Abstract:
A coaxial communications cable has a center conductor extending coaxially of the longitudinal axis of the cable with a low loss foam dielectric surrounding the inner conductor and bonded thereto. An electrically and mechanically continuous sheath surrounds the foam dielectric. The sheath is a smooth-walled longitudinally welded tube formed of a bimetallic material, which in one embodiment has an inwardly facing copper layer and an outwardly facing aluminum layer. A polymeric jacket surrounds the tubular sheath and is bonded thereto.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a coaxial cable, and more particularly to an improved low-loss coaxial cable having enhanced attenuation and mechanical bending properties. 
     BACKGROUND OF THE INVENTION 
     Coaxial cables are commonly used today in the transmission of broadband signals, such as cable television signals and cellular telephone broadcast signals, for example. One typical type of coaxial cable includes a core containing an inner conductor, an aluminum sheath surrounding the core and serving as an outer conductor, and a foam polymer dielectric which surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. A protective jacket is often provided surrounding the metallic sheath. 
     Coaxial cable manufacturers continue to strive to improve the electrical performance of the cable, and in particular, to lower the signal attenuation at high frequency. At the same time, any alterations in the cable design must maintain adequate mechanical characteristics, such as cable bending performance and resistance to unwanted deformation during installation, which can impair the electrical performance. U.S. Pat. No. 4,104,481 addressed these concerns by improving the composition of the foam dielectric. U.S. Pat. No. 4,472,595 provided improvements in cable performance by reducing the stiffness of the tubular sheath in relation to the stiffness of the cable core. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved cable with excellent mechanical performance and with lowered attenuation at high frequency. In accordance with the present invention, the cable uses an outer tubular sheath formed of a bimetallic material of two different metals. 
     The cable comprises at least one inner conductor, a foam dielectric surrounding this inner conductor, and an electrically and mechanically continuous tubular sheath formed of a bimetallic material closely surrounding the foam dielectric and being adhesively bonded thereto. The bimetallic tubular sheath includes an inwardly facing layer of a first metal bonded to the dielectric and an outwardly facing layer of a second metal different from the first metal. The inwardly facing first metal layer preferably has a lower resistivity than the outwardly facing second metal layer. 
     The wall thickness of the tubular metallic sheath is suitably less than about 750 micrometers and the first metal layer may have a thickness less than about 100 micrometers. In a further more specific aspect, the first metal is copper and the second metal is aluminum. 
     The coaxial cable may further include a protective outer jacket surrounding the sheath. Preferably, the tubular metallic sheath has a thickness of no greater than about 2.5 percent of its outer diameter. 
     In one specific embodiment, the coaxial communications cable comprises a center conductor extending coaxially of the longitudinal axis of the cable and formed of a copper-clad aluminum bimetallic conductor, a low loss foam dielectric surrounding the inner conductor, and an electrically and mechanically continuous smooth-walled tubular sheath formed of a bimetallic material closely surrounding said foam dielectric. The bimetallic tubular sheath includes an inwardly facing copper layer and an outwardly facing aluminum layer metallurgically bonded to the copper layer. The sheath has a wall thickness of less than 750 micrometers and the wall thickness is no greater than about 2.5 percent of its outer diameter. A thin continuous layer of adhesive is disposed between the foam dielectric and the sheath and serves to bond the foam dielectric to the inwardly facing copper layer to form a structural composite. A polymeric jacket surrounds the tubular sheath and is bonded to the outwardly facing aluminum layer. 
     These and other features of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description which describes both the preferred and alternative embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawing FIGURE is a perspective view showing a coaxial cable in accordance with the present invention in cross-section and with portions of the cable broken away for purposes of clarity of illustration. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The drawing illustrates a coaxial cable produced in accordance with the present invention. The coaxial cable comprises a core  10  which includes an inner conductor  11  of a suitable electrically conductive material, and a surrounding continuous cylindrical wall of expanded foam plastic dielectric material  12 . Preferably, the foam dielectric  12  is adhesively bonded to the inner conductor  11  by a thin layer of adhesive  13  such that the bond between the inner conductor  11  and dielectric  12  is stronger than the dielectric material. The inner conductor  11  may be formed of solid copper, copper tubing or of copper-clad aluminum. The inner conductor  11  preferably has a smooth surface and is not corrugated. In the embodiment illustrated, only a single inner conductor  11  is shown, but it is to be understood that the present invention is applicable also to cables having more than one inner conductor insulated from one another and forming a part of the core  10 . Furthermore, in the illustrated embodiment, the inner conductor  11  is a wire formed of an aluminum core  11   a  with a copper outer cladding layer  11   b.    
     The dielectric  12  is a low loss dielectric formed of a suitable plastic such as polyethylene. Preferably, in order to reduce the mass of the dielectric per unit length and hence reduce the dielectric constant, the dielectric material should be of an expanded cellular foam composition, and in particular, a closed cell foam composition is preferred because of its resistance to moisture transmission. Preferably, the cells of the dielectric  12  are uniform in size and less than 200 microns in diameter. One suitable foam dielectric is an expanded high density polyethylene polymer such as described in commonly owned U.S. Pat. No. 4,104,481, issued Aug. 1, 1978. Additionally, expanded blends of high and low density polyethylene are preferred for use as the foam dielectric. The foam dielectric has a density of less than about 0.28 g/cc, preferably, less than about 0.25 g/cc. 
     Although the dielectric  12  of the invention generally consists of a uniform layer of foam material, the dielectric  12  may have a gradient or graduated density such that the density of the dielectric increases radially from the inner conductor  11  to the outside surface of the dielectric, either in a continuous or a step-wise fashion. For example, a foam-solid laminate dielectric can be used wherein the dielectric  12  comprises a low density foam dielectric layer surrounded by a solid dielectric layer. These constructions can be used to enhance the compressive strength and bending properties of the cable and permit reduced densities as low as 0.10 g/cc along the inner conductor  11 . The lower density of the foam dielectric  12  along the inner conductor  11  enhances the velocity of RF signal propagation and reduces signal attenuation. 
     Closely surrounding the core is a continuous tubular smooth-walled sheath  14 . The sheath  14  is characterized by being both mechanically and electrically continuous. This allows the sheath  14  to effectively serve to mechanically and electrically seal the cable against outside influences as well as to seal the cable against leakage of RF radiation. The tubular sheath  14  has a wall thickness selected so as to maintain a T/D ratio (ratio of wall thickness to outer diameter) of less than 2.5 percent. Preferably, the thickness of the bimetallic sheath  14  is less than 2.5% of its outer diameter to provide the desired bending and electrical properties of the invention. In addition, the tubular bimetallic sheath  14  is smooth-walled and not corrugated. The smooth-walled construction optimizes the geometry of the cable to reduce contact resistance and variability of the cable when connectorized and to eliminate signal leakage at the connector. 
     In the preferred embodiment illustrated, the tubular bimetallic sheath  14  is made from a bimetallic strip formed into a tubular configuration with the opposing side edges of the strip butted together, and with the butted edges continuously joined by a continuous longitudinal weld, indicated at  15 . The welding may be carried out generally as described in U.S. Pat. Nos. 4,472,595 and 5,926,949, which are incorporated herein by reference. While production of the sheath  14  by longitudinal welding has been illustrated as preferred, persons skilled in the art will recognize that other methods for producing a mechanically and electrically continuous thin walled tubular bimetallic sheath could also be employed. 
     The bimetallic strip from which the sheath is formed is composed of two metal layers metallurgically bonded to one another to form a integral unitary metal strip. The two metal layers are formed of different metals having different electrical resistivities. In producing the tubular sheath, the metal layers are preferably oriented so that the lower resistivity metal layer  14   a  is inwardly facing and the higher resistivity metal layer  14   b  faces outwardly of the tubular sheath in order to improve the attenuation properties of the cable. While various different metals could be selected, in a preferred embodiment, the invention uses a bimetallic strip of copper and aluminum. The thickness of the strip is less than about 750 micrometers (desirably less than about 500 micrometers) and the copper layer has a thickness less than about 100 micrometers. Most desirably, the thickness of the copper is such that in the sheath, after fabrication and sinking onto the cable core, the copper layer has a thickness between 25 and 75 micrometers. In certain other specific applications, it may be desirable for the copper layer to be oriented outwardly, e.g. for compatibility with connectors (providing a copper-to-copper connection) or for improved mechanical performance. 
     The inner surface of the tubular sheath  14  is continuously bonded throughout its length and throughout its circumferential extent to the outer surface of the foam dielectric  12  by a thin layer of adhesive  16 . A preferred class of adhesive for this purpose is a random copolymer of ethylene and acrylic acid (EAA). The adhesive layer  16  should be made as thin as possible so as to avoid adversely affecting the electrical characteristics of the cable. Desirably, the adhesive layer  16  should have a thickness of about 25 micrometers or less. 
     The outer surface of the sheath  14  is surrounded by a protective jacket  18 . Suitable compositions for the outer protective jacket  18  include thermoplastic coating materials such as polyethylene, polyvinyl chloride, polyurethane and rubbers. Although the jacket  18  illustrated in FIG. 1 consists of only one layer of material, laminated multiple jacket layers may also be employed to improve toughness, strippability, burn resistance, the reduction of smoke generation, ultraviolet and weatherability resistance, protection against rodent gnaw-through, strength resistance, chemical resistance and/or cut-through resistance. In the embodiment illustrated, the protective jacket  18  is bonded to the outer surface of the sheath  14  by an adhesive layer  19  to thereby increase the bending properties of the coaxial cable. Preferably, the adhesive layer  19  is a thin layer of adhesive, such as the EAA copolymer described above. Although an adhesive layer  19  is illustrated in the drawing, the protective jacket  18  can also be directly bonded to the outer surface of the sheath  14 . 
     The coaxial cables of the present invention are beneficially designed to limit buckling of the bimetallic sheath during bending of the cable. During bending of the cable, one side of the cable is stretched and subject to tensile stress and the opposite side of the cable is compressed and subject to compressive stress. If the core is sufficiently stiff in radial compression and the local compressive yield load of the sheath is sufficiently low, the tensioned side of the sheath will elongate by yielding in the longitudinal direction to accommodate the bending of the cable. Accordingly, the compression side of the sheath preferably shortens to allow bending of the cable. If the compression side of the sheath does not shorten, the compressive stress caused by bending the cable can result in buckling of the sheath. 
     The ability of the sheath to bend without buckling depends on the ability of the sheath to elongate or shorten by plastic material flow. Typically, this is not a problem on the tensioned side of the cable. On the compression side of the tube, however, the sheath will compress only if the local compressive yield load of the sheath is less than the local critical buckling load. Otherwise, the cable will be more likely to buckle thereby negatively affecting the mechanical and electrical properties of the cable. 
     The coaxial cables of the present invention have enhanced bending characteristics over conventional coaxial cables. One feature which enhances the bending characteristics of the cable is the use of a very thin bimetallic sheath  14 . In an aluminum/copper bimetallic sheath, the relatively lower compressive yield strength of the aluminum component contributes to the avoidance of buckling failures during bending. The copper component, which has a higher compressive yield strength, is of such thinness that it does not adversely impact the overall compressive yield strength of the bimetallic sheath and the presence of the copper component of the bimetallic sheath contributes significantly to enhanced electrical performance, i.e. attenuation values. Preferably, the aluminum layer is of such a thickness as to constitute more than half, and preferably more than three-fourths of the overall cross sectional thickness of the bimetallic strip from which the sheath is formed 
     Another feature which enhances the bending characteristics of the coaxial cable of the invention is that the sheath  14  is adhesively bonded to the foam dielectric  12  and the protective jacket  18 . In this relationship, the foam dielectric  12  and the jacket  18  support the sheath  14  in bending to prevent damage to the coaxial cable. The bending characteristics of the coaxial cable are further improved by providing an adhesive layer  19  between the tubular bimetallic sheath  14  and the outer protective jacket  18 . 
     Furthermore, increased core stiffness in relation to sheath stiffness is beneficial to the bending characteristics of the coaxial cable. Specifically, the coaxial cables of the invention have a core to sheath stiffness ratio of at least 5, and preferably of at least 10. In addition, the minimum bend radius in the coaxial cables of the invention is significantly less than 10 cable diameters, more on the order of about 7 cable diameters or lower. The reduction of the tubular sheath wall thickness is such that the ratio of the wall thickness to its outer diameter (T/D ratio) is no greater than about 2.5 percent and preferably no greater than about 1.6 percent. The reduced wall thickness of the sheath contributes to the bending properties of the coaxial cable and advantageously reduces the attenuation of RF signals in the coaxial cable. The combination of these features and the properties of the sheath  14  described above results in a cable with a unique combination of electrical performance (e.g. low attenuation values) and mechanical bending performance. 
     It is understood that upon reading the above description of the present invention, one skilled in the art could make changes and variations therefrom. These changes and variations are included in the spirit and scope of the following appended claims.