Abstract:
A coaxial cable compatible with standard 735 type performance parameters has a smaller diameter and greater flexibility at a lower cost.

Description:
CROSS-REFERENCED TO RELATED APPLICATION  
       [0001]    The present application claims domestic priority under 35 U.S.C. §119(e) to copending U.S. provisional patent application serial No. 60/322,641 filed Sep. 17, 2001, incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to coaxial cables. More particularly, the present invention relates to miniature coaxial cables used in digital signal transmission applications, such as in telephone switching applications.  
           [0004]    2. Related Art  
           [0005]    In some digital communication networks, certain digital signals are carried from one piece of central office equipment to another on miniature coaxial cables. In North America, standards for such cables have been set by Bell Communications Research, Inc. (“Bellcore”), the former Central Services Organization of AT&amp;T, spun off during the 1984 divestiture, and now operating as Telcordia Technologies (“Telcordia”). The most common types of digital signals carried by coaxial cables are produced by so-called DS3 and STS-1 signal sources.  
           [0006]    One standard promulgated by Telcordia is GR-139-CORE, “Generic Requirements for Central Office Coaxial Cable”, Issue 1, October 1996, incorporated herein by reference. Among other things, with respect to electrical parametric requirements Telcordia GR-139-CORE defines a cable having a 75 ohm characteristic impedance referred to as 735 coaxial cable, which is suitable for connecting a digital source and a digital receiver less than 225 feet apart. The construction of conventional 735 coaxial cable as shown in FIG. 1, is now described.  
           [0007]    Conventional 735 coaxial cable  100  has a 26 AWG silver-plated copper center conductor  101 . The center conductor  101  is surrounded by several successive layers of materials in the order recited.  
           [0008]    The center conductor is surrounded by a high density polyethylene (HDPE) foam with a blow ratio of about 37% and an outer diameter of about 0.077 inches, an inner shield  103  of 0.002 inches of aluminum on 0.001 inches of polyester tape, an outer shield  104  of flat bundles of 6 strands of 38 AWG tinned copper braided to provide a 90% coverage and a polyvinyl chloride (PVC) outer jacket  105 . The structure has an outer diameter of 0.129 inches. A bundle of twelve conventional 735 coaxial cables  100 , enclosed in an outer PVC jacket  201  is shown in FIG. 2. This construction has an outer diameter of 0.600″.  
         SUMMARY OF THE INVENTION  
         [0009]    According to one embodiment, the invention comprises a miniature coaxial cable having a characteristic impedance of about 75 ohms, comprising: a center conductor having an outer diameter of about 0.0159 inches, a first dielectric layer disposed about the center conductor having a dielectric constant less than about 1.7 and an outer diameter less than about 0.077 inches, a conductive tape including a second dielectric layer less than 0.001 inches thick on which is disposed a metal layer less than 0.002 inches thick where the conductive tape is applied to the first dielectric layer such that the second dielectric layer is adjacent to it, a braided conductor disposed about and in contact with the metal layer of the conductive tape, and an insulating jacket disposed about the braided conductor.  
           [0010]    In another embodiment, the invention comprises a miniature coaxial cable with a characteristic impedance of about 75 ohms, including: a center conductor, a dielectric layer having a dielectric constant value of less than 1.7, an outer conductor structure, and a jacket; wherein the outside diameter of the miniature coaxial cable is less than 0.122 inches.  
           [0011]    In a further embodiment, the invention comprises a bundle of coaxial cables enclosed in a jacket, wherein the coaxial cables enclosed are miniature coaxial cables which meet the GR-139-CORE (Issue 1, 1996) standard having a characteristic impedance of 75 ohms and wherein a miniature cable includes a center conductor, a dielectric layer having a dielectric constant value of less than 1.7, an outer conductor structure, and a jacket, wherein the outside diameter of the miniature coaxial cable is less than 0.122 inches.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    In the drawings, in which like reference designations indicate like elements:  
         [0013]    [0013]FIG. 3 is a cross sectional view of a coaxial cable embodying one aspect of the invention;  
         [0014]    [0014]FIG. 4 is a cross sectional view of a bundled cable of coaxial cables embodying another aspect of the invention;  
         [0015]    [0015]FIG. 1 is a cross sectional view of a conventional coaxial cable;  
         [0016]    [0016]FIG. 2 is a cross sectional view of a bundled cable of conventional coaxial cables;  
         [0017]    [0017]FIG. 5 is a perspective view of peeled-back layers of the cable of FIG. 1 in which a conductive tape is applied longitudinally; and  
         [0018]    [0018]FIG. 6 is a perspective view of peeled-back layers of the cable of FIG. 1 in which a conductive tape is applied spirally. 
     
    
     DETAILED DESCRIPTION  
       [0019]    The present invention will be better understood upon reading the following detailed description of various embodiments and aspects thereof, in connection with the figures.  
         [0020]    Embodiments of aspects of the present invention can replace conventional 735 cables at a lower production cost and take up less space in existing cable trays and runs than the conventional 735 cable replaced. The invention meets the afforementioned Telcordia GR-139-CORE 735 cable standard. The invention is suitable for carrying digital signals for at least 223 feet and up to about 225 feet. It can be used to carry DS3 and STS-1 digital signals, as well as other digital and analog signals of similar spectral bandwidth. An example use for such cable is in central office applications.  
         [0021]    The center conductor  301  of the exemplary cable  300  shown in FIG. 3 is a 26 AWG silver plated copper wire. This wire has an outside diameter of between 0.0157 and 0.0162 inches. The particular materials and dimension of the center conductor  301  are selected to have a conductivity which limits at which permits run lengths of at least 223 feet and up to about 225 feet for DS-3 signals. The center conductor  301  may be constructed of wire having a different base material and/or plating, or of unplated wire known in the art to have an adequate conductivity for the purpose. As is known, other construction can provide the same or other conductivities, as may be desired.  
         [0022]    The center conductor  301  is surrounded by a dielectric material  302  having a dielectric constant, ∈, less than or equal 1.7, and an outer diameter less than 0.077 inches. The dielectric  302  may be formed of a polyolefin. An example of a polyolefin suitable for use is a foamed high density polyethylene (HDPE) material having a blow ratio greater than 37%. In a particular exemplary embodiment, the dielectric is HDPE foamed with a blow ratio of 45%, resulting in an outer diameter of 0.073 inches.  
         [0023]    Foamed HDPE can be made by chemical foaming or by gas injection foaming. Chemical foaming is used in the exemplary embodiment because it can be used in conventional coating equipment, without the addition of gas injection facilities.  
         [0024]    Other materials can be used in place of the HDPE foam. For example, fluorinated ethylene propylene (FEP) could be used, yielding a smaller outer diameter, in order to maintain the characteristic impedance desired of about 75 ohms. However, foamed HDPE is a low cost material presently available for this application.  
         [0025]    In this cable, there is a relationship between the inner diameter, d, the outer diameter, D, the desired characteristic impedance, Z, and the dielectric constant of the foamed HDPE, ∈ r , which governs. Namely,  
             Z   :=       138       ɛ   r              log        (     D   d     )                 (   1   )                               
 
         [0026]    or, the outer diameter desired, D, is:  
             D   =     d   ·     10       Z          ɛ   r         138                 (   2   )                               
 
         [0027]    It is well known that foamed HDPE, at a blow ratio of 37% has ∈ r =1.7.. In contrast, at a blow ratio of 45%, ∈ r =1.5. The value of ∈ r  varies with blow ratio, hence for fixed d and Z, D must vary accordingly.  
         [0028]    Surrounding the dielectric layer  302  is an outer conductor structure including an inner shield formed of a conductive tape  303  and an outer shield formed of a braided conductor  304 . The conductive tape  303  is constructed of a layer of polyester less than 0.001 inches thick on which is disposed a metal layer less than 0.002 inches thick. The conductive tape  303  may be applied to the outside of the dielectric layer  302  longitudinally, as shown in FIG. 5, with or without overlap applied in a spiral fashion, as shown in FIG. 6, also with or without overlapping seams  501 . An edge of the conductive tape may be folded under  502 , as shown in FIGS. 5 and 6, so as to provide continuous contact along the longitudinal or spiral seams  501 .  
         [0029]    In the exemplary embodiment, the polyester film has a thickness of about 0.00045 inches thick and the conductive metal layer is about 0.001 inches thick of aluminum. The aluminum layer can be as thin as 0.0005 inches thick, if desired.  
         [0030]    The outer shield of braided conductor  304  of the exemplary embodiment is constructed of flat bundles of six strands each of 38 AWG tin plated copper, braided to provide 95% coverage, in the manner known to the skilled artisan. The outer conductor structure  304  brings the outside diameter of the coaxial cable to 0.092 inches in diameter. Finally, a jacket  305  of any suitable material is applied to the cable, bringing the total diameter 0.114 inches. PVC is an example of suitable material from which to form the jacket. The outer jacket  305  can be treated, for example by fluorination, or other materials can be selected for the outer jacket  305  to meet various structural, wear and fire resistance requirements as may be desired.  
         [0031]    As shown in FIG. 4, a bundle of 12 cables  300  according to the above-described embodiment can be further surrounded by a jacket  401  of PVC or any other suitable material, producing a bundled cable of 0.515 inches diameter. The cross-sectional area of such a cable is 0.209 square inches. The resulting dimensions of this cable  400  are significantly smaller than those of a similar performing conventional cable (FIG. 2, 200). Moreover, the individual coaxial cables  300  and the bundle  400  described herein are substantially more flexible than conventional cable (FIG. 1, 100) and bundle (FIG. 2, 200), as a result of the higher blow ratio and thinner outer conductive structure. For example, the minimum radius of the individual coaxial cable described above is at most 10 times its diameter.  
         [0032]    The present invention has now been described in connection with a number of specific embodiments thereof. However, numerous modifications which are contemplated as falling within the scope of the present invention should now be apparent to those skilled in the art. The features of this invention in connection with the various embodiments can all be combined and recombined in various ways. Therefore, it is intended that the scope of the present invention be limited only by the scope of the claims appended hereto.