GB2163285A

GB2163285A - Screened electric cable

Info

Publication number: GB2163285A
Application number: GB08520441A
Authority: GB
Inventors: Nicholas Henry Neely
Original assignee: Volex Group PLC
Current assignee: Volex PLC
Priority date: 1984-08-16
Filing date: 1985-08-15
Publication date: 1986-02-19
Also published as: GB2163285B; GB8520441D0

Abstract

A screened cable has a screen 14 formed from longitudinally applied aluminium tape and bonded to a sheath 16 made from polyethylene or a laminate of polyethylene with a hard skin of polypropylene bonded by a mixture of the polymers. Overlapping edges of the tape are bonded together but the tape is not bonded to the underlying dielectric layer 12 which may be air spaced. The tape may have an outer skin of polyethylene or polyethylene/acrylic blend. The use of aluminum tape is stated to be cheaper that copper tape and braid screens and provides better screening. The cable is used for TV communication. <IMAGE>

Description

SPECIFICATION Cable This invention relates to a screened cable.

The conventional form of screened communications cable comprises a conductor surrounded by a dielectric layer. A screen comprising a copper tape and a copper braid encioses the dielectric layer and a plastic sheath, for example of polyethylene, encloses the tape and braid screen.

The conventional cables are reasonably effective but they are increasinngly expensive to produce to meet the requirements of British Standard 5425 particularly because of the intrinsic cost of the copper metal for the tape and braid and because braiding is a relatively expensive procedure.

With the introduction of cable television and the need for greater security in communications the effectiveness of the screen in the cable is of utmost importance.However, the relationship between improved screening and the percentage cover of braid in the screen is substantially exponential so that to improve the screen of conventional cable by even a small amount involves a very heavy increase in cost.

The present invention has been made from a consideration of this problem.

According to the invention there is provided a screened cable comprising a conductor, a dielectric layer around the conductor, an aluminium, or aluminium based, tape screen wrapped substantially longitudinally around the dielectric layer and a sheath around the screen and firmly bonded to the screen.

It might be expected that the provision of an aluminium screen would make the cable so inflexible as to make it difficult if not impossible to install it for example in ducts that include changes of direction. Surprisingly it has been found that the cable of the invention is quite flexible. Moreover, unlike copper tape screens which can crack on repeated flexing, thus seriously reducing their screening effectiveness, th aluminium screen in the cable of the invention does not crack even upon repeated flexing of the cable.

Perhaps the most extraordinary discovery, however, was to find that the cable of the invention using only an aluminium screen of the same resistance equivalent as a copper tape and braid screen shows a very marked improvement in isolation. Isolation is a measure of the effectiveness of a cable screen and can be represented as 20 log,0Xzt/zo) where Zt is the transfer impedance and Zo is the characteristic impedance. The isolation at 5 megahertz of a 1 metre sample of cable having a 50/1 copper tape and braid (Kf = 0.3 as defined in British Standard 2316) tested in accordance with British Standard 2316 is 80 dB. The cable of the invention having an aluminium screen of substantially the same resistance equivalent as the copper tape and braid screen tested under the same conditions had an isolation of about 100 dB.It is interesting to note that to improve the isolation of the cable with copper tape and braid screen to 100 dB the braid would need to be replaced by a braid of Kf 0.7 involving an increase in ciost of about 15%. But not only does the cable of the invention have improved isolation in comparison to the equivalent cable with a copper tape and braid screen but it is up to 30% cheaper.

Yet a further surprising advantage of the cable of the invention is that the return loss ratio is improved by at least 3 dB as compared with the equivalent cable with copper tape and braid. It is not certain why this should be but it could be that the aluminium screen behaves more like a tube than the copper tape. The pressure of the braid on the copper tape can cause small distortions to the tape and the dielectric which changes the impedance of the cable so that when a signal is fed to the cable a standing wave may be set up with a consequential increase in the power demand. That increase can result in damage to electric components such as amplifiers to which the signal is fed and which may not be adequately rated for the aforesaid increased power.

In a preferred embodiment of the invention the aluminium screen is in the form of a tape applied longitudinally around the dielectric so that opposite edges of the tape overlap. The tape is preferably not bonded to the dielectric layer. The tape has on its outer side a thin skin or coating of plastics such as polyethylene or a polyethylene/acrylic blend. Substantially immediately after the tape is positioned around the dielectric the sheath is extruded onto the screen. The material of the sheath should be one that will bond to the plastic coating on the screen, for example polyethylene.

A particularly advantageous cable construction can be obtained by forming a laminated sheath as described in our British Patent No.

2084927 with a relatively soft inner layer, for example of polyethylene and a relatively hard, thin, outer layer for example of polypropylene, the two layers being bonded together by an intermediate layer which is a homogeneous mixture of the plastics of the inner and outer layers. If water can gain access through a sheath damaged during installation, it will corrode the aluminium screen and could thus seriously reduce the isolation. The laminated sheath is less likely to be damaged during installation than a sheath which is composed only of polyethylene.

Moreover a smaller overall thickness of laminated sheath is required, in order to protect the cable, than a sheath of a single softer material such as polyethylene so that the use of the laminated sheath provides a material saving as well and aiso results in a cable of smaller overall diameter very useful advantage where the cable must be installed in a restricted space.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing which is a diagrammatic view in side elevation of a cable.

Referring to the drawing the cable comprises a conductor 10 surrounded by a dielectric layer 1 2 which is preferably air spaced. A typical overall diameter of the dielectric is 10 mm. A longitudinally applied tape of aluminium 14 for example of thickness at least 1 00it, preferably of 1 50it having a 40CL coating of polyethylene or polyethylene/acrylic blend is longitudinally wrapped around the dielectric with the plastic coating outermost and overlapped by about 6 mm. The aluminium tape is not bonded to the dielectric layer but the overlapping area of the tape can be bonded tothe tape beneath it if desired. A sheath 1 6 is extruded-onto the screen and bonded thereto immediately after the screen is positioned around the dielectric. The sheath can be a layer of polyethylene or a thinner laminate of polyethylene having a hard thin skin of polypropylene bonded thereto by a homogeneous mixture of the polyethylene and the polypropylene.

Claims

1. A screened cable comprising a conductor, a dielectric layer around the conductor, a screen consisting of an aluminium or aluminium based tape wrapped substantially longitudinally around the dielectric layer and a sheath around the screen and firmly bonded to the screen.

2. A screened cable as claimed in Claim 1, wherein opposite edges of the tape overlap.

3. A screened cable as claimed in Claim 1 or Claim 2, wherein a coating of plastics material is provided on the outer side of the tape which will bond to the sheath.

4. A screened cable as claimed in Claim 3, wherein the coating is of polyethylene or a polyethylene acrylic blend.

5. A screened cable as claimed in any preceding. claim, wherein the sheath comprises a laminate of a relatively soft inner layer and a-relatively hard outer layer bonded together by an intermediate layer, said intermediate layer being a homogeneous mixture of the plastics of the inner and outer layers.

6. A screened cable as claimed in Claim 5, wherein the inner layer of the sheath is of polyethylene.

7. A screened cable as claimed in Claim 5 or Claim 6, wherein the outer layer of the sheath is of polypropylene.

8. A screened cable as claimed in any preceding claim, wherein the screen has a thickness of at least 100it.

9. A screened cable substantially as described herein with reference to the accompanying drawing.