Abstract:
A communications cable comprises an elongate cable jacket having an internal cavity and a plurality of twisted pairs of insulated conductors disposed in the internal cavity of the cable jacket, each of the conductors being insulated with a polymeric layer. Each of the insulated conductors within each of the twisted pairs of conductors defines a twinning helix having a first rotative direction, and each of the twisted pairs defines a bunching helix having a second rotative direction, the second rotative direction being opposite that of the first rotative direction. In this configuration, the communications cable can provide acceptable crosstalk and attenuation performance, even with foamed insulators that have demonstrated unacceptable performance when twinned and bunched in the same rotative direction.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates broadly to communications cable and, more particularly, to communications cable containing at least one twisted pair of insulated conductors.  
         BACKGROUND OF THE INVENTION  
         [0002]    Insulated conductors such as those used in communications cable are often provided as twisted pairs of insulated conductors having two insulated conductors twisted, or “twinned”, about each other to form a dual conductor group. A typical assembly for these communications cables comprises two or more twisted pairs of insulated conductors “bunched” together (i.e., further twisted and in some instances captured with a binder thread or cable) and contained in a cable jacket. The twisting and bundling of the conductors can facilitate the installation of the cable and connection between insulated conductors. Twisted pair conductors are commonly used in applications such as local area network (LAN) cables and wireless cable network architectures.  
           [0003]    One problem associated with communications cable produced with the conventional twisted pair assembly is that crosstalk can occur between twisted pairs of insulated conductors that can negatively affect the signals transmitted by these conductors. Crosstalk may especially present a problem in high frequency applications because crosstalk may increase logarithmically as the frequency of the transmission increases. Some twisted pairs are sufficiently impacted by crosstalk that insulating spacers are positioned between pairs within the same cable. See, e.g., U.S. Pat. No. 5,969,295 to Boucino et al. Another technique for adjusting crosstalk performance involves twinning the conductors of different pairs so that they have different lay lengths and carefully selecting the lay length for bunching.  
           [0004]    The insulation employed for conductors is typically a polymeric material. Exemplary insulating materials includes but are not limited to, polyvinylchloride, polyvinylchloride alloys, polyethylene, polypropylene, and flame retardant materials such as fluorinated polymers. Exemplary fluorinated polymers, include but are not limited to, fluorinated ethylene-propylene (FEP), ethylenetrifluoroethylene (ETFE), ethylene chlorotrifluoroethylene (ECTFE), perfluoroalkoxypolymers (PFA&#39;s) like tetrafluoroethylene and perfluoropropylvinylether (e.g., Teflon PFA 340), and mixtures thereof.  
           [0005]    In an effort to reduce the weight and cost of insulation, conductors with foamed polymer insulation, and particularly foamed FEP insulation, have been constructed. The foaming process introduces air into the dielectric medium. Air having a lower dielectric constant increases the velocity of propagation (Vp). Higher Vp typically translates to improved signal transmission speed for high speed data or communications systems. However, the resulting foamed medium tends to become more susceptible to crushing during the twinning and bunching processes. Such crushing can undesirably raise the capacitance and lower the impedance of the finished cable, which can consequently degrade attenuation performance. In order to provide foamed dielectric insulation with sufficient crush resistance to provide adequate cable performance, additional dielectric material has been required, thereby negating some or all of the weight, cost and performance advantages of using a foamed dielectric. Accordingly, it would be desirable to provide a cable having a foamed dielectric with acceptable performance properties while reducing material weight and cost.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is directed to a communications cable and an associated manufacturing method therefore that can utilize foamed insulators for electrical conductors and still provide acceptable performance. According to certain embodiments of the invention, a communications cable comprises: an elongate cable jacket having an internal cavity; and a plurality of twisted pairs of insulated conductors disposed in the internal cavity of the cable jacket, each of the conductors being insulated with a polymeric layer. Each of the insulated conductors within each of the twisted pairs of conductors defines a twinning helix having a first rotative direction, and each of the twisted pairs defines a bunching helix having a second rotative direction, the second rotative direction being opposite that of the first rotative direction. In this configuration, the communications cable can provide acceptable crosstalk and attenuation performance, even with foamed insulators that have demonstrated unacceptable performance when twinned and bunched in the same rotative direction.  
           [0007]    It is preferred that at least one, and more preferably all, of the polymeric layers are formed of a foamed polymeric material (as used herein, a “foamed” polymeric material means both foamed and foam skin materials). It is also preferred that the twinning helices have different lay lengths, and the bunching helix also has a different lay length. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0008]    [0008]FIG. 1 is a perspective cutaway view of an embodiment of a twinned pair cable of the present invention.  
         [0009]    [0009]FIG. 2A is a section view of the cable of FIG. 1 taken along lines  2 A- 2 A thereof.  
         [0010]    [0010]FIG. 2B is a section view of the cable of FIG. 1 taken along lines  2 B- 2 B thereof.  
         [0011]    [0011]FIG. 3 is a perspective cutaway view of another embodiment of a twinned pair cable of the present invention, wherein the cable includes an insulating spacer.  
         [0012]    [0012]FIG. 4A is a section view of the cable of FIG. 3 taken along lines  4 A- 4 A thereof.  
         [0013]    [0013]FIG. 4B is a section view of the cable of FIG. 3 taken along lines  4 B- 4 B thereof.  
         [0014]    [0014]FIG. 5 is a perspective cutaway view of another embodiment of a twinned pair cable of the present invention.  
         [0015]    [0015]FIG. 6 is a graph plotting attenuation as a function of frequency for a cable sample twinned in a counterclockwise direction and bunched in a clockwise direction.  
         [0016]    [0016]FIG. 7 is a graph plotting near end crosstalk as a function of frequency for a cable sample twinned in a counterclockwise direction and bunched in a clockwise direction.  
         [0017]    [0017]FIG. 8 is a graph plotting attenuation as a function of frequency for a cable sample twinned in a counterclockwise direction and bunched in a counterclockwise direction.  
         [0018]    [0018]FIG. 9 is a graph plotting near end crosstalk as a function of frequency for a cable sample twinned in a counterclockwise direction and bunched in a counterclockwise direction. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Instead, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. It will be understood that when an element (e.g., cable jacket) is referred to as being “connected to” another element, it can be directly connected to the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly connected to” another element, there are no intervening elements present. Like numbers refer to like elements throughout. Some dimensions and thicknesses may be exaggerated for clarity.  
         [0020]    Referring now to the figures, a twinned pair cable, designated broadly at  20 , is illustrated in FIGS. 1, 2A and  2 B. The cable  20  comprises two twinned pairs  22 ,  28  of conductors, with the first pair  22  including conductors  24 ,  26  and the second pair  28  including conductors  30 ,  32 . The conductors  24 ,  26 ,  30 ,  32  are covered with, respectively, insulators  25 ,  27 ,  31 ,  33 . The conductors  24 ,  26 ,  30 ,  32  may be a metallic wire of any of the well-known metallic conductors used in wire and cable applications, such as copper, aluminum, copper-clad aluminum and/or copper-clad steel. Preferably, the wire is 18 to 26 AWG gauge.  
         [0021]    Suitable insulating materials for the insulators  25 ,  27 ,  31 ,  33  include polyvinylchloride, polyvinylchloride alloys, polyethylene, polypropylene, and flame retardant materials such as fluorinated polymers. Exemplary fluorinated polymers for use in the invention include FEP, ETFE, ECTFE, PFA&#39;s, and mixtures thereof. Exemplary PFA&#39;s include copolymers of tetrafluoroethylene and perfluoropropylvinylether (e.g., Teflon PFA 340) and copolymers of tetrafluoroethylene and perfluoromethylvinylether (MFA copolymers, which are available from Ausimont S.P.A.). In addition, the material of the insulators  25 ,  27 ,  31 ,  33  may contain conventional additives such as pigments, nucleating agents, thermal stabilizers, acid acceptors, processing aids, and/or flame retardant compositions (e.g., antimony oxide). If desired, the insulating material may not be the same for each twisted pair  22 ,  28 . In accordance with the present invention, some or all of the insulators  25 ,  27 ,  31 ,  33  may be formed of polymeric materials that have been foamed or that have a foam skin structure, such as FEP or polyethylene. Typically, these materials are foamed to a density of between about 50 and 80 percent of their solid volume.  
         [0022]    As illustrated in FIGS. 1, 2A and  2 B, the conductors  24 ,  26  of the pair  22  are twinned about a twin axis T 1  and follow a counterclockwise twinning helix when viewed from the viewing direction indicated in FIG. 1 and from the vantage point of FIGS.  2 A- 2 B. Likewise, the conductors  30 ,  32  of the pair  28  are twinned about a twin axis T 2  and follow a counterclockwise twinning helix when view from the viewing direction indicated in FIG. 1 and from the vantage point of FIGS.  2 A- 2 B. However, the pairs  22 ,  28  are bunched about a bunching axis B 1  and follow a clockwise bunching helix when viewed from the viewing direction indicated in FIG. 1 and from the vantage point of FIGS.  2 A- 2 B. It has been discovered that, when conductors with insulation are helically twinned in one rotative direction and helically bunched in the opposite rotative direction, there can be reduced crushing of the insulators  25 ,  27 ,  31 ,  33  without the expected corresponding reduction in cross-talk performance.  
         [0023]    Typically, the pairs  22 ,  28  are twinned such that the “lay length” (defined as the distance along each conductor required for the conductor to travel one complete circumference of the helix) of twinning is between about 0.25 and 1.0 inches. In some embodiments, the lay lengths of the pairs  22 ,  28  will differ from one another (usually by about 20 to 50 percent). The pairs  22 ,  28  are typically bunched so that the lay length of bunching is between about 2.5 and 6.0 inches.  
         [0024]    Those skilled in this art will recognize that, although the cable  20  is illustrated with pairs  22 ,  28  being twinned in a counterclockwise helix and being bunched in a clockwise helix, cables can also be constructed with pairs being twinned in a clockwise helix and bunched in a counterclockwise helix.  
         [0025]    The pairs  22 ,  28  are enclosed within the cavity  35  of a jacket  34 . Preferably, the jacket  34  is made of a flexible polymer material and is formed by melt extrusion. As will be understood by those of skill in the art, any of the polymer materials conventionally used in cable construction may be suitably employed; these include, but are not limited to, polyvinylchloride, polyvinylchloride alloys, polyethylene, polypropylene and flame retardant materials such as FEP or another fluorinated polymer. Moreover, other materials and/or fabrication methods may be used. Preferably, the cable jacket  34  is extruded to a thickness of between 15 and 25 mils (thousandths of an inch), which may facilitate stripping the cable jacket  34  away from the twisted pairs  22 ,  28 . However, other dimensions may be used. The jacket may overlie one or more optional shielding layers  36 ; these are typically formed of a wide variety of known conductive and/or nonconductive materials such as nonconductive polymeric tape, conductive tape, braid, a combination of nonconductive polymeric tape, conductive tape and/or braid, and/or other such materials as will be understood to one of skill in the art using conventional fabrication techniques.  
         [0026]    The cable  20  may be used in a variety of computer, communication, and telecommuncation environments, including residential and commercial buildings.  
         [0027]    Another cable embodiment of the present invention, designated broadly at  50 , is illustrated in FIG. 5. The cable  50  includes four twisted conductor pairs  52 ,  58 ,  64 ,  70 , which comprise, respectively, conductors  54  and  56  (insulated by insulators  55  and  57 ), conductors  60  and  62  (insulated by insulators  61  and  63 ), conductors  66  and  68  (insulated by insulators  67  and  69 ), and conductors  72  and  74  (insulated by insulators  73  and  75 ). Like the cable  20  illustrated in FIGS. 1, 2A and  2 B, the pairs  52 ,  58 ,  64 ,  70  are covered by a jacket  76  and an optional shielding layer  78 . The description of the materials appropriate for use in the conductors, insulators, jacket and shield of the cable  20  are equally applicable to these components of the cable  50  and need not be repeated here.  
         [0028]    The pairs  52 ,  58 ,  64 ,  70  are twinned such that they form clockwise helices along their respective twinning axes T 3 , T 4 , T 5 , T 6 , and are bunched such that they form counterclockwise helices along the bunching axis B 2 . Lay lengths of the twinning and bunching helices are as described above for the cable  20 .  
         [0029]    A further cable embodiment of the present invention, designated broadly at  150 , is illustrated in FIGS. 3, 4A and  4 B. The cable  150  includes four twisted conductor pairs  152 ,  158 ,  164 ,  170  which comprise, respectively, conductors  154  and  156  (insulated by insulators  155  and  157 ), conductors  160  and  162  (insulated by insulators  161  and  163 ), conductors  166  and  168  (insulated by insulators  167  and  169 ), and conductors  172  and  174  (insulated by insulators  173  and  175 ). The cable  150  also includes a jacket  176  and an optional shielding layer  178 . The discussions hereinabove regarding the materials and construction of the conductors, insulators, jacket and shield layers are equally applicable to the cable  150  and need not be repeated here.  
         [0030]    Unlike the cable  50 , the cable  150  also includes a spacer  151  that extends the length of the cable  150  and separates the internal cavity of the cable  150  into four compartments  153   a,    153   b,    153   c,    153   d.  Each of the pairs  152 ,  158 ,  164 ,  170  resides in a respective one of the compartments  153   a,    153   b,    153   c,    153   d.  The spacer  151  is typically included in a cable in order to regulate the distance between twisted pairs, which in turn can render crosstalk performance more consistent. Suitable different spacer configurations and materials are discussed in detail in U.S. Pat. No. 5,789,711 to Gaeris et al., U.S. Pat. No. 5,969,295 to Boucino et al. and co-pending and co-assigned U.S. patent application Ser. No. 09/591,349, filed Jun. 9, 2000 and entitled Communications Cables with Isolators; the contents of each of these documents are hereby incorporated herein by reference in their entireties.  
         [0031]    The invention will now be described in great detail in the following non-limiting example.  
       EXAMPLE 1  
       [0032]    Testing was conducted comparing the performance of cables employing oppositely twinned and bunched conductors with cables having similarly twinned and bunched conductors.  
         [0033]    Two cable samples were constructed, each having four twisted pairs of insulated conductors and having the specifications set forth in Table 1. 
                   TABLE 1                       Property   Value                   Conductor Dimensions   24 gauge       Conductor Material   AWG copper wire       Insulator Material   3 pairs foam/skin FEP; 1 pair foam/           skin PE       Insulator Thickness   0.007 in       Insulator Coaxial Capacitance   FEP 52 min., 57 max; PB 61 (pf/ft)       Cable Length   328 ft       Jacket Material   PVC Alloy (plenum rated)                  
 
         [0034]    The twisted pairs of each cable were twinned in a counterclockwise direction at a lay length of between 0.45 and 0.8 inches. One cable (Cable 1) was bunched in a clockwise direction at a lay length of 6 inches (such that the twinning and bunching were in opposite rotative directions), and the other cable (Cable 2) was bunched in a counterclockwise direction at a lay length of 6 inches (such that twinning and bunching were in the same rotative direction). The cables were evaluated under testing conditions set forth in ASTM-D4566-2000.  
         [0035]    Results of the evaluations are set forth in FIGS.  6 - 9 . FIGS. 6 and 7 are graphs illustrating the performance of Cable 1. FIG. 6 is a plot of cable attenuation as a function of frequency of Cable 1 and the permissible attenuation per specification. FIG. 6 demonstrates that the plot of Cable 1 falls below the specification (i.e., is acceptable) for attenuation performance. FIG. 7 is a plot of near end crosstalk as a function of frequency for Cable 1 and specification. FIG. 7 shows that the plot for Cable 1 is positioned above the specification curve, thereby indicating acceptable performance. These results compare favorably to FIGS. 8 and 9, which show that Cable 2, while having acceptable crosstalk performance, was not able to meet the specification for attenuation.  
         [0036]    The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.