GB2137907A

GB2137907A - Coaxial Cables

Info

Publication number: GB2137907A
Application number: GB08309595A
Authority: GB
Inventors: L R Spicer
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-04-08
Filing date: 1983-04-08
Publication date: 1984-10-17
Also published as: FR2544124A1; GB2137907B; JPS6145512A

Abstract

A method of manufacturing low attenuation flexible centre conductors for radio frequency flexible coaxial cables. A centre conductor member is manufactured by stranding seven or less circularly cross-sectioned conductor elements together and then compacting the stranded member, in order to substantially fill the interstitial spaces therein with conductor element material and to produce an overall substantially circularly cross- sectioned member. <IMAGE>

Description

SPECIFICATION Coaxial Cables This invention relates to coaxial cables and in particular, but not exclusively, to radio frequency flexible coaxial cables.

According to the present invention there is provided a method of manufacturing a centre conductor for a flexible coaxial cable comprising the steps of stranding a plurality of conductor elements together to form a stranded member, and compacting the stranded member whereby to cause any interstitial spaces therein to become substantially filled with conductor element material and to cause the compacted stranded member to present a substantially circular crosssection.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows a cross-section through a conventional seven strand centre conductor, and Fig. 2 shows a cross-section through a seven strand centre conductor according to an embodiment of the present invention.

The attenuation a, due to the conductors (centre and outer coaxial) of a flexible coaxial cable, assuming both conductors are of the same material, the outer being braided, is given by the equation:

wnere: E is the dielectric constant of the dielectric p is the resistivity of the conductors f is the frequency d is the diameter of the centre conductor D is the diameter over the dielectric Ksis a factor (stranding factor) determined by the physical form of the centre conductor, and Kb is a braiding factor and equals (Kl/Kf) where KI is the lay factor and Kf is the filling factor.

For a-solid copper centre conductor, wire Ks=1, and for a conventional seven strand conductor, six round-sectiohed wires stranded around a central rqund-sectioned wire, Ks=1.25.

However, the effective electrical diameter of a seven strand flexible conductor equals Q.939 times the actual physical diameter d. Thus the factor Ks/d for a seven strand conductor becomes 1.25/(0.939 d) which is approximately 1.33/do in contrast to the factor KsId for a solid conductor of 1/d. Therefore when using a seven strand donventional conductor the attenuation due to the centre conductor is approximately 33% higher than when using a solid conductor.

Nearly all conventional radio frequency flexible coaxial cables, including TV downleads, employ solid circular centre conductors, or conventional seven strand conductors, where high flexlife and more reliable terminations than achievabie with a solid conductor are required.

Whilst the increased flexibility provided by the conventional seven conductor is desirable, the resultant increased attenuation of the centre conductor is undesirable. However, a centre conductor with the flexibility of the conventional seven strand conductor and the attenuation of a solid conductor can be achieved by forcing (compacting) the individual strands of a conventional seven strand conductor into closer contact with one another whereby to substantially fill up the interstitial spaces therebetween and present a substantially circular section. Fig. 1 illustrates a cross-section through a conventional seven strand conductor, which is the same as a cross-section through a seven strand conductor according to the present invention before compaction to the cross-section shown in Fig. 2.

The compacted conductor is thus dimensionally and electrically equivalent to a solid conductor, so that the factor 1.25/(0.939 d) becomes lid. Thus a centre conductor with the same dimensional accuracy as can be achieved for solid conductor and which provides minimum attenuation without loss of flexlife can be obtained.

The compaction may be achieved by drawing a seven strand conductor, each strand being substantially circular in cross-section before compaction, through a suitable forming die, that is one which is smaller than that employed to produce a conventional non-compacted conductor. Conductors comprising less than seven strands may be similarly compacted, for example three or four strands. This may be advantageous for certain applications since larger basic strands (wires) can be used. The larger the number of strands the more costly will be the basic wire drawing process and the handling process before stranding-compaction. Whereas arrangements with more than seven strands may be envisaged, the electrical advantages of using more than seven strands are minimal.

Preferably the strands are of copper, although they may alternatively be comprised of aluminium, copperclad steel wire, copperclad aluminium wire, for example. The compacted centre conductor may be employed in any coaxial cable whose dielectric is comprised of an airspace, semi-airspaced, solid or cellular material, whose outer conductor is comprised of but not necessarily braided, copper wires or elements and having sheating comprised of extruded plastics or plastics materials.

-By means of the compacted centre conductor there may thus be provided a flexible radio frequency coaxial cable having a centre conductor, comprised of seven or less compacted strands, which will give a similar flexibility and fatigue life as a conventional seven strand conductor, yet have the same electrical properties both at low and high frequencies as a solid circular conductor. Whereas the invention is not to be considered so limited, typically it is concerned with the manufacture of very small conductors, for use as coaxial cable centre conductors, which are of the order of 0.2 mm (0.008") diameter.

Claims

1. A method of manufacturing a centre conductor for a flexible coaxial cable comprising the steps of stranding a plurality of conductor elements together to form a stranded member, and compacting the stranded member whereby to cause any interstitial spaces therein to become substantially filled with conductor element material and to cause the compacted stranded member to present a substantially circular crosssection.

2. A method as claimed in claim 1, wherein six conductor elements are stranded around a central conductor element, all of said conductor elements being of substantially circular cross-section prior to the compacting step.

3. A method as claimed in claim 1 or claim 2, wherein the stranded member is compacted by drawing it through a forming die.

4. A method of manufacturing a flexible coaxial cable including manufacturing a-centre conductor by a method as claimed in any one of the preceding claims.

5. A method as claimed in claim 4, including the steps of providing dielectric over the compacted stranded member and forming a cylindrical outer conductor on the dielectric and coaxial with the compacted stranded member.

6. A method as claimed in claim 5 including the step of extruding a sheathing layer onto the outer conductor.

7. A centre conductor for a flexible coaxial cable made by a method as claimed in any one of claims 1 to 3.

8. A flexible coaxial cable made by a method as claimed in any one of claims 4 to 6.

9. A minimum attenuation flexible stranded centre conductor for a flexible coaxial cable substantially as herein described with reference to Fig. 2.

10. A method of manufacturing a minimum attenuation flexible stranded centre conductor for a flexible coaxial cable substantially as herein described with reference to the accompanying drawings.

New Claims or Amendments to Claims filed on 20th October, 1983.

Superseded Claims 1 to 10.

New or Amended Claims

1. A flexible coaxial cable including a minimum attenuation, flexible, compacted centre conductor

2. A coaxial cable as claimed in claim 1, wherein the centre conductor is comprised by six conductor elements disposed around a central conductor element.

3. A method of manufacturing a flexible coaxial cable comprising the step of forming a flexible centre conductor by stranding a plurality of conductor elements together to form a stranded member, and compacting the stranded member whereby to cause any interstitial spaces therein to become substantially filled with conductor element material and to cause the compacted stranded member to present a substantially circular cross-section.

4. A method as claimed in claim 3, wherein six conductor elements are stranded around a central conductor element, all of said conductor elements being of substantially circular cross-section prior to the compacting step.

5. A method as claimed in claim 3 or claim 4, wherein the stranded member is compacted by drawing it through a forming die.

6. A method as claimed in any one of claims 3 to 5, wherein the centre conductor is of minimum attenuation.

7. A method as claimed in any one of claims 3 to 6, including the steps of providing dielectric over the compacted stranded member and forming a cylindrical outer conductor on the dielectric and coaxial with the compacted stranded member.

8. A method as claimed in claim 7 including the step of extruding a sheathing layer onto the outer conductor.

9. A flexible coaxial cable made by a method as claimed in any one of claims 3 to 8.

1 0. A flexible coaxial cable including a minimum attenuation flexible stranded centre conductor for a substantially as herein described with reference to Fig. 2.

11. A method of manufacturing a flexible coaxial cable having a minimum attenuation flexible stranded centre conductor a substantially as herein described with reference to the accompanying drawings.