US2580838A - Coaxial cable with helical insulation - Google Patents

Description

Jan. 1, 1952 T, J, RHODES 2,580,838

COAXIAL CABLE WITH HELICAL INSULATION Filed Jan. 2, 1947 T EZ? BYQ'JWB .A TTORNEr Patented Jan. 41, 1952 COAXIAL CABLE WITH HELICAL INSULATION Thomas J. Rhodes, Ramsey, N. J., assignor to United States Rubber Company, New York, N. Y., a corporation of New Jersey Application January 2, 1947, Serial No. 719,868

2 Claims.

This invention relates to electric cable, and more particularly to coaxial cable for the transmission of high frequency current.

In certain types of coaxial cable constructions now on the market the outer tube or braid of conductive material forming the outer conductor of the cable is vsupported upon and held in -spaced concentric relationship with the inner conductor by a series of disc-type spacers located along the length of the inner conductor at spaced intervals. Since the main dielectric material in such a cable is air, the cable exhibits exceptionally low loss characteristics. However, there is a tendency for the spacers to become displaced whenever the cable is subjected to bending so that the concentricity of the inner and outer conductors is not maintained which adversely af- Iects the electrical characteristics of the cable. Moreover, the method of manufacturing such a cable is rather costly inasmuch as the spacers must be rst placed on the wire and then held in position during assembly of the outer conductor. Thus, this type of coaxial cable has found favor chieiiy due to the air dielectric which gives it low loss characteristics. But it is open to the objection that it is relatively high in cost and there is considerable danger that the electrical characteristics of the cable may vary at the' points where the cable is bent.

In another type of high frequency coaxial cable on the market the dielectric material comprises a solid or sheeted plastic, such as polyethylene, surrounding the inner conductor and on which the outer conductor rests, Since the outer concentric conductor is supported at every point along its length by the plastic dielectric material this cable has the advantage that it is not subject to distortion upon bending so that the electrical characteristics of the cable are not impaired at those points. Moreover. the extruding process by which the dielectric material is applied tothe inner conductor maybe performed quickly and at low cost. However, a, considerable amount of the plastic dielectric material is required per unit length of cable thereby making the cable more rigid and increasing its weight with a corresponding increase in material cost.

According tothe present invention, however, a coaxial cable is provided which embodies the advantages of each type of cable without the corresponding disadvantages. For example, the cable is one which essentially employs air, or other suitable gas, as a dielectric along its entire length but in which a supporting structure or web is provided for maintaining the inner and outer conductors in concentric relationship. The supporting structure is obtained by means .of a special extruding process which lends itself to high speed operation whereby the cable may be manufactured in any desired length at comparatively low cost. Moreover, the construction makes possible a considerable saving in material. The web or supporting structure is so designed as to maintain the inner and outer conductors in concentric relationship irrespective of any bends or twists to which the cable may be subjected during installation.

Therefore, it is an object of the invention to provide a new and improved coaxial cable which has exceptionally low loss characteristics.

It is another object of the invention to provide a new and improved coaxial cable so constructed as to give excellent flexibility without displacement of the conductors making it suitable for use in a wide variety of installations.

It is still another" object of the invention to provide a new and improved core for use in coaxial cable constructions, the core providing a means whereby accurate and permanent alignment of the inner conductor is obtained under all normal conditions of use.

In the accompanying drawing, Fig. 1 illustrates part of the cable constructed in accordance with my invention showing one suitable method of making the cable; Fig. 2 is a side view of a guide die utilized in performing the method; Fig. 2a is an end view, and Fig. 2b is a perspective view of such die; Fig. 3 is a cross-sectional view through the cable core taken along the line 3-3 of Fig. l; Fig. 4 and Fig. 4a illustrate cross-sectional views of modified forms of cable constructions; and Fig. 5 is a view, partly in section, of a coaxial cable constructed in accordance with the present invention.

Referring to the drawing, Fig. 5 illustrates a coaxial cable constructed in accordance with my invention and which includes an inner conductor I, a concentric sleeve 2 of insulating material supported on a web 3 twisted around the inner conductor, and an outer conductor 4 supported on the insulating sleeve 2. It will be seen that the space between the inner conductor I andthe sleeve 2 forms an air chamber 5 extending along the length of the cable and providing a substantially uniform air dielectric medium between the inner and outer` conductors except for the small amount of space taken up by the web 3. The inner conductor l, sleeve 2, and web 3 form, in effect, al cable core 6 which may be made in any at IB lnFig. 3.

desired length by means of an extruding process to be described below. *Y

Fig. 1 also illustrates the cable core which has Just been formed by an extruding machine l. In the illustration, the extruding machine 'l Vhas 5 been shown in schematic form with only its es-l sential elements, inasmuch`as it formsv the subject matter of my United States Patent No. 2,465,482 issued March 29, 1949. It includes a tubing head 8 provided with a chamber 9 for 10 receiving a rotating feed screw I0. Plastic insulating material, such as` polyethylene or polystyrene, enters the tubing head through an inlet port II and is forced through the head to a nozzle I2 by means of ,the helical feed screw 15 I0. The plastic insulating material is then forced `outwardly through an annular orice I3 in the nozzle I2 around the center conducting wire I in a manner now to be described. Since the insulating material is thermoplastic, it is heated 20 to plastic condition before being extruded, and

it then solidiiles on leaving the tuber.

The inner conductor I is drawn continuously through the lower end of thel tuber in which position it is centered in the orifice I3. The 25 wire extends through a rotating guide die I4 having an enlarged head I5 which likewise is centered in the orifice I3. The walls of the head` I5 and nozzle I2 cooperate to form the annular orifice I3 through which the insulating material 30 is extruded to form the sleeve 2. In order to form the web 3 which supports the sleeve 2 on -the inner conductor, the head I5 of the die is provided with an angularly disposed slot I6 formed integrally with the sleeve 2. Also, sincer a portion of the plastic material ows through the slot I6 into the opening of the bore il, a 45 small amount of the plastic material will be forced outwardly from' the opening il! around the wire I thereby firmly anchoring the web 3 to the inner conductor I, in the manner indicated 5 The die Il is rotated during the extruding o operation by means of a drive wheel I9. In this way the web 3 is formed as a helicoid around the length of the wire I as it travels through the extruder. Since it encireles or extends 5;, around the wire it supports the outer` insulating sleeve 2 continuously along the length of the wire.

, This means that the wire i is firmly supported the number of turns of web 3 per unit length of cable, it is simply necessary to increase the 70 speed of rotation of the die I4.

In constructing the die I4 it is important that the dimensions of the slot I6 be so related to the dimensions of the annular orifice i3 -that the material forming the web and the material 75 forming the sleeve shall issue from the extruder head at substantially the same linear rate. For this purpose it is found in practice that the op posing walls of the slot I8 should be spaced apart by a distance substantially the same as or slightly`- greater than the radial width of the annular orifice I3. It is then found that the thickness flows through the slot I8, as in the event that the slot IB is made too large, a helical ridge will form along the length of the sleeve 2 at the line of juncture of the web 3 with the sleeve. Conversely, if the rate of ow is too small, as in the event that the slot is too narrow, there will be a tendency for the sleeve 2 to be drawn inwardly toward its axis, resulting in a helical depression being formed along the line of juncture of the sleeve 2.

When the cable core is extruded in extremely long lengths there is some likelihood that a partial vacuum may be formed in the chamber 5 between the wire I and the outer sleeve 2,- which might result in some collapse or distortion of the sleeve. The tendency toward forming a vacyuum will be considerable, unless means are provided to introduce air, or other gaseous dielectric medium, into the interior of the core during the extruding operation, To this end, the die I4 is provided with an extension 20 for cooperating with an air supply pipe line 2I.v A combined coupling and packing 22 seals the pipe line to the die and at the same time provides a means of permitting rotation of the die I4 relative to the gas supply pipe 2I. The bore I'l extending through the die is made slightly larger than the diameter of the conductor I so that the conductor may be easily pulled therethrough, but it is small enough to serve 'as a guiding means for the conductor as it travels through the extruder. As thus constructed, the bore is still large enough to provide for a flow of air along the length of the die I4 to an opening 23 formed in the head l5 of the die. The opening or passageway 23 extends at an angle to the bore Il and opens upon the interior of the channel 2 as the sleeve is extruded from the orifice I3.

During the extruding process, air or other dielectric gas such as nitrogen, at a pressure equal to or slightly above atmospheric, is forced along the length. of the die I4, in the manner' described, to the interior of the sleeve 2. In addition to providing an insulating medium Within the cable, the gas pressure serves to maintain the cylindrical configuration of the sleeve 2. If aflowering of the gas pressure occurs within the core, there is a tendency for the sleeve 2 to be drawn inwardly toward the wire due to the difference in pressure between the outside and the inside of thev sleeve. This tendency is overcome by introducing the gas under a pressure equal to ory slightly above atmospheric to the interior of the sleeve so as to overcome any tendency of the sleeve to collapse. It should be noted that the pressure of the gas supplied to the interior of the sleeve through the oriiice 23 should be carefully regulated to assure a uniform product. If the pressure of the gas introduced to the interior of the core is too high there is a tendency for the side walls 2 of the sleeve to expand thereby producing a core of uneven configuration. The gas pressure within the sleeve 2 may be more closely regulated during actual manufacture if, at the outset of the manufacturing run. the end of the sleeve 2 is sealed down around the conductor I to close the end 0f the core to prevent the escape of air.

It has been found during actual manufacture that the quality of the product can be maintained uniform and the manufacturing process speeded up if air is supplied to the interior of the core in the manner indicated. In this way. the sleeve 2 is formed as a smooth uniform dielectric covering around the wire l. Although, theuse of the air feed greatly facilitates production, it should be manifest that in certain instances the air feed may be omitted. For example, if low extruding speeds are used together with a quick-setting thermoplastic insulating material, it. is possible to extrude the sleeve and have it set in position Without distortion without the use of the air supply.

After the core 6 has been formed in the manner indicated, it may be wound on a reel for further processing or it may be passed directly to a conventional braiding machine for application of the outer braid 4. It should be manifest that other forms of outer conductors may be used, for example, a thin copper sleeve ,or tape may be wrapped around the core in a manner well understood in the'art. Also, the core may be sprayed or painted with a metallic covering directly on the exterior of the sleeve 2.

In the cable construction thus far described, a single helicoidal web I is formed about the conductor and within the sleeve. The advantage of this construction is that a minimum amount of material is required to support thesleeve 2, and the sleeve is held in spaced concentric relationship with the inner conductor I throughout its length. However, in certain instances, particularly where large diameters of sleeves 2 are required, it may be desirable to extend the web I completely across the sleeve in the form of a diametrically disposed web 24, as indicated in Fig. 4. In other instances three or more webs may be used as indicated in Fig. 4a. The various forms of the webs are constructed by-providing additional slots in the rotating die plug` il, preferably with equiangular spacing aboutthe axis.

A cable constructed in accordanceffwith my invention is one which gives unusually low capacitance and attenuation as compared with a coaxial cable having av solid dielectric between the inner and outer conductors. Moreover a con-l siderable saving in material is obtained.: and the cable is of light weight. At the same time, the two concentric conductors are maintained in accurately spaced relationship with each other throughout the entire length of the cable due to the continuous supporting action ofthe sleeve 2 and web l. Thus; when the cable is subjected to twists and bends during installation, the web I prevents displacement of the inner conductor I relative to the outer conductor, due to the fact that the'portion Il of the web completely surrounds the conductor and prevents it from pulling away from the web. Since the web extends continuously along the length of the,` cable the inner conductor is supported at every point, which would not be the case if it were supported on spaced insulating disks. The sleeve 2 forms` an accurately located and continuous support for the outer conductor 4 thereby providing a cable having good uniform electrical characteristics throughout its entire length, particularly in those installations in which the cable is bent in sharp angles. a

Another advantage of the construction is that by twisting the web around the center conductor, the flexibility of thecable is greatly increased over constructions in which the webs extend in a single plane parallel to the center conductor. Also, the center conductor is supported at every point along its length irrespective of the direction in which the cable is bent.

Although my cable construction is one which is particularly well suited for the transmission of high frequency current it is not necessarily limited to such use. It may be used in the transmission of all types of electric currents wherever applicable.

Having thus described my invention what I claim and desire to protect by Letters Patent is;

1. In a cable for transmission of high frequency current, a flexible conductor. a surface coating of plastic insulating material extending along the length of and'covering said conductor. an outer continuous tubular sleeve of plastic insulating material spaced from said conductor and extending along the length thereof, and a helicoidal web of plastic-insulating material twisted around and extending along the length of said conductor between said coating and said sleeve, said web being formed integrally with both said coating and said sleeve along their lengths to hold said insulating sleeve in spaced concentric relationship with said conductor.

2. A cable for transmission of high frequency tor, and an outer conductor carried by said sleeve.

THOMAS J. RHODES.

REFERENCES CITED I The following references are of record in the ville of this patent:

UNITED STATES PATENTS Number Name Date 30 1,976,804 Ringel Oct. 16. 1984 2,204,737 Swallow et al June 18, 1940 2,481,834 Morin Feb. 15. 1949 y. FOREIGN PATENTS Number ICountry Date 16,542 England July 18, 1912 of 1911 681.787 Germany Sept. 30. 193i

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