US2830219A - Traveling-wave tube - Google Patents

Description

APl'llfsl 1958 v l s. E; wEBBER l 2,830,219

v -TRAvELING-WAVE TUBE.'

Filed June 29, 195o Figi.

Inventor: Stanleg B'Webber,

bg His Attorneg.

TRAVELlNG-WAVE Stanley E. Webber, Ballston Lake, N. Y., assigner to General Electric Company, a corporation of New York l Application June 29, 1950, Serial No. 171,178

Claims. (Cl. S15-6.5)

This invention relates to improvements in'electron dis-Y charge devices of the type generally called traveling-wave tubes; and its principal object is to increase the maximum power output, the gain, and the efficiency Vor such tubes, especially for use aspower ampliers. Other objects and advantages will appearzas the description proceeds.

The features of this invention which are believed to be novel and patentable are pointed out in the claims which form a part of this specification. For a better understanding of the invention, reference is made in the following description to the accompanying drawing, in which Fig. l is a schematic longitudinal section of a travelingwave tube embodying principles of this invention; Fig. 2-

is a fragmentary side view of the same tube showing details of the attenuator construction; and Fig. 3 is a fragmentary longitudinal section of a modiiication of the improved traveling-wave tube.-

Referring now to Fig. 1 of the drawing, the traveling-f wave tube amplies an electromagnetic Wave transmitted between input terminal 1 and output terminal 2. Connecting these terminals is a kstructure which transmits electromagnetic waves at relatively low velocity. This structure may be an electrically conductive wire helix 3, as shown in the drawing. Since an electromagnetic wave travels along the wire of helix 3 at substantially the speed of light, the velocity of the wave along the axial direction of the helix isless than the speed of light byafactor substantially proportionalto the ratiobetween the pitch of helix 3 and its diameter. It is known in the, art Athat other structures whichA transmit electromagnetic waves at relatively low velocity may befused in place of the helix, and it will be understood that such other structures may be used in the practice of my invention. Examples ofstructures which have beenY proposed for such use arel a rod which is electrically loadedbwith a plurality of rather closely spaced transverse metal kdisks,. or a cylindrical wave guideloaded with a' plurality oflapertured disks.

United States Patent C theeld of the transmitted electromagnetic wave is Connection of terminals 1 and 2 to external circuits may be made by direct connections to the wire, or by inductive orV capacitative coupling, e. g., by a probe, cavitylor wave-guide connection, as is known in the art.

Closely surrounding and enclosing helix 31is a tubular envelope 4 which is usually made of electrically 'noncon-Y This envelope is evacu ductive material, such as glass. ated in the customary manner. An alternative structure is one in ywhich the evacuated envelope is a metal cylinder at some distance outside thehelix, within which the helix is supported by insulatingrods.

Within theleft-hand end of envelope 4V there is an electron gun comprising a cathode 5, a cathode heater 6, and a focusing electrode 7. Electrode 7 may be tubular in shape, so that a beam of electrons, indicated by dotted lines 8, is directed axially through helix V3 to a collector plate 9 positioned `within the right-hand end of envelope 4. Means, n ot shown, may be provided to produce a longitudinallmagnetic eldthrough the envelope to keep strongest. A battery 10, or other suitable voltage source, is connected to maintain electrode 7 and collector plate 9 at a large positive potential with respect to cathode 5, and a batttery'll, or other current source, is provided to supply operating current to heater 6. Y v Y.

The ratio of pitch to diameter of helix 3 and the electron accelerating potential provided by battery 10 areso related that electron beam 8 travels in the same direction and at near the same velocity as electromagnetic waves transmitted by-helix 3 between the input terminal 1 and output terminal 2. lThe adjustment is such that the veloc ity of the electron beam is slightly greater than'that of the wave type with which it is to react. Under these conditions, it is known that interaction between the beam and the wave occurs in such a way that the electromagnetic wave is amplified as it travels along the helix. One exf planation of this interaction, by l. R. Pierce, may be found in The Bell System Technical Journal, vol. 29,` No. 1

(January 1950),- pp. 6-19.

Because the traveling-wave tube is capable of amplifying a very broad band of frequencies, it is ditiicult to match exactly the impedances of the connecting circuits Y at input terminal 1 and output terminal 2 to the impedance of the helix at all frequencies which the tubecan amplify. Therefore, at certain frequencies there may be reliectio-ns whereby an amplied wave is rellected from the output back to the input, where it is reilected again and further amplified as it travels back toward the output. This maylproduce oscillations unless an attenuator is 'providedY such that the attenuation of the wavein the back# ward direction exceeds the gain in the forward direction. Several ways of providing this attenuation have been suggested, such as makingthe helix itself of a lossy material.

However, the most convenient way of providing the neces' sary attenuation is to provide an attenuator 12 of lossy,

electrically conductive material about the outside, or the' inside or as a part, of envelope 4. This attenuator may,

for example, be a platinum band positioned about the out side of the envelope, or an Aquadag coating 'applied to the surface of the envelope.

the envelope, an attenuator so placed absorbs energy from the field,'and can easily be made to provide the requiredA attenuation. Some of the attenuators previously used ex-v tend substantially throughout the entire length of the helix.rv However, it has been found that a relatively short attenuator can be employed to greater advantage.

According to known prior art, the location of the atten-- uator along the helix was believed to be unimportant.

When a short attenuator was employed, it was usually placed near the center of the helix. According to the presfent invention, however, the location of the attenuator has an important eifect upon the maximum power output,

gain, and etliciency of the tube, as will now be explained. Best results can be obtained by placing attenuator 12k near the input terminal of the helix, as shown in Fig. l, rather than near the center of the helix as in previous practice. It has been found that maximum power output from the tube can be obtained when attenuator 12 is placed as close as possible to input terminal 1, and that` Y the maximum output power decreases as the attenuator Patented Apr. 8, 1958 Since a portion of the field of the transmitted electromagnetic Waveextends through helix. Therefore, for a given percentage attenuation of the' wave, less power isl absorbed when the attenuator is nal, as' shown inA Fig. l. However, since the attenuatorl is still relatively close to the inputterminal, the maximum poweroutput obtainable' is' not" far below the optimum value. Therefore, by'placing the attenuator for maximum gain, themaximum'powerj output is alsoA increased over that obtainable with' previousarrangements,"and is veryV near the optimum value. The eiciency Vof the tube is also increased, since more; power output is obtained without increasinguthe input power supplied to the electronbeam. This is an especially important advantage when the traveling-wave tube is to be used asY a power amplifier, since, previously, aserious` objection to traveling-wave tubes for such use has been-their relatively low eciency.

T heioptimum spacing for maximum gain between input terminal 1 and attenuator 12 isinthe order of one wavelength of `the electromagnetic wave as transmitted by helix 3. Since the traveling-wave tube is an extremely broad-band device, this distance, expressed in wavelengths, may vary considerably, even for the-same tube when Ytransmitting different wavelengths. A spacing as small as. one-half wavelength, or as large as two wavelengths of `the transmitted wave may giveV good results. However, in all cases the position of the attenuator is relatively close to input terminal 1, since the total length of the helix` in an efficientA traveling-wave tube may be seven or more wavelengths of the electromagnetic wave n as transmitted by the helix.

The attenuator itself should be as short as it' can be made while still providing. a sutcient amount of attenuation to prevent oscillations. In apractical tube, the spacingbetween inputv terminal 1 andthe attenuator may be in theV order of 20%, or less, of the helix length, while the spacing between the attenuator and output terminal 2 may be in the order of 60%, or more, of the helix length. Thus, the latter spacing ismuch. greater than the former.

A possible` explanation of theincreascdgain obtained by placing the attenuator a short. distance fromV the input terminal follows: In the space between the inputv terminal' andthe attenuator, the electron beam. is velocityfmodulated tosome extent. by the electromagnetic wave hansmitted by the helix. This. is` especialy true when the signal is Vrelatively large, as in power amplifier applications oi the tube. That portion of the helix adjacentthe attenuator. then actsas a drift space in which'electron bunching takes place. The hunched electrons induce an elec-` tromagnetic wave Vin'thatportion of the helix between the attenuator and output 'terminal` 2, thereby providing considerable amplification of` the transmitted wave. If the attenuator is placed as close as possible to input terminal 1, the immediate attenuation of the input signalV reduces this'. velocity modulation of thef beamV by an amount which depends uponthe rate of attenuation,` and therefore the gain` obtained from bunching'of the electrons is reduced. The wave can 'be attenuated soV fast that there is no` velocity modulation of thebeam. On the other hand, the necessary `"velocity modulation occurs within a relatively short distance, in the order of one wavelength of the transmitted wave, and any additional spacing` between the `input terminal and the attenuator produces no useful result, but actually reduces the gain, as well as the maximum output power and eiiiciency.`

To prevent successive reflections between. the oufpu terminal and the right-hand edge of the attenuator, which could produce undesired oscillations, 1h@ fight-hand edge of the attenuator is preferably serrated, as shown in Fig.

2, so that there is no abrupt impedance discontinuity to create reflections at the right-hand edge of the attenuator.

Traveling-wave tubes embodying this invention may be used either as amplifiers or as oscillators, depending upon the external circuit employed with the tube. When used in an oscillator, a positive feedback circuit is provided between the output and input terminals. Since the tube does not oscillate in itselhthe frequency of oscillation can easily and accurately be controlled by the feedback circuit.

It should be understood that Vvthe theoretical explanations given in this specicaticn are for clarity only, `and that applicants invention` is not limited to any particular theory of operation of the traveling-wave tube.

Having described the Vprinciple of this invention, and the best mode in which I'have contemplated applying that principle, I wish it to be understood that the apparatus described isv illustrative'only, and that other means can be employed without departing from the true scope of the invention.

What 1 claim asl new and desire to secure by Letters Patent of the United States is:

l. An electron discharge device comprising a transmission structure having input and output terminals between which electromagnetic waves are transmitted at low velocity, means providing an electron beam in proximity to said structure traveling in the same direction and at near the same velocity as an electromagnetic wave so transmitted, whereby interaction between said beam and said wave occurs'to amplify the wave, and attenuator means placed in proximity to said structure within the eld of electromagnetic waves-transmitted thereby having a length substantially less than half that of said transmission structure, characterized in that said attenuator is spaced from the input terminal of said structure a distance not exceeding two wavelengths of the electromagnetic wave as transymitted by said structure and is spaced from the output terminal of said structure by a much greater distance, said attenuator means beingthe only attenuating means in proximity to said transmission structure between the nput and output terminalsv thereof so that substantially more attenuation is present inthe input half. of said transmission structure than in the output halfV of said transmission structure. i v

2. Anelectron discharge device comprising a transmission structure having input and output terminals between which electromagnetic waves` are transmitted at low velocity, means providing an electron beam in proximity to said structure traveling in the same directionV and at near'the-samevelocity as-an electromagnetic wave so transmitted, whereby interaction between said beam VVand said waveV occurs to'amplify thev wave, and attenuator means placed in proximity to said Ystructure within Vthetield ofelectromagnetic waves transmitted thereby,

the lengthY `of vsaid attenuator being substantially less than half'the length of said structurecharacterized in that said attenuator is spaced `from the input terminal of said structure a Vdistance not exceeding one-fifth the total length of said'` structure-and from. said output terminal a distance exceeding 3A; the total length ofsaid transmission structure, said attenuator means being, the only `attenuating means in proximity to said transmission structure between the input and output terminals thereof so that substantially moreattenuation is present in the input half of said transmission structure than in the output half of said transmission structure. i

3. An electron discharge device comprising a transmissionstructure having input. and output terminals between which electromagnetic waves are. transmitted at low velocity, means providing an electron beam in proximity to said structuretraveling in the same direction and at near the same velocity as an electromagnetic wave so transmitted, whereby intersection between said beam Aand' said wave o ccurs toamplify the wave, and

attenuator means placed in proximity to said structure Within the field of electromagnetic Waves transmitted thereby, the length of said attenuator being short compared to the length of said structure, characterized in that said attenuator is spaced from the input terminal of said structure a distancein the order of one wavelength of the electromagnetic wave as transmitted by said structure, said attenuator alsoV having a gradually decreasing volume toward the edge thereof lnearer said output terminal to eliminate abrupt impedance discontinuity, said attenuator means being the only' attenuating means in proximity to said transmission structure between the input and output terminals thereof so that substantially more attenuation is present in the input half of said transmission structure than in the output half of said transmission structure.

4. A traveling-wave tube comprising input and output terminals, a long, electrically conductive helix connected between said terminals for transmitting electromagnetic waves therebetween at low velocity, an electrically nonconductive, tubular, evacuated envelope closely surrounding and enclosing said helix, means providing an electron beam axially through said helix, said last-named means comprising an electron gun within one end of said tubular envelope and a collector plate within the other end of said envelope, a cylindrical attenuator of lossy, electrically conductive material about said envelope and Within the eld of electromagnetic waves transmitted by said helix, said attenuator being short compared to the length of said helix, said attenuator being spaced from said input terminal a distance not exceeding one-iifth the length of said helix, the edge of said attenuator nearest said output terminal being serrated to prevent the occurrence of an abrupt impedance discontinuity which might produce reflections, said cylindrical attenuator being the only attenuator associated` with said helix so lthat substantially more attenuation is present in the half of said helix adjacent said input terminal than is present in the half of said helix adjacent said output terminal. 5. A11 electron discharge device comprising a transmission structure having input and output terminals between which electromagnetic waves are transmitted at low velocity, means providing an electron beam in proximity to said structure traveling in the same direction and at near the same velocity as an electromagnetic wave so transmitted, whereby interaction between said beam and said wave occurs to amplify the wave, and a single cylindrical attenuator placed in proximity to and surrounding said structure within the eld of electromagnetic Waves transmitted thereby, the length of said attenuator being short compared to the length of said structure, characterized in that said attenuator is placed immediately adjacent said input terminal said cylindrical attenuator being the only attenuator associated with said transmission structure so that substantially more attenuation is present in the half ofthe transmission structure adjacent said input terminal than is present in the half of said transmission structure adjacent said output terminal.

References Cited in the file of this patent UNITED STATES PATENTS 2,122,538 Potter July 5, 1938 2,278,210 Morton Mar. 31, 1942 2,516,944 Barnett Aug. l, 1950 2,585,582 Pierce Feb. 12, 1952 2,660,689 Tourston et al Nov. 24, 1953 2,669,674 Diemer Feb. 16, 1954

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