US2416979A - Electric cable - Google Patents

Description

Mmh 4, 1947. J, C, BURLEY 2,416,979

` LEcTRl CABLE Filed April 27.' 194s Eye.

[3l/venan' Patented Mar. 4, 1947 UNITED srA'rEsPATENT OFFICE ELECTRIC CABLE Joseph Ciney Burley, Minori, Mass.

Application April 21, 194s, serial No. 484,691;

7 Claims. (Cl. 174-29) This invention relates to electric cables.

The invention and its aims and objects will be readily understood from thefollowing description, taken in connection with the accompanying drawing, of embodiments of the invention herein given for illustrative purposes.

In the drawing:

Fig. 1 shows one illustrative embodiment of the invention in its application to dual conductor coaxial cables; l

Fig. 2 is a longitudinal section of Fig. 1;

Fig. 3 is a cross section on line 3-3 of Fig. l;

Fig. 4 shows another illustrative embodiment of the invention in its application to dual conductor coaxial cables;

Fig. 5 is a longitudinal section of Fig. 4;

Fig. 6 is a cross section on line B-B of Fig. 4, and

Fig. 'l is a cross section of an illustrative embodiment of the invention in its application to multiple conductor cables. 4

The illustrative embodiment of the invention shown in Fig. 1 comprises a. substantially central conductor 2 surrounded by a tubular conductor 4 which may be substantially coaxial with said conductor 2. Said conductor 4 herein {illustratively consists of metal wire braid. These two conductors 2 and 4 are separated by an air space insulation. To this end said conductor 2 is surrounded by an insulating covering of braided material possessing the desirable physical and electrical properties. Said insulating covering may comprise one or more layers or sheets superimposed one upon another. In the illustrative embodiment of Jghe invention shown in Fig. 1 two such layers B and 8 are provided. The numerous points of Contact of the filaments or threads of the braided material with the conductors 2 and 4 are sufliciently close to one another to maintain said two conductors adequately spaced from each other, and to preclude any contact of said two conductors with each other, no matter how sharply the cable may be flexed or bent. The design and character of the braided covering allows a considerable portion of the cross-sectional area between said Iconductors to be air, and this fact, in conjunction with the numerous contacts of the filaments provides a very eilicient low loss insulation.

This invention is superior in one respect or another to low loss insulation heretofore used, consisting of solid paper or other wrappings, or rubber or rubberlike compounds, solidly covering the conductor, or of rigid aligned insulating elements of beads or discs or the like placed upon 2 the conductor. It will be understood that in the case of paper wrappings and rubber insulatons, the low loss factor is primarily governed by the properties of the materiaL'with a very small, if any, inclusion of air in the cross-section.

On the other hand while the spaced discs include a large portion of air, they are generally used for rigid coaxial cables, and are not satisfactory for flexible coaxial cables. While the cable using small beads provides greater flexibility owing to the increased'number of smaller elements, said cable is more costly to produce and includes less and less air. In the present construction, on the contrary, the flexibility is still greater, being uninterrupted throughout the length of the cable, and the spa-:ing of the two conductors remains substantially uniform.

The cable Will preferably be provided with an out/er protective sheath I0 of any suitable conventional material and construction, moisture-resisting cotton braid, for example.

In the illustrative embodiment of the invention shown in Figs. 4, 5 and 6, one of the layers or sheets of insulating braided material shown in Figs. 1, 2 and 3 is omitted and is replaced by a filament I2 helically wound upon the central conductor 2 and consisting of material having physical and electric properties similar to those possessed by said braided material. In general it will b e found convenient to make the braided material and said filament I.2 of the same insulating material although by no means necessary. It will be seen that this modification provides additional air space insulation next to the conductor without materially increasing the bulk and without sacrificing flexibility. The advantage of this combination of coverings over the single spiral can be readily seen when the cable is bent sharply, for in the latter case the wide openings in the outer portion of the bent helical spiral would alone offer no support for the outer tubular conductor.

In the illustrative embodiment of the invention shown in Fig. 7 three conductors I 4, I6 and I8 are provided. Each of the conductors I4 and I6 is surrounded by an air space insulation comprising preferably two layers or sheets 20 and 22 ofsaid insulating braided material and these are in turn surrounded by a further air space insulation including a layer 24 of said braided material. The conductor I8 surrounds said insulating layer 24 and is in turn surrounded by any suitable, conventional protective insulating sheath 26, which may be similar to the sheath III of Fig. 1.

such as atraer@ The present invention contemplates the use of any suitable material for the construction of the point of view of braiding it is desirable to haveA strong, pliable, threadlike material, such that it can be lsatisfactorily braided on the present day braiding machines generally used in wire manufacture.

From the electrical standpoint it is desirable tc have a low dielectric constant and high electrical resistance, so that the cable will have low capacity and small loss angle. Another desirable feature is low water absorption such that the material will not be aiected by high humidity conditions. For this reason it is desirable that the `material be as non hygroscopic as is feasible. Another factor is a certain amount of rigidity or springiness in the material in order that air spaces will be formed between the braided strands instead of braiding these closely together as would be the ca seif soft cotton thread or twine were used.

Considering all of the above, I preferably use plastics as a very suitable material as these possess the desired physical and electrical properties. There are many variations in the different types of plastics, such that some are more suitable than others. This is true of plastic material of the groups comprising (l) cellulose derivatives, particularly cellulose aceto-buytrate; (2) vinyl resins plastics, more particularly vinylidene chloride; (3) polystyrenes, more particularly polystyrene and polystyrol. I

As compared to cables heretofore used, cables embodying the present invention possess other advantages in addition to those previously referred to. Small diameter low loss cables are required for high frequency applications having a high degree of ilexibility, to be twisted in and about apparatus, in particular radar equipment. Cables embodying the present invention are ideally suited for this service as no reasonable amount of bending will impair the good properties they possess as coaxial cables. Furthermore, ythey will stand the continued vibration and jarring to which they are often subjected in service, with no signs of failure. Beaded cables are not suited for sharp bending, and constant vibration wears the surfaces and causes fracture of the insulating elements. Only rubberlike compounds possess equally good exing and abrasion-resisting properties, but these have other characteristics which seriously impair their value, such as cold flow distortion of shape and position, and in addition the dielectric constant of the material is at present limited to about 2.6.

In evaluating low loss cables, it should be recognized that a. material may be used with a relatively high dielectric constant, such as ceramic at 5 for example, or a plastic at 6, which, by proper design in keeping the air space to a maximum and placing the minimum amount of material next to the conductor, will possess lower loss factors than a solid dielectric of 2.6. I calculate this for complex cable constructions and term the result equivalent specific inductive capacity although in reality it is the dielectric I constant of what a solid dielectric would have on features of various constructions of coaxial cable. Thus, a certain shaped ceramic bead might be 2.0 and a similar polystyrene bead might be 1.4 but the value of the braided cable of my invention runs about 1.8. Different modifications, however, of my invention give different values and it is not necessary to list here all the possibilities that this invention covers. Suillce it to say that diiferent braid constructions and diner- `ent materials may be combined in various layers to. give a great variety of electrical and mechanical characteristics to make suitable and excellent coaxial cables.

It will be apparent that cables herein shown can be spirally cabled together to form multiple conductor cables having any number and types of conductors desired.

I am aware that the present invention may be embodied in other specic forms without departing `from the spirit or essential attributes thereof. and I therefore desire the present embodiments of said invention to be considered in all respects as illustrative and not restrictive, reference being had to the appended claims rather than to the foregoing description to indicate .the scope of the invention.

I claim:

1. An electric cable having low capacity and small loss angle, comprising a wire conductor; a tubular stranded iiexible conductor surrounding said wire conductor and co-axial therewith; and insulating spacing means` separating said two conductors and comprising aspacing helix wound upon the wire conductor aid an insulating, open braided covering surrounding and supported by said helix, said helix and said braidedy covering being each composed oi' polystyrene, the dielectric between the conductors consisting largely of air.

2. An electric cable having low capacity and small loss angle, comprising a wire conductor; a tubular braided flexible conductor surrounding said Wire conductor and co-axial therewith; and insulatingspacing means separating said two conductors and comprising two insulating open braided sleeves surrounding said wire conductor and composed of vinyl resin plastic, whereby the dielectric between said conductors consists largely of air.

3. An electric cable having low capacity and small loss angle, comprising a wire conductor; a tubular woven flexible conductor surrounding said wire conductor and co-axial therewith; and insulating spacing means separating said two conductors and comprising twoinsulating open braided sleeves surrounding said wire conductor and composed of polystyrene, whereby the dielfectric between said conductors consists largely o air.

4. An electric cable having low capacity and low loss angle, comprising a wire conductor; a tubular braided ilexible conductor surrounding said wire conductor; and insulating spacing means' separating said two conductors and comprising two insulating layers surrounding said 5. An e1ectric cable, as set forth in claim 4,'

wherein said open braided sleeve is composed of polystyrene.

6. An electric cable having low capacity. and low loss angle, comprising two parallel wire conductors: a tubular braided nexibie conductor surrounding both of said wire conductors; and insulating spacing means separating -the two wire conductors from each other and from the tubular conductor and comprising at least one open braided sleeve surrounding each wire conductor and an open braided sleeve surrounding the sleeves on both wire conductors and all composed of vinyl resin plastic, whereby the dielectric between al1 of the conductors is composed largely of air.

7. An electric cable, as set forth in claim 6,

A wherein all of said open braided sleeves are composed of polystyrene. i

JOSEPH C. BURLEY.

6 REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,987,442 Harris Jan. 8, 1935 2,116,268 Klmmer May 3, 1938 2,116,267 Klmmer May 3, 1938 2,197,616 Lehne Apr. 16, 1940 2,253,967 Carl Aug. 26, 1941 FOREIGN PATENTS Number Country Date 476,110 British Nov. 29, 1937 360,743 Italian July 5, 1938 776017 French Oct. 22. 1934

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