the-range of the tube-resonances.

= While-it may-be very desirable -and-even necessary-tense the -resonancesuppressing rings in-the end zones of the magnetrons-of some specific wave 7:5 *length,-=the -'-physieal-relationships between the anode'g'the anode frequencies and the resonating properties of the'end space inthe magnetrons of some other wave length'may be such that high operating eiiiciency, frequency tracking, low pulling figure, etc.,' that is general stability of the system, may be obtained without the use of the resonance suppressing rings. Therefore the fact that the resonance suppressing ring may: prove to be an especially efifective and desirable device in the case of the magnetrons of some specific wave length'region, it does not necessarily follow that the ring has a-general application and will produce the universal beneficial results. The governing factor in either accepting or rejecting the use of the ring is the previously mentioned relationship between the operating wave length (or some higher modes) of the magnetron and the electrical properties of the end space and of the pins.

In describing the pin structure it has been mentioned that they are made of conductive material. Since pins 12 introduce losses and reduce the Q value of the anode, it is important that the pins should have the maximum obtainable conductivity. From a purely mechanical point of view it is desirable for the pins and the pin holder 14 to represent a structure with maximum possible rigidity, the rigidity of this structure; together with the back lash of the moving elements, primarily determining the resettability to a given Wave length from the dial readings on knob 80. To satisfy the mechanical rigidity the pins may be made of copper-plated tungsten. However, tungsten is very brittle and difficult to work and, therefore, equally successful results may be obtained but with a lesser degree of mechanical effort if the pins are made of copper-plated stainless steel; A measurement of losses in the pin material has indicated that the high temperature to which these pins must be subjected in assembly increases their resistivity, presumably by difiusion of the components of the stainless steel into copper. Improvements in assembly technique eventually permitted the use of solid pure copper pins in spite of their tendency to distort under very small forces; with the space and highest obtainable conductivity at a premium in the disclosed tuning mechanism, pure copper is the commercial metal with the best conductivity per unit of volume. 7

While the invention has been specifically disclosed in connection with the hole-and-slot anode, it will be apparent to those skilledin the art that the invention is applicable to the anode structures of different types, such as vane type or Rising Sun type anodes, as disclosed in my co-pending application titled Tunable magnetron, Serial No. 623,422, filed October 19, 1945. The only requirement is that there is a sufficient electrical coupling between the inductive portion of the resonatorand the pin so that the pin may act as a short-circuiting device for the distributed inductance of the resonator. The pins having a cylindrical shape have been disclosed because it is obvious that the pins of this shape will be especially advantageous in short-circuiting theinductance of the cylindrical holes. When the shape of the resonating cavity is other than the cylindrical hole, the shape of the tuning pin may be modified so as to engage the inductive portion of the resonator in as efiective a manner as the round pin engages the round .hole. I It is to be noted however that any sharpdiscontinuities in the-surface should be avoided in order to prevent excessive losses'in the structure and accidental '8 flash-overs. The advantages obtained with the scalloped anodes as discussed in'the co-pending application SerialNo. 623,422, are equally applicable to the principal case.

While the invention has been described with reference to several particular embodiments, it will be understood that various modifications of the apparatus shown may be made within the scope of the following claims. l

1. An ultra-high frequency magnetron including an anode, a pole piece mounted in spaced relationship with respect to said anode, a plurality of metallic members mounted within said pole piece, mechanically instrumentalities for lowering said members from their normal position within said pole pieceinto positions producting varying degrees of coupling between said members and said anode to vary the operating frequency of said anode, said pole piece having a well for housing said instrumentalities.

2. A tunable magnetron having an anode, a pole piece, said pole piece having a well, a holder mounted within said well, a plurality of tuning members attached to said holder, a corresponding plurality of openings in said pole'piece, said tuning members fitting into said openings, and means joining said holder with an external controlling element for varying the position of said holder within said well for varying the degree of electrical engagement between the anode of said magnetron and said tuning members.

3. A magnetron having a slot-and-hole anode, a cathode centrally mounted with respect to said anode, pole pieces mounted in spaced relationship with respect to said anode, and a plurality of round metallic pins adjustably mounted within one of said pole pieces and protruding with their ends through said pole piece so as to be insertable into the holes 'of said anode for' varying the operating frequency of said magnetron.

4. A magnetron as defined in claim 3 which further includes instrumentalities for keeping the respective axes of said pins in alignment with the respective axes of said holes in said anode through the traveling range of said pins.

' 5. An ultra-high frequency tunable magnetron including a multicavity anode, a pole piece mounted in spaced relationship with respect to said anode,-said pole piece defining an end space of said magnetron, said pole piece having a well, a pin holder mounted within said well, a plurality of metallic-pins mechanically connected to said pin holder, 3, corresponding plurality of holes in said pole piece, said pins protruding through said holes into said end space, metallic bellows interconnecting said well and said pin holder, and a drive screw connected to a rotatable knob and to said pin holder, said drive screw forming a threaded engagement with said pin holder whereby turning of said knob raises or lowers said pins from or into the cavities of said anode for varying the degree of electrical engagement between the anode of said magnetron and said pins, the variation in said electrical engagement varying the operating frequency of said magnetron.

'6. An ultra-high frequency tunable magnetron including a cathode, a multicavity-multiresonator anode concentrically mounted with respect to said cathode, first and second pole pieces mounted in spaced relationship with respect to said anode, a concentric line coaxially mounted within said first pole piece, said line being connected to and supporting said cathode, said second pole piece having a well, a holder mounted within said well, a plurality of metallic tuning members fastened to said holder, a corresponding plurality of openings in said second pole piece, said tuning members passing through said openings, metallic bellows interconnecting said well and said holder, a rotatable knob, a drive member interconnecting said knob and said holder, and a ball bearing mounted on said second pole piece and having means for holding said tuning members in alignment with said cavities through said drive member and said holder, whereby turning of said knob in one direction lowers said tunin members into the respective cavities of said anode, and turning of said knob in the opposite direction withdraws said tuning members from said cavities and draws them into said openings.

'7. An ultra high frequency tunable magnetron including an electron-emissive cathode, an anode having a plurality of resonators and having at least one flat end surface, said cathode and said anode having a common longitudinal axis; first and second pole pieces mounted in spaced relationship with respect to said anode, said pole pieces defining first and second end spaces, respectively, above and below the end surfaces of said anode of said magnetron; said first pole piece having an axially positioned well, a pin holder mounted within said well, a plurality of metallic pins mechanically connected to said pin holder, said pole piece defining a corresponding plurality of holes therein, said pins protruding through said holes into said end space, metallic bellows hermetically sealed within said first pole piece interconnecting said well and said pin holder, and a drive screw connected to an external controlling element and to said pin holder, said drive screw forming a threaded engagement with said pin holder, a ball bearing mounted on said first pole piece and having means for holding members in alignment with said resonators through said drive member and said holder, whereby turning of said controlling element raises or lowers said pins from or into the cavities of said anode for varying the degree of electrical coupling between the anode of said magnetron and said pins, the variation in said electrical coupling varying the frequency of oscillations of said magnetron.

SIMON SONKIN.

REFERENCES CITED The following references are of record in the

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