Patent Publication Number: US-5293001-A

Title: Flexible shielded cable

Description:
The present invention relates to electrical cables and, more specifically, to a flexible coaxial cable. 
     BACKGROUND OF THE INVENTION 
     Various type of flexible cables are heretofore known. U.S. Pat. No. 4,486,252, for example, discloses a coaxial cable comprising a conductor core, a layer of dielectric disposed about the core, and a layer of conductive or semi-conductive matter surrounding the dielectric. A copper shield, which may be braided, is embedded in the layer, and the layer is softened by heating. 
     Another type of flexible cable heretofore known is disclosed in U.S. Pat. No. 4,694,122. It comprises a flexible metal conductor core, a layer of dielectric positioned about the conductor core, and a flexible copper shield disposed about the dielectric. The metallic shield has a copper foil with overlapping edges and a copper braid disposed about the foil. A layer of metal such as solder, bonds the overlapping edges, bonds the braid and the foil, and encloses the openings of the braids. Although the heretofore known coaxial cables have achieved commercial success, a certain degree of movement of the core relative to the cable still may occur, which may especially be a problem in connection with shorter cable assemblies. 
     Accordingly, it is an object of this invention to provide a flexible cable assembly that improves upon the prior art. 
     It is further object to provide a flexible cable assembly that provides a stronger adhesive between the metal shield and the conductor. 
     SUMMARY OF THE INVENTION 
     In accordance with these and other objects, a flexible cable assembly is provided that includes a flexible metal conductor, a layer of dielectric positioned about the conductor, and a flexible metallic shield disposed about the dielectric. The shield preferably employs a very thin metallic foil having a thickness less than 0.0030 inches, and a metallic braid, serve, flat/ribbon tape or other type of braid disposed about the foil. Because of the thinness of the foil, a better fit between the braid and the dielectric occurs, which limits relative movement of the dielectric relative to the shield. 
     In accordance with the invention, the metallic foil or the metallic braid may comprise a metal that is noncopper. For example, a copper foil may be used with a braid that is bronze, plated bronze, nickel, plated nickel, silver, or gold, or instead, a copper braid may be used with a foil that is bronze, silver, nickel or gold. 
     A preferred embodiment of the invention is seen in the attached drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a cross section view of a shielded cable in accordance with the invention; 
     FIG. 2 is a perspective view of the cable of FIG. 1 showing in broken view the various components; 
     FIG. 3 is a cross-section view of another shield cable in accordance with the invention; and 
     FIG. 4 is an enlarged perspective view of a composite metal foil tape in accordance with one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A flexible shielded cable 10 in accordance with the invention is shown in FIGS. 1 and 2. The cable 10 comprises a core 12 defined by at least one elongate flexible metallic conductor 14 which is preferably copper. In the event that the core 12 is defined by a plurality of conductors 14, preferably, each conductor is insulated from each other. 
     Disposed about the core 12 is a flexible layer of dielectric material 20 preferably in the form of fluorinated ethylene polymer i.e. Teflon® (Teflon® is a registered trademark of DuPont), or polyethylene, polypropylene, polytetrafluoroethylene, perfluoroalkoxy or cellular forms thereof. Disposed about the dielectric layer 20 is a flexible shield 22. 
     The flexible shield is composed of: (a) a metal foil tape 24, a composite foil tape 25 or conductive epoxy; (b) a metal braid, serve or flat/ribbon tape, other type of braid 26 wrapped around the foil 24; and (c) a bonding layer 28. The braid may be single or double served. 
     The metallic foil tape 24 or the metal on the composite metal foil tape 25 has a thickness of less than 0.0030 inches. This small thickness provides a better fit between the braid 26 and dielectric 20 which prevents or reduces the movement of the dielectric 20 relative to the shield 22. As shown in FIG. 2, the metallic foil 24 may have overlapping longitudinally extending edges 40, which are bonded together by a layer of the bonding layer 28. The composite metal foil tape 25, as shown in FIG. 4, is generally prepared by laminating the metal foil 25a to a dielectric polymer substrate 25b. The substrate 25b may have a bonding agent thereon to permit the substrate of the foil tape to be bonded to the dielectric 20 with the metal foil 25a facing the braid 26. 
     The metallic foil 24 functions to limit high frequency signal penetration, and the metallic braid functions to limit penetration of low frequency signals. The employment of the braid 26 over the foil 24 results in low radio frequency leakage and low susceptibility to electrical noise. The braid 26 being bonded to the foil 24 by the bonding layer 28 also offers several mechanical advantages. The presence of the braid prevents tearing of the foil when the cable 10 is bent. Furthermore, the braid offers a degree of elasticity, permitting the cable to have a higher operating temperature than an otherwise comparable semirigid cable incorporating a shield of copper tubing. 
     The dielectric layer 20 is preferably formed of flexible thermoplastic polymer such as fluorinated ethylene polymers, i.e. Teflon (a registered trademark of DuPont for synthetic resins containing fluorine), polytetrafluoroethylene, perfluoroalkoxy, polyethylene, polypropylene and cellular forms thereof. 
     The bonding layer 28 may be a conductive metal or a plastic conductive material such as an epoxy polymer containing conductive agents. The layer of metal 28 is applied by passing the incipient cable through a molten bath of tin or solder. This causes the molten metal (which is drawn in by wicking action--capillary attraction) to fill the braid openings and to close any hairline opening between the overlapping edges 40. During the application of the molten tin or solder component, the foil 24 functions as a heat barrier to insulate the dielectric 20 from a high temperature of the molten metal. But for the foil, the molten metal would directly contact the core insulation material. The use of the foil 24 allows polymers having less heat resistance than Teflon to be used for dielectric layer 20 because the foil conducts heat away from dielectric 20. 
     The cable 10 is flexible and can be bent without the use of special tools such as are required to prevent kinking or breaking of the cable having a copper tubing shield. Due to its flexible components, the bend radius of the cable 10 is approximately equal to the outside diameter of the cable which is preferably in the range of one to two times the diameter of the cable 10. 
     The metallic braid 26 includes a flat or ribbon metal type braid or any other suitable configuration rather than the standard round multi-wire braid. 
     Both the metallic foil 24 and metallic braid 26 may be copper. In accordance with the preferred embodiment of the invention, however, either the metallic foil 24 or metallic braid 26 may be constructed of a metal other than copper. For example, the metallic foil may be copper and the metallic braid may be bronze, plated bronze, nickel, plated nickel, silver or gold. Instead, the metallic braid may be copper and the metallic foil may be bronze, silver, nickel or gold. The metallic foil metallic braid may also be a metal having a copper component such as copper alloy, plated copper, or copper or copper plated covered steel. 
     Referring to FIG. 3, another embodiment of our invention is shown. In this embodiment, the shielded cable 30 has a plastic jacket 31 extruded thereover. The plastic jacket may be formed of any appropriate plastic material such as polyvinyl chloride, polyethylene, fluorinated ethylene polymers, polytetrafluoroethylene, perfluoroalkoxy, and flame retardant plastic cable jackets. 
     Those who are skilled in the art will readily perceive how to modify the invention. Therefore, the appended claims are to be construed to cover all equivalent structures which fall within the true scope and spirit of the invention.