US2928979A - Travelling-wave tube - Google Patents

Description

2 Sheets-Sheet 1 Filed Nov. 22, 1957 FIG. 1.

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OVERLAP (cms IN VENTOR by RUDOLF KOMPFN'ER "Zm, A TTORNEYS' March 15, 1960 KOMPFNER 2,928,979

TRAVELLING-WAVE TUBE Filed Nov. 22, 1957 2 Sheets-Sheet 2 INVENTOR Run 01.! KOMPFNE 5%, wamwm ATTORNEYS United States Patent TRAVELLING-WAVE Application November 22, 19 57, Serial No. 698,137

Claims priority, application Great Britain November 3, 1951 :9 ltit- This invention relates to travelling wave tubes of the type having, in a vacuum envelope, means for setting up an electron beam and one or more helical conductors around the beam, which tubes are used for generating, amplifying, attenuating and modulating microwave radio frequency signals, and is particularly directed to the provision of improved coupling and attenuating means for such tubes.

Of the various known forms of travelling wave tubes, thoseof the type most widely used consist of a metallic helix inside an evacuated envelope along which a sloweddown electromagnetic wave is propagated while an electron beam is projected along the axis of the helix with a velocity approximately equal to that of'the'wave. In

b a a d e Wav od ces a increase in signal amplitude along the length of thehelix. In order to be of practical value, such a device must be stable and oscillation-free in operation. However, these requirements present several problems ofwhich two in particular have to be satisfactorily solved before the tube can be regarded as satisfactory: first, there must-be good,

, o n nd] able residual reflections must be absorbed Somewhere along the middle of the tube so that energy from the output cannot get back to the input and: cause oscillationsand instability. I l i v V Prior to the presentinvention, the attempted solutions of these two problems have been mainly alongthe fol lowinglines Coupling to the helix hasbeen achieved by attaching antenna-like structures to the ends of the helix So arranging them as'to radiate into waveguides. Alterna vely, the ends of 'the helix wire have been brought out through the envelope by means ofspecial s eals, and direct contact has been made with the inner conductors of coaxial lines. In eitherof these couplingarrange mer ts, the dimensions of the helix, par,ticularlyits length, must be held to very close tolerances if a satisfactory match to the external fixed circuits, whether waveguides or coaxial lines, is to be obtained. Otherwise, it would be necessary to make mechanical adjustmentsof the external circuits each time a tube is replaced. Another disadvantage of these prior expedients, particularly when waveguides are used, "is that they take' up considerable space and makeit "diflicult to provide simple compact magnetic circuits, either electromagnets or perinanent magnets, for beam focusing purposes. Furthermore, it has been found. tliiliciilt to provide a suificiently good wi a enatch over a wide band of frequencies mer ts of the character described.

-I'hesecond problem, namely, that of providing absorp. tion or attenuation along the middle of the tube, has been met in the past byv providing material of suitable re tivi ty nearthe helix, as inthe. form of a thin metallic film'on a glass "tube, or graphite layers on dielectrici'rods.

2,928,979 a ents! Mat- 52.

2, which means that in practicethey have to beinside the, vacuum envelope. Consequently, once the tube'hasbe'en sealed off, the attenuating material is inaccessible'and 'it is impossible to change either its amount or its location. The present invention overcomes the defects and disadvantages of the coupling and attenuating devices of the. prior art, and provides new and advantageous solutions of these two important problems inherent in travellingwave tubes of the character described, by utilizationofi my discovery that an extremely strong, highly efficient coupling can be produced by winding two concentric: helices in opposite senses, and; so adjusting the physical characteristics of the outer'helix, i.e., its diameter and pitch, that its inherent phase velocity; in the uncoupled State. is approximately equal to that 'of the. inner helix. A travelling-wave tube according to the invention thus comprises a vacuum envelope, a helix inside the envelope; electron gun and collector means for setting up a beam along the longitudinal outer helix around the envelope, wound in opposite sense to the helix inside the envelope and overlapping a part thereof, and having a phase velocity state which is as close as practically possible to that-0f, the inner helix.

In addition, when the tube includes two outer helices adapted to serve as input and output couplings, an improved decoupling or attenuating means may be provided in the form of a third helix outside the envelopev em: bedded in lossy material and like-wise Wound in opposite; sense to the inner helix.

Another embodiment of the invention comprises. a. travelling-wave tube having a helical coupling. consisting. of the inner conductor of a coaxial line extended past the: end of the outer conductor of said line and formed into; a helix around the envelope of the tube wound in oppo: site sense to thehelix inside the envelope. Such an arrangement may be termed a coaxial-to-helix to-helix transformer. A modification of this arrangement may" consist in the provision of a flange termination to the, outer conductor of the coaxial line. A coaxial-to-helim to-helix transformer provides broad band couplingf'and is useful in connection with Brillouin-flow focusing which requires a magnetic field configuration unsuited to wave guide coupling.

By adjusting the length of the outer helix, it is possible to ensure that substantially all the power travelling along. the coaxial line and fed into the t mer helix istrans: ferred into the inner helix within the vacuum envelope. and is propagated there preferentially in one direction. only. In this form, the coaxial-helix-helix transformer;- acts as a directional coupler. Conversely, power propajs gated in the inner helix towards such a coupler is trans-, mitted to the coaxial line via a COQXlQlnhfiliX-hlbl tra former substantially without loss or reflection It is, possible, however, to obtain anydesired amount of di'g rectivity or reflectivity with the type of transformer described merely by lengthening or shortening the outerhelix by requisite amounts. The invention thus meets the need for coupling rangements for travelling-wave tubes that allow for. ad: justment 0f the performance of the tube from ontside; the vacuum envelope. For example, tolerances inthe manufacture of the inner helix may be more liberal and; limited deficiencies in-a tube caused by'the constru eti n of the inner helix may "be made good by adjustment an outer helix by alteration of its length, pitch or ameter. The invention will now be further described with refaxis of said helix, and at least one.

in the uncoupled.

curves showing the relationship between insertion or input power loss and length of overlap between the helices.

Fig. 3 is a diagrammatic view of the apparatus used in obtaining the data plotted in Figs. 1 and 2.

. Fig. 4 is a cross-sectional, partially diagrammatic view of a travelling-wave tube according to the invention having input and output helices.

Fig. 5 is a view similar to that of Fig. 4 of a modified embodiment of the invention having input and output helices and a decoupling helix.

Fig. 6 is a view similar to that of Fig. 4 wherein the tube is provided with a graded pitch output helix.

. Fig. 7 is a view similar to that of Fig. 4 wherein the tube is provided with a series of output coupling helices for taking the power off in stages.

Fig. 8 is a perspective viewof a coaxial-helix-helix transformer arrangement embodying the invention.

The character of the coupling between coaxial helices Wound respectively in the same sense and in the opposite sense (i.e., as left-hand and right-hand threads) is illustrated in Figs. 1 and 2 which graphically indicate test results obtained with an experimental. apparatus of the type diagrammatically illustrated in Fig. 3. As shown, the apparatus comprised an inner helix 21 mounted within a glass tube 22 and connected at one end to a suitable power input circuit, such as a coaxial line 23, and an outer helix 24 surrounding the tube 22 with one end thereof in overlapping relationship to inner helix 21 and connected at the other end to a power output circuit, such as a coaxial line 25. The outer helix 24 was slidable axially along tube 22 so as to vary the length of the overlap between the two helices.

The graph of Fig. l was obtained by measuring the insertion or power input loss for different lengths of overlap between the inner helix 21 and an outer helix 24 wound in the same sense as the inner helix. It will be seen from Fig. 1 that in such an arrangement the loss approaches zero only with a substantial overlap and that the points of minimum loss are narrowly defined. The graph of Fig. 2 shows the results obtained with an outer helix wound in opposite sense to the inner helix. The curve of Fig. 2 distinguishes from that of Fig. l in that a substantially loss-free coupling is obtained with only a small overlap between helices and that the length of overlapfor minimum loss is not narrowly defined.

-In the travelling-wave tube illustrated in Fig. 4, an electron beam 30 is generated at a source 31 and collected at a collector 32, and passes along the longitudinal axis of an inner helix 33 wholly contained inside avacuum envelope 34. Surrounding the envelope 34 adjacent the ends thereof are an input outer helix 35 and an output outer helix 36, both short in comparison with and wound in opposite sense to inner helix 33. The outer helices 35 and 36 are coaxial with and in close proximity to inner helix 33 and overlap parts of the ends of the latter adjacent the source 31 and collector 32, respectively, forming power transfer means. The diameter and pitch of each of the outer helices are so selected that the phase velocity of each helix in the uncoupled state is preferably substantially equal to that of the inner helix. The length of each outer helix is on the order of, i.e., between 0.5 and 1.5 times, its diameter. The inner helix 31 and outer helices 35 and 36 are preferably capable of rela-. tive movement in an axial direction.

In a tube of the character represented by Fig. 4 wherein the inherent phase velocities of propagation U, and U of the oppositely wound inner and outer helices are approximately equal, it has been found experimentally that the coupling between the helices is so strong that a virtually complete power transfer from one helix to the other takes place in a few helix wavelengths. Since the wavelength on the helix is only a fraction (normally less than $4 of the free-space wavelength, this meansthat complete power transfer can be achieved in a very short distance, e.g., a distance on the order of the diameter of 1'5 the outer helix. Consequently, if a coaxial line is connected to one end of outer helix 35, as indicated in Fig. 8 hereinafter described, and power is fed from the coaxial line into the outer helix, this power will be gradually transferred from the outer helix, through the glass envelope, to inner helix 33, and at one point along the length of the coupled helices all the power will be on the inner helix and none will be left on the outer. If the outer helix is terminated at that point, the coupling provided by the overlapping helices will be substantially refiectionless because, there being no power at the end of the outer helix, none can be reflected therefrom.

The arrangement just described thus provides a microwave coupling device from an external coaxial line to a helix inside an evacuated glass envelope which has been found to be an efiicient' coupler over abroad band of frequencies. It has also been found that substantially all of the power is transferred from the coaxial line to the inner helix in the form of a wave travelling in one direction only. Conversely, a wave travelling on the inner helix can be transferred to a similar coupling device, such as the outer output helix 36. The coupling properties of such a device are not affected by relative axial displacement of the inner helix and the coupler, provided there is a complete overlap of the outer and inner helices, i.e., all portions of the outer helix are in overlapping relationship to the inner helix. Consequently, as long as the inner helix is longer than the overall distance between the outer helices forming the input and output couplers, its exact length and axial position relative to the latter are not critical, a fact which greatiy facilitates manufacture and assembly of the tube.

Inasmuch as coupling devices of the character described can be made very small, physically, the present invention enables the use of close-fitting focusing electromagnets or permanent magnets, with attendant savings in space and weight.

The power fed into the outer helix 35 may be made disposable along any or all of three paths by adjustment of the features of the outer helix; that is, it may be re flected back, it may be propagated in one direction along the tube, or it may be propagated in the opposite direc tion along the tube. The ratio of powers propagated in each direction along the tube may be made variable substantially between zero and infinity; that is, for zero input loss the input power can be propagated in one direction or the other, or it can be divided equally in both directions. Similarly, power may be coupled out of the tube dependent upon directional properties in the tube.

It, as' is normally the case in practice, the input and output couplings are not substantially reflectionless, the embodiment of the invention illustrated in Fig. 5 provides effective means for decoupling the output from the input to prevent oscillation. As shown diagrammatically in Fig. 5, the travelling-wave tube comprises an electron gun 40, an inner helix 41 and a collector 42 for the electron beam 43, all positioned inside an evacuated envelope. Surrounding the envelope in overlapping relationship with the end portions of inner helix 41 are outer helices 44 and 45 adapted to serve as input and output couplings, respectively. As in the tube of Fig. 4, outer helices 44 and 45 are wound in opposite sense to that of inner helix 41 and have inherent phase velocities substantially equal to that of the latter, and their lengthsare on the order of their diameters. In order to decouple output helix 45 from input helix 44, a third outer helix 46 embedded in lossy material 47 surrounds the tube envelope intermediate helices 44 and 45. The material 47 is provided with tapered ends 48, 49 to avoid reflections at those ends, and the composite decoupling structure formed by helix 46 and material 47 is preferably so arranged as to be movable axially along the envelope for adjustment purposes.

With this construction, a signal reflected at the output coupling is absorbed in the he ix 46 and the radio frequency power is strongly attenuated on arrival at the input end of the tube by an amount adequate to prevent oscillation. Being in close proximity to the path taken by the radio frequency wave, decoupling helix 46 exerts a powerful attenuating efiect, and being on the outside of the evacuated envelope, it not only is adjustable, but also can dissipate substantially more heat than the attenuating means heretofore commonly provided inside the envelope.

The travelling-wave tube shown in Fig. 6, which may the velocity of the beam as radio frequency power is extracted therefrom. heating in the output than at the other end.

A more efiicient arrangement is obtained by providing, as shown in Fig. 7, a series of output graded pitch helices 64a, 64b, etc., overlapping successive parts of the inner helix adjacent the collector end of the tube so as to take the power ofi the inner helix in stages. The input helix 65 of this embodiment is similar in characteristics to input helix 35 of Fig. 4.

In either of the arrangements illustrated in Figs. 6 and 7, the inner helix is preferably of graded pitch, but the grading need not be critically defined because a modifying efiect can be introduced by adjustment of the features of the outer helices.

Fig. 8 shows a coaxial-helix-helix transformer arrangement comprising a coaxial line 80 having an outer conductor 81 and a flange termination therefor in the form of a metal disc 82 of a diameter equal to one half-wavelength. The inner conductor 83 of the line 80 is extended past the end of the outer conductor and formed into a helix 84 wound in opposite sense to, and having a phase velocity in the uncoupled state approximately equal to that of, the helix 85 inside a vacuum envelope 86. Trans former arrangements of this construction are especially well adapted for use as the input and output couplings of tubes of the character illustrated in Figs. 4-7.

This is a continuation-in-part of application Serial No. 318,453, filed November 3, 1952, and now abandoned.

What is claimed is: V r

1. A travelling-wave tube comprising a vacuum envelope, an inner helix wholly contained inside the envelope, means for setting up an electron beam along the longitudinal axis of said inner helix, and means for transferring power to or from said inner helix including an outer helix of uniform diameter short in comparison with the inner helix and wound around the envelope in the opposite sense to and overlapping the inner helix but not conductively connected therewith, the phase velocity of propagation of said outer helix in the uncoupled state being substantially equal to that of said inner helix and the length of said outer helix being so adjusted as to provide a substantially 1oss-free coupling between said outer and inner helices.

2. A travelling-wave tube according to claim 1 having an outer helix adjacent each end of the tube to provide input and output couplings respectively.

3. A travelling-wave tube according to claim 1 wherein the length of said outer helix is on the order of its diameter.

4. A travelling-wave tube according to claim 1 wherein said inner and outer helices are movable axially relative to one another.

5. A travelling-wave tube according to claim 2 including attenuation means comprising a third helix outside the envelope wound in opposite sense to the helix In this arrangement, there is greater helix 54 at the end of closer pitch lapping successive inside the envelope and embedded in a mass of lossy material, said third helix being located between said input and output coupling helices.

6. A travelling-wave tube according to claim 5 wherein said attenuation means is movable axially relative to said inner helix.

7. A travelling-wave tube according to claim 2 wherein said outer helix overlapping the output end of the tube has a diminishing pitch in the direction of movement of the beam. I

8. A travelling-wave tube comprising a vacuum envelope, a helix wholly contained inside the envelope, electron gun and collector means for setting up an electron beam along the longitudinal axis of said helix, and

a plurality of outer helices around the envelope wound in opposite sense to the helix inside the envelope but not conductively connected with said helix, one of said outer helices overlapping the gun end of the inner helix to provide an input coupling therewith andthe remainder of said outer helices overlapping successive parts of the collector end of the inner helix to provide output couplings therewith.

9. A travelling-wave tube comprising a vacuum envelope, a helix wholly contained inside the envelope, means for setting up an electron beam along the longitudinal axis of said helix, and a coupling for said tube comprising a coaxialline having its inner conductor extended past the end of the outer conductor of said line and formed as a helix around the envelope of the tube wound in opposite sense to the helix inside the envelope and overlapping a part of said inner helix but not conductively connected therewith. J

10. A travelling-wave tube according to claim 9 wherein the outer conductorof said coaxial line includes a terminating flange having a diameter of about one halfwavelength.

ll. A travelling-wave tube comprising a vacuum tube envelope, means for setting up an electron beam along the longitudinalaxis of said envelope, a helix wholly contained inside said envelope arranged for microwave coupling with said electron beam, and input and output coupling helices of uniform diameter outside said envelope coaxial with, and arranged for microwave coupling with, the helix inside said envelope but not conductively connected therewith, each of said input and output coupling helices being short in comparison with and wound in opposite sense to said helix inside said envelope, the diameter and pitch of each of said coupling helices being such that the phase velocity of propagation thereof in the uncoupled state is substantially equal to that of the helix inside said envelope.

12. A travelling-wave tube according to'claim 11 including attenuation means comprising a further helix outside said envelope, intermediate said input and output coupling helices, embedded in lossy material and wound in opposite sense to the helix inside said envelope.

13. A travelling-wave tube according to'claim 11 wherein said output coupling helix has a diminishing pitch in r the direction of movement of the beam.

14. A travelling-wave tube according to claim 11 including a plurality of additional coaxial helices intermediate the input and output coupling helices and overparts of the collector end of the helix inside said envelope.

15. A travelling-wave tube according to claim 11 wherein said input and output coupling helices comprise the inner conductors of coaxial lines ends of the outer conductors of said lines.

References Cited in the file of this patent UNITED STATES PATENTS extended beyond the

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