GB2230151A

GB2230151A - Electromagnetic shielding of cable termination

Info

Publication number: GB2230151A
Application number: GB8906132A
Authority: GB
Inventors: Graham Peace
Original assignee: Telent Technologies Services Ltd; Plessey Telecommunications Ltd
Current assignee: Telent Technologies Services Ltd; Plessey Telecommunications Ltd
Priority date: 1989-03-17
Filing date: 1989-03-17
Publication date: 1990-10-10
Also published as: GB8906132D0

Abstract

A cable termination for a coaxial cable is designed to minimise leakage of electromagnetic radiation which can arise from the region at which the cable is coupled to a connector. The cable includes a conductive foil screen (4) which surrounds an insulated cable core (5), and a conductive sleeve (7) is fitted to this screen so that it extends back from the end of the cable to overlie the outer insulation layer (2) of the cable. The casing (9) of a connector is coupled to this sleeve by means of a further conductive sleeve (12), with both sleeves being heat-shrunk into position to form a complete electrical screen in the region of the termination. Sleeve (7) consists of wound, self-adhesive copper foil, contacting the inner of two conductive foil layers (4) and a heat-shrunk insulating sleeve (6). Sleeve (7) receives a plastics ferrule (10) retaining the cable in the conductive hood (9) of a connector housing (8). <IMAGE>

Description

Cable Termination This invention relates to cable terminations of the kind which do not give rise to excessive levels of electromagnetic radiation. The requirements to minimise the emission of electromagnetic radiation are becoming more stringent, and it is proving difficult to effectively suppress leakage of such radiation at an acceptable cost. Conventionally, high performance cables of the kind designed to carry high frequency information signals are provided with an outer conductive copper braid which completely surrounds the cores of the cable, and these have been used with expensive cable connectors with some success. The present invention seeks to provide an improved cable termination in which a less expensive cable can be used, but which is suitable for operation at high frequencies.

This invention enables systems utilising specialised foil screen cables to use the same cables as part of an EMC hardened (electromagnetic compatibility) system, thus maintaining the same transmission properties of the said cable conductors.

A cost saving is therefore available, as a new braided screened cable does not have to be designed, manufactured or tested and existing cable stock does not have to be scrapped. There is also the opportunity to retrospectively update existing cables to meet new EMC requirements.

According to a first aspect of this Invention a cable termination includes a length of cable having a conductive foil screen which completely surrounds an inner insulated conductive core, and which foil screen is surrounded by a cable sheath; a first conductive member adjacent to the end of the cable and arranged to overlie and surround a length of said cable sheath and to surround an exposed length of said foil so as to be electrically in contact therewith; and means for electrically coupling said conductive member to a conductive shell of an electrical connector which terminates said cable.

The shell typically forms part of the casing of the connector and acts as part of an electromagnetic screen for the connector itself.

Preferably said conductive member comprises a shrunk-on material, a portion of which firmly grips said exposed length of foil, said material extending back over said cable sheath for a distance substantially greater than the diameter of said cable sheath.

Preferably again said shell includes an electrically conductive portion which surrounds said cable in the vicinity-of the end of the cabls sheath, and which is connected electrically to said fi rst conductive member by a second conductive member which is shrunk-on to said conductive portion and to said first conductive member.

Conveniently both first and second members are heatwshrunk into position, since such a method is simple and reliable and results in firm mechanical and electrical joints.

According to a second aspect of this invention, a method of forming a cable termination for a cable having a conductive foil screen which completely surrounds an inner insulated conductive core and which foil screen is surrounded by a cable sheath, includes the steps of; removing an end portion of the foil screen to expose an end length of insulated conductive core and removing an end portion of the cable sheath to expose an end length of the foil screen; placing a first conductive member adjacent to the end of the cable so that it overlies and surrounds the remaining end of the cable sheath and so that it surrounds the exposed length of said foil so as to be electrically in contact therewith; and introducing the end of the cable into the conductive shell of an electrical connector so that a portion of the shell surrounds said first conductive member so as to make good electrical connection therewith.

The cable may be of the kind which includes a conductive drain wire In contact with the conductive foil. Although the drain wire is intended to contrlbute to the screening connection of the cable and to constitute an earth terminal, in practice the degree of screening which can be achieved by connecting this drain wire to the conductive shell of the connector is relatively poor, as the drain wire presents a high Impedance connection at high frequencies of the order of 1MHz to 300 MHz. This results in a large electromagnetic emission from the cable screen. Thus, even though a drain wire may be present in the cable, it does not in itself contribute significantly to the shielding of the cable termination.

Effective electromagnetic shielding of a cable termination assembly can be achieved by means of a 360" bond between the cable screen and the connector. This however is not generally achieved at present using cables having a foil screen. Although the performance requirements may be met using a cable having an outer braided screen formed of a conductive mesh, such a cable may be rather more expensive than a cable utilising a foil screen, and may exhibit different shielding properties.

The invention is further described by way of example with reference to the accompanying drawings in which Figures 1 to 4 show sequential steps in the process of connecting a screened cable to an electrical connector.

Referring to Figure 1, there is shown therein a cable 1 comprising an outer cable sheath 2 which surrounds a screen which consists of two conductive foil layers wound in opposition to one another with a drain wire 3 sandwiched between them, only the inner foil 4 being visible. The outer layer of foil has a plastic film insulator laminated to Its outer face; this outer layer of foil has been removed to expose the conducting face of the inner foil 4 and the drain wire 3. An end portion of the inner foil 4 also has been removed to expose the Inner core comprising a plurality of cable wires 5, each of which consists of an insulated electrical conductor.

The next step in making the cable termination is to effectively extend the electrical conduction of the exposed foil 4 back over the surface of the outer cable sheath 2. As shown in Figure 2 a short length of heat-shrink sleeve 6 Is placed over the outer surface of the end of the cable sheath 2 and heat-shrunk Into position so as to form a tight grip with the outer surface of the sheath. The sleeve 6 Is formed of an electrically insulating material. Its purpose is to increase the outer diameter of the cable so as to match it to the inside diameter of a ferrule which forms part of the connector, to be described subsequently.

A length of self-adhesive copper foil tape 7 is wound over the outer surface of the sleeve 6 and the exposed end of the conductive foil 4 so as to completely encircle the cable. The adhesive material forming part of this copper foil tape 7 is itself conductive so that the tape forms an electrically continuous path with the inner foil 4. The tape extends back from the end of the sheath 2 for a distance considerably greater than twice the diameter of the sheath.

Referring to Figure 3, part of the outer shell of an electrical connector is shown; the inner electrical connnector whereby the individual cable wires 5 are terminated is not however illustrated as this aspect is incidental to the present invention.

The conductive outer shell consists of two parts, an angled conductive member 8 which forms part of the connector housing, and a conductive hood 9 to which It Is firmly secured. The hood 9 is in the form of a short hollow cylinder, and the end of the cable is inserted through the hood 9 as shown, after which a retaining ferrule 10 is placed over the outer surface of the conductive tape 7. This ferrule 10 is of non-conductive plastics material and is provided with a longitudinal slit which in operation engages with an indent 11 in the conductive hood. The ferrule 10 is inserted into the hood 9 so as to retain the cable end firmly in position with respect to the conductive member 8.

As is illustrated in Figure 4, after the conductive shell has been assembled, a length of conductive heat-shrink tubing 12 Is placed over the conductive hood 9 and the exposed portion of the tape 7. This heat-shrink tubing 12 Is of the kind having an inner conductive layer and after being placed in position, it is heated so as to shrink and form a firm conductive bond with the outer surface of the tape 7 and the conductive hood 9.

Thus In this way a 3600 bond between the cable screen and the connector Is achieved via the conductive heat-shrink material and the conductive tape. As mentioned above the diameter of the heat-shrink sleeve 6 Is chosen so as to increase the outer diameter of the cable to the extent necessary to match the inside diameter of the plastic ferrule 10.

The connection made in accordance with this invention provides a very high degree of protection again emission of electromagnetic energy whilst enabling a relatively inexpensive high frequency cable to be used, thereby providing economy of installation.

Claims

1. A cable termination including a length of cable having a conductive foil screen which completely surrounds an inner insulated conductive core, and which foil screen is surrounded by a cable sheath; a first conductive member adjacent to the end of the cable and arranged to overlie and surround a length of said cable sheath and to surround an exposed length of said foil so as to be electrically In contact therewith; and means for electrically coupling said conductive member to a conductive shell of an electrical connector which terminates said cable.

2. A cable termination as claimed In Claim 1, and wherein said conductive member comprises a flexible conductive material which is bonded to the said length of foil by means of a conductive adhesive.

3. A cable termination as claimed in Claim 1 or 2, and wherein said conductive member comprises a shrunk-on material, a portion of which firmly grips said exposed length of foil, said material extending back over the cable sheath for a distance substantially greater than twice the diameter of said cable sheath.

4. A cable termination as claimed in Claim 1 or 2,and wherein a spacer sleeve is interposed between the cable sheath and said conductive member.

5. A cable termination as claimed in any of the preceding claims and wherein said means for electrically coupling said conductive member to said shell comprises a second conductive member which is shrunk-on to said first conductive member so as to completely surround it, and which also contacts a portion of said shell.

6. A cable termination as claimed in Claim 5, and wherein said shell includes an electrically conductive rigid hood portion which surrounds said cable in the vicinity of the end of the cable sheath, with said second conductive member also being shrunk-on to said portion so as to surround it.

7. A cable termination as claimed in Claim 5 or 6, and wherein both first and second conductive members are shrunk into position by the application of heat.

8. A method of forming a cable termination for a cable having a conductive foil screen which completely surrounds an Inner insulated conductive core and which foil screen Is surrounded by a cable sheath, said method including the steps of removing an end portion of the foil screen to expose an end length of insulated conductive core and removing an end portion of the cable sheath to expose an end length of the foil screen; placing a first conductive member adjacent to the end of the cable so that It overlies and surrounds the remaining end of the cable sheath and so that it surrounds the exposed length of said foil so as to be electrically in contact therewith; and Introducing the end of the cable into the conductive shell of an electrical connector so that a portion of the shell surrounds said first conductive member so as to make good electrical connection therewith.

9. A method as claimed In Claim 8, and wherein said first conductive member comprises a flexible sheet having a conductive adhesive surface which is wrapped around the exposed length of said foil and the end of the cable sheath.

10. A method as claimed in Claim 9, and wherein said flexible sheet Is heated so as to cause It to shrink Into position.

11. A method as claimed in Claim 8, 9 or 10, and wherein said cable is inserted into the conductive shell of a connector, and a heat shrinkable electrically conductive sleeve is positioned over said first conductive member so as to surround a conductive portion of the shell, after which said sleeve Is heated so as to cause It to shrink into close physical and electrical contact with said first member and said shell.

12. A cable termination substantially as illustrated in and descrlbed with reference to the accompanying drawings.