US 2901712 A
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Description (OCR text may contain errors)
Aug. 25, 1959 A. G. HOGG MATCHING DEVICE Filed Ma rch 22, 1957 INVENTOR. HHRUN E. H0513 BY M a United States The invention relates to an impedancematching device fora cavity coupler, and has for an object'to provide an improved impedance match between a cavity coupler and its associated coaxial transmission line.
The invention is particularly adapted for'use witha traveling wave tube coupler: having a hollow inner poratent tion positioned within a hollow outer portion, and a coaxial transmission line which provides a connection between the traveling wave tu-be' coupler and external circuitry. The outer conductor of the coaxial line is terminated and connected to'the outer portion ofthe coupler. The inner conductor of the coaxial lineextends within the hollow outer portion of the coupler and is terminated at and connected to the inner portion of the coupler. The delay line or helical portion of a traveling wave tube fits within the hollow innerportion of the coupler, and radio frequency energy is applied to or derived frorn'the helical portion of the traveling wave tube through the coupler. The other end of the transmission line isconnected' to external circuitry; In the use of the coupler just described, it has been found that the impedance of the coaxial transmission line is not always-matched'tothe coupler impedance aswell asmight be desiredlj The invention provides means for matching these impedances, and comprises a metallic strip or tab which surroundsa portion of, but is insulated from, the inner conductor of the line within the outer portion of the coupler, and which is connected to the outer portion; The'strip ofconducting material provides a capacitance between the inner conductor of the coaxial line and the outer portion of the coupler. If the ends of the strip are passed through a slot in the outer portion of the coupler, it is possible to vary the capacitanceprovided by the strip by moving the strip along the slot. When the optimum capacitance is determined, the strip may be fastened and connected by any suitable means. The invention is explained in detail in connection with the accompanying drawing,inwhich: 1 Fig. 1 shows a-longitudinal sectional view of the coupler utilizing the invention; and Fig. 2 shows a perspective view of the tuning strip a it is used with the coaxial transmission line and-the cylindrical outer portion of the coupler, with most of the coupler broken away to more clearly illustrate the manner of connection of the strip. v
Fig. 1 shows a coupler 10 positioned on a traveling wave tube 12. While the coupler 10 is positioned at the input end of the traveling wave tube 12, the coupler 10 can be used equally well at the output end of the traveling wave tube 12. The traveling wave tube 12 comprises a socket 14, an enlarged bulb 16 containing heater elements and an electron gun for the traveling wave tube 10, and an elongated cylindrical neck or tube '18. The elongated tube 18 contains the delay line of the traveling wave tube 12, which in this case is shown in the form of a helix 20. Input radio frequency signals are applied to the helix by means of an antenna-like element 22 which is connected to the end of the helix 20 and folded back in the directionof the output end of the traveling wave tube 12. The element 22 is terminated in a sleeve 24 which surrounds the helix 22 and which serves to provide an effective coupling between the antenna-like element 22 and the coupler 10.
The coupler 10 comprises a metallic, cylindrically shaped outer portion or casing 30, and two coaxially arranged, metallic, cylindrically shaped inner portions 32 34. The coupler 10 is closed at its ends by metallic end plates 38, and surrounds the neck or tube 18 of the travel ing wave tube 12. The confronting edges of the inner portions 32, 34 are separated by a gap 36. The outer portion 30 of the coupler 10 is provided with a longitudinal slot 39 for reasons which will be explained. The inner portions 32, 34 of the coupler 10 are dimensioned so that the'neck or tube 18 of the traveling wave tube 12 passes through the inner portions 32, 34 with no more clearance than necessary to permit expansion because of changes in temperature. The gap 36 permits radio frequency energy in the coupler 10 to be coupled to the traveling wave tube 12. Thisvradio frequency energy is applied to the coupler by means of a coaxial transmis- 'sion line 40. The coaxial transmission line 40 enters the coupler 10 through one end. At the point of entrance of the coaxial transmission line 40, the outer conductor 42 is terminated and connected to the coupler 10. The inner conductor 44 of the coaxial transmission line 40 continues within the coupler 10 toward the other end, and terminates at and connects to the cylindrical inner portion 32 of the coupler 1G. The dielectric sleeve cover material 46 surrounding the inner conductor 44 of the coaxial transmission line 40 also extends along the inner conductor 44 to its point of termination. For convenience, the one inner portion 32 is fixeg within the 'coupler10, and the other inner portion 34 may be adjusted in a' lengthwise direction to vary the length of gap '36; A set screw 43 is provided to holdthe inner portion 34in a fixed position relative to the outer portion 30. This'feature permits the gap 36 between the two inner portions 32, 34 of the coupler 10 to be adjusted to provide the desired amount of coupling between the coupler 10 and the traveling wave tube 12. The coupling between the coupler 10 and the traveling wave tube 12 results from capacity coupling or coupling by the coupler 10 acting as a cavity resonator, or a combination of both means of coupling; However, the operation of the invention does not depend upon the exact nature of the coupling between the coupler 10 and the traveling wave tube 12.
'As"previously mentioned, ithas been found that the impedance match between the coupler 10 and the travelingwavef tube'12 is not always optimum, particularly where manufacturing conditions permit certain tolerances in the traveling wave tube 12 and in the coupler 10. In order that .the impedance match between the coupler lll'and the traveling wave tube 12 may be improved, the invention provides a capacitance between the inner conductor 44 of the coaxial transmission line 40 and the outer portion30 of the coupler 10. This. capacitance comprises a strip 50 of conductive metal, such as copper, which is ibent'or'shaped to form a generally circular configuration which surrounds the dielectric material 46 of the coaxial transmission line 40 and is positioned so that the ends 52 of the strip 50 extend through the longitudinal slot 39 in the outer portion 30 of the coupler 10. In Fig. l, the strip 50 is shown in dotted lines with its ends 52 extending through the slot 39 in the outer portion 30 of the coupler =10. Once the desired position of the strip 50 is found, the strip 50 is soldered to the coupler 10 and its ends 52 are clipped oil to provide a smooth and compact arrangement. This feature is shown in detail in Fig. 2. The capacitance is provided by the portion of the strip 50 that surrounds the dielectric material 46 of the coaxial transmission line 40 serving as one plate of the capacitance, the dielectric material 46 of the coaxial transmission line 40 serving as the dielectric material for the capacitance, and the inner conductor 44 of the coaxial transmission 'line 40 serving as the other plate of the capacitance. The magnitude of capacitance provided by the strip may be varied by using strips of difierent widths, or by' using several strips. The ettective phase of the susceptance produced by the capacitance, as viewed from any fixed point on the coaxial transmission line 40, may be varied by moving the strip 50 to the left or right as indicated by the arrows shown in Fig. 1.
The matching device in accordance with the invention has been found to give a very good impedance match between the coupler and the coaxial transmission line 40 over a very wide band of frequencies. For example, in apparatus usingthe RCA 8681 traveling wave tube, it was found that by the use of this matching device, a voltage standing wave ratio of less than 1.5 could be obtained in the frequency band between 2500 megacycles and 3500 megacycles. However, without the matching device the best standing wave ratio that could be obtained over this same band was 2.0. Furthermore,
since the invention permits the capacitance to be located within the coupler as close as possible to the discontinuity to be matched, the maximum possible bandwidth can be obtained. The matching device has proved useful in obtaining improved impedance matches over wider bands of frequencies, and in compensating for the differences between individual traveling wave tubes and couplers which arise through manufacturing tolerances.
The matching device for traveling wave tube couplers is easily adjusted, and is low in cost. It provides a means of introducing an adjustable capacity within the coupler to reduce reflections caused by impedance mismatches in the traveling wave tubes in the couplers.
The invention claimed is:
' 1. In combination, a hollow coupler having a hollow metallic casing, a transmission line having an inner conductor covered with insulation extending within and coupled to said coupler, and means for matching the impedance of said coupler to said transmission line, said means comprising a metallic strip wrapped around the insulated portion of said conductor within the coupler and in close proximity to the insulation, said strip having closely spaced straight portions extending to and directly connected at their ends to said casing.
2. In combination, a hollow metallic cavity coupler having a hollow metallic casing and an inner portion positioned within said hollow casing, a transmission line having an inner conductor, an outer conductor, and a di electric sleeve therebetween and covering said inner conductor, said inner conductor and said dielectric material extending within said hollow coupler, said inner conductor being arranged in physically parallel relation and coupled to said inner portion, and means for matching the impedance of said hollow coupler to said transmission line at'the frequency of the waves to be carried by said line, said means comprising a metallic strip wrapped around and in close proximity to said dielectric material within said hollow coupler, the ends of such strip being directly connected to saidcasing.
3. A matching device for use with a coaxial transmission line and a traveling wave tube coupler, said coaxial line having an inner conductor, an outer conductor, and dielectric material positioned therebetween, and said coupler having a hollow cylindrically shaped outer portion and having a cylindrically shaped inner portion positioned within said outer portion and extending longitudinally therein for at least a part of the length of said outer portion, comprising means terminating said outer conductor at a first point on said outer portion and connecting said outer conductorto said outer portion at said first point, said inner conductor and said dielectric material being positioned within said outer portion between said first point and said inner portion, means terminating said inner conductor at a second point on said inner portion and connecting said inner conductor to said inner portion at said second point, said dielectric material extending between said first and said second points, a conductive strip positioned around said dielectric material between said first and second points, and means connecting said strip to said outer portion.
p 4. Amatchingdevice for a traveling wave tube coupler having an electrically conducting, cylindrically shaped, hollow outer portion that has a longitudinal slot therein for at least apart of the length thereof and having an electrically conducting, cylindrically shaped hollow inner portion positioned within said outerportion and extending longitudinally therein for at least a part of the length of said outer portion, said inner portion being adapted to surround a portion of said traveling wave tube, comprising a coaxial transmission line having its outer conductor terminated at and connected to said outer portion and its'inner conductor extending within said coupler and terminated at and connected to said inner portion, and a conductive strip positioned within said hollow outer portion around and adjacent .to but insulated from said inner conductor with the ends ofv said strip passing through said slot and connected to said outer portion.
5. -A device for matching the impedance of a coaxial transmission line to a traveling wave tube coupler, said transmission line having an innerconductor, an outer conductor, and a dielectric material therebetween, and said coupler having a hollow outer metallic portion that has a longitudinal slot therein for at least a part of the length thereof and having a hollow inner metallic portion positioned within said outer portion and extending longitudinally therein for at least a part of the length of said outer portion, said innenportion being adapted to surround a portion of said traveling wave tube, said coaxial transmission line having its outer, conductor terminated at and connected to said outer portion, its inner conductor terminated at and connected to said inner portion at a location within said outer portion, and its dielectric material extending along said inner conductor between said inner and outer portions, comprising a conductive metallic strip positioned within said outer portion around said dielectric material and said inner conductor with the ends of said strip passing through said slot, and means connect: ing said strip to said outer portion at theedges of said slot.
References Cited in the tile of thispatent V FOREIGN PATENTS France i Aug. 13, 1945 Aus r lia .H- ..Nov. 28, 1946