US2405437A

US2405437A - Impedance matching transformer

Info

Publication number: US2405437A
Application number: US456941A
Authority: US
Inventors: Laurance M Leeds
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1942-09-01
Filing date: 1942-09-01
Publication date: 1946-08-06
Anticipated expiration: 1963-08-06
Also published as: FR949099A; BE478854A

Description

Aug. 6, 1946. L. M. LEEDS IMPEDANCE MATCHING TRANSFORMER Filed Sept. 1, 1942 w is Inventor: Laurence M. Leeds, b Wan 17 2M4,

His Attorney.

Patented Aug. 6, 1946 IIVIPEDANCE MATCHING TRANSFORMER Laurance M. Leeds, Rotterdam Junction, N. Y., assignor to General Electric Company, a corporation of New York Application September 1, 1942, Serial No. 456,941

2 Claims.

My invention relates to impedance matching transformers for a concentric transmission line and it has for its object to provide an improved impedance matching means for two dissimilar sections of such a line. I The use of a transmission line having a length equal to a quarter of a wave length, or any odd multiple thereof, of the wave to be transmitted, to match two unequal'impedances is well known. In the past various types of balanced and unbalanced lines, such as a two-wire balanced open line, two bar or strip balanced open lines, coaxial unbalanced lines, and similar composite lines have been employed for this purpose. In general, in most practical applications, it is necessary to vary the surge impedance of the quarter wave line in order to obtain precise impedance matching. This has been accomplished, in the case of the two-wire line, by varying the spacing of the conductor elements. line, variation of the surge impedance has been accomplished by varying the ratio of the diameters of the conductors. This method of varying the surge impedance of a coaxial line is cumbersome, requires disassembly of the line, and the variation of surge impedance is not continuous. To obtain a continuous variation of the surge impedance of an impedance matching section and one which does not require disassembl of the line, my invention employs a shielded line in which the inner conductor. may or may not be coaxial.

It is an object of my invention, therefore, to provide in a transmission line an eccentric section for matching the impedance of one section of the linewith the impedanc of an adjacent section, the impedance of th eccentric section being the geometric mean of the impedances of the two sections,

It is a further object of my invention to provide an unbalanced impedance matching transformer which may be manufactured at low cost, which may be inserted in a transmission line without difliculty, and which may be adjusted with ease to provide precise impedance matchmg. I a t The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advan: tages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 is a section of a transmission line employing the impedance matching transformer ofmy invention; Fig. 2 is a sectional view of the transformer of Fig. 1 on the line 2-2, and Figs. 3 and 4 are modifications of my impedance matching transformer.

In the transmission line of Fig. 1, adjacent 580- For the concentric tions In and II, having different values of surge impedance, are connected together by means of the impedance matching transformer l2. As shown, the sections I0 and H comprise coaxial tubular outer conductors l3 and [4 having the same diameter and coaxial inner conductors l5 and I6 of different diameters. The inner conductors are held in spaced relation with respect to the outer conductors by means of suitable insulators 28. Section I 0 of the transmission line may be connected to some source of voltage, such as a radio transmitter, and section I l to some utilization means, for example an antenna. It will be realized of course that instead of connecting the transformer 12 to the section ll, it may be desired to connect this section directly to a load impedance.

In the above described transmission line, if the impedance of section II, that is the load impedance, is Zr and the impedance of section Ill, that is the sending end impedance, is ZS, the surge impedance Zn of the connecting line or quarter wave transformer, in accordance with well known transmission line theory, is related to the other impedances by the relation ZO ZsZT. In practical application it is difficult to .obtain the desired value of surge impedance for the matching line. Considerable experimentation and consequent disassembly of the transmission line are required before a satisfactory arrangement is ob tained.

In accordance with my invention, a precise value of Zn for-the impedance transformer I2 is obtained by making the electrical length of the section l2 equal to a quarter wave length at the angular velocity of the desired operating frequency and by providing in th section l2 an inner conductor I! which is eccentrically disposed with respect to the tubular outer conductor l8 and means foradjusting the amountof this eccentricity, conductor I! being connected to conductors l5 and U5 by means of the flexible cable l9. 1

Adjustment of 1 the position of conductor I! with respect tov the tubular conductor I8 is secured by means of the triadic insulator support shown in Fig. 2. The members 20 suitably secured to the outer conductor I8, as by brazing or welding, have internally threaded bores 2| in alignment with the three equally spaced coplanar v threaded openings 22 in the outer conductor I8.

Rodlike members 23 of suitable insulating material, havingenlarged head portions 24 externally threaded for engagement with the threads 2!, hold conductor I! in spaced relation with outer conductor [8. By adjustment of the threaded heads 24 in members 20, the position of conductor I! with respect to conductor 18 may be adjusted as desired. Member 20 is likewise threaded overa portion. of its outer surface for engagement with cooperating threads on the interior of cap member 25. For installations in which a gas-filled transmission line is used, sealing gasket 25 is provided between cap member 25 and a shoulder portion El on member 20.

The ratio of the outside diameter of conductor I! to the inside diameter of conductor 18 may or may not be different from the ratio of corresponding diameters in sections iii and H of the transmission line. In general, the impedance transformer I2 is used to match two lines or two sections of a line of different characteristic impedance and the above-mentioned diameter ratio for the section i2 is chosen to give substantially the correct surge, or characteristic, impedance for this matching section when the position of conductor l? with respect to conductor i8 is at approximately the midpoint of eccentric movement. In this way the surge impedance of the matching section l2 may be Varied above or below a calculated and expected value in order to obtain precise matching with sections it and H. Movement of the inner conductor I! from the midpoint of its eccentric path toward the axis of the transmission line is efi'ective to increase the surge impedance of the matching section it, while movement further from the axis is effective to decrease the surge impedance of this section.

The expression for the surge impedance Z of the matching section l2 for any eccentricity may be derived in the following manner. It can be shown mathematically that the capacitance C of such a line is where a=the outside diameter of the inner conductor l'l, b=the inside diameter of the outer conductor I8,

and c=the eccentricity of the conductor I'i, that is,

the distance between the axis of conductor l1 and the axis of conductor l8.

Since, at high frequencies the surge impedance and the velocity of propagation,

v= vi? the surge impedance can be determined. Taking suitable regard for the units, it is found that 4 33 of the section 32 in a non-coaxial position.

Conductor 33 is eccentrically mounted on bearing rods 34 non-coaxially supported in

inner conductors

35 and 36. This particular construction is especially suitable for use in a cable which is not gas-filled. In such a case, the eccentricity of conductor 33 may be adjusted by inserting a suitable tool through opening 3! in the outer conductor 38 and rotating the conductor 33 on the bearings 34. The triadic insulator support shown in Fig. 2, of course, may be used in conjunction with matching section 32 as a means for more precisely determining and maintaining the proper eccentricity of conductor 33.

In the modification shown in Fig. 4, the inner conductors of the sections 40 and 4! and the matching section 42 are rigidly connected together and precise impedance matching of section 42 with sections 40 and 4! is obtained by moving the position of the tubular-outer conductor 13 with respect to thesolid inner conductor 46. This construction is particularly desirable Where the conductors of the respective sections are of different diameter. In such a case, the inner conductors 4 45, and M are rigidly connected together, as by brazing or welding. The tubular outer conductor 43 is joined to outer conductors 37 and 43 by means of the Sylphons or corrugated sections l9 and 59. Movement of the outer conductor of one section relative to the outer conductors of the other sections is prevented by means of th supporting arrangement comprising the plate member 5!, the clamping rings 52 and 53 and the triadic supporting means 54. The rings 52 and 53 may be Welded to the plate 51 and ma be either brazed or welded to the outer conductors M and 48 or clamped thereto in any well known manner. The triadic supporting means comprises the ring member 55 suitably se- 40 cured to plate 5! and the thumb-screw members as threadedly engaging ring 55. By adjustment of the screws 56 in a well known manner, the position of the outer conductor 43 with respect to the inner conductor in may be varied to obtai precise impedance matching.

It is readily apparent that the equation for Z0 given above for the arrangement of Fig. v1 applies equally well to the arrangement of Figs. 3 and 4.

It is thus seen that my invention provides an unbalanced impedance matching transformer in which the surge impedance of the transformer can be varied for matching purposes, the variation being accomplished by moving one of the conductors of the matching transformer to .a non-coaxial position. In the transformers described above, continuous variation of the surge impedance of the matching section is provided without disassembly of the transmission line. Moreover, the value of the required matching impedance may be obtained with speed, with ease, and with precision.

While I have shown particular embodiments In the transmission line of Fig. 3, the sections 3E! and 3| of different impedance value and the matching section 32 have inner conductors of equal diameter and outer conductors of unequal diameter. In this figure, too, there is shown another method of supporting the inner conductor of my invention, it will of course be understood that I do not wishto be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any 7 such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

A transmission line section adapted for connection between adjacent ends of two concentric transmission lines of unequal surge impedance to match the impedance of one of said lines to that of the other, said transmission line section comprising an outer conductor adapted for connection between the outer conductors of said concentric lines and an inner conductor adapted for connection between the inner conductors of said concentric lines, means to move one of said conductors of said section in a direction transverse to the other conductor while maintaining said one conductor substantially parallel therewith to vary the eccentricity of said section to match said impedances, and yielding electrical connections between said one conductor and the conductors of said concentric lines to which it is connected to allow said movement by said means in said transverse direction.

2. In a concentric transmission line system, the

combination of a continuous and substantially rigid inner conductor, a pair of tubular outer conductors coaxial with said inner conductor and forming therewith two sections of concentric transmission line having difierent characteristic impedances, said outer conductors having their adjacent ends spaced apart by a distance equal to a quarter wave length at the operating frequency of said system, a third tubular outer conductor flexibly connected between said ends, said third conductor having a length equal to a quarter wave length at said frequenc and forming with said inner conductor an impedance matching section of transmission line connected between said two sections, and means for moving said third conductor in a direction transverse to said inner conductor to vary the eccentricity of said impedance matching section to match said impedances, said pair of outer conductors and said third conductor having substantially uniform thickness throughout their lengths.

LAURANCE M. LEEDS.