COAXIAL CABLE INCLUDING TUBULAR BIMETALLIC OUTER LAYER WITH BEVELLED EDGE JOINT AND ASSOCIATED METHODS

Field of the Invention

[0001] The present invention relates to the field of communications, and, more particularly, to coaxial cables and associated methods for making the coaxial cables.

Background of the Invention

[0002] Coaxial cables are widely used to carry high frequency electrical signals. Coaxial cables enjoy a relatively high bandwidth, low signal losses, are mechanically robust, and are relatively low cost. A coaxial cable typically includes an elongate inner conductor, a tubular outer conductor, and dielectric separating the inner and outer conductors. For example, the dielectric may be a plastic foam materia!. An outer insulating jacket may also be applied to surround the outer conductor.

[0003] One particularly advantageous use of coaxial cable is for connecting electronics at a cellular or wireless base station to an antenna mounted at the top of a nearby antenna tower. For example, the transmitter and receiver located in an equipment shelter may be coupled via coaxial cables to antennas carried by the antenna tower. A typicai installation includes a relatively large diameter main coaxial cable extending between the equipment shelter and the top of the antenna tower to thereby reduce signal losses. For example, CommScope, Inc. of Hickory, N. C. offers its CellReach® coaxial cable for such applications.

[0004] In larger diameter coaxial cables, which are commonly used in cellular communication as described above, the elongate inner conductor can be tubular in shape. The tubular inner conductor may also surround an inner

dielectric materia!. The inner conductor is typically manufactured by forming a flat layer or sheet of conductive material into a tube with a longitudinal seam and welding the seam to form a continuous joint. The outer conductor is also similarly manufactured by forming a flat layer or metal sheet into a tube with a longitudinal seam that is welded to form a continuous joint, [0005] The high frequency signals carried by the coaxial cable are concentrated in only a small portion, radially outermost, of the inner conductor, and a correspondingly small radially innermost portion of the outer conductor. This characteristic is attributed to the electromagnetic phenomenon called the skin effect. Therefore, only the thin outer radial portion of the tubular inner conductor carries the high frequency transmission. Conversely, the outer tubular conductor also carries the high frequency signals in the thin radially innermost portion.

[0006] Bimetallic layers have been used for the inner and/or outer tubular conductors in a coaxial cable where a higher conductivity and more expensive metal is used to provide the radially outermost portion of an inner conductor, and is used to provide the radially innermost portion of the outer conductor. For example, the outermost layer of the inner conductor may include a relatively costly and highly conductive metal such as copper, and the inner layer of the inner conductor may include a less costly and less conductive metal, such as aluminum. For example, U.S. Patent No. 6,717,493 B2 to Chopra et al. and U.S. Patent Application No. 2004/0118591 A1 to Bufanda et al. each discloses a coaxial cable with such bimetallic tubular inner conductors.

[0007] Notwithstanding the benefits of a bimetal tubular inner conductor, there may be some shortcomings. For example, the manufacture of a bimetal tubular inner conductor usually involves some form of heat based welding, such as for example, conventional induction welding, to weld the seam to form a weided joint. Unfortunately, the two metals that form the bimetal tubular inner conductor usually have different melting temperatures. For example, copper and aluminum are commonly used as the outer and inner layers of the

inner conductor, respectively. Copper has a melting point of 1100 ⁰ C and a conductivity of 59.6 * 10 ⁶ S m ^'1 , while aluminum has a lower melting point of 66O ⁰ C and a lower conductivity of 37.8 * 10 ⁶ S m ^"1 . This disparity in melting points makes welding of the joint relatively difficult. [0008] In response to this particular shortcoming in manufacture of bimetal tubular inner conductors, coaxial cable manufacturers have developed a coaxial cable with a bimetal tubuiar inner conductor comprising an inlaid bimetallic layer, such as disclosed, for example, in U.S. Patent No. 6,342,677 to Lee. This coaxiai cable is more easily welded since only the inner metal layer is welded during manufacture of the bimetal tubular inner conductor. Nonetheless, the inlaid bimetal inner conductor is relatively costly to manufacture. Of course, similar considerations apply to the outer conductor of a coaxial cable. That is a conventional bimetallic layer may be difficult to weid, and an inlaid bimetallic layer may be relatively expensive.

Summary of the Invention

[0009] In view of the foregoing background, it is therefore an object of the present invention to provide a coaxial cable including an outer conductor fabricated using a less expensive tubular bimetallic layer that is also readily welded at its longitudinal seam

[0010] This and other objects, features and advantages in accordance with the present invention are provided by a coaxial cable comprising an outer conductor including a tubular bimetallic iayer having a pair of opposing longitudinal edges at a longitudinal seam. The tubuiar bimetallic layer may comprise an inner metal layer and an outer metal layer bonded thereto with the outer metal layer having a lower melting temperature than the inner metal layer. At least one of the opposing longitudinal edges of the tubular bimetallic layer may be at least partially bevelled. In addition, the longitudinal seam may comprise a welded joint between at least portions of the opposing longitudinal edges. Accordingly, a less expensive starting materia! may be used for the outer conductor, that is, a simple bimetallic strip with one or both edges bevelled, as compared to the more expensive inlaid bimetallic strip, for

example. The at least one bevelled edge may advantageously permit the lower melting temperature metal edges to be softened and pressed together to form the welded joint, while the higher melting temperature edges may remain solid, at least initially separated, and thereby not interfere with the welding of the lower temperature metal edges.

[0011] In some embodiments, each longitudinal edge of the tubular bimetallic layer may be at least partially bevelled to define a notch at the longitudinal seam. For example, the at least one opposing longitudinal edge may be at least partially bevelled at an angle from normal to an outer surface of the tubular bimetallic layer in a range of between 20 and 70 degrees. [0012] The inner metal layer may have a greater electrical conductivity than the outer metal layer. For example, the outer metal layer may comprise aluminum, and the inner metal layer may comprise copper. [0013] The tubular bimetallic layer may have a thickness in a range of about 0.005 to 0.050 inches. In addition, the inner metal layer may have a percentage thickness relative to an overall thickness of the tubular bimetallic layer in a range of about 1 to 30%.

[0014] The coaxial cable may further comprise another dielectric material layer filling the inner conductor. The cable may also further include an insulating jacket surrounding the outer conductor. [0015] A method aspect is for making a coaxial cable comprising an inner conductor, an outer conductor and a dielectric material layer therebetween. The method may include forming the inner conductor, forming the dielectric layer surrounding the inner conductor and forming the outer conductor surrounding the dielectric layer. Forming the outer conductor may include forming at least forming a bimetallic strip into a tubular bimetallic layer having a pair of opposing longitudinal edges at a longitudinal seam with the tubular bimetallic layer comprising an inner metal layer and an outer metal layer bonded thereto and with the outer metal layer having a lower melting temperature than the inner metal layer, and at least one of the opposing longitudinal edges of the tubular bimetallic layer being at least partially

bevelled. Forming the outer conductor may further include welding the longitudinal seam to form a welded joint between at least portions of the opposing longitudinal edges.

Brief Description of the Drawings

[0016] FIG. 1 is a perspective end view of a coaxiai cable in accordance with the present invention.

[0017] FiG. 2 is an enlarged cross-sectiona! view of a portion of a tubular bimetallic outer conductor of the coaxial cabie of FIG. 2 shown prior to welding.

[0018] FIG. 3 is a cross-sectional view of the portion of the tubular bimetallic outer conductor of FiG. 2 shown after welding. [0019] FIG. 4 is an enlarged cross-sectional view of a portion of a tubular bimetallic outer conductor of a coaxial cabie of another embodiment shown prior to welding.

[0020] FiG. 5 is a cross-sectional view of the portion of the tubular bimetallic outer conductor of FIG. 4 shown after welding. [0021] FIG. 6 is a schematic diagram of a portion of yet another embodiment of an apparatus for making the coaxial cable in accordance with the present invention.

Detailed Description of the Preferred Embodiments [0022] The present invention will now 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. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and wil! fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternative embodiments.

[0023] Referring initially to FiGS. 1-3, an embodiment of a cable 51 is described. The coaxial cable 51 illustratively includes an outer conductor 52

comprising a tubular bimetallic layer 84. The coaxial cable 51 illustratively includes an inner conductor 54 and an outer conductor 52 with a dielectric material layer 53 between the conductors. A dielectric rod or filling 23 is illustratively within the tubular inner conductor 54, although in other embodiments, the tubular inner conductor may be hollow or solid as wiii be appreciated by those skilled in the art. The outer conductor 52 illustratively includes a tubular bimetallic layer 84 having a pair of opposing longitudinal edges 81, 82 at a longitudinal seam 55.

[0024] The tubular bimetallic layer 84 illustratively comprises an inner metal layer 57 and an outer metal layer 56 bonded thereto. The metai comprising the outer metal layer 56 may have a lower melting point, lower electrical conductivity, and lower cost, for example, than the metal comprising the inner metal layer 57. The lower melting temperature of the outer metal layer 56 facilitates welding, for example, as described in detail below. The outer metal layer 56 may comprise aluminum or any other suitable metai as will be appreciated by one skilled in the art. The inner metal layer 57 may comprise copper or any other suitable metal as will be appreciated by one skilled in the art. The inner metal layer 57 may have a higher electrical conductivity than the outer metal layer 56 to facilitate signal carrying ability at the skin depth, for example.

[0025] In the illustrated embodiment, both of the opposing longitudinal edges 81, 82 of the tubular bimetallic layer 84 are bevelled at least prior to welding, and the longitudinal seam 55 illustratively includes a welded joint 91 (FIGS. 1 and 3) between at least portions of the outer metai layer 56. Prior to welding, as illustrated in FIG. 2, the edges of the outer metal layer 56 may be touching or more closely spaced than the edges of the inner metal layer 57. Furthermore, when both of the opposing longitudinal edges of the tubular bimetallic layer 84 are bevelled, the bevelled opposing longitudinal edges may define an inverted V-shaped notch 83 at the longitudinal seam 55. The opposing longitudinal edges 81, 82 may be bevelled at an angle from normal

to an outer surface of the tubular bimetallic layer 84 in a range of between 20 and 70 degrees.

[0026] The welding of the opposing longitudinal edges 81 , 82 of the tubular bimetallic layer 84 results in a welded joint 91. The welding can be accomplished through induction welding or any other suitable method as will be appreciated by one skilled in the art. Alternatively, the opposing longitudinal edges 81, 82 of the tubular bimetallic layer 84 may define a flush meeting joint, an inwardly protruding outer metal layer portion, or an inwardly protruding outer metal layer portion that is subsequently trimmed in other embodiments.

[0027] The bevelled edges may advantageously permit the lower melting temperature metal edges to be softened and pressed together to form the welded joint 91, while the higher melting temperature edges may remain solid, at least initially separated, and thereby not interfere with the welding of the lower temperature metal edges. Advantageously, the welding process can be matched to the characteristics of oniy the outer metal layer 56, which is typically more easily welded due to the lower melting temperature. The coaxial cable illustratively includes an insulating jacket 98 surrounding the outer conductor 52.

[0028] The exemplary dimensions of the tubular bimetallic layer 84 are as follows. The tubular bimetallic layer 84 may have a thickness in a range of about 0.005 to 0.050 inches, in addition, the inner metal layer 57 may have a percentage thickness relative to an overall thickness of the tubular bimetallic layer 84 in a range of about 1 to 30%.

[0029] Referring now additionally to FIGS. 4 and 5, another embodiment is now described. In this embodiment of the coaxial cable 51', those elements already discussed above with respect to FIGS. 1-3 are given prime notation and most require no further discussion herein. This embodiment differs from the previous embodiment in that only one of the longitudinal edges 81', 82' of the tubular bimetallic layer 84' is bevelled.

[0030] Referring now additionally to FIG. 6, another apparatus and method aspect is directed to making the coaxial cable 51 including an outer conductor 52 comprising tubular bimetallic layer 84, and with at least one bevelled edge. The supply reei 142 of bimetallic strip, which can be provided with at least one longitudinal edge being bevelled, and inner conductor 144 are fed into the tube former 143. The tube former 143 illustratively forms the bimetallic strip into an outer conductor having tubular bimetallic layer having a pair of opposing longitudinal edges at a longitudinal seam. The method can be modified to provide a supply reel of bimetallic strip and a beveiler 145, shown with phantom lines, to illustratively bevel at least one longitudinal edge of the bimetallic strip.

[0031] Many modifications and other embodiments of the invention wiil come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.