US3465198A

US3465198A - Coaxial attenuator for traveling wave devices

Info

Publication number: US3465198A
Application number: US590901A
Authority: US
Inventors: Arthur H Downing; Peter Janis
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1966-10-31
Filing date: 1966-10-31
Publication date: 1969-09-02
Anticipated expiration: 1986-09-02

Description

COAXIAL ATTENUATOR FOR TRAVELING WAVE DEVICES Filed 0012. 31, 1966 P 969 A. H. DOWNING E AL 5 Sheets-Sheet l flaszS/l ve Maia/ml uvvmrans ARTHUR H. DOWN/N6 55m? JAN/S/ ATTORNEY Sept. 2, 1969 COAXIAL ATTENUATOR FOR TRAVELING WAVE DEVICES I Filed Oct; 31, 1966 A. H. DOWNING ET L 5 Sheets-Sheet 2 I fisA z ATTORNEY p 969 A DOWNING ET AL 3,465,198

COAXIAL ATTENUATOR FOR TRAVELING WAVE DEVICES Filed Oct. 31, 1966 5 Sheets-Sheet 5 l/VVE/VTO/PS ARTHUR H. DOWN/N5 75/? JAIN/5 5y ATTORNEY United States Patent M 3,465,198 COAXIAL ATTENUATOR FOR TRAVELING WAVE DEVICES Arthur H. Downing, Woburn, and Peter Janis, Auburndale, Mass., assignors to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed Oct. 31, 1966, Ser. No. 590,901 Int. Cl. H01j 25/34 U.S. Cl. 315--3.5 7 Claims ABSTRACT OF THE DISCLOSURE The present invention relates generally to traveling wave devices having a slow wave transmission circuit and more particularly to a coaxial termination for dissipation and absorption of electromagnetic energy propagated in such circuits. In addition impedance matching means are provided for the reduction of anomalous oscillations arising from either forward or background wave interaction between the beam of electrons and the energy propagating on the slow wave circuit.

Traveling wave devices of the type under consideration may be employed as amplifiers or oscillators and such devices conventionally include a periodic slow wave transmission structure such as an interdigital delay line disposed within an evacuated envelope. A sole electrode of either a linear or circular configuration is spaced from the slow wave circuit in a coplanar manner and defines therewith the boundaries of an interaction space. Means for the generation of an electron beam is disposed at one end thereof and means for the collection of electrons is located at the opposing end. Suitable electrical biasing potentials are established between the sole electrode and slow wave circuit and external magnets provide for the establishment of a magnetic field transverse to the electric field. Hence, such devices 'are often referred to as being of the crossed-field type. Under the combined influence of the electric and magnetic fields electromagnetic wave energy propagating along the slow wave circuit interacts with the electron beam traversing the interaction space in an energy exchanging manner. Suitable adjustment of the field parameters as well as beam trajectory will result in forward or backward wave interaction with oscillation and/or amplification of radio frequency energy.

An exemplary slow wave circuit of the interdigital finger type is disclosed in US. Patent No. 2,925,518. issued Feb. 16, 1960 to John M. Osepchuk and assigned to the assignee of the present invention. Specifically, the finger elements comprise a first portion extending perpend-icular to a common base, or as sometimes referred to, back wall, and attached thereto. A second portion extends perpendicular to the first portion and parallel to the common base, while a third portion extends perpendicular to the second portion in a direction extending back toward the common base and defining a free space at the extremity thereof. Such interdigital delay line elements are referred to as I-fingers and as a result of this configuration the over-all electromagnetic wave energy path has been lengthened while maintaining the 3,465,198 Patented Sept. 2, 1969 over-all magnetic field gap constant to result in much lower frequency of operation and higher power handling capabilities.

All traveling wave devices of the crossed-field type are desirably operated in a nonreentrant manner to achieve power output stability as well as eliminate spurious or anomalous oscillations during operation. For the purposes of this description the term nonreentrant refers to the requirement that the electron beam traverse the interaction space only once in an energy exchanging relationship and be as completely expended as possible after the first traverse. In addition, the term is utilized in referring to the slow Wave transmission circuit to indicate that electromagnetic wave energy propagated thereon is permitted to traverse the circuit in only one direction without any reflection of such energy by any discontinuities or (mismatching of impedances. In prior art devices preselected portions of the delay line structure have functioned as electromagnetic energy absorption means and still be required to maintain the electrical characteristics of the over-all slow wave structure at a predetermined value. Energy absorbing coatings of materials such as iron or other suitable lossy materials have been conventionally disposed on a plurality of the delay line elements near the terminal portion of the interaction space. Such coatings of iron or other resistive materials have been difficult to apply uniformly to the delay line elements with resultant poor adherence. Further, such coatings which are inherently of a carbon composition create problems when the devices are evacuated. The introduction of coating materials such as iron also varies the magnetic field parameters which must be carefully controlled for suitable operation.

At the high power levels of operation in a large number of such devices the heat resulting from the absorption of the electromagnetic wave energy, particularly when continuous wave operation is anticipated, has oftentimes resulted in deformation of the interdigital delay line elements thereby introducing limitation in usefulness. Rather lengthy externally coupled attenuating structures have therefore evolved in the art in an attempt to further extend the range of the power handling capabilities of subject devices, particularly those operating in the lower end of the microwave frequency spectrum. It has been noted, however, that such elongated or lumped lossy structures for the absorption and attenuation of the undesired electromagnetic wave energy and electron beam residual electrons introduces many variables in the capacitance and/or inductance of the over-all slow wave delay line design parameters, particularly when attenuators of many wavelengths in over-all dimension are appended to the slow wave circuit structure.

In the present invention a new and novel type attenuator is disclosed having one member directly coupled to the terminal end of a delay line and defining with a coaxial member transformation means for matching impedances. Further, the over-all length of the coaxial attenuation arrangement is a small fraction of the operating wavelength to thereby provide an essentially pure resistive component in the slow wave circuit. Additionally, selected materials are disclosed of a wholly resistive characteristic to further enhance the attenuation capabilities in the absorption of heat and rapid conduction of said heat away from the critical region. In illustrative embodiments of the present invention bulk as well as thin film attenuation means are disclosed having high heat handling capabilities to enable generation of higher output power. The average coaxial impedances. of the attenuator structure are suitably matched by transformation means to the impedance characteristics of the applicable slow wave transmission circuit. The invention is equally applicable to many delay line configurations including the J-shaped elements in lower frequency devices. Throughout the description it will be noted that the illustrative configurations of the invention are provided having a very short over-all dimension. External coolant means communicating with the coaxial attenuator will also be described to further aid in the rapid conduction of the heat generated by the attenuation of the undesirable electromagnetic wave energy.

A primary object of the present invention is the provision of an improved resistor coaxial attenuator for traveling wave devices having a slow wave conducting structure.

Another object of the present invention is the provision of a coaxial attenuator for crossed-field traveling Wave devices having incorporated therein means for the matching of the impedances of the coaxial structure to the slow wave transmission circuit.

Still another object of the present invention is the provision of electromagnetic energy absorbing means in a crossed-field device having an interdigital delay line structure to thereby provide for more efficient nonreentrant operation with regard to the electromagnetic Wave conducting path as well as the electron beam path.

A still further object of the present invention is the provision of a coaxial attenuator for crossed-field traveling wave devices wherein the energy absorbing means comprise a relatively thin layer of a resistive material having a high microwave energy absorption capability deposited on a bulk material having a rapid heat conduction capability to permit the operation of such devices at higher output powers for both intermittent and continuous operation.

Further objects, features and advantages of the present invention will be readily apparent after consideration of the following detailed description together with the accompanying drawings in which:

FIG. 1 is a longitudinal cross-sectional view of an illustrative embodiment of the invention;

FIG. 2 is a detailed cross-sectional view along the line 2-2 in FIG. 1;

FIG. 3 is a fragmentary cross-sectional view of another embodiment of the invention;

FIG. 4 is an enlarged fragmentary view partially in cross-section of an auxiliary coolant circulation means which may be utilized in conjunction with the embodiment of the invention;

FIG. 5 is a fragmentary cross-sectional view of still another embodiment of the invention;

FIG. 6 is a graph of the standing wave ratio in relation to the ratio of the diameter of the inner and outer members of the coaxial attenuator;

FIG. 7 is a fragmentary view of a portion of the slow wave structure in FIG. 2 with an additional modification;

FIG. 8 is a partial cross-sectional view of the utilization of the embodiment of the invention in a linear interdigital delay line;

FIG. 9 is a fragmentary cross-sectional view of another alternative embodiment of the invention; and

FIG. 10 is a similar view illustrative of a modification in the embodiment shown in FIG. 9.

Referring to the drawings, FIGS. 1 and 2 illustrate a backward wave crossed-field device 10 incorporating a slow wave propagating structure 14 of the interdigital delay line type with the individual delay line elements being of the I-shaped configuration disclosed in the aforementioned United States patent. Sole electrode 12 is disposed concentrically with respect to the delay line 14 and is normally maintained at a negative potential with respect thereto. An input electrical lead assembly 16, electron gun assembly 18 including an indirectly heated cathode 19, magnetic field producing means 20 and output coupling means 22 complete the major subassemblies of the over-all embodiment. The arcuate interdigital delay line 14 comprises a plurality of interdigital elements secured to cylindrical base member 24 which together with the oppositely disposed

cover plates

26 and 28 hermetically sealed thereto form the walls of evacuated envelope of the over-all embodiment 10.

The sole electrode 12 comprises a cylindrical member of an electrically conductive material and includes a web portion 30 bounded by an arcuate portion 31 defining a channel 32 for the purpose of confining the electron beam within the interaction space 33 defined by the channel wall surfaces and the delay line assembly 14. One end of a hollow supporting member 34 is inserted within a tubular member 35 which is in turn secured to the sole electrode web portion 30. Member 34 in addition to supporting the sole electrode forms a portion of the electrical lead assembl 16 and permits the introduction of external circuit connecting leads to appropriate electrodes within the over-all device. A slotted section 36 is defined within the sole electrode and the electron gun assembly 18 is disposed therein. A mounting plate 37 provides means for attachment to the web portion 30. The gun assembly includes a cathode, heater, grid and accelerating electrodes of the known configuration and the details have not been enumerated herein for the sake of clarity.

The input electrical lead assembly 16 comprises a sleeve member 38 secured to cover plate 26 together with a dielectric sealing member 39 joined at its outer end to a second electrical conductive sleeve member 40. A terminal glass bead seal 41 supports the electrical leads in spaced relationship and hermetically seal the tube envelope. Electrical energy from appropriate sources is supplied to the gun assembly electrodes by way of electrical

lead wires

42, 43, 44 and 45 which extend through the glass bead 41.

The required electric field between the slow wave structure 14 and sole electrode 12 is supplied by means of a unidirectional voltage supplied therebetween, such as, for example, a battery 47. The sole electrode 12 is preferably biased negatively with respect to the cathode 19 by means of a source 48 connected between the cathode lead 43 and the sole electrode supporting member 34 by way of sleeve member 40. Similarly, the line 14 will be maintained at a positive potential relative to both the sole electrode 12 and cathode 19 by the source of unidirectional voltage 47 connected between the sleeve 38 and cathode lead 43 with the sleeve member 38 being connected through base member 24 to the delay line. Lead 44 may for example be utilized to supply a positive potential relative to the cathode to the accelerating electrode by means of a source 50 connected between leads 43 and 44. The remaining electrical lead member 45 may be connected by way of a terminal 51 to an appropriate energy source for controlling the magnitude of the electron beam current in the oscillator 10.

Output coupling means 22 comprise a coaxial transmission line having an outer conductor 52 and an inner conductor 53 with its inner end secured illustratively to delay line element 54 adjacent to the electron gun assembly 18. In this manner of output coupling the cross-field traveling wave device will operate in the backward wave mode of oscillation. A collector electrode 55 is disposed at the opposing end of the interaction space 33 and provides for the interception of the residual electrons in the beam after the first traversal of the interaction space. Collector electrode 55 may be tapered and is joined to the base member 24.

A uniform magnetic field transverse to the direction of the electron beam and electric fields is provided by the magnet assembly 20 including

pole piece members

56 and 57 with the major interaction components being disposed in the magnetic gap defined therebetween. Permanent magnet members or any other suitable electromagnetic means will contact the pole piece members.

The coaxial attenuation arrangement in accordance with the principles of the invention is incorporated in the structure designated generally by the numeral 60.

This arrangement provides for the termination of the electromagnetic energy propagating path by a short-circuited coaxial transmission line having a resistance element disposed between the inner conductor member and the short circuit end with the length of the attenuator being a small fraction of a Wavelength at the frequency of operation. The coaxial termination and attenuator 60 includes a center conductor member 61 which is joined at one end to the delay line finger element 62 disposed at the end of the interaction path adjacent to the collector electrode 55. Inner conductor member 61 may be fabricated of any metallic material having a coeflicient of expansion compatible with the electromagnetic energy absorbing attenuating material and in an illustrative embodiment molybdenum was selected as having the requisite characteristics. An ideal resistive material for attenuation of the electromagnetic Wave energy was found to be silicon carbide. Cylindrical member 63 composed of this material is disposed in contact With the inner conductor member 61 and the short circuit end 64 of the outer conductor member 65 which is mounted and appended to the envelope wall member 24. Outer conductor member 65 is preferably of a high conductivity metal such as copper which is similar to the conductive material commonly employed in traveling wave tube envelope walls.

In the electrical considerations for the coaxial attenuator structure it is of paramount importance that the impedance of the coaxial line be closely matched to that of the substantially parallel plate delay line in order to eliminate reflections of electromagnetic wave energy which will result in undesirable spurious oscillations. In the termination of electromagnetic energy transmission circuits with a resistor element the characteristic impedance of a line in ohms (Z when the value of R for the series resistance of the line is zero is derived from the classical equation log where D is the inside diameter of the outer conductor and d is the diameter of the resistor. In oscillator devices of the type illustrated in FIGS. 1 and 2 when operated in a microwave radio frequency range of 900 to 1500 megacycles the characteristic impedances of the delay line propagating structure is approximately 150 to 180 ohms. The coaxial attenuator termination therefore to be provided with a matching impedance requires the ratio of the diameters of the inner and outer conductors to be such that either the outer conductor will be unusually large or the center conductor would be unusually small. The combined configuration would destroy the desired circuit parameters and would not dissipate the heat generated at the high power levels desired. A step or impedance matching transformation means is therefore desirable to couple the high delay line impedances to a more useful coaxial impedance, illustratively 50 ohms. Such transformation means are provided by a stepped portion 66 provided in the inner walls of outer conductor 65 concentrically disposed about the inner conductor member 61. The lumped attenuator material rapidly absorbs the electromagnetic energy and the heat generated is rapidly dissipated by the thermally conductive copper material of the outer conductor walls at the short circuit end.

To further increase the heat dissipation capability the inner walls of outer conductor member 65 may be gradually tapered adjacent the terminal end 64 as at 67 to provide a larger bulk of conductive material as shown in FIG. 3. In this as well as subsequent illustrations similar or corresponding parts have been designated by the same reference numerals as those shown in FIGS. 1 and 2.

In FIG. 4 the thermal and power handling capability of the attenuator arrangement 60 incorporating resistor element 63 is enhanced by external coolant circulation means designated generally 70 which will now be described. In this embodiment a coolant jacket is defined by opposing upper and lower wall members 71 and 72 together with a lateral wall member 73 joined to the outer peripheral Wall of the envelope member 24. Inlet port 74 and outlet port 75 are provided in wall 73 for ingress and egress of the selected coolant which is circulated by conventional means (not shown) in contact with the outer walls of the outer conductor member of the coaxial attenuator 65. By means of any of the known coolants the traveling wave device may be utilized at even higher power levels than those attainable without conduction cooling. To further enhance the efficiency of the thermal conduction the outer walls of member 65 are provided with fins 76 circumferentially disposed about this member. The increased radiative surfaces exposed to the coolant will permit higher power levels of operation.

In FIG. 5 still another configuration of a lumped coaxial attenuator configuration is shown. The delay line finger element 62 is terminated by inner conductor member 61 which is in turn joined to a bulk resistive attenuator structure 80 of a silicon' carbide composition having tapered walls 81. The outer conductor member 82 is provided with similar tapered walls 83. The resistor 80 is terminated in wall member 84 which is joined to the outer conductor 82 by any conventional means. In this embodiment the resistor 80 is firmly bonded as by brazing to a recessed portion of member 84 to provide for good heat conductivity. The tapered or flared configuration may illustratively be from a diameter at the apex of resistor 80 of .130 inch to a value of .350 inch at the base of the resistor.

When resistors are utilized for terminating an electromagnetic wave energy propagating transmission line the principal cause of standing waves is the presence of reactance. In a paper entitled Radio-Frequency Resistors As Uniform Transmission Lines by D. R. Crosby and C. H. Penneypacker, Proceedings of the I.R.E., February 1946, pp. 62P-66P, a theoretical analysis of the classical transmission-line equation includes a plot of curves shOW- ing how standing-wave ratio varies with frequency. FIG. 6 of the drawings is now referred to with the standing wave ratio plotted along the vertical coordinate and a ratio R/Z plotted along the horizontal coordinate. The value of R/Z is determined by the ratio of the resistor diameter to the diameter of the transmission line surrounding the resistor. From this group of curves it will be noted that the standing wave ratio approaches unity at a value of approximately R/Z 3 which is approximately 1.73. From the referenced paper it is ascertained that the term R/Z is independent of frequency. In the determination of the frequency characteristics of the resistor then another term U). which is proportional to frequency may be calculated from the equation:

l fmc 11,800

where l is the length of the resistor in inches, A is the free space Wavelength in inches and fmc is the operating frequency in megacycles. It is desirable to operate in a frequency range where the standing ratio will be at the highest value approaching unity. Curve 86 or the value of l/)\ equal to 0.05 indicates that the standing Wave ratio will be 0.97 at the optimum value of R/z If this value of l/ is put in the foregoing equation it will be noted that a resistor 1 inch long would have good characteristics up to 600 megacycles. Since most high power traveling wave tubes operate up to from 1200 to 1500 megacycles a resistor 63 in FIGS. 1 through 4 would have an approximate length of .400 to .500 inch. Further, the optimum diameter proportions would be fixed at .130 inch for the resistor element and approximately .260 inch for the inner diameter of the surrounding conductor to result in a value approximating the /3 to achieve the optimum standing wave ratio. It will also be evident that as the length of the resistor is increased, for example to 12 inches, the coaxial termination would result in maximum operation at frequencies of only up to 50 megacycles.

Another modification which may be practiced in the present invention is illustrated in FIG. 7. In this embodiment the coaxial attenuator termination 90 in a device similar to that illustrated in FIGS. 1 and 2 is directly coupled to the next to the last delay line finger element 91 and the coupling point is spaced approximately onequarter of an electrical wavelength away from the terminal portion of the over-all delay line structure. Such one-quarter wavelength spacing provides a radio frequency choke arrangement which coupled with the impedance matching transformer means 66 will further aid in the reduction of the undesired reflections of electromagnetic wave energy.

Referring next to FIG. 8, this configuration is a linear delay line including two

parallel bars

93 and 94 which support opposing series of straight

interdigital finger members

95 and 96 to thereby define the serpentine electromagnetic energy path. The coaxial attenuator 97 is provided with an inner conductor 98, energy absorbing resistor 99 and outer conductor 100 with the impedance matching transformer means defined by the wall 101. The numerous other previously described modifications may be employed for the removal of the heat generated in the walls of the attenuator, such as a surrounding coolant jacket.

Referring now to FIG. 9, a modification of a bulk type attenuator structure having tapered walls similar to that shown in FIG. will now be described. A lossy ceramic member 102, for example alumina, is provided with tapered walls 103. The inner conductor 104 joined to the appropriate delay line element is disposed in contact with the apex of the tapered dielectric body. The bonding of the lossy material to the metallic inner conductor 104 to withstand the wide temperature range of operation will be aided by a serrated conductive sealing member 105, illustratively of copper, united by known brazing techniques to the bulk material. One of the inherent qualities of electromagnetic wave energy is the relatively shallow depth of penetration of the current which enables relatively thin films to be employed in the provision of a resistance in the transmission line. A resistive layer 106 deposited on the tapered walls of the bulk lossy material 102 may be for example carbon having a resistance value of approximately 50 ohms. Such a thin film layer 106 where a high density alumina material is employed may be deposited by exposing the ceramic material to a heated atmosphere of benzene. The deposition of the thin resistive film may be accurately controlled to achieve any desired values of resistance. Another sealing member 107 of the serrated configuration and similar material unites the high density bulk member 102 to the short circuit end 108 of a highly conductive metal for the rapid removal of the heat absorbed in the resistance layer and ceramic member.

FIG. illustrates a modification in the structure shown in FIG. 9 and where applicable similar numerals will indicate similar structure. A hollow lossy dielectric member 109 which may be provided in a tapered or circular configuration is mounted concentrically on a relatively large conductive body 110 of a high thermal conductivity material such as copper. The short circuit end of the body 110 is indicated at 111. The heat generated in the lossy material by the energy absorbed in the thin film layer 112 deposited on the outer walls of the hollow ceramic body 109 will be rapidly conducted away from the terminal end of the slow wave structure in communication with the inner conductor 104.

There is thus disclosed a novel coaxial attenuator of the bulk or thin film type for absorption of electromagnetic energy propagating in a slow wave transmission circuit. Reflections of such energy have been substantially reduced and through the provision of transformer means matching the impedances of the coaxial attenuator structure to the impedances of the slow wave circuit no redesign of the latter is required. The primary advantage of the present invention resides in the fact that through the provision of a resistor element at the terminal end of the energy path the overall length of the coaxial attenuator arrangement will be only a fractional part of the operating wavelength of a traveling wave device. Hence, structures as short as one inch are capable of dissipating extremely high temperatures and many watts of DC electrical energy generated in the terminal components. Such disclosed structure has enabled traveling wave devices to be operated efiiciently in continuous wave operation in the generation of high output power, particularly at the lower frequency band of the electromagnetic wave energy spectrum. The disadvantages of the prior art plating techniques requiring closer delay line element spacings as well as smaller cross-sectional areas for optimizing of the electrical characteristics of the slow wave transmission circuit when the attenuating material is incorporated directly in the finger elements have been substatnially eliminated. Numerous other materials such as pyrographite film layer coatings as well as beryllium oxide ceramic materials may also be utilized in the practice of the invention to further enhance the absorption and thermal conductivity capabilities.

The invention including the numerous modifications and alterations evident to those skilled in the art is accordingly to be interpreted broadly in accordance wth the scope and spirit as set forth and defined in the appended claims.

What is claimed is:

1. In combination:

a metallic envelope;

an interdigital delay line within said envelope for propagating electromagnetic energy at a predetermined frequency;

means for generating and directing a beam of electrons along a path adjacent to said delay line;

and means for terminating said delay line and absorbing electromagnetic energy comprising a coaxial attenuator extending laterally outside said envelope and having an inner conductive member connected to a discrete point of said delay line;

an outer conductive member concentrically disposed about said inner conductor and having a short-circuited end wall;

said outer conductive member further defining wall structure for substantially matching the impedances of said delay line and coaxial attenuator;

a member of a highly resistive material disposed coaxially with said inner conductor and contacting said short-circuited end wall;

said coaxial attenuator having a length of a fractional part of a wavelength of said predetermined frequency;

and means for circulation of a coolant along the outer walls of said outer conductor concentrically disposed about said envelope.

2. In combination:

a metallic envelope;

and means for terminating said delay line and absorbing electromagnetic energy comprising a coaxial attenuator having an inner conductive member connected to a discrete point of said delay line;

an outer conductive member concentrically disposed about said inner conductive member and having a short-circuited end wall;

a member of a highly resistive material disposed coaxially between said inner conductive member and said short-circuited end wall, the diameter of said resistive member being substantially equal to the diameter of said inner conductive member and the ratio of the resistance of said resistive member to the line impedance determined by the ratio of the diameters of said resistive member and said outer conductive member being substantially equal to the square root of 3;

said resistive member further having an over-all length of a fractional part of a wavelength of said predetermined frequency.

3. In a traveling wave device having a slow wave transmission structure for propagating electromagnetic energy at a predetermined frequency, means for absorbing said energy directly coupled to said slow wave structure comprising:

a coaxial attenuator including an inner metallic member connected at one end to a discrete point of said slow wave structure;

an outer cylindrical metallic member concentrically disposed about said inner member and terminating in a short-circuited end wall;

a member having a high resistance value to electrical energy joined at one end to said inner member and at the opposing end to said end wall;

said resistive member having an over-all length of a fractional part of a wavelength of said frequency and the ratio of the resistance of said member to the line impedance determined by the ratio of the diameters of said resistive member and said outer member being substantially equal to the square root of 3.

4. A- traveling wave device according to claim 3 wherein said resistive member has tapered walls and the inner walls of said outer conductor are similarly tapered and coextensive with said member.

5. A traveling wave device according to claim 3 wherein said resistive member consists of silicon carbide.

6. A traveling wave device according to claim 3 wherein said resistive member comprises a body of a lossy dielectric material having a substantially thin layer of a high electrical resistance material deposited on the outer peripheral Walls.

7. A traveling wave device according to claim 3 wherein said resistive member comprises a hollow cylindrical body of a lossy dielectric material mounted on a highly conductive metallic member and a substantially thin layer of a high electrical resistance material deposited on the outer peripheral walls of said hollow body.

References Cited UNITED STATES PATENTS 2,839,730 6/1958 Rosenberg 33322 2,863,092 12/1958 Dench 3153.5 X 2,922,918 1/1960 Wasserman 3 l53.5

HERMAN KARL SAALBACH, Primary Examiner PAUL L. GENSLER, Assistant Examiner US. Cl. X.R. 31539.3; 333-81