GB2166984A - Cables - Google Patents

Abstract

A method of forming a cable having inner and outer members (1, 3) separated by a medium (2), the inner member and the medium forming a core, the method comprising the step of inserting the core into the outer member which comprises a tube of a ductile material, drawing the outer member so that it grips the core, and subjecting the so formed cable to a rolling operation to form convolutions (4, 5) along the length of the outer member. <IMAGE>

Description

SPECIFICATION Cables This invention relates to cables and a method of producing the same, and has particular reference to cables having inner and outer conductor means separated by dielectric means. The invention has particular, but not exclusive, reference to such cables as used to transmit microwave electrical signals.

In microwave systems, cable runs very often are complex and are other than in a single plane.

When semi-rigid cables are used, it is necessary first to produce a three-dimensional representation of each run and then carefully to form the cable in accordance therewith and connect it into the system. Whilst minor adjustments to a run can be effected, any significant changes cannot be made because once the cable has been bent, it cannot be straightened and reformed because a permanent kink is introduced into the cable by each bend and each kink adversely affects the transmission properties of the cable to the extent that the latter is rendered useless.Ignoring any wastage which occurs due to kinking, this form of cabling is expensive, not because of the unit cost of the cable per se but because of the cost of preparing three-dimensional representations of the runs, forming the cable accordingly and delivering the formed cables intact for assembly into the system for which they are required. Extra cost is often involved in temperature cycling the cable before formation into a run and sometimes even temperature cycling the formed run. This is because the cable needs to be highly temperature stable, otherwise the least variation in temperature causes expansion or contraction with an attendant change in transmission properties which cannot be tolerated.

In an attempt to obviate the aforementioned problems the Applicants introduced a flexible cable which does not kink on being bent and then straightened and has excellent temperature and phase stability. With this cable, there is no need to produce a three-dimensional representation of a run because all that assembly requires is for one end of the cable to be connected to the required point, the cable then bent in the required run, and the other end connected as necessary. If any bending is inaccurately executed, then the cable can be straightened and reformed without problems.

Thus, the relatively high cost of pre-forming each run according to a representation and then delivering the or each run to the system is avoided, as well as wastage due to kinking and the cost of temperature cycling which is unnecessary except in very specialised applications. There is some offset to this saving in cost by the increased cost of producing the cable which results from having to provide a composite tubular outer conductor into which the inner conductor means and surrounding dielectric (referred to as the core) is placed. The thus basically assembled cable is then subjected to a rolling operation which produces convolutions (preferably helical) in the outer conductor, which convolutions produce the flexure characteristic of the cable.The extent of the rolling in terms of depth of the convolutions has to be carefully controlled so as not to work harden the outer conductor which would otherwise fracture on being flexed. Also, when the convolutions take the form of a helix, the pitch of the latter has to be chosen carefully in relation to the overall diameter of the cable.

The outer conductor of this flexible cable has to date consisted of a fused composite of a soft copper tube surrounded by a tube of stainless steel which it has been found necessary to employ in order to impart structural strength to the copper tube. This composite material is relatively expensive because the stainless steel tube has to be specially treated so that it is malleable. Also, the cable can only be produced in relatively short lengths of about 3 metres because the necessary quality control as regards the relative despostion of the outer conductor and the core is difficult to achieve with long lengths.Furthermore, the tolerance of the inner diameter of the outer conductor has to be held within limits so as to be able to insert the core therewithin with relative ease but not with such a clearance that relatively deep convolutions have to be formed in the outer conductor to enable the latter to bite into the core dielectric which is necessary in order to secure together these two components. If such deep convolutions have to be formed, then there is the danger that work hardening will result, leaving the finished cable liable to fracturing.

According to the present invention there is provided a method of forming a cable having inner and outer members separated by a medium, the inner member and the medium forming a core, the method comprising the steps of inserting the core into the outer member which comprises a tube of a ductile material, drawing the outer member so that it grips the core, and subjecting the so formed cable to a rolling operation to form convolutions along the length of the outer member.

Conveniently, the outer member consists of soft copper which has excellent electrical conductive properties and lends itself well to the drawing step.

It has not been thought possible previously to achieve the necessary structural strength with copper alone which tends readily to work harden and hence fracture when flexed. However, it has been found that the step of drawing the copper outer member onto the core disposed therewithin gives substantially uniform contact with the core throughout the length of the cable, even after the rolling operation as the core tends to "flow" into the peaks of the convolutions produced on rolling.

This contact and the convolutions enable the cable to be bent, straightened and then reformed (to a reasonable extent) without the outer member fracturing and without a permanent kink being produced in the cable.

The cable produced by the method of the present invention has excellent temperature and phase stability as does the Applicants' prior cable but it is cheaper than the latter because a composite outer member is avoided and there are no serious limitions in the length of cable which can be produced.

According to another aspect of the present invention, there is provided a cable comprising a core of an inner member surrounded by a medium, and an outer member surrounding the core, the outer member consisting of a uniform ductile material formed with convolutions along its length and substantially in contact with the core along its length.

In both the method and cable aspects of the present invention the inner and outer members may be electrically conductive and the medium may be a dielectric, and furthermore, the convolutions in the outer member preferably take the form of a helix extending along the full length of the cable. However, the term "convolutions" as used throughout this specification is not limited to a helix, this just happening to be a convenient form in terms of manufacture. For example, the convolutions may be in the form of discrete peaks and troughs either generally at right angles, or otherwise inclined, to the axis of the cable.

The present invention will now be described in further detail, by way of example, with reference to the accompanying drawings in which: Figure 1 is an enlarged diagrammatic cross-sectional view of a microwave cable in accordance with the present invention, Figure 2 is a tabulation of dimensions indicated in Figure 1 for three different sizes of cable, and Figures 3, 4 and 5 are respective cross-sectional views of three different sizes of cable showing the varying contours thereof, below which are shown the corresponding side views.

Referring to Figure 1, the microwave cable comprises a single inner conductor 1 which may be a silver plated, copper covered steel wire or a silver plated solid copper wire, for example. The inner conductor 1 is surrounded by a cylinder of a dielectric material, such as PTFE, and these two components form a core which is a standard available component. Finally, the cable comprises an outer conductor 3 of soft copper, substantially in contact with the core throughout its length and being formed with convolutions along its length. In this embodiment of the invention, the inner and outer conductors 1 and 3 are coaxial, and the convolutions are in the form of a helix.

The cable of Figure 1 is manufactured in accordance with the method of the present invention by first inserting the core 1, 2 into the soft copper outer conductor 3 which initially is in the form of a tube having an inner diameter of the order of 0.002 inches (0.025mm) greater in diameter than that of the core so that it receives the latter relatively easily. In this respect, the core needs to be manufactured to a tight tolerance. The thus assembled cable is then drawn through a series of polished dies of successively decreasing diameter until the outer conductor 3 grips the core substantially along its length. The ductibility of the soft copper enables this drawing operation to be accomplished and the cable is then subjected to the final manufacturing step of rolling to produce the helical convolution along its length.

This rolling operation can be effected either by rotating a tool relative to the cable, or vice versa, the tool utilising one or more ball bearings or rollers, for example, to produce the convolution. With the outer conductor being in close contact with the core dielectric before the rolling operation, the latter increases that contact at least in the "troughs" 4 of the convolution, the dielectric tending to "flow" from the troughs 4 into the peaks 5, whereby the outer conductor remains in contact with the core substantially along its length although in this respect, small air or gas pockets at the peaks of the convolution between the outer conductor 3 and the dielectric 2 can be tolerated.

The dimensions A to D of the cable components and the pitch E of the helix are indicated in Figure 1, and Figure 2 is a tabulation showing how these values (all of which are nominal) vary for three sizes X, Y and Z of cable. It will be seen that the pitch E of the helix decreases as the overall diameter A of the cable decreases. However, it has been found that the precise shape of the convolution is preferably varied according to the diameter of the cable being produced in order to maximise the flexure characteristic. Figures 4, 5 and 6 show the precise contours of the cables X, Y, Z and the full side views thereof.

Cables in accordance with the present invention exhibit excellent characteristics are regards temperature and phase stability and can be bent, straightened and reformed without kin king. The method of manufacture is simpler, and hence less expensive, than that of the Applicants' previous flexible cable, whereby the invention represents a considerable advance in the art. The invention lends itself particularly well to cables for transmitting electrical microwave signals, operating up to 30 GHz. However, the invention is applicable in other areas where there is a need for complicated cable runs. It will be appreciated that whilst cables in accordance with the invention are flexible, they are not resiliently, whereby they retain a given set imparted thereto by bending or forming.

The inner conductor means 1 need not be of solid construction and may comprise a straight or twisted multi-stranded conductor. Furthermore, the inner conductor means need not be coaxial with the outer conductor means and the former may comprise more than one conductor.

After manufacture, the outer conductor means may be provided with a protective coating, such as a plated layer for example.

Alternative materials for the outer conductor means other than copper are aluminium and brass, for example. The dielectric means may be of any conventional form, i.e. solid or gas filled.

Claims (15)

1. A method of forming a cable having inner and outer members separated by a medium, the inner member and the medium forming a core, the method comprising the steps of inserting the core into the outer member which comprises a tube of ductile material, drawing the outer member so that it grips the core, and subjecting the so formed cable to a rolling operation to form convolutions along-the length of the outer member.

2. A method according to claim 1, wherein the step of rolling the outer member produces convolutions in the form of a helix.

3. A method according to claim 1 or 2, wherein the method comprises the step of plating the rolled cable.

4. A cable comprising a core of an inner member surrounded by a medium, and an outer member consisting of a uniform ductile material formed with convolutions along its length.

5. A cable according to claim 4, wherein the convolutions are in the form of a helix.

6. A cable according to claim 4 or 5, wherein the inner and outer members are electrically conductive and the medium is in the form of a dielectric.

7. A cable according to claim 6, wherein the outer member is formed from soft copper.

8. A cable according to claim 6, wherein the outer member is formed from aluminium.

9. A cable according to claim 6, wherein the outer member is formed from brass.

10. A cable according to any one of claims 6 to 9, wherein the dielectric material is solid.

11. A cable according to any of claims 6 to 9, wherein the dielectric is gas filled.

12. A cable according to any of claims 4 to 11, wherein the inner member is multi-stranded.

13. A cable according to any of claims 4 to 12, wherein the rolled cable is plated over its exterior surface.

14. A cable substantially as herein particularly described with reference to the accompanying drawings.

15. A method of forming a cable substantially as herein particularly described with reference to the accompanying drawings.