US2809321A

US2809321A - Traveling-wave tube

Info

Publication number: US2809321A
Application number: US401303A
Authority: US
Inventors: Horace R Johnson; Dean A Watkins
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1953-12-30
Filing date: 1953-12-30
Publication date: 1957-10-08
Anticipated expiration: 1974-10-08
Also published as: GB776952A

Description

' oct. s, 1957 H. R JOHNSON E'rAL TRAVELINGWAVE TUBE Filed Dec. 50, 1953 3 Sheets-Sheet 1 j OC- 8, 1957 H. R. JOHNSON Erm. 2,809,321

TRAvELING-WAVE TUBE:

I 3 Sheets-Sheet 2 Filed Dec. 30, 1953 in :wig/w', anni:

745/4 nam/isc OC- 8, 1957 H. R. JOHNSON Erm. 2,809,321

TRAVELING-WAVE TUBE Filed Dec.

3 Sheets-Sheet 5 i Q i an@ Zgfll e E "si l iii we'/iie-" Patented Get. 3, 9%?

When the opening in the helix is made larger while maintaining a fixed pitch angle, the axial electric fields of both the fundamental and backward waves are of subasaaszi L.

staniially the same magnitude and exist, ror the most part, TRAVEL TF1-WAVE TUBE 5 in the regions adjacent to the wires of the helix. Hence, Horace R. Johnson, Venice, and Dean A. Watkins, Menlo ll/hen afhehhavllng an nlrgd Opdlmg I ilsed Gilly tile Park, Calif., assignors, by mesne assignrr: ts, to Hughes .)W O striam .e ectrons m he. reglims macelli LO t le icraft Company, a crpmn of Lsewge wires of the helix may constructively interact with one `of the electric fields of the waves propagated by the helix. Application December 3G, 19539 Serial No- 491,303 10 This, however, creates the basic problem of having the f 4 electron stream flow in regions where the electric fields 7 Claims' (CL 315 am) of the fundamental and backward waves are of the same order of magnitude. Under normal circumstances, this condition would result in the start of backward wave oscil- Tlie Preseni inveniion relnieS i irVeling-Wve inbeS 15 lations when any substantial amount of current is made to and more particularly to an electron stream amplifier tube ow in the electron Stream, For a tube operating as a incorporating a helix with an enlarged opening for aC- forward wave amplifier, however, it is necessary to precOni-rncrlaiin' a lnrger Sirenni cnrreni and linving a Siep vent these backward wave oscillations. For a traveling band for suppressing backward Wave Oscillniions in se wave tube amplifier having a helix of typical pitch angle lected modes. and operating in the region where its helix is non-dis- AS iS CGInrnOnly known, conveniienl irveling-Wave persive, it has been found that the frequency of a backtubes have found widespread use as microwave amplifiers Ward wave propagated by the baux is Such that its circumbecause 0f ilieir abiliiy i0 Provide gain over n broad ference is either approximately one-half wavelength of bs-nliviililr Present dily traveling-Wave inbes generally the wave or approximately one wavelength corresponding comprise n helical sirncinre capable 0f prpgaiing an 25 to what may be called the low and the high frequency electromagnetic signal wave at a velocity considerably backward Wave modes 0f propagation 0n the helix. in less ilinn ille velocity 0f liiii- The free'siiace vevelengili addition, for a predetermined electron stream velocity, reclnced by e fncior equal io ine rail@ of illis velociiy oscillations will normally occur within a very narrow i0 ine velnciiy Gf liglii is linoWn as ine guide fvevelengiliband of frequencies for the low frequency backward waves in their operation, an electron stream is directed through because of the greater gain in this mode. ille helical sirneinre prope'eilng ine Wave ei en anpro- As is well known, a helix with a typical pitch angle pl'li VGlOCl'Cy S0 aS O lIlBIElCt COSTllClVSly With the that is large compared t0 the free spacg Wavelength of a Wave- In Order fOr ille Wave propngeiefl by ille lieliX propagated wave is non-dispersive. Thus, when a helix of to interact efficiently with the electron stream, it is necesthis type is used in a travelingwave tube Operating as a sary for the axial components of the electric iield of the forward Wave amplifier, tbe electron Stream will be at a Wave 0 XiS in ille region 0f eleciron lloW' To accom* substantially constant velocity for frequencies over its enplisli iliis, ilie general precedere is i0 nse 21 lieliX having tire range of operation. Therefore, it is evident that a circumference less than two guide wavelengths at the means fOr attenuating th@ frequencies at which backward Signal frequency so inni ine eleeirie neld of ille Wave has wave oscillations occur for an electron stream velocity axial components across the entire opening of the helix. 40 corresponding to the forward wave mode of operation may ifi/'hen this is done, all of the cross sectional area of an be provided by an appropriate Stop band. i electron stream directed through the helix is in the electric This Stop band may be provided, for example, by disfield of the Wave, ilins eiieciin eliicieni inieiecilon be posing periodic discontinuities along the turns of the helix LV/CSD the CleCrOn sirern and ille Wave separated by a multiple of one-half free space wavelength Traveling-Wave inlies of illls iype have cel'ialn obvious 4D for the frequency to be attenuated. ln the event that it is limitat'OnS. AS au @Xarnple7 ii is nppnreni illai as higher desired to attenuate a broader band of frequencies, it is e-nfi higher frequencies are amplified: ille necessary open' only necessary to increase the length of such disconing in the helix to eect optimum interaction between the tinuity along the periphery of the helix. These discon- Wnle and ille eleciron sirenm becomes lilcl'easmgly tinuities, because of their periodicity and proximity to smaller. The Srnnller Openlnl in ille helix of conventional the helix, will form either a spiral or a line along the tubes designed to operate at higher frequencies reduces helix; hence they may be provided by a Single road or ille Sirein Cnrreni ilioi may be accommode-led: thereby other elongated member. ln order to prevent additional refilllCrng ille available Pon/ er oniPPiextraneous modes of propagation through the helix, the 11h@ aforementwped 11mm@ lsderfenmveiel 1H the 55 rod is preferabiy made with its conductivity in the longtinne Of'ilne present lnvenilon blfeml'ioilog a han( having tudinal direction interrupted at intervals of less than the a considerably enlarged opening. This, however, inpitch of the helix* troduces other basic problems, as will be explained, which One embcdmem of the mbe of the present iwenion f l de Y P A b kv d V 60 t e order of .1.6 or a free space wavelength of the signal es ille fundamental PTV/3f W3 L' m1 al? jar Ware 1S a to be amplified ann a stop band to prevent backward wave ggsithclrlenlgsg gocl frfnlletlar oscillations. 1nlnits Operation, ahollow Cylindrical elecwave which has phase and group velocities traveling in the gon samf hal/lili? diamiiterfshgmly les? han the mue; same direction. As previously stated, when the helix has lame/[efr O t e i 1X 1S. dlrcd oncenlncauy througn a circumference of less than two guide wavelengths'of 65 the hehx to effect amplicauon Oi thi? Signal Because the propagated Wavs, axial Components of the electric of the greater. cross sectional area 'available for the elec` field of the fundamental wave exist across the entire opention Stream 1t 15.56 that the 11511K Incorporated m thls ing of the helix. The axial component of the electric tube has an Openmg that may acccmmodale considerably field of the backward wave space harmonic, however, is larger sireani currenis than il'loSe normally nsed- Assumvery small and exists only in the regions contiguous to lng an Upper liinii of benrn cnrreni densiiy, illis lnrser the wires of the helix. Hence, under normal circumstances, the backward wave may generally be neglected.

stream current enables the traveling-wave tube of the present invention to provide a power output that is considerably greater than that available from conventional traveling-wave tubes.

ln addition, the tube disclosed'here has a basic fundamental advantage in that the larger sized helix'enables signals of higher frequencies to be amplified. A- concomitant structural advantage is, of course, that fora givenfrequency, a larger helix may be utilized.

A further advantage ofthe disclosed-device is that the circumference of the helix and the beam voltage of the traveling-wave tube may be chosen so as to provide a predetermined power output within a desired frequency range. As is generally known, optimum gain occurs at a frequency where the difference between the circumference of the helix and the circumference ofthe electron stream is of the order of from 1.8 to-2.0 guide wavelengths. Thus, the circumference of the electron stream may be chosen-so that the frequency at which optimum gain occurs coincides with the mean frequency of the operating frequency range.

ln a traveling-waveV tube having a solid electron stream, on the other hand, one must choose a helix having a circumference less than OLZ of a freerspace wavelength at the operating frequency in order toavoid backward wave oscillations. In order to obtain a desired power output, it

is necessary for the beam voltagerto' be in excess of a Y certain predetermined value in that the beam-current is limited by itsperveance. The choice of these parameters in a solid stream tube, however, also determines the frequency at which optimum gain occurs. In the case of traveling-wave tubes designed for substantially large power outputs, this frequency is substantially higher than the operating frequency. This results in the tube having high gain in a frequency range yfor which there would generally be a poor impedance match at its output, thus destroying the utility of the tube by making it have a tendency to commence into uncontrolled oscillations at frequencies within this range.

A particular application of the tube ofthe present invention is that the tube may be made to oscillate on the high frequency backward'wave modes by the use of an appropriate stop band to attenuate the low frequency backward waves. When operating in this manner, the tube described here may be used' to generate higher frequencies than heretofore possible with conventional helical-type traveling-wave tubes.

An additional application of the tube of the present invention is that it is adaptable to operation as a voltage tuned amplier through a frequency range substantially greater than 300() megacycles. Heretofore, travelingwave type voltage tuned amplifiers haveV been made by operating a conventional traveling-wave tube in the dispersive range of its helix operating on its fundamental forward wave mode. At a frequency of 3000 megacycles, a helix having anextrernely small circumference was required, and at higher frequencies it was found to be entirely impractical to make a voltage tuned amplifier of this type. The traveling-wave tube of the present'invention, on the other hand, may be operated so as to arnplify by means of interaction of the electron stream with a highly dispersive component of a new mode of propagation covering a frequency range substantially higher than Y heretofore possible by merely increasing the electron stream velocity. inasmuchV as dispersion is the rate` of change of the phase velocity of a wave with respect to Y frequency, the tube of the present invention may be tuned through the band of frequencies corresponding to the portion of the frequency spectrum where the aforementioned component is highlyV dispersive by merely varying the velocity of the electron stream through a range that is coextensive with the range covered by the phase velocity of the wave.

Additional embodiments of the tube of the present invention may incorporate a multitilar helix for providing an even larger opening than in the'aforernentioned ernbcdiment. A traveling-wave tube having an enlarged bi- 4 lilar helix is shown and described as an example of this type of tube.

It is therefore an object of this invention to provide an improved stream-type amplifier tube.

Another object of this invention is to provide an electron stream-type tube incorporating an enlarged helical structure capable of accommodating greater stream currents and thus produce more power output than heretofore possible with traveling-wave tubes employing helices of conventional size.

Still another object of this invention is to provide an electron stream-type amplitier vtube including a helical wave propagating structure of relatively large dimensions as compared to the free space operating wavelength.

A further object of this invention is to provide an electron strearn-type amplifier tube incorporating an enlarged N-iilar helical wave propagating structure having stop bands for suppressing backward wave oscillations in selected modes.

A still further object of this invention is to provide a microwave oscillator by employing a stream-amplifier tube incorporating an N-tilar helix with appropriate stop bands to enable it to oscillate on the higher order backward wave mode.

An additional object of this invention is to provide astream-type amplifier tube incorporating an N-tilar helix of appropriate pitch and diameter for adapting it to'operate as a voltage-tuned amplifier for frequencies as high as 12,00() megacycles.

' The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood fromV the following description considered in connection with the accompanying drawings, in which several embodiments of the invention are illustrated by way of examples. t is to be expressly understood, however, that the drawings are for thepurpose of illustration and description only, and are not intended as a definition of the limits -of the invention. Y

Figs. l and 7 are diagrammatic sectional views of two embodiments of the invention together with associated circuitry;

Figs. 2, 3 and 9 areV alternate forms of the apparatus for providing the stop band in the tubes of Figs. 1 and 7; Figs. 4 and 6 show preferred locations of stop bands and operating ranges on plots of the phase velocity of a wave propagated on a helix versus the ratio of the circumference of the helix to the wavelength at the frequency of operation;

Fig. 5 is a plot of gain and power output versus electron stream current for a typical electron stream amplifier tube; and

Fig. S is a cross sectional View of the tube of Fig. 7.y

Referring now to Fig. 1, there is shown a diagrammatic sectional view of an electron stream-type amplifier tube of the present invention. An envelope 10,'which provides the necessary evacuated chamber, consists of a long cylindrical structure having an enlarged portion at the left extremity, as viewed in the figure. Within the enlarged portion of Aenvelope 10, there is disposed an electron gun 12 for producing a hollow cylindrical electron stream. Electron gun 12 comprises an annular cathode 14 with a heater 16, a focusing electrode 18, and an accelerating electrode 20, the electrodes 18 and 20 being provided with conformal apertures to allow passage therethrough of the electron stream.

Cathode 14y is disposed in a plane normal to the'longi'- tudinal axis of envelope 16. Heater 16 of cathode 14 is connected across a source of potential, such as la battery 22, one terminal of which is connected to cathode-14, as shown. The electrode 18, disposed adjacent to and to the right of the cathode 14.-, as shown inthe figure, has an inner and an outer surface of revolution at an acute angle about the longitudinal axis of envelope 10' to provide focusing for the electron stream. Cathode 14 and focusing electrode 18 are connected together and are, in turn, connected to the negative terminal of a source of potential such as a battery 24, the positive terminal of which is connected to ground. The potential provided by battery 24 may be of the order of i500 volts with respect to ground.

Accelerating electrode 20 is disposed in a plane normal to the longitudinal axis of envelope l@ and to the right of focusing electrode 18 as viewed in the drawing. Electrode Ztl is maintained at a potential that is positive with respect to the potential of cathode 14 by means of a connection to a battery 26 connected from electrode 2t) to ground. This potential may be of the order of 300 volts positive with respect to ground.

A solenoid 23 is positioned symmetrically about the complete length of envelope lil. An appropriate direct current is made to flow through solenoid 2S by means of a battery 3-9 so as to produce a magnetic field, which may be of the order of 600 gauss, extending axially along the tube. The purpose of this magnetic eld is to keep the electron stream constrained throughout the active length of the tube.

Positioned concentrically about the electron stream in the direction of the electron how is a tape helix 32 which has a circumference of the order of one-half Wavelength at the operating frequency. In general, the inner diameter of the hollow electron stream should be at least 0.70 the inner diameter of helix 32 in order that the helix present a high impedance to the electron stream. A material such as tungsten or molybdenum is suitable for making the helix, the main prerequisites being that it main tain its form, especially with respect to its pitch and diameter, and that it be a good electrical conductor. Positioned about the outer periphery of helix 32 is a thin glass sheath 34, the purpose of the glass sheath 34 being to support the helix 32 throughout its length so that it retains its form.

A discontinuity is provided on each turn of helix 32 yby means of a spiral dielectric rod 36 having small isolated areas 37 or 38 of conductive material disposed over the greater portion of its surface, as illustrated in Figs. 2 and 3. Rod 36 is inserted between glass sheath 34 and envelope lil to hold helix 32 in a concentric position about the electron stream. Alternate forms of member are shown in Figs. 2 and 3. Referring to Fig. 2, there is illustrated a short section of the dielectric rod 35. Conductivity in the ransverse direction is provided on this rod oy closely spaced, narrow metal bands 37, the approximate space between bands being @.l the width of a band which is pre erably less than the pitch of helix 32. A rod of this type may 'be made, for example, by first plating a dielectric rod with a metal, and then removing the metal at periodic intervals along the rod by means of an etching process. A short section of an alternate form of rod 36 is shown in Fig. 3. This figure shows a multitude of small isolated retiecting elements 33 deposited on the surface of a dielectric rod. A rod of this type may be made, for example, by attaching ilalies of conductive material such as, for example, aluminum to the rod with a suitable binder.

Referring again to Fig. 1, a conductive coating is deposited on the inside of glass envelope l@ which extends from accelerating electrode 2@ to the beginning of helix 32. This conductive coating 39 provides a drift space for the elecron stream in this region and is maintained at ground potential by means of a connection thereto.

An input to the extremity of tape helix 32 nearest electron gun 12 is provided by a section of coaxial cable 46. A center conductor 41 of the coaxial cable 4l? connects from the input terminal through the envelope lll to the first turn of helix 32. The outer conductor of cable 4% is connected to a ferrule which is disposed concentrically about the rst several turns of helix 32 on the outside of envelope lil. The extremity of ferrule 42 farthest away from electron gun 12 is flared out in order to improve the `impedance match between coaxial cable 40 and helix 32. The outer conductor of cable 40 together with ferrule 42 may be maintained at any substantially fixed potential such as, for example, at ground by means of a connection thereto.

An output is provided from helix 32 at the farthest extremity from electron gun 12 by a section of coaxial cable 4S. A center conductor 46 of cable 45 connects from the last turn of helix 32 through envelope 10 to an output terminal 47. The outer conductor of cable d5 connects to a ferrule 49 which is disposed concentrically about the last few turns of helix 32 on the outsidev of envelope l0. As before, the impedance match between helix 32 and coaxial cable 45 is improved by daring out the extremity of ferrule 49 nearest the electron gun l2. The outer conductor of cable 45, together with ferrule 49, are maintained at ground potential by means of a suitable connection thereto. Helix 32 is also operated at ground potential by means of a connection from output terminal 47 through an isolation resistor 51 to ground.

Disposed so as to intercept the electron stream after it has progressed through the helix 32, is a collector electrode 53. Electrode 53 is maintained at a potential that is several hundred volts negative with respect to the potential of helix 32 in order that maximum energy may be extracted from the stream electrons. Accordingly, a potential of the order of 300 with respect to ground is applied to electrode 53 by means of a connection through envelope 10 to the negative terminal of a battery 55, the positive terminal of which is connected to ground.

In the operation of the traveling-wave tube of the present invention, a signal to be amplified is first applied to the input terminal. This signal is, in turn, impressed on the rst turn of helix 32 through coaxial cable 40. rlhe comparatively large circumference of the helix employed relative to the free space wavelength of the signal at the operating frequency enables an impedance match to be obtained between conventional coaxial cable and the helix.

The signal is propagated along the helix 32 as an electromagnetic wave having a velocity that is slightly less than the velocity of the electron stream so as to eect a transfer of energy from the stream to the wave. Inasmuch as the axial components of the wave are, for the most part, in the region contiguous to the periphery of the helix 32, it is necessary to restrict the ow of stream electrons to this region. Hence, a hollow cylindrical electron stream is employed to effect efficient interaction with the electric field of the traveling wave.

The electric fields of the backward wave modes of propagation, however, also exist in the region contiguous to the periphery of the helix 32. Since these backward waves are inherently regenerative in character, it is necessary to provide stop bands to prevent backward wave oscillations that would result from the above structure. The relationship between these backward wave modes and the fundamental forward wave mode of propagation on the helix is shown in Fig. 4. Referring to this figure, there is plotted the phase velocity of the waves as a fraction of the velocity of light versus the helix circumference in free space wavelengths at the signal frequency, the latter generally being designated as kaf In this ligure, line 6i) represents the fundamental forward wave mode, and lines 62, ed represent the low and high frequency backward wave modes, respectively. The tube of the present invention, when employed as an amplifier, operates in the region where the helix 32 is non-dispersive. This region is indicated by portion 6l of the line e@ representing the fundamental forward wave mode. Because of the non-dispersive characteristic of the helix 32 throughout this portion 61, the phase velocity, vp', of

the'wave remains substantially constant, thus requiring aT substantially constant'electron stream velocity. It is possible, however, for, the tube Yto commence backward wave oscillations at the. points 68, 69 where this electron s tream velocity is common with the phase velocity of the backward wave modes of propagation.Y Y

Therefore, in order to prevent 'backward oscillations, stopbands are provided for the

portions

71, 72 between kal, ka2 and Zka'l, 21mg, respectively, of the

lines

62, 64 representing the backward waves on the helix 32'. The limits of portions 71,.72'will be apparent from the nature and characteristics of the type of stop. band used.

AsY previously mentioned, one form of stop band may be provided by disposing discontinuities atV periodic intervals alongY the turns of the: helix. When the interval between subsequent discontinuities is one-half wavelength or multiple thereof, a stop band is formed for the corresponding frequencies.. The reason for this is be cause the reflected portion of the waveV has progressed through a complete cycle plus an additional 180 upon being reflected, and thus tends to cancel unreftected portions of the Wave. The width of the stop band may be broadened by extending the discontinuities about the periphery of the helix. A stop band'of this type has the characteristic of being recurrent at double the frequency of the fundamental stop band.

In the present instance, it is desiredto provide stop bands for the backward waves having phase velocities in theinterval between the'dashedlines 74 of Fig. 4 in order to prevent backward wave oscillations when amplifying signals in the non-dispersiveV portion 61 of line 60. It is seen from the drawing that in order to include the desired interval about vp', it is necessary to extend the 'stopband toV include the

portions

71, 72 on the

lines

62, 64, respectively, representing the backward Vwave modes. Y

. In the tube described here, it isnecessary to locate the stop bands to attenuate frequencies corresponding to wavelengths that are slightly less than the circumference vofthe helixY in the high Vfrequency backward wave mode, and slightly less than'twice the circumference of the helix in the low frequency backward wave mode. It is Ytherefore necessary for the helical spiral rode'36 to extend between points on the helix 32 that are slightly greater than arcomplete turnrapart. Hence, in the present case, the spiral rod 36 and helix 32 Yrotatein the same direction in proceeding along the length of the tube.

The. tube of the present invention may thus be utilized to amplify signals of frequencies within the non-dispersive band ofthe helix that are notincluded within the stop band. The signal is propagated along the helix 32 as a growingV wave which continuously interacts with the electron stream. AtV the extremity of helix 32 farthest from electron gun 12, the amplified wave is impressed on coaxial cable 45 to provide an amplified output signal at'output terminal 47. Collector electrodeV S3 is maintained negative relative to the potential of helix 32 'so as to veffect maximum transfer of the signal energy propagated by the electron stream to the electromagnetic wave on the'helix prior to dissipation of all the energy of the electron stream at the collector. Y

An illustration'of the substantial increase Vin the power output available from a tube of the present invention incorporating a stop band of the type described, to prevent backward wave oscillations, is illustrated in Fig. 5 which shows gain and power output Versus beam current represented by lines Si), 82, respectively. It is seen that the gain in decibels increases rapidly in the region where the beam current issmall and less rapidly for the larger values of beam current. VTherpower output, on the other hand,

increases approximately as the three-halves power of beam current. Thus it is apparent that'the stop bands, by preventing the propagation .of'waves at whichbackward wave oscillations occur, lallow greater current to ow without backward wave oscillations commencing,y and therefore` greatly increase thepower output available from the tube.4

As previously stated; it is considered that the use of stopl bands to causethe tubes of the present invention to. operate as backward wave oscillators on the high fre-A quency backward wave mode of propagation on the helix, is within the teachings of this invention. An illustration. of the manner inwhich this maybe accomplished is shown' in Fig. 6. Referring kto Fig. 6, there is shown a plot` of` the phase Velocity of an electromagnetic Waveversus the ratio the circumference of the helix to the free space wavelength at the frequency of the wave for the funda-- mental forward Wave and backward wave modes propagation represented

by'lines

60, 62 and 64 in the same manner as in Fig. 4. Y

Under normal circumstances, the impedance of the helix for the backward wave mode represented by vline 62 is greater than its impedance for the mode represented by line 64. Hence, when possible, backward wave oscillations Will be on the low frequency mode represented by line 62. As seen in Fig. 6, the high frequency backward wave mode represented by line 64 commences at a phase velocity of vp. In order to prevent oscillations on the low frequency backward wave mode for phase velocities higher than vp, it is necessary to employ a stop band toy Y from the tube, in this case, appears at the input terminal.

Further, as previously set forth, it is considered that the operation of the tubes of the disclosed invention asA voltage tuned amplifiers is within the scope of the teachings of the present specification. This mode of operation is accomplished by simply increasing the velocity of the electron stream soas to effect the amplification of a highly dispersive component of the forward waveV mode rather than the fundamental component. The characteristics of this dispersive component of the forward Wave mode are described in a doctoral thesis entitled, Electromagnetic Wave Propagation on Helical Conductors, by Samuel Sensiper which was published by the Research Laboratory of Electronics, Massachusetts Institute of Technology on May 16, 1951, in Cambridge, Massachusetts, as'Technical Report No. 194. In this report, the aforementioned highly dispersive component is referred to as the -1 component of the hn' mode. A plot of the phase velocity of this component versus the ka ofthehelix appears on.

page 69 in Fig. 30(b) of the report. The disclosed tubes are operated on the -1 component of the hn mode by merely increasing the velocity of the electron stream so that it is substantially equal to this phase velocity. The velocity of the electron stream may be increasedby merely increasing the voltage provided by potential surce 24 which is impressed on cathode 14 of electron gun 12 in the tubeof Fig. l. A stop band is necessary to prevent backward wave oscillations from commencing as inthe previous cases.

It is also apparent, in operating on the -1 component of the im' mode, that gain is'also provided at the low frequency end of the conventional forward wave mode which is designated as the fundamental component of the hm mode in the aforementioned report. Since the output circuit is not designed to provide an impedance match for` electromagnetic wave corresponding to these low fre-J quencies propagated on the helix extends radially outwards therefrom for several diameters of the helix. Thus, oscillations in the conventional forward wave mode may be prevented by disposing a lossy coating concentrically about the helix and spaced several diameters therefrom so as to reduce the loop gain in this mode to less than unity. Inasmuch as the electric fields for the highly dispersive -l component of the hn' mode are very close to the wires of the helix, propagation of this mode is not aected by the lossy coating.

In designating the helix of a traveling-wave tube for operation as a voltage tuned amplifier employing the -1 component of the hn' mode of propagation, the restriction placed on the pitch angle, gb, of the helix is that it should satisfy the relation wherein Vois the voltage through which the stream electrons are accelerated,

ka is the circumference of the helix in free space wavelengths,

a is the radius of the helix,

c is the velocity of light, and

fu is the mean frequency of the band of frequencies amplified.

The above relation is approximate in that the assumption is made that the velocity of the electron stream may be substituted for the phase velocity of the wave. Further, the relation is only valid for ka greater than 1.0 and Vo less than 20,000 volts.

In addition to the above, it is also considered that the use of multi-filar helix, as the wave-propagating structure in the tube described here, is also within the scope of the teachings of the present specification. As an example of an embodiment of this type, a tube incorporating a parallel fed bifilar helix is shown and described. The use of a parallel fed biiilar helix enables the same phase relations to be maintained about its periphery as for uniiilar helix of only half the circumference. Hence, the employment of a biiilar helix makes it possible to use larger beam currents and thus increase the power output of the tube or, alternatively, to construct a tube for amplifying signals of higher frequency.

Referring to Fig. 7, there is shown an embodiment of the tube of the present invention incorporating a biiilar helix. The envelope lil, electron gun 12, solenoid 28, conductive coating 39, and collector electrode S3, together with associated circuitry of this tube, are the same as for the corresponding elements described in connection with the tube of Fig. l. A bilar helix 161) is disposed concentrically about the path of the electron stream and extends approximately from the termination of conductive coating 39 to the collector electrode 53. A thin glass sheath 12 is disposed concentrically about helix in order that it retain its form. Helix is held in a position concentric about the path of the electron stream by means of two dielectric rods 1G43, and a transversely conducting elongated member i435 (see Fig. 8). Rods 194, 195 and member 165 are angularly spaced at approximately 129 from each other so as to hold the h ix 1% rigidly in place. rThe ang l r spacing is not critical and may be varied with the wi of member A. cross sectional view of the tube Mien showing this arrangement is illustrated in 8.

Elongated member 166, in ade to holding the helix 19@ in place, rrovides a stop band. Member 106 has a kidney-shaped cross section, as shown in Figs. 8 so as to extend pardy around the periphery of the helix and thus increase the Width of the stop band. Member 196 may comprise, for example, a dielectric rod 197 of appropriate cross section, having a series of metal bands 163 disposed along its entire length. An enlarged view of 1. 1, ,a taxen t member 106, showing this construction, is illustrated in Fig. 9.

Referring again to Fig. 7, means for providing a parallel feed from input terminal 119 to biiilar helix 160 is provided by equal lengths of coaxial cable H2, 114 connecting the extremities of the two helices nearest electron gun 12 forming bilar helix 160 to a common Jiunction 116 which in turn is connected to input terminal 119. A ferrule 118 is disposed about the first few turns of helix il outside of envelope 10 and flared out at the extremity farthest from electron gun 12 in the same manner as before, to improve the impedance match between the coaxial cables 112, 114 and helix 160. The outer couductors of cables 112, 114 and ferrule 118 are maintained at ground potential by a connection thereto.

An output circuit for the tube is provided by equal lengths of coaxial cable 119, 120 connected from the two extremities of the helices of bilar helix 1% farthest from electron gun 12 to a common junction 122 which is, in turn, connected to an output terminal 124. A matching ferrule is disposed about the last few turns of helix 100 on the outside of envelope 10 and connected to the outer conductors of coaxial cables 119, 12?. These outer conductors and ferrule 125 are maintained at ground potential by a connection thereto. Helix ltltl is also maintained at ground potential by means of a connection from output terminal 124 through an isolation resistor 126 to ground.

The Operation of the tube of the present invention incorporating a biiilar helix is essentially the same as for the previously described tube employing a uniilar helix. It is to be noted, however, that in the present case, modes of propagation comparable with those of a uniiilar helix are attained by making the circumference of the biiilar helix twice that of the unilar helix. Thus, the tube incorporating the bitilar helix illustrated in Fig. 7 may be adapted to operate either as a high power travelingwave amplifier tube, a backward wave oscillator, or a voltage-tuned oscillator in the same manner as the tube illustrated in Fig. l. Further, it is also apparent that it is within the teachings of the present specification that the disclosed techniques may be employed so as to operate any traveling-wave tube incorporating an N-lar helix in the aforementioned modes.

What is claimed as new is:

1. A traveling wave tube comprising an N-filar conductive helix capable of propagating a wave having a phase velocity substantially less than the velocity of light, where N is an integer equal to or greater than one, and the circumference per turn of said helix is greater than 02N free space wavelength of said wave; means for producing an electron stream; means for directing said electron stream along a path parallel to the longitudinal axis of and contiguous to said helix to increase the amplitude of said wave; an elongated dielectric member disposed along and immediately adjacent periodically spaced points on successive turns of said helix, said elongated dielectric member having a pitch substantially greater than the pitch of said helix and a plurality of relatively small isolated areas of conductive material disposed on said elon ated dielecrie member to provide electrical reflectors whereby an electromagnetic wave being propagated around the turns of said helix is partially reflected at periodically spaced points therealong thereby to attenuate waves within a predetermined band of frequencies capable of being propagated by said helix including the frequency at which oscillations on the low frequency backward Wave mode would occur.

2. The traveling wave tube as claimed in claim l wherein the width of said elongated member is increased whereby said member extends about a larger portion of the periphery of said helix thereby to increase the width of said predetermined band of frequencies.

3. The traveling wave tube as claimed in claim 1, wherein said elongated dielectric member comprises a rod hav- `1.71 ing a. series ofy narrow conductive bands disposedsuccessively along its entire length, the width of said bands being less than the Ydistance between Successive turns of ,said' N-larconductive helix.

4. The traveling wave tube as claimed'in claimr l, where- Y N-lar conductive helix in the same direction and at aV greater pitchthan the turnsV of said N-lar helix.

6. The traveling wave tube as claimed in claim l wherein N=1 making said N-lar helix a unifilar helix, said wave capable of being propagated' by said helix isa forward wave, said helix has a circumference per turn of from 0.2 to 0.9 of a free space wavelength at the frequency. of said forward wave, said electron stream is a hollow,gcy lindrical stream having a diameter of atleast 0.6, the inner diameter ofV said helix, said electron stream is directed concentrically through said helix thereby to cumulatively increase the magnitude ofl said forward wave as it is propagated along said helix, and said elongated member is adapted to attenuate a predetermined bandV of frequencies which includes the frequencies at which oscillations in both the low and the high frequency backward wave modes would occur and which does not include theA frequency of said forward wave.

7. The traveling Wave tube as claimed in claim 1 wherein said wave capable of being propagatedby said helix is a high frequency backward wave, said electron stream is a hollow cylindrical stream having a diameter of at least 0.6 the inner diameter of said helix, said electron stream is directed concentrically through said helix thereby to cumulatively increase the magnitude of said high ,frequency backward wave as it is propagated along said helix, and said reflectors being adapted to attenuate rst and second predetermined bands of frequencies which in-,

clude, respectively, a first band of frequencies in which oscillations on the low frequency backward wave mode would occur and a portion of a second band of frequencies wherein oscillations on the high frequency backward wave mode would occur, the frequency of said backwardvwave of increasing magnitude being within the Vremaining portion of said second band of frequencies.

References Cited in the file of this patent UNiTED STATES PATENTS 2,575,383 Field Nov. 20, 1951 2,626,371 Barnett et al. Jan; 20, 1953Y l2,654,047 Clavier Sept. 29, 1953 2,660,689 T ouraton et al Nov. 24, 1953 2,720,609 Bruck et al. Oct. l1, 1955 2,773,213 Dodds Dec. 4, 1956 OTHER REFERENCES Traveling Waves, by I. R. Pierce, pages 158-159, published 1950, by D. Van Nostrand Co., Inc., New York.

Article by E. C. Dench, entitled Carcinotron, pp. 64-66 and 157-162, Tele-tech for November 1953.