US2602156A - Modulated microwave generator - Google Patents

Description

July l, 1952 J. S. DQNAL, JR., ET AL MODULATED MICROWAVE GENERATOR Filed June 28, 1947 Patented July 1, 1952 l l 2,602,156 MODULATED. MICROWAVE GENERATOR John S. Donal, Jr., and Rohert'R. Bush, Princeton, N. .1.assignors to Radio` Corporation of America, a corporation of Delaware- Application June 28, 1947, SerialNo; 757,755

invention relates generally to modulated microwave generators and more particularlyy to improvedA methodsv of and means for amplitudemod'ulating'and frequency-controlling microwave generators, especially of the magnetron type;

In the copending applications of Lloydv P. Smith, Serial Numbery 563,732, filed November 16, 1944, and entitledl High Frequency Apparatus, and John -S. Donal, J-r;, etal., Serial Number 757 ,756, led June 28, 1947 and entitled Frequency Mdulated Magnetron Microwave Generator, now Patent No. 2,534,503, granted December 19, 1950, there is disclosed a system for modulating yor controllingfthe frequency of. a magnetron microwavefgeneratorv by projecting an electron beam along'a` constant magnetic eldand through a cavity resonator which may be either a cavity resonator of a magnetron` generator or anexternal cavity A resonator coupled to a magnetron genera-tor; Control of the energy of theV modulating electron beam provides a variable shunt reactancev'forthefrequency determining param'- eters of the magnetron generator;

' The instant invention is an improvement upon `the devicesdisclosed insaid copending applica'- ti'ons yin'` -that applicants have found that.V either frequency or-amplitude-modulation ofthe magnetron generator may be provided substantially to the exclusion of, or independently of, the other,4 depending upon the intensity of they applied unidirectional magnetic ield'. Furthermore, applicants have found that optimum magnetic field-intensities-exist for most efcient magnetron microwave generation', for amplitude modulation tothe exclusion of frequency modulation, andffor frequency modulation to the exclusion ofi amplitude-jm'odulation.

The-instant invention utilizes these realizations of'optimum magnetronmodulating and' operating characteristics and comprises improved: methods ofi and4v means for providing an efficient amplitude-modulatedv` magnetron generator which may, if desired, include frequency stabilization.

Claims.

Y "A first embodiment ofthe invention comprises l amagnetron generator ofthe multi-cavity type which includes meansI for providing a frequencystabilizing electron beam` projected through one or" more of the magnetron resonant cavities and additional'. means for providing an amplitudemodulating electron beam projected through one orrmore of said magnetron resonant cavities, and thus eiectively directly in shunt with the load.

In order to provide optimum operating characteristics forthe generation of microwaveoscillations in the` magnetron proper, as well as to` provide optimum frequency-stabilization and amplitudemodulation of the microwave oscillations, the-intensities of the unidirectional' axial lmagnetic el'ds applied to the magnetron proper and to the modulatingfand controlelectron beamstructures-are adjusted tooptimumvalues by proper proportioningof the magnetic structure-operable upon-thedifferent portions of the system.

A- modification of the invention contemplates the use` of an external amplitude-modulating electron beamdevice vconnected to the output transmission line connecting amicrowave generator to itsload. The amplitude-modulatingfelectron beam device comprises an electron beam generating gun'which projects a modulating electron beam through a coaxial" cavity resonator toa collector electrode. The electron gun includes a control grid to vary the beam current When a unidirectional axial magnetic ield` of proper' intensity is applied to the device, the latter operates as a variablenon-reaetive vshunt conductance connected to the microwave load transmission line. The variable shunt conductance is connected, either through aA half-wave transmission line, or through a quarter wave transmissionV line, to'a point on the-load trans'- mission line removed; some odd multiple ofv a quarter wavelength at the operating frequency,

from the microwave generator. Y

The amplitude -modulating systems comprising the instant invention provide a much more eilicient method of amplitude-modulating.` microwave oscillations than ispossible by grid modulation of triode generators orv by anode voltage modulation of magnetron or Klystron generators. Furthermore, in addition to .more eflicientoperations, the amplitude modulation of` microwave oscillations may be accomplished. without the usual undesirable frequency-modulation asis the case-when a magnetron is modulated byv varying the anode-cathode potential. Also, any'residual frequency-modulation or frequency instability in the device may be compensated for bythe inclusion ofa separate 'frequency control electron beam device responsive to frequency deviationsl in the modulated generatoroscillations.

A-mong the objects of thev invention arey to provide improved methods of and: means for modulating microwave oscillations. Another object is to provide improvedmethods oiv and means for amplitude-modulating magnetron generators. Afurther object is to provideL animproved method of' andmeans for amplitude-modulating a magnetronl generator ati a high modulation percentage. f An additional object is. toV provide improved methods of and means for amplitude-modulating microwave generators and for simultaneously stabilizing the frequency of the generated oscillations. Another object is to provide methods of and means for selecting optimum magnetic eld intensities for microwave magnetron generating and modulating devices. A further object of the invention is to provide improved methods of and means for insuring optimum operating parameters in an amplitude-modulated magnetron generator by providing optimum unidirectional axial magnetic eld intensities for the magnetron generator per se, and for the amplitude or frequencymodulating or control electron beam devices associated with the magnetron generator. A still further object of the invention is to provide irnproved methods of and means for coupling a variable conductance device to the load circuit of a microwave generating system for providing amplitude-modulation of the generated microwave oscillations.

Other objects of the invention include improved methods of and means for amplitude-modulating microwave oscillations substantially to the yeX- clusion of accompanying resultant Afrequency modulation. Another object is to provide improved methods of and means for frequencymodulating, stabilizing, or controlling microwave oscillations substantially to the exclusion of accompanying resultant amplitude-modulation. Another object of the invention is to provide an improved magnetron microwave generator including an amplitude-modulating electron beam device projected through one of the magnetron cavity resonators and a frequency-stabilizing electron beam device also projected through one of the magnetron cavity resonators, said magnetron generator including means for applying unidirectional coaxial magnetic elds of diierent intensities to the generating and modulating elements ofthe device.

The invention will be described in greater detail by reference to the accompanying drawings of which Figure l is a family of graphs illustrating the operating. characteristics of the system; Figure 2 is a partially schematic, cross-sectional elevational view of a first embodiment of the invention; Figure 3 is a fragmentary, cross-sectional plan view taken alongv the section line III-III of Figure 2; Figure 4 is a schematic circuit diagram to be referred to in the description of the operation of said first embodiment; Figure 5 is a schematic circuit diagram of a modification of said rst embodiment of the invention; and Figure 6 is a perspective, partially sectional View of a modulating electron beam device comprising the variable conductance GM lof the circuit of Figure 5. Similar reference characters are applied to similar elements throughout the drawing.

Referring to Figure l of the drawing, the graph 1 illustrates the variation in frequency of microwaves generated by a multi-cavity magnetron asa function of the unidirectional axial magnetic field intensity applied to a modulating electron beam coupled into or projected through one of the magnetron cavity resonators. Graph 3 illustrates the radio frequency loading of the magnetron by the modulating beam as a function of 'said unidirectional magnetic field intensity. Comparison of the graphs indicates that maximum frequency modulation is obtained for values A1 and A2 of magnetic eld intensity although somey radio frequency loading occurs for these values. Appreciable frequency modulation may be provided for values B1 and B2 with negligible t v radio frequency loading. Also substantially no frequency modulation will obtain and a maximum of radio frequency loading will be provided for a magnetic field intensity indicated by the value C. in addition, it has been determined that most magnetrons function most efficiently as a microwave generator for even higher values of magnetic field intensity as indicated by the value D.

As a result of the information deduced from the graphs l and 3, applicants have provided a multi-cavity magnetron microwave generator in which .the magnetron cathode and anode are operated at a magnetic field intensity of a value D to obtain. good operating efficiency, a frequency control electron beam is .projected through one of the magnetron cavities for stabilizing the operating frequency, and a second ymodulating electron beam is projected through another of the magnetron cavities for modulating the amplitude of the generated microwave oscillations in response to modulating signals. The frequency control electron .beamv preferably is subjected Ato an axial magnetic field intensity of the value B2 which provides substantially negligible radio frequency loading vwith appreciable frequency control. The amplitude modulating electron beam is subjected to a lesser magnetic eld intensity corresponding preferably tothe value C which provides maximum radio frequency loading and negligible frequency modulation. A unitary structure including these features is shown in Figures 2 and 3 4of the drawing.

The magnetron generator. comprises a central axial cathode 5 surrounded by a plurality of radially extending anode cavity resonator vanes I which are alternately connected together by anode Vane straps 8, and are positively biased with respect to the cathode 5. The inner ends ofthe vanes 'I provide anode segments. The spaces between adjacent radially extending anode varies comprise anode cavityresonators tuned to the desired operating. microwave frequency. An axial unidirectional magnetic field is derived by an external magnetic structure, not shown, terminated in pole pieces 9, `9 which include small pole tips .I l, Il', of highly permeableV material, located adjacent to the central axis of .the magnetron. Y

The frequency control electron beam generating device comprises a first auxiliary Vcathode I3 and a frequency control grid I5 located above one of the spaces between two adjacent anode cavity resonator vanes for projecting an electron beam I 'I through a cavity resonator to a collector electrode i9 which is positively biased with respect to the cathode I3. Similarly the amplitude modulation control electron beam devicecomprises a second auxiliary cathode 2| ,with an amplitude control grid 23 located above the space between two other adjacent anode vanes for projecting a second electron beam 24 through an anode cavity resonator to a second collector electrode 25 which is positively biased with respect to the second auxiliary cathode 2|. The auxiliary cathodes I3 and 2I may be either directly or indirectly heated in accordance with known thermionic tube technique.

The pole pieces 9, il are so positioned with acca, rse.

anodevarres: I5 andtheirequencyi control :and control; electron. beamadevices alli are enclosed within arrev'lanuated.I ermelope 7,11? through whichzsuitable-'operatingfleads arerprovideda` Qutput'. energy' is'derived from an. outputfccupling loop 29 coupled into one of theranode cavityV resonators. .-,fxllhe output 11m@ -zf may.; be coupled through any: suitabietype ot microwase transmissiom lineff3'l toztheiloadi: -1/ L f The stabilizatiom oithemperatingfrequencyby the frequency controlielectronbeam lfkisf eiected by connecting a disoriminator', .fouexample'of the magic T3 waveguide typeitmthe output line 3.1i and'. deriving'. control volta'ges'tronrther discrimirmrtoirwhinh are appliedibetweenlthe cathode ha" and; gridi |5zot' .thefrequencyY cont'rohelectron beam device..z .amplitude rrrodulationfofi the ontplut. sig-naief. is. eifectertv by applyinggvdesired modulation signalsztozthe'cathode: 2:1fv and gridz2i oathex secondauxiliary electrony ,beamvv device whichz projects' thel beanr .241 throughsl the maganode cavity. resoriatton. ThexrrequencycontroLbeanr I;1;. being'. operated1 atz'arr axialmage netic field intensity Bz, provides substantially; only reactive; effects: upon' thei magnetronoscillations with negligible radiov frequencyr loading thereof'. The amplitude-contrai bea11`i..2!l provides .max-imum* loading; or: the# generated*y microwavefzoscillations wtih negligible frequencyf` modulation thereof, since it is operated with an axial magneti'celd' havingthe valu'czG In`l operation,v the amplitude modulation electron beam 24 effectively-.providesa nonrreactiy-.evariable shunt. conductance.. GM indicated. by thedashlineresistor connected across.. the output transmission line. Varyingthe controLgridLpotentialyariesthe beam current, which varies the energy absoubingability oi the.. beam.. The. theory andoperation'oflthis embodiment of the invention yis described in greater detail by reference to Figure 4 of the drawing which follows.. It should be understood. however, that the amplitude modulaiton electron beam may alternatively be incorporated in an external variable conductance=- device as illustrated and .lescribedbyJ referenceftofFigure 6.- of the drawing, and such an. external variable conductance device would" be' coupled directly into one of the magnetron anodefcavitiesfontconnected to the load transmission line as will be discussed in greater detail hereinafter.

Reference is made to thet'circuityof Figurefel. where O is the magnetron generator, Gr. the load conductance, GM the-variableor modulating conductance, and T a lossless transformer. By varyingr Giu; thefdry-meansf of theoontrol grid Z3',`

the-"power in'Gm will be varied, if the?.generadaorV power output and thus the power output- 'oi" the transformer are constant. In this case, if a is a fraction and I Y GM.=VCLGL then the power in the load is Y 'Prandi-mer p l (2)"l where P0 is the power output? ofthe-.magnetron generator.

I rAlternatively, if. the transformer were not present, and if the generator were a constant voltage generator; the power in Gry would` remainxconfstant regardless of the valueor? fore, to= determine the eme'ctiveness-f offfsuchf a modulation'. scheme, the powerV output charac.- terist-ic as avfunctionoff'the load mus-tvbe-deterrmined;and the eiectof various"types'-.oi'transformersfmust',beiconsidered.V r L l li' "f l General chemioteria-ticsy f firmed by experimental data. Power output and frequency may be -measured as a function of load "impedance, fromjstanding@ wave? measurements, and thed'ata plotted c'm a; familiar Smith chart or Rieke diagram. Frorrr these data', thepower output is noted as a function of RL/Uu, where Rr. is the load resistance. and Zo the surge impedance` of the line. The zero. reactance line is Vdetermined from the frequency contours. Data taken indicates that the assumed relation of' Equation' 3 does provide. a good approximation to actual characteristics and that uvar'i'es from about`0.1 to 0.6 fordifferent types of magnetrons.

yPower relations without tmnsjfrmner fectively in direct shunt with the generator. If'

the modulating conductance, GM, were increased under these conditions, itV would absorb more power from the load, Gr., but the conductance, Grrr., seen by the generator iss increased` and therefore from (3)the power'. output of the magnetron generator increases. Therefore, the decrease in power in Gi. is` thel difference be.- tween the additional power absorbed lin Aandv the increase in poweroutputV of the magnetron; ThisY condition yobtains when the' ampi-rende modulating electron beam is projected through one' of the magnetron cavity resonators as in the device described by reference to .Figures 2 and 3. ff/.The conductance seen by' the generator is GLL-=GL 1+a v r4) from Equation 3 the-magnetron power output is P0=KGtu(1-lrr)1L (5) and the; power inf the; load is, from-Equation 2;

PL'=KGLH Y1+aw-1 (s).

The magnetron eiciency is defined by ulatiohl, varies' between'z'erof and' this maximum value. f v

In practice, a` modulation 'eiici'ency' 'i's' fre."- quentlyf'defihed 'as half1 the maxinrur'npowr u n u-l v u -GL [1 (Ll-) l l (10) where it is again to be understood that a has its maximum value. Expressing this modulation eiiciency in terms of the maximum magnetron eiiiciencyffrom Equation 8,

rwm, Haw-(Ham '1(11) This function has a maximum with respect to a which occurs whenk of a for different Values of u.

` It is frequently convenient to dene a modulation factor, 1n, as the ratio of the maximum voltageV lamplitude of the envelope to the average voltage'of the carrier. This may be written lll-1 From Equations 14 and 16, m may be calculated as a function of a for different values of u;

The value of M2 when a has the value, given by Equation 12, for maximum g, is

M2=1/u (17) and the value of m for maximum is 1 1# mma, (18) Eect of transformers .As discussed heretofore, the power characteristic of the magnetron works in opposition to that of the shunt combination of GM and GL. VIt is therefore desirable to introduce between the magnetron and the two conductances, a transformer T which will effectively invert the characteristic of the generator;

This is most easily accomplished by the use of an external modulator and a quarter-wave transformer, a transmission line of an odd number of quarter wavelengths long, between the generator and the junction of the load and modulator lines, as shown in Figure 5, since an increase in the conductance at the `junction of the load and modulator lines will result in a decrease in the conductance seen by the generatorO. This .will decrease the power output of the generator, and so the power in the load is decreased for' this reason in addition to the decrease from increasing GM. f

Any length of transmission line transformer other than an integral number of half or quarter wavelengths would reflect a. reactive component back to the generator, and since this would result in frequency modulation, it is not feasible. for amplitude y mo dulaton. It is also possible to inserta transformer between the modulating conductance, GM, and the point of junction with the load line as shown in Fig. 5. If this were a half-wave transformer, no change would result, since it would have the same effect as a direct shunt across theload. If it were a quarter-wave transformer, van inversion would result such that when GM is'zero, the load would be short-circuited. This would reflect an open circuit to the generator, with a quarter wave transformer between the generator and the junction.

' It is assumed that the load conductance, GL, is equal to the surge admittance, Yo, of the line and so the distance between the actual load and the junction point with the Vmodulating branch is not important.

Power relations with, quarter-wave load-Zine transformer represented by Figure 5.

The conductance seen by the generator will be but, in contrast tothe transformerless situation, the maximum tube efficiency occurs when ais zero, giving As defined by Equation 9, the modulation efciency becomes =1/211mX[1-(1+a)*1ul (25) This function does not have a maximum for finite a, but increases with a. The ratio M2 is M2=(1-|a)1+ Erom Equations 26? and 14 m as a function of a.

Comparison between the two types of loadline transformer connection may be easily made. As an example, if u=0.3 and a=1, the half-wave transformer connection gives a value of 5 `of 0.15611max, while thev quarter-wave transformer v (26) may be calculated .aerien-se connection .gives l`a value .of ig of i0g29711max. For the half-Wave transformer connection and u=0'.3, the maximum value of is 0.20911max and occurs when a=4.6. This .same efficiency obtains in the quarter-Wave transformer connection when 0:0;6.

The quarter-Wave yload line transformer connection gives .an 'appreciably .higher modulation efficiency for agi-ven range inmodulating conductance and will give'the same efficiency as the half-,wave transformer connection lat a much vlovrer.frange -of modulating conductance.

Moduatm'g device It is, of course, desirable to var y the conduct tance. GM, electronically.' y'b/l'oreove'r, it is desirable that GM be .real at all times, sthat no frequency modulation occurs, and that GM'be 'a function of an independent parameter, such as a control grid potential, arr'dJindependent of the average voltage appearing across it.l A high plate conductance triode would provide the means of control but its 'conductancefdecreases lwith'decreasing plate voltage. -As Atheconductance is increased by decreasing Lthe `negative--grid voltage; the "powerf'in jthe load ldecreases,v =but this decreases 'the zradio* frequencyvoltage -o'n the `platelofI the triode, vtending to-decrease its-conductance. f

A type `of variable conductance ldevice which does meet all "of the above requirements 'and is more 'eicientthan va `trio de is lone "similar to "the spiral .beam reactance 4tube described in said coe pending applications "and used for'frequ' ency incuiul'atngv magnetrons. Such adev'ice is shown 'in Figure 6;

- aSince'the admittance Aacross the device"^isjtofbe real, "and has Vbeen *assumed itc-be equal'to'Yo only sfor sake ,o-"s`implicity; 'the magneticeld; vI-I, yshould'b'eaijustedsothat e wHm ce Where w is the angular operating' frequency and e/m is the charge to massratio of the electrons. The quantity in Equation?? 'is 'the 4angular velocity of cyclotron 'trequency we lbf 'e`l'ec tr'ons 'traveling Sincircular "Dat-hsjiin a magentic field' ofjs'trength when the Amagnetic-r1erein is aejujstesto--i1aire the angular Acyclotron'frequencywe equal to the Where :ipo is .therradiofrrequency;amplitude. 'The ff .energyabsorbed lbyfeach'lelectron is `then Y By proper selection of f and d, the termin `brack- 1i'0i ets can easily :be made. appreciably greater than unity. Since the `energy Aabsbrbed by each electron in atriode is simply ooe, the .triodewould be less 'efficient @sean energy .absorbing device.

If itbegrequired thatthe electrons noltfbe :cap- .tured by fthe plates, Vbut be collected at the `end of the tube `rby ra collector, y"then f :is independent.l 'of the radio frequency voltage between the plates. Since Io will .be .determined 'by the :accelerating beam voltage Vb, Ywhich .is iin the .direction :of the magnetic eld, and a control-grid `voltage Vg, 1G will .'be independent :of the Eradio frequency evoltage.

.To .meetlthe .requirement that no :electrons.are captured y:by 'the lplates, it Lis necessary 'that fthe spiral deflection be less'` than 1/2(`d'lt)f, Where t 'isfthefcathode width. `It iskno'wn thatthefamplitude of the lspiral is. v

AWhere Z -isl the length --o'f Vtlie plates. Eo'is `the amplitude fo'f the rad-io frequency iiiel'd between 'the plates. -Whengy is set equal to Meid-tj), 'aY maximum value of r results "which Wheninserted in 'Equation A2 fgives`for the maximum obtainable conductance The quantity in brackets is maximum when `t/LL: 1/3 and has a-rnaximum value of In order for space charge to permit the current fdensity, Jo, lto passthroug'h the cavity, the Vexpression must hold. Fmax isa -function of t/d and varies from 2 for t/d=1to '0.1866 for t/d`=0\. When -t/d= 13, Fmax=1.10.-

EromLEquation 34., it :ifs seenthat'lif Gmaxlis to -be made flarge, .and :1i must fbe large. From Equation 35, large values of d will require a large Vb, and since Equations ,29 and 30 must be such that y=1/2 (d-t), a large Vb will mean a large value of Z. However, a large wmeans a large Io, which with a large Vb means high total beam power. Therefore, it is desirable to use as low a value of G, as possible, but it is limited only by practical factors.

Finally, it should be pointed out that if the electrode; arrangement considered is to be part of a resonant cavity, `the G calculated above will be the conductance which appears between the pole faces of the cavity and will be transformed to a new value, vappearing across the transmission line, by agcoupling arrangement, such asa loopvor window. It is this conductance which appears across the line as GM in the foregoing analysis. In general, it will be possible tomake GM appreciably higher than the electronic conductance, G, and if the proper coupling is used, GM will be directly proportional to G. Since the gap filled by the beam is an integral part of the resonant circuit, kthe capacitance of the gap must be considered in the design.

However, in estimating the maximum value of GM from Equation 34, the coupling problem need not be considered since co2 appears in the denominator of Equation 34. The radio frequency power dissipated in the beam is 1/2 G po2 and this must be equal to the power which would be dissipated in a conductance GM `across the end of the line where an,R.-F. voltage, ai appears. Thus Equation 34will give the maximum value of GM directly, if o`is replaced b`y 1.`

If the beam is to be placed inside the magnetron, as 4in the system of Figure 2, a direct shunt with lthe load results and the rst or transformerless analysis is applicable, and lower modulation eiciency is .obtained than if a quarter-wave load line transformer is used.

Lineavrzty Thedegree of overfall linearity of the external modulator may-be determined by expressing the amplitude of the radio frequency Voltage across the load as a function ofthe applied signal voltage. This radio frequency voltage amplitude is I v a may be related ytolthe beam current by using Equations 1 and 28:

a5=BIo (39) where Mdm Presumably, a beam forming gun will be used in the modulating tube. If Vg is the control gridjvoltage, Va the accelerating voltage, and c When Vg=0, la has its `maximum value since it is assumed that Vg is never to be positive and so Equation 42 maybe rewritten Using this expressionv in Equation 38,v

. .B Vv

where the -land -signs again refer` to the quarter and half-wave connections, respectively. This modulation characteristic is valid only at low modulating frequencies; no attempt has been made to consider the band width rof kthe overall system Which, to a great'extent, will depend on the particular circuit conguration used.

kThe systems of Figures 2, 3 and 4 and the modication of the circuit of Figure 5 employ-T ing a coupling line having an eiective length of any multiple of one-half wavelength between the modulating conductance device GM and the quarter wave connection on the load transmission line all have the disadvantage of relatively low maximum. depth of modulation. An important improvement of the devices described involves the use of a coupling transmission' line, between the quarter wave pointV on the load transmission line andthe modulating conductance GM, of any odd number of quarter wavelengths load line to a veryrhigh resistance at the magnetron generator output coupling loop. 'I'hus not only is the loadimpedance substantially vshort-circuited by the modulating conductance butthe magnetron output energy is reduced to an extremely low value. Y This arrangement pro,- vides extremely eicient operation and, good .depth of modulation. `In several tests Vof the system the decrease in output energy provided by keying the modulating beam 23 of the device GM on and off corresponded to a voltage modulation of Without consideration of modulator tube losses, the effective voltage modulation has been found tobe ofthe order of Thus the improvement over the circuits described heretofore insofar as modulation percentage is concerned is of the order of two or three times. I 1 f GIL: Grt

But #Gtia-GL :by 'despiden -of twend @para (assumed) I Magnetron ,generator zoutput: Bel mari-Gm -cfrom 'experimented data, egoista To. evaluate 'PMt I fimismaximu'mwhen orisfmaximum); y c

- To evaluate modulationjeniciencywhiciijisf dened as when@ is maximum.v Af in terms of @rmx for a amaaim-um, seeEduatlon 48'.

From Equations J9 4and -'8:

lReferringtojFignre 6,' the 'external amplitude modulating or absorption device.' corresponding to the' element GM Oif jtle circuits. described hle'rel toore c mayV comprise. kan evacilateol gias'senveljope 1H enclosing an electron '.bealnQgiun comprising a Ithern0nic`cathode 1l'3,f"a` control grid 145;, Va screnggrid'l, and anacpe1eratinggria 'as .j electronlgun elements are suitably biasedjtfo gpfro-r jecty 'the electron 'beam 13'? toa' positively "biased collectorieleotrodeT51; The evacuated' envelo `e 4 I is surrounded 'by a, coakialfcavity( reson'd including Aarcuate' electrodes `55,;f51. nator 5.3v istuned 'to vthe' voperati;:1gI'niicrw'ave frequency l thus providingf'by microwave energy absorption; a transverse microwave electric y'ield between the electrodes T55, 51 and 'causing'the electron beamy to `iollo'wjjtheY directrix of'a hollow cone. having a .radiu'sfdetermined byjthe magnitudeof the energy absorption; Itsliold be understood 'that the entire .assembly kInayloe enclosed within a lrnjetalenvelope; and that the glass. envelope `described merely comprises; a convenient demountable design. 'A coupling loop 59 coupled into the resonator'ii and terminated on thebracket 6I supporting Ithe electrodefl delivers energy from the coupling transmission linef 'to the cavity .resonator and'tothe,e1ectron'beam"2'3f1, .axial unidirectional magnetic field, of suitable intensity C as described heretofore "by reference tothegra'phs off Figure '1, is applied to -thevariable cgnduotance device 'GM asfindicated by thearrow Tt should 'be understood'ithat theinventionjis not to be considered as limited tothe speciic microwave -structuresdescribed herein .and that the frequency-stabilizing 'and amplitude-modd- Alating :features may be employed'eith'er in combination, or as separate adjuncts toknown types A'o'iimiczifovvave generators. Y 'Thusthe invention described and/claimed he-'re in r comprises improved methods lof j andmeans formodulazting the amplitudes of microwave oscillations especially as provided 'ov-magnetron microwave generators.v and additional means for controlling the operating microwave Ifrequency independentlvofv the amplitude modulation. The relations between frequency' variation and R.F.

" The [reso-f loading as a function of applied axial Vunidirec;

tional magnetic field intensity are discussed and utilized in the various embodiments of the invention. Various circuits and structures are disclosed which provide .improved amplitude modulation efficiency as well as substantial freedom from undesired frequency modulation.

We claim as our invention:y

1. A microwave system including a generator adapted to provide microwave oscillations, a variable-conductance electron beam modulating device coupled to said generator, means for providing a constant magnetic field coaxial with the beam of said device, the cyclotron. frequency of said magnetic field being substantially equal to the angular frequency of said oscillations, and means for varying the conductance of said beam to modulate the amplitude of said oscillations with negligible frequency modulation thereof wherein said generator is coupled to said variable conductance electron beam modulating device by means of a load line and impedance inversion means coupling saidy variable conductance device to an intermediate point cn `said load line.

2. A microwave system in accordance with claim l, wherein said intermediate point is removed an odd number of quarter wavelengths at the operating frequency from said` generator.

3, Avmagnetron generator including Aa cathode,

an anode cavity resonator adjacent said cathode, A

means for providing a first magnetic 'eld coaxial withsaid cathode, said generatorvbeing 4,adapted to be connected to a source of operating potentials to provide oscillations therefrom, means forpro'- jecting an electron beamthrcugh saidresonator, means for providing a second magnetic field Vco'- axial with said beam, the'cyclotron frequencypf saidfsecond constant magnetic "field :being subvs'tafr'itially equal'tothe angular 4frequency of said oscillations, whereby; said beam is substantially nonreactivathe intensity of said first magnetic field being substantially greater than that of said second magnetic field, and means vfor varying the conductance of said electron beam to' provide amplitude modulation of said .oscillations 4. A magnetronzgenerator including' a; central cathode surrounded byaj'plurality of anode cavity Y resonators,y means "forV providing a first' magnetic field coaxial to said cathode, said generator/being adapted ytojbe connected to a source of.` operating potentials to provide oscillationstherefroln; means adjacent to onefo'f said resonators for projecting an electron beam along a pathextending there.- thrdugh, means for providing a secondY magnetic field coaxial with said path, the cyclotron jfrequency of said second magnetic ,field "being substantially' equal toV the generated angular',fre-A quency of said magnetron generator, whereby'said beam loads said generator non-reactively,v the' intensity of said first magnetic field being substantially greater than that cfzsaid second magnetic field.' 1.

v5.v A magnetron generator'accrding to`claim`4,

further includingv means l vforf fvaryinglffthe.. conductance of said' electron beam'to `axriplitudel.modulate saidoscillations. y 'l 6. A microwave system includingagenerator adapted to provide microwave oscillationsa variablefconductance electron ,beam-modulating, de

vice coupledfto said generatonmeans for provid-- ing a constant magnetic field coaxial with the beam of said device, the cyclotron frequency :of said magnetic field being substantially equalto the angularfrequency of said oscillations, means for varying the conductance of saidV beamjto 1,6 modulate-.the .amplitude of said oscillations with negligible vfrequency modulation thereof, asecond electron beam modulating devicecoupled to said generator, means forproviding a second constant magnetic eldcoaxiallyof sai'dsecond modulating device and of such strength relative'to theoperating frequency that said second beam device provides appreciable reactive effect upon and negligible loading of said oscillations, and means for varying the energy absorbing ability of said second beam modulating device to provide frequency control of said oscillations.

7. A microwave magnetron generator including a cathode,'an anode in operative relation to said i cathode, said cathode andlanode being adapted to be connected to a sourceof operating potentials to provide oscillations, a pair of electron modulating beam devices coupled tosaid anode, means for providing magnetic fields coaxially of said cathode and said modulating beam devices, said field providing means being constructed and arranged to provide portions of different intensity substantially to Lprovide simultaneous maximum loading of and negligible reactive effect upon said oscillations by one o fr'said electron beams, to provide appreciable reactive effectupon and negligible loading of said oscillations by the other of said electron beams and toprovide high eiiiciency of oscillation generation by said cathode and anode, means for varying the energy-absorbing ability of said one of said electron beams to provide amplitude modulation of said oscillations, and means for varying. the energy-absorbing ability of said other of said beams to provide frequency control of said oscillations.

8. An amplitude modulated microwave magnetron generator including a cathode, an anode having a plurality of radially disposed cavity resonators in operative relation tosaid cathode, said cathode and anode being adapted to be connected to a source of operating potentials to provide microwave oscillations, a pair of electron beam modulating devices directed through different ones of said resonators, means for providing a magnetic field coaxially of Vsaid cathode and said modulating beam devices, said eld providing means being constructed and arranged to provide portions of dilferent intensity substantially to provide simultaneous maximum loading of and negligible reactive effect upon said oscillations by one of said electron beams, to provide appreci-Y able reactive effect upon and negligible loading of said oscillations by the other of said electron beams'and to provide high efficiency of oscillation generation, means for varying the energy-absorbing ability of said one of` said electron beams to provide amplitude modulation of said oscillations. and means for varying the energy-absorbing ability of said other of said beams to provide frequency control of ksaid oscillations.

9. A microwave system including a generator adapted to provide microwave oscillations, a variable-conductance electron beam modulating device coupled to said generator, means for providing -v a constant magnetic rfield coaxial with the beam of said device, the cyclotron frequency of said magnetic field being substantially equal to the angular frequency of said oscillations, and means foi*` varying 4theconductance of said vbeam to modulate the amplitude'o'f said oscillations with negligible frequency modulation thereof, wherein said generator is coupled to said variable conductance device ybymeans of a load line and a transmission line having an electrical length equal to an integral number'of .quarter wavelengths at 17 the operating frequency coupling said variable conductance device to an intermediate point on said load line.

10. A microwave system in accordance with claim 9, wherein said intermediate point is removed an odd number of quarter wavelengths at the operating frequency from said generator.

.I JOHN S. DONAL.'JR.

ROBERT R. BUSH;

REFERENCESl CITED The following references are of record in the file of this patent: 4

Number 18 UNITED STATES PATENTS Name Date Habann et al Mar. 28, 1939 Christ Nov. 26, 1940 Blewett et al. May 13, 1941 Hansen et al Apr. 13, 1948 Spencer Apr. 26, 1949 Everhart June 7, 1949 Spencer July 26, 1949 Peters Nov. 29, 1949 Donal Mar. 28, 1950 Peters et al. Oct. 31, 1950 Cuccia Feb. 20, 1951

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