Oscillation suppressor for ring-bar slow wave structure
&#39; United States Patent 1191 Hiramatsu et al.  
 [111 3,876,962 [4 1 Apr.8, 1975 [22] Filed:  
 [ 1 OSCILLATION SUPPRESSOR FOR RING-BAR SLOW WAVE STRUCTURE [75] Inventors: Yukio Hiramatsu, Los Altos; Louis Y. Lau, Cupertino. both of Calif.  
 [52] US. Cl. 333/31 A; 333/98 M [51] Int. Cl H0lj 25/36;H()1p 1/16 [58] Field of Search 333/98 M, 31 A [56] References Cited UNITED STATES PATENTS 2,869,085 1/1959 Pritchard et al. 333/98 M 3.221205 11/1965 Sensiper 333/31 A 3,365.607 1/1968 Ruetz et al 333/31 A 3.562.679 2/1971 Coalc et a1. 333/98 M 3.593.221) 7/1971 Rooney 333/98 M 3,693,038 9/1972 Scott et a1 333/31 A Primary Examiner-Alfred E. Smith Assistant E.\&#39;aminerWm. H. Punter Attorney, Agent, or Firm-Stanley Z. Cole; D. R. Pressman; Robert K. Stoddard [57] ABSTRACT A longitudinally directed comb supported ring bar slow wave structure includes a pair of elongated waveguides. Energy from the structure is coupled to the waveguides to propagate transversely of the structure. The waveguides extend longitudinally of the structure and are positioned so they abut against and span the external sides of the structure. The waveguides are dimensioned so that they are cut off over the operating bandwidth of the circuit but are capable of supporting spurious microwave energy extant in the structure above the operating bandwidth. The spurious energy is coupled by the waveguides to elongated transversely tapered loads at the far ends of the waveguides. The length of the waveguides in the propagation direction is at least three times the waveguide height.  
 7 Claims, 7 Drawing Figures PATENTEDAPR 81975 SHEET 1 2 PER PERIOD (RADIANS) PATEHTEBAPR 81975 FIG. 4A  
 FIG.6  
 OSCILLATION SUPPRESSOR FOR RING-BAR SLOW WAVE STRUCTURE FIELD OF THE INVENTION The present invention relates generally to ring bar circuits of the type used in travelling wave tubes and more particularly to a microwave load device for suppressing spurious oscillation in said circuit which is sufficiently removed from said circuit to avoid disturbing the intended circuit operation.  
 BACKGROUND OF THE INVENTION The background of the present invention centers primarily on the recognition of problems associated with the unexpectedly inadequate operation of a prior device for suppressing spurious oscillations in a comblsupported ring bar (CSRB) slow wave structure. This prior device, disclosed in US. Pat. No. 3,693,038, to Scott et al., filed May 3, 1971, and issued Sept. 19, I972, has one inventor in common with the present invention. These problems are best understood with reference to FIG. 1 of the drawing herein which is an isometric view of the prior art device. In FIG. 1, broad wall 13a of a longitudinally directed hollow rectangular waveguide 13 is provided to abut against external side 25 of a conventional comb-supported ring bar structure 24 for suppressing spurious oscillations. Vertically elongated high pass coupling apertures 15 in the broad wall 13a are in register with the usual vertical openings 25b in the support structure sides 25 which communicate with the ring bar circuit 19. The entire interior surface area 14 of waveguide 13 is coated with lossy material. We have now found that this lossy material unexpectedly attenuates microwave energy in the operating band of ring bar circuit 19 despite the fact that slots 15 have a height 15a to couple only microwave energy above the operating bandwidth into the lossy waveguide 13.  
 SUMMARY OF THE INVENTION We have concluded that the prior device does not operate as expected because the lossy coating on the waveguide interior surfaces 14, particularly that portion of the coating on the inside of the apertured waveguide broad wall 13a, is too close to the slots 25b defined by the CSRB comb side 25. Viewed differently, the apertures 15 do not exhibit as sharp a high pass frequency cutoff as expected because fringing fields from slots 25b traverse through resonant apertures 15 in the relatively thin broad wall 13a.  
  To avoid this attenuation in the operating band, one feature of the present invention is that a transversely directed waveguide coupling means is provided, instead of the frequency determining slots 15 in the thin broad wall 13a, so that the cutoff frequency determining vertical dimension exists over a transverse distance of approximately three times the vertical dimension to isolate the microwave load from fringing fields.  
  It is another feature of the present invention that the transverse waveguide means is elongated over the length of the CSRB structure for ease of manufacture.  
  It is still another feature of the present invention that an elongated microwave load which is tranversely tapered is provided at the far transverse end of said waveguide means for minimizing reflection of the spurious energy from the load.  
  It is a further feature of the present invention that said waveguide means contains a dielectric with a di electric constant greater than unity for providing an electrical length of the waveguide means which is greater than its physical length, thereby reducing the size of the oscillation suppressing device.  
 OBJECTS OF THE INVENTION It is an object of the present invention to provide a new improved spurious osciallation suppression device for a ring bar slow wave circuit, wherein the device does not disturb the operation of the slow wave circuit in its operating bandwidth.  
 It is a further object of the present invention to pro vide a new and improved comb-supported ring bar slow wave oscillation suppression structure which is easy to manufacture and is not substantially larger than conventional ring bar slow wave structures.  
  Other objects and features of one main embodiment of the present invention will become apparent upon a perusal of the following detailed description of the invention in conjunction with the appended drawing.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1, as previously indicated, is an isometric view I ofa prior art CSRB slow wave and oscillation suppressing device. I  
 FIG. 2 is a portion of an w-B diagram wherein frequency of operation is plotted against phase shift per 1 period of the prior art CSRB circuit.  
  FIG. 3 is an isometric, partially explodedview of an oscillation suppression structure of the present invention including a CSRB structure portion.  
  FIG. 4 is an exploded, isometric view of the CSRB structure portion showing top and bottom halves.  
  FIG. 4A is a plan view of a portion of one of the halves of FIG. 4.  
  FIG. 5 is a cross-sectional end view of the structure of FIG. 3 along the lines 55 of FIG. 4A. FIG. 5 illustrates one embodiment of microwave loads.  
 FIG. 6 is an end view of an alternate embodiment of a microwave load for the structure of FIGS. 3-5.  
 DETAILED DESCRIPTION OF THE PREFERRED I EMBODIMENT Referring to the (0-13 diagram of FIG. 2, the CSRB slow wave structure 24 of the FIG. 1 travelling wave tube (TWT) includes a ring bar circuit 19 having a plurality of frequency passbands 10a, 10b and which alternate with stopbands 12a and 12b. Ring bar circuit.  
 19 further has an intended operating bandwidth 0,, of frequencies within the lowest. frequency, i.e., fundamental, passband 10a. Various different spurious oscillation frequencies are associated with frequencies in the various passbands 10a, 10b and 100 above the operating bandwidth O These oscillation frequencies are excited as a function of the velocity or voltage of the electrom beam (not shown longitudinally passing through the center of ring bar circuit 19. At the lowest electrom beam voltages and velocities, the frequency of 11&#39; phase point b of fundamental passband 0,, is an oscillating frequency. At still higher electron beam velocities, such as generally used, the spurious oscillation frequencies are at frequencies in the second passband 10b characterized by greater than 11&#39; radian phase shift, between points a and 0. At higher beam voltages and velocities than normally used, the oscillation frequencies increase with increasing velocity from points (I to e in the third passband 100. Thus it is apparent that a broadband microwave load is required for absorbing spurious oscillation energy above the operating bandwidth O produced by a broad range of electron beam voltages such as encountered in the course of turning on a TWT. The straight lines v and v represent phase velocities of forward and backward waves equal to the velocity of light.  
  The present invention provides means for absorbing spurious oscillation energy from any helix-derived circuit which has no fundamental transverse component of electric field. The ring-bar circuit is the best known example of this class and the invention is illustrated by a comb-supported ring-bar circuit adapted for TWTs with high average power in which spurious oscillations are particularly harmful.  
  Referring particularly to FIGS. 3-5, as in the prior art the CSRB structure 24 of the present invention includes opposed CSRB top and bottom halves 16a and 16b, respectively including top and bottom sinuous ceramic support structure halves 17a and 17b for supporting opposed longitudinally repeating meander line conductor portions 190 and 19b in transverse registration upon the concave semicircular trough surfaces of the support structure halves. The meander line portions 19a and 19b are held in registration with each other by the support structure halves 17a and 17b to form the cylindrical ring bar circuit 19. Each of the support structure halves 17a and 17b includes a meandering vertical wall 26 having transverse wall portions 28 alternating with longitudinal side wall portions extending from top and bottom horizontal support metal slabs 23a and 23b. Transverse conductive vanes 21 are provided and arranged to extend from the support slabs 23a and 23b in the spaces formed between adjacent transverse wall portions 28 for broadbanding the CSRB structure. The provision of such vanes is disclosed and claimed in US Pat. No. 3,654,509 to Scott et al., filed Dec. 14, 1970 and issued Apr. 4, 1972 which is assigned to the same assignee as the present application.  
  The assembled CSRB circuit 24 has a generally rectangular external cross-section defined by the top and bottom support slabs 23a and 23b and the support structure side wall portions 25. A plurality of longitudinally repeating vertical slots 27 between adjacent support structure side wall portions 25 defined by the usual spaces between adjacent transverse wall portions 28 communicate with the ring bar circuit 19. These slots 27 on both sides of CSRB structures provide passageways for more strongly coupling spurious oscillation microwave energy out from the ring bar circuit 19.  
  Referring to FIGS. 3 and 5, one embodiment of the invention to prevent the spurious energy from being coupled to a load responsive to the slow wave structure output includes a pair of transversely directed, longitudinally elongated open waveguides 29 and 29&#39;. Waveguides 28 and 29, formed as rectangular crosssectional depressions in the usual side walls 30 and 30, are brazed to the sides of the upper and lower support slabs 23a and 23b. The side walls 30 and 30&#39; and open ends of the waveguides 29 and 29 abut against opposite side portions 25 and span the length of CSRB structure 24. The waveguides 29 and 29 are high pass transmission lines serving as coupling means having a constant vertical dimension h which is sufficiently small to cut off the waveguides to frequencies in the operating bandwidth 0,, while permitting propagation of frequencies above O The waveguides 29 and 29 couple spurious oscillation microwave energy out of the ring bar circuit 19 with the aid of the slots 27. Elongated, transversely tapered, microwave loads 31 and 31 are provided respectively abutting the closed far ends 33 and 33 of the waveguides 29 and 29&#39; for absorbing the coupled spurious oscillation microwave energy. The distributed loads 31 and 31&#39; are tapered so that loss increases as spurious microwave energy advances down waveguides 29 and 29&#39; toward the waveguide closed ends 33 and 33 in order to minimize reflections from the loads.  
  The transverse length of waveguides 29 and 29&#39; is preferably at least three times the height dimension I! so that dimension 11 extends for a sufficient distance to provide the waveguides 29 and 29&#39; with a sharp high pass cutoff characteristic and to isolate the loads 31 and 31 from any fringing fields in the operating bandwidth 0 The waveguides 29 and 29&#39; are substantially filled with a ceramic dielectric 35 having a dielectric constant greater than unity for increasing the electrical dimensions of the waveguide with respect to the physical dimensions, enabling the waveguides to be relatively compact. The triangular cross-section loads 31 and 31&#39; and the dielectric material may either be configured as mating parts having an overall rectangular crosssection when mated together for filling the waveguides, or the dielectric material may be configured as porous and rectangular, whereby the dielectric itself fills the waveguides. For the latter case the triangular crosssection loads 31 and 31 are formed by selectively impregnating a lossy material into the pores of dielectric 35.  
  An alternate load 39, illustrated in FIG. 6, is transversely tapered having a single angled planar surface 4] cutting the interior of the waveguides 31 and 31&#39;. This embodiment is particularly suited for production of the loads by selective impregnation into a porous rectangular cross-section dielectric 37 filling the waveguides. The rectangular cross-section dielectric rod 37 is soaked in a sugar solution up to the level of surface 41 and then fired to deposit carbon load 39 in porous dielectric 37. For this embodiment, the loads 39 and dielectric 37 may also be separate pieces mating along surface 41 to form an overall rectangular cross-section.  
  In testing the oscillation suppression technique in an X-band TWT CSRB structure, it was found that approximately two decibels of attenuation was provided in each period of the CSRB structure to a band of frequencies from approximately 16 to 40 GHz without influencing the characteristics of the CSRB structure within its operating bandwidth.  
  Having described several preferred embodiments of the present invention it should be apparent that numerous modifications are possible within its spirit and scope. Accordingly, it is intended that the material specifically described be interpreted as illustrative and not in a limiting sense.  
 What is claimed is:  
 1. A slow wave structure adapted to interact over a preselected band of frequencies with a linear beam of sageway containing an axis of symmetry, each loop conductively joined to one adjacent loop at one side of said passageway and to the other adjacent loop at a symmetrically opposite side, an envelope surrounding said circuit, dielectric means supporting said loops from said envelope, said envelope including a conducting wall, a waveguide forming part of said wall and having an open end extending in said axial direction overa plurality of periods of said circuit, said waveguide having two conducting walls parallel to said axis, the spacing between said. two waveguide walls being such that said waveguide is cut off for transmission transverse to said axis of wave energy in said band of frequencies, and wave absorptive means in said waveguide spaced from said open end.  
  2. The apparatus of claim 1 wherein said dielectric means comprises a comb with teeth supporting said loops.  
  3. The slow wave structure of claim 1 wherein two said waveguide walls are equally spaced from a plane containing said axis.  
  4. The apparatus of claim ll wherein said wave absorptive means has distributed loss increasing with distance from said open end.  
  5. The slow wave structure of claim 1 wherein the transverse electric length of said waveguide is at least three times the electric vertical height of said waveguide.  
  6. The slow wave structure of claim 1 wherein said wave absorptive means comprises lossy material substantially spanning the axial extent of said waveguide.  
  7. The apparatus of claim 6 wherein said lossy material is transversely tapered to a lesser height toward said open end and wherein said structure further comprises an elongated transversely tapered dielectric material between said lossy material and said open end, the tapers of said dielectric material and said lossy material mating so that the two said materials substantially fill said waveguide means.