GB2170945A

GB2170945A - Coaxial cables

Info

Publication number: GB2170945A
Application number: GB08527789A
Authority: GB
Inventors: Ronald Yaxley Gill; Clifford Heywood
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1985-02-01
Filing date: 1985-11-11
Publication date: 1986-08-13
Also published as: GB8527789D0; GB8502566D0

Abstract

A coaxial cable comprises an extrudate of insulating material 3 having a central core 4 in which the inner conductor 2 is embedded and a plurality of radial spokes 5 extending from the core to an outer tubular wall 6 surrounded by the outer conductor (not shown), each of the longitudinal channels formed between the spokes being blocked at intervals along its length, either by the deformation of the outer wall or the introduction of a blocking material into the channel. <IMAGE>

Description

SPECIFICATION Coaxial cables This invention relates to coaxial cables of the type having air as the main dielectric between the two conductors.

It is an object of the present invention to provide such a coaxial cable in which water or gas flow in the airspace between the conduc tors is prevented or at least substantially re duced.

Accordingly, the invention provides a coaxial cable comprising a central conductor, an ex trudate of electrically insulating material having a central core within which the central con ductor is embedded and a plurality of radial spokes extending from the core to an outer tubular wall thus providing a plurality of longi tudinally extending closed channels along the cable, and an outer conductor surrounding the outer wall of the extrudate, each of said chan nels being blocked at intervals along its length.

Preferably each channel is blocked at a point longitudinally spaced from the point at which the other channels are blocked.

The channels may be blocked either by de forming the outer wall of the extrudate in the region of each of the channels so that it col apses and blocks each channel in turn, or the channels may be blocked by inserting a blockting material into the channels at appropriate locations.

The blocking material is preferably a viscous quick-setting material having a suitable dielec trip value and may be injected into the appro .priate channel during the extrusion process.

The material may, for example, be a UV-cured resin, or it may be a thermoplastic material.

Preferably, the coaxial cable further includes a moisture barrier, such as an aluminium/ polythene laminated tape surrounding the outer conductor and a plastics sheath may op tionally surround the laminated tape.

One embodiment of a coaxial cable accord ing to the invention will now be more fully described by way of example with reference to the accompanying drawings of which: Figure 1 shows in perspective a coaxial cable according to the invention but without the outer tubular conductor or any of the outer layers; and Figure 2 shows a cross-sectional view through a complete coaxial cable of the type illustrated in Fig. 1.

Thus, as shown in the drawings, a coaxial cable 1 has a central conductor 2 embedded within an extrudate 3 having a central core 4, radial spokes 5 and on outer tubular wall 6.

The extrudate 3 thus has several longitudinal channels which provide the dielectric between the central conductor 2 and an outer conduc tor 7.

In order to reduce the possibility of gas and/or water flow through the channels, the cable according to the present invention has each channel blocked at jntervals along its length. Before the outer conductor 7 is positioned around the extrudate 3, the plastic section defining each of the channels is heated to a controlled temperature over a short longitudinal distance, the position of the channels first having been detected and related to the tooling and the position of the heated section of one channel being longitudinally staggered with respect to the position of the corresponding sections of the other channels. The outer walls 6 of the heated section is then deformed by collapsing it inwards untii it reaches the central core 4 and thus seals off the channel at that point.

Subsequently, an electrically conducting tape is applied longitudinslly to the deformed extrudate so as to form the outer conductor 7, and suitably sheathed, with a moisture barrier such as an aluminium 8/polythene 9 laminated tape.

A protective plastic sheath 10 may then finally cover the laminated tape 8, 9 to complete the cable.

In an alternative embodiment (not illustrated) a viscous, quick-setting material having a suitable dielectric value is injected into each channel at the desired position so as to block the channel to water or gas flow.

The material may be injected into the appropriate channel during the process of extruding the dielectric section, by means of a small diameter hyperdermic tubing extending through the die head and into each channel of the extrudate for a suitable distance. A controlled amount of the blocking material is forced, at pedetermined intervals, through the hyperdermic tubing under pressure and at a velocity greater than the extruding speed so as to form. a bead of the material in the channel.

The material may, for example, be UV-curable resin, in which case the extrudate is then exposed to UV radiation to harden the bead before the outer conductor is applied around the extrudate in a similar manner to that described above.

The introduced blocking material or the collapsed walls as the case may be need not necessarily completely seal the respective channels as long as they provide a substantial restriction to gas-or liquid flow.

1. A coaxial cable comprising a central conductor, an extrudate of electrically insulating material having a central core within which the central conductor is embedded and a plurality of radial spokes extending from the core to an outer tubular wall thus providing a plurality of longitudinally extending closed channels along the cable, and an outer conductor surrounding the outer wall of the extrudate, each of said channels being blocked at intervals along its length.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Coaxial cables This invention relates to coaxial cables of the type having air as the main dielectric between the two conductors. It is an object of the present invention to provide such a coaxial cable in which water or gas flow in the airspace between the conduc tors is prevented or at least substantially re duced. Accordingly, the invention provides a coaxial cable comprising a central conductor, an ex trudate of electrically insulating material having a central core within which the central con ductor is embedded and a plurality of radial spokes extending from the core to an outer tubular wall thus providing a plurality of longi tudinally extending closed channels along the cable, and an outer conductor surrounding the outer wall of the extrudate, each of said chan nels being blocked at intervals along its length. Preferably each channel is blocked at a point longitudinally spaced from the point at which the other channels are blocked. The channels may be blocked either by de forming the outer wall of the extrudate in the region of each of the channels so that it col apses and blocks each channel in turn, or the channels may be blocked by inserting a blockting material into the channels at appropriate locations. The blocking material is preferably a viscous quick-setting material having a suitable dielec trip value and may be injected into the appro .priate channel during the extrusion process. The material may, for example, be a UV-cured resin, or it may be a thermoplastic material. Preferably, the coaxial cable further includes a moisture barrier, such as an aluminium/ polythene laminated tape surrounding the outer conductor and a plastics sheath may op tionally surround the laminated tape. One embodiment of a coaxial cable accord ing to the invention will now be more fully described by way of example with reference to the accompanying drawings of which: Figure 1 shows in perspective a coaxial cable according to the invention but without the outer tubular conductor or any of the outer layers; and Figure 2 shows a cross-sectional view through a complete coaxial cable of the type illustrated in Fig. 1. Thus, as shown in the drawings, a coaxial cable 1 has a central conductor 2 embedded within an extrudate 3 having a central core 4, radial spokes 5 and on outer tubular wall 6. The extrudate 3 thus has several longitudinal channels which provide the dielectric between the central conductor 2 and an outer conduc tor 7. In order to reduce the possibility of gas and/or water flow through the channels, the cable according to the present invention has each channel blocked at jntervals along its length. Before the outer conductor 7 is positioned around the extrudate 3, the plastic section defining each of the channels is heated to a controlled temperature over a short longitudinal distance, the position of the channels first having been detected and related to the tooling and the position of the heated section of one channel being longitudinally staggered with respect to the position of the corresponding sections of the other channels. The outer walls 6 of the heated section is then deformed by collapsing it inwards untii it reaches the central core 4 and thus seals off the channel at that point. Subsequently, an electrically conducting tape is applied longitudinslly to the deformed extrudate so as to form the outer conductor 7, and suitably sheathed, with a moisture barrier such as an aluminium 8/polythene 9 laminated tape. A protective plastic sheath 10 may then finally cover the laminated tape 8, 9 to complete the cable. In an alternative embodiment (not illustrated) a viscous, quick-setting material having a suitable dielectric value is injected into each channel at the desired position so as to block the channel to water or gas flow. The material may be injected into the appropriate channel during the process of extruding the dielectric section, by means of a small diameter hyperdermic tubing extending through the die head and into each channel of the extrudate for a suitable distance. A controlled amount of the blocking material is forced, at pedetermined intervals, through the hyperdermic tubing under pressure and at a velocity greater than the extruding speed so as to form. a bead of the material in the channel. The material may, for example, be UV-curable resin, in which case the extrudate is then exposed to UV radiation to harden the bead before the outer conductor is applied around the extrudate in a similar manner to that described above. The introduced blocking material or the collapsed walls as the case may be need not necessarily completely seal the respective channels as long as they provide a substantial restriction to gas-or liquid flow. CLAIMS

2. A coaxial cable according to Claim 1, wherein each channel is blocked at a point longitudinally spaced from the point at which the other channels are blocked.

3. A-coaxial cable according to Claim 1 or 2 wherein the channels are blocked by inward deformation of the outer wall of the extrudate at appropriate locations.

4. A coaxial cable according to Claims 1 or 2 wherein the channels are blocked by a blocking material which has been inserted into the channels at appropriate locations.

5. A coaxial cable according to Claim 4 wherein the blocking material is a viscous quick-setting dielectric material.

6. A coaxial cable according to Claims 4 or 5 wherein the blocking material is a UV-cured resin.

7. A coaxial cable according to Claims 4 or 5 wherein the blocking material is a thermoplastic material.

8. A coaxial cable according-to any preceding claim wherein a moisture barrier surrounds the outer conductor.

9. A coaxial cable according to Claim 8 wherein the moisture barrier is an aluminium/polythene laminated tape.

10. A coaxial cable acording to Claim 9 wherein the aluminium/polythene laminated tape is surrounded by a plastics sheath.

11. A coaxial cable substantially as herein described with reference to Figs. 1 and 2 of the- accompanying drawings.

12. The manufacture of a coaxial cable in cluding the steps of- extruding around a central conductor a core of thermoplastic electrically insulating material having an outer wall spaced from the conductor by a plurality of radial spokes, defining between them a plurality of longitudinally extending closed channels, deforming the outer wall of the cover inwards at a plurality of locations along each said channel to block the channel at said locations, and applying an electrically conducting tape around the core to provide the outer conductor of the cable.

13. The manufacture of coaxial cable including the steps of extruding around a central conductor a core of thermoplastic electrically insulating -material having an outer wall spaced from the conductor by a plurality of radial spokes, defining between them a plurality of longitudinally extending closed channels injecting intobeach said channel at intervals along its length during the extrusion process, a quick-setting material so that it blocks the channels at those regions, and applying-an electically conducting tape around the core to provide the outer conductor of the cable.

14. The-manufacture according to Claim 4 wherein the blocking material is a thermoplastic material or a UV-curable resin.

15. The manufacture according to Claims 13, 14- or 15 wherein a layer of material forming a moisture barrier is applied aroundthe outer conductor.

16. The manufacture of a coaxial cable substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawings.