US2661441A

US2661441A - High-frequency amplifier

Info

Publication number: US2661441A
Application number: US794939A
Authority: US
Inventors: George E Mueller
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1947-12-31
Filing date: 1947-12-31
Publication date: 1953-12-01
Anticipated expiration: 1970-12-01
Also published as: BE486035A; FR969267A

Description

Dec. l, 1953 G, E, MUELLER l 2,661,441.

HIGH-FREQUENCY AMPLIFIER Filed Dec. 3l. 1947 2 /5 1.- yELEcrfe/c vEcTo/e /a 1: /o /3 40 lil' FIGS. OUTPUT A 7' TURA/EV Patented Dec. l, 1953 UNITED STATE HIGH-FREQUENCY AMPLIFIER Application December 31, 1947, Serial No. 794,939

18 Claims. l.

This invention relates to devices for amplifying high frequency electrical waves, particularly such devices in which amplincation is had through interaction between an electron stream and a high frequency electric field associated with the waves to be amplied over an extended distance such as a distance of more than a wavelength along the transmission path of the wave.

It is an object of the invention to provide devices capable of producing amplification of high frequency electrical waves over a wide band of frequencies.

Another object of the invention is to provide a high frequency wave amplifying device which shall be simple, rugged, and relatively easy of manufacture.

There have been proposed electron beam amplifying devices wherein amplification is achieved by the interaction between an electron stream and apure traveling wave, that is, a wave which progresses continually in a single direction without reflection, along an extended path. The arrangement is such that the traveling wave has electric eld components in the direction of the n electron stream and there is substantial equality between the speed of propagation of the wave in the direction of the stream and the speed of the electrons in the electron stream. Under such conditions the electron stream reacts on the electric field and the electric field reacts on the electron stream in'a manner such that the wave increases in amplitude as it progresses along the path. Since the interaction is continuous, the amplifying action may take place over a relatively wide range of frequencies.

The present invention is concerned with means whereby the substantial equality between the speed of the electrons and the speed of the traveling waves may be achieved. Particularly, the invention contemplates a wave transmission path incorporating material having a high dielectric constant along the path traveled by the waves to be amplified whereby the phase velocity oi the waves may be reduced to a magnitude which may be readily produced in a concentrated stream of electrons. One embodiment of the invention utilizes a wave transmission path composed of a circular wave guide having a hollow core of high dielectric material such as titanium dioxide. `The wave to be amplified is propagated along this transmission path in a predominantly transverse magnetic mode and longitudinal coinpcnents of electric intensity associated with the wave interact with an electron stream passing through the hollow central portion of the core to produce the amplifying action. Another embodiment of the invention utilizes a wave transmission path composed of a rectangular wave guide having a hollow core of the high dielectric material and propagating the traveling wave in a predominantly transverse electric mode.

It is a feature of the invention that the transmission path is capable of propagating the waves to be amplified in the desired mode over a wide range of frequencies so that the inherent wide band amplication capabilities of the amplifying device may be more fully utilized.

The novel aspects of this invention are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may be better understood by reference to the following detailed description taken in connection with the accompanying drawing, in which:

Fig. l shows an embodiment of the invention in which the wave transmission path through the amplifying device comprises a rectangular wave guide having a hollow dielectric core;

Fig. 2 is a cross-sectional View of the wave amplifying device of Fig. 1 taken along the line 2-2 and which further illustrates the input wave guide and the wave transmission path;

Fig. 3 shows an embodiment of the invention in which the wave transmission path through the amplifying device comprises a circular wave guide having a hollow dielectric core; and

Fig. 4 is a cross-sectional view of the wave amplifying device of Fig. 3 taken along the line 4 4 and which further illustrates the input Wave guide and the wave transmission path.

Referring now to Fig. l, there is shown a first embodiment of the traveling wave amplifying device of the invention. The arrangement shown comprises an electron beam tube including a Wave transmission path I0 in the form of dielectric wave guide comprising a rectangular metallic shell II having a dielectric core I2 and a hollow interior region I3 about the longitudinal axis I4 of the guide. One end of the wave transmission path IU may be designated as the input end and is provided with an input dielectric wave guide I5 whereby high frequency electrical waves may be introduced into the wave transmission path. The input wave guide I5 may preferably be of rectangular cross-section and joined to the transmission path wave guide I I so as to form an electrically conductive connection. Similarly, at an output end of the wave transmission path I0 there is provided an output dielectric wave guide I6 of rectangular cross-section and conductively joined to the transmission path wave guide I I.

The junctions formed by the wave transmission path I and the input Wave guide I5 and the output Wave guide I5 form right angle corner bends for Waves traveling through the amplifying device. In order that reflection effects at these corners be minimized over a wide band of frequencies, it is desirable that additional means be provided to improve the impedance match at the corners. art and may take the form of metallic corner strips I'I or tuning screws I8 as is shown in the drawing. Impedance match and coupling is also` enhanced by a section I9 of. the dielectric core I2 which extends through the inputvvave guide I5, to a far Wall of the guide. Similarly, at the output end a section 2| of the-dielectric core extends through the output wave guide IB. to a farv wall 22 of the output guide. It may be noted that oi these three dielectric wave guides (I0, the transmission path having the dielectric core I2, the input guide I5, and the output guideA I6), the input and output guides I5 and I6, which may be as shown withv the interior dielectric air having a dielectric constant substantially unity, may have quite different impedance characteristics than the guide I0, which employs the dielectric material I2 having a dielectric constant many times unity.

A source of electrons, which may be an electron gun 23 of well-known type, is provided at the input end of the wave transmission path I of the amplifying device. The electron gun 23 is enclosed by an envelope 24 of glass or quarta which may be joined to the input wave guide I5 by means of a projecting strip 25. A t the output end of the wave transmission path, a collector anode 25 is enclosed by a second similar envelope 21 which may be joined to the output wave guide I6 by means of a projecting strip 28. The input wave guide I5 is closed off by a window 23 of mica or similar material while the output wave guide I5 is similarly closed oii by a window 30.

The entire device is thus sealed off and. may be evacuatedY to the necessary degree.

The electron gun 23 comprises a cathode 3| having a heater 32, a focussing electrode 33, and an accelerating electrode 34. The heater 32 is supplied by a source conventionally represented as a battery 35 While the focussing electrode 33 is maintained at a proper potential for the formation of a concentrated stream of electrons by a source 36. The accelerating electrode 34 and the collector anode 2B are maintained at potentials suitable to produce an appropriate electron stream velocity through the interior region I3 of the wave transmission path I0 by a high potential source 31. The metallic wave guide structure may, if necessary, be maintained at a high potential by a connection 'II to the high potential source 3'I. In practical devices suitable electron stream velocities may be produced by potentials in the order of 1500 to 3000 volts although the exact value will be dependent upon the design of the wave transmission path I0.

The action of the electrodes of the device in forming a concentrated electron stream along the longitudinally axis I4 is aided by a solenoid 3B surrounding the wave transmission path I0 and energized by current from a source 39 so as to provide a strong unidirectional magnetic field along the axis of the device. The field due to the solenoid serves also to prevent deviation of the electron stream from its desired course due to an Such means are well known in theoutside magnetic influence. It will be noted that in order to facilitate the action of the solenoid 38, the wave guides and other metallic portions of the device must be made of non-magnetic materials.

Referring now to Fig. 2 there is shown an illustrative cross-sectional view of the device of Fig. l taken along the line 2-2 through the input wave guide I5. The input Wave guide I5, the mica window 29, and the dielectric core I2 are shown in section while the transmission path guide shell I I is shown in dotted outline surrounding the dielectric core. It will thus be noted that the dielectric guide of the Wave transmission path I0 is.. a. composite structure including a metallic shell lvl, the dielectric core I2, and the hollow interior region I3 surrounding the longitudinal axis I4.

In the operation of the device just described, high requency Waves from an external source are introduced through the input wave guide I5 in such a manner that there. is, induced. the guide, a. traveling Wave having a predominantly transverse electric mode, preferably a TEM; In de. The waves travel through the input wave guide and are bent around the corner formed by the junction of the input wave guide and the Wave transmission path I0. The Wave is then propagated along the wave transmissionV path as a TEN wave having electric vectors such as those illustrated in Fig. 2 as dotted lines.v It Will be realized that the electric waves traveling along the hollow interior region I3 of the dielectric core will beA purely transverse only along the longitudinal axis I4... In the regions between the walls ofthe dielectric core I2 and the longitudinal axis I4, however, there will be longitudinal components of electric eld.

When there is n o wave traveling along the wave transmission path II) the electron stream willv move along the longitudinal central axis I4. However, when a wave is propagated .alongV the Wave transmission path at the sameA order ofv velocity as the electron stream it will be seen that the transverse component of electric field will serve to deflect portions of the electron stream to either side of the longitudinal axis. As the electron stream is deflected from the axial region into the region nearer the walls of the dielectric core, it will be seen that the electrons will enter regions having a longitudinal component of electric intensity. The electron stream will then be velocity varied in accordance with the variations of amplitude of the electric Waves traveling along the wave transmission path. The initial interaction between the electron stream is slight, the wave serving initially only to produce Waves of charge density and velocity in the electron stream. However, as the wave and the electron stream travel along the length of the Wave transmission path and a wave is established in the electron stream, a condition is established in which the wave travels somewhat slower than the electrons forming the modulated stream andthe electrons irnpart energy to the wave in a manner which lncreases the amplitude of the wave at a rapidly increasing rate. Thus the wave reaching the output wave guide I5 has an amplitude greatly in excess of that of the Wave at the input Wave guide I5.

The wave transmission path I0 serves as a path along which the waves to be amplified are propagated in order that they may react with the electron stream iioWing therethrough. In order to perform these functions the wave'transmission path must have characteristics such that the waves to be amplified travel along the path with a phase velocity of the same order as the velocity of the electron stream, and, the wave path must be of sufficient length so that a cumulative interaction between the electron stream and the wave take place for a suiiicient period of time. The rst requirement is satisfied in accordance with the invention by utilizing a wave transmission path having a high dielectric constant. The phase velocity of a wave propagating through a guiding medium is inversely proportional to the square root of the dielectric constant of the medium and it will be seen that reasonable values of phase velocities may be achieved by the use of materials having dielectric constants in the order of 100 at the frequency of the waves to be amplifled. The second requirement is satised by the use of a wave transmission path having a length in the order of thirty to forty wavelengths of the wave to be amplified.

In order to achieve the desired high dielectric constant along the wave transmission path I il, the dielectric core I2 may be formed of a ceramic dielectric material such as, for example, titanium dioxide. Titanium dioxide will exhibit dielectrc constants in the order of 8O to 100 at high frequencies with relatively low losses. While other materials may well be utilized to achieve the desired dielectric characteristics, the alkaline earth titanates are particularly desirable in that they combine a high dielectric constant at high frequencies with a high order of dimensional stability and low attenuation factors. Furthermore, particular impedance characteristics along the wave transmission path may be achieved by appropriate mixtures of the titanates of barium, strontium, or calcium or combinations of those materials with titanium dioxide.

. The amplified wave reaching the output'wave guide i6 may, due to impedance discontinuities, be partially reflected so that a second wave is propagated along the wave transmission path in a direction opposite to the motion of the electron stream. While the mere presence of the reflected wave does not greatly affect the interaction between the electron stream and the wave traveling in the direction of the electron stream, it is essential that suiiicient energy should not reach the input end of the wave transmission path so that'a second reflection would cause the generation of self-sustaining oscillations. It will be realized that by the very nature of the dielectric core l2, a certain amount of dissipation of the wave reflected from the output end will take place `along the length Vof `the transmission path. However, in the event that the `attenuation should be insufficient to dissipate the reflected wave, there may be introduced along the wave transmission path Il] an additional means for absorbing the reflected wave. This alternative means may talee the form of a graphite coating on the dielectric core or an actual admixture of the graphite or similarwave absorbing material with the core material in the process of manufacture. Alternatively, where the core is formed of a ceramic such as titanium dioxide the appropriate dissipative action may be obtained by firing portions of the. cors under reducing conditions.

The dissipative material may preferably be introduced along the` central portion of the core as is shown in Fig. i by the dotted lines 4D and 4I. That such a distribution of attenuation along the wave transmission path is preferable will be apparent from a consideration of the fact that suflicient attenuation may be provided such that both the wave traveling in the direction of the electron stream and the reflected wave will be reduc-ed to very low amplitudes after passing the attenuative portion. The wave traveling in the direction of the electron stream will, however, rapidly increase in amplitude after traversing the attenuating portion of the path due to the cumulative effect of the density variations of the electron stream previously produced by that wave, so that the effective amplification of the device is not reduced. of the action of dissip-ative materials along the wave transmission path of traveling wave amplifying devices generally, reference may be made to United States Patent 2,575,383, issued November 11, 1951 to Lester M. Field.

Referring now to Fig. 3, there is illustrated an embodiment of the invention in which the wave transmission path of the amplifying device comprises a dielectric wave guide of circular crosssection and having a hollow dielectric core. There is shown an electron beam tube which comprises an evacuated container 42 enclosing therein an electron gun d3 of known construction and a collector anode 44 for forming a concentrated stream of electrons along a longitudinal axis l5 of the device. The electron gun 43 comprises a cathode d5 having a heater 4l, a focussing electrode 4S and an accelerating electrode 49. The heater is supplied by a source conventionally represented as a battery while the focussing electrode t8 is maintained at a proper potential for the formation of a concentrated stream of electrons by a source 5I. The accelerating electrode 49 and the collector anode 44 are maintained at potentials suitable to produce anappropriate electron stream velocity along the length of the device by a high potential source 52. A solenoid 53 supplied by a source 54 serves to provide a strong unidirectional magnetic field along the axis of the device.

- There is provided a dielectric wave guide having a shell 55 in the form of a metallic cylinder surrounding the evacuated container 42. Within the metallic cylinder 55 and the -container 42 there is a dielectric core 5S, also of circular crosssection, and having a hollow interior region 51 about the longitudinal axis d5. The wave transmission path thus formed is provided at an input end with an input dielectric Wave guide 53 whereby high frequency electrical waves to be amplified may be introduced into the path and at an output end with an output dielectric wave guide 59 whereby the amplied waves may be transmitted to a load. The portions of the structure just described may be more readily visualized by reference to Fig. 4, wherein a cross-sectional view taken along the line 4 4 of Fig. 3 illustrates the input wave guide 58 and the container 42 in cross-section and the transmission path metallic shell and the dielectric core 56 in outline.

The input wave guide 58 is provided with a projecting cylindrical strip 60 which cooperates with a cylindrical metallic strip 6| within the container l2 to effectively form an open cir-- cuited quarter wavelength stub and hence a low impedance path for waves flowing along an end wall 62 of the input wave guide. Similarly, the output wave guide 59 is provided along an end wall 63 with a projecting strip 64 which in turn cooperates with a cylindrical metallic strip 65 within the container 42.

Optimum coupling and impedance matching between the wave transmission path, formed by For a more detailed description the metallic. guide 55 and the dielectric core 5.5. and the input and .output wave guides 58 and 5.9. over a wide range of frequencies may be obtained bv spacing the longitudinal axis .of the device aftdistance of one-.fourth to one-fifth Wavelength from the ends 66 and 51 of the two guides respectively. Impedance matching and coupling may also be enhanced by projecting portions of the dielectric c ore 56 into the input and output wave guides as is illustrated in Fig. ,3.

In the operation of the amplifying device of Eig. 3, the high frequency waves to be ampliiied are introduced into the input wave guide 58 in such a manner that the waves are propagated along the transmission path formed by the wave guide shell and the dielectric core 56 in a pre.- dominantly transverse magnetic mode. The illustrative arrangement is such that when a wave having a 'IEm mode is induced in the input wave guide 5B, there will be produced in the wave transmission path a wave traveling in a TM01 mode and having electric eld lines such as is show-n by means of dashed lines at 68. The electric field of the traveling wave in the interior region 5T is then predominantly in the direction of the longitudinal axis 45 and the interaction with the electron stream is identical with that which takes place between the longitudinal components ofelectric field and the electron stream in the device of Figs. l and 2.

It is to be understood that the dielectric core 5.6 of the device ofv Fig. 3 is to be formed of titanium dioxide or otherV material having high dielectric constant and low loss in accordance with the considerations set forth with respect to the dielectric core I2 of Fig. 1. Likewise, the dielectric core 56y may be provided with dissipative central portions 68 and 'l0 following the same general consideration discussed in connection with the dissipative portions 40 and 4| of the dielectric core l2.

It will be realized that while the amplifying devices of the invention have been described and illustrated with reference to the lower modes of propagation of the traveling wave along the wave transmission path, higher modes of propagation may be utilized within the spirit of the invention so long as the electric field requirements of the devices are satisfied. Under such conditions it may be desirable to utilize mode limiting barriersv in the wave transmission path in a fashion well known in the art It will also be realized that the rectangular input and output wave guides shown are by way of illustration and that circular wave guides or concentric transmission lines may readily be utilized.

What is claimed is:

l. An electron discharge device comprising a length of dielectric wave guide having, extending therealong, a hollow central core composed of a tubular member of material having a dielectric constant substantially greater than unity and forming a portion of a non-resonant high irequency wave transmission circuit, s aid length of wave guide having an input end and an output end and substantially impedance matching terminations at each said end, means for applying an electric wave to said input end, whereby the wave is transmitted through said length of delectrio guide to the output end at a phase velocity dependent upon the magnitude of said dielectric constant, and means for producing a stream of electrons along a path through said length of wave guide in the region of the traveling electric eld associated with said transmitted wave and in the 'direction of travel of said field at a velocity substantially the same as said phase velocity.

2. An electron discharge device. comprising a length of dielectric wave guide having, extending therealong, a hollow central core composed of a tubular member of material having a dielectric constant substantially greater than unity and forming a portion of a non-resonant high frequency wave transmission circuit, said length of wave guide having an input end and an output end and substantially impedance matching terminations at each said end, means for applying an electr-ic wave to said input endy whereby the wave is transmitted through said length of dielectric guide to the output end at a phase velocity dependent upon the magnitude of said dielectric constant, means for producing a stream of electrons alonga path through said length of wave guide in the region of the traveling electric field associated with said transmitted wave and in the direction of travel of said field at a velocity substantially the same as said phase velocity, and high frequency wave attenuatng means distributed along said path and coupled to said eld therealong in the region traversed by said electron stream.

3. An electron discharge device comprising a length of dielectric wave guide of material having a dielectric constant substantially greater than unity and forming a portion of a nonresonant high frequency wave transmission circuit, said length of Wave guide having an input end and an output end and substantially impedance matching terminations at each said end, means for applying an electric wave to said input end, whereby the wave is transmitted through said length of dielectric guide to the output end at a phase velocity dependent upon the magnitude of 4said dielectric constant, and means for producing a stream of electrons along a path through said length of Wave guide in the region of the traveling electric field associated with said transmitted Wave andA in the direction of travel of said field at a velocity substantially the same as said phase velocity, one of said impedance matching terminations comprising another length of dielectric wave guide.

4. An electron discharge device comprising a length of dielectric wave guide of material having a dielectric constant substantially greater than unity and forming a portionY of a nonresonant high frequency wave transmission circuit, said length of wave guide having an inputr end' and an output end and substantially irnpedance matching terminations at each said end, means for applying an electric wave to said input end, whereby the wave is transmitted through said length of dielectric guide to the output end at a phase velocity dependent upon thcmaenitude of said dielectric constantend means fpr producing a stream o electrons along a path through said length o f wave guide in the region of the traveling electric tield associated with Said transmitted Wave and in the direction of travel of said ekld at a velocity substantially the same as said phase velocity, one of said impedanceY matching terminations comprising another length of dielectric Wave guide of material having a di. electric constant substantially lower than thatof the material of the first-mentioned length of wave guide coupled to said first-mentioned length of wave guide and having a conductingr shell into which an end portion of the dielectric material of the first-mentioned length o1 wave guide projects.

5. The invention in accordance with claim 1, the said dielectric material of the said wave guide being substantially composed of titanium dioxide.

6. An electron discharge device comprising a length of dielectric wave guide having a central core, extending therealong, composed of a tubular member of barium titanate and forming a portion of a non-resonant high frequency wave transmission circuit, said length of wave guide having an input end and an output end and substantially impedance matching terminations at each said end, means for applying an electric wave to said input end, whereby the wave is transmitted through said length of dielectric guide to the output end at a phase velocity dependent upon the magnitude of said dielectric constant, and means for producing a stream of electrons along a path through said length of wave guide in the region of the traveling electric eld associated with said transmitted wave and in the direction of travel of said field at a velocity substantially the same as said phase velocity.

7. An electron discharge device comprising a length of dielectric wave guide having a central core, extending therealong, composed of a tubular member of barium-strontium titanate and forming a portion of a non-resonant high frequency Wave transmission circuit, said length of wave guide having an input end and an output end and substantially impedance matching terminations at each said end, means for applying an electric wave to said input end, whereby the wave is transmitted through said length of dielectric guide to the output end at a phase velocity dependent upon the magnitude of said dielectric constant, and means for producing a stream of electrons along a path through said length of wave guide in the region of the traveling electric eld associated with said transmitted wave and in the direction of travel of said eld at a velocity substantially the same as said phase velocity,

8. A device according to claim 3 in which said other length of wave guide and the rst-mentioned length of wave guide are coupled effectively end to end, whereby energy transfer between them is longitudinal in both and the electric mode of transmission is the same in both said lengths of guide.

9. A device according to claim 3 in which said other length of wave guide and said rst-mentioned length of wave guide are coupled side to end, whereby energy transfer between them is laterally in one guide and longitudinal in the other and the electric mode of transmission is diierent in the two.

10. The invention in accordance with claim 2, said wave attenuating means comprising a layer of graphite incorporated in said guide along the central portion of said extended path.

11. In a wave amplifier, a wave guide having extending therealong a hollow central core composed of a tubular member of material having a dielectric constant substantially greater than unity, means for producing a concentrated stream of electrons through the hollow core of said guide, means for transmitting high frequency waves to be amplied along said guide, said guide being adapted to cause a relatively continuous inter- 10 action between the electric field of said waves and said electron stream, and wave dissipative material incorporated along said guide in the region of said interaction.

12. The invention in accordance with claim 11, the said dissipative material being embodied in said dielectric core.

13. A device according to claim 3 in which along the center of the guide, the lines of force of the traveling electric field are directed predominantly transverse to the direction of travel.

14. A device according to claim 3 in which along the center of the guide, the lines of force of the traveling electric eld are directed predominantly in the direction of travel.

15. In a wave amplifier, a dielectric wave guide having, extending therealong, a hollow central core composed of a tubular member of material having a dielectric constant substantially greater than unity, means for producing a concentrated stream of electrons through the hollow core of said guide, means comprising input and output dielectric wave guide portions coupled to said first-mentioned wave guide for transmitting high frequency waves into and out of said rst-mentioned guide, said first-mentioned guide being adapted to cause a relatively continuous interaction between the electric field of said waves and said electron stream, and wave dissipative material incorporated along said guide in the region of said interaction.

16. A Wave amplifier according to claim 15 in which the input and output ends of the firstmentioned guide are coupled to the sides of the input and output guide portions, respectively.

17. An amplifier according to claim 15 in which the lines of force of the electric eld are substantially transverse along the center of the firstmentioned guide.

18. An amplifier according to claim 15 in which the lines of force of the electric field along the center of the rst mentioned guide are substantially in the direction of the longitudinal axis of that guide.

GEORGE E. MUELLER.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,957,538 Jensen May 8, 1934 1,979,668 Boddie Nov. 6, 1934 2,122,538 Potter July 5, 1938 2,153,728 Southworth Apr. 11, 1939 2,300,052 Lindenblad Oct. 27, 1942 2,304,540 Cassen Dec. 8, 1942 2,367,295 Llewellyn Jan. 16, 1945 2,413,608 Di Toro Dec. 31, 1946 2,575,383 Field Nov. 20, 1951 FOREIGN PATENTS Number Country Date 508,354 Great Britain June 29, 1939 OTHER REFERENCES Article by J. R. Pierce, pp. 439-442, inclusive, Bell Lab. Record for December 1946. Copy in Div. 54.