US2436284A - Coaxial transmission line - Google Patents

Description

Feb. 17, 1948. L. A. BONDON 2,436,284

/ COAXIAL TRANSMISSION LINE Filed Dec. 26, 1946 IETET-EJ POL YTETEHFL 008057197 [/Vf Lewis A. Bondon Patented Feb. 17, 1948 UNITED STATES PATENT OFFICE COAXIAL TRANSMISSION LINE Lewis A. Bonclon, Arlington, N. J.

Application December 26, 1946, Serial No. 718,489 Claims. (01. lie-2e) This invention relates to coaxial transmission lines and has more particular reference to insulating assemblies for maintaining the conductors of such lines in concentric relation.

Conventional forms of coaxial transmission lines usually comprise a tubular inner conductor disposed in a tubular outer conductor and spaced concentrically therefrom by insulating spacing devices. One conventional type of spacing means comprises insulating rings or discs usually made of ceramic material and mounted on the periphery of the inner conductor at intervals therealong. The rings or discs are usually secured on the inner conductor by one of the following means:

(a) Rolled annular beads or expanded shoulders formed on the inner conductor on both sides of the insulating ring; (1)) soldered collars 0n the inner conductor on both sides of the insulating ring; (0) annular springs compressed to fit in undercuts in the discs and inner conductor; ((1) annular spring supported locking pins indexing in the inner conductor and with recesses in the discs or rings; and (e) friction.

Another conventional type of spacing means comprises radial insulating rods or pins usually made of ceramic material and secured to the inner conductor at intervals therealong. The rods or pins are usually secured to the inner conductor by one of the following means:

(a) Pins secured by cement to the surface of the inner conductor; (b) pins secured by annular staking at inner conductor; (0) pins supported by outer conductor and not attached to inner conductor; (d) pins secured by threaded inserts in inner conductor; (e) pins supported by supports through their body, then staked to the inner conductor.

All of these assemblies, whether of the insulating ring type or insulating pin type, are defic ent for one or more of following reasons:

a) Large proportions of solid dielectric; (b) sensitive to thermal shocks; (0) undesirable diamctric irregularities; (11) poor physical properties: (e) reduced amount of radio frequency conducting surface; (1) unpredictable dielectric sta bility; (g) diflicult to repair after assembly; and (h) costly and difficult to manufacture.

The object of all the foregoing methods of coax al line construction is to obtain a signal propagating medium having a propagating constant that is close to that provid d by air. This propagation constant is reduced as soon as dielectrics having specific inductive capacity values higher than air are introduced into the propagating medium. Therefore the desirable coaxial transmission line must have a minimum mass of low dielectric body as supporting and spacing elements as will allow maximum physical and thermal performance under various service conditions for long periods of time without need for replacement or maintenance.

In addition. to the foregoing, the inner and outer conductors must he maintained as accurately as possible in concentric relation throughout the transmission system in order to provide undistorted signal propagation from the signal source to the terminating elements. When momentary or sustained high voltage gradients exist along the line it is necessary to have this accurate concentric relation to provide a uniformly distributed electrical stress at the points of support to prevent localized dielectric heating and possible thermal failure and to prevent lower corona initiation voltages producing electrical flash-over at these points and a subsequent inoperative system. At these points of support it is necessary that all air be excluded between the dielectric and the inner conductor, chiefly because the voltage gradient is maximum at these points and will often cause local ionization, thereby reducing the corona initiation voltage of the tran mission line as well as introducing distortional effects on the propagating signals.

It is an object of the present invention to provide a coaxial transmission line in which the inner and outer conductors are held in accurate concentric relationship, throughout their length, by longitudinally spaced, angularly disposed insulating spacing assemblies, each of which comprises a pair of diametrically opposed insulating spacing members positioned between the conductors and held in position by an insulating pin passing through the inner conductor with its ends engaging the spacing members.

Another object of the invention is to provide a coaxial transmission line, as characterized above, in which the pins and spacing members are made of insulating material of such character and are assembled in such manner as to provide great resistance to thermal and physical shock.

Another object of the invention is to provide a coaxial transmission line, as characterized above. in which the spacing members and supporting pins are made of insulating materials which provide a propagating medium substantially the same as air, thus providing higher signal transmission efiiciency to permit its use at frequencies beyond the range of the conventional air dielectric coaxial transmission line.

Another object of the invention is to provide a coaxial transmission line, as characterized above, in which the supporting pins and the spacing members are made of insulating material of such character and are assembled in such manner as to obviate the necessity for the usual linear dimensional tol rances, thereby providing a. differential impedance. limit heretofore unobtainable for commercial application.

A further object of the invention is the provision of a coaxial transmission line that can be readily assembled and repaired in both factory and field and one that is relatively inexpensive tomanufacture.

Other objects and advantages will be apparent in the following specification, when considered together with the accompanying drawings, in

I which:

Fig. 1 is an elevational view with parts broken away of a coaxial transmission line constructed in accordance with the present invention;

Fig. 2 is an end view with certain parts shown in section, of the coaxial transmission line shown in Fig. 1;

Fig. 3 is an end view of one of the insulating spacers shown in Figs. 1 and 2; and

Fig. 4 is an elevational view of the insulating spacer shown in Fig. 3.

7 Referring to Figs. 1 and 2, there is shown a. coaxial transmission line comprising an outer conductor l and an inner conductor l l. The conductors are held in concentric relationship, throughout their length, by means of longitudinally spaced, angularly disposed insulating spacing assemblies mounted on the inner conductor.

Each insulating assembly comprises a diametrically opposed pair of spacing members or caps l2 interposed between the inner and outer con ductors and engaging diametrically opposed sections on the inner surface of the outer conductor and held in position by means of an elongated pin l3 extending through diametrically opposed holes H-il formed in the inner conductor with its projecting ends |5l5 press-fitted into recesses iii-l6 formed in the caps l2.

Each of the caps I2 is identical in construction and, in the particular embodiment illustrated, is shown as a cylindrical member having a coaxial cylindrical recess l6 provided with a conically shaped bottom I! formed therein. One end of the cylinder is wedge-shaped, as indicated at l8, and the other end is made concave, as indicated at IS, with a surface curvature the same as the outer surface curvature of the inner conductor. The caps are preferably made of a compressible plastic organic insulating material such as polytetrafiuoroethylene. This is an extremely lowloss material. It is physically capable of maintaining operating efliciency at the elevated temperatures encountered in normal service and is also physically capable of maintaining intimate contact with the conducting surface of the inner conductor throughout the service life of the line. Cap members made of this material will not carbon-track on flash-over and do not require surface impregnating treatments to reduce its moisture absorption factor as the material is normally nonhydroscopic.

Each of the pins I3 is identical in construction and in the particular embodiment illustrated is shown as an elongated cylindrical member having conically shaped ends l5-l5. The pins are made of an organic insulating material, The material preferred is a thermosetting styrene copolymer such as ethyl vinyl benzine div nyl ben- 4 zene styrene copolymer. Such a copolymer is disclosed in U. 8. Patent No. 2,256,160, issued Septempber 16, 1941. The pins may also be made of methyl methacrylate or of ceramic material composed principally of magnesium oxide. Pins made ofthe foregoing named materials are not subject to deformation, cracking or plastic flow. They have great resistance to physical shock and are physically capable of maintainin maximum operative efliciency at the elevated temperatures encountered in normal service exposures. The diameters of the pins are such that they fit tightly in the holes i6, I4 formed in the inner conductor to eliminate any air therebetween.

In the preferred construction, the pins are approximately one-half the diameter of the caps and approximately one-fourth the diameter oi. the inner conductor.

In assembling the transmission line, the supporting pins i3 are fitted in the openings i4, H in the inner conductor. Then, the caps 12 with their wedge-shaped ends positioned longitudinally, are press-fitted onto the projecting ends of the pins and against the outer surface of the inner conductor.

The concave end IQ of each cap provides an intimate physical air-tight saddle fit with the outer surface of the inner conductor, as indicated at 20. Then, the inner conductor, with the spacing assemblies in place, is press-fitted into the outer conductor, the plastic material of which the cups are made being slightly compressed to make an intimate physical lit with the inner surface of the outer conductor. To facilitate the insertion of the cap member into the outer conductor, the longitudinal ends of the wedge-shaped end l8 may be bevelled. as indicated at 2|.

The insulating assemblies are longitudinally spaced at desired intervals and may be disposed on the inner conductor in any desired angular relation. In the particular embodiment illustrated, they are shown as being angularly disposed relative to adjacent assemblies.

Spacing assemblies employing a combination of extremely low-loss dielectrics, as above set forth, and assembled in the manner described, substantially eliminate all air in the dielectric field at the points of assembly and reduce the fiux density in the electric field at these points, The elimination of air in the dielectric field at the points of assembly minimizes the probability of any local corona. formation at these points. The low-loss properties of the dielectric combination at any service frequency encountered in the application of an air dielectric transmission line affords stable dielectric performance, which, with the positive centering of the inner conductor, provides a stable and continuously uniform propagating medium free from diametric discontinuities.

The spacing members or caps l2, being made of compressible plastic material press-fitted between the inner and outer conductors, permit the use of dimensional tolerances which cannot be effected with conventional inorganic materials without a large increase in processing cost and subsequent attendant assembly difficulties increasing the final cost.

A coaxial transmission line constructed in accordance with this invention, does not require the usual linear dimensional tolerances, thus providing a differential impedance limit heretofore unobtainable for commercial application. This provision will permit a wider field of application for coaxial transmission lines as such dimensional limitations restrict the development of high outer conductor along diametrically opposed lonstanding waves to less than 10% of the voltage gitudinal sections thereof and wherein said elonstanding wave ratio introduced at the signal gated insulating member is cylindrical and has source. its ends contoured to conform to the contour of In addition, such a construction provides a the inner surfaces of said recesses. minimum signal attenuation for a transmission 3. A coaxial transmission line, as set forth in line of a given impedance. This is due to the claim 1, wherein said elongated member is made low-loss dielectric combination, which will, for a oi a thermcsetting styrene copolymer.

given line impedance, permit the use or a larger 4. A coaxial transmission line, as set forth in center conductor to provide an increase in radio claim 1, wherein said elongated member is made frequency conducting surface with attending inof ethyl vinyl benzine divinyl benzine styrene cocrease in power handling capacity. polymer.

From the foregoing, it will be seen that there 5. A coaxial transmission line comprising tubuhas been provided a coaxial transmission line lar inner and outer conductors and spacing ashaving an impedance and providing a propagatsemblies radially disposed within said outer coning medium substantially the same as that of a ductor at predetermined spaced points throughtheoretical coaxial transmission line of the same out its length for maintaining said conductors in dimensions in which the inner conductor is supconcentric relation, each of said assemblies comported solely by air. prising a pair of diametrically opposed cylindrical While the invention is particularly described in 2 can members provided with coaxial recesses and connection with a coaxial transmission line havp ess-fit ed b ween the inner surface of the ing a tubular inner conductor, obviously, it is outer conductor and the outer surface of the equally well suited to a coaxial transmission line inner conductor, sa d p members being a e having a solid inner conductor. of polytetrafluoroethylene and having concave Obviously, the invention is not restricted to the inner ends to provide intimate physical contact particular modifications herein shown and dewith the outer surface of the inner conductor and scribed. wedge-shaped outer ends to provide intimate I claim: physical longitudinal line contact with the enter 1. A coaxial transmission line comprising tubuconductor, and an elongated cylindrical pin exlar inner and outer conductors and spacing astending diametrically through Said n r semblies radially. disposed within said outer conductor and having its ends press-fitted into the ductor at predetermined spaced points throughrecesses formed in said cap members, said pin out its length for maintaining said conductors in being made of ethyl vinyl benzene v ny benconcentric relation, each of said assemblies comzene styrene c D Y and having t 011- prising a pair of diametrically opposed insulating g toured to conform to the contour of the inner members press-fitted between the inner surface surfaces of said recesses.

of the outer conductor and the outer surface of LEWIS a nonnon. the inner conductor to provide intimate surface contacts between said insulating members and REFERENCES CITED said inner conductor, said members 00 1 1 1 1 8 0f 40 The following references are of record in the polytetrafluoroethylene having coaxial recesses m of this patent; formed therein and an elongated insulating member extending diametrically through said inner UNITED STATES PATENTS conductor and having its ends press-fitted into Number Name Date the recesses formed in said first named members. 46 2,216,893 Smith Oct. 8, 1940 2. A coaxial transmission line, as set forth in 2,218,921 Mueller Oct. 22, 1940 claim 1, wherein said first named insulating mem- 2,256,160 Britton Sept. 16, 1941 bers are cylindrical and have concave inner ends 2,287,201 Scott et al June 23, 1942 and wedge-shaped outer ends, said concave ends 2,397,568 Seaman Apr. 2, 1948 having a curvature substantially the same as the curvature of the outer surface of the inner con- OTHER REFERENCES ductor to provide intimate physical contact there- Article entitled Design data for beaded coaxial with, thereby excluding all air therebetween and lines," by Cox, published in Electronics Magazine said wedge-shaped outer ends provide intimate for May. 1946, pages -136. physical contactwith the inner surface of the $6

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