US2626371A

US2626371A - Traveling wave tube attenuator

Info

Publication number: US2626371A
Application number: US39117A
Authority: US
Inventors: Guy F Barnett; Jr Walter H Chudleigh
Original assignee: Philco Ford Corp
Current assignee: Space Systems Loral LLC
Priority date: 1948-07-16
Filing date: 1948-07-16
Publication date: 1953-01-20
Anticipated expiration: 1970-01-20

Description

Patented Jan. 20, 1953 TRAVELHNG WAVE TUBE ATTENUATOR Guy F. Barnett and Walter H. Chudleigh, Jr.,

Philadelphia, Pa., assignors to Philco Corporation, Philadelphia, Pa., a corporation of Penn- Sylvania Application July 16, 1948, Serial No. 39,117

Claims.

The invention herein described and claimed relates to improvements in helical wire transmission lines adapted for the propagation of electromagnetic microwaves and, more particularly, to improvements in helical wire transmission lines as used in beam traveling wave tubes.

The present invention constitutes an improvement over the traveling wave tube described in the co-pending application of William Bradley and William H. Forster, Serial No. 763,010, filed July 23, 1947.

The operation of the traveling wave tube depends upon the interaction between an electric field and an electron stream, both of which are traveling in the same direction and at approximately the same velocity. While an electric iield ordinarily travels at about the speed of light, electrons ordinarily travel at a much slower speed. Extremely high voltages of the order of 10,000,000 volts are required to propel an electron stream at a velocity approaching that of light. In order that the velocity of the traveling wave be of the same order as that of an electron beam obtainable with a reasonable voltage, there is included within the evacuated envelope of the traveling wave tube a transmission line or helix whose function is to produce an axial electric field whose velocity is substantially less than that of the applied electric field. The applied electric iield still travels at approximately the velocity of light, but in following the turns of the helix its axial velocity is reduced by a factor of approximately the ratio of the length of helical path to the length of axial path; for example. in some of the traveling wave tubes now being tested and used experimentally, the helical path is 15 times longer than the axis of the helix, hence the axial electric field travels down the helix at about oneflfteenth the speed of light. This is substantially the saine as the velocity of electrons propelled by a potential of the order of 1160 volts.

The traveling wave tube appears to be particularly suited for use as an ampliiier of microwaves over an extremely wide band of frequencies, the width of the operating band being largely determined, in the present state of the art, by the impedance matches obtainable at the input and output ends of the helix, i. e., by the impedance matches at the junction between each end of the helix and the external wave guides, coaxial transmission lines, or other external system. ln the present state of the art it is impossible, or at any rate impractical, to match impedances at both ends of the helix over the whole band of frequencies involved. Reflection of energy of substantial magnitude at some frequencies within or without the intended operating band is therefore practically unavoidable.

Accordingly, to prevent the development of oscillations caused by such impedance mismatches and/or to prevent non-uniform gain at frequencies without the intended operating band, the traveling wave tube described in the above-identified copending application employs a helix constructed of two different materials having respectively a high attenuation constant and a low attenuation constant, the material having the high attenuation constant being placed near the central portion of the helix and the material having the low attenuation constant being placed near the ends of the helix. Alternatively a single wire having a high attenuation constant may be used, the sections of said wire located at the ends of the helix being coated with a material having low attenuation constant. As the gain of the traveling wave tube is increased to practical values the required aforementioned attenuation increases to such high values that this attenuation can no longer be obtained by prior art methods without prohibitively increasing the length of the helix.

In accordance with the present invention improvements are made in the helical transmission line, or helix, whereby a high value of attenuation can be introduced therein Without substantially increasing the length of the helix. These improvements are made without increasing the tendency of the traveling wave tube to oscillate and without impairing the uniformity of amplication over its operating band.

It is therefore an object of the invention to provide a helical wire transmission line having a higher attenuation per unit length than has been attainable heretofore with previously known arrangements.

It is another object of the present invention to construct an improved traveling wave tube capable of providing increased gain without oscillation.

The invention has as a further object the provision of an improved traveling wave tube capable of providing more uniform gain.

It is a further object of the present invention to construct an improved traveling wave tube capable of providing increased and more uniform gain over a wider range of frequencies.

A still further object of the present invention resides in the provision-in a traveling wave tube-of a helical Wire transmission line, or helix, so constructed and arranged as to attenuate. in

comparison with prior art helices, a considerably larger portion of the energy reected from the output end to the input end.

To the foregoing general ends it is a specific feature of the present invention to provide, in a beam traveling wave tube. a helical conductor having a high-resistance attenuating section, and shunt means providing leakage conductance between the turns of the attenuating section only of said helical conductor.

These and other objects and features of the present invention will become apparent from a detailed consideration of the following description and accompanying drawings wherein:

Figure 1 is an illustration, partly in section, of a traveling wave tube embodying the present invention;

Figure 2 is a sectional View taken along line 2-2 of Figure l;

Figure 3 is a representation on an enlarged scale, and in perspective, of a specic embodiment of the helical transmission line provided by the present invention; and

Figure 4 shows in perspective an alternative embodiment of the improved helical transmission line of the present invention.

Referring now to Figure l, there is shown a traveling wave tube I comprising a heater II, cathode I2, accelerator anode I3, collector anode I4, a helical transmission line I5 and a plurality of supporting rods I6, I1 and I8 constructed of a suitable dielectric material, such as glass, all contained within an evacuated glass envelope. A portion of the envelope is in the form of a long barrel I9. Heater I I, cathode I2 and accelerating anode I3 constitute an electron gun assembly similar to that used in conventional cathode-ray tubes. The helix I5, the supporting rods I6, I1 and I8 and the collector anode I4 are contained Within the barrel I9, as shown, and are centrally l supported within said barrel by conventional means (not shown).

An input wave guide 20 and an output wave guide 2| terminate at the tube in circular sections through which the barrel I9 of tube I0 passes. While not essential, a tubular shield 22 may, if desired, connect the circular terminating sections of the input and output wave guides. Furthermore, magnetic focusing coils, not shown, will ordinarily be located on the gun side of the input wave guide 20 and also along barrel I9 between the input and output wave guides.

Accelerator anode I3 is connected to a source of suitable positive potential B+, collector anode I4 and helical transmission line or helix I5 are connected to ground. Cathode I2 is connected to a suitable negative potential B-, by way of heater II; and heater II is connected to a conventional A battery, or its equivalent.

The microwaves to be ampliiied are applied to the traveling wave tube by way of wave guide 20 and are taken from the tube by way of output wave guide 2|. It will be understood, of course, that, so far as the present invention is concerned, transmission lines or resonant structures may be employed in lieu of the wave guides shown.

The helical transmission line, or helix, I5 comprises a coil of wire supported in a coaxial position within the glass barrel I9 of tube I0, as by glass rods I6, I1 and I8 disposed parallel to the axis of the tube. The helix I5 terminates at both ends in short stubs 23 and 24, formed of highly conductive material. The stubs 23 and 24, which project from the helix in a direction parallel to the axis of the helix, are so located with respect to the input and output guides as to project into said guides in a direction parallel to the electric fields. The stubs are held in place, close to the glass envelope, by

metallic collars

25 and 26.

As thus far described, the apparatus identified above is well known, and it is deemed unnecessary to describe it in further detail. However, because the operation of the prior type of tube is similar, in many respects, to that of the improved traveling-wave tube, presently to be described, and since an understanding of the operation of the prior-art tube will aid in understanding the present invention, a brief description of the operation of the device, as thus far described, will be given.

As shown in Figure 1, a beam of electrons 21 is projected from the gun structure down the center axis of the helix I5. An electromagnetic field, or wave, is propagated along input wave guide 20 in the direction of tube I0, as shown by the arrow in Figure 1. This wave enters tube I0 and is picked up by stub 23 functioning as a receiving antenna. The wave then travels along the helix I5 toward the right, in the direction in which the electrons of beam 21 are moving. The wave increases in amplitude as it travels along the helix, and the wave is reradiated in amplied form into output wave guide 2I by stub 24 functioning as a transmitting antenna.

The increase in the amplitude of the electromagnetic wave hereinbefore described as it travels along the helix is derived from the inter-change of energy between the electron stream traveling axially down the helix and the electric field of the wave established along the axis of the helix, the interaction being such that at any instant more electrons are decelerated than are accelerated; hence, more energy is absorbed by the wave from the electron beam than is given up by the wave to the electron beam.

As previously indicated, the traveling-wave tube is capable of operation as an amplifier of microwaves over an extremely wide band. For example, a prior-art tube has a substantially flat frequency response over a range of 800 megacycles. Outside this range, the output drops off primarily because of impedance mismatch at the ends of the helix. It has ben found either irnpossible, or infeasible, to match the impedances of the helix and external wave guides over the entire range of frequencies for which the gain of the tube may be greater than one; and this includes frequencies, such as noise frequencies, outside the preselected or intended operation band. It is contemplated, therefore, both by the prior art and by the present invention, that matching of impedances at the input and/or` output ends of the helix will not be ordinarily achieved over the wide band of frequencies involved and that reflections of substantial magnitude will occur at some of the frequencies.

In view of the above, the helical transmission line of the prior art traveling-wave tube is so constructed that, in the absence of the electron stream, the transmission loss along the helix, at any particular frequency, is greater than the gain obtained from the tube at that frequency when the tube is operating in a normal manner, i. e., operating with a beam of normal current. A transmission loss of .this relative magnitude appears to be necessary, at least in most cases, in order to insure that energy reilected from the output end of the helix, upon reaching the input end, is of substantially smaller magnitude than .the input energy there obtaining at that frequency, thus to avoid the development of oscillations at frequencies, both within and outside ythe normal operating band.

This transmission loss may be obtained by constructing selected portions of the helical transmission line or helix of a material having a high attenuation constant, as described in the aboveidentified copending application of William E. Bradley and William H. Forster.

As the art advanced, however, higher and higher values of gain were obtained, until it was no longer possible to insert suiiiciently high attenuation by these methods. With the prior art methods, even when using materials with the highest known attenuation constants in the construction of the helix, it would have been necessary still further to increase the number of attenuating turns in the helix to obtain sumcient attenuation. Such an increase in the section of ythe helix made of attenuating material would have resulted either in a shortening of the section over which gain is obtained, and therefore in a decrease in gain, or alternatively in a lengthening of the entire helix, and a consequent decrease in bandwidth when used as a wideband microwave amplifier as hereinbefore described.

Referring now to Figure 3, there is shown, on an enlarged scale, a helix with supporting rods constructed in accordance with the present invention. The helix is illustrated as having seven turns. However, the number shown has no limiting sgnicance the seven turns shown are merely representative of the substantially larger number of turns of which the helix will ordinarily be comprised. For example, one tube with which We have been experimenting has a helix of 280 turns.

The helix illustrated in Figure 3 is comprised at each end, of material having a relatively low attenuation constant, and in the central section, of material having a relatively high attenuation constant, as described in the above-identied copending application. In particular accordance with the present invention, however, the supporting rods are coated With a material having a high attenuation constant, such as a finely divided carbon, over substantially the range occupied by those turns of the helix which are made of material having a high attenuation constant. Satisfactory results have, in practicey been obtained by the use of a carbon suspension available commercially under the trade name Aquadag. A suitable adhesive material may be admixed with said coa-ting to maintain the coating in position on said supporting rods. This adhesive material does not, however, have any substantial eiect upon the electrical performance of the device.

The supporting rods are so coated that the attenuating material contacts each turn of the helix within the range of the coated section, thus providing short connecting paths, or leakage conductance, between the turns of the helix. The section of helix constructed of material having a high attenuation constant, and the coated sections of the supporting rods associated therewith, will hereinafter be referred to as the attenuating section of the helix.

In Figure 2 there is shown a sectional View of the helix and supporting rods, taken along line 2 2 of Figure l. rhe coating of attenuating material on the supporting rods is shown to be in contact with the Wire of the helix and to extend a predetermined distance around the circumference of said supporting rods. The leakage conductance increases as the proportion of coated circumference is increased.

Reference may now be had to Figure 4, which illustrates `an alternative embodiment of the attenuating section of the helix. As is shown in Figure 4, the helix may be enclosed in an envelope of a suitable dielectric material, preferably glass, similar to the barrel I@ of Figure 1. In the present embodiment, however, barrel is serves also as a support for the helix and is at least partially coated, over its inner surface, with a suitable attenuating material, preferably carbon, which contacts the helix. Here again the attenuating material should Contact each turn of the helix over substantially the range occupied by those turns of the helix which are made of material having a high attenuation constant.

Construction of the attenuating section as described hereinbefore, yields much higher attenuation than could have been obtained by using attenuating sections of equal length constructed according to prior art methods. In addition, the attenuating section of the helix when constructed in accordance with the present invention also presents the following advantages.

Considering the helix as a transmission line, it is obvious that a transition, from a section of helix having a low attenuation constant to one having a high attenuation constant or vice versa, would ordinarily introduce reflections which might add to those already present due to the impedance mismatch at the ends of the helix, thus defeating the principal purpose of the attenuating section, which is to `attenuate reiiections. To prevent such reflections it is necessary to make the characteristic impedance of the attenuating section equal to the characteristic impedance of the remainder of the helical transmission line.

As is well known, the characteristic impedance of a transmission line can ybe expressed by the formula,

7| 7'u l Z0:

g-iywc wherein Ze is the above mentioned characteristic impedance, r is the series resistance per unit length of line, g -is the parallel or leakage conductance per unit length, l is the inductance per unit length, and c is the capacitance per unit length of line, The terms 7' and w are standard and well known in the iield of transmission lines and need no further definition here.

Before the introduction of an attenuating section, the r and y of the line may be considered substantially negligible compared to the values of y'wl and y'wc. In accordance with the present invention, an appreciable value of series resistance r is introduced in combination with parallel conductance g having values such as to maintain the characteristic impedance Ze unchanged. This result will be accomplished if for a value of series resistance 1- introduced in the transmission line, a value of g be also introduced sufficient to make the ratio of r to g equal to the ratio of Z to c. In the present invention, the added r is provided by the increase in the attenuation con stant of the material of which the helix is constructed over the range of the attenuating section, and the compensating added g is provided by the attenuating coating on the supporting rods of the helix over the range of the attenuating section.

It is not necessary, in practicing the present invention, that the attenuating material cover an unvarying portion of the circumference of the supporting rods over the entire length of the attenuating section of the helix. Instead the attenuating material may be tapered down to cover a smaller portion of the circumference-or it may even come to 'a point-at either or both ends of the attenuating section. This construction presents certain advantages. Where the attenuating material covers a uniform portion of the above-mentioned circumference, the abrupt change in attenuation constant requires relatively high accuracy in the design and application of the attenuating coating to effect the aforementioned impedance match. With a suitably tapered coating, the permissible tolerances in the design and application of the attenuating coating are considerably increased.

It can be seen that by the proper choice of materials in the attenuating section of the helix and in the supporting rods, as well as by the proper choice of the dimensions of the components of the attenuating section of the helix, a wide range, including very high values, of attenuation can be introduced into the helical transmission line, without causing reflections of energy with the attendant malfunctioning of the traveling wave tube.

Although the present invention has been described with particular reference to certain specific embodiments, it will be understood that the invention ds capable of still other forms of physical expression, and consequently is not limited to the specific disclosure, but only by the scope of the appended claims.

We claim:

1. An electromagnetic microwave transmission line comprising, in combination, a helical wire, an intermediate section of said helical wire being constructed of material having a high attenuation constant, and a plurality of supporting rods for said helical wire, said rods being constructed of a dielectric material and coated with an attenuating material, said attenuating material being in contact with each turn of said intermediate section of helical wire.

2. In an electromagnetic microwave traveling wave tube, a helical wire transmission line comprising, in combination, a helical wire, an intermediate section of said helical wire being con-I structed of material having a high attenuation constant, a plurality of dielectric rods disposed parallel to the axis of said helical wire and supporting said helical Wire in axial alignment, and an attenuating material coating at least a portion of the surface of said support-rods, said attenuating material being in contact with each turn of said intermediate section of helical wire.

3. In an electromagnetic microwave system, a traveling wave tube comprising a helical wire transmission line and means for projecting an electron stream into one end of said helical wire transmission line and along the axis thereof; means for applying an electromagnetic wave to said transmission line at said end; means for radiating said wave in amplified form from the other end of said transmission line; said helical wire transmission line comprising, in combination, a helical wire, an intermediate section of said helical wire being constructed of material having a high attenuation constant, and a plurality of dielectric rods disposed externally adjacent to said transmission line and parallel to the axis thereof and supporting said helical wire in axial alignment, and an attenuating material coating at least a portion of the surface of said support-rods, said attenuating material being in contact with each turn of the intermediate section of said helical wire.

4. In an electromagnetic microwave system, a traveling wave tube comprising a helical wire transmission line having an input section at one end and an output section at the other end, means for projecting a beam of electrons into one end of said helical wire transmission line and along the axis thereof, means for applying an electromagnetic wave to the input section of said transmission line, and means for deriving said wave in amplified form from the output section of said transmission line, said helical wire transmission line comprising, in combination, a helical conductor including a high-resistance intermediate section in which the series resistance r1 per unit length of conductor is high compared to the series resistance r2 in the output section of said helical line, and shunt means providing leakage conductance g1 between the turns of the high-resistance intermediate section of said helical conductor, the leakage conductance g1 being related to the series resistance r1 in the high resistance section in substantial accordance with the formula T1/g1=Z/c, l and c representing, respectively, the distributed inductance and capacitance of the transmission line, the characteristic impedance of the said two sections of transmission line being thereby maintained in substantial equality.

5. An electromagnetic microwave traveling wave tube comprising: a helical wire transmission line; means for projecting an electron stream into one end of said transmission line and along the axis thereof; means for applying an electromagnetic wave t0 said transmission line at said one end; means for radiating said wave from the other end of said transmission line; said helical transmission line comprising, in combination, a helical wire, an intermediate section of said wire being constructed of a material having a high attenuation constant, and means at least partly surrounding said helical wire, said last-named means comprising a dielectric material, a portion of the area of said dielectric material being coated with an attenuating material in contact with each turn of said intermediate section of said helical wire.

GUY F. BARNETT. WALTER H. CHUDLEIGH, JR.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,957,538 Jensen May 8, 1934 2,064,469 Haeff Dec. 15, 1936 2,300,052 Lindenblad Oct. 27, 1942 2,413,608 Di Toro Dec. 31, 1946 2,441,047 Wall May 4, 1948 2,541,843 Tiley Feb. 13, 1951 2,575,383 Field Nov. 20, 1951 2,602,148 Pierce July l, 1952