EP0015816B1 - Method of manufacturing a coaxial cable - Google Patents

Description

The present invention relates to a method of manufacturing a coaxial cable and more particularly a small cable intended in particular for transmission systems which do not require rigorous shielding, that is to say in practice for digital transmission systems. It is, in fact, known that the specifications of such systems can be fulfilled using cables whose external conductor alone acts as a screen.

The present invention is particularly motivated by the reduction in the cost of production by using cheaper constituent materials and reduction in the cost of labor. The problem of economical manufacture of small coaxial pairs has been the subject of numerous studies, one of which was published on page 243 of the July 1971 issue of the review "Cables and Transmission". The article entitled "1.2 / 4.4 mm coaxial pair for digital transmission" describes a structure with four coaxial pairs assembled like the conductors of a quarter, the outer conductors of which consist of smooth ribbons formed into an overlapping cylinder around the insulation surrounding the central conductor, during the assembly operation. The outer conductors are kept cylindrical and the mechanical rigidity of all four pairs is ensured by the forces applied by the pairs after assembly. It is therefore not possible to obtain by this process unit pairs with final dimensions.

It has also been proposed to manufacture coaxial pairs by welding the edges facing the external conductor. Such a solution leads to high production costs as a result of the care and investment necessary for the implementation of longitudinal welding. Such techniques are described in FR-A 2 224 844 (filed by the Applicant) and FR-A 1 526 643 (filed by WESTINGHOUSE).

FR-A 2 325 164 discloses a method for manufacturing electrical cables comprising operations of winding a laminated ribbon around a cable core. It comprises a first operation of hot bonding of a metal strip with a plastic strip wider than the latter and comprising flaps which are secured by hot forming with the opposite face of the metal strip. The overlapping edges of the laminated tape are joined together during the extrusion of an outer plastic sheath around this tape placed lengthwise around the core of the cable. First of all, this manufacture absolutely requires the installation of an extruded outer sheath, which is not the case with the present invention which, on the contrary, is aimed at an unsheathed coaxial pair. On the other hand, the laminated ribbon can only be held in place approximately in the extruder since there is an additional material supply around it.

Finally, GB-A 1 059438 describes a simple electrical conductor which is insulated by means of a paper tape laid lengthwise so as to form a tube around the conductor, the edges of the ribbon being laid overlapping. This is the installation of insulation in the form of a simple tape and not at all the realization of an outer conductor of a coaxial cable. The only production constraint for this insulation of an electric wire is to obtain a good bonding of the edges of the paper tape. On the other hand, in the case of a coaxial cable, the production constraints are located at the level of the external conductor itself so as to obtain satisfactory geometrical qualities allowing the production of a surf cable without any defects causing absorption of the telecommunication signals it transmits. GB-A 1 059 438 absolutely does not suggest giving up the principle of the longitudinal welding of an external conductor of a coaxial telecommunication cable.

The present invention is essentially characterized in that the outer conductor is a composite ribbon, subsequently formed into a cylinder around the central conductor carrying an insulation, consisting essentially of a metallic ribbon, the width of which is close to the circumference of the circle circumscribed to said insulation of the central conductor, made integral over its entire surface with a dielectric tape of width greater than its own. During the forming of the composite tape, metal inside and dielectric outside, the dielectric part of the tape constitutes a lap joint and the dielectric tape allows the longitudinal sealing of the outer conductor over the entire width of the lap, by passing through a heating die, which ensures the cylindrical shape of the ribbon and the constancy of the impedance along the cable without external action. In addition, the outer dielectric tape mechanically protects the outer conductor.

The invention thus relates to a method of manufacturing a coaxial cable consisting of a central conductor associated with insulating means ensuring the spacing of an external conductor. It is characterized in that the laying of the latter comprises a first step in which a composite tape consisting of a metallic tape with a width at least equal to the circumference of the insulating means secured to a first tape is arranged lengthwise around said insulating means dielectric of greater width, and a second step in which the overlapping edges of the dielectric tape are joined using a heating die, the composite tape thus being formed into a cylinder around said insulating means carried by the central conductor.

The overlapping edges of the dielectric tape are thus held very precisely in position and therefore also the metallic strip, which gives the cable thus produced excellent transmission properties since the coaxial cable is mechanically stable and cylindrical.

In addition, this manufacturing process proposes to allow the manufacture of coaxial cables and more particularly of small coaxial cables and allowing a reduction of the production cost by using on the one hand materials constituting less expensive and on the other hand implementing installations allowing a faster manufacturing speed and a simple operation whose automation can be carried out without problem.

• - le câble coaxial peut être assemblé avec un nombre quelconque de paires identiques soit en faisceaux soit en couches concentriques ce qui est particulièrement favorable à la réalisation de câbles de grande capacité ;
• - le conducteur externe peut être au choix en cuivre ou en aluminium ;
• - l'augmentation de diamètre de la paire téléphonique isolée est négligeable par rapport à la valeur du diamètre d'une même paire nue.

The method according to the invention also has the following advantages:

• - The coaxial cable can be assembled with any number of identical pairs either in bundles or in concentric layers which is particularly favorable for the production of high capacity cables;
• - the external conductor can be either copper or aluminum;
• - the increase in diameter of the isolated telephone pair is negligible compared to the value of the diameter of the same bare pair.

• la figure 1 représente la suite des opérations du procédé,
• la figure 2 représente le ruban composite utilisé,
• la figure 3 représente schématiquement la fabrication du câble coaxial par le procédé selon l'invention,
• la figure 4 représente la section agrandie du câble coaxial fabriqué par le procédé selon l'invention.

In what follows, the method according to the invention is described in detail with particular reference to FIGS. 1 to 4 given by way of illustration, in no way limiting, and in which:

• FIG. 1 represents the continuation of the operations of the method,
• FIG. 2 represents the composite ribbon used,
• FIG. 3 schematically represents the manufacture of the coaxial cable by the method according to the invention,
• FIG. 4 represents the enlarged section of the coaxial cable manufactured by the method according to the invention.

FIG. 1 represents the rest of the manufacturing operations for the preferred variant of the coaxial cable according to the invention.

Operation 1 is the realization by means known in themselves of the insulation of the central copper conductor, for example by continuous molding of calibrated discs of polyethylene of diameter d corresponding to the nominal characteristic impedance of the coaxial cable counts given the permittivity of the dielectric used.

In order to avoid short-circuits which could result from a mechanical deformation bringing the external conductor into contact with the central conductor, between two discs, provision is made to lay lengthwise (operation 2) a dielectric tape around the discs.

Operation 3 is the production of the composite ribbon which will be used to produce the outer envelope of a coaxial cable and the description of which is given below.

Operation 4 is the shaping of the composite ribbon around the insulated central conductor in order to produce the coaxial cable according to the invention and the sealing of the ribbon.

FIG. 2A represents one of the ends of a composite ribbon allowing the implementation of the method according to the invention. This composite ribbon comprises a thin copper strip 8, for example of thickness equal to 0.1 millimeter, the width I of which is equal to πd + b where d is the outside diameter of the central conductor provided with the disks and the ribbon insulation and b is small in front of πd and corresponds to the desired overlap (cf. figure 3 _{/ J} for metal. The copper strip 8 rests on its entire underside on a dielectric strip 5, for example made of kraft paper. The dielectric strip 5 has a width L = πd + R with R> b. It carries on its upper face a coating of ethylene copolymer 6 arbitrarily enlarged for the needs of the representation. According to other variants the dielectric is a plastic material and the adhesion between the metallic tape and the dielectric tape is obtained without interposition of adhesion agent by calendering for example As shown in FIG. 2A, the two tapes are laterally offset (a, c) in order to avoid the overlapping overlapping areas ( figure 3 _{/ J.}

FIG. ₂₈ represents a variant of the composite tape corresponding to an edge-to-edge laying of the metal part 8 and overlapping of the dielectric part 5. In this variant 1 = ₇ rd and L = πd + R using the same notations as above. It is advantageous to produce the composite tape by providing for the displacement of the dielectric tape 5 on either side of the metal tape 8 so as to seal the external conductor even if the edge-to-edge covering is not straight.

Figures 3 show sections of two variants of coaxial cable manufactured by the method according to the invention. The central core of this cable consists of a copper conductor 30 on which the dielectric disc 31 is molded. A dielectric strip 32 is placed in a manner known per se on said disc 31. It is understood that this strip does not is not essential for carrying out the cable. The outer conductor is constituted by the metallic strip 8 of the composite strip 8-5. It is laid overlapping in the variant of FIG. 3A and edge to edge in the variant of FIG. _3B .

A cable made from a composite tape laid edge to edge has not been shown, although this variant may have an advantage from the point of view of cost price.

FIG. 4 schematically represents your production line for a coaxial cable according to FIG. 3A. The central conductor 30 made of copper, supplied by the reel 10, enters the molding station represented diagrammatically by a chain 11 of the type described in French patent 2 108 142 and its first addition 2361 728 ô where it is provided with insulation discs 31 in diameter d. There is shown at 12 the disc diameter control station and at 13 the station for installing the dielectric insulating tape 32 supplied by a reel 14. A reel 15 supplies the installation with the metal strip 8, for example made of copper, neces be able to produce the composite tape at the running speed of the central conductor 30 at the outlet of the molding station 11. A reel 17 supplies the installation with kraft paper 5 previously coated on the face coming into contact with the metal strip. Two rollers 19 and 20 having a suitable speed of rotation ensure the adhesion of the metal strip 8 and of the paper 5 constituting the composite tape 21. The former 22 surrounds the core of the coaxial cable of the composite tape 21. A heating die 26 ensures sealing the composite tape. A pulling member 24 associated with the drum 25 constitutes the reception of the finished cable. We did not specify the enslavements between the various positions of the manufacturing line.

• - diamètre du conducteur en cuivre intérieur : 1,2mm
• - nature du diélectrique, et des disques : polyéthylène et polypropylène respectivement
• - diamètre interne du conducteur externe : 4,4 mm
• - épaisseur du conducteur externe : 0,1 mm
• - largeur du ruban isolant : 18 mm
• - épaisseur de l'isolant externe : 0,1 mm
• - largeur de la zone de recouvrement : 3 mm
• - impédance du câble coaxial : 75 ohm
• - variations extrêmes de l'impédance : ± 1 ohm.

By way of illustration, the applicant realizes a first type of coaxial cable called 1.2 / 4.4 mm cable, the characteristics of which are as follows:

• - inner copper conductor diameter: 1.2mm
• - nature of the dielectric, and of the discs: polyethylene and polypropylene respectively
• - internal diameter of the external conductor: 4.4 mm
• - thickness of the external conductor: 0.1 mm
• - width of the insulating tape: 18 mm
• - thickness of external insulation: 0.1 mm
• - width of the overlap area: 3 mm
• - coaxial cable impedance: 75 ohm
• - extreme variations in impedance: ± 1 ohm.

The applicant also produces a second type of coaxial cable differing from the previous one only in the nature of the external insulation which is a polyester, for example that sold under the name of "Mylar" by Dupont de Nemours.

The copper strip can be replaced by an aluminum strip of similar thickness and of the same width or a bimetallic strip known per se.

Claims (5)

1. Method of producing a coaxial cable composed of a central conductor (30) associated with insulation means (31) assuring the spacing from an outer conductor, characterized in that the application of the latter comprises a first step wherein a composite tape is disposed in longitudinal direction about said insulation means (31), said composite tape being composed of a metallic ribbon (8) having a width at least equal to the periphery of the insulation means (31) and joined with a first dielectric tape (5) of larger width, and a second step wherein the mutually overlapping edges of the dielectric tape (5) are joined with each other using a heating draw-head (26), the composite tape being thus cylindrically shaped about said insulation means (31) carried by the central conductor (30).

2. Method according to claim 1, characterized in that it comprises, in a step preceding the first step, the lengthwise application of a second dielectric tape (32).

3. Method according to claim 1 or 2, characterized in that the first dielectric tape (5) is of paper having a face coated with a layer (6) of plastics material.

4. Method according to claim 3, characterized in that said plastics material comprises an ethylene copolymer.

5. Method according to any of claims 1 to 4, characterized in that the metallic strip (8) and the dielectric tape (5) are laterally shifted with respect to one another.

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