US2038973A - Electrical conducting system - Google Patents

Description

April 28, 1936. .1. F. WENTZ 2,038,973

ELECTRICAL CONDUCTING SYSTEM Filed Jan. 10, 1935 FIG.

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A B C D E INVENTOR J. F. WENTZ A T TORNE Y Patented Apr. 28, 1936 UNITED STATES PATENT OFFICE ELECTRICAL CONDUCTING SYSTEM Application January 10, 1935, Serial No. 1,149

In Great Britain and Japan March 16, 1933 7 Claims.

This invention relates to electrical conducting systems and more particularly to the mechanical structure of systems in which the conductors are in coaxial relation.

An object of the invention is to improve the mechanical and electrical properties of a coaxial conductor system.

A more particular object of the invention is to increase the ruggedness and flexibility of the in- 10 sulating structure separating the members of a coaxial conductor system, and to reduce the energy loss attributable to said structure.

For electrical signaling systems operating at frequencies extending into the radio frequency range a transmission system comprising a central conductor and a tubular return conductor coaxial therewith is especially well adapted. One

of the chief advantages of the structure is that the dielectric between the conductors is not subject to varying atmospheric conditions but can be maintained in a uniformly-dry condition. The

ideal dielectric from an electrical standpoint would be purely gaseous or vacuum. There would then be no solid material traversed by the electromagnetic fields'associated with the signaling currents, and the signal attenuation due to shunt impedance or dielectric losses would accordingly be low. The capacitance, another factor affecting signal attenuation, should likewise be as low as possible and for this reason the effective dielectric constant of the medium separating the conductors should be as nearly unity as possible.

The mechanical requirements of the system, however, demand that a suflicient amount of solid material be used to maintain the inner conductor in a central position withinthe outer conductor. Since the conducting pair is subject to repeated bending during manufacture and installation a certain degree of resiliency and ruggedness in the separating member is necessary.

In accordance with the present invention there is provided an insulating or separating member for coaxial conductors that is well adapted to meet the electrical and mechanical requirements above set forth. In brief, the separator comprises a pair of cords, filaments or the like, of insulating material twisted together and wound helically about the central conductor. This invention is H) disclosed but not claimed in applicants copending application for patent, Serial No. 630,552,

filed August 26, 1932 which has matured to Patent No, 2,018,477 dated October 22, 1935.

A feature of the foregoing construction is that 55 contact between, the separator and the conductors is not continuous. This not only tends to reduce conductive leakance but also reduces the amount of solid insulating material traversed by the highly concentrated electric field immediately around the central conductor. 5

Another feature of the invention is that in the preferred embodiment the insulating cords comprise partially acetylated cotton. Best results have been obtained by employing partially acetyl- I ated cotton in the form'of tape rolled to cylin- 10 drical shape. Acetylation, it has been found, increases the resiliency of the cotton so that air spaces are retained within the cords despite the repeated deformation of the separator that is incident to reeling, handling and installing the 1 conductor pair.

Other objects and features of the invention will appear from a consideration of the specific illustrative embodiment which is shown in the accompanying drawing and which is now to be described. 20 Fig. 1 shows a. partial longitudinal section of the conductor assemblage and Fig. 2 shows diagrammatically a plurality of plane transverse sections thereof.

Referring to the drawing, there is shown a 00- 25 axial conductor transmission line comprising a central copper wire I and a tubular composite return conductor 2 made up of profiled strips of the type shown in applicant's copending application for patent, supra. Thin iron tapes 3 are employed to bind the profiled strips together and the whole is encased in a lead sheath 4. The internal diameter of the outer conductor may range from a quarter inch to a half inch or more. The conducting pair is connected to the terminal circuits 5 of a carrier wave signaling system adapted to operate over a wide frequency range extending upwards to a maximum frequency of the order of megacycles per second.

Conductor l is maintained in its central position by means of the two similar cylindrical fila ments 6, 6, which are twisted together and wrapped helically around the central conductor.

In the preferred form, the helix is opposite in sense to the twist and its pitch is approximately four times the internal diameter of the outer conductor and twice the pitch of the twist. Point I support approximately every ninety degrees around the central conductor is thereby provided. 59

Considering the geometry of the cross-section of the assemblage, as shown in Fig. 2, it is obvious that less cross-sectional area is occupied by the double composite separating member than would be the case if the separator were a single helical filament of larger diameter or a triple or quadruple member. The most important advantage lies in the fact that the points of contact with the central conductor are less numerous and of smaller area than with a single filament or with three or more. In this respect the single filament member is poorest, the double filament member best, and members comprising three or more filaments approaching a stranded single filament, intermediate. Inter-conductor capacitance and dielectric losses accordingly are smallest for the double filament separator. As a practical matter, however, neither the outer conductor nor the individual filaments retain at all times the ideal circular cross-section since the conducting pair is subjected to bending and crushing during manufacture and installation. It is therefore necessary to exercise considerable care in the choice of the insulating material comprising the separator, with regard not simply to intrinsic dielectric constant and dielectric loss angle but to such factors as the ability of the material, or of material and structure, to withstand crushing and to restore itself to circular crosssection, the amount of material it is necessary to use to fill a given volume, that is, the density of the separator as distinguished from the density of the material comprising it, and flexibility.

Fibrous cellulosic materials such as cotton and silk meet, with varying degrees of perfection, the several mechanical and electrical requirements placed on the separator.

Thoroughly washed cotton spun into yarn has been found to make a Satisfactory filament for use in the composite separator.

In the preferred embodiment of applicants invention, the filaments of the separator are made up of partially acetylated cotton either in the form of yarn or as fabric tape rolled into cylindrical shape. Acetylation of the cotton has been found to improve markedly the electrical and mechanical properties of the separator. The acetylated cotton is less affected by moisture than other textile materials. Its dielectric constant and power factor are low. The resiliency of the cotton fibres is increased, so that particularly in the case of the rolled tape, the filaments more perfectly resume their original shape after any deforming stress is removed. For the same reason, air spaces within the filaments are retained to a greater degree and the dielectric properties of the separator are thereby enhanced.

To obtain best results, that is, minimum signal attenuation, attention should be given to the density of the filaments comprising the separator. Extremely compact, corded filaments would be desirable if there were only the mechanical requirements to be met; but because of the high frequency field traversing the space between conductors, dielectric losses might well be found to ofiset'to a large degree the advantages that the configuration ofiers.

0n the other hand, the filaments must not be so lacking in substance that they do not retain their shape when not under stress. Between these two extremes a fairly wide range of variation in density is practical, especially since the separator is not required to assist in maintaining the form of the outer conductor.

Other embodiments of the invention herein set forth will be obvious to those skilled in the art and it is intended that all such other embodiments be comprehended within the scope of the appended claims.

What is claimed is:

1. A radio frequency transmission system com prising a pair of coaxial conductors connected one as a return for the other, the outer of said conductors comprising a hollow self-supporting tube, and an elongated separating member consisting of two filaments of insulating material twisted together and spiralled about the inner of said conductors so that said member supports said inner conductor only at recurrent points where the longer axis of the cross-section of said member lies in a radial position between said conductors, the remainder of the space between said conductors being substantially gaseous.

2. A conducting system for transmission at frequencies of the order of megacycles per second comprising a central conductor and a self-supporting tubular return conductor coaxial therewith, a separator for maintaining said conductors in coaxial relation consisting of two like strands of cellulosic textile material, substantially circular in cross-section, twisted together and applied helically to said central conductor in such manner that said separator is in discontinuous contact with both of said conductors, the remainder of the space between said conductors being substantially gaseous.

3. A high frequency transmission system comprising a pair of coaxial conductors connected one as a return for the other, the outer of said conductors being self-supporting, and means for maintaining said conductors in coaxial relation consisting of a pair of circular strands of textile material twisted together and wound helically about the inner of said conductors.

4. A combination in accordance with claim 3 in which said material comprises partially acetylated cotton.

5. In a multiplex carrier telephone system operating over a wide range of frequencies extending upwards to a maximum of the order of megacycles per second, a transmission line comprising a central wire and a self-sustaining coaxial tubular conductor made up of inter-engaging profil'ed strips, means for supporting said central wire consisting of a pair of insulating filaments of equal diameter twisted together and wrapped helically around said wire in a helical sense opposite to that of the twist, the pitch of said twist and the pitch of the helical wrapping being of the order of two and four times, respectively, the internal diameter of said tubular conductor, and said filaments comprising partially acetylated cotton.

6. A combination in accordance with the claim next preceding in which each of said filaments comprises a partially acetylated cotton fabric strip rolled into substantially cylindrical form.

7. In a radio frequency transmission system, a transmission line comprising a central conductor and a self-sustaining tubular return conductor coaxial therewith, and means for supporting said central conductor comprising two, and only two, insulating filaments of substantially equal diameter twisted together and wrapped helically around said central conductor, the pitch of the helical wrapping being several times the internal diameter of said return conductor.

JESSE F. WEN'IZ.

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