EP1076343A1

EP1076343A1 - Cable shielding

Info

Publication number: EP1076343A1
Application number: EP99115171A
Authority: EP
Inventors: Heinz Braungart; Jürgen Klier; Roland Stöhr
Original assignee: WL Gore and Associates GmbH; WL Gore and Associates Inc
Current assignee: WL Gore and Associates GmbH; WL Gore and Associates Inc
Priority date: 1999-08-13
Filing date: 1999-08-13
Publication date: 2001-02-14

Abstract

An electrical cable (10) with a core (20) and a shield (30) surrounding the core (20) is disclosed. The shield (30) is made up of a first shielding layer (40) surrounding the core (20) which is made of conductive metal wire, a first conducting layer (50) surrounding the first shielding layer (40) which is made of semi-conducting expanded polytetrafluoroethylene (ePTFE), a second shielding layer (60) surrounding the first conducting layer (50) which is made of a metallised fabric, a second conducting layer (70) surrounding the second shielding layer (60) which is also made of semi-conducting ePTFE, and a third shielding layer (80) surrounding the second conducting layer (70) which is also made of conductive metal wire. The semi-conducting ePTFE is made in the disclosed embodiment of graphite-filled ePTFE and the metallised fabric of copper-coated polyamide fabric. The electrical cable (10) of the invention has a flex-life of at least 1.2 million cycles and a shielding effectiveness of more than 80 dB at 1 GHz.

Description

Field of the Invention

The invention relates to an electrical cable for transmitting electrical signals.

Description of the Prior Art

Electrical cables for transmitting electrical signals are well-known in the prior art. Conventional coaxial cables comprise a central conductor surrounded by a dielectric material and a shield made from braided wire. The shield is used to ensure that the electrical signals are little affected by external electromagnetic interference. One problem encountered by such conventional cables is that the flex-life of the cables, i.e. their ability to flex from side to side is limited over time as the resistance of the cable increases.

W.L.Gore and Associates GmbH, Pleinfeld. Germany, sells electrical cables in which the single shield of the above mentioned prior art cables is replaced by two braided shields separated by a semiconducting layer made of graphite filled expanded polytetrafluoroethylene (ePTFE). The ePTFE layer has excellent lubrication properties and improves the flex-life of the cable by reducing the wear and tear on the braided shields during flexing.

Also known in the art is a cable of the design shown in US-A-5 477 011 (Singles et al) assigned to W.L.Gore & Associates, Inc. which has an insulating layer of ePTFE bonded to a shielded layer by means of an adhesive.

It is an object of the invention to provide an improved electrical cable.

It is furthermore an object of the invention to provide an electrical cable with excellent shielding properties.

It is furthermore an object of the invention to provide an electrical cable with excellent flex-life properties.

Summary of the Invention

The present invention provides for an electrical cable which offers excellent shielding properties even after multiple flexing. This is achieved by providing an electrical cable with a core having a central electrical conductor or a twisted pair and a shield surrounding the core. The shield of the inventive electrical cable has a first shielding layer surrounding the core, a first conducting layer surrounding the first shielding layer, a second shielding layer surrounding the first conducting layer, a second conducting layer surrounding the second shielding layer, and finally a third shielding layer surrounding the second conducting layer. The use of three shielding layers separated by two semiconducting layers provides an effective shielding effect for the cable's signal conducting core.

The first conducting layer and/or said second conducting layer are semiconducting layers having a conductivity between 2.5 and 5 Ω/cm². The use of semiconducting layers allows the dissipation of electrical charges which may build up within the shields when the cables are flexed. It has been found that a surface conductivity of 0.388 Ω/cm² which corresponds to a volume conductivity of 6.945 milli-Siemens is optimal for this purpose. In the preferred embodiment of the invention, the first conducting layer and the second conducting layer are made from filled fluoropolymer materials such as graphite-filled polytetrafluoroethylene (PTFE), copper-filled expanded PTFE (ePTFE) or PTFE tapes with a metal filling such as silver, copper, graphite or gold. Most preferably the first conducting layer and the second conducting layer are made from graphite-filled ePTFE which has good lubricating properties and allows the shielding layers to slip with respect to each other. As a result there is also less wear and tear on the shielding layers during flexing of the cable.

The first shielding layer, the second shielding layer and/or the third shielding layer are made from a metal coated fabric in one embodiment of the invention since this can be used in the cable to completely isolate the internal signal carrying core from external electromagnetic interference. One such fabric which has been found to have extremely effective shielding properties is a metal coated polyamide fabric. Most preferably the second shielding layer is made from a copper-coated fabric.

The first shielding layer, the second shielding layer and/or the third shielding layer can be made from a braided or served shield which is constructed to provide optimum shielding and flex-life properties.

Description of the Drawings

Fig. 1 shows the design of a cable in accordance with the invention

Fig. 2 shows the design of a coaxial cable in accordance with the invention.

Fig. 3 shows the design of a twisted pair with a shield in accordance with the invention.

Fig. 4 shows the testing method for the flex-life of the cable of the invention.

Fig. 5 shows the shield effectiveness of the inventive cable compared with a prior art cable.

Detailed Description of the Invention

Fig. 1 shows an electrical cable 10 in accordance with the invention. The electrical cable 10 comprises a signal carrying core 20 as will be described in more detail later and a shield 30. The shield 30 has a first shielding layer 40 surrounding the core 20. A first semiconducting layer 50 surrounds the first shielding layer 40 and is in turn surrounded by a second shielding layer 60. A second semiconducting layer 70 surrounds the second shielding layer 60 and a third shielding layer 80 surrounds the second semiconducting layer 70. A binder 90 is wrapped about the third shielding layer 80 and finally a jacket 100 is placed about the binder 90.

The first semiconducting layer 50 and the second semiconducting layer 70 can be made from a variety of materials and incorporating a variety of different properties. Examples of such layers include: dense carbon filled materials, metal filled or metal plated materials. Suitable materials include metal or carbon filled fluoropolymer material such as metal (gold, copper or silver) or carbon filled expanded polytetrafluoroethylene (ePTFE) and metal or carbon coated polyester or other polymer film. The first semiconducting layer 50 and the second semiconducting layer 70 are preferably made from ePTFE which has extremely good lubricating properties and thus improves the flex-life of the electrical cable 10.

The first shielding layer 40, the second shielding layer 60 and the third shielding layer 80 can be made from served or braided wire. The wire used can be copper wire, silver plated copper wire or tin-plated copper wire. Alternatively they can be made from a metallised textile or fleece such as an aluminised textile or a copper-plated polyester or polyamide textile web. A conductive polymer shield can also be used. In the preferred embodiment of the invention the first shielding layer 40 and the third shielding layer 80 are provided with a braided shield whilst the second shielding layer 80 is made of a metallised textile. The use of a metallised textile allows a complete electromagnetic insulation of the core 20 of the cable 10 even when the cable 10 is bent. Braiding of the first shielding layer 40 and the third shielding layer 80 is carried out on a conventional braiding machine and the parameters are chosen to ensure that the cable 10 has good flex life properties whilst maintaining the electrical shielding characteristics.

The binder 90 is made from a dielectric material such as polyethylene, polyester, perfluoralkoxy, fluoroethylenepropylene, polypropylene, polymethylpentene, polytetrafluoroethylene or expanded polytetrafluoroethylene. Preferably ePTFE tapes such as that described in US-A-3 953 556, US-A-4 187 390 or US-A-4 443 657 are used and are wrapped about the third shielding layer. The binder 90 serves to fix the third shielding layer in place and because of its lubricating properties ensures that the cable 10 has good flex-life properties.

The jacket 100 can be made from an insulating material such as a fluorothermoplast, polyurethane, rubber, polyamide, polyimide, polyester, polyvinylchloride (PVC), polypropylene or polytetrafluoroethylene. Preferably a polyurethane material is used which is extruded over the binder 90. Alternatively tapes made of polyester, polyimide or PTFE could be wrapped, foamed or extruded about the binder 90.

The core 20 of the cable 10 can comprise either a central conductor 110 surrounded by a dielectric material 120 as shown in Fig. 2. The central conductor 110 can be made from any conducting material such as copper, silver, gold, nickel-plated copper, tin-plated copper, silver-plated copper, tin-plated alloys, silver-plated alloys or copper alloys. Hybride conductive materials could also be used for the central conductor 110. The dielectric material 120 is made from polyethylene, polyester, perfluoralkoxy, fluoroethylene-propylene, polypropylene, polymethylpentene, polytetrafluoroethylene or expanded polytetrafluoroethylene. Preferably expanded polytetrafluoroethylene such as that described in US-A-3 953 556, US-A-4 187 390 or US-A-4 443 657 is used.

Alternatively the core 20 of the cable 10 can be in the form of a twisted pair as shown in Fig. 3. The twisted pair comprises a first insulated conductor 130 twisted about a second insulated conductor 140. The conductors are made from a conducting material as described above. The insulation is coated over the conductors are is made of PVC, polyethylene, PTFE, fluoro-ethylene polymer (FEP), polyester or polypropylene. Preferably they are made of foamed, extruded or wrapped polyurethane.

Testing of the flex-life properties of the cable 10 is carried out using the apparatus shown in Fig. 4. The cable 10 is held between a fixed end 150 and a movable end 160 and is bent around in a curve 170 in the form of a half-circle and having a bending radius a. The movable end 160 of the cable 10 is moved horizontally in a cyclical manner over a distance b for a measured number of cycles. The distance b is at least twice that of the bending radius a so that at least part of the cable 10 is moved through the complete curve 170. In one typical embodiment of the testing apparatus the bending radius a was 50 mm and the distance b was 300 mm. At the commencement of the measurement cycle the resistance of the cable 10 was measured and the resistance is monitored throughout the test. It is found that for a large number of cycles there is no change in resistance. The resistance then increases in an approximately exponential manner until finally it becomes infinite at the point at which either the central conductor 110 or the shield 30 breaks. The aim of the current invention is to have a flex-life of three million cycles before the resistance becomes infinite.

Example 1

A cable core 20 is constructed from a central conductor 110 made of silver plated copper wire having an AWG 2619 (outside diameter of 0.508 mm). Expanded PTFE tapes having a thickness of 0.022 mil (5.6 µm) and a dielectric constant of approx. 1.5 are wrapped about the central conductor 110 to form a dielectric layer 120 of outside diameter 1.63 mm. A first shielding layer 40 made of 64 silver plated copper wires having an AWG 38 (0.1 mm diameter) is braided about the dielectric layer 120 with 10 picks per inch (2.54 cm) to form a shield of 2.0 mm outside diameter. Graphite filled ePTFE tapes of thickness 0.076 mm with a surface conductivity of 0.388 _Ω/cm² and thus a volume conductivity of 6.945 milli Siemens are then wrapped about the first shielding layer 40 to form the first semiconducting layer 50. A copper coated polyamide mesh foil obtainable from the STATEX company in Hamburg, Germany, is wrapped about the first semiconducting layer 50 to form the second shielding layer with an outside diameter of 2.53 mm.

The second semiconducting layer 70 is made in the same manner as the first semiconducting layer 50 and has an outside diameter of 2.73 mm. The third shielding layer 80 is made made of 112 silver plated copper wires having an AWG 38 (0.1 mm diameter) is braided about the dielectric layer 120 with 10 picks per inch (2.54 cm) to form a shield of 3.20 mm outside diameter. The binder 90 comprises two ePTFE tapes of 0.076 mm thickness are wrapped about the third shielding layer 80 and has an outside diameter of 3.40 mm. The polyurethane jacket 100 with a thickness of 0.2032 mm is extruded about the binder 90. The cable 10 has an outside diameter of 3.8 mm.

Comparative Example 1

A conventional cable made of a copper central conductor 120 of AWG 26 (0.508 mm diameter ) surrounded by an ePTFE dielectric layer of thickness 0.056 has a double braided shield. The first shielding layer of the double braided shield is made of silver plate copper wire of AWG 38 (0.102 mm diameter) and has a diameter of 2.03 mm. The second shielding layer of the double braided shield is made in the same manner and has a diameter of 2.73 mm. The first and second shielding layers are separated from each other by a semiconducting layer made from two graphite filled ePTFE tapes of thickness 0.076 mm to give a wall thickness of 0.3 mm. The overall diameter of the conventional cable is approx. 3.23 mm.

Attenuation Measurements

These were carried out in accordance with the IEC 96-1 standard and the results of the shield effectiveness against frequency for both the inventive cable (lower line) and the prior art cable (upper line) are shown in Fig. 5. It will be observed that at 1 GHz the cable has a shielding effectiveness of more than 130 dB.

Flex-Life Measurement

This was carried out using the apparatus of Fig. 4 as described above. The bending radius a was 50 mm and the distance b was 300 mm. The resistance of the cable 10 of example 1 was measured after 1.2 million cycles and was found not to have increased from the initial resistance.

Claims

Electrical cable (10) with

a core (20), and

a shield (30) surrounding the core (20);

whereby said shield (30) comprises

a first shielding layer (40) surrounding the core (20),

a first conducting layer (50) surrounding the first shielding layer (40),

a second shielding layer (60) surrounding the first conducting layer (50),

a second conducting layer (70 materials) surrounding the second shielding layer (60), and

a third shielding layer (80) surrounding the second conducting layer (70).
Electrical cable (10) according to claim 1 wherein said first conducting layer (50) and/or said second conducting layer (70) are semi-conducting having a surface conductivity of between 2.5 and 5 _Ω/cm².
Electrical cable (10) according to claim 2 wherein said first conducting layer (50) and/or said second conducting layer (70) are semi-conducting having a surface conductivity of 0.388 _Ω/cm².
Electrical cable (10) according to claim 1 wherein the first conducting layer (50) and the second conducting layer (70) are made from filled fluoropolymer materials.
Electrical cable (10) according to claim 3 wherein the first conducting layer (50) and the second conducting layer (70) are made from graphite-filled expanded polytetrafluoroethylene (PTFE).
Electrical cable (10) according to claim 1 wherein the first shielding layer (40), the second shielding layer (60) and/or the third shielding layer (80) are made from a conductive metal wire.
Electrical cable (10) according to claim 1 wherein the first shielding layer (40), the second shielding layer (60) and/or the third shielding layer (80) are made from a metal coated fabric.
Electrical cable (10) according to claim 7 wherein the second shielding layer (60) is made from a metal coated fabric.
Electrical cable (10) according to claim 9 wherein the metal coated fabric is a copper-coated polyamide fabric.
Electrical cable (10) according to claim 1 wherein the first shielding layer (40), the second shielding layer (60) and/or the third shielding layer (80) are a braided or surfed shield.
Electrical cable (10) according to claim 1 wherein the core (20) comprises a central conductor (110) surrounded by a dielectric material (120).
Electrical cable (10) according to claim 1 wherein the core (20) comprises a first insulated conductor (130) twisted about a second insulated conductor (140).
Electrical cable (10) according to claim 1 wherein the cable (10) has a flex-life of at least 1.2 million cycles.
Electrical cable (10) according to claim 1 wherein the cable (10) has a shielding effectiveness of more than 80 dB at 1 GHz.