US3361926A - Interdigital stripline teeth forming shunt capacitive elements and an array of inductive stubs connected to adjacent teeth - Google Patents

Description

Jan. 2, 1968 G. K. FARNEY 3,361,926

INTERDIGITAL STRIPLINE TEETH FORMING SHUNT GAPACITIVE ELEMENTS AND AN ARRAY OF INDUCTIVE STUBS CONNECTED To ADJACENT TEETH Filed March 9, 1964 5 Sheets-Sheet l FIG.2I

INVENTOR. GEORGE K. FARNEY W fa;

ATTORNEY Jan. 2, 1968 s. K. FARNEY 3,3

INTERDIGITAL STRIPLINE TEETH FORMING SHUNT CAPACITIVE ELEMENTS AND AN ARRAY OF INDUCTIVE STUBS CONNECTED TO ADJACENT TEETH Filed March 9, 1964 5 Sheefcs-Sheet 2 FIG. I20

A \f O 4 q 3 i T m W 0" c ARI- n l 4," Ea I: m 2 m m l ll. 125W A 6%- i. 1 j L F A v /q E 1b 24 4 G B w INVENTOR. GEORGE K. FARNEY FIG."

ATTORNEY Jan. 2, 1968 G. K. FARNEY 3,361,926

INTERDIGITAL STRIPLINE TEETH FORMING SHUNT CAPACITIVE ELEMENTS AND AN ARRAY OF INDUCTIVE STUBS CONNECTED TO ADJACENT TEETH Filed March 9, 1964 3 Sheets-Sheet 3 FIG. l3

FIG.l5 25 INVENTOR.

GEORGE K.FARNEY ATTORNEY United States Patent INTERDIGITAL STRIPLINE TEETH FORMING SHUNT CAPACITIVE ELEMENTS AND AN ARRAY 0F INDUCTIVE STUBS CONNECTED TO ADJACENT TEETH George K. Farney, New Providence, N.J., assignor to S-F-D Laboratories, Inc., Union, NJ., a corporation of New Jersey Filed Mar. 9, 1964, Ser. No. 350,516 18 Claims. (Cl. 315-35) ABSTRACT OF THE DISCLOSURE An interdigital stripline slow wave circuit is disclosed which is especially suited for use in microwave crossedfield tubes. The stripline slow wave circuit is formed by a pair of comb-like structures having their teeth interdigitated. The interdigitated teeth form iterative shunt capacitive elements in the stripline. An array of inductive stubs connect adjacent teeth to a conductive support wall. The adjacent stubs, which are interconnected at the support wall, form iterative inductive elements connected in shunt with the stripline for resonating the capacitive elements at a frequency determinative of the low frequency cut-off of the passband of the slow wave circuit. An elec tron stream is electronically interacted with the electric fields of the resonated shunt capacitive element to produce a microwave output signal. In one embodiment, a transmission line is connected to the slow wave circuit via a mitered tooth thereof for obtaining a broadband match to the slow wave circuit. In another embodiment, the interdigital stripline is recessed from the electron stream at the wave turn-around portions of the meandering wave path, formed by the circuit, to reduce electronic interaction in the turn-around field regions of the circuit for increased efficiency.

The present invention relates in general to slow wave circuits and more particularly to an improved interdigital slow wave circuit and impedance matching circuit for same. Use of the improved interdigital circuit in slow wave tubes, and particularly in crossed field tubes of circular geometry, yields tu-bes having increased gain, efficiency and power output. Such tubes are used as transmitting tubes in radar, communication links, radio astronomy, etc.

Heretofore tubes have been built using interdigital slow Wave circuits, see US. 2,816,248 issued Dec. 10, 1957. Typically, an interdigital line is used for backward wave tubes such as backward wave amplifiers or oscillators. Recently it has been proposed to reactively load the conductors in series therewith and to interact the beam alternately with both the series and shunt voltages of the interdigital line, see US. patent application No. 350,504 filed Mar. 9, 1964 and assigned to the same assignee as the present invention. This latter type of circuit yields a fundamental forward wave space harmonic with improved electronic bandwidth. Thus, the interdigital slow wave circuit can now be used tin both forward and backward wave tubes operating in the fundamental space harmonic mode.

One of the problems associated with interdigital line circuits is their relatively low interaction impedance of 20-30 ohms which leads to rather low electronic efficiencies for crossed field tubes of this type of 20-30%. Another problem common to amplifiers using interdigital lines is to find matching structures that will permit coupling wave energy onto and from the slow wave circuit without introducing unwanted reflections of wave energy which often lead to unwanted oscillation of the tube thereby restricting the useful bandwidth of the amplifier to some value substantially below the electronic bandwidth of the circuit obtainable in the presence of an improved match.

Another problem encountered in crossed filed tubes using interdigital lines is that the meandering circuit formed by the interdigital line contains portions where the electric field vector of the wave energy on the circuit is parallel to the static magnetic field lines. For such a case, the electrons of the stream which see this electric field are driven axially out of the interaction region and are lost from the stream thereby reducing electronic efficiency of the tube.

In the present invention an iterative inductive reactive loading is added to the interdigital line in shunt with the iterative shunt capacity elements of the line to increase the electronic interaction impedance thereof. In a preferred embodiment the reactive loading takes the form of stub supports for the line which stubs also serve to cool the circuit by conduction of heat from the circuit to a heat sink. In addition, the present invention provides an improved matching structure which permits impedance matching to and from the interdigital line over a broad band of frequencies as of :25% of the center band frequency of the tube over which power output varies by only 3 db. Furthermore, the present invention provides centrally located tabs on the interaction side of the interdigital line in crossed field tubes such that those side portions of the meandering line where the electric field vector is parallel to the magnetic field are removed from the stream of charged particles whereby stream particles are not lost from the stream due to electric fields in the turn around portions of the interdigital line.

It has been discovered in a stub supported interdigital two-wire line, in the limit of maximum width stubs, i.e., wherein the stubs have the same width as the interdigitated fingers, that the dispersion characteristic for the interdigital line aprpoaches the dispersion characteristics for a split-folded waveguide as described in a publication titled, Study of Interaction Structures, technical document report ASDTDR62-8l3, dated November 1963, and available from the U.S. Government Defense Documentation Center. For such a limiting case the interaction impedance is certainly increased over that available from a nonstub supported or non-inductively shunt loaded interdigital line but the bandwidth is reduced to a minimum value. Therefore, such a limiting case structure is relatively inflexible and does not permit a tube designer to trade bandwidth for impedance to any desired extent. In the case of the split folded waveguide, the inductive shunt loading is continuously distributed over the interdigital line and does not constitute an iterative inductive shunt loading as provided by the inductive stubs herein proposed.

The iterative inductive stubs connected in shunt with the iterative capacitive shunting elements of the interdigital line, as herein proposed, permits the tube designer to trade bandwidth for impedance to any desired extent between the limits of that of a non-shunt inductively loaded interdigital line and that of the split folded waveguide. The tube designer need merely alter the relative dimensions of the stub supports and thereby alter the amount of inductance connected in shunt with the shunting capacity elements and correspondingly shift the low frequency cut off of the slow wave circuit to any extent desired.

The term interdigital line as used herein means that the two-wire line is of the strip line type wherein the conductors forming the meandering transmission line have a width taken in a direction normal to the .dominant mode electric field vector which is more than half the characteristic distance between the conductors taken along the direction of the electric field vector. This excludes strapped vane magnetron structures wherein the straps have a width less than one-half the spacing between vanes. Such strapped magnetrons are narrow band devices because the two wire'transmission line formed by such narrow straps sets up strong standing wave patterns because of the iterative large jumps in characteristic impedance of the two-wire line formed by such narrow straps and wide vanes.

The principal object of the present invention is to provide improved interdigital slow wave circuits and im-' pedance matches for same whereby tubes with enhanced performance are obtained.

One feature of the present invention is the provision of an interdigital line circuit with iterative inductive reactive loading connected in shunt with the iterative capacity portions of the circuit whereby the electronic interaction impedance of the circuit is increased leading to increased efficiency and gain.

Another feature of the present invention is the same as the preceding feature wherein the iterative reactive loading comprises inductive thermally conductive stub members interconnecting the circuit to a heat sink whereby cooling of the circuit is enhanced.

Another feature of the present invention is a novel strip line impedance matching structure for matching into and/or out of an interdigital line, the novel strip line being characterized by one of the conductors having a mitered corner at the point of connection to the interdigital line whereby a substantially refiectionless broad band connection to the circuit is obtained.

Another feature of the present invention is the provision of centrally disposed tabs on the interaction side of the interdigital line such tabs extending from the line toward the cathode electrode whereby the Wave turn around portions of the interdigital line, containing electric field lines parallel to the static magnetic field of crossed field tubes, are removed from the beam field interaction region to enhance electronic efficiency.

Other features and advantages of the present invention will become apparent upon perusal of the specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a perspective view of a prior art interdigital slow wave circuit;

FIG. 2 is an equivalent circuit for the prior art interdigital circuit of FIG. 1;

FIG. 3 is an w-B diagram contrasting the dispersion characteristics of the prior art with those of the present invention;

FIG. 4 is a transverse sectional view of two alternative embodiments of the interdigital slow wave circuit of the present invention, both embodiments having the same transverse section;

FIG. 5 is a longitudinal sectional view of the structure of FIG. 4 taken along lines 55 in the direction of the arrows showing one embodiment of the present invention;

FIG. 6 is a longitudinal view, partly broken away, of the structure of FIG. 4 taken along lines 66, showing the embodiment of FIG. 5;

FIG. 7 is an equivalent circuit for the structure of FIGS. 46;

FIG. 8 is a longitudinal sectional view of the structure of FIG. 4 taken along lines 88 in the direction of the arrows, showing the second alternative embodiment of the present invention;

FIG. 9 is a longitudinal view, partly broken away, of the structure of FIG. 4 taken along lines 99, showing the second alternative embodiment;

FIG. 10 is an equivalent circuit for the structure of FIGS. 4, 8 and 9;

FIG. 11 is an enlarged perspective view, partly in phantom, showing the matching structure of the present invention;

FIG. 12(a1) is a transverse view of the structure of FIG. 11 taken alonglines 12--12;

FIG. 12(b) is a reduced flattened out plan view of the meander strip line of FIG. 12(a);

FIG. 12(0) is a transverse sectional view of the structure of FIG. 12(5) taken along lines 12(c)12(c) in the direction of the arrows;

FIG. 13 is a longitudinal sectional view of a tube using the slow wave circuit of the present invention;

FIG. 14 is a transverse sectional view of the structure of FIG. 13 taken along lines 14-14; and

FIG. 15 is a transverse sectional view of the structure of FIG. 14 taken along lines 15-15.

Referring now to FIG. 1, there is shown the prior art interdigital line slow wave circuit. The circuit consists of a pair of interdigitated conductive combs A and B, each having a backbone with depending tooth portion. One comb Aforms one conductor of a strip transmission line. The other comb B forms the other conductor of the strip transmission line. Wave energy propagating on the circuit meanders back and forth across a stream of charged particles, typically electrons, indicated at line 1, which is projected adjacent the structure or through aligned openings in the structure, not shown.

An equivalent circuit forv the interdigital line of FIG. 1 is shown at FIG. 2.'The dotted line represents the path of charged particles and they alternately interact with the capacitive shunt voltages of the circuit developed between the relatively large mutually opposed areas of adjacent teeth of the interdigitated conductive combs A and B and forming the iterative capacity shunting elements of the circuit.

The dispersion characteristic for the circuit of FIG. 1 is shown by dotted line 2 in the 40-13 diagram of FIG. 3. Curve 2 shows the circuit to have a fundamental backward wave space harmonic mode of operation and, of course, it is accompanied by other higher order space harmonics. It is well known, for example that forward wave interaction can be obtained with the first forward space harmonic, at reduced voltage and interaction impedance from that of the fundamental backward wave. From the dispersion characteristic it can be seen that the circuit is extremely broad band extending from DC. or near DC. to an upper cut-off frequency 01 The upper cutoff frequency m corresponds to resonance of the series inductive elements L with the shunting capacity elements C in the equivalent circuit and corresponds to 11' phase shift along path I, see FIG. 1. The useable part of the dispersion curve is defined by that portion of line 2 between points X and Y. This portion of the curve corresponds to practical beam voltages and has a rather high group velocity, i.e., proportional to the slope of curve 2. The electronic interaction impedance of the circuit is inversely proportional to the group velocity and, therefore, the interdigital line of FIG. 1 has a typical interaction impedance of 20-3O ohms which leads to relatively low efficiencies of 20 to 30%.

Referring now to FIGS. 47, there is shown one embodiment of the present invention. More specifically, an inductive reactive load L is connected across the shunting capacitance C of the transmission line. The inductance L of the shunting element is proportioned to resonate with the shunting capacitance C of the line at a frequency, m which determines the low frequency cutoff for the transmission line. A transmission line cannot propagate unless the series reactance, such as produced by L is of opposite sign to the reactance of the shunting elements, L and C. Therefore, since L is connected in parallel with C the line must be operated above the parallel resonance frequency of elements L and C or else the combined reactance of the shunting-elements L and C is inductive which is not of opposite sign to the reactance of the series inductance L The inductive element L is preferably provided by shorted sections of two-wire line formed by generally parallelconductive stubs 3, as of copper, projecting away from the circuit.Stubs 3 interconnect theconductive teeth 4 near the central region thereof, with aconductive wall member 5 as of copper. Thewall 5 shorts the strip transmission line formed by adjacentparallel stubs 3. Thestubs 3 are proportioned in length d, and width, w, such that the total electrical length, A from the tip of the vane liketeeth 4 to thewall 5, is less than A /4 at the low frequency cut-off of the passband of the interdigital circuit. This dimension, a, is characteristically equal to x /4 for the split folded waveguide. Wall preferably forms the vacuum envelope of the tube thereby also serving as a heat sink for cooling of theconductive teeth 4. Thestubs 3 also give greater mechanical rigidity to the interdigital line.

The dispersion curve for the stub supported interdigital line of FIGS. 4-7 is shown bycurve 6 of FIG. 3. It will be seen that the useable portion of this curve between points x'-y' has a slope which is substantially less than the slope of the non-inductively shunted transmission line dispersion curve 2. Therefore, the group velocity of the stub supported strip line has been reduced and the electronic interaction impedance substantially increased. In a typical example thestubs 3 increased the interaction impedance from 20-30 ohms to 60-80 ohms and produced a commensurate increase in electronic efliciency from 20-30% to 45-50%.

Referring now to FIGS. 4, and 8-10, there is shown an alternative embodiment of the present invention. In this embodiment the slow wave circuit includes an interdigital meander line having a structure quite similar to that of FIGS. 4, 5 and 6 except that theteeth 4 of the interdigitated combs A and B are bifurcated at 7. The bifurcation of theteeth 4 introduces an additional inductive reactive loading in series with the two conductors A and B of the strip line. This reactive loading at 7 puts the stream of charged particles into electromagnetic interaction with the voltages developed across the series inductance L' as indicated in FIG. 10 Where the dotted line indicates the beam path. The type of electronic interaction obtained in this type of circuit will be called alternating series and shunt interaction because the beam particles alternately interact with the series and shunt voltages developed in the circuit by elements U and L -C, respectively.

Stub supports 3 are used as above described with regard to FIGS. 4-6 to interconnect eachtooth 4 to theback wall 5.

The dispersion curve for the circuit of FIGS. 4 and 8-10, without stub supports 3, is as shown by dottedline 8 of FIG. 3. From this dispersion curve it is seen that this alternate series and shunt interaction circuit operates in a fundamental forward wave space harmonic with the same phase shift per section falling in the range of 1r/2 to 11' over the wide operating band of D.C. to w The derivation of this circuit and the theory behind why it has a forward Wave dispersion curve rather than a backward wave characteristic is described and the circuit claimed in applicants copending US. application Ser. No. 350,504 filed Mar. 9, 1964 and assigned to the same assignee as the present invention.

The stub supports 3, as above described, add an inductive element L in shunt with the shunting capacitive element C to resonate the shunting capacity at a frequency w' which is determinative of the low frequency cut off of the circuit. In addition, the stub supports 3 also introduce an additional inductance in series with conductors A and B. This additional inductance gets added in the equivalent circuit to L and the inductance of reactive element 7 to produce L' L is substantially greater than L such that the high frequency cut off m is substantially reduced to w' and the new dispersion curve is as shown in solid line at 9. Therefore, the electrical result of thestub support 3 is to raise the low frequency cut off w' and lower the high frequency cut off thereby substantially decreasing the slope of the dispersion curve and raising the interaction impedance with attendant increase in efficiency and gain. Thestubs 3 also have the effect of placing theentire dispersion curve 9 within the range of practical beam voltages between V and V such that substantially the entire bandwidth of the circuit may be used for electronic interaction. As before, thestubs 3 serve to cool the circuit by conduction to thewall 5. Also, the amount of inductance added by thestubs 3 is regulated by judicious choice of dimensions a, d, and w, as before described.

Referring now to FIG. 11, there is shown the matching embodiment of the present invention. In this embodimentadjacent stub members 3 which are in shunt with the line form a strip line feed. The vane ortooth 4 connected to one of the stubs is mitered in height h to form a mitered corner at 11 such that it has substantially reduced height at the edge remote from thesupport wall 5 as compared to its height at the edge adjacent thewall 5. In a preferred embodiment, thestub 3 is also mitered in width w as produced by a straight line continuation of the mitered corner 11 of thetooth 4. Preferably thestub 3 at its connection to thetooth 4 is mitered at an angle of 45 to the axis of the longitudinal axis or centerline of thestub 3, as more clearly shown in FIG. 12, with the mitered edge 11 passing through the centerline Q of the stub at the point of itsconnection 12 to thetooth 4.

For ease of explanation the mitered corner matching structure has initially been described in FIG. 11 with regard to an unsevered interdigital line. However, in most instances the interdigital line will be severed and the matching structure will be disposed adjacent the sever 31 (see FIG. 14) either at the input and/or output terminal of the slow wave circuit. Thesever 31 is typically a solid conductive block with transverse section the same as that of the circuit. Therefore, in practice thesever 31 serves as thelast tooth 4" of the slow wave circuit, such severtooth 4" being preferably much thicker than theadjacent circuit teeth 4. As used herein the term mitered tooth will be deemed to include such a mitered portion of the circuit sever. This severstructure 31 is shown by phantom lines in FIG. 11 and is shown in the tube structure of FIG. 14. Note that the severblock 31 is mitered at 11 only for that portion immediately adjacent the slow wave circuit. The depth of the mitered corner 11 in a typical embodiment is only twice the tooth thickness of the other teeth of the slow wave circuit.

Awaveguide 13, indicated in phantom, communicates with the stub formed strip line via the intermediary of a dumbbell-shapediris 14 cut through theconductive wall 5. Acapacitive matching ramp 15 is centrally disposed of thewaveguide 13 for matching the impedance of theguide 13 to the impedance of theiris 14.

Wave energy is coupled from thewaveguide 13 viairis 14 into the strip line betweenstubs 3. At thejunction 12 of the stubs andteeth 4 the mitered corner 11 ofstub 3 andtooth 4 causes the wave energy to make a right angle turn as indicated by the arrow of FIG. 11 and thence it continues around the interdigital line in the conventional manner.

The mitered corner 11 provides a near reflectionless match, i.e. less than 1% reflected energy corresponding to a VSWR less than 1.2 over a 25-50% bandwidth. A similar matching structure without the mitered corner 11 has a VSWR of 2:1 corresponding to 10% reflected energy which would be considered generally unsatisfactory for broadband, high gain tube operation.

Referring now to FIG. 12 there is shown another embodiment of the present invention. More particularly there is shown the provision oftab portions 4 of theteeth 4 which extend out from the teeth 4 a distance, g, toward the cathode structure and thereby define the interaction side edge portion of the interdigital strip line.

The tabs 4' extend over the central portion of the interdigital line and terminate in axial extent at or near the end of the overlapping portion of theteeth 4. In other words the spacing, k, between the axial end of the tabs 4' and the backbone portion of the interdigital line is the same as the spacing, 0, between the free end portions of theconductive teeth 4 and the other conductor of the meandering strip line, see FIGS. 5 and 8. In a preferred embodiment g is approximately equal to O, and k is approximately equal to O, and p, the width of the strip meander line, is at least three times greater than g. The reason for these proportional relationships can be seen by assuming that the strip meander line is flattened out, neglecting thestubs 3. Such a flattened out line would appear as shown in FIGS. 12(b) and 12(c).

The characteristic impedance Z of the strip line of FIGS. 12(b) and (c) is given by the following expression where O is the spacing between conductors of the strip line and b is the transverse extent of the conductors. In one portion of the strip line of FIG. 12(0) b is equal to p, for another portion b is equal to p+g.

Therefore, the characteristic impedance of the meandering strip line of FIG. 12(1)) will vary from In a typical example Z varies from 709 to 909 yielding a VSWR of 1.28 which corresponds to approximately 1% reflected power at the discontinuities in Z This amount of reflected power is acceptable. However, of reflected power corresponding to a 100% jump in Z would not be considered acceptable for broad band operation.

The effect of the tab portions 4' is to remove the turn.

around portion of the electric field E within the strip line from theelectronic interaction region 20. In this manner the electric field lines E which are parallel to the static magnetic field B in crossed field tubes are removed from the stream of electrons such that electrons of thestream 20 are not driven out of the stream along the magnetic field lines B.

Referring now to FIGS. 13 and 14, there is shown a tube structure employing the features of the present invention. More specifically a typical X-band crossed field amplifier tube structure is shown. The tube includes a hollow cylindrical vacuumenvelope Wall member 5 as of copper. An array ofstub 3 supported vane orteeth 4 project radially inwardly from the inner side of thewall 5 to form with annular conductors A and B an interdigital line of the type shown and previously described with respect to FIGS. 4-7. Conductors A and B, as of copper, extend over to and are conductively joined, as by brazing, to the wall '5.

A hollow cylindrical cold cathode emitter 17 is coaxially disposed centrally of the interdigital circuit. The cathode is made of a material having a high secondary electron emission ratio such as berryllium copper. A pair ofannular end hats 18 are disposed at the axial ends of the emitter 17. Conventional high voltage feed through insulator assemblies, not shown, bring thecathode stem 24 through the vacuum envelope to the emitter.

A pair of hollow cylindricalmagnetic pole pieces 25 as of soft iron are axially spaced apart on opposite sides of the emitter 17 and interdigital lines A and B to provide an axial magnetic field as of 5000 gauss throughout theelectrons interaction gap 26 between emitter 17 and the interdigital line. Permanent C-shaped magnets, not shown, join to thepole pieces 25 externally of the tubes vacuum envelope to provide the magnetomotive force for the axial magnetic field.

A pair ofwaveguides 13 project radially away from the mainbody wall member 5. Suitable gas-tight wave permeable window members, not shown, seal off the outer extremities of thewaveguides 13 and include flange assemblies to permit the tube to be connected to waveguides for applying to and or extracting wave energy from the tube. Dumbbell irises 14 extend substantially entirely across thewaveguide 13 and communicate withstubs 3 throughwall 5.Irises 14 are matched to thewaveguide 13 via capacitive matching ramps 15 which have a slope, S, of approximately Theirises 14 are relatively thick, i.e., they have a length, 1, which is more than a small fraction of their minimum transverse dimensions, c, in order to provide broadband low Q coupling since the Q of theiris 14 is inversely proportional to its thickness to transverse ratio, t/c.

The circuit is severed at 31 by a metallic sector subtending approximately 54 of arc to provide adrift space 32 such that the .re-entrant beam can debunch before reentering into interaction with the circuit. In a typical tube example of the structure of FIGS. l3-l5, the tube had an operating range of 8.5 to 9.6 gc. as a backward Wave amplifier, an average power output of 1 kw., cathode to anode voltage of 28 to 35 kv., a synchronous voltage of 35 kv., and a gain of 20 db. Thecircuit teeth 4 had a height, h, of 0.280", a total length, a, including stu-b of 0.285, a stub length, d, of 0.125", a stub width, w, of 0.125", and a tooth thickness of 0.030", and 0.030" spacing between teeth, a tab length, g, of 0.040, and a total of 34 teeth in the circuit.

Since many changes can be made in the above construction and many apparently widely ditferent embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high frequency tube apparatus including, an interdigital stripline slow Wave circuit having an operating passband, said slow wave circuit including a pair of interdigitated conductive comb structures having interdigitated tooth portions and a pair of spaced backbone portions, said interdigitated tooth portions of said circuit forming iterative capacitive means for developing a series of capacitive shunt voltages in said circuit, inductive stub means forming iterative inductive reactive sections of transmission line interconnecting teeth of one comb to the adjacent teeth of the other comb for developing inductive reactive loading connected in shunt with said stripline for resonating said capacitative means with said inductive stub means at a frequency determinative of the low frequency cut-off of said passband, means for projecting a stream of charged particles adjacent said slow wave circuit for cumulative electronic interaction with successive voltages developed by said iterative resonated shunting means, whereby the electronic interaction impedance of said slow Wave circuit is increased, means forming a conductive Wall extending adjacent to and along said interdigitated slow wave circuit, said stub means inter-connecting said teeth portions of said comb structures and said Wall to enhance cooling of said teeth by thermal conduction therefrom via said stub means to said wall, and means for extracting wave energy from said slow wave circuit for transmission to a suitable utilization device.

2. The apparatus according to claim'l wherein said interconnecting sections of transmission line include conductive stub members connected to said teeth in a plane generally midway between the backbones of said combs and extending away from said teeth in said plane to form generally T shaped composite tooth and stub members, and conductive means interconnecting adjacent ones of said stub members toform the shunting inductive reactance in parallel with the capacitance between adjacent teeth.

3. The apparatus according to claim 1 wherein said wall serves as said stub shorting means for shorting adjacent stub members.

4. The apparatus according toclaim 3 wherein said backbone portions of said comb structures extend over to and are conductively connected to said wall for enhancing thermal conduction from said circuit to said wall.

5. The apparatus according toclaim 3 wherein said interdigitated teeth of said comb structures are bifurcated to provide said slow wave circuit with series induct ve reactive loading in both conductors of said interdigital lme for developing a succession of series inductive voltages, and wherein said means for projecting said stream of charged particles projects said stream alternately through the series and shunt voltages of said interdigital line whereby said line is caused to have a fundamental forward wave space harmonic for electronic interaction with said stream of charged particles.

6. A high frequency tube apparatus including, an interdigital strip line slow wave circuit, said slow wave circuit formed by a pair of interdigitated conductive comb structures, each of said combs having a backbone portion and an array of teeth projecting from said backbone portion, and said interdigitated teeth defining a meandering wave path for wave energy propagating on said slow wave circuit, means forming a second transmission line connected to one of said teeth of said slow wave circuit along a first side edge of said one connected tooth, and said one connected tooth having a mitered corner along a side edge opposite from said first side edge for turning the direction of wave energy propagation on said slow wave circuit into the direction of wave propagation on said second transmission line and vice versa, whereby wave reflections associated with the change in direction of wave propagation are substantially reduced.

7. The apparatus according toclaim 6 wherein said second transmission line is a strip line.

8. The apparatus according toclaim 6 wherein said teeth are plate-like members, with their broad faces disposed in mutually opposed relation, and including a plurality of conductive stub members connected to the side edge portions of said plate-like teeth along one side thereof and extending away from said teeth in the same plane thereof, and wherein at least one conductor of said second transmission line is defined by one of said stub members.

9. The apparatus according toclaim 8 wherein said teeth are elongated, and said stub members are connected generally centrally of said elongated teeth and extend away substantially normally to the longitudinal axes of said teeth to form an array of generally T-shaped composite tooth and stub members with the stub forming the base leg of the T and with the tooth defining the two cross arm portions of the T-shaped structure, and wherein said mitered corner of said one connected tooth substantially deletes one cross arm of said T-shaped tooth.

'10. The apparatus according toclaim 9 wherein said one connected mitered corner cuts across said tooth and a portion of said stub at an angle of approximately 45 to the longitudinal axis of said tooth.

11. The apparatus according to claim 10 wherein said mitered stub forms one conductor of a strip line.

12. The apparatus according toclaim 8 including, a conductive Wall directed along said slow wave circuit,

and wherein said stub members interconnect said teeth of said interdigital line with said wall for thermal conduction from said teeth to said wall.

13. The apparatus according to claim 12 including an iris communicating through said wall in registry with the space between said two conductors of said strip line for Wave energy coupling thereto.

14. The apparatus according to claim 13 including a hollow rectangular waveguide disposed in registry with said coupling iris for wave energy communication therewith, and means disposed in said waveguide for matching the impedance of said waveguide to the impedance of said lI'lS.

15. The apparatus according to claim '14 wherein said iris is dumbbell shaped and said matching means in said waveguide includes a conductive ramp directed longitudinally of said waveguide with tapered height which increases in the direction moving toward said iris.

16. A high frequency tube apparatus including, an interdigital strip line slow wave circuit to define a meandering wave path for wave energy propagating on said slow wave circuit, means for projecting a stream of charged particles adjacent said slow wave circuit for cumulative electronic interaction between the fringing fields of the wave energy on said slow wave circuit and the charged particles of said stream to efiect a net transfer of energy from the stream to the wave, said meandering wave path having wave turn around portions on opposite sides of the mean direction of wave travel on said circuit, and means for removing a substantial proportion of the fringing electric field lines of the wave energy in said turn around portions of said wave path from electronic interaction with said stream whereby electronic efliciency of said slow wave circuit is enhanced.

17. The apparatus according to claim 16 wherein said turn around fringing field removing means comprises means for causing edge portions of said interdigital slow wave circuit, which are removed from said turn around portions of said wave path, to project inwardly toward said stream more than circuit edge portions at the turn around portions of the wave path.

18. The apparatus according to claim 17 wherein said inwardly projecting edge portions are tabs which inwardly project from said strip line to an extent which is less than 50% of the width of the strip line at said turn around portions, whereby wave reflections along said meandered strip line are reduced.

References Cited UNITED STATES PATENTS 2,679,615 5/1954 Bowie 315-39.73 2,881,348 4/1959 Palluel SIS-3.6 2,920,227 1/ 1960 Dohler et al 315-393 HERMAN SAALBAOH, Primary Examiner, S. CHATMON, 1a., Assistant Examiner,

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