WO1989002646A1

WO1989002646A1 - Cable dielectric

Info

Publication number: WO1989002646A1
Application number: PCT/GB1988/000732
Authority: WO
Inventors: David Curtis
Original assignee: Volex Group PLC
Current assignee: Volex PLC
Priority date: 1987-09-08
Filing date: 1988-09-08
Publication date: 1989-03-23
Anticipated expiration: 1990-03-08
Also published as: GB8721072D0; CN1031904A; AU2328588A

Abstract

A dielectric cable comprising a conductor (10) surrounded by an inner layer (14) and an outer layer (12). The inner layer extends between the outer layer and the conductor in a longitudinally sinusoidal form thereby defining a plurality of closed cells (18) of the same shape. The structure is formed by concentrically extruding the conductor and the inner and outer layers and repeatedly changing the pressure difference between either side of the inner layer.

Description

CABLE DIELECTRIC

This invention relates to the dielectric structure of cables.

The most common kind of co-axial cables comprise an inner conductor surrounded by dielectric and the dielectric is itself surrounded by an outer conductor. In one construction the dielectric is formed by a plurality of radially extending walls whose planes are parallel to the inner conductor and which between them define air spaces of substantially segmental cross-section. While this construction is economical to manufacture and electrically successful it has the disadvantage that if the cable is damaged in such a way as to permit ingress of moisture to the airspace structure, the resulting degradation of cable performance may not be confined to the local area of the damage. To deal with that, constructions have been proposed which include walls transverse to the inner conductor axis located at intervals along the cable so that the longitudinal lengths of the air spaces are limited. This kind of construction involves an additional operation and special equipment for forming the transverse walls which increases the expense of the product, limits the range of cable sizes that can be economicallv manufactured and does not lend itself to high speed cable production.

The present invention has been made from a consideration of these problems.

According to the present invention there is provided a dielectric structure for a cable, comprising an outer layer which extends substantially parallel to and spaced from a conductor and an inner layer extending in a substantially zig-zag fashion between the outer layer and the conductor, so as to define a plurality of closed cells.

In a preferred embodiment of the invention the outer and inner dielectric layers are extruded at the same time around a conductor. By adjusting the relative air pressure on either side of the inner layer during extrusion, the inner layer is.caused to be diverted towards the conductor or towards the outer dielectric layer. Thus the inner layer can be made to take up a longitudinally zig-zag or sinusoidal path.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Pig.l is a diagrammatic longitudinal axial section through a co-axial cable; Fig.2 is a diagrammatic part sectional illustration of apparatus for producing the cable of Fig.l; and

Fig.3 is a perspective view of the cable of Fig.l, with the outer layer cut back.

Referring to Figs.l and 3 of the drawing the cable comprises an inner conductor 10 which is surrounded by a two layer tubular polyethylene dielectric. The outer layer 12 is uniformly spaced from the conductor while the inner layer 14, as it extends along the length of the cable is alternatel - engaged against the inner surface of the outer layer and against the conductor. The inner conductor thereby defines a series of closed inner cells 16 and a series of closed outer cells 18. The outer layer is surrounded by an outer conductor for example a braid which is itself covered by a sheath. These items are conventional and not shown in the drawing.

The dielectric structure is preferably produced by simultaneous extrusion of both layers around the inner conductor 10. By increasing the air pressure inside the inner layer the said layer is caused to be moved into contact with the outer layer. And by reducing the pressure inside the inner layer below the air pressure between the inner and outer layers the inner layer is caused to move into contact with the conductor. The pressure changes can be controlled so that the disposition of the inner layer is substantially regular. It is desirable for good electrical performance to have as nearly a periodic structure as possible. The actual shape or form of each "cell" is of little importance electrically providing each one is nearly identical.

A convenient method of manufacture will now be described with reference to Fig.2. A pair of concentric polyethylene tubes 12, 14 are extruded through die 20 around a copper conductor 10. The tubes are circular and the outer one 12 is supported during its cooling phase by a die tube 22 in a vacuum trough. (This means of forming a tube is quite common in the hosepipe and cable industry) .

The radius of the inner tube 14 is controlled by the air pressure at point A. By raising the pressure at A above that of ambient the inner tube 14 tends to expand until touching the outer tube 12. Subsequently decreasing the pressure at A to below ambient causes the inner tube to decrease in radius until it touches the conductor. The hole 24 in die 20 for the conductor is made sufficiently large to allow the air between the inner tube and the conductor to inflow and exhaust freely.

Thus by cyclically changing the air pressure at point A in this manner a dielectric structure similar to that shown in Figs.l and 3 can be made. The wavelength of the inner tube is controlled by the speed at which tubes 12, 14 are extruded from die 20, and/or the frequency at which the air pressure at point A is changed.

The air supply comprises a pneumatic valve 24 controlled by an electrical oscillator 26. The valve 24 is fed by two air supplies 28, 30, one above ambient and one below ambient pressure. The output of the valve is switched between the two inputs by the oscillator.

The alternating air supply from the valve 24 is led to the die 20 as by line 32.

Although in this arrangement the air between the inner and the outer tubes 12, 14 is free to inflow and exhaust freely it is possible, if desired, to control the air pressure between them, for example driving it in antiphase to the pressure between the inner tube 14 and the conductor 10. In this embodiment air is used as the controlling medium, but any gas that is safe to handle and does not adversely affect the performance of the finished cable can be used.

An example of approximate operating conditions for producing cable using the above described assembly were as follows:-

Wire diameter Outer tube diameter Outer tube thickness

Inner tube mean thickness Extrusion speed Oscillator frequency

Air supplies of ambient +/-0.4 Bar were used together with regulators to restrict the air flow.

The actual pressure needed to make the product depends very much on the type of polyethylene compound used and on the running conditions of the extruder.

The example given is typical but by no means the only size that can be made.

It is possible that inventive concept of blocking the dielectric structure to water ingress can be combined with other more traditional methods of dielectric construction, e.g. 5 cell, to exploit the benefits of both techniques. In the case of a 5 cell/blocked cell hybrid, the benefit inherited from the 5 cell design is that of better support for the conductor, particularly when the cable is flexed in use.

The invention is not limited to the above described embodiments and many other variations and modifications can be made.

Claims

1. A dielectric structure for a cable comprising an outer layer which extends substantially parallel to and spaced from a conductor and an inner layer extending in a substantially zig-zag fashion between the outer layer and the conductor so as to define a plurality of closed cells.

2. A dielectric structure for a cable as claimed in Claim 1, wherein the shape of the closed cells is substantially the same.

3. A dielectric structure for a cable as claimed in Claim 1 or Claim 2 , wherein the inner layer is of sinusoidal form in longitudinal, axial section.

4. A cable comprising a dielectric. structure as claimed in any preceding claim.

5. A method for producing a dielectric structure as claimed in Claim 1 comprising concentrically extruding an outer layer and an inner layer around a conductor, repeatedly changing the fluid pressure diference on either side of the extruded inner layer to cause the inner layer to be deflected in a zig-zag fashion berween the conductor and the outer layer thereby forming a plurality of closed cells.

6. A method as claimed in Claim 5, wherein the pressure changes on either side of the inner layer are controlled so that cells of substantially the same size are formed.

7. A method as claimed in Claim 5 or Claim 6, wherein the pressure changes on either side of the inner layer are controlled so that the inner layer is given a sinusoidal shape in longitudinal, axial section.

8. A method as claimed in any of Claims 5 to 7, wherein the fluid pressure difference on either side of the inner tube is changed by increasing and decreasing the fluid pressure above and below ambient on one side of the inner layer, fluid pressure on the other side of the inner layer being substantially constant.

9. A method as claimed in any of Claims 5 to 7 wherein the fluid pressure difference on either side of the inner layer is changed by increasing and decreasing the fluid pressure on one side of the inner layer and decreasing and increasing the fluid pressure in antiphase on the other side of the inner layer.

10. A dielectric structure for a cable substantially as described herein with reference to Figs.l and 3 of the accompanying drawings. - lO -

ll. A method for producing a dielectric structure substantially as described herein with reference to Fig.2 of the accompanying drawings.