COAXIAL CABLE INCLUDING TUBULAR BIMETALLIC INNER 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 material. 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 typical 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 CeilReach® 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 fiat 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 fiat iayer or metal sheet into a tube with a longitudinal seam that is weided 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 meta! 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 welded 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 tubular inner conductor comprising an inlaid bimetallic layer, such as disclosed, for example, in U.S. Patent No. 6,342,677 to Lee. This coaxial 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 weld, 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 inner 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 inner conductor including a tubular bimetallic layer having a pair of opposing longitudinal edges at a longitudinal seam. The tubular bimetallic layer may comprise an inner metal layer and an outer metal layer bonded thereto with the inner metal layer having a lower melting temperature than the outer metai 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 material may be used for the inner 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 weiding 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 inner surface of the tubular bimetallic layer in a range of between 20 and 70 degrees. [0012] The outer metai layer may have a greater electrical conductivity than the inner metal layer. For example, the inner metal layer may comprise aluminum, and the outer 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 outer 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 tubular bimetallic layer. 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 by 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 metai layer and an outer metal layer bonded thereto and with the inner metal layer having a lower melting temperature than the outer metal layer, and at least one of the opposing longitudinal edges of the tubular bimetallic layer being at least partially bevelled. Forming the inner conductor may further include welding the longitudinal seam to form a welded joint between at least portions of the opposing longitudinal edges. The

method may further comprise forming the dielectric material layer surrounding the inner conductor, and forming the outer conductor surrounding the dielectric material layer.

Brief Description of the Drawings

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

[0017] FIG. 2 is an enlarged cross-sectional view of a portion of the tubular bimetallic inner conductor of the coaxial cable of FIG. 1 shown prior to welding.

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

[0020] FIG. 5 is a cross-sectional view of the portion of the tubular bimetallic inner conductor of FIG. 4 shown after welding. [0021] FIG. 6 is schematic diagram of an apparatus for making the coaxial cable in accordance with the present invention. [0022] FiG. 7 is a schematic diagram of a portion of another apparatus for making the coaxial cable in accordance with the present invention.

Detailed Description of the Preferred Embodiments [0023] 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 will 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.

[0024] Referring initially to FIGS. 1-3, a coaxial cable 20 in accordance with the present invention is now described. The coaxial cable 20 illustratively includes an outer conductor 21 , and a dielectric material layer 26 formed between an inner conductor 22 and the outer conductor. [0025] The inner conductor 22 illustratively includes a tubular bimetallic layer 34 having a pair of opposing longitudinal edges 31, 32 at a longitudinal seam 27. The tubular bimetailic layer 34 illustratively includes an inner meta! layer 24 and an outer metal layer 25 bonded thereto. The inner meta! layer 24 may have a lower electrical conductivity and a lower melting temperature, for example, than the outer metal layer 25 of the tubular bimetailic layer 34, such as to facilitate welding, for example, as described in greater detail below. For example, the inner metal layer 24 may comprise aluminum or any other suitable metal as will be appreciated by one skilled in the art. The outer metal layer 25 may have a higher electrical conductivity than the inner metal layer 24 to facilitate signal carrying ability at the skin depth, for example. The outer metal layer 25 may comprise copper, for example, or any other suitable metal as will be appreciated by one skilled in the art. Aluminum is also less expensive than copper.

[0026] In the illustrated embodiment, both of the opposing longitudinal edges 31 _» 32 of the tubular bimetailic layer 34 are bevelled, and the longitudinal seam 27 illustratively includes a welded joint 41 (FIGS. 1 and 3) between at least portions of the inner metal layer 24. Prior to welding, as illustrated in FIG. 2, the edges of the inner metal layer 24 may be touching or more closely spaced than the edges of the outer metal layer 25. Furthermore, when both of the opposing longitudinal edges 31, 32 of the tubular bimetallic layer 34 are bevelled; they may define a V-shaped notch 33 at the longitudinal seam 27 prior to welding. Both of the opposing longitudinal edges 31, 32 may be bevelled at an angle from normal to an inner surface of the tubular bimetallic layer 34 in a range of between 20 and 70 degrees, for example. [0027] The welding of the opposing longitudinal edges 31 , 32 of the tubular bimetailic layer 34 results in a welded joint 41. The welding can be

accomplished through induction welding, for example, or any other suitable method as wiii be appreciated by a person skilled in the art. Both of the opposing longitudinal edges 31, 32 being bevelled may advantageously permit the lower melting temperature metal edges to be softened and pressed together to form the welded joint 41 , while the higher melting temperature edges may remain solid, at least initially separated, and thereby not interfere with the welding of the lower temperature metai edges. Advantageously, the welding process can be matched to the characteristics of only the inner metai layer 24, which is typically more easily welded due to the lower melting temperature.

[0028] After welding, as illustrated in FIG. 3, the opposing longitudinal edges 31, 32 of the tubular bimetallic layer 34 may define a smaller, partially closed notch at the welded joint 41. Alternatively, the opposing longitudinal edges 31, 32 of the tubular bimetallic layer 34 may define a flush meeting joint, an outwardly protruding outer metal layer portion, or an outwardly protruding outer metal layer portion that is subsequently trimmed in other embodiments. [0029] The exemplary dimensions of the tubular bimetallic layer 34 are as follows. The tubular bimetallic layer 34 may have a thickness in a range of about 0.005 to 0.050 inches. In addition, the outer metal layer 25 may have a percentage thickness relative to an overall thickness of the tubular bimetallic layer in a range of about 1 to 30%.

[0030] The coaxial cable illustratively includes another dielectric material layer 23 filling the inner conductor 22. As appreciated by one skilled in the art, this dielectric material may be a rod formed prior to forming the inner conductor 22, or thereafter using settable material as described in U.S. Patent No. 6,915,564, also assigned to the assignee of the present invention, the entire contents of which are incorporated here by reference. The coaxial cable illustratively includes an insulating jacket 29 surrounding the outer conductor 21.

[0031] Referring now additionally to FIGS. 4 and 5, another embodiment is now described, fn this embodiment of the cable 20', 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 31', 32' of the tubular bimetallic layer 34' is bevelled. [0032] Illustrated in FIG. 6, another aspect relates to an apparatus 120 and a method for making a coaxial cable 20 including the inner conductor 22 comprising the tubular bimetallic layer 34. A dielectric material rod 123 and a supply reel 121 for the bimetallic strip are provided. The bimetallic strip may have a pair of longitudinal edges with at least one edge being bevelled prior to being wound onto the supply reel 121.

[0033] The longitudinal edges may be bevelled at an angle from normal to an inner surface of the bimetallic strip. The bevelled angle may be in the range of between 20 and 70 degrees. The inner metal layer may comprise a metal of lower melting temperature and lower electrical conductivity than the metal of the outer metal layer. The inner metal layer may comprise aluminum or any other suitable metal as will be appreciated by one skilled in the art. The outer metal layer may comprise copper or any other suitable metal as will be appreciated by one skilled in the art. The dielectric material rod 123 and the bimetallic strip are fed into the tube former 122. The tube former 122 illustratively forms the inner conductor surrounding the dielectric material rod 123 and having a tubular bimetallic layer having a pair of opposing longitudinal edges at a longitudinal seam.

[0034] The inner conductor is then fed into the induction welder 124, which illustratively welds the longitudinal seam to form a welded joint between at least portions of the inner metal layer of the tubular bimetallic layer. As will be appreciated by one skilled in the art, the dielectric material may be disposed inside the inner conductor downstream from the tube former 122, or thereafter using settable material as described in U.S. Patent No. 6,915,564. [0035] The inner conductor is fed into the dielectric extruder 125, which illustratively forms a dielectric material layer surrounding the inner conductor. The outer conductor is illustratively formed surrounding the dielectric material

layer by feeding a supply reel 126 of conductor strip and the output of the dielectric extruder 125 into a second tube former 127. [0036] The output of the second tube former 127 is fed into the second induction welder 161, which illustratively welds the outer conductor. The coaxial cable 163 is complete when the jacket extruder 162 illustratively forms an insulating jacket surrounding the outer conductor. The coaxial cable 163 is output from the jacket extruder 162 for take-up on a suitable take-up reel, not shown.

[0037] With additional reference to FIG. 7, the above apparatus and method can be modified to use a supply reel 131 of bimetallic strip without at least one longitudinal edge bevelled. In other words, the strip from the supply reel 131 is first fed into a beveller 132, which bevels at least one longitudinal edge of the bimetallic strip. The bimetallic strip with at least one longitudinal edge being bevelled 135 is input into the tube former 133 with the dielectric materia! rod 134.

[0038] Many modifications and other embodiments of the invention will 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.