GB2130430A

GB2130430A - Cable screen

Info

Publication number: GB2130430A
Application number: GB08229480A
Authority: GB
Inventors: Neil Mcdonald
Original assignee: Raydex International Ltd
Current assignee: Raydex International Ltd
Priority date: 1982-10-15
Filing date: 1982-10-15
Publication date: 1984-05-31

Abstract

It has been found difficult to provide a screen for a cable which will remain effective yet will permit the cable to flex. To overcome this problem the present invention proposes a cable screen consisting of an inner metal tape 14 longitudinally wrapped around a cable 10, 12 and an outer metal tape 18 helically wound on the inner tape with an overlap between adjacent turns. Further metal tapes may be wound helically around the cable. A conductive braid may also be provided. The tape may be a copper-plastics laminate. An outer plastics sheath may be present. <IMAGE>

Description

SPECIFICATION Cable screen This invention relates to screened cable and more particularly, but not exclusively, to coaxial communications cable.

The primary purpose of a cable screen is to ensure that the signals being carried by the cable do not interfere with any external signals and that any external signals do not in turn interfere with the signals being carried by the cable. As the bandwidth utilisation of cables increases, then the interference problems multiply. In addition there is a growing requirement that the information carried on the cable is secure and cannot be readily picked up as a radiated signal from the cable.

The simplest and most effective form of screening is to enclose the unit to be screened in a solid extruded or swaged down conducting metal tube which has an effective cover of 100%. For example, a 0.65 mm wall tube thickness of, say aluminium, would have an isolation of better than 140db. (All isolation measurements in this specification are based on one metre samples at a frequency of 100MHz). This type of screen is perfectly adequate as long as there is not a requirement for the cable to be flexible. Attempts have been made to improve the flexibility by corrugating the metal tube, but the resultant cable still tends to be rigid, and a varying outer conductor diameter is not desirable since that can affect the electrical characteristics of the cable.

For flexible applications, wire braids are used extensively, but the screening efficiency of a single braid is relatively low because of the gaps between groups of strands making up the braid. For example, a copper braid with a 55% cover would have an isolation of around 40db. If the cover of the braid is increased to 95%, the isolation is only improved to around 60-70db. Higher isolation levels can be achieved, for example, from 90 to 100db, by applying two braids of around 95% cover, one on top of the other, but unfortunately such a construction is very expensive.

Flexible cables using a metal foil or tape as the screen are manufactured quite extensively. The advantage of the foil is that 100% screening can be achieved. However, flexing the cable generally results in a rapid deterioration in its screening performance. This is particularly the case with coaxial cables, where through necessity, the tape is applied longitudinally along the cable. Longitudinally applied tapes are very susceptible to cracking on flexing with a resultant deterioration in the screening performance.If the tape were not applied longitudinally, for example, if the tape was helically wrapped instead, then as the contact resistance between the tape layers increases with age, the radiation also increases due to the spiral path of the current in the outer conductor, i.e. the magnetic field set up by the longitudinal current in the inner conductor can only cancel the magnetic field set up by the longitudinal component of the current in the outer conductor. To give an example of the performance of an overlapped longitudinally applied metal tape, a 0.075 mm thickness copper tape applied in such a manner has an isolation of around 90db in the virgin state, but this deteriorates to around 75db after being severely flexed.

One solution that has been proposed is to produce a metal foil in a form which can be applied longitu dinally to the cable and yet will not fracture, or alternatively fracture very little on flexing. This can be achieved with a number of metals by laminating the metal to plastics material such as a polyester.

This is particularly applicable to aluminium, which performs very well in laminate form. A 0.1 mm thickness aluminium laminate has an isolation in the order of 90db in the virgin form and which after severe flexing is only reduced to around 85db.

However, aluminium or other metals which perform well as laminates are not necessarily the preferred material for example because of problems arising from corrosion and creep.

The present invention has been made from a consideration of these problems. The main object of the invention is to provide a longitudinal screen on a cable, particularly but not exclusively of those metals which do not have good flexing characteristics when laminated, in order that when the cable is flexed, there is not a major deterioration in screening effectiveness as one would experience with a conventional longitudinally applied overlapped tape.

According to the invention, the conventional metal tape is effectively split into two, not necessarily equal thinner tapes, the inner tape being applied longitudinally and either overlapped or welded at the seam and the second outer tape being applied helically on top of the first tape with an overlap. As a result of skin effect, the majority of the signal in the outer conductor is concentrated in the inner part of the split tape and hence the current path is longitudinal as required. The outer part of the split tape has excellent flexing characteristics due to its helical formation and acts as a secondary shield, bridging any cracks or breaks in the underlying longitudinal part of the split tape.

It is essential that adjacent turns of the outer tape overlap one another, the amount of overlap being preferabiy from 0.01 to 50%, more preferably from 40 to 50% but the overlap may exceed 10% if desired.

An open spiral tape makes substantially no contribution to screening. To illustrate the effect of the invention a single copper tape of 0.075 mm thickness, longitudinally applied has an isolation of about 90db. After vigorous flexing the isolation is reduced to about 75db. When the tape is split into two tapes of 0.0375 mm thickness and applied in accordance with the invention the isolation in the virgin state remains at about 90db but on vigorous flexing is only slightly reduced to about 86db.

In one embodiment of the invention a conductive braid surrounds the lapped tape. The braid is not essential. Other additional layers of screening can be added if desired for example a second lapped tape wound in the opposite or same sense to the first lapped tape. If lapped in the same sense it is important that the second lapped tape should cover the overlap region of the first lapped tape.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 is a communications cable in side elevation and partly in section; and Figure 2 is a view similar to that of Figure 1 of another embodiment of a communications cable.

Referring to the drawing the cable comprises a conductor 10 for example of copper surrounded by a di-eiectric 12. Thus far the construction of the cable is known per se.

A first flexible tape 14 is wrapped around the di-electric, the tape 14 extending longitudinally along the cable, that is to say with the tape axis substantially parallel to the cable axis. The tape may be of any conductive material, for example copper or other metal or a metal/plastics laminate. The thickness of the tape is selected having regard to the frequencies of the signals to be transmitted along the cable; the higher the frequency the thinner the tape may be. Generally tape having a thickness of from 0.001 to 0.003 inches (0.025 - 0.075 mm) may be used.

In order to preclude leakage of radiation or interference from external signals the tape should be sufficiently wide that an overlap 16 is formed when wrapped around the di-electric. The performance of the screen is improved the greater the overlap. The overlap is chosen having regard to cable size and screening requirements. Generally the overlap is not less than 3 mm.

A second tape 18 is lapped around the first tape 16.

The second tape may be the same as or different to the first tape as regards thickness and width and material. The material of each tape may be homogenous, e.g. a single metal or may be a composite such as tinned copper or plastics/metal laminate. The purpose of the second tape is to provide a screen for any radiation that leaks through cracks or breaks that might appear in the first tape and a screen for interference from external signals.

The second tape is wound so as to overlap the preceding turn of tape. The extent of overlap can vary, for example from 50% to 0%.

In the embodiment of Figure 2 a braided metal cover 20 surrounds the lapped tape 18. As mentioned previously the braid is optional and provides a simple means of making a connection to the screen for example by forming a "pigtail" at the cable ends.

Although not shown in the drawing the cable may be provided with an outer protective sheath of plastics or other material.

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

CLAIMS (Filed on 14 Oct 1983) 1. A screened cable in which the screen comprises an inner metal tape applied substantially longitudinally along the cable and overlapped or welded at the seam and an outer metal tape applied helically over said inner tape, there being an overlap between adjacent turns of the outer tape.

2. A screened cable as claimed in Claim 1, wherein the amount of overlap of adjacent turns of the outer tape is from 0.01 to 50%.

3. A screened cable as claimed in Claim 2, wherein the amount of overlap of adjacent turns of the outer tape is from 40 to 50%.

4. A screened cable as claimed in any preceding claim, wherein a conductive braid surrounds the outer tape.

5. A screened cable as claimed in any preceding claim, and further comprising one or more additional layers of screening.

6. A screened cable as claimed in Claim 5, wherein an additional layer of screening comprises a metal tape helically wound on the cable.

7 A screened cable as claimed in Claim 6, wherein the additional layer of screening is wound in the same sense as the outer tape, the overlap regions of the outer tape being covered by the turns of additional screening.

8. A screened cable as claimed in any preceding claim, wherein the thickness of the inner tape is from 0.025 to 0.075 mm.

9. A screened cable as claimed in any preceding claim wherein the outer tape is of the same material as the inner tape.

10. A screened cable as claimed in any preceding claim, wherein the material of the inner tape and/or the outer tape is homogeneous.

11. A screened cable as claimed in any of Claims 1 to 9 wherein the inner tape and/or the outer tape is composed of more than one material.

12. A screened cable substantially as described herein with reference to Figure 1 or Figure 2 of the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. be described by way of example with reference to the accompanying drawing in which: Figure 1 is a communications cable in side elevation and partly in section; and Figure 2 is a view similar to that of Figure 1 of another embodiment of a communications cable. Referring to the drawing the cable comprises a conductor 10 for example of copper surrounded by a di-eiectric 12. Thus far the construction of the cable is known per se. A first flexible tape 14 is wrapped around the di-electric, the tape 14 extending longitudinally along the cable, that is to say with the tape axis substantially parallel to the cable axis. The tape may be of any conductive material, for example copper or other metal or a metal/plastics laminate. The thickness of the tape is selected having regard to the frequencies of the signals to be transmitted along the cable; the higher the frequency the thinner the tape may be. Generally tape having a thickness of from 0.001 to 0.003 inches (0.025 - 0.075 mm) may be used. In order to preclude leakage of radiation or interference from external signals the tape should be sufficiently wide that an overlap 16 is formed when wrapped around the di-electric. The performance of the screen is improved the greater the overlap. The overlap is chosen having regard to cable size and screening requirements. Generally the overlap is not less than 3 mm. A second tape 18 is lapped around the first tape 16. The second tape may be the same as or different to the first tape as regards thickness and width and material. The material of each tape may be homogenous, e.g. a single metal or may be a composite such as tinned copper or plastics/metal laminate. The purpose of the second tape is to provide a screen for any radiation that leaks through cracks or breaks that might appear in the first tape and a screen for interference from external signals. The second tape is wound so as to overlap the preceding turn of tape. The extent of overlap can vary, for example from 50% to 0%. In the embodiment of Figure 2 a braided metal cover 20 surrounds the lapped tape 18. As mentioned previously the braid is optional and provides a simple means of making a connection to the screen for example by forming a "pigtail" at the cable ends. Although not shown in the drawing the cable may be provided with an outer protective sheath of plastics or other material. The invention is not restricted to the above described embodiments and many variations and modifications can be made. CLAIMS (Filed on 14 Oct 1983)

1. A screened cable in which the screen comprises an inner metal tape applied substantially longitudinally along the cable and overlapped or welded at the seam and an outer metal tape applied helically over said inner tape, there being an overlap between adjacent turns of the outer tape.