US2641731A - Wave propagating electron discharge device - Google Patents

Description

June 9, 1953 A. W.'LINES 2,641,731

' WAVE PROPAGATING ELECTRON DISCHARGE DEVICE Filed Sept. 27. 1,948 5 Sheets-Sheet 1 FIG. 2.

z I LINE;

"W2" 0! FIG. 3.

Atforne 15 June 9, 1953 Y .A. w. LINES 2,641,731

WAVE PROPAGATING ELECTRON DISCHARGE DEVICE Filed Sept. 27, 1948 5 sheetsesheet '3 Inventor Afforneys Uune 9, 1953 A. WQLINES 2,641,731

WAVE PROPAGATING ELECTRON DISCHARGE DEViCE filed Sept. 27, 1948 I 5 Sheets-Sheet 4 I A lavas Inventor Attorneys June 9, 1953 A. w. LINES WAVE PROPAGATING ELECTRON DISCHARGE DEVICE 5 Sheets-Sheet 5 Filed Sept. 2'7, 1948 A// ////i// ////i Fig; ll

Adi/N43 {avenhr Attorney.

Patented June 9, 1953 wAv'E EROPAGATING ELECTRON DISCHARGE DEVICE Albert Walter Lines, Straf" land, assignonby mesne lish Electric Valve Compa arts, to he? ited, London,

ny' Inm England, a corporation of England Application September 27, 51 ,439

' In Great'Britain October 6, 1947 7" Claims. (01.- 315 -4) The present invention relates to wave-propagating structures such as waveguide, strip transmission line and like devices of the type in which the phase velocity of electromagnetic waves is less than the free-space velocity, so that, for example, coupling may be effected to a" stream of electrons falling through a potential difference of some few thousand volts, or less.

A known arrangement employing such a structure comprises an evacuated envelope containing a wire helix arranged axially in a metal tube. Such a construction results in a reduced phase velocity, and coupling may be obtained to a stream of electrons from a suitable gun located at one'end of the tube: the structure is excited at the gun end of the tube, and by virtue of the coupling of the electromagnetic wave with the electron stream, an amplified wave is available at the other end of the tube. The phase velocity may be of the order of one tenth freespace velocity, and electrons falling through a potential difference of about 2 kv. couple satisfactorily with the wave. When the device is employed as an amplifier in the cm. band-a bandwidth of the order of '800 inc/s. between 3 db points maybe realised. By introducing a suitable feedback coupling, the device may be made to serve as an oscillator.

In adapting an arrangement such as is described in the preceding paragraph for use at higher frequencies or for working at higher power levels, certain constructional and other diiilculties are encountered, and the present invention has for one of its objects to provide a waveguide, strip transmission line or like device which is capable of employment at such higherfrequencies or higher powers, but whose constructionis relatively straightforward.

The invention is based on the fact that the phase velocity of a component of an electromage netic wave in a waveguide or strip transmission line with a uniformly-corrugated wall or walls can be made less than'free-space velocity. (A component of the electromagnetic wave is one of the Fourier space harmonics of the wave, sometimes called Hartree harmonics.) In general, however, it is found that th'edispersion of such a guide or transmission-line for'the largest amplitude components of the wave is high; that is to say, within a given frequency band, the phase velocity of the largest amplitude compo nents at low phase velocities varies'continuously between wide limits. This feature of corrugated structures is also'a feature of any {guide or like structureso formed that-it is effectively-a series 2 "torsfarr'a'nged uniformly end d -with respect to one another, the uni formly-f orrugated' guide and strip transmission v ng egampres of this class.

The principal object of the invention is to provideawave propa'gating structure suchthattlie dispeifsion 'pflthje structure to a large amplitude component 'of electromagnetic waves propagated ther is"low. large component in" this context s-cn wim which a substantial fraction of the total energy in the'wave is associated, and a low dispersion exists when, over a substantial frequency band, the variation in phase velocity iss'mall; for example, over a frequency band of sor'nel0% of the mid frequency, the phase velocity may vary byno more than 1%. I

According to'th'e invention in oneaspect there is provided an electromagnetic wave propagating structure adapted to support electromagnetic waves having aphafse velocity less than the freespace velocity'of propagation, and having a low dispersion; to a large amplitude component of waves propagated therein over a substantial band of frequency, said structure having at least one boundary consisting of a conducting surface formed with a series of repeated like groups of resonators, the smallest repeated'pattern of such resonators forming a group which is delineated by a particular pattern of variation in at least one of the'factors, the natural frequency and the -spacing, of theresonators forming the group. 7 :In another aspect, -the invention comprises an electrical transmission system comprising a source of energy to'be propagated, an electromagnetic wave propagating'structure coupled to said source so that in the operative condition there are propagated in said structure electromagneticwaves having a phase velocity less than the free sp'ace velocity of propagation, and havinga flow dispersionto a large amplitude" component of waves propagated therein over a substantial b'and of frequency, Said structure having at least one' 'boundary consisting of a conducting-surface formed with a series of repeated likegroups of resonators, the smallest repeated pattern of such resonators forming a group wh ch is deuneateeb'y a particular patternof variation in atleast one of thefactors, the natural frequency and the spacing, of the resonators forming thegroup, and'a'n output circuit coupled to said structure to receive energy from said source.

In yet another aspect the invention resides in an electromagnetic wave propagating structure adapted to support an electromagnetic wave having a phase velocity less than the free-space velocity of propagation, said structure comprising a conductive boundary surface formed with a series of resonators such as slots or cavities distributed along its length said resonators being arranged in recurrent, like, regularly spaced groups of three, the resonators of a group comprising two resonators of like natural frequency, and one resonator of different natural frequency from the other two. 1

A further aspect of the invention contemplates an electromagnetic wave propagating structure adapted to support an electromagnetic wave having a phase velocity less than the free-space velocity of propagation, said structure compris ing a conductive boundary surface formed with a series of resonators such as slots or cavities distributed along its length, the spacing between successive resonators being varied in cyclic manner.

Yet another aspect of the invention involves an electromagnetic wave propagating structure adapted to support an electromagnetic wave having a phase velocity less than the free-space velocity of propagation, said structure comprising a conductive boundary surface formed with a series of resonators such as slots or cavities distributed along its length, every nth space between adjacent resonators being made equal to one another, but different from the remaining spaces, which are themselves equal, as being greater than unity.

The nature of the invention will be more clearly understood from the following description, and explanation of the behaviour of periodic structures propagating electro-rnagnetic waves, given with reference to the accompanying drawings, in which:

Figs. 1 to 8 are explanatory diagrams whose significance will appear hereinafter,

Fig. 9 is a diagrammatic representation of an electronic device according to the invention suitable for use on an amplifier or oscillator,

Fig. 10 is anenlarged cross-section View of a part of the device in Fig. 9, and

Figures lla-e are representations of further embodiments of my invention.

If for an unloaded transmission line, the operating frequency f is plotted against 1/)\g where )q; is the Wavelength in the line, a straight line I Fig. 1 is produced. The slope of this line f)\g will represent the free-space velocity of propagation of electro-rnagnetic waves, usually termed c. The same characteristic plotted for a waveguide will be as shown in curve 2. The phase velocity of the wave for any given frequency will be represented by the slope of the line joining the equivalent point on the curve to the origin. It will be observed that a low-frequency cut-ofi exists and the curve tends towards the free-space characteristic as the frequency is raised. The characteristic of a uniformly-corrugated strip transmissio line shown in Fig. 2 is indicated in curve 3 of Fig. 1, the phase velocity decreasing from about the freespace value C as the frequency is increased from zero; phase velocity of {e C may be reached, for example, at the point a. Consideration of the behaviour of the corrugated waveguide may be simplified if the scaleso-f the curves of Fig. 1 are changed to the alternatives shown, and

(which is proportional to f), where d is the spacing between adjacent corrugations, slots or res- 4 onators, and A is the free-space wavelength, is plotted against M where )\g is the wavelength in the loaded guide.

The uniformly corrugated guide, such as the strip transmission line with equal resonators spaced at equal intervals along one wall, is equivaand the attenuation'in the guide is then infinite. From the cut-off frequency to the frequency at which the resonator length L is the phase change along one section of the guide remains at the value 11'. At the resonant frequency the loading on the guide changes from inductive to capacitive, and the phase difference across a section changes by Jr. The phase change remains constant at zero until the next pass band is reached. At the beginning of the second pass band the loading is capacitive, and the frequency spectrum curve is indicated at 4, Fig. 1. When the effective length of the resonator becomes it is again resonant with a node of the longitudinal electric held at the mouth of the resonator and )\g=\. For a higher frequency the loading is inductive and acut off occurs at a frequency approaching the value for which the resonator length is V The same cycle of events is repeated at all res onant lengths 'nli A 2 Z At frequencies for which the effective length of the resonator is the loading changes from capacitive to inductive and /\g=)\.

At frequencies such that further out off values,not indicated in Fig. 1 occur. The stop band associated with a cut off of this type is thesame as the stop bands obtained with inductive or capacitive loading, No resonance occurs in the cavities, and the phase change acrossa section of the guide remains constant throughout the stop band. On both sides of the stop band the pass bands are either both inductive or both capacitive. In the former case :12 is a low frequency cut ofrufor the pass bandand inthe latteriitis'a high'frequency cut off."

- For a transmission line the frequency spectrum.

curve is given by"0=; (or X=A The phase velocity, and groupvelocityfiefined (in terms of free-space velocitylby are equal tounity-and independentof frequency.

The transmission line is thus non-dispersive.

With the introduction-of lumped impedances orresonators at regular intervals along the -line,ua structure is" produced exhibiting: thecharacter istics of a dispersive medium; Phase and group velocities are different and'dependent on frequency, the medium: becoming more dispersive for the resonator case-with increase of the ratio:

For the first pass band .of'the corrugated strip transmission line. phase velocity decreases from free-space velocity more and more rapidly with increasing frequency; at the same group velocity,

decreases to zero at the cut off frequency, a standing wave being producedwith adjacent resonators oscillating in antiphase (x =2d); The occurrence of this standing wave indicates that the illustration of frequencyspectragiveninFig; l are incomplete;

The longitudinal component of electric field E2 of the wave, which must exist along the-corrugated surface at anynparticular instant of time is illustrated in Fig. 3, and so far only the funda mental component of thiswave, shown by the, dotted line, has been considered. Theothencomponents or space harmonics, have valuesof dif'-' fering by 21m, Where'n=0, i1, :2 the co-m.-.

plete spectrum beingas illustrated in Fig. 4. Thus for any frequency (f).the wave propagated in'the" corrugated structure-may be analysed into aseries of space harmonics, a, he, .Fig; 4.

The relative amplitudes of these space harmonics depend on the specific boundary condi' tions enforced by, the corrugated structure, and the amplitudes of the harmonics decrease'with distance from the corrugated surface, the field; due to the higher harmonics being of secondary importance as the aXis of the-main guide is ap.- proached.

Curve i-of Fig. 1 indicates the characteristics of a rectangular Waveguide with one wall cor= rugated, a structure which may be'obtained'by, fitting side plates to the corrugated strip trans The frequency spectra for the rec-' tangular guide may be obtained'from the spectra:

mission line.

for the corrugated strip by replacing.

where A w is the wave-length in the resonators. The further explanation and description whichfollows isbased; for simplicity, upon the characteristics of the corrugated strip transmission line; but-the reasoning is equally applicable to the -caseof corrugated rectangular or circular f waveguides and co-axials:

They showonly a single phase velocity at )\g=2d, whereas two equal Waveswith- In the simple structureor: Fig. 2a standingwave canvbe' set up for a-nylfrequency by-a suite able arrangement: of generators at the two-ends ofthe structure Then for all; wavelengths-in the structure, except )\g=2d, the wave pattern can; occupy an arbitrary position with respect to the'structure: In the case of )\g=2d, forwhich-a standing: wave occurs Whatever the method of excitation; the pattern isvfixed relative to the structure: Antinodesoccur atthe cavities, and nodes at points between the cavities. If now' every; Nth-cavity, ismodified by, for example, increasin or decreasing the length; then-stand-- ing waves with the same wavelength but different frequencies can be obtained. Thus a standing wave can occur either with nodes or antinodes at the modified' caviti'es." Evidently,jthe frequency: of the standing wave with nodes at the modified cavities will be independent of this modification. However, the frequency" of the standing wave with antinodes at the modified cavities-will beincreased'or decreased according to whether the cavity has been decreasedor increased in length.- Each of these standing'wave patterns will be fixed relative to the structure of the guide, and willnot depend on the method of excitation. They. will occur when the phase difference between the modified cavities is mr, or the distance between the modified cavities is ch- 2 l v thereflections from the groups of cavities ,(N

cavities per group) .'adding'in phase and'produc- 1 ing" a standing wave. Thewavelength in'the structure at'whic'h these waves'will occur are given by 2 *N I Or I 7\ =2lyd where 'N'is thenumber of cavities in a group.

d-is the distance apartof the cavities,- and guide, the slots or resonators are grouped inthrees, one resonator in each group having its length increased or decreased to a valuel as indicated in Fig;- 5. The effect of the introduction of this periodicity-intothe corrugated structure is to cause thephase 'velocity' of frequency characteristic to break up, the lowest frequency pass band of thesm'oothly corrugated structure now-being replaced by threeseparate pass bands separated by stop bands.

It-is evident that standingwavez will occur inthis structure fo1" =6d-, 3d and 2d.- In'th'e case" of both- Ag=6d -and 3d, two'standing waves arepossiblehaving'either'nodes-or antinodes at the modified resonators. Thefrequencies of 5 the standing waves with nodes at the modified resonators' will lie on the-spectrum curvefor the sim-- pIecQrrugated' structure. The frequencies of the standing *wave" with :antinodes at the modi-- fied resonators will be higher or lower than these depending on whetherthe modified resonators have been -';made shlorterbr'longer than the-- others; Hence two":types"or spectra are possible" spectrum of the corrugated guide with the'periodic structure, the higher order space harmonies should be included. The curves forthe various harmonics are of the same form as for the fundamental illustrated in Fig. 6 or Fig. 7.

For each value of space harmonics are given by values of differing by g 3 x v the curve of Fig. 6 or 7 being repeated every as illustrated in Fig. 8 which indicates the first few space hormonics in the first, second and third pass bands when the modified resonator length Z is shorter than the unmodified length L. The repetition of the pattern occurs every 211' if the groups of three resonators are considered as a single composite resonator, and the value of d adjusted appropriately. For the spectrum-to be complete the space harmonics travelling in the reverse direction would have to be shown, as would also the fundamentals and harmonics corresponding to the higher frequency pass bands. For the purposes of the invention, however, only the low order harmonics in the low frequency pass band will, in general, be of importance as the amplitudes of the higher order harmonics are small and also the amplitude of thefundamental component is considerably greater than the amplitudes of the harmonics.

Consideration of the characteristics illustrated in Fig. 8 or Fig. 6 or 7 will show that particular frequency ranges can be selected over which one of the fundamentalor space harmonic com onents exhibits non-dispersive transmission. Since non-dispersive transmission corresponds to uniform phase, and group velocities defined re-' spectively by the ratio 9/(1), and the slope of the characteristic (19/115!) respectively, the non-dispersive regions will be characterised by coincidence of the tangent to the 0/ curve and line joining the origin to the point of contact of the tangent. In other words, a non-dispersive region is defined by a substantially straight portion of the e/ oharacteristicwhich, when produced, passes substantially through the origin.

Such a result may be obtained with the described arrangement employing groups of three stant phase-velocity wave. Again, non-dispersive regions may be located in higher main pass bands of the device which are not indicated in Fig. 8.

A frequency spectrum similar to that obtained with the structure illustrated in Fig. 5 may be obtained with an arrangement comprising groups of three resonators in which all resonators of a group are of different lengths, or resonant frequencies. The form of the characteristic will lie between those obtained for the three slot structure when one slot is detuned above and below the other two slots of each group. An arrangement alternative to the employment of groups of three equally spaced slots with the periodicity defined by modifications in the resonant frequencies of the slots is an arrangement of groups of three slots with irregular spacing. This arrangement produces the same type of We characteristic as is produced by the structure of Fig. 5, and so long as the periodicity of three remains,

the narrow stop band with'the first main pass band are equally spaced, but the spacing at is (measured as one third the spacing between the groups of three resonators.

Other groupings of the slots or resonators to provide the periodic structure are possible, and an arrangement embodying groups of two dissimilar slots may be employed, or similar slots :may be arranged with alternately different spacings between successive pairs of slots. With these structures based on a periodicity of 2, only one minor stop band is produced within each major pass band and, in general, a non-dispersive condition will not be realised with the fundamental component of the wave. However, on the higher space harmonicwaves the 0/ characteristics will dispersive transmission at a particular 'quency and velocity, each group of resonators beslots or cavities, one in each group being detuned,

by operating about the frequency corresponding to point a indicated in Fig. 8. In this frequency region, the phase velocity of the fundamental wave may have a' velocity of, say. re C over a point a. As previously stated, however, the fieldv strength provided by the second space harmonic wave will be smaller than that provided by the fundamental and for a given R. F. power may not be adequate to provide the required degree of coupling to an electron beam which is the ing delineated by a particular pattern of variation in the natural frequency, the spacing, or both the natural frequency and the spacing of the resonators forming the group.

1 The constant phase velocity result may be obtained if the corrugated waveguide with periodic structure is made as a corrugated strip transmission line or surface or as a rectangular waveguide with one or two o posite walls corrugated to form slots or resonators. The structure may also take the form of a co-axial line with the corrugation formed on the inner or outer conductor. A circular waveguide with the desired form of wall may also be employed, but in this case, the inner diameter of the waveguide (measured to the surface of the corrugations or slots) has of necessity to be made so large that the transmission properties approximate those of a corrugated strip surface.

The corrugated waveguide structures according to the invention have a considerable advantage over known forms of travelling wave devices with low phasevelocity as the attenuation is considerably reduced.

The device according to the invention may be constructed in any of a wide variety of diiferent ways, provided that the structure effectively comprises a periodic structure of groups of adjacent slots, corrugations or resonators as set fre-.

*iorth above. 'The' most convenient form: consists of a rectangular waveguidewith one wall corrugated, and may conveniently be formed by joiningtogether a-seriesof rectangular sections-"of {appropriate 'crosssectional dimensions. Other forms, such as the co-a-Xial line ='orcircular'wave guide, may be conveniently formed as suitably turned surfaces. I

"The various groupings of resonators andspaces which-may be used in carrying-out 'the' invention are typifiedby the examplesillustrated-"in Fig. 11. In'this drawing Fig. lla shows a 'group which corresponds to that described above with reference to Fig. 5, more groups, however, being shown and the vertical chain dotted lines showing the sub clivision into-groups. In" Fig. 11b three groups are shown of a series inwhicheach group comprises three equal'spaces' separating threeresonators each of a differentnaturalfrequency. In the form shown inFig: "llcfthe resonators areall of thesame naturahfrequency but every third'space'is shown enlarged. In Fig. 11d againall the resonators are of the same natural frequency but the spaces vary in cyclic manner in a periodicity of"three;"each"successive space being different from iitsipredecessor. In'Fig. lle groups ofthree resonators areshown in'which thesecond space in the groupis'made larger'than the other two and the third resonator in "the group is ofa different'natur'al frequency from the other two. Obviously otherr'combinations of thetwo expedients of varyin'g the spaces and resonator natural frequency imay'bedevised and periodicitiesof more than three may be employed. V p

The electron beam, withwhich the selected travelling wave in the corrugated"- waveguide is required to couple, to m ake efiective-usebf the properties of the device'in the apparatus-with which the invention is primarily concerned; may be provided by any known or suitable means, In the case of the rectangular'structures, the electron beam may conveniently be of strip cross-sectional form, while-if thestru tureisTOrmed as a corrugated co-axiealline, their an electron beam of annular cross section maybe employed, Suit: able focusing and directing, means may be'provided as desired to maintain'the'form' of the electron beam and its transverse position in the guide structure; for example, vthe waveguide structure may have an arcuate form, or may be constructed as a closed ring, in-which case-asteady magnetic field-isset up, in known manner, to constrain the beam to follow the desired arcuate path. I The electron beam may in some cases be focused by means of crossed magnetic and electric fi'el'ds, as in amagnetron oscillator. I l

1 By way of-example, oneform=ofdeviceaccordingzto the invention, which has .the meritthatiit lends itself -to simplicity in. manufacture is adiagrammatically illustrated in alongitudinahand transverse section in Figs. 9 and 10 respectively. This device, comprises a rectangular wave guide, II, one wall of which is provided with slots 1), q, r, 11 q r etc., equally spaced along its length and arranged as a periodic series in groups of three. The slots :0, p etc., and q, q etc., being of equal length, the slots 1, r etc., being of a different length. It will be seen that the smallest repeated pattern of slots is, therefore, a group of three, two long and one short and the total length of the waveguide is such that a large number of cycles of this pattern is included.

At the left hand end of the guide is an electron gun 12, arranged in a glass envelope l1 sealed to the "end of the 'guide' and adapted reproduce a widefthin beam ofelectrons ofthe*g ezi '"a r'm indicated'at 18 in Fig. '10 when fed with sui able potentials via terminals I 3,1 4, 15 and HiI The electron gun "l2 and the guidestr'ucture roman "enclosed space whichis'evacuated.

Coupling means, in the form of short lengths or waveguidei' 'ztl and 2 l ,"sealed 1 err by gla'sswindows- 22 and23 and coupledthrough slottype matching transformers 24' and 25' with 'th "end resonators are provided 'for 'feeding'R i'lF" nergy :into a'nd extracting R. *'F. energy from theleft hand'and right hand ends "of the-waveguide'structure respectively. 7 I Y *Wheri' the device is -'energise d at the'prop'ei potentials; an electron stream will-pass t rough the "device at a jvelocity which maybe 'arraii'ged-fto approximate to the 'phasevelocity "of-an -R F.

wave numbed in= wry-"mernp t waveguid to at the left hand end. V The waveguide structure I conveniently constructed of metallic stampingslof any suita ble non-magnetic material,"the-thicknesses of which are arranged to provide the" desiredfslofi'aipertu'r'es, or the appropriate spacings betweenthe slots and are stamped "out to sizes' ap'lfllopri'ate to "20 an m 1 ts;""r s1ots or spam: "gs i'between slots. A stack of such stampingsis- "sembledandbondedintoa c'ontinuously 'condu ye. structure for whichpurpose the meeting faces of-the-stampings *aresilver plated or 'tlier- 'wise' treated to ensure-low" resistance con'tact conditions between them.

It will be appreciated*-'that=-the arrangement illustratedds' diagrammatic {andmay= be"m difl'ed considerably in practice. '-*For"example, the method of coupling-to the waveguide "I 1 1m varied considerably according "to the adopted for thestructure andr'nay co coupling loops or probes associated eithe appropriate-'resonators 'or' with the fwaveguide itself.

One such structure whi'ch-has beenfrhaide was built up'from cooper stampings 0:080 inch thick,

silver'platd on one side, thecomplete assembly being held clamped ftightly together eurmg-== -a baking process carried out a temperature of "about 800 C." in a-' nitrogen atmosphere for a time sufficient for the formation of" copper/- silver eutectic bonds between adjacent stampings'. In this example-the waveguide was" 4.81 inches wide; 'the free-s ace depth of the guide was'0f67- inch, the depth of the 10" 'and ffq slo ts (from' the-smooth wallof-'-"the guide) was 2 .6 inches;"and the depth of the r 'slotspfron f fthe smooth wall: of the" guide)-- was 2200 inches. The waveguide when complete was found -to-*have j a non-dispersive: pass" band at" a phase "velocity of approximately-02850; from 12.-75cm.'-'*to norm. wavelength. V w

The particular application in whichf the de vice 'a ccording-td the invention finds" "i1sefufemployment is the realization of a broad-band amplifier or oscillator for centimetric and millimetric wavelengths. In such as amplifier a device such as that described above with reference to Figs. 9 and 10 is employed, and the electron stream is fired axially down the corrugated as it travels through the guide structure, owing to anenergy interchange between the electron stream and the field, and a considerable amplification of the wave is obtained. The amplifled R;;F. output is obtained by suitable coupling arrangements at the output end of the structure. Such a device may also be employed as a radio frequency. oscillator in the centimetric and milliinetric bands. In such an oscillator an arrangement similar to that outlined above as an amplifier is provided with a feedback coupling between input and output. Such an oscillator will have the advantage that its frequency can be adjusted over the band of frequencies for which the phasevelocity is constant simply by theihclusion in the feedback path of a phase.

shifting device, without the necessity of readjustingtheelectron velocity, since the phase velocity is constant over the band of frequencies over which the oscillator is to beused.

I claim: a r 1.1 An electromagnetic wave propagating structure adapted to support an electromagnetic wavehaving a phasevelocity less than the free- 7 support an electromagnetic wave having aphase velocity less than the free-space velocity of propagation, .said structure comprising a conductive boundary surfaceformed with a series of resonators distributed along its length, every nth space between adjacent.v resonators being 1 made equal to one another, but different from the, remaining spaces, which are themselves equal, n being an integer greater than unity; 3. An electromagnetic I wave propagating plurality of groups of resonators which are spaced alongfthe boundary in the direction of.

propagation with one group of at least three resonators for each half wavelength of a wave to be propagated, each group being identical With the others and each group being characterized by a predetermined cyclic variation in resonator spacing, the spacing between adjacent resonators being, defined by longitudinal portions of said conductive boundary substantially parallel to the axis of said structure, the spacing between certain adjacent resonators differing from the spacing between other adjacent resonators. f

6. In a structure for propagating electromagnetic waves, a conductive boundary surface de-- fining a plurality of resonators spaced along the boundary in the direction of propagation, said resonators being ingroups of at least three with one group for each halfwavelength of apropagatedwave, all the groups being identical with a variation between resonators of a group, said variation being cyclically repeated in said groups. said variation comprising a difference in spac ing of the resonators of a group.

7. In a broad band'high'gain amplifying system, a path for radio frequency signal energy comprising a signal input, a signal output and an amplifying chamber .directly connecting said input and said output, electron gun means having accelerating means positioned to project .an electron beam through saidchamber, said chamber having resonant portions acting to retard the velocity. of radio frequency signal energy along said beam so that the component of velocity of said signal along said beam is 'of the same order of magnitude as the velocity of said beam and amplification of said radio frequency signal is obtained over a broad band and is produced by a continuously interacting energy interchange between theenergy of said beam and the radio frequency signal energy, said resonant portions being arranged in groups ofat least three resonant-I portions spaced from each other, the

structure as set forth in claim 1 wherein every third resonatoris of a' different natural frequency from the natural frequency of'the remaining resonators. v V

4. An electromagnetic wave propagating structure adapted to support'an electromagnetic wave having a phase velocity lessthan the free spacevelocity of propagation, said structure comprising a conductive boundary surface formed with. a series of resonators distributed along its length said resonators, being arranged in cyclically repeated, like, regularly spaced groups of three resonators and separating spaces between resonators, each resonator of a group being of a different natural frequency.

. 5. In a structure for propagating electromagnetic waves, a conductive boundary defining a spacing of selected resonant portions differing from the spacing of the others to form a cyclic pattern of'variation in spacing, said chamber having a plurality of such groups.

I ALBERT WALTER LINES.

' References Cited in the file of this patent UNITED STATES PATENTS

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