US2376725A

US2376725A - End seal for transmission lines

Info

Publication number: US2376725A
Application number: US432844A
Authority: US
Inventors: Richardson Marie Maurer; Marvel W Scheldorf
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1942-02-28
Filing date: 1942-02-28
Publication date: 1945-05-22
Anticipated expiration: 1962-05-22

Description

y 1945- L. w. RICHARDSON EI AL 2,376,725

END SEAL FQR TRANSMISSION LINES I Filed Feb. 28, 1942 Lycurgus W. RichardsomDecease'd, Marie Maurer Richardson, Administratr'ix,

Marvel WScheldorf,

b WW6. XLAJW Their Attorney.

Patented May 22, 1945 END SEAL FOR TRANSMISSION LINES Lycurgus W. Richardson, deceased, late of Scotia, N. Y., by Marie Maurer Richardson, administratrix, Scotia, N. Y., and Marvel W. Scheldorf,

Schenectady, N. Y., assignors to General Electrlc Company, a corporation of New York Application February 28, 1942, Serial No.'432,844

2 Claims. 174-19) This invention relates to transmission lines, and more particularly to mechanical connections between the two conductors thereof.

Conductors which are utilized for the transmission of high frequency waves are generally used in pairs spaced relatively closely to minimize radiation therefrom. Spacing between such pairs of conductors, and the sizes of the individual conductors, determine certain transmission characteristics of the transmission line, one of which is known as the surge impedance, or characteristic impedance. Abrupt changes in such impedance along a transmission line generally produce deleterious effects, as is well known in the art. When two such conductors are spaced closely together, it is therefore necessary to use insulating means between the conductors to maintain proper spacing therebetween under the effect of external forces.

It is an object of the invention to provide new and improved means whereby the two conductors of such a transmission line may be maintained in proper spaced relation with minimum variation in impedance along the line.

It is a further object of the invention to provide such means which is especially useful for maintaining the proper spacing between two such conductors at the ends thereof.

It is an additional object of the invention to provide such spacing means which; are rugged and highly resistant to th effect of external forces and the elements, and which is at the same time simple and easily constructed and assembled. I

The invention is particularly useful with coaxial transmission lines, in which the spacing means may be utilized for closing the passage between conductors against the passage of gas under pressure, used for insulation purposes, and against moisture and the like. It is accordingly also an object of the invention to provide such insulating means, particularly useful in coaxial transmission lines, Whose configuration is such, in conjunction with the coaxial lines, as to make the total length of each line, including its end connections to associated apparatus, substantially equal to the total length of the other line.

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself. both as to its organization and manner of operation. together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing, in which as Fig. 1 of the drawing is a sectional view along the axis of a transmission line embodying the invention, and Fig. 2 is a modification thereof.

In Fig. 1 a coaxial transmission line including an outer cylindrical conductor in and an inner cylindrical conductor I I is terminated near a generally disc-shaped insulator l2. A threaded member liextends through the insulator II and has a plug-shaped end It soldered or brazed within the end of the inner conductor H. A metal washer I5 is placed over the threaded member l3 adjacent the plug l4 and firmly holds a disc I8 of packing material within the recess 16a in the insulator II. The disc I6 is preferably made of synthetic rubber, as for example, polyisobutylene compound, which is soft enough to spread and make a gas-tight seal between the center conductor II and the insulator I2. At the same time such material has long life when subjected to high frequency currents, light, oil, heat and most gases.

On the outside of the insulator l2 a disc ll, of some soft material such as pure aluminum or annealed copper, is placed over the threaded member l3, followed by a metal washer 08, a lock washer l9, and a nut 20 which is screwed on the threaded member l3 to compress the insulator l2 and the discs l6 and I1 tightly between the plug l4 and the nut 20. Due to their softness, the discs l6 and I1 distribute pressure uniformly over the central surface of the insulator i2, and minimize the chances of breakage. A second nut 2| is provided for convenience in making electric connection through the threaded member l3 to the center conductor 5 I.

The insulator H, as is clearly shown in the cross-sectional view thereof is made somewhat dome-shaped, the under side of the dome facing the interior of the transmission line. The outer part of the under face of the domed insulator is made flat in a plane perpendicular to the conductor ID to form a circular sealing surface 3 2.

The cylindrical outer conductor I0 is spun at its end portion near the insulator l2 into a conical configuration 30 and the extreme end of the coned portion 30 so formed is further spun into a flange Si in a plane substantially perpendicular to the conductors Ill and II. The flange 3| is shaped thus parallel to surface 32' to conform thereto and provide good gas sealing therebetween. This flange 3| and surface 32 may, if desired, take other mutually conforming shapes. A ring 33 of synthetic rubber, or other suitable material, is placed between the flange 3| and the sealing surface 32 of the insulator l2, and a metal clamping ring N is pressed toward the insulator l2 byscrews 35 suitably spaced therearound and extending through the ring 34, the flange 3|, the ring 33, and into threaded holes in the insulator l2.

, The dome shape of the insulator i2 provides sumcient mechanical strength so that gas under pressure may be confined between the conductors I. and II to increase the breakdown voltage therebetween, and also to prevent breaking of the line with the consequent introduction of moisture.

the rubber disc It provide sufllcient sealing between the insulator l2 and the conductors Hi and II so that such gas under pressure suffers no leakage.

The use of the insulator 12 provides a dielectric path between the screw member II and the flange II, which path is sumciently long to withstand the voltage between conductors in and II without breakdown, and due to the shape of the insulator I! there is less capacity between the conductors It and H at the insulator than would be the case if the usual type of insulator were provided between the conductors l and II without altering their sizes. Although the use of this long ceramic path between the flange 3| and the screw member It tends to minimize this capacity, yet this capacity is still larger than the capacity existing in a similar length of the transmission line between the conductors in and II where no insulator is present. If suitable provision were not made for counteracting the effect of this added capacity between the conductors ill and II, there would be a discontinuity in the transmission line, with consequent deleterious eflects.

In the section of the transmission line including the coned portion 30 of the outer conductor ID, the two conductors l0 and II are spaced farther apart than at other sections of the trans mission line. Due to the fact that they are so spaced, there is less capacity between the coned portion 30 of the outer conductor l0 and the inner conductor H than there is between other portions of the outer conductor l0 and the inner conductor I i. At the same time, the space en- Furthermore, the rubber sealing ring 33 and v closed by the conductors l0 and II in that section of the transmission line including the conical section 30 is greater than the space enclosed by a similar length of the transmission line in which the conductor I0 is uniform in diameter. Due to the fact that the conductors l0 and II enclose such additional space at the conical portion 30, the inductance of the transmission line is greater at this conical portion 30 than at other portions of the line.

At each incremental section of the coned portion 30 of the transmission line there is a particular characteristic impedance, which-varies from a value equal to that of the transmission line where the coned portion 30 is joined to the uniform portion of the transmission line to a higher surge impedance at the open end of the coned portion 30. Due to the fact that this coned portion 30 thus has a smoothly varying characteristic impedance, it tends to act as an impedance transformer.

When a particular material is used for the insulator l2 and a particular shape therefor is utilized, the amount of capacitance introduced between the flange 3| of the outer conductor Iiiand the threaded member II is fixed. This does not mean that the capacity between the coned portion 30 and the inner conductor H is fixed,

insulator l2. small, the coned portion 3! becomes very long,

for that capacity depends upon the angle to which the coned portion 30 is flared. If this angle, formed by the inner surface of the coned portion "with the axis of the transmission line is too great, there is increased capacity between the coned portion at through the insulator l2 and the threaded member I3. I! this angle be too small, the inductance added by the flaring of. the coned portion 30 may be more than enough to compensate for capacity effect caused by the Furthermore, if this angle is too and may approach a length equal to a substantial portion of a wave length of a wave transmitted through the transmission line. It is not desirable that the length of the coned portion 30 should be so great.

In any particular situation, where the dimensions of the inner and outer conductors II and III, and the shape and material of the insulator I! are chosen, a suitable choice of the angle made by the coned section 30 with the axis of the transmission line may be made, So that the progressive simultaneous decrease in capacity and increase in inductance of the coned portion 30, progressing from the transmission line toward the insulator l2, may be made effective just to counteract the changein surge impedance of the transmission line at the insulator l2 due to the increased capacity between .the conductors l0 and II through the insulator i2.

Although the optimum angle between the inner surface of the conical portion 30 and the axis of. the transmission line generally lies somewhere between 20 and 60 degrees in any particular case, this angle depends entirely on the particular structure utilized for the insulator l2 and the transmission line I, II. The dimensions of the transmission line III, II are generally fixed by considerations not mentioned herein, such as the characteristic impedance of the source of oscillations, or the load therefor. It is generally desirable to make the insulator l2 as thin as possible to reduce capacitance therethrough, and

yet thick enough to have sufflcient strength to maintain proper spacing between the conductors l0 and II and to resist the pressure of gas in the transmission line.

It is generally desirable that the overall length of the conical portion 30 and the insulator 12 be a small portion of a wave length of a wave transmitted through the transmission line l0, II. If the particular structure chosen for the insulator l2 be such as to'require a very small angle between the inner surface of the coned portion 30 and the axis of the transmission line, thereby making the coned portion 30 of considerable length with respect to a wave length, it is generally desirable to change the formation, and possibly the material, of the insulator If in such fashion that the angle between the inner surface of the coned portion 30 and the axis of the transmission line may be increased substantially.

When the invention is used in an end connection for a coaxial transmission line, as illustrated in Fig. 1, especially advantageous connections may be made to desired apparatus. Such connections are made by means of a short transmission line comprising two parallel conductors without introducing large amounts of inductance, and consequent discontinuity, at any part of the coaxial transmission line or of the parallel conductor connections. In Fig, 1 a wide flat conductor 40 is fastened to the conducting ring 34 by means of a screw I, and a parallel wide, flat conductor 42 is fastened upon the threaded member l3 by means of the nut 2|. If desired, the characteristic impedance between the wide flat conductors 4e and 42 may be made the same as the characteristic impedance between the conductors ill and l i. To this end, the spacing between the

conductors

40 and 42, and the width of these conductors, may be given such values that there is any desired characteristic impedance therebetween. It should be noted particularly that the conductor 45 extends only a small distance beyond the end of the conductor 42, so that no substantial extra inductance is introduced between the conductors 45 and 42, as is the case in connections made with end terminals provided previously for coaxial transmission lines.

In Fig. 2 a connecting arrangement is shown in which a connection is made between a coaxial transmission line including the conductors ill and II and a second similar coaxial transmission line including an outer conductor 50 and an inner conductor 5|. Certain elements of the device of Fig. 2 are like those of the arrangement illustrated in Fig. 1, and are designated by like reference numerals. A flat insulating disc-52 is interposed between the two coaxial transmission lines, the flat flanged portion 3| of the outer conductor bearing through washer 33 on one side of the disc 52, and a similar flat flanged portion 53 of the outer conductor 56 bearing through a washer 54 against the other side of the insulating disc 52.

The outer conductors I0 and 50 of the two transmission lines are held together by 'bolts 55 extending through the conducting ring 34, flange 3 I, washer 33, insulating disc 52, washer 54, flange 53, and another conducting ring 56 similar to conducting ring 34.

The threaded member l3 at the end of the inner conductor H extends through thecenter of the insulating disc 52 and through washers 51 and 58, respectively, similar to washers l6 and I5, into a metal plug 59 fastened into the end of the inner conductor The threaded member 13 is screwed tightly into the plug 59 to maintain a flrm'electrical connection between the inner conductors l l and 5|.

A hole Gll is provided through the insulating disc 52 to allow for the passage of fluid therethrough. Where gas under pressure is maintained between the inner conductors I l and 5! and the outer conductors l0 and 50, the pressure of the gas in various sections of the line is equalized through the hole 60. There may, if desired, be more than one of the holes 60, as for example, three such holes.

The electrical structure of the joint including the insulating disc 52 between the twocoaxial transmission lines in Fig. 2 is similar to that of the arrangement of Fig. 1. The disc 52 is of larger diameter than the outer conductors i0 and 50, thereby minimizing the capacitance added between the inner and outer conductors of the two transmission lines. This added capacitance is compensated by the inductance added between the inner and outer conductors of the two transmission lines by the conical portion 35 of the outer conductor l0 and by a similar conical portion 6| of the outer conductor 50.

Where, as in the arrangement of Fig. 2, there are two conical portions 30 and it, each of which introduces added inductance into its coaxial transmission line, the inductance added by each of these conical portions 30 and 5! need not be so great as the inductance added by the single conical portion in the arrangement of Fig. 1. Generally, therefore, the conical portion 3a in the arrangement of Fig. 2 need not be as long as the conical portion 35 in the arrangement of Fig. 1.

While we have shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and we, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent in the United States is:

1. An end seal for a high frequency transmission line of the type having a tubular outer conductor and a coaxially disposed inner conductor comprising, a disc of insulating material afixed centrally to one end of said inner conductor and having a diameter larger than the diameter of said outer conductor ,a section of said outer conductor at the end thereof adjacent said one end of said inner conductor being flared outwardly at an angle to the periphery of said disc and being affixed thereto, said flared section of said outer conductor having a. length substantially smaller than the length of a wave transmitted by said transmission line, the angle of said flared section being such that the increased inductance introduced between said conductors by said flared section is sufiicient to compensate substantially completely for the increased capacity introduced therebetween by said insulating disc, said disc being of a thin domelike shape to afiord maximum mechanical strength to resist mechanical forces between said conductors.

2. An end seal for a gas filled high frequency transmission line of the type having a tubular outer conductor and a coaxially disposed inner conductor comprising, a disc of insulating material aflixed centrally to one end of said inner conductor and having a diameter larger than the diameter of said outer conductor, a section of said outer conductor at the end thereof adjacent said one end of said inner conductor being flared outwardly at an angle between 20 and to the periphery of said disc and being atflxed thereto, said flared section of said outer conductor having a length substantially smaller than the length of a wave transmitted by said transmission line, the angle of said flared section being such that the increased inductance introduced between said conductors by said flared section'is suflicient to compensate substantially completely for the increased capacity introduced therebetween by said insulating disc, and fluid sealing means interposed between said flared section and said disc and between said inner conductor and said disc, said disc being of a thin domelike shape to afford maximum mechanical strength to resistmechanical forces between said conductors and the pressure of said gas within said line.

MARIE MAURER. RICHARDSON, Administratria: of the Estate of Lycurgus W. Richardson, Deceased.

MARVEL W. SCHELDORF.