Shielded wire or cable

Abstract: A shielded wire and cable article generally comprises an inner conductive core (11) of one or more wires (12). The conductive core (11) is surrounded by one or more thin layer(s) (13) of insulation about which shielding (14) is applied. The shielding comprises a braided or served mesh or woven yarn of metallically coated fibres, having a high tensile strength and flexibility, which permits a thinner fibre to be utilized, so a greater shield weight reduction can be realised.

Description: SHIELDED CABLE The invention relates to shielded wire and cable, and more particularly to improved shielded wire and cable providing several orders of magnitude of shielding improvement over standard shielded wire and cable, and additionally, shielded wire and cable that is lighter in weight than conventional shielded wire and cable articles. Advanced technological uses for wire and cable have imposed many new requirements upon traditional wire and cable specifications and functions. In missile and aerospace environments, for example, the need for lighter weight cabling is directly related to aircraft performance and operating cost. Also, wiring is often required to meet stringent shielding specifications, since it is contemplated that the missile or aircraft will have to fly through radiation and electrical interference fields without compromising the on-board electronics. Presently, wire and cables are shielded electrically by braiding wire mesh shields about the primary wire core and insulation. This shielding is meant to prevent Radio Frequency Interference and Electromagnetic Interference (RFI and EMI) disturbances from influencing the signals in the cable. As the advanced technology requirements impose greater stringency in shielding and weight specifications, these previously functional braided articles become unacceptable. Shielding leakages occur in these conventional cables by virtue of the looseness by which the wire mesh is braided, leaving holes in the shield web. In addition, the stiffness of the metal wire used in braiding makes it difficult to conform the mesh to the insulation core surfaces, leaving small gaps. Such gaps limit the frequency range in which the cable or wire can be operationally effective. While it may be possible to use finer wire mesh to resolve some of the above-mentioned shielding problems, it is still necessary to contend with the lower weight requirements that these environments impose. The lower weight requirements cannot be practically met by using wire mesh braiding techniques. The present invention has resolved the aforementioned problems by the development of a new type of shielded wire and cable article. The new article of this invention contemplates the use of shielding composed of fine mesh yarns or fibres made of aramid that have been metallically coated with an extremely thin layer of material. The metallic layer is coated upon the fibres in thin layers. The yarns contemplated for use in the invention have high tensile strength and flexibility. Aramid is a polyaromatic amide and is sold under the trade name Kevlar (RTM). The high tensile strength and flexibility of the fibers of this invention ensures that the fibers can be made thin without losing structural integrity. The thinner the fiber, the tighter it can be braided or woven; and hence, the greater the shielding effectiveness. Also, the greater flexibility of the fiber mesh, as compared to wire mesh, provides a greater conformity to the surface of the underlying insulation. Such improved conformity further improves the closeness and tightness of the mesh shield. This also contributes to a higher shielding frequency range capability. The fibres have a clear weight advantage over that of metallic wire, providing the solution to the most vexing aspect of the new aerospace specifications. It is known in the art to coat fibers with metal, and to braid these fibers into a wire article. Such a teaching is shown in United States Patent No. 4,634,805, issued to Ralph Orban on January 6, 1987, entitled "Conductive Cable or Fabric". The patent also suggests that a mesh can be manufactured utilizing the metal coated fibers. But the use of metallic coated fibers is not taught therein for the purposes of fabricating shielded wire and cable. Nor does the patent teach the use of yarn, nor the yarn sizes and metal thicknesses necessary to accomplish the shielding frequency ranges contemplated by this invention. U.S-A-4 822 950 discloses a cable article comprising a conductive core member, at least one layer of insulation disposed over said conductive core member, and a layer of shield material disposed over the insulated conductive core member comprising a plurality of metallic coated insulating fibres interwined to form a protective shield layer against radio frequency interference (RFI) and electromagnetic Interference (EMI) disturbances influencing signals passing through the cable, and a jacket disposed over the protective shield layer. However, the cable article disclosed comprises a layer of shield material braided from yarns each comprising 800-12000 fibres in a size range from 5 to 10µm. Thus, the minimum yarn diameter is about 4mm. The thickness of the yarn limits the surface coverage of the conductor and reduces its shielding efficiency. Accordingly, the invention provides a cable article comprising a conductive core member, at least one layer of insulation disposed over said conductive core member, and a layer of shield material disposed over the insulated conductive core member comprising a plurality of metallic coated fibres intertwined to form a protective shield layer against radio frequency interference (RFI) and electromagnetic Interference (EMI) disturbances influencing signals passing through the cable, and a jacket disposed over the protective shield layer, characterised in that said fibres are aramid, in that the weight of the fibres is between 50 and 10,000 denier, and in that the layer of shield material covers 96% or more of the insulated conductive core member. The strength of the aramid fibres allows the fibres to be relatively thin which allows a tight weave or braiding of the shielding layer. This increases the shielding efficiency. A particular problem is to effectively coat the fibres with metal. If this is not done properly the electrical continuity of the metal layer will be poor and cracking may occur. U.S. A-4 822 950 uses nickel plated carbon fibres formed by electrodeposition. In a preferred arrangement of the present invention the fibres are coated with silver. The silver coat of the invention is chemically anchored to the fibres rather than merely physically deposited. This difference is significant, since it provides the fibres of this invention with electrical continuity and prevents the coating from cracking. In accordance with the present invention, there is provided a shielded wire and cable article capable of meeting stringent aerospace specifications and requirements, particularly that of low weight. The article generally comprises an inner conductive core of one or more wires that can be twisted or braided, and which can be individually insulated. The conductive core is surrounded by one or more thin layer(s) of insulation about which the shielding of this invention is applied. The shielding comprises a braided or served mesh or woven yarn of metallically coated aramid fibres. The fibres of the yarn or mesh are characterised by high tensile strength and flexibility. Where the fibres thmselves are braided, the resulting mesh can be braided more tightly about the interior insulation surface than can conventional meshes. Also, the high tensile strength requirement for the yarn provides that a thinner fibre can be utilized, wherein a greater shield weight reduction can be realised. The metal coating upon the shield fibres is approximately in a thickness range of a few to a few tens nanometres (a few tens to a few hundred angstroms). The thinner metal coating greatly reduces the shielding weight of the shield mesh. The yarn is fabricated from aramid fibres, having a weight in an approximate range of about 50 to a few hundred denier, and in some cases up to 10,000 denier. The denier rating of a fibre is the weight in grams of 9000m of the fibre. About the fibre shield, a thin insulative jacket is disposed to complete the shielded wire or cable article of this invention. The shielding effectiveness (operational frequency range) of the resulting inventive article is comparable to that of conventional shielded cable. The surface transfer impedance of the shielded wire and cable of the invention is approximately in a range approaching a few hundred milliohms/meters over a frequency range of 100 KHz to 1 GHz. A typical total cable weight for the silver coated nylon braided shield utilized in the wire and cable article of the invention is approximately 0.6g/m (0.4 lbs per 1,000 feet), as compared to a tin-copper braided wire mesh cable having a total weight of 1.14g/m (0.76 lbs per 1,000 feet). A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed descripton, in which: Fig.1 is a schematic, cutaway, perspective view of the shielded wire or cable article of this invention; and Fig.2 through 6 represent graphical representations of shielding data obtained for various shielded wire and cable articles fabricated in accordance with the invention, and compared with standard wire braided shield articles. Generally speaking, the present invention features a shielded wire and cable article whose shielding is fabricated from metallic coated fibres woven into a yarn or braided into a mesh. The shielding layer of the invention utilizes highly flexible fibres with a high tensile strength. The yarn or braided mesh is disposed about the inner insulated core of the wire or cable. The metallic coating upon the fibres is very thin, and comprises a layer of approximately between a few to a few tens of nanometres (a few tens to a few hundreds angstroms in thickness). The weight of the braided fibres is as low as 22% of the conventional metallic mesh, and provides shielding effectiveness comparable to that of conventional metallic mesh. Now referring to Figure 1, a typical shielded wire or cable article 10 of this invention is illustrated in schematic, cutaway perspective view. The inner, electrically conductive core 11 of the wire or cable 10 is composed of one or more metallic wires 12, usually of copper. The wires 12 can be straight, twisted or braided, as is conventionally known in the art, and may be bare or individually insulated. The conductive core 11 is covered by one or more thin insulation layer(s) 13, which insulation can be any suitable material as befits the utility and specifications sought to be met. One of the insulation layers 13 may contain ferrite powder. About the insulation layer(s) 13, the shielding layer 14 of this invention is overlaid. The shielding layer 14 can be applied in one of two ways; a) as a thin layer of woven yarn, or b) as a braided or served layer of aramid fibres. The fibres of the yarn or braid are coated with a metal usually silver. The fibres are characterised by their high tensile strength and flexibility, thus allowing a tightly woven yarn or braided mesh. Because of their high tensile strength and flexibility, the fibres can be made thinner, thus reducing their weight and providing for a tighter weave or braiding about the insulation layer 13. The fibres are aramid. The fibres generally have a weight range of approximately between 50 to a few hundred denier, and in some cases up to 10,000 denier. The metallic coating is applied by a proprietary process, commercially available from Sauquoit Industries, Inc., of Scranton, Pennsylvania. Other commercially available processes that can be utilized in coating the metal on the fibres are known, such as electrostatic deposition, dielectric deposition, vapor deposition, etc. Over the shield layer 14 is generally disposed one or more jacket layers 15 of insulation. The jacket layer(s) 15 can be any number of materials, again befitting the intended purposes and specifications designated for the final cable product. EXAMPLE 1 A wire construction was fabricated utilizing the following materials: For the conductive core, a centre conductor was utilized, comprising 0.63mm diameter (AWG 22) tin-coated copper wire manufactured by Hudson Wire Company. The conductive core was overlaid with a layer of primary insulation of Kynar 460 polyvinylidene fluoride supplied by Atochem Company. About this primary insulation was overlaid a second insulation layer of Viton fluorinated rubber filled with ferrite powder (82%) supplied by DuPont. The second layer was then overlaid with Exrad, an irradiated, cross-linked ethylene tetrafluoroethylene copolymer manufactured by Champlain Cable Corporation, Winooski, Vermont. The third layer was overlaid with the shielding of this invention. The final wire was not jacketed. The total outside diameter was 1.75mm (0.069''). The shielding consisted of silver-coated Kevlar fibres whose weight was approximately 0.6g/m (0.4 lbs per 1,000 feet), braided into a mesh about the insulation layers. Conventional tin-copper braided wire has twice the weight of the metallic coated fibre shielding of the invention. The shielding effectiveness of the fabricated article in EXAMPLE 1 was measured via surface transfer impedance measurement, and was compared to cable fabricated with the conventional shield of tin-copper braid. The results are shown in Figures 2 and 3, respectively. The tin-coated copper braid provided 92% coverage, whereas that of the silver-coated Kevlar produced a 99% coverage of the underlying insulation. The resulting shielding of the invention shows an effectiveness comparable to that of the conventional shielding. Attenuation measurement were the same as those obtained with a metal braided shield (FIGURE 4). EXAMPLE 2 A second cable was fabricated utilizing the silver-plated copper core (AWG 22) of EXAMPLE 1. About the conductive core was overlaid an insulation layer of irradiation cross-linked ethylene tetrafluoroethylene copolymer. The insulated conductive core consisted of a twisted pair whose length of lay is about 25.4mm (one inch) (lefthanded lay). A shield was disposed over the twisted pair, and consisted of the same silver-coated Kevlar braid, having a 96% coverage. Over this was jacketed a layer of cast tape (FEP-coated teflon, teflon is a registered trade mark). A counterpart to this cable was fabricated with metal braided silver-plated copper flat mesh consisting of a twisted pair (two conductors) whose length of lay was about 25.4mm (1") (left-hand lay) having an 86% coverage. The results of the shielding effectiveness of the inventive article compared to the conventional cable is illustrated in Figures 5 and 6, respectively. Comparison of the cable weight for the shield is as follows: Kevlar-braided fibre cable weighed 1.09 g/m (0.735 1b/1,000') silver-plated copper cable weighed 1.28 g/m (0.86 1b/1,000'). The insulation thickness on each of the wires of the twisted pair was 0.17mm (0.0065'') and the FEP tape thickness (jacket) was 0.04mm (0.0014''). The conductive core of the cable of this invention can comprise one or more bare metallic wires or metallic wires having individual layers of insulation. These wires may be straight, twisted or braided, and then covered with a layer of insulation and jacketing. The cable article of this invention may be fabricated as a cable pair. Insulated cores can themselves be paired or be formed into a multicore member, which can then be shielded and jacketed. The jacket layer(s) can comprise at least one material selected from a group of materials consisting of: fluoropolymer, fluorocopolymer, polyimide, halogen-free insulation, and irradiated, cross-linked ethylenetetrafluoroethylene polymer.

Claims: A cable article comprising a conductive core member (11), at least one layer (13) of insulation disposed over said conductive core member, and a layer (14) of shield material disposed over the insulated conductive core member comprising a plurality of metallic coated fibres intertwined to form a protective shield layer against radio frequency interference (RFI) and electromagnetic Interference (EMI) disturbances influencing signals passing through the cable, and a jacket (15) disposed over the protective shield layer, characterised in that said fibres are aramid, in that the weight of the fibres is between 50 and 10,000 denier, and in that the layer of shield material covers 96% or more of the insulated conductive core member (11,13). A cable article according to claim 1, wherein the fibres are coated with silver. A cable article according to claims 1 or 2, wherein the surface impedance of the article is above 10 milliohms/meter over a frequency range of 100 kHz to 10 MHz. A cable article according to any preceding claim, wherein the fibres are braided or served into a mesh. A cable article according to any of claims 1 to 3, wherein the fibres comprise a thin layer of woven yarn. A cable article according to any preceding claim, wherein the conductive core member (11) comprises a multicore member. A cable article according to claim 6, wherein the conductive core member (11) comprises a plurality of metallic wires (12) which are straight, braided or twisted. A cable article according to claim 7, wherein the conductive core member (11) comprises a plurality of metallic wires (12) that are individually insulated. A cable article according to any preceding claim, wherein said jacket (15) is selected from at least one material from a group of materials consisting of: fluoropolymer, fluorocopolymer, polyimide, halogen-free insulation; and irradiated, cross-linked ethylenetetrafluoroethylene polymer. A cable article according to any preceding claim, wherein said at least one insulation layer (13) is selected from at least one material from a group of materials consisting of: fluoropolymer; fluorocopolymer; polyimide; halogen-free insulation, and irradiated, crosslinked ethylenetetrafluoroethylene polymer. A cable article according to any preceding claim, wherein one of the insulation layers (13) contains ferrite powder. A flex comprising a pair of cable articles according to any preceding claim.

Inventors: ALDISSI MAHMOUD, ALDISSI, MAHMOUD
Applicants: CHAMPLAIN CABLE CORP, CHAMPLAIN CABLE CORPORATION
Application year: 1992
Main classification: H01B11
IPC codes: H01B11