Abstract:
A flexible coaxial cable includes an inner conductor, and a dielectric layer generally surrounding the inner conductor. A tape outer conductor generally surrounds the dielectric layer, and does not underlie another electrically conductive layer such as a braided wire layer. The tape outer conductor includes a surface defining a plurality of indentations for minimizing damage to the tape outer conductor resulting from repeated flexing of the cable. The cable may include an insulating jacket generally surrounding the tape outer conductor.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to coaxial cables, and more particularly to flexible coaxial cables with tape outer conductor layers having surfaces defining a plurality or pattern of indentations.  
         BACKGROUND OF THE INVENTION  
         [0002]    Coaxial cables have employed several different types of outer conductors. Four types of outer conductors commonly used are as follows:  
           [0003]    1) braided wire employed for the outer conductor providing excellent flexibility, but resulting in cables with high attenuation and poor RF shielding at a relatively high cost;  
           [0004]    2) flat tape with braid applied over it providing lower attenuation and better RF shielding, but not having as good flexibility as the braided wire outer conductor, and having a slightly higher cost;  
           [0005]    3) corrugated copper or aluminum tubes providing excellent shielding and low loss, but being stiff and expensive; and  
           [0006]    4) smooth wall copper or aluminum tubes providing the lowest loss and excellent shielding, but being extremely stiff and expensive.  
           [0007]    Smooth aluminum or copper tapes such as those commonly applied underneath a braid can be applied without a braid, but the resulting cable is stiff and has a very limited flex life.  
           [0008]    It is a general object of the present invention to provide a flexible coaxial cable that avoids the above-mentioned drawbacks.  
         SUMMARY OF THE INVENTION  
         [0009]    In a first aspect of the present invention, a flexible coaxial cable includes an inner conductor, and a dielectric layer generally surrounding the inner conductor. A generally flat outer conductor generally surrounds the dielectric layer, and does not underlie another electrically conductive layer such as a braided wire layer. The generally flat outer conductor includes a surface defining a plurality of indentations for minimizing damage to the generally flat outer conductor resulting from bending or repeated flexing of the cable.  
           [0010]    In a second aspect of the present invention, a flexible coaxial cable includes an inner conductor, and a dielectric layer generally surrounding the inner conductor. A tape outer conductor generally surrounds the dielectric layer, and does not underlie another electrically conductive layer such as a braided wire layer. The tape outer conductor includes a surface defining a plurality of indentations for minimizing damage to the tape outer conductor resulting from bending or repeated flexing of the cable. The cable may include an insulating jacket generally surrounding the tape outer conductor.  
           [0011]    A first advantage of the present invention is that the coaxial cable has an increased flex life relative to a coaxial cable having a smooth-surfaced tape outer conductor.  
           [0012]    A second advantage of the present invention is that the coaxial cable has excellent RF shielding.  
           [0013]    A third advantage of the present invention is that the coaxial cable is inexpensive relative to a coaxial cable having a braided wire layer.  
           [0014]    A fourth advantage of the present invention is that the coaxial cable is smaller in diameter and of lower weight relative to a coaxial cable having a braided wire layer.  
           [0015]    A fifth advantage of the present invention is the relatively small diameter cable lends itself to ease of installation  
           [0016]    Other advantages will be made apparent with reference to the description and accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a cross-sectional end view of a flexible coaxial cable in accordance with the present invention.  
         [0018]    [0018]FIG. 2 is a side view of the cable of FIG. 1 showing a first embodiment of an oblique angled crisscross pattern of indentations defined by a tape outer conductor.  
         [0019]    [0019]FIG. 3 is a side view of the cable of FIG. 1 showing a second embodiment of a crisscross pattern defined by the tape outer conductor extending in directions parallel and transversely to a longitudinal axis of the cable.  
         [0020]    [0020]FIG. 4 is a side view of the cable of FIG. 1 showing a third embodiment of a parallel line pattern of indentations defined by the tape outer conductor.  
         [0021]    [0021]FIG. 5 is a cross-sectional end view of a flexible coaxial cable having an insulator jacket surrounding the tape outer conductor in accordance with the present invention.  
         [0022]    [0022]FIG. 6 is a cross-sectional end view of a flexible coaxial cable having an additional layer interposed between the tape outer conductor and the dielectric. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    With reference to FIG. 1, a flexible coaxial cable embodying the present invention is generally designated by the reference number  10 . The cable  10  includes an inner conductor  12 , a dielectric layer  14  generally circumferentially surrounding the inner conductor, and a generally flat outer conductor  16 , such as but not limited to, an electrically conductive tape generally circumferentially surrounding and bonded or unbonded to the dielectric layer, and not underlying another electrically conductive layer such as a braided wire layer. The inner conductor  12  may be any electrically conductive material such as, but not limited to, copper and aluminum, and in exceptional cases may be gold and silver. Moreover, the inner conductor  12  may be in either solid, stranded or tube form. The dielectric layer  14  may be any electrically insulating material such as, but not limited to, foam or solid polyethylene. The generally flat outer conductor  16  may be any electrically conductive material such as, but not limited to, aluminum, copper, silver and gold, as well as composites and laminates thereof.  
         [0024]    By way of example of a generally flat outer conductor, a flat tape outer conductor will be explained and illustrated in several embodiments. However, other types of generally flat outer conductors may be substituted without departing from the scope of the present invention. Tape outer conductors are employed herein without an overlying braided wire layer in order to lower attenuation, cost and size of the cable, as well as to improve RF shielding. However, a tape outer conductor does not have as good flexibility as a braided wire outer conductor. Tape outer conductors typically have little elasticity when bent and consequently tend to crack or otherwise be damaged when repeatedly bent or flexed such that the fatigue life of cables having tape outer conductors is lower relative to cables having braided wire outer conductors. Moreover, cracking of the tape outer conductor is detrimental to the electrical performance (such as shielding and attenuation loss) of the coaxial cable.  
         [0025]    It has been discovered that embossing or otherwise defining a plurality of indentations throughout a surface of a tape outer conductor reduces the bending moment of the cable and significantly increases its flex life without adversely affecting the electrical performance of the cable. More specifically, the indentations provide an elasticity to it when flexed so as to prevent the development of cracks in the tape outer conductor which otherwise would cause the conductor to suffer from high attenuation loss and degraded shielding. The resulting cable has the advantages of low loss, excellent RF shielding, low cost, small diameter and low weight as compared to braided wire cables, cables having flat tape with braided wire applied over it, corrugated cables, and smooth wall copper or aluminum cables.  
         [0026]    Referring now to FIG. 2, the flexible coaxial cable  10  including a tape outer conductor  16   a  in accordance with the present invention is illustrated. The tape outer conductor  16   a  includes a surface  18  defining a plurality of indentations generally in the form of a crisscross pattern  20 . Preferably, the surface  18  defining the indentations is facing radially outwardly from a central longitudinal axis A of the cable, but may be facing radially inwardly without departing from the scope of the present invention. The crisscross pattern  20  includes a plurality of lines  22 ,  24  extending along oblique angles relative to the central longitudinal axis A of the cable.  
         [0027]    With reference to FIG. 3, the flexible coaxial cable  10  including a tape outer conductor  16   b  in accordance with another embodiment of the present invention is illustrated. The tape outer conductor  16   b  includes a surface  18 , preferably facing radially outwardly, defining a plurality of indentations also generally in the form of a crisscross pattern  26 . The crisscross pattern  26  includes a plurality of lines  28 ,  30  extending generally along parallel and transverse directions relative to that of the central longitudinal axis A of the cable.  
         [0028]    Turning now to FIG. 4, the flexible coaxial cable  10  including a tape outer conductor  16   c  in accordance with a further embodiment of the present invention will be explained. The tape outer conductor  16   c  includes a surface  18 , preferably facing radially outwardly, defining a plurality of indentations in the form of a parallel spaced lines  32  extending generally in a direction along the central longitudinal axis A of the cable.  
         [0029]    [0029]FIG. 5 illustrates a flexible coaxial cable  100  in accordance with another embodiment of the present invention. The cable  100  is generally the same as the cable  10  of FIG. 1, except that the cable  100  includes an insulator jacket  102  generally circumferentially surrounding the tape outer conductor  16 . The jacket is fabricated from an electrical insulator, such as but not limited to, polyethylene and polyvinyl chloride (PVC).  
         [0030]    With reference to FIG. 6, a flexible coaxial cable in accordance with a further embodiment of the present invention is generally designated by the reference number  200 . The cable  200  is similar to the cable shown and described with respect to FIG. 1, except that at least one additional layer  202  may be interposed between the dielectric layer  14  and the tape outer conductor  18 . The additional layer  202  may be another layer of the tape outer conductor  18  or may be electrically non-conductive material such as, but not limited to, polyester, polypropylene or other polymer substrates applied to one or more layers of the tape outer conductor to add stability to the tape outer conductor when the coaxial cable  200  is being flexed. Moreover, the at least one additional layer  202  may be an adhesive layer such as, but not limited to, a low molecular weight polyethylene or polyethylene copolymer such as ethylene acrylic acid (EAA) or ethylene ethyl acrylate (EEA) to adhere the tape outer conductor  18  to the dielectric layer  14 . When employing a plurality of layers of electrically conductive tape, the plurality of indentations are preferably defined by the layer of tape farthest from the dielectric layer. However, the plurality of indentations may also be defined on all of the layers of tape without departing from the scope of the present invention.  
         [0031]    Although the invention has been shown and described above, it should be understood that numerous modifications can be made without departing from the spirit and scope of the present invention. For example, the flexible coaxial cable having the flat outer conductor defining a plurality of indentations may be covered with a braided layer to improve flexibility and performance over conventional braided coaxial cables. Accordingly, the present invention has been shown and described in several embodiments by way of illustration rather than limitation.