EP0803878A1 - Process and device for manufacturing an aerated casing made of insulating material around a conductor, and cable with such a casing - Google Patents

Abstract

The annular passage (34, 35) in the extruding head (3), surrounding the core conductor (20) guide (2), contains viscous insulant, eg. ethylene-propylene fluoride, under pressure. Following a conical reducing section (22, 32), this passage divides into eg. 4 or 5 equally spaced T-section apertures, the T cross-bars forming a divided outer annulus connected by the T legs to a complete inner annulus surrounding the guide. After drawing out and cooling, the cross-bar material unites, forming the tubular outer part of the core insulation, that emerging from the legs forming partitions between longitudinal air-filled voids. Opt., by extruding in a gas-chamber, other fillings are possible. A draw-down ratio of 25 or more is achieved by delaying conductor-insulant contact.

Description

The present invention relates to a method and a device for manufacturing a ventilated sheath made of an insulating material around a conductor. It relates more particularly, but not limited to, the manufacture of intermediate dielectric sheaths of coaxial cables.

It is well known that coaxial cables generally comprise a central conductor (solid or stranded) surrounded by an intermediate sheath made of an insulating dielectric material, itself surrounded by an outer conductor protected by an outer sheath of protection. The intermediate dielectric sheath must have specific dielectric properties in order to obtain the required attenuation characteristics for the cable, in particular at high frequencies. More specifically, it is generally requested that this sheath have a dielectric constant of less than about 1.8, and as close as possible to 1. The closer the dielectric constant is to 1, the more the cable can be used at high frequencies.

The insulating materials conventionally used in cables do not have such dielectric constants when they are used in massive form. Their dielectric constants are generally close to 2. This is particularly the case for polyethylene and polytetrafluoroethylene (PTFE). To lower this dielectric constant, it is known to use these materials to form cellular or aerated sheaths.

Cellular sheaths are those into which are introduced, during the processing (generally by extrusion) of the insulating material in the form of a sheath and most often by the effect of a chemical reaction, a plurality of bubbles filled with air. or a gas with a dielectric constant close to 1. This type of sheath is not concerned with the present invention.

The ventilated sheaths, in which the present invention is concerned, are those having cells extending longitudinally (in a straight line or in a helix) along the cable and separated from each other by radial walls, the cells being obtained by placing in the form of the insulating material used, which in this case is massive, using an extrusion device having for this purpose the appropriate openings and passages. These cells are fully closed, so that the intermediate sheath is cylindrical or polygonal and that its cross section has substantially the shape of a spoke wheel.

A method of manufacturing an aerated cylindrical intermediate sheath in which the cells are completely closed is described in US Pat. No. 3,771,934. It generally consists in extruding the insulating material, in the viscous state of course, into it giving the desired shape using shaping means to form the cells, then applying this material thus formed on the central conductor, and finally cooling this insulating material to obtain the sheath.

In this process, the material formed by extrusion is applied to the central conductor immediately at the outlet of the forming means. Therefore, in order to avoid sagging of the very thin upper walls (those intended to come into contact with the outer conductor of the cable) of the formed cells, to introduce an overpressure inside the latter at the during manufacturing. This makes manufacturing complex.

In addition, the shaped material suddenly passes from the guide to the conductor, which makes it undergo a significant variation in diameter which can cause longitudinal cracks in the sheath formed.

Finally, this method does not make it possible to manufacture coaxial cables with ventilated insulation having a low dielectric constant and a small diameter on an intermediate sheath.

A first object of the present invention is therefore to develop a method of manufacturing an aerated sheath around a conductor which makes it possible to dispense with the use of an overpressure in the cells.

Another object of the present invention is to develop such a method which does not involve the risk of cracking of the sheath formed.

• on extrude ledit matériau isolant à l'état visqueux en lui donnant la forme souhaitée à l'aide de moyens de mise en forme pour former lesdites alvéoles,
• on applique ledit matériau isolant ainsi formé sur ledit conducteur,
• on fait refroidir ledit matériau isolant pour obtenir ladite gaine,

caractérisé en ce que ledit matériau isolant est appliqué sur ledit conducteur à une distance de la sortie des moyens de mise en forme telle que ledit matériau est suffisamment étiré pour que les parois des alvéoles ne s'affaissent pas, sans utilisation d'une surpression à l'intérieur desdites alvéoles.The present invention proposes for this purpose a method of manufacturing a ventilated sheath made of an insulating material around a conductor, said sheath comprising a longitudinal passage in which said conductor is housed as well as closed cells extending longitudinally and separated from each other. to each other by radial walls, said method comprising the following operations:

• said insulating material is extruded in the viscous state, giving it the desired shape using shaping means to form said cells,
• applying said insulating material thus formed on said conductor,
• said insulating material is cooled to obtain said sheath,

characterized in that said insulating material is applied to said conductor at a distance from the outlet of the shaping means such that said material is sufficiently stretched so that the walls of the cells do not collapse, without using overpressure to the interior of said cells.

Thanks to the method according to the invention, since the shaped material is not applied to the conductor immediately after the exit from the shaping means, the stretching of this material is sufficient to prevent the walls of the cells to collapse, and it is therefore no longer necessary to use an overpressure inside them. The method according to the invention is therefore much simpler to implement than that of the prior art.

In addition, due to stretching, there can be no cracking of the manufactured sheath.

Furthermore, the method according to the invention makes it possible to manufacture the insulating intermediate sheath of a coaxial cable of small diameter on intermediate sheath (less than 5 mm) and of low dielectric constant (less than 1.7), which does not could have been obtained so far.

The device for implementing the method described in US Pat. No. 3,771,934 comprises a guide having an inner longitudinal channel intended for the passage of the central conductor of the cable, and a die coaxial with the guide, surrounding the latter and defining with the outer surface of the guide a passage for the insulating material in the viscous state, the shape of the sheath being obtained by means of openings made in the guide itself, so that the shape of the cross section of the intermediate sheath obtained is substantially identical to that of the guide openings associated with that of the passage defined between the die and the guide.

This arrangement does not make it possible to manufacture coaxial cables of small diameter on an intermediate sheath, typically less than 5 mm, used in particular in the medical field. Indeed, to manufacture the intermediate sheath of such cables according to the method described in the previous patent, in which the shape of the intermediate sheath obtained is a "photograph" without reduction of the empty parts of the guide, it would be necessary to use a guide of very small size, in order to immediately obtain at the outlet of the device a sheath having the desired dimensions. Now, it is desired to obtain, in a cross section of the sheath, a high ratio between the total surface of the empty parts of material and the total surface, typically greater than 40, that is to say a low dielectric constant, typically less than 1.7. It is not possible to make in a guide of such small size openings making it possible to obtain such a ratio, as this would lead to manufacturing a guide having insufficient mechanical strength to be used in the manufacture of the intermediate sheath.

Another object of the present invention is therefore to provide a device for implementing the above method making it possible to manufacture cables having both a small diameter on an intermediate sheath and a low dielectric constant.

• un guide présentant un canal longitudinal intérieur destiné au passage dudit conducteur,
• une filière coaxiale audit guide, entourant ce dernier et définissant avec la surface extérieure dudit guide un passage pour ledit matériau isolant à l'état visqueux,

caractérisé en ce que ladite filière comporte au moins une ouverture communiquant avec ledit passage et dans laquelle peut être introduit ledit matériau à l'état visqueux, la disposition autour dudit passage et la forme de la ou desdites ouvertures étant adaptée pour qu'en sortie de ladite filière, ledit matériau comporte lesdites alvéoles.The present invention thus provides for this purpose a device for implementing the method according to the invention, comprising:

• a guide having an internal longitudinal channel intended for the passage of said conductor,
• a die coaxial with said guide, surrounding the latter and defining with the external surface of said guide a passage for said insulating material in the viscous state,

characterized in that said die comprises at least one opening communicating with said passage and into which can be introduced said material in a viscous state, the arrangement around said passage and the shape of said opening (s) being adapted so that at the outlet of said die, said material comprises said cells.

• utilisation d'un dispositif dans lequel les ouvertures sont pratiquées dans la filière et non pas dans le guide,
• étirage du matériau avant son application sur le conducteur,

permet d'obtenir des gaines intermédiaires de dimensions aussi faibles qu'on le souhaite, et notamment des dimensions compatibles avec les applications dans le domaine médical.The combination of the following two characteristics:

• use of a device in which the openings are made in the die and not in the guide,
• stretching of the material before its application on the conductor,

makes it possible to obtain intermediate sheaths of dimensions as small as desired, and in particular of dimensions compatible with applications in the medical field.

According to a particularly advantageous embodiment of the device according to the invention, the die comprises a plurality of identical openings and arranged symmetrically around its longitudinal axis, the cross section of each of these openings having substantially the shape of a T whose horizontal bar is curved around the longitudinal axis, the horizontal curved bars of the T all belonging to the same cylinder and the extensions of their vertical bars crossing on the longitudinal axis.

With such a device, given that the shaped insulating material is stretched before being applied to the conductor, it undergoes pressure at the outlet of the die which tends to bring the horizontal bars of the T in contact with each other. others which makes it possible to obtain the desired sheath.

With the device and the method according to the invention, according to the stretching rate applied, it is possible to manufacture intermediate sheaths having either a substantially identical shape, except for a ratio of homothety, to that of the die (when the latter has T-shaped openings, the shape of the sheath is identical to that of the die after approximation of the horizontal bars of the T), or quite different.

• un conducteur central,
• une gaine intermédiaire isolante en un matériau diélectrique comportant un passage longitudinal dans lequel est logé ledit conducteur ainsi que des alvéoles fermées s'étendant longitudinalement et séparées les unes des autres par des parois radiales,
• un conducteur extérieur,
• une gaine extérieure de protection,

caractérisé en ce que le diamètre extérieur de ladite gaine intermédiaire est inférieur à 5 mm et en ce que sa constante diélectrique est inférieure à 1,7.Finally, the present invention relates to a coaxial cable, preferably obtained according to the method of the invention, comprising, arranged coaxially from the inside to the outside:

• a central conductor,
• an insulating intermediate sheath of a dielectric material comprising a longitudinal passage in which said conductor is housed as well as closed cells extending longitudinally and separated from each other by radial walls,
• an external conductor,
• an outer protective sheath,

characterized in that the outside diameter of said intermediate sheath is less than 5 mm and in that its dielectric constant is less than 1.7.

The method and the device according to the invention made it possible for the first time to manufacture such a cable.

Other characteristics and advantages of the present invention will appear in the following description of a method and a device according to the invention, given by way of illustration and in no way limitative.

• la figure 1 est une vue en perspective avec arrachements partiels d'un câble coaxial à gaine intermédiaire aérée obtenu selon l'invention,
• la figure 2 représente une vue de côté schématique d'un dispositif selon l'invention,
• la figure 3 est une coupe transversale de la figure 1 dans laquelle on voit uniquement le conducteur intérieur et la gaine intermédiaire,
• la figure 4 est une coupe transversale de la figure 2 au niveau du guide et de la filière permettant d'obtenir la gaine intermédiaire représentée en figure 3,
• la figure 5 est une coupe transversale de la figure 1 dans laquelle on voit uniquement le conducteur intérieur et une variante de la gaine intermédiaire.
• la figure 6 est une coupe transversale de la figure 2 au niveau du guide et d'une variante de la filière permettant d'obtenir la gaine intermédiaire représentée en figure 5.

In the following figures:

• FIG. 1 is a perspective view with partial cutaway of a coaxial cable with ventilated intermediate sheath obtained according to the invention,
• FIG. 2 represents a schematic side view of a device according to the invention,
• FIG. 3 is a cross section of FIG. 1 in which only the inner conductor and the intermediate sheath are seen,
• FIG. 4 is a cross section of FIG. 2 at the level of the guide and of the die making it possible to obtain the intermediate sheath shown in FIG. 3,
• Figure 5 is a cross section of Figure 1 in which only the inner conductor and a variant of the intermediate sheath are seen.
• FIG. 6 is a cross section of FIG. 2 at the level of the guide and of a variant of the die making it possible to obtain the intermediate sheath shown in FIG. 5.

In all these figures, the common elements have the same reference numbers.

• un conducteur électrique intérieur 4 en cuivre constitué par exemple d'un toron de fils conducteurs,
• une gaine intermédiaire 5 en un matériau isolant diélectrique, par exemple en Fluorure d'Ethylène et de Propylène (FEP), comprenant une partie tubulaire cylindrique 51 en contact avec le conducteur 4, une pluralité d'alvéoles 52 s'étendant longitudinalement et de manière rectiligne le long de l'axe X et séparées les unes des autres par des parois radiales 53, ainsi qu'une partie tubulaire cylindrique 54 entourant les parois radiales 53, de sorte que la gaine intermédiaire 5 a sensiblement une section transversale en forme de roue à rayons,
• un conducteur extérieur 6 sous forme de tresse ou de guipage, qui s'appuie sur la partie tubulaire cylindrique 54 de la gaine 5,
• une gaine extérieure de protection 7.

A coaxial cable 10 which can be obtained according to the invention is shown in FIG. 1. It comprises, arranged coaxially from the inside to the outside around a longitudinal axis X:

• an interior electrical conductor 4 made of copper, for example a strand of conductive wires,
• an intermediate sheath 5 made of a dielectric insulating material, for example made of Ethylene and Propylene Fluoride (FEP), comprising a cylindrical tubular part 51 in contact with the conductor 4, a plurality of cells 52 extending longitudinally and rectilinearly along the X axis and separated from each other by radial walls 53, as well as a cylindrical tubular part 54 surrounding the radial walls 53, so that the intermediate sheath 5 has substantially a cross section in shape spoke wheel,
• an outer conductor 6 in the form of a braid or covering, which rests on the cylindrical tubular part 54 of the sheath 5,
• an outer protective sheath 7.

Typical dimensions are given below for two coaxial cables of the type described above.

• diamètre du conducteur intérieur 4 : 0,12 mm
• diamètre extérieur de la gaine intermédiaire 5 : 0,51 mm
• conducteur extérieur 6 constitué d'un guipage de brins de 0,03 mm de diamètre avec un taux de recouvrement de 98%,
• diamètre extérieur de la gaine extérieure 7 : 0,55 mm.

A coaxial cable usable in the medical field, that is to say of small dimensions, has the following dimensions:

• inner conductor 4 diameter: 0.12mm
• outer diameter of intermediate sheath 5: 0.51 mm
• outer conductor 6 consisting of a covering of strands of 0.03 mm in diameter with a recovery rate of 98%,
• outer diameter of the outer sheath 7: 0.55 mm.

• diamètre du conducteur intérieur 4 : 0,25 mm
• diamètre extérieur de la gaine intermédiaire 5 : 1,20 mm
• conducteur extérieur 6 constitué d'une tresse de brins de 0,10 mm de diamètre avec un taux de recouvrement de 66%,
• diamètre extérieur de la gaine extérieure 7 : 2 mm.

A coaxial cable usable in telecommunications, that is to say of medium dimensions, has the following dimensions:

• inner conductor 4 diameter: 0.25 mm
• outer diameter of the intermediate sheath 5: 1.20 mm
• outer conductor 6 consisting of a braid of strands 0.10 mm in diameter with a recovery rate of 66%,
• outer diameter of the outer sheath 7: 2 mm.

FIG. 2 shows an extrusion device 1 according to the invention, making it possible to manufacture the intermediate sheath 5 of the cable 10 of FIG. 1. This device comprises a guide 2 and a die 3.

The guide 2 is provided with a cylindrical inner channel 20 around the longitudinal axis Y of the guide. This channel 20 allows the conductor 4 to pass. The guide 2 comprises a substantially cylindrical part 21 extended by a frustoconical part 22 whose base of smaller diameter has a diameter equal to that of the cylindrical part 21.

The die 3 surrounds the guide 2 and is coaxial with it. Its external surface is cylindrical, while its internal surface 30 has a cylindrical part 31 extended by a frustoconical part 32. The internal surface 30 of the die 3 defines with the guide 2 a cylindrical passage 34 for the insulating material 35 intended for constitute the intermediate sheath 5. This insulating material 35 comes from the angle head (not shown) of the extrusion device, located downstream of the die-guide assembly.

Openings (not shown in Figure 2) communicating with the passage 34 are made in the cylindrical part 31 of the die 3 to give the insulating material 35 the desired shape so that the sheath 5 has a cross section in the shape of a spoke wheel . These openings could also be made in guide 2, but we will see later why it is preferable that they be made in die 3.

To manufacture the insulating intermediate sheath 5 around the conductor 4, the latter is made to scroll inside the channel 20 in the direction indicated by the arrow F in FIG. 2, that is to say in the direction of reduction of diameter of the frustoconical parts 21 and 31 of the guide 2 and of the die 3 respectively. Simultaneously, the insulating material 35 is introduced in the viscous state under pressure, so that it fills the passage 34 as well as the openings of the die 3.

According to the invention, the material thus formed does not come into contact with the conductor 4 immediately at the outlet 37 of the die 3 (in the direction of arrow F). but at a non-zero distance from this outlet 37, so that it undergoes a stretching before being applied to the conductor 4. It is this stretching which makes it possible to prevent the walls 53 and 54 of the cells 52 from s collapse while the material constituting them is still viscous, without the need, as in the prior art, to introduce an overpressure in the cells 52.

The distance between the outlet 37 of the die 3 and the contact zone between the sheath formed and the conductor 4 is a function of the desired stretching rate. For a given stretching rate, it is fixed as a function of the running speed of the conductor 4. As an indication, it can vary between 2 times and 20 times the internal diameter of the die 3.

According to the invention, the distance between the outlet 37 of the die 3 and the point of application of the sheath being formed on the conductor 4 must be such that the stretching rate is at least equal to 25.

où D_F est le diamètre extérieur des ouvertures de la filière 3, D_G est le diamètre extérieur de la partie cylindrique 21 du guide 2, D_f est le diamètre extérieur de la gaine 5 et D_g le diamètre extérieur de la partie tubulaire 51 de la gaine 5.It is recalled that the stretch rate (DDR for Draw Down Ratio in English) is given by the following formula: $$\text{DDR =}\frac{{\text{D}}_{\text{F}}^{\text{2}}{\text{- D}}_{\text{G}}^{\text{2}}}{{\text{D}}_{\text{f}}^{\text{2}}{\text{- <figure-callout id="2" label="D g" filenames="imgf0001.png,imgf0002.png" state="{{state}}">D </figure-callout>}}_{\text{g}}^{}}\text{,}$$

where D _F is the outside diameter of the die openings 3, D _G is the outside diameter of the cylindrical part 21 of the guide 2, D _f is the outside diameter of the sheath 5 and D _g the outside diameter of the tubular part 51 sheath 5.

Since there is a stretching of the insulating material formed before its application on the conductor 4, the cross section of the intermediate sheath obtained is necessarily less than that of the empty parts defined by the openings for the passage of the material in the state viscous, and homothetic to the latter. In this way, when the openings are made in the die, which necessarily has a larger surface area than that of the guide, it is possible, by choosing an appropriate stretching rate, to produce an intermediate sheath having very small dimensions and a low dielectric constant, adjusting the size of the openings so that the cells have a large section.

There is shown in Figure 4 the cross section of the guide 2 and a die 3 'according to the invention. The four openings 38 'of this die 3' cross it longitudinally right through at its cylindrical part 31 and communicate with the passage 34. The openings 38 'each have substantially the shape of a T whose horizontal bar 39' is curved around the Y axis. They all belong to the same cylinder with a Y axis. The vertical bars 40 ′ of the Ts communicate with the passage 34 and their extensions cross on the Y axis.

The diameter at the top of the curved horizontal parts 39 'is 8 mm, and their diameter at the base is 6.4 mm, so that they have a thickness of 0.8 mm.

The die 3 'makes it possible to obtain the intermediate sheath 5' shown in FIG. 3, when the stretching rate is 235. It can be seen in FIG. 3 that the parts of the sheath 5 'coming from the horizontal bars of the T 39 'came into contact with each other to form the substantially cylindrical outer tubular part 54' of the sheath 5 '. We also observe in this figure that the cross section of the sheath 5 'is practically identical to that of the empty parts (openings 38' and passage 34) of the die 3 ', apart from the fact that the horizontal bars of the T have come in contact with each other. This occurs when the stretching rate is high, in practice greater than 150. In this case, one can precisely control the volume of air present in the sheath 5 ', since the latter is almost homothetic to the empty parts of the die 3 '. This type of die, used with a high stretching rate, makes it possible to obtain coaxial cables of small dimensions, usable in particular in the medical field.

Thus, one can obtain an intermediate sheath of small dimensions (external diameter: 0.51 mm) having a low dielectric constant (1.57).

FIG. 6 shows the cross section of the guide 2 and of another die 3 "according to the invention. The four openings 38" of this die 3 "pass through it longitudinally right through at its cylindrical part 31 and communicate with the passage 34. The openings 38 "each have substantially the shape of a T whose horizontal bar 39" is curved around the axis Y. They all belong to the same cylinder of axis Y. The vertical bars 40 "T's communicate with passage 34 and their extensions cross on the Y axis.

The diameter at the top of the curved horizontal parts 39 "is 7 mm and their diameter at the base of 4.37 mm, so that they have a thickness of 1.315 mm, that is to say that they are much thicker than the curved horizontal parts 39 'of the openings 38' of the die of FIG. 3.

The die 3 "makes it possible to obtain the intermediate sheath 5" shown in FIG. 5, when the stretching rate is 32. It can be seen in FIG. 5 that not only the parts of the sheath 5 "coming from the horizontal bars of the T 39 "came into contact with each other, but also that there was interpenetration between them, to form the substantially cylindrical outer tubular part 54" of the sheath 5 ". It is also observed in this figure that the cross section of the sheath 5 "is quite different from that of the empty parts (openings 38" and passage 34) of the die 3 ". This occurs when the stretching rate is lower, in practice of the order of 50. In this case, the volume of air present in the duct 5 "is less precisely controlled, since the latter is not homothetic to the empty parts of the die 3". This type of die , used with a lower stretching rate, is rather intended for the manufacture of intermediate sheaths of medium dimensions, for the coaxial cables used in telecommunications.

Thus, it is possible to obtain an intermediate sheath with an external diameter of 1.2 mm, having a low dielectric constant (1.56).

The coaxial cables obtained with the method and device of the present invention have electrical characteristics which meet the requirements generally required for the applications for which they are intended. Their impedance is close to 75 Ω.

Their intermediate sheaths are as easily stripped as solid insulation. The external cylindricity of these intermediate sheaths is sufficient to allow the external conductor to be cut quickly and precisely. In addition, these sheaths are homogeneous and not cracked.

The cables obtained resist crushing and bending stresses well.

Finally, the method according to the invention makes it possible to use the same type of device as those used for the extrusion of massive insulating sheaths, with the difference close to the machining of the die.

Of course, the present invention is not limited to the embodiments which have just been described.

Firstly, it can be used not only for the manufacture of ventilated sheaths of coaxial cables, but also for the manufacture of ventilated sheaths in any type of cable requiring this form of sheath, and for example in cables with pairs or quads of twisted conductors.

Furthermore, the material used to manufacture the sheath can be any type of extrudable material, and in particular of thermoplastic material, capable of undergoing stretching rates such as those necessary for the implementation of the invention. It can be in particular FEP, but also Ethylene Tetrafluoroethylene (ETFE), Polyvinylidenedifluoride (PVDF) or even Perfluoroalkoxy (PFA) ®, trademark registered by the company Du Pont de Nemours.

The cells can be filled with air or any other gas allowing the dielectric constant of the sheath to be lowered. For this, the extrusion is then carried out under an atmosphere of gas filling the cells.

Furthermore, by animating the die with a rotational movement around its longitudinal axis, it is possible to obtain helical cells which allow the cable to withstand even better the bending stresses.

The geometry of the die openings can be any as long as it allows the desired sheath shape to be obtained. In particular, the die may include an opening having a shape strictly identical to the cross section of the sheath to be manufactured.

Finally, any means can be replaced by equivalent means without departing from the scope of the present invention.

Claims (8)

Method for manufacturing an aerated sheath (5) made of an insulating material around a conductor (4), said sheath comprising a longitudinal passage (51) in which said conductor (4) is housed as well as closed cells (52) extending longitudinally and separated from each other by radial walls (53), said method comprising the following operations: - said insulating material (35) is extruded in the viscous state, giving it the desired shape using shaping means (2, 3) to form said cells (52), applying said insulating material thus formed on said conductor, - said insulating material is cooled to obtain said sheath, characterized in that said insulating material is applied to said conductor at a distance from the outlet of the shaping means such that said material is sufficiently stretched so that the walls (53, 54) of the cells (52) do not collapse , without using an overpressure inside said cells. A method according to claim 1 characterized in that said distance is such that the stretching rate of said material is at least equal to 25. Method according to one of claims 1 or 2 characterized in that said insulating material (35) is a thermoplastic material. A method according to claim 3 characterized in that said insulating material (35) is ethylene and propylene fluoride. Device for implementing the method according to one of claims 1 to 4, comprising: - a guide (2) having an internal longitudinal channel (20) intended for the passage of said conductor (4), a die (3) coaxial with said guide (2), surrounding the latter and defining with the outside surface of said guide a passage (34) for said insulating material (35) in the viscous state, characterized in that said die (3) has at least one opening (38 '; 38 ") communicating with said passage (34) and into which said material (35) can be introduced in the viscous state, the arrangement around said passage (34) and the shape of said opening (s) (38 '; 38 ") being adapted so that, at the outlet of said die (3), said material comprises said cells (52). Device according to claim 5 characterized in that said die (3 '; 3 ") has a plurality of identical openings (38';38") and arranged symmetrically around its longitudinal axis (Y), the cross section of each of said openings (38 '; 38 ") having substantially the shape of a T whose horizontal bar (39';39") is curved around said longitudinal axis (Y), the curved horizontal bars of different T all belonging to the same cylinder and the extensions of their vertical bars (40 '; 40 ") crossing on said longitudinal axis (Y). Coaxial cable comprising, arranged coaxially from the inside to the outside: - a central conductor (4), - an insulating intermediate sheath (5) of a dielectric material comprising a longitudinal passage (51) in which said conductor (4) is housed as well as closed cells (52) extending longitudinally and separated from each other by radial walls (53), - an external conductor (6), - an outer protective sheath (7), characterized in that the outside diameter of said intermediate sheath (5) is less than 5 mm and in that its dielectric constant is less than 1.7. Coaxial cable according to claim 7, obtained according to the method of one of claims 1 to 6.

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