US2870367A - Low-noise microwave tube - Google Patents

Description

Jan. 20, 1959 'T. E. EVERHART EIAL 2,370,367

LOW-NOISE MICROWAVE TUBE 3 Sheets-Sheet 1 Filed Aug. 1. 1955 Q gag/ giiii? ig lz g AMI/v1: 720% .6. fuzz/1x7, 6/1/15: ,6 5/1041,

Irma [K United States Patent O LOW-N OISE MICROWAVE TUBE Thomas E. Everhart, Santa Monica, and Charles K.

Birdsall, Menlo Park, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application August 1, 1955, Serial No. 525,663

8 Claims. c1. sis-3.6

This invention relates to electron stream amplifiers, and more particularly to a low-noise traveling-wave tube.

Traveling-wave tubes generally comprise an evacuated envelope, a slow-wave structure disposed within the envelope for propagating electromagnetic waves at a velocity substantially less than the velocity of light, and an electron gun disposed at one end of the envelope for projecting an electron stream in an interacting relationship with the waves propagated along the slow-wave structure.

Traveling-wave tubes are known to amplify waves with in an extremely broad band of frequencies and, for this reason, are considered to be very useful in the amplification of microwaves. The broad spectrum noise figure of a traveling-wave tube, however, is also normally relatively high; therefore, selecting an input signal out of tube-noise is especially diflicult in the broad-band amplification of signals having a relatively low input power.

It is, therefore, an object of the present invention to provide a low-noise microwave tube.

It is another object of the invention to provide means whereby the noise on the slow-wave circuit due to the noise in the electron stream of a traveling-wave tube may be reduced.

It is another object of this invention to reduce the ratio of noise power due to the beam to thermal noise power (kTB), or in other words, to reduce the noise figure, of a traveling-wave tube.

Briefly, in accordance with the present invention, use is made of the Kompfner dip phenomenon which is a null in the amplitude of the traveling-wave at.a stationary position along the helix, determined by the frequency of R. F. energy in the traveling-wave, beam current, and various other parameters. At the Kompfner dip, all the input signal power on the circuit has been transferred to the beam, hence the circuit may have a discontinuity at this point, and while the noise power impressed on the circuit by the beam will be reflected, the input signal power will not. In one embodiment of the present invention means are provided at the point of the Kompfner dip to reflect waves of noise induced on the slow-wave structure by the noise on the electron stream back toward the electron gun. Attenuating means are provided to selectively attenuate these reflected waves whereby the stream may be demodulated of noise to a maximum extent, so that the waves of noise frequencies induced on the slow- ""wave structure may be substantially eliminated.

In another embodiment an auxiliary slow-wave structure is disposed about the electron stream between the electron gun and a principal or amplifying slow-wave structure. Means are then provided at the after end of the auxiliary slow-wave structure to either reflect or absorb waves of noise frequencies induced on the auxiliary slow-wave structure by the noise. modulation on the electron stream.

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 2,870,367 Patented Jan. 20, 1959 advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 is a diagrammatic sectional view of a low-noise traveling-wave tube in which one embodiment of the present invention is illustrated;

Fig. 2 is a sectional view of a low-noise traveling-wave tube showing an additional embodiment of the present invention; and

Figs. 3, 4, 5, 6, 7, 8, and 9 are abbreviated schematic diagrams of further embodiments of the invention.

Referring to the drawings and particularly Fig. 1, a traveling-wave tube 10 is shown comprising an evacuated envelope 12 which has an elongated portion 14 at its right end and a relatively short enlarged portion 16 at its left end. Within the enlarged portion 16 of the envelope 12, an electron gun 18 is shown comprising a cathode 20 which is provided with a filament 22, a focusing electrode 24 and an accelerating anode 26. Filament 22 is heated with direct current which is provided by a filament source of potential 28 the negative side of which is connected to cathode 20 through the negative side of filament 22. Focusing electrode 24 is maintained at the same potential as that of cathode 20 by an appropriate connection thereto. Focusing electrode 24 has a frustoconical configuration with an internal surface of revolution disposed at an angle of 67%. degrees from its axis of symmetry. Anode 26 is maintained a few hundred volts positive with respect to cathode 20 by means of an accelerating source of potential 30, anode 26 being connected to a tap 32 on source of potential 30 the positive terminal of which is grounded. Adjacent anode 26 is shown a noise re- I antenna lead 42 to an output ferrule 44. A collector electrode 46 is positioned at the right end of the elongated portion 14 of the envelope 12 to intercept the stream electrons. Input ferrule 34, antenna lead 36 and auxiliary helix 38 are maintained at a potential somewhat positive with respect to that of the anode 26 by means of a connection toa tap 48 on accelerating source of potential 30. Ferrule 44, antenna lead 42, and principal helix 40 are maintained at a potential somewhat positive with respect to ground by a collector source of potential 50 which is connected between collector 46 and ground. Ferrule 44 is'thus connected to a tap 52 on collector source of potential 50. In this manner collector 46 is maintained at a potential positive with respect to that of output ferrule 44 in order to prevent secondary electrons, which may be produced by the stream electrons impinging upon the collector surface, from reaching the output ferrule 44.

Auxiliary and principal helices 38 and 40, which serve as the slow-wave circuit for the traveling-wave tube 10, preferably-are made of a material such as tungsten or molybdenum, the principal requirement being that they retain their form especially with respect to the pitches and diameters. In accordance with the present invention, auxiliary helix 38 is maintained at a potential for demodulating the stream produced by the electron gun 18 in the band in which the tube 10 is intended to operate. Maximum demodulation ordinarily occurs when the poisease? tential of the auxiliary helix 38 is somewhat below that of the potential of principal helix 40, principal helix 40 being maintained at a potential for maximum amplification of the wave launched along the auxiliary helix 38 to be amplified.

Shown disposed about the elongated portion 14 of the envelope 12 adjacent to and to the right of input cavity 54 a helix 74! is shown which may be made of a ferromagnetic material known simply as ferrite material. When the ferrite material is operated with a predetermined D. C. magnetic field, it may have unilateral wave propagation properties at frequencies within the band of operation of the traveling-wave tube 110, that is, as a unilateral device .may attenuate waves propagated in only one direction. In the case of the instant invention, it is employed to attenuate waves propagated only in the direction toward the electron gun ll8.

Disposed contiguous to the envelope 12 about the left end of principal helix d there is shown a resistance termination 76 which is employed to suppress oscillations within the tube caused by the reflection of waves propagated along principal helix 40 at its opposite ends. The use of resistive termination 76 is not absolutely necessary although it will be found that tube operation will ordinarily be much enhanced by its use.

The traveling-wave tube 10 is provided with an input matching cavity 54 external to the evacuated envelope 12, having a coaxial input cable 56 connected thereto and an output matching cavity 58 connected to a coaxial output cable 60. As previously mentioned, auxiliary and principal helices 38 and 40 are connected to ferrules 34 and 44 by leads 36 and 42, respectively. Leads 36 and 42 are located parallel to the electric fields excited within matching cavities 54 and 58. Matching cavity 54 has a configuration of a rectangular toroid with a concentric collar 62 disposed about and spaced from matching ferrule 34. An opening 64 in the end plate of cavity 54 facing the left end of auxiliary helix 38 allows the full length of lead 36 to be energized, and, in addition increases the tendency of the electric fields produced in the cavity to afiect or modulate the flow of electrons in the stream. Cavity 58 is similarly constructed having a corresponding concentric collar 66 disposed about and spaced from matching ferrule 44, and an opening 68 in the plate facing the right end of principal helix 430.

A center conductor 70 of input coaxial cable 56 extends through an aperture in the annular wall of cavity 54 and is connected to concentric collar 62 while the outer conductor of cable 56 is bonded to the periphery of the aperture. Likewise, the center conductor 72 of output coaxial cable 58 extends through an aperture in the annular wall of cavity 58 and is bonded to the periphery of the aperture in the same manner as before or viceversa. Cavities 54 and 58 are fabricated with an inner surface composed of a highly conductive material and are broadly resonant so as not to limit the frequency of operation of the traveling-wave tube 10. The configuration shown and described for the cavities 54 and 58 provides suitable impedance matching from auxiliary and principal helices 38 and 40 to coaxial cables 56 and 60, respectively, over a range of frequencies such as, for example, 2000 to 4000 megacycles per second.

A solenoid 78 is coaxially positioned symmetrically about the envelope 12 and appropriate direct current is maintained in solenoid 78 by means of a potential source such as battery 80 so as to produce an axial magnetic field of the order of more than 1000 gauss to constrain the electron stream produced by the gun 118 and to provide a D. C. field within the ferromagnetic helix 74 whereby waves of noise frequencies within the operating band of the tube 10 may be attenuated along auxiliary helix 38 in the backward direction, or toward electron gun l8.

In the operation of the traveling-wave tube 10, electron gun 118 projects an electron stream through auxiliary J5 in which it shown in Fig. 1.

and principal helices 38 and 30 to collector 4d. The antenna lead 36 connected between input ferrule 3d and 38 is then energized by an input signal arriving at input cavity 54 through input coaxial cable 56. The signal wave is thus launched along the auxiliary helix 36. Under certain specified conditions well known in the art, a phenomenon known as the Kompfner null or dip takes place at the end of the auxiliary helix 38, i. e., the circuit voltage representing the energy of the input wave propagated along-the auxiliary helix 38 will be zero at the end of the auxiliary helix. The fact that the auxiliary helix 38 ends abruptly at its right end will not, therefore, reflect electromagnetic energy of the input signal Wave. However, demodulation of the stream produced by the electron gun 118 will occur within the auxiliary helix 38, and waves of noise frequencies within the operating band of the tube 10 will be induced on the auxilairy helix and will be reflected at the right end of auxiliary helix 38.

Due to its unilateral wave propagation properties, ferromagnetic helix 74 does not impede the input signal modulation of the stream and does not impede the demodulation of the stream of noise originally existing in it. However, the stream will continue to be modulated as it passes between auxiliary helix 38 and principal helix 40 by the input signal although all the noise energy represented by waves induced on the auxiliary helix 38 will be reflected (or radiated) at its right end due simply to its abrupt discontinuity. The noise energy propagated in the direction of electron gun 18 will then be attenuated by the unilateral attenuation characteristics of the ferromagnetic helix 741.

The waves passing from auxiliary helix 38 to principal helix 30 are amplified within the helix 40; and waves reflected at the output end of principal helix 40 are attenuated by the resistive termination 76 of principal helix 40. At the end of principal helix 40, the amplified electromagnetic waves in flowing along output antenna lead 42, connecting principal helix 40 to output ferrule 44, excite an electric field in output matching cavity 58. This electric field then induces a corresponding output signal on the center conductor 72 of output coaxial cable 60.

The traveling-Wave tube of Fig. 1 is again shown in a diagrammatic sectional view in Fig. 2 with all of the same associated circuitry and structure although some of the structure has been changed in position. First, the input matching ferrule 34 is connected over input antenna lead 36 to principal helix 40 whereas auxiliary helix 38 is unconnected and then terminated at its opposite ends except for the connection to the tap 48 on accelerating source of potential 30. Input ferrule 34 is likewise disposed between input antenna lead 36 and auxiliary helix 38 and not adjacent to anode 26 of gun 18 as shown in Fig. 1. On the contrary, from the gun 18 in Fig. 2 there is shown disposed along elongated portion lll of envelope 112 auxiliary helix 38, input ferrule 3 3, input antenna lead 36, principal helix 40, output antenna lead 42, output matching ferrule 44, and collector 46. The position of ferromagnetic helix 74 is likewise changed although it is still positioned about auxiliary helix 38. Ferromagnetic helix 74 is thus positioned adjacent to and to the left of input matching cavity 54.

Resistive material 76 in Fig. 2 is positioned approximately at the center of principal helix 40 and disposed about a portion of the elongated portion 114 of the envelope 12. Resistive material 76 thereby serves the same purpose as described in connection with Fig. 1 although in Fig. 2 it is obvious that it is unnecessary to terminate either end of principal helix 40 because each end may be matched to the input and output coaxial cables 56 and 60 by appropriate means including input and output matching cavities 54 and 58. Solenoid 78 is disposed about the envelope 12 in substantially the same position Solenoid 78 is employed to produce an axial magnetic field as before and, to this end, battery 80 is also again connected to solenoid 78. The operation of the low-noise tube in Fig. 2 is substantially the same as the operation of the tube 10 shown in Fig. 1 although the existence of the phenomena known as the Kompfner dip is not necessarily relied upon.

In the operation of the tube 10 in Fig. 2, an electron stream is projected through auxiliary helix 38 and principal helix 40 from gun 18. Waves of noise frequencies within the operating band of the tube 10 are launched along auxiliary helix 38 when the electron stream is projected through auxiliary helix 38. At the right end of auxiliary helix 38 there is an abrupt discontinuity and therefore waves of noise frequencies within the operating band of the tube 10 will be reflected as before. Ferromagnetic helix 74, as before, has unilateral wave propagation properties whereby maximum electromagnetic induction along the auxiliary helix 38 may take place as waves progress in a direction of electron flow and waves reflected at the right end of auxiliary helix 38 are attenuated as they are reflected back toward the electron gun 18. A low-noise stream is thus projected through input ferrule 34 and thence modulated by an input signal.

Resistive material 76 is placed at the Kompfner dip so as to further eliminate, by attenuation, any noise energy while not affecting the wave energy desired to be amplified.

Referring to Fig. 3, there is shown a third embodiment which is a modification of the embodiment of Fig. l in which the center section of the tube has been modified. Resistive attenuator 76 and ferrite helix 74 are shown removed and replaced by a nonreciprocal attenuator 75 which, as one unit, performs both functions of the two replaced elements 76 .and 74. Attenuator 75 extends to cover the forward end of helix 40 as well as the after end of helix 38. In operation of this embodiment the nonreciprocal attenuator 75 in a conventional way attenuates only energy traveling in the backward direction, toward the electron gun, and, therefore, precludes oscillation in traveling-wave tube 10 without attenuating any forward energy and without affecting forward gun energy in any other way as by adding noise.

Referring to Fig. 4, there is shown a modification of the embodiment shown in Fig. 1 in which the auxiliary helix 38 is terminated by a resistive load 71 which absorbs and dissipates the noise energy induced on auxiliaryhelix 38 from the electron stream instead of refleeting it and dissipating it in a nonreciprocal attenuator 74, as described in connection with Fig. 1.

Referring to Fig. 5, there is shown another modification of the auxiliary helix 38 of Fig. 1 which is neither terminated nor surrounded by a nonreciprocal attenuator 74, but which has a discontinuity at the after end of helix 38 and an absorptive resistive termination 73 on the forward end of the auxiliary helix 38. In operation, the discontinuity on the afterend of helix 38 causes the noise signals to be reflected back toward the electron gun and they are absorbed in load 73.

Referring to Fig. 6, there is shown a further modification of the embodiment of Fig. 1 in which auxiliary helix 38 is terminated on the after end by resistive load 71 and 'on' the forward end by resistive load 73 in a manner to absorb forward traveling noise energy in resistor 71 while absorbing any reflected noise energy in resistive load 73.

Fig. 7 shows a modification of the embodiment of Fig. 1 in which without an actual break in helix 40' there is placed a reflector 77 at the Kompfner dip point while nonreciprocal attenuator 74 absorbs energy reflected thereby in the same manner as described in connection with Fig. l. I

Fig. 8 shows another modification of the embodiment of Fig. 1 in which there is an actual D. C. break between helix 38 and helix 40 with resistive load 79 coupled to the after end of helix 38 and resistive termination 81 coupled to the forward end of helix 40.

Fig. 9 is yet another modification of the embodiment 5 of Fig. 1 in which, as in Fig. 7, a reflector 77 is placed 10 and a resistive termination 81-is provided for the input end of helix 40.

There have thus been shown embodiments of the present invention which provide a microwave tube having a low-noise figure by virtue of the means disclosed for removing noise from the electron stream.

What is claimed is:

1. A low-noise traveling-wave tube comprising: an electron gun for producing a stream of electrons; a slowwave structure disposed along said stream of electrons for propagating a predetermined traveling-wave signal therealong; means coupled to said slow-wave structure at the Kompfner dip point on said slow-wave structure where substantially all the energy of said predetermined traveling-wave signal has been transferred to the stream of 5 electrons, for selectively deriving and dissipating undesired noise wave energy from said stream and from said slow-wave structure while permitting desired wave energy to pass said Kompfner dip point substantially unaffected.

2. The traveling-wave tube as defined in claim 1 wherein the means for deriving and dissipating the noise wave energy from the stream includes a reflector and a nonreciprocal attenuator for dissipating noise energy traveling back toward the electron gun.

3. In a traveling-wave tube having a principal slowwave structure and an electron gun for projecting an electron stream through the principal slow-wave structure, an auxiliary slow-wave structure disposed about the stream between the principal slow-wave structure and the electron gun immediately adjacent to the principal 0 slow-wave structure to be electromagnetically coupled thereto and being of such a length relative to that of the principal slow-wave structure that the two slow-wave structures electromagnetically constitute one slow-wave structure with a direct-current discontinuity between the two slow-wave structures, means for reflecting undesired electromagnetic noise wave energy towards the electron gun from the end of said auxiliary slow-wave structure opposite the electron gun, and means for selectively attenuating said electromagnetic waves propagated only toward the electron gun along said auxiliary slow-wave structure.

4. In a low-noise traveling-wave tube having a principal helical conductor and an electron gun for projecting an electron stream through a principal helix: an auxiliary 65 helical conductor disposed about the stream between the v principal helix and the electron gun immediately adjacent to the principal helix and electromagnetically coupled thereto, said auxiliary helical conductor having unconnected and unterminated ends for reflecting undesired electromagnetic noise energy, and a ferromagnetic helical conductor disposed about said auxiliary helix to attenuate electromagnetic waves propagated along said auxiliary helical conductor in the direction of the electron gun, whereby said noise energy may be substantially eliminated.

5. A low-noise traveling-wave tube comprising: a principal slow-wave structure; an electron gun for projecting an electron stream through said principal slow-wave structure; magnetic means for producing an axial constraining.magnetic field along said electron stream; an

such a length relative to that of said principal slo structure that the two slow-wave structures electromagnetically constitute one slow-wave structure with a direct-current discontinuity between'them; means for refiecting electromagnetic Wave energy toward said electron gun from the end of said auxiliary slow-wave structure opposite said electron gun; and ferrite helix means disposed about said auxiliary slow-wave structure for selectively attenuating electromagnetic waves propagated in the direction of said electron gun along said auxiliary slow-wave structure, said ferrite helix means being adapted to resolve from said axial constraining magnetic field a transverse component for producing said attenuating.

6. A low-noise traveling-wave tube comprising: a principal helix; an electron gun for projecting an electron stream through said principal helix; an auxiliary helical conductor disposed about said stream between said electron gun and said principal helix and disposed immediately adjacent thereto and electromagnetically coupled thereto and being of such a length relative to that of said principal helix that said principal helix and said auxiliary helical conductor electromagnetically constitute one slow-wave structurewith a direct-current discon tinuity at their juncture, said auxiliary helical conductor having unconnected and unterminated ends for reflecting electromagnetic energy; and a ferromagnetic helix disposed about said auxiliary helical conductor to attenuate electromagnetic waves propagated along said auxiliary helical conductor in the backward direction toward said electron gun.

7. The traveling-wave tube as defined in claim 6, wherein means are coupled to the end of said auxiliary helical conductor adjacent said electron gun for modulating said electron stream with an electromagnetic input signal wave.

8. The traveling-wave tube as defined in claim 7, wherein resistive means are disposed about the end of said principal helix adjacent said auxiliary helical conductor to prevent said traveling-wave tube from oscillating because of the propagation of electromagnetic waves along said principal helix reflected at its opposite ends.

References Cited in the file of this patent UNITED STATES PATENTS 2,584,597 Landauer Feb. 5, 1952 2,602,148 Pierce .luly l, 1952 2,798,183 Sensiper July 2, 1957 2,798,203 Robertson luly 2, 1957 OTHER REFERENCES Article entitled The Microwave Gyrator," Bell Sys- 25 tern Tech. lour. for January 1952, pages 22 to 27.

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