GB2169437A

GB2169437A - Coaxial cable

Info

Publication number: GB2169437A
Application number: GB08526106A
Authority: GB
Inventors: Ronald Yaxley Gill
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1985-01-02
Filing date: 1985-10-23
Publication date: 1986-07-09
Also published as: GB2169437B; GB8500034D0; GB8526106D0

Abstract

An air spaced coaxial cable has a strip of plastics insulating material 5 folded around the central conductor 1 and provided, at spaced positions along its length, with sets of projections 4 which extend inwards and contact the surface of the conductor so as to hold the strip coaxially with respect to it, the outer conductor 2 being carried by the outer surface of the strip; the projections are preferably of sector shape so that at each position they co-operate to form an effectively complete disc 3. <IMAGE>

Description

SPECIFICATION Coaxial cables This invention relates to air-spaced coaxial cables that is to say cables having an outer tubular conductor and a central inner conductor coaxial with the outer conductor, and with the dielectric between them consisting mainly of air, such cables being hereinafter referred to as being of the type specified. More particularly, but not exclusively, the invention relates to such types of cables for use with cable television or CATV.

Problems can arise when water enters the cable and flows in the space between the inner and outer conductors, such as may occur, for example when the cable is buried underground for connecting domestic houses to a CATV network.

It is thus an object of the present invention to provide a coaxial cable of the type specified which can be constructed to provide a high resistance to longitudinal water flow.

Accordingly, the invention provides a coaxial cable of the type specified wherein the central conductor is separated from the outer conductor by plastics spacers bonded to the central conductor at intervals along its length and sealed to a plastics strip which completely covers the inside surface of the tubular conductor.

Preferably, the plastics strip carries, at spaced intervals along its length, a series of projections extending inwards and which constitute said spacers, the projections conveniently being of sector shape and being of dimensions such that at each of the spaced positions the projections co-operate to form an effectively complete disc.

The tubular conductor may have a plastics binding tape wound around it and this may in turn be surrounded by a Polythene/aluminium laminate. The cable may finally have a plastics sheath extruded around it.

According to another aspect of the invention, there is provided a method of manufacturing coaxial cables of the type specified, comprising the steps of forming a strip of plastics material having a plurality of sets of projections on one side thereof, each set extending transversely along the strip and the sets being spaced at intervals along the length of the strip, bonding the other side of the strip of plastics material to a strip of metal and folding the composite strip longitudinally around the central conductor with the projections extending inwards so as to contact the conductor and support the composite strip coaxially around it.

In a preferred method according to the invention, the central conductor is first heated so that as the projections contact the conductor, they become welded to it.

A plastics tape is preferably then wound around the composite strip to keep it in place and further sheaths such as Polythene/aluminium laminate and/or a plastics extruded sheath may also be provided.

One embodiment of a coaxial cable according to the invention will now be more fully described by way of example with reference to the drawings of which: Figure 1 shows one step in the process of manufacturing a cable according to the invention, and Figure 2 shows a perspective cut-away view of a completed cable.

A coaxial cable comprises a central conductor 1 coaxial with a tubular outer conductor 2, the two conductors being separated by plastic spacers 3. The spacers 3 are made up of a plurality of segments 4 radiating inwards from a plastic strip 5 folded longitudinally around the central conductor.

As can best be seen in Fig. 1, the plastic strip 5 is formed with a series of sets of approximately sector-shaped segments 4 projecting from one side of the strip 5, each set running across the width of the strip and the sets being spaced at intervals along the length of the strip. Each segment 4 is generally pyramid-shaped with one face 6, adjacent the next segment, being slightly wedge-shaped as shown in the drawing. The top of each segment is cut away to form a concave face 7 with a radius smaller than that of the central conductor 1 for reasons that will become apparent. The strip 5 also has strengthening ribs 8 extending longitudinally between respective segments 4 of adjacent sets so as to improve the bending characteristics of the strip.

During the manufacturing process, a strip 5 such as that described above, is bonded, via its flat face 9, to the plastic side 10 of a strip of plastic/metal laminate 11 which is slightly wider than the strip 5 to provide an overlap region 12. The metal side 13 is preferably either copper or aluminium and will form the tubular conductor 2 in a manner to be described below.

The composite strip 5, 11 is then folded longitudinally around the central conductor 1 using the overlap region 12 to fully seal the plastic strip 5. Prior to this operation the central conductor 1 is heated so that as the composite strip is fold around it and, due to the radius of the top face of the segments 4 being smaller than that of the conductor, as described above, the segments are slightly deformed and bonded to the said conductor 1.

As the composite strip 5, 11 is formed into a tube, the metal side 13, is thus on the outside and constitutes the outer conductor 2.

The dimensions of the segments are such that at each position the wedge shaped side 6 of each of the sector shaped segments 4 bears against the facing side of the adjacent segment so that the segments co-operate to form a complete disc.

A plastics tape 16 is then wound around the outer conductor 2 and a Polythene/aluminium laminate 14 is provided around the tape 13. The cable is then sheathed in plastic 15 to seal the conductor from moisture.

The advantages of such a cable are that flow of water within the cable is prevented because the segments 4 are sealed to both the central conductor and the plastic strip 5, and the outer conductor 2 is totally sealed from moisture by plastics material on both sides of it.

Claims

1. A coaxial cable of the type specified wherein the central conductor is separated from the outer conductor by plastics spacers bonded to the central conductor at intervals along its length and sealed to a plastics strip which completely covers the inside surface of the tubular conductor.

2. A coaxial cable according to Claim 1, wherein the plastics strip carries, at spaced intervals along its length, a series of projections extending inwards and which constitute said spacers, the projections being of sector shape and being of dimensions such that at each of the spaced positions the projections co-operate to form an effectively complete disc.

3. A coaxial cable according to Claim 1, wherein the outer conductor has a plastics binding tape wound around it.

4. A coaxial cable according to Claim 3, wherein the plastics binding tape is surrounded by a Polythene/Aluminium laminate.

5. A coaxial cable according to Claim 4, wherein the Polythene/Aluminium laminate has a plastics sheath extruded around it.

6. A method of manufacturing coaxial cables of the type specified comprising the steps of forming a strip of plastics material having a plurality of sets of projections on one side thereof, each set extending transversely along the strip and the sets being spaced at intervals along the length of the strip, bonding the other side of the strip of plastics material to a strip of metal and folding the composite strip longitudinally around the central conductor with the projections extending inwards so as to contact the conductor and support the composite strip coaxially around it.

7. A method of manufacturing coaxial cables according to Claim 6, wherein the central conductor is first heated so that as the projections contact the conductor, they become welded to it.

8. A method of manufacturing coaxial cables according to Claim 7, wherein a plastics tape is then wound around the composite strip to keep it in place.

9. A method of manufacturing coaxial cables according to Claim 8, wherein at least one further sheath such as Polythene/Aluminium laminate and/or plastics extruded sheath is applied around the tape-wound composite strip.

10. A coaxial cable substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawing.

11. A method of manufacturing a coaxial cable substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawing.