GB2160718A

GB2160718A - Elastomeric screen for coaxial cable

Info

Publication number: GB2160718A
Application number: GB08503641A
Authority: GB
Inventors: Stanley Ralphs
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1984-06-20
Filing date: 1985-02-13
Publication date: 1985-12-24
Also published as: GB8503641D0; GB8415746D0

Abstract

An earthing connection to one or more coaxial cable screens (4,5,6) is made by an electrically conductive elastomer (19) which is preferably in the form of a sheet provided with apertures for gripping the screens. The sheet is preferably formed in situ around the exposed coaxial screens (on a circuit board or electrical connector (1) for example) by applying a settable liquid or semi-liquid elastomer composition around them. A silicone-based, silver-loaded elastomer may be used. A lead (22) of a common earth terminal (9) also contacts the conductive elastomer (19). <IMAGE>

Description

SPECIFICATION Screened cable termination The present invention relates to electrical screening particularly of the terminations of screened coaxial cables. Such cables generally comprise one or more conductors located within an insulating core, a braided screen of fine copper wire surrounding the core and an outer insulating sleeve.

Hitherto, such cables have been connected to pins on circuit boards by removing the outer cable insulation to expose the braided screen, unravelling the end of the braided screen and twisting the unravelled strands into a "pigtail", removing the end of the insulating core to expose the conductor end(s), and soldering the "pigtail" and the or each conductor end to respective pins on the circuit board.

Unravelling the strands of the braid and forming the "pigtail" is awkward and timeconsuming, and has the further disadvantage that the length of cable from which the braid is unravelled (which may be as long as 25mm) is left unscreened. Both these problems are particularly acute when a large number of pieces of equipment need to be connected by short lengths of screened cable, since the unscreened cable ends then represent an appreciable proportion of the total cable length.

An object of the present invention is to alleviate such disadvantages, particularly when a multiplicity of cable screens have to be connected to a common earthing point.

A further object of the invention is to provide a compact earthing arrangement for a coaxial screen.

According to one aspect of the present invention, a method of making an electrical connection between a terminal and an exposed length of electrical screen at the termination of a screened cable comprises the steps of gripping said exposed length of screen with an electrically conductive elastomeric member to form an electrical connection therewith and forming a further electrical connection between said elastomeric member and said terminal.

The screen may be the braided screen of a conventional coaxial cable. However the screen may instead be a thin metal layer on a tubular sleeve (of heat-shrinkable tubing for example) surrounding one or more conductors of the cable. The screen may even be a selfsupporting metal tube.

The elastomeric member may be loaded with a metal such as nickel or copper to make it electrically conductive. The exposed length of cable screen is preferably gripped within an aperture in the elastomeric member, so that the latter surrounds the cable termination.

The elastomeric member may incorporate a plurality of apertures for gripping the screens of respective cable terminations. In such a case the elastomeric member is suitably in the form of a sheet which may be supported on, and provide electrical screening for, a circuit board for example. The elastomeric member may incorporate an embedded metal pin to enable a common soldered electrical connection to be made with all the screens.

The elastomeric member may be formed in situ by potting the or each exposed length of coaxial screen in a settable liquid or semiliquid electrically conductive elastomer composition. Such a technique is particularly useful when applied to cable terminations at multiple connectors for example. Thus the inner conductors of the coaxial cables may be initially soldered to connection pins on the back of the connector, a layer of insulating material may be deposited so as to cover the soldered connections but not the exposed lengths of coaxial screen, and the layer of electrically conductive elastomer may then be deposited on this insulating layer to surround and make electrical contact with the screens.

A connecting pin may be arranged to protrude through the insulating layer to make contact with the elastomeric layer to form the earth connection.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing, Fig. 1.

Figure 1 shows, partly in section, a five-pin connector 1 attached to single-core coaxial cables 2, 3 and a twin-core coaxial cable 4.

The five pins 7, 8, 9, 10 and 11 of the connector protrude through an insulating base 12.

In accordance with the invention, the conductors 1 3, 14, 1 5 and 1 6 and braided coaxial screens 5, 6, 20 and 21 are connected to respective connector pins as follows.

Initially, conductors 1 3, 14, 1 5 and 16 are soldered to pins 7, 8, 10 and 11 respectively.

A bare lead 22 is soldered to pin 9 (an earthing pin). A collar 1 7 is then fitted on top of base 1 2 to form a mould and Berger PR 1 201 Q elastomer is then deposited with a syringe between the protruding ends of the pins until they and the ends of cable conductors 13, 14, 1 5 and 1 6 are completely covered by an insulating layer 1 8 of the elastomer. This insulating layer is typically 3mm to 5mm thick. When this layer has set a further layer 1 9 of CHOBOND 1 038 RTV sealant (a silicone based, silver-loaded elastomer supplied by Chomerics UK Ltd.) is similarly deposited around the braided screens and the protruding end of lead 22, so as to form a common earth connection to terminal 9.Elastomer layer 1 9 sets in air to the consistency of soft rubber and is then adhesive, bonding firmly to layer 1 8. Layer 1 9 has a resistivity of the order of 1 Qm and shields pins 7, 8, 10 and 11 as well as the cable conductors. Collar 17 may either be removed or incorporated in the finished connector.

It will be appreciated that cables 2, 3 and 4 could be provided with alternative coaxial screens to the braided screen incorporated in conventional coaxial cable. Thus the cable conductors could be shielded with heat-shrunk sleeving provided with a silver coating on its outer surface. This silver coating would form a good electrical connection with the silver loaded elastomer layer 1 9.

It will be appreciated that the method of the invention is particularly suited for making an electrical connection between a coaxial screen and a metal track of a printed circuit board or to a pin soldered to such a track.

Claims

1. A method of making an electrical connection between a terminal and an exposed length of electrical screen at the termination of a screened cable, comprising the steps of gripping said exposed length of screen with an electrically conductive elastomeric member to form an electrical connection therewith and forming a further electrical connection between said elastomeric member and said terminal.

2. A method as claimed in Claim 1 wherein said screen is a braided screen of a coaxial cable.

3. A method as claimed in Claim 1 wherein said screen is a thin metal layer on a tubular sleeve surrounding one or more conductors of said cable.

4. A method as claimed in Claim 1 wherein said screen is a self-supporting metal tube surrounding one or more conductors of said cable.

5. A method as claimed in any preceding Claim wherein said exposed length of screen is gripped within an aperture in said elastomeric member.

6. A method as claimed in Claim 5 wherein exposed lengths of electrical screen at the terminations of a plurality of respective screened cables are gripped within respective apertures of a common electrically conductive elastomeric member and a common electrical connection is made between said terminal and said elastomeric member.

7. A method as claimed in Claim 6 wherein the inner conductors of said cables are soldered to electrically conductive tracks of a circuit board, said elastomeric member is earthed by said further electrical connection and is formed as a sheet on the surface of said circuit board to screen the tracks thereof.

8. A method as claimed in Claim 6, wherein the inner conductors of said cables are connected to pins of an electrical connector, said elastomeric member is earthed by said further electrical connection and forms at least a partial screen for said pins.

9. A method as claimed in Claim 8 wherein said inner conductors are intially soldered to said pins, a layer of settable insulating material is deposited so as to cover the soldered connections to said pins but not said exposed lengths of screen, and a layer of electrically conductive elastomer is attached to said insulating layer to surround and make electrical contact with said screens.

10. A method according to any of Claims 5 to 9 wherein said elastomeric member is formed in situ by potting the or each said exposed length of screen in a settable liquid or semi-liquid elastomer composition.

11. A method according to any preceding Claim wherein said electrically conductive elastomeric member is loaded with finely divided metal.

1 2. A method according to Claim 11 wherein said electrically conductive elastomeric member is composed of silicone-based material and is located with finely divided silver.

1 3. An arrangement comprising a plurality of screened cables extending through a body of electrically conductive elastomeric material with their screens in contact with said body and their inner conductors terminating at an array of circuit connections, said body being so shaped and disposed as to screen the terminations of said inner conductors.

1 4. An electrical connector substantially as described hereinabove with reference to Fig. 1 of the accompanying drawings.