US3735188A

US3735188A - Traveling wave tube with coax to helix impedance matching sections

Info

Publication number: US3735188A
Application number: US00268797A
Authority: US
Inventors: C Anderson; C Cardoza
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1972-07-03
Filing date: 1972-07-03
Publication date: 1973-05-22
Anticipated expiration: 1990-05-22
Also published as: GB1424745A; DE2327484A1; DE2327484B2; JPS4946371A; JPS5244697B2

Abstract

A traveling wave tube includes an elongated hollow cylinder of nonmagnetic metal which serves as a container and an elongated conductive wire helix of predetermined diameter which serves as a slow wave structure, located in the cylinder. A plurality of straight elongated dielectric support rods of a length greater than the helix length are spaced about, contact, and extend along the inner walls of the cylinder substantially parallel with the axis of the cylinder. The rods contact the outer periphery of the helix and thereby support the helix concentric with the axis of the cylinder. A matching helix section of a relatively small number of turns and relatively short length is located at an end of the elongated helix and is supported by the support rods. A coaxial input connector includes a center conductor which extends into the metal cylinder and is connected to one end of the matching helix. The other end of the matching helix is connected to an end of the elongated helix. The matching helix is of a diameter larger than the diameter of the elongated helix to permit the turns of the matching helix to be more proximate the inner surface of the metal cylinder than the turns of the elongated helix and the matching helix includes indented portions which fold around each of said plurality of dielectric support rods.

Description

United States Patent 1 Anderson et al.

[ 51 May 22, 1973 I [54] TRAVELING WAVE TUBE WITH COAX TO HELIX IMPEDANCE MATCHING SECTIONS [75] Inventors: Charles Elmore Anderson, San Car- 10s; Clarence Anthony Cardoza, Menlo Park, both of Calif.

[73] Assignee: Litton Systems, Inc., San Carlos,

Calif.

[22] Filed: July 3, 1972 [21] Appl. No.: 268,797

[52] US. Cl ..315/3.5, 333/31 A, 333/34, 315/36 [51] Int. Cl ..H0lj 25/34 [58] Field of Search ..3l5/3.5, 3.6, 39.3; 333/34 [56] References Cited UNITED STATES PATENTS 3,201,720 8/1965 Bradford et al ..3l5/3.5 X 2,922,067 1/1960 Van Dien ..333/3l A X 2,922,068 l/l960 Kennedy ..315/3.6 2,891,190 6/1969 Cohn ..3l5/3.5 3,070,725 12/1962 Lee et al ..3l5/3.5 3,013,177 12/1961 Minakovic ..3l5/3.5

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Saxfield Chatmon, .Ir. AtzorneyRonald M. Goldman [57] ABSTRACT A traveling wave tube includes an elongated hollow cylinder of nonmagnetic metal which serves as a container and an elongated conductive wire helix of predetermined diameter which serves as a slow wave structure, located in the cylinder. A plurality of straight elongated dielectric support rods of a length greater than the helix length are spaced about, contact, and extend along the inner walls of the cylinder substantially parallel with the axis of the cylinder. The rods contact the outer periphery of the helix and thereby support the helix concentric with the axis of the cylinder. A matching helix section of a relatively small number of turns and relatively short length is located at an end of the elongated helix and is supported by the support rods. A coaxial input connector includes a center conductor which extends into the metal cylinder and is connected to one end of the matching helix. The other end of the matching helix is connected to an end of the elongated helix. The matching helix is of a diameter larger than the diameter of the elongated helix to permit the turns of the matching helix to be more proximate the inner surface of the metal cylinder than the turns of the elongated helix and the matching helix includes indented portions which fold around each of said plurality of dielectric support rods.

10 Claims, 5 Drawing Figures Patented May 22, 1973 FREQUENCY (GHZ) TRAVELING WAVE TUBE WITH COAX TO HELIX IMPEDANCE MATCHING SECTIONS FIELD OF THE INVENTION This invention relates to an improvement in O-type traveling wave tubes and, more particularly, to atraveling wave tube having a helix slow wave structure and means to match the low impedance characteristic of the helix slow wave structure.

BACKGROUND OF THE INVENTION One common and well-known variety of microwave tubes, having application as a microwave signal amplifier, is the O-type traveling wave tube. In its essentials an O-type traveling wave tube includes in an evacuated housing, asource of electrons, such as a cathode; a collector electrode spaced from said cathode and defining therebetween a region, termed an interaction region; a slow wave structure, such as an elongated wire helix, located in the interaction region; means for forming the electrons from the cathode into a beam which is directed into said interaction region in a path through the center of said helix structure to said collector; a magnetic focusing structure of either the permanent magnetic or electromagnetic type for maintaining favorable focusing of electrons in the interaction region; a microwave energy coupling means, such as a coaxial connector, that is coupled to the input end of said helix and another microwave energy coupling means, also suitably a coaxial connector, that is coupled to the output end of said helix.

Connected to a suitable power source and source of microwave signals, a microwave signal from the source is applied to the input coupling and travels around and around along the turns of the helix to the output coupling. By design, the axial velocity of the microwave signal is substantially synchronous with the velocity of electrons in the electron beam passing through the center of the helix. By a phenomenon referred to as electronic interaction, the microwave signal on the helix interacts with the electrons in the electron beam as the microwave signal propagates along such helix. The signal undergoes a net gain in energy and appears at the output coupling amplified in level from the level as applied as the input. Concurrently the electrons undergo a net loss of energy. The particular details and theory of operation of such devices as well as innumerable variations in the structure of such devices are well known and very well explained in the prior art literature, including the patent literature, to which reference is made.

Practical aspects of traveling wave tube design requires a proper match of impedances between the input and output couplings and the helix slow wave structure of the traveling wave tube. A coaxial connector which has an impedance characteristic on the order of 50 ohms is coupled to a helix which has a higher characteristic impedance on the order of 200 ohms. Obviously without an appropriate match of impedances, microwave energy coupled from an external source to the input connector for application to the helix slow wave structure is in great part reflected, causing losses in the signal and resulting in undesired frequency sensitivity. Initially with such an impedance mismatch the traveling wave tube amplifier is self-defeating; the microwave signal is reduced in level at the input of the traveling wave tube. And such reduction in level could be so large that it might not be made up by amplification provided by the tube. Secondly, not only is an impedance match desired which provides good signal coupling and hence good amplification at one given frequency, it is also necessary to provide a good match of impedances over as large a range of microwave frequencies as is available in order to allow different microwave frequency signals to be uniformly amplified.

Ideally, a traveling wave tube should provide uniform amplification independent of the frequency of the microwave signal over a broad band of frequencies. The helix structure of the traveling wave tube is one of the most broad band, slow wave transmission lines known. Theoretically it is capable of providing a transmission path that is nondispersive or non-frequency selective over a band of frequencies of many octaves. However, if the electrical characteristics of the coupling means varies greatly with signal frequency within the frequency band, i.e., is dispersive, the tube provides different levels of amplification, dependent upon the frequency of the input signal. The electrical transmission characteristics of the coupling means is a limiting factor to the broad band capability available with the helix type slow wave structure. The coupling means forms essentially, by analogy, a weak link in the structural chain.

To avoid serious impedance mismatches, various types of impedance matching structures have been used in the prior art to form a transition between the helix and the coaxial connector coupling. These are exemplified by patents U.S. Pat. No. 2,905,858 to Cutler: U.S. Pat. No. 2,849,651 to Robertson; U.S. Pat. N0. 2,863,085 to Robertson; U.S. Pat. No. 2,673,900 to Mumford; and U.S. Pat. No. 2,987,644 to C. E. Anderson, to which reference is made. In the structure disclosed in Mumford the impedance match is in part obtained by varying the periodicity of a few end turns of a uniform diameter helix; in Robertson a tapered dielectric is employed or additional wire configurations are used; in Cutler a tape conductor overlying the helix is used; and in Anderson a conductive tape helix forming a frustro conical surface is coupled to the ends of the main helix. While each of these structures appear to have been suitable for these purposes, they necessarily involve complex structural components or structural features which are difficult to manufacture and adjust, or which impose additional limitations on the operation of the tube or upon the design of other elements of the traveling wave tube or require still additional tube structure. Indeed none appear to have the advantage of providing the full flexibility of adjustment over wide ranges of impedance with minimal criticality, capable of full integration within a traveling wave tube with minimal modification required of the conventional tube elements and be relatively immune to shock and vibration. Further to our knowledge none of the prior art impedance matching structures have been capable of providing a good impedance match, without loss of power, that remains essentially constant over more than 1% octaves of frequencies.

OBJECTS OF THE INVENTION Accordingly, it is an object of the invention to provide a traveling wave tube of novel structure capable of broad band operation.

It is an additional object of the invention to provide an impedance matching element in an O-type traveling wave tube which matches the 50-ohm impedance of a coaxial connector to the high ZOO-ohm impedance of a helix slow wave structure uniformly over a wide band of frequencies.

And it is a still further object of the invention to provide an impedance matching structure in an O-type traveling wave tube which provides a uniform match over approximately a frequency band of 2 octaves and which is simple in structure, easy to manufacture, relatively simple to adjust, reliable in operation and which does not involve extensive modifications to conventional tube elements.

BRIEF SUMMARY OF THE INVENTION In accordance with the foregoing objects and the invention, and O-type traveling wave tube includes an elongated hollow cylinder of nonmagnetic metal which serves as a container and an elongated conductive wire helix of predetermined diameter which serves as a slow wave structure, located in the cylinder. A plurality of straight elongated dielectric support rods of length greater than the helix'length are spaced about, contact, and extend along the inner walls of the cylinder substantially parallel with the axis of the cylinder. The rods contact the outer periphery of the helix and thereby support the helix concentric with the axis of the cylinder. A matching helix section ofa relatively small number of turns and relatively short length is located at an end of the elongated helix and is supported by the ceramic rods. A coaxial connector includes a center conductor which extends into the metal cylinder and is connected to one end of the matching helix and the other end of the matching helix is connected to an end of the elongated helix. The matching helix is of a diameter larger than the diameter of the elongated helix to permit the turns of the matching helix to be more proximate the inner surface of the metal cylinder than the turns of the elongated helix and the matching helix includes indented portions which fold around each of said plurality of dielectric support rods.

In accordance with a further aspect of the invention, the matching helix is of a tapelike form and is ofa width which progressively reduces over its length to approximately the same width as the wire of the elongated helix. The end of greatest width is connected to the center conductor of the coaxial connector and the remaining end of smallest width is connected to an end of the elongated helix.

In accordance with another aspect of the invention, a second coaxial connector having a center conductor is coupled through the metal cylinder and is coupled electrically to the other end of the elongated helix by means of a matching helix of substantially similar configuration as that of the first matching helix.

The foregoing objects and advantages of the invention as well as additional advantages and the elements and functional cooperation of the elements which are characteristic of the invention, together with obvious substitutions and equivalents therefor, are more clearly understood from the description ofa preferred embdiment which follows taken together with the illustrations of the drawings.

DESCRIPTION OF DRAWINGS In the drawings: FIG. 1 illustrates in section and in elevation a preferred embodiment of the invention;

FIG. 2 illustrates a section of the embodiment of FIG. 1 taken along the lines A-A;

FIG. 3 is an enlarged section and elevation of the section of FIG. 2; FIG. 4 illustrates a step for initially assembling an element of the invention; and

FIG. 5 illustrates graphically the resulting VSWR of one specific example of the invention.

DETAILED DESCRIPTION OF INVENTION As is recognized by those skilled in the art, the embodiment of FIG. 1 is illustrated in section and discloses in a greatly simplified illustration a helix type traveling wave tube. And this section is of exaggerated scale and proportion in order to permit clear illustration of the particularly novel aspects of the invention and their functional inter-relations with the more conventional elements normally found in the helix type of traveling wave tube without presenting wholly unnecessary and well-known mechanical detail. The tube is conventional in structure except where it is modified to incorporate the novel impedance matching sections characteristic of the invention, hereinafter explained in greater detail. The traveling wave tube of FIG. 1 includes an electron gun assembly 3, a collector electrode 5 located spaced from the electron gun across an interaction region 7 located between the collector and electron gun. An elongated electrically conductive wire helix 9, which forms the slow wave structure, is located in the interaction region. A helix shaped matching section 13 is located to the left of helix 9, and another helix shaped matching section 17 is located to the right of helix 9. A coaxial connector 11 has a center conductor 12 coupled to one end of matching section 13. A second coaxial connector 15 has a center conductor 16 coupled to one end of the second helix shaped impedance matching section 17. Ceramic dielectric spacers l0 and 14 support the respective center conductor in electrically insulated relationship with the outer cylindrical connector wall and form a vacuum tight seal therebetween. Each of the impedance matching sections, 13 and 17, is of a metal material and is geometrically broadly defined as a helix. As is better illustrated in the succeeding figures, the matching helixes l3 and 17 are larger in diameter than elongated helix 9. The remaining end of each of the matching helixes l3 and 17 is connected to a respective end of helix 9.

Helix matching sections

13 and 17 and the slow wave structure, helix 9, are supported by three elongated straight support rods, 19, 21, and 23, the latter one of which is not illustrated in this figure. The rods are of an electrically insulative (dielectric) material, suitably a ceramic such as aluminum oxide or boron nitride as is conventional. The dielectric support rods are evenly spaced about the outer surface of helix 9, extend substantially parallel to the axis of the helix and are sufficiently greater in length than elongated helix 9 to accommodate extending beyond each helix matching section. The support rods are in turn mechanically attached or supported within the hollow cylinder 25, suitably of a nonmagnetic metal which shields helix 9 from external RF fields, abut the inner cylindrical wall of cylinder 25 and are oriented parallel to the axis of the cylinder. A ceramic ring insulator is coupled between collector electrode 5 and one end of cylinder 25 in an electrically insulated and vacuum tight relationship.

This attachment is customarily accomplished by wellknown brazing techniques.

The electron gun 3 is located in a housing or chamber 28, suitably of metal or ceramic, which maintains a vacuum tight connection to the exterior. The electron gun contains a cathode and a pair of electrical leads 2 and 4, the latter of which permit application of suitable filament voltage and cathode voltages from an external source to the electron gun. A metal end plate 31 is joined between cylinder 25 and cavity 28 and forms an accelerating anode for the electron gun. The end plate includes a passage 33 with which to permit electrons from the electron gun to enter interaction region 7.

A conventional magnetic focusing arrangement is normally include in a traveling wave tube to focus electrons traveling in the interaction region in a favorable manner but this structure is not here illustrated. One typical magnetic focusing arrangement comprises an electric coil of wire formed into a solenoid which is wound around cylinder 25 so as to maintain an axial magnetic field along the center of the helix. Inasmuch as cylinder 25 is of a nonmagnetic material, the magnetic fields penetrate from outside the tube body into the interaction region. An alternative magnetic focusing structure incorporates a series of ring shaped magnets mounted about the outer surface of and surrounding the cylinder 25 and axially poled and oriented so that the polarity of adjacent magnets is the same, such as described in pages 55-61, Proceedings of the IRE, Vol. 44, No. 1, Jan. 1956. This conventional magnet focusing structure, referred to as periodic permanent magnet focusing, provides a magnetic field along the axis of helix 9 that reverses in direction periodically along the length of the helix. Inasmuch as such types of magnetic focusing structures are very conventional and well-known they are not illustrated in the figures in order to eliminate elements not necessary to an understanding of the invention. It is understood however that a preferred embodiment of a traveling wave tube includes such a magnetic focusing system.

It is understood that the details of the electron gun, collector electrode, the slow wave structure, the coaxial connectors and the housing, and the exact details of construction and assembly of those elements into a traveling wave tube are all well known in the art, the presentation of which are not necessary to the complete disclosure of the invention or to an understanding of the invention. In the interests of clarity and conciseness of this specification such details and description are necessarily omitted. The reader understands also that there are many variations of helix type traveling wave tubes which differ from the disclosed embodiment both in details of construction and in the nature of the disclosed elements. Thus some helix type traveling wave tubes have additional grid or focusing electrodes, have the pitch of the elongated helix tapered, include lossy or attenuative material to provide certain attenuation characteristics to microwave energy within the tube, have a severed helix structure with the helix formed in two spaced parts or severed by the application of loss or attenuative material to the helix. Notwithstanding such variations in the traveling wave tube, as becomes apparent, the modifications of the invention are useful and applicable each to those various O-type traveling wave tubes. Thus although the invention is explained in connection with one of the conventional traveling wave tube types, that is in turn simply illustrated, it is understood that the modifications of the invention are not so limited in application and are not intended to be construed in such a limited sense.

For the readers convenience, the designation of elements in the figures of the drawings which follow is the same as the designations assigned to those same elements as they have been identified in FIG. 1.

FIG. 2 illustrates a section of the traveling wave tube of FIG. 1 taken along the lines A-A. In this view the geometry of matching helix section 13 becomes more apparent. Metal cylinder 25, coaxial connector 11, the three straight evenly spaced rectangular shaped

ceramic rods

19, 21 and 23, helix 9, and helix matching section 13 are visible in this view. The rods support the helix 9 and helix 13 to the inner cylindrical metal walls of cylinder 25 and maintains those helices rigidly in an electrically insulated relationship thereto. As is evident, the end profile of helix 9 is a circle. On the other hand, the end profile of matching helix 13 resembles a cover leaf. One end of helix 13 is connected electrically to center conductor 12 of connector 11 and thereupon the electrical conductor form-ing matching helix 13 extends in a circular path up to the region of support rod 23. The conductor then becomes reduced in diameter, forming a concave indentation in the helix, and folds around the outer surface of ceramic rod 23. The conductor then extends up to the larger diameter and extends in a circular path up to the vicinity of support rod 19. The helix then undergoes a similar radially concave indentation and the conductor is folded around the end of rod 19. From there the conductor extends to the larger diameter and extends in the circular path adjacent the walls of cylinder 25 up to the vicinity of ceramic support 21. Helix 13 then becomes reduced in diameter and is concavely indented to fit and fold around ceramic support rod 21. The conductor of the matching helix then extends up to the larger diameter and extends in the circular path completing at least 360 or one turn about the central axis of the tube. As is more clearly shown in FIG. 1 the helix l3 continues in such a geometric assembly for the requisite number of turns and length desired. In the embodiment of FIG. 1 and by way of example only, it is seen that the matching helix 13 is sightly greater in length than 1% turns from the end coupled to conductor' 12 to the point where it is electrically joined to helix portion 9 and is considerably smaller in length than elongated helix 9.

As is evident, if a particular traveling wave tube contains more than three spaced ceramic support rods, such as four evenly spaced support rods, the end profile of the matching section 13 will have a corresponding larger number of indentations to accommodate such additional support rods and accordingly the profile view of FIG. 2 would be modified to appear as a four leaf clover geometry, etc.

The illustration of FIG. 3 provides another view of the section of FIG. 2 in perspective elevation. Moreover, the illustration is enlarged slightly from the scale than that used in connection with FIGS. 1 and 2 to better illustrate the elements of the helix and matching section. Thus the illustration of FIG. 3 includes the metal cylinder 25, coaxial connector 11, with center conductor 12,

ceramic support rods

19, 21 and 23, helix 9 and helix matching section 13. The helix matching section 13 is in electrical contact with and is connected at one end, 20, to center conductor 12 and extends in a spiral helix for the requisite number of turns,

approximately 1% turns in this embodiment, and is connected electrically at its other end 22 to the begin ning turn of helix 9. As is evident from this figure, the helix section 13 is looped around each of the ceramic support rods by means of a concave indentation in the helix conductor. The support rod is brazed to the helix section at these locations to form a rigid support.

As is noted in this figure and as is also brought out in FIG. 1, the helix matching section is in the form of a tape having a progressively decreasing width. The tape conductor is of greatest width at the end where it is coupled to the coaxial connector center conductor and tapers progressively over the predetermined number of turns, here 1%, to a width approximately equal to the width of the wire in the helix 9. Considered as a transmission line, the tape possesses its lowest impedance, approximately 50 ohms at the input end, and at its other end is narrow in width and matches the impedance of helix 9. In this manner the impedance change of the line is gradual, resulting in a smooth transition and minimum reflections of microwave energy.

For purposes of illustration, FIGS. 2 and 3 illustrate only helix matching section 13 of FIG. 1. As is apparent, helix matching section 17 is similarly constructed.

Each of the helix matching sections is constructed in a very simple manner as is illustrated in part in FIG. 4. In this the requisite number of turns of metal tape, such as 13 in FIG. 4, is wound around a mandrel 35 with the turns being spaced by the desired pitch or turn-to-turn spacing. The mandrel 35 is specially constructed to be of the desired outer diameter and to have a series of indentations, 36, 37 and 38, of the predetermined width and depth. A series of shaping rods, 39, 40 and 41, are then moved in radially at each of the locations of the indentations of the mandrel 35 and press against the overlying portions of wire 13. The wire 13 deforms and becomes indented, as is illustrated by the dotted lines which indicate the profile of the tape 13 after the completion of the operation. The slow wave structure helix 9 is manufactured in a relatively conventional and well known manner and thus need not be fully described. Briefly, making reference again to FIG. 1 and FIG. 2, the elongated helix 9 is formed and joined with the

helix matching sections

13 and 17. The

ceramic rods

19, 21 and 23 are then mounted around the helix and matching section assemblies, with the rods being positioned in the indented portions of the matching

helixes

13 and 17. The entire assembly of rods and helix are then inserted into the metal cylinder 25 and the metal cylinder is shrunk or sized so as to reduce its diameter and to create a slight pressure on the ceramic support rods. In this way the elongated helix and matching helix sections are rigidly maintained in place in the cylinder. The remaining construction and assembly necessary to the completion of traveling wave tube is very well known and is not here described.

Conventionally, in the operation of a helix type traveling wave tube the electron gun 3 is maintained at a high negative voltage and the metal cylinder 25 as well as helix 9 is maintained at electrical ground potential and collector electrode is maintained at some intermediate negative potential. With the suitable source of voltages connected, electrons are emitted from the electron gun 3 and are accelerated up to a predetermined velocity as they pass through passage 33 in plate 31. These electrons are said to be formed into an electron beam and the electrons travel down the axis of the helix 9 to electrode 5 where they are returned to the power supply via external circuitry of conventional structure. Microwave energy signals to be amplified are coupled to the input coaxial connector 11 and are thusly coupled via center electrode 12 through helix matching section 13 to the helix 9. As is conventionally understood, the microwave signals propagate around the turns of the helix to the output connector 15. Any suitable electrical load is coupled to the output connector and the amplified microwave signal is passed from this end of helix 9 and matching helix 17 to such a load. The pitch of the helix 9 is such that the effective velocity of propagation of the microwave signal over the length of the helix is effectively reduced because of the tortuous path which the signal must propagate around the turns of the helix, typically on the order of l/lOth of the normal free space propagation velocity. The propagation velocity by design is such that it is in synchronism or just slightly lower than the velocity of the electrons in the beam previously described. The electric fields of the microwave signal on the helix extend into the helix center and electronically interacts" with the electron beam. The mode and theoretical mechanics of operation of the helix traveling wave tube is of course well known and amply described in the literature and is only briefly described here for background only. As a result of this electronic interaction, the net velocity of the electrons decreases slightly, resulting in a net loss of energy, whereas the electromagnetic signal is increased in intensity resulting in a net gain in energy to the microwave signal, hence amplification.

One of the characteristic features of a helix, such as helix 9, is that it is a relatively broad band device which, with certain limitations, is essentially independent of frequency. Thus signals over a wide range of frequencies may be coupled to the helix and those signals will propagate along the helix at essentially the same predetermined velocity. Ideally, therefore, the helix type traveling wave tube should provide uniform amplification over a very wide range of frequencies of microwave signals applied at the tube input. A limiting factor, however, is the frequency attenuation and reflection characteristics of the electrical couplings between the source of microwave signals and the helix structure. One initial problem is that the coaxial connector has one characteristic impedance, typically on the order of 50 ohms, while the helix has a characteristic impedance on the order of 200 ohms. Any direct coupling between those two elements results in an impedance mismatch that reflects and attenuates microwave energy to a degree heavily dependent upon the frequency of the microwave signal. As in prior art devices, impedance matching is required to minimize electrical signal reflections and attenuation. And an impedance transforming section is employed to provide the impedance match; one that has a gradually changing impedance characteristic along its length and provides a smooth electrical impedance transition between the impedance at one end and the impedance at its other end.

Considering the outer metal cylinder 25 as the electrical ground plane, electrically grounded at zero volts, it is apparent that the turns of the matching helix section are placed or located at an optimum distance from the cylinder walls in order to obtain the desired low impedance characteristic. This is obtained irrespective of the fact that the ceramic support rods provide obstructions at their locations. The impedance of the matching section at the support rod can also be made the correct value to minimize reflections by taking into account the dielectric constant of the ceramic support rod. To a first approximation the spacing between the cylinder walls and the matching helix conductor at the support rod is effectively electrically decreased by a factor proportional to the square root of the dielectric constant of the support rod material.

It is within the scope of this invention to permit additional design changes and variations depending upon the particular tube in which the matching section is incorporated. Thus while we have illustrated a tape of a progressively decreasing width from its point of connection with the coaxial connector to the point of connection to the regular helix, the rate of taper can be structured differently from that illustrated. Likewise, it may be desired to use a matching section in which the diameter of the helix is greater or lesser than that illustrated. In adjusting the diameter of the matching section to be larger and hence more proximate the metal walls of cylinder 25, the impedance of the line decreases and conversely by decreasing the diameter of the helix the tape is more remote from the ground plane formed by cylinder walls 25 and the effective impedance of the section is increased, other factors being held constant. Likewise, the width of the tape can be made wider and hence effectively lower the effective impedance, or more narrow to thereby increase the effective impedance, other variables being held constant, and as in the prior art the pitch of the helix matching section may be varied to adjust impedances, and as is obvious the number of turns in the helix as well as its overall length form additional variables that can be used to provide a correct impedance match.

In one example, the impedance matching section provided uniform matching with power reflection of less than 4 percent over a frequency range of 3 GHz to 12 GHz. This corresponds to a voltage standing wave ratio, VSWR, of 1.5.

In one example of a traveling wave tube constructed in accordance with the teachings of the invention, the elongated helix 9 comprised a helix of tungsten metal of an overall length of 6.5 inches, an outer diameter of 0.075 inches, and each of the

helix matching sections

13 and 17 was formed of a length of platinum tape which tapered in width from 0.012 inches to 0.080 inches over a length of 0.650 inches. This tape was formed into the helix configuration consisting of ap proximately 1% turns with a turn-to-turn spacing of 0.100 inches and an overall helix length of 0.250 inches. As is brought out in the graphical illustration of FIG. 5, the results rather surprisingly disclosed a uniform impedance matching characteristic over a frequency range of 3 to 12 GHz as represented in Curve A. This contrasted with the gain characteristic illustrated in Curve B of FIG. 5, obtained from a traveling wave tube similarly constructed but having impedance matching sections conventional in the prior art wherein a helical coil of wire was used to connect the helical slow wave structure tothe center conductor of the coaxial input line. This coil of wire served as a quarter wave transformer such as described in FIG. 1 of US. Pat. No. 2,849,651 previously referred to.

Given the helix assembly as described, it is apparent that the slow wave structure so formed is relatively simple in construction and except for the fabrication of the matching sections uses standard parts and results in a very rugged structure'Thus the straight ceramic rods are used as well as the conventional helix 9. As is apparent, the ceramic rods need not be cut to permit a helix matching section of a larger effective diameter than that of helix section 9, and that the matching section such as 13 is substantially of a larger diameter than helix 9. Thus any need to cut away sections of the ceramic support rods or to drill holes through ceramic support rods, which are very impractical procedures, is entirely avoided. At the same time, a matching section capable of greater variation in impedance and successfully obtaining a low impedance input is obtained.

The preceding description of an embodiment of our invention clearly presents one skilled in the art the necessary details which enable one to make and use our invention. However it is expressly understood that the presentation of those details is not to be construed in the sense of limiting our invention, inasmuch as many other equivalent elements that may be substituted for elements of such embodiment as well as modifications and improvements to the disclosed structure, suggest themselves to one skilled in the art upon reading this specification. Accordingly, it is expressly understood that my invention is to be broadly construed within the spirit and scope of the appended claims.

What we claim is:

1. In a traveling wave tube of the type which contains:

a source of electrons, a collector, and an interaction region therebetween;

a means for forming electrons from said source into a beam which travels in a path through said interaction region to said collector;

a slow wave structure located in said interaction region;

input means for coupling microwave energy to be amplified to one end of said slow wave structure and output coupling means coupled to the other end of said slow wave structure for coupling amplified microwave energy therefrom, and a cylindrical metal sleeve surrounding and housing said slow wave structure;

the improvement wherein said slow wave structure comprises:

a first elongated electrically conductive helix portion of a predetermined radius;

a plurality of elongated dielectric support members spaced about the inner surface of said sleeve and extending axially thereof, said members being coupled between the inner surface of said metal sleeve and the outer surfaces of said first helix portion to thereby support said helix portion in electrically insulated relationship with and in said metal sleeve;

a second electrically conductive helix portion having a larger radius than said first helix portion but less than the radius of said metal sleeve, said second helix portion including a plurality of concave indentations corresponding in number and location to said plurality of dielectric support members with each respective one of said support members with each respective one of said support members being fitted into a corresponding one of said concave indentations to thereby support said second helix portion in addition to said first helix portion while permitting peripheral surface portions of said second helix portion to be located substantially more proximate to the inner walls of said metal sleeve than peripheral surfaces of said first helix portion.

2. The invention as defined in claim 1 wherein said first coupling means comprises a coaxial type connector having a center conductor and a surrounding outer conductor and wherein said center conductor is coupled in electrical contact with an end of said second helix portion.

3. The invention as defined in claim 2 wherein said second helix portion comprises a tapelike metal conductor of a width which progressively decreases between its widest portion at one end to its narrowest portion at the other end to thereby provide a smoothly tapering impedance characteristic.

4. The invention as defined in claim 3 whereby said second helix portion is of a constant pitch.

5. The invention as defined in claim 3 whereby said second helix portion is of a nonconstant pitch.

6. The invention as defined in claim 3 wherein said second helix portion is of a progressively varying pitch.

7. The invention as defined in claim 3 wherein said second helix portion is of a substantially constant radius exclusive of said indented portions.

8. The invention as defined in claim 3 wherein the radius of said second helix portion is varied.

9. In an O-type traveling wave tube which includes a helix slow wave structure located in a cylindrical nonmagnetic metal sleeve and a plurality of straight elongated insulative support members for supporting said helix structure within said metal sleeve in electrically insulated relationship thereto, a first coaxial connector coupled to one end of said helix and a second coaxial connector coupled to the other end of said helix; the improvement wherein an end portion of said helix structure, comprising a predetermined number of turns, is of a radius greater than the radius of the major portion of said helix structure but less than the radius of said cylindrical metal sleeve and contains a plurality of indented portions, indented concavely to a radius substantially equal to that of said major portion of said helix structure, with each of said indented portions being fitted onto a corresponding one of said plurality of insulative support rods.

10. An O-type traveling wave tube of the type which includes an elongated metal slow wave structure located within an elongated hollow nonmagnetic metal cylinder; a plurality of straight elongated support members of electrically insulative material evenly spaced about said elongated slow wave structure and oriented substantially parallel to the axis thereof for supporting said slow wave structure within said metal cylinder in electrically insulated relationship with the inner wall surface of said metal cylinder; a first coaxial connector for coupling microwave energy to one end of said slow wave structure; and a second coaxial connector for coupling microwave energy from the other end of said slow wave structure; the improvement comprising in combination therewith:

a first helix of electrically conductive material, said first helix being substantially shorter in length than said elongated slow wave structure and one end of said first helix being coupled to said coaxial connector and the other end of said first helix being coupled to one end of said elongated helix for providing an impedance matching section therebetween; a second helix of electrically conductive material, said second helix being substantially shorter in length than said elongated slow wave structure, one end of said second helix being coupled to said second coaxial connector and the other end of said second helix being coupled to the other end of said helix for providing an impedance matching section therebetween; each of said helix matching sections comprising a maximum diameter greater than twice the distance between the axis of said cylinder and a most adjacent edge of a support rod and having a plurality of indented portions for receiving respective corresponding ones of said plurality of spaced support rods.

ggz gy m me STATES PATENT @FFNIE CE TEWQATE GE CGR EQEWN Patent No. 3,735,188 Dated May 22, 1973 Inventor(s) Charles Elmore Anderson et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In Column 10, delete line 61 which reads as follows: each respective one of said support members w1th" Signed and sealed this 5th day of February 1974.

(SEAL) Attest:

EDWARD M. FLETCHER,'JR. Attesting Officer RENE D. TEGTMEYER Acting Commissioner of Patents

Claims

1. In a traveling wave tube of the type which contains: a source of electrons, a collector, and an interaction region therebetween; a means for forming electrons from said source into a beam which travels in a path through said interaction region to said collector; a slow wave structure located in said interaction region; input means for coupling microwave energy to be amplified to one end of said slow wave structure and output coupling means coupled to the other end of said slow wave structure for coupling amplified microwave energy therefrom, and a cylindrical metal sleeve surrounding and housing said slow wave structure; the improvement wherein said slow wave structure comprises: a first elongated electrically conductive helix portion of a predetermined radius; a pluraLity of elongated dielectric support members spaced about the inner surface of said sleeve and extending axially thereof, said members being coupled between the inner surface of said metal sleeve and the outer surfaces of said first helix portion to thereby support said helix portion in electrically insulated relationship with and in said metal sleeve; a second electrically conductive helix portion having a larger radius than said first helix portion but less than the radius of said metal sleeve, said second helix portion including a plurality of concave indentations corresponding in number and location to said plurality of dielectric support members with each respective one of said support members with each respective one of said support members being fitted into a corresponding one of said concave indentations to thereby support said second helix portion in addition to said first helix portion while permitting peripheral surface portions of said second helix portion to be located substantially more proximate to the inner walls of said metal sleeve than peripheral surfaces of said first helix portion.

10. An O-type traveling wave tube of the type which includes an elongated metal slow wave structure located within an elongated hollow nonmagnetic metal cylinder; a plurality of straight elongated support members of electrically insulative material evenly spaced about said elongated slow wave structure and oriented substantially parallel to the axis thereof for supporting said slow wave structure within said metal cylinder in electrically insulated relationship with the inner wall surface of said metal cylinder; a first coaxial connector for coupling microwave energy to one end of said slow wave structure; and a second coaxial connector for coupling microwave energy from the other end of said slow wave structure; the improvement comprising in combination therewith: a first helix of electrically Conductive material, said first helix being substantially shorter in length than said elongated slow wave structure and one end of said first helix being coupled to said coaxial connector and the other end of said first helix being coupled to one end of said elongated helix for providing an impedance matching section therebetween; a second helix of electrically conductive material, said second helix being substantially shorter in length than said elongated slow wave structure, one end of said second helix being coupled to said second coaxial connector and the other end of said second helix being coupled to the other end of said helix for providing an impedance matching section therebetween; each of said helix matching sections comprising a maximum diameter greater than twice the distance between the axis of said cylinder and a most adjacent edge of a support rod and having a plurality of indented portions for receiving respective corresponding ones of said plurality of spaced support rods.