US3181090A - Delay line for travelling wave tube - Google Patents

Description

April 27, 1965 E. A. ASH 3,181,090

DELAY LINE FQR TRAVELLING WAVE TUBE Filed 001;. 27. 1960 4 Sheets-Sheet 1 H61. FIG. F1 6;. v v 4 Inventor JJSH April 27, 1965 E. A. ASH 3,181,090

DELAY LINE FOR TRAVELLING WAVE TUBE Filed Oct. 27, 1960 4 Sheets-Sheet 2 FEGES.

l I400 I600 [800 'ZOOO 2200 I n venlor A Home y April 27, 1965 E. A. ASH 3,181,090

DELAY LINE FOR TRAVELLING WAVE TUBE Filed Oct. 2'7, 1960 4 Sheets-Sheet 5 Inventor E-A-ASH By P Attorney April 21, 1965 E, A, ASH 3,181,090

I DELAY LINE FOR TRAVELLING WAVE TUBE Filed Oct. 27. 1960 4 Sheets-Sheet 4 /7 mm 3 we A Home y United States Patent 3,181,090 DELAY LINE FOR TRAVELLING WAVE TUBE Eric Albert Ash, London, England, assignor to International Standard Electric Corporation, New York, N.Y. Filed Oct. 27, 1960, Ser. No. 65,304 Claims priority, application Great Britain, Nov. 30, 1959, 40,545/59 7 Claims. (Cl. 333-31) The present invention relates to slow Wave structures such as are incorporated in travelling wave tubes and backward wave oscillators.

The conventional helix slow wave structure has many known disadvantages for tubes of the highest frequencies, particularly from the point of view of manufacturing difificulties. An alternative slow wave structure, or" which there are several variants, consists, basically, in a ladderlike arrangement of half wave resonators, in the form of bars or wires, stretched between parallel plane conducting side walls. A perfectly symmetrical arrangement of this type does not propagate over a finite band of frequencies, but may be made to propagate by introducing some asymmetry in the geometrical arrangement which will make the magnetic and electric couplings between adjacent resonators unequal. The present invention may be regarded, in one aspect, from the viewpoint of this asymmetrical class of slow wave structure. According to the present invention there is provided a slow wave structure which will propagate radio frequency waves over a frequency band whose width exceeds four percent of the mid-band frequency comprising a pair of parallel plane conducting walls and a grating joining the two side Walls along the length of the structure in a plane perpendicular to the side walls with the side walls extending above and below the plane of the grating, and in which the grating is composed of intersecting conducting strands arranged to form a regular pattern of staggered apertures.

Many possible patterns of grating may be used, some more advantageously than others from the point of view of either mechanical construction or of bandwidth. In general it may be taken that for travelling wave ampliher or backward wave oscillator use, a frequency band less than 4% of the midband frequency is of little value. At present we prefer one of two general types of grating pattern: in the first, the pattern is of diamonds, produced by the intersection of two oppositely inclined sets of equidistant parallel strands, with, as a special case, what may be called a half diamond pattern, where the oppositely inclined strands intersect only at the side walls.

The other preferred pattern, which may be regarded as derived from a meander line, has a zig-zag conducting path extending from end to end of the slow wave structure in between the side walls and supporting strands join this meander line at regular intervals to the side walls.

A variant of the meander line structure is produced by a set of equidistant parallel strands stretched from side wall to side wall at right angles to them and shorter strands, parallel to the side walls, each joining an adjacent pair of the transverse strands and off-set from one another so as to form a pattern of apertures similar to the conventional arrangements of bricks in a wall, there being normally a fraction of a whole brick terminating each row. This last mentioned variant can be regarded as a version of the diamond pattern in which the diamonds are distorted into rectangles.

The slow wave structures of the invention have mechanical and electrical characteristics which render them particularly suitable for use either for comparatively low frequency, high power, tubes or for millimetre wave tubes.

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Unlike the helix, no insulation is required between the slow wave structure and surrounding parts. The dispersion characteristics are such as would be suitable for a slow wave structure used in conjunction with a high voltage electron beam, while the strands of a structure can be made in the form of hollow pipes so that it is eminently suitable for use in a high power low frequency travelling wave tube. On the other hand, small gratings can readily be produced by photo-etching techniques, and the structures exhibit strong spatial harmonic components in their propagation characteristics; they are thus very suitable for use as slow wave structures at the shortest wavelengths, for example in a backward wave oscillator at millimetre wavelengths.

The invention will be further described with reference to the accompanying diagrammatic drawings in which:

FIG. 1 represents a perspective view of one end of a slow wave structure according to the invention;

FiGS. 2 to 5 illustrate plan views of the grating of PEG. 1, each showing a different diamond pattern of apertures;

FIG. 6 shows, qualitatively, dispersion characteristics of diamond-apertured structures of the invention;

FIGURE 7 illustrates a meander line pattern of grating according to the invention;

FIG. 8 illustrates a modification of the arrangement of FIG. 7 suitable for water cooling;

FIG. 9 illustrates a brick wall variant of the meander line structure of FIG. 7;

FIGS. 10 to 12 illustrate respective variants of the arrangement'of FIG. 9; I

FIGS. 13 and 14 show measured dispersion characteristics of variously dimensioned diamond and meander line structures respectively illustrated in FIGS. 13a and 14a; 7

FIG. 15 illustrates a means of coupling a slow wave structure according to the invention to a hollow rectangular waveguide; and

PEG. 16 illustrates an embodiment of a slow wave structure according to the invention in a travelling wave tube.

In all its variants the slow wave structure of the present invention comprises, as indicated in FIG. 1, a pair of plane parallel side walls 1 and 2, indicated as spaced apart a distance a, with a conducting grating 3 joining the walls in a plane perpendicular to them and running the length of the structure. The strands of the grating are arranged to produce an array of intersecting conductors forming a regular pattern of staggered apertures. A typical type of pattern is indicated in FIGS. 2 to 5, in all of which there are two sets of equidistant parallel strands stretching from side wall to side wall, the two sets making equal and opposite angles with the normals to the side walls, and in which, except for the extreme end strands of the pattern, each strand is joined at its ends to respective strands of the other set.

Mention has been made above of the resonator type of slow wave structure in which an array of similar half wave resonators in stretched between a pair of parallel side walls. By inclining the resonators to the walls an asymmetry in coupling of the electric and magnetic fields of adjacent resonators is produced and propagation can occur over a limited bandwith. A bandwidth of such or Hartree harmonics, while that-illustrated in FIG. 2

has also a fundamental backward mode which the contrawound helix does not possess. The .side walls are essential constituents of the slow wave structure and determine the low frequency cut-off of the propagation characteristics.

In the pattern of FIG. 2 one set of equidistant parallel strands 4 stretches from side wall to side wall and is joined, at its ends by respective strands of a similar set, strands 4 and 5 being oppositely inclined to the walls. In FIG. 2 the strands of the two sets intersect only at the side Walls and so produce a pattern or triangles which, compared with the other patterns of FIGS. 3 to 5, can be regarded as being a pattern of half diamonds, as indicated by the symbolic legend to the rightof the figm'e.

In FIG. 3 the strands 4 and 5 of the respective two sets intersect one another once intermediate the side walls so as to produce full diamond-shaped apertures between the walls. This pattern we refer to as a single diamond pattern, a complete diamond reaching from side wall to side wall, as represented by the single diamond symbol to the right of the figure. 7

' With each strand of one set intersected twice by strands of the other set, we obtain 'the one and a' half diamond pattern of FIG..4, while with three intersections of each strand the two diamond pattern of FIG. Sis obtained.

It is possible, by increasing the number of intersections, to produce further diamond patterns in whichany number of complete diamonds or of complete diamonds plus a half diamond, joined end to end, fit in the transverse dimension of the slow wave structure. As the number of intersections is increased, however, the thickness of the strands reduces the open area of the grating occupied by apertures. i

The dispersion characteristics of the diamond structures of the present invention are illustrated qualitatively in the Brillouin plot of FIG. 6. Here the angular frequency a of the propagated waves is plotted as ordinate against the phase constant e as abscissa. namely those typical of the half-diamond, one-diamond, and twodiamond patterns respectively. All have the same lower cut-off frequency, denoted by the ordinate 1rC/a, where c is the velocity of light and a is the separation between the conducting planes 1 and 2 of FIG. 1. At this cut-off frequency the phase velocity is equal to the velocity of light, so that the curves intersect the straight line v=c passing through the origin as indicated in FIG. '6, v

being the velocity of propagation along the line. In the Brillouin plot the velocity of propagation v=w/;8 while the group velocity is equal to 7 the slope of the curve. For the half-diamond structure the dispersioncurve of the Brillouin plot is of low slope and is characterised by a branch, shown dotted, in which the slope is negativei.e. the group velocity is oppositely directed to the phase velocity; thus the half-diamond structure exhibits a fundamental backward mode of propagation. The other patterns of FIGS. 3 to 5 do not have a fundamental backward mode of propagatiombut all exhibit strong first spatial harmonic modes. For these harmonic modes the phase'constant ,8 is simply related to the phase constant in the fundamental mode,'so that the Brillouin plots are generally similar to those for the fundamental modes. As the number of diamonds across the pattern is increased; so there is an increase in the upper frequencycut-oif of the slow wave structure- As the pitch p between adjacent wires (FIG. 2) is increased,

Three curves are shown,

in any of the patterns, the velocity of propagation is increased, the velocity increase being nearly proportional to the increase in pitch (except near the cut-off frequencies).

The characteristic impedance of the slow wave structures is high, and, although the bandwidth of, for example, the half-diamond structure, is. somewhat:low, the product of bandwidth and impedance of the circuit turns out to be nearthe optimum for use in a travelling wave amplifier.

Mention has been made above of the mechanical difficulty which may be encountered when an attempt is made to increase the number of diamonds across the width of the slow wave structure. This may be overcome, to a large extent, by modifying the pattern so as to obtain, for example, an equivalent arrangement of rectangles, such as illustrated in FIG. 9. These modified structures can also be regarded as derived from a meander line stretching from end to end of the slow wave structure. In FIG.

7 there is shown a pattern in which a rectangular zig-zag continuous conductor 6 stretches from end to end of the structure midway between the sidewalls 7 and-8. The conductor 6 is supported at each bend by a conducting stub 9 joining. the meander line to the respective adjacent side walls of the structure.

The arrangement of FIG. 7 can be compared to the half-diamond structure of FIG. 2, and its propagation characteristics are generally similar.

There is no need for the meander line to be of strictly rectangular shape; the bends may be rounded and, as is illustrated very diagrammatically in FIG. 8, the grating may be made of hollow piping rather than fine strands. In FIG. 8 a sinous piping 10 replaces the meander line 6 and the struts 9 are replaced by further pipes 11 which join the piping 10 to hollow side walls-12 and 13. Cooling liquid is pumped into the side wall 12 and flows through the piping network to the opposite hollow wall 13.

The pattern of FIG. 7 may be modified by arranging that the side struts extend from wall to wall. We then obtain a pattern such as that of FIG. 9. In FIG. 9 the struts 9 become portions of a set of parallel equidistant strands 14 stretchingnormally between the'side walls, anda set of short strands 15 is arranged parallel to the side walls with each strand 15 joining two adjacent strands of the set 14, adjacent strands 15 being staggered sothat the pattern of apertures resembles that of the conventional briclobuilt wall, each row containing one and a half bricks. In the drawing the strands 15 and the portion of the strands 14 between them has been thickened to show the correspondence with the meander line 6 of FIG. 7. On the other hand, comparison with FIG. 4 shows that the pattern'of FIG. 9 can be regarded as a modified one and a half diamond structure, to which it has similar propagation characteristics. More than one strand 15 can be placed between adjacent strands 14, as for example is illustrated in the FIG. 10, which would correspond to a two and a half diamond structure. .Alternatively the pattern of FIG. 10 can be regarded as a double meander line structure in which two meander lines such as 6 of FIG. 7 can be traced out side by side.

In arrangements otherwisesimilar to those of FIGS. 7 to 10 it is not essential that the grating pattern be orthogonal to the side walls. Thus in FIG. 11 the strands 14 are inclined to the side walls, the strands 15 remaining parallel to the side walls, so that we now have a staggered pattern of parallelograms corresponding to the one and a half diamond pattern of FIG. 4. In the arrangement of FIG. 12 the strands 15 have also been inclined to the side walls, so that there is no longer a complete analogy with the patterns of FIGS. 2 to 5. The propagation characteristics. of an arrangement such as that of FIG. 12 will, however, not be altogether dissimilar to those of the other structures so far described, although the bandwidth will be considerably less than that of the straightforward diamond structures.

Measured dispersion characteristics of several diamond and meander line structures according to the invention are shown in FIGS. 13 and 14 respectively, the size of structure and the frequency range having been chosen purely from the point of view ofease of measurement. In FIG. 13 the ratio v/c of phase velocity to the velocity of light is plotted against frequency in megacycles per second. The curves lettered A to C correspond to the similarly lettered structures illustrated diagrammatically in FIG. 13a. In each case the spacing between the side walls, indicated by a in FIG. 13a, was 12 cm. The distance between adjacent wires is indicated in FIG. 13a by the dimension p, /217, or 2p as the case may be; p=1 cm. in all cases.

The curves of FIG. 13 show, as has been discussed previously in connection with FIG. 6, how the bandwidth increases with an increasing number of intersections of the strands and also shows, in curves A and B and in E and F, the somewhat slight increase in phase velocity obtained when the pitch is increased. Thus curves E and F both relate to a one-and-a-half diamond structure but in curve E the pitch is /2 cm. and in curve F 1 cm.

In FIG. 14 the curves A, B and C relate to structures arranged as indicated schematically in FIG. 14a. In all these the pitch p was 1.7 cms. and the separation between sidewalls 12 cms.; in A and B the dimension d was 6 ems. and b 3 cms., but in C b and d were each 4 cms. Structure A has analogies to the half-diamond structure of FIG. 13, B can be regarded as a modified one-and-aquarter diamond structure, and C as a modification of the one-and-a-half diamond structure.

In connection with the curves of FIGS. 13 and 14 and the experiment structures to which they relate, it is to be emphasized that the absolute values of the frequencies plotted and of the dimensions of the structures given above should not be taken as illustrative of any preferred embodiments of the invention; the frequencies and dimensions were chosen purely from the point of view of convenience in making the measurements involved and in setting up different patterns of grating. Useful design data may, however, be derived from these curves by considering the variation of v/c in terms of the ratio of the frequency of measurement to the lower cut-off frequency, f,,, which, from FIG. 6, is seen to be given by and by using, instead of the pitch parameter p, a generalised parameter obtained by expressing p as a portion of the free space wavelength.

Slow wave structures according to the present invention are very readily coupled to hollow rectangular waveguides. Basically, all that is necessary is to continue the grating a little way beyond the ends of the side walls so that it projects through a slot cut transversely in the broad side of the waveguide. Thus in FIG. 15 a hollow rectangular waveguide is represented at 16 having a transverse slot 17 cut in a broad side 18. The side walls 1 and 2 of a slow wave structure according to the invention are brought up to the wall 18 so that an extension 19 of the grating 3 projects through the slot 17 about half way into the waveguide. The lips of the slot 17 are provided with curved fins 20 which help to provide a smooth impedance transformation between the waveguide and slow wave structure and also act as chokes preventing radiation through the slot 20.

FIG. 16 illustrates the use of a slow wave structure according to the invention in a travelling wave amplifier tube. Since there is no need for any insulation in association with the slow wave structure the tube may be of all metal construction having a rectangular envelope 20 whose sides 21 function as the side walls of the slow wave structure with the grating 3 joined between them. The upper and lower walls should be a wavelength or more spaced from the grating 3 so as not to interfere with the performance of the slow wave structure. At each end of the slow wave structure a waveguide 16 is arranged as in FIG. 15 to serve as input and output feeds respectively. The waveguides are represented as having tuning pistons 22 for adjusting the match between slow wave structure and waveguide and are indicated as being hermetically sealed by means of waveguide windows 23. The waveguides 16 are apertured for passage of an electron beam above and below the grating 3 and are secured to extensions 24 and 25, respectively, of the envelope 20. An electron gun 26 is indicated within the end enclosure 24 and an electron collector electrode 27 is represented within the extension 25.

In the case of applicationof the invention to a backward wave oscillator, the arrangement is generally similar to that of FIG.v 16 except that the waveguide adjacent the electron collector electrode is omitted and the end portion of the grating 3 adjacent the electron collector electrode is coated with lossy material to provide a refiectionless termination for the slow wave structure.

Although several embodiments have been illustrated in the accompanying drawings, numerous other variants of the invention will occur to those skilled in the art. Although the propagation constants will, in all cases, conform to the general characteristics as described with reference to FIG. 6 and to FIGS. 13 and 14, it does not follow that all structures whose geometry appears to :fall within the general geometrical arrangement of embodiments of the invention will necessarily have useful band- Widths; each structure would need to be examined individually to determine the exact shape of the characteristic curves and to determine whether the bandwidth available, at the fundamental mode, falls within the four percent limit which we consider to be requisite for useful application.

It may be mentioned that an experimental backward Wave oscillator has been made using a structure similar to C, FIG. 14a, scaled down, however, to the 3 cm. band thus the 12 centimetre width between sidewalls was reduced to 15 millimetres and the pitch between wires reduced accordingly. Oscillations on the first backward mode were obtained, the voltage and frequency characteristics being in good agreement with the figures predicted from cold tests.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

1. A slow wave structure which will propagate radio frequency waves over a frequency band whose width exceeds four percent of the mid-band frequency comprising a pair of parallel plane conducting side walls and a grating connecting the two side walls along the length of the structure in a common plane perpendicular to the side walls with the side walls extending above and below the plane of the grating, and in which the grating is composed of continuous intersecting conducting strands arranged to form a regular pattern of staggered apertures, said grating being operablerin both amplifier and backward wave tubes.

2. A slow wave structure according to claim 1 in which the strands of the first set are each normal to the side walls and the strands of the second set are perpendicular to those of the first set and arranged to form a brick-like pattern with a fraction of a brick terminating each row.

3. A slow wave structure which will propagate radio frequency waves over a frequency band whose width exceeds four percent comprising a pair of parallel plane conducting side walls and a grating connecting the side walls along the length of the structure in a common plane perpendicular to the side walls with the side walls extending above and below the plane of the grating in which 7 the grating is composed of a'first set of equidistant continuous intersecting conducting strands stretching obliquely from sidewall to side walland a second set of conducting strands similarly arranged at equal and opposite angles ofinclinationtothe side walls, each strand of one set intermediate the end strands of the grating being joined at both ends tothe ends of respective strands of the other set, said grating being operable in both 7 amplifier and backward tubes.

4. A slow wave structure which will propagate radio frequency waves over a frequency band whose width exceeds four percentof the mid band frequency comprising a pair of parallel plane conducting side walls and a "grating connecting the two side walls alongthe length of the structure in a common plane perpendicular to the side walls with the side walls extending above and below the plane. of the grating, and in which the grating is composed of a-regular continuous zig-zag conducting strand centrally disposed from end to end of the grating and an 6. A slow wave structure according to claim 1 in which the grating is composed of a first set of equidistant conducting strands stretching from side wall to side wall and a second set of shorter conducting strands each joining a respective one pair only of the strands of the first set intermediate the ends of the pair.

7. A slow wave structure according to claim 3 in which the strands of the two sets intersect each other only at their ends.

10 References Cited by the Examiner UNITED STATES PATENTS 2,708,236 5/55 Pierce 333-31 X 2,823,332 2/58 'Fletcher 315-3.5 15 2,831,142 4/58 Kazan 3153.6 2,834,915 5/58 Dench 33331 X 2,882,440 4/59 Mourier 333-31 X 2,890,384 6/59 Dench 3333l X 2,920,227 1/60 Dohler 333--31 20 2,932,761 4/60 Epsztein 315--3.6 2,989,661 6/61 Cutler 3153.6

FOREIGN PATENTS 814,120 5/ 59 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner.

, ELI J. SAX, Examiner.

Claims (1)

1. A SLOW WAVE STRUCTURE WHICH WILL PROPAGATE RADIO FREQUENCY WAVES OVER A FREQUENCY BAND WHOSE WIDTH EXCEEDS FOUR PERCENT OF THE MID-BAND FREQUENCY COMPRISING A PAIR OF PARALLEL PLANE CONDUCTIING SIDE WALLS AND A GRATING CONNECTING THE TWO SIDE WALLS ALONG THE LENGTH OF THE STRUCTURE IN A COMMON PLANE PERPENDICULAR TO THE SIDE WALLS WITH THE SIDE WALLS EXTENDING ABOVE AND BELOW THE PLANE OF THE GRATING, AND IN WHICH THE GRATING IS COMPOSED OF CONTINUOUS INTERSECTING CONDUCTING STRANDS ARRANGED TO FORM A REGULAR PATTERN OF STAGGERED APERTURES, SAID GRATING BEING OPERABLE IN BOTH AMPLIFIER AND BACKWARD WAVE TUBES.

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