Copper clad aluminum strips and a process for making copper clad aluminum strips

[0001] The present invention relates generally to copper clad aluminum strips for use as conductors in coaxial cables. More particularly, the present invention relates to a copper clad aluminum strip capable of being formed into a tube and used as both the inner and outer conductors of a coaxial cable, wherein the strip has a first edge, a second edge, and a middle portion disposed between the first edge and the second edge and being clad with copper.

[0002] Coaxial radio frequency cables traditionally consist of an internal copper conductor and external copper conductor with a dielectric in between. The dielectric surrounds and electrically insulates the inner conductor. The dielectric is usually a foam material. The copper conductors may be either smooth or corrugated.

[0003] Because copper is a costly metal, alternatives to pure copper are often sought. In the cable television industry, it is possible to replace the copper conductor on the inside with a copper clad aluminum conductor. That is possible because, due to the radio frequencies, the current in the internal conductor does not fill the total cross-section of the conductor. Instead, the current creeps to the outside of the conductor. That phenomenon is known as the skin effect. The skin effect allows the copper clad aluminum conductor to function like a conductor of pure copper.

[0004] A copper clad aluminum conductor is cheaper than a pure copper conductor because of the higher price of copper per weight unit versus the price of aluminum for the same weight unit. Moreover, there is a substantial difference in specific weight between the two metals. Copper has a specific weight which is more than three times higher than the specific weight of the aluminum. As a result, the use of copper clad aluminum wire is well established in radio frequency cables only where the internal conductor (in the form of a wire) has a diameter of less than 10 mm. Use of a copper clad aluminum for both the outer conductor and for an inner conductor greater than 10 mm in diameter has not proved practical because these conductors use a copper clad strip material that is seam welded into a tube to construct the wires of the coaxial cable. The copper clad aluminum strip is brittle at the copper/aluminum intermetallic weld seam which cracks and breaks when the cable is flexed. Because a copper clad aluminum strip would offer the substantial cost savings over pure copper conductors at the larger diameters, the need exists for a copper clad aluminum strip that can be formed into a conductor of sufficient diameter to be used in larger coaxial cables, i.e. cables having an inner diameter greater than 10 mm.

[0005] The present invention provides for copper clad aluminum strip capable of being formed into a tube, having a first edge and a second edge, wherein the width of the aluminum strip is wider than the copper portion, so that the first edge and the second edge consist entirely of aluminum.

[0006] Additionally, the present invention provides for an aluminum strip comprised of a first edge portion, a second edge portion, and a middle portion disposed between the first edge portion and the second edge portion and being clad with copper.

[0007] The present invention also provides for a copper clad aluminum conductor wherein at least 70% of the outer circumference of the conductor is clad with copper.

[0008] Further, the present invention provides for a copper clad aluminum conductor wherein at least 70% of the inner circumference of the conductor is clad with copper.

[0009] The present invention provides for a method for making a copper clad aluminum strip, the method comprising the steps of, processing an aluminum strip in a chamber under shield gas to be substantially free from contaminants, processing a copper strip in a chamber under shield gas to be substantially free from contaminants, and bonding the aluminum strip and the copper strip together to form a copper clad aluminum strip.

[0010] The present invention also provides for a method for making a copper clad aluminum strip, the method comprising the steps of, processing an aluminum strip in a chamber under shield gas to be substantially free from contaminants, processing a copper strip in a chamber under shield gas to be substantially free from contaminants, and pressure-temperature bonding the aluminum strip and the copper strip together to form a copper clad aluminum strip.

[0011] The present invention provides the advantages of lower cost and lighter weight. Those, and other advantages and benefits will become apparent from the Detailed Description of the Invention.

[0012] For the present invention to be easily understood and readily practiced, the present invention will now be described, for purposes of illustration and not limitation, wherein:

[0013] FIG. 1 shows a prior art cable design with solid copper tubing for both the inner and outer conductors.

[0014] FIG. 2 shows a cladding apparatus of the present invention used to create the copper clad aluminum strips of the present invention which may then be shaped into tubes for use as the inner and outer conductors of a coaxial cable.

[0015] FIG. 3 shows a cross-sectional end view of the copper clad aluminum strips which may then be shaped into tubes for use as the inner and outer conductors of a coaxial cable.

[0016] FIG. 4 shows a forming process used to form the copper clad aluminum strips into tubes for use as the inner and outer conductors of a coaxial cable.

[0017] FIG. 5 shows a cross-sectional view of a copper clad strip formed into the shape of a tube which may be used as the outer conductor of a coaxial cable, and specifically shows the weld seam.

[0018] FIG. 6 shows a cross-sectional view of a coaxial cable resulting from the present invention with copper clad aluminum tubing for both the inner and outer conductors.

[0019] FIG. 7 shows a graph plotting the attenuation of a coaxial cable constructed using conductors according to the present invention and the attenuation of a prior art coaxial cable versus the width of the weld seam.

[0020] FIG. 1 shows a prior art cable design with solid copper tubing for both the inner and outer conductors. As previously discussed, copper is a costly metal and therefore for cost reduction purposes it is of interest to use an alternative to pure copper conductors.

[0021] FIG. 2 shows a cladding apparatus according to the present invention which may be used to create the copper clad aluminum strips, which strips may then be shaped into tubes for use as the inner and outer conductors of, for example, a coaxial cable. As shown in FIG. 2, aluminum strip 1 and copper strip 2 are moved by support rolls 8. It is understood that aluminum strip 1 may be pure aluminum, one of the series 3000 aluminum alloys, or a high strength high magnesium strength aluminum, among others. Aluminum strip 1 and copper strip 2, processed to be free from organic contaminations, are paid off in a continuous manner from their pay-off's (not shown). In two separate chambers 13, 14, copper strip 2 and aluminum strip 1, respectively, are brushed by brushes 9 under a shield gas or reducing gas to prevent the formation of oxide at the bond interface. It is preferable that chambers 13, 14 be substantially enclosed so that the pressure inside chamber 13, 14 is above atmospheric pressure. That is, chambers 13, 14 may be partially sealed at chamber entrances 40 by any suitable mechanism that will reduce the amount of escaping gas while not damaging the incoming strip, such as a felt pad. This will allow strips 1 and 2 to enter their respective chambers with minimal pressure loss inside chambers 13 and 14. Strips 1 and 2 exit their respective chambers 13, 14 through chamber exits 41 under cover of the exiting pressurized shield gas. Chamber exits 41 may simply be narrow slots or any other suitable mechanism to provide for minimal pressure loss inside chambers 13,14. The average pressure inside chambers 13,14 is preferably greater than atmospheric and may be 1.013 bars. As previously stated, this ensures that the activated surfaces of strips 1 and 2 are protected from oxidation. The shield gas may be an inert gas. It is preferable, but not necessary, that the shield gas be argon or helium, or a mixture of argon or helium. It is also preferable that the chamber 13, 14 contain less than 8 ppm oxygen.

[0022] Strips 1 and 2 exit chambers 13, 14 and meet in the slot of rolling mill 10 where they are crush bonded to form an overlaid copper clad aluminum strip 3. It is preferable that rolling mill 10 be physically close to exits 41 so that the activated surfaces of copper strip 2 and aluminum strip 1 are protected by the shield gas from oxidation until they meet in the slot of rolling mill 10. As strips 1 and 2 meet in the slot of rolling mill 10, the pressure of the rolling mill 10 causes an increase in temperature, and as a result of the pressure and temperature a bond is formed between strips 1 and 2. However, it is understood that other known bonding methods may be used to create the bond between strips 1 and 2, such as a bond where heat is added to create a temperature higher than that resulting from pressure alone. A reduction of the total thickness of the two metals can be in a range of 25% to 65% to give the desired bond without intermediate annealing and with the exactly required strip width. It is preferable, but not necessary, that the thickness of the copper portion 2 is less than approximately 12% of the total thickness of the copper clad aluminum strip 3. The thinner the copper, the less expensive the final product.

[0023] Common prior art processes to clad strips of different metals are known to use activated strips. These activated strips are reduced together by approximately 60% or more reduction of the combined total thickness of both metals in an air atmosphere. This high reduction breaks the oxide layers on the contact surfaces and makes the virgin areas of the two metals bond. However, sometimes that is not sufficient to create the desired bond and an annealing process is necessary to improve the bond strength. Additionally, the high reduction can cause the edges of the strip to crack which then requires that a trimming step be implemented. The additional trimming step results in high costs as a result of the scrap. Also, aluminum oxide, which can remain at the bond interface, can cause fracturing of the thin copper cladding during subsequent cold working operations. Therefore, this prior art process is not economically very viable. Additionally, this prior art process has a low efficiency due to the annealing cost for the bond. Therefore, the above identified process of the present invention provides for many advantages over the prior art process as discussed.

[0024] FIG. 3 shows a cross-sectional end view of copper clad aluminum strip 3 which may then be shaped into a tube to serve as either the inner or outer conductor of a coaxial cable. As shown in FIG. 3, it is preferable that the width of aluminum portion 1 of copper clad aluminum strip 3 be wider than the width of copper portion 2 of copper clad aluminum strip 3. More specifically, it is preferable to have the edges 11, 12 of aluminum portion 1 extend beyond the ends 21, 22, respectively, of the copper portion 2 of copper clad aluminum strip 3. The width of copper portion 2 of copper clad aluminum strip 3 is preferably no greater than approximately 30% less than the width of aluminum portion 1 of copper clad aluminum strip 3. It is preferable to have edges 11, 12 of aluminum which are of a minimal size. Edges 11, 12 of aluminum should be just wide enough to produce the smallest possible weld seam or gap when ends 31, 32 of aluminum portion 1 are brought into contact as described below. The dimensions of edges 11, 12 may be optimized so that after manufacturing of conductors of a radio frequency cable therefrom, the increase in the attenuation of resulting cable is less than 0.8% compared to an all copper conductor coaxial radio frequency cable for all frequencies of interest.

[0025] FIG. 4 shows a forming process which may be used to form the copper clad aluminum strips into tubes for use as the inner and outer conductors of a coaxial cable. As shown in FIG. 4, copper clad aluminum strip 3 is folded to form a tube bringing ends 31, 32 into contact. Because copper portion 2 is not as wide as aluminum portion 1, ends 21, 22 are not brought into contact. Any known folding method may be used. The excess aluminum portions 11, 12 of the copper clad aluminum strip are then passed under a welding electrode 6 to form a weld seam 5. That is, aluminum portions 11, 12 are welded together to keep copper clad strip 3 in the shape of the formed tube. FIG. 4 shows the tube being formed with copper portion 2 of copper clad aluminum strip 3 being on the interior of the resultant formed tube. It is understood that the tube may also be formed so that copper portion 2 of copper clad aluminum strip 3 is on the exterior of the resultant formed tube (not shown). In either case, the excess aluminum portions 11, 12 of copper clad aluminum strip 3 are welded together to form weld seam 5.

[0026] FIG. 5 shows a cross-sectional view of a copper clad strip 3 formed into the shape of a tube which may be used as the outer conductor of a coaxial cable, and specifically shows weld seam 5. Also shown in FIG. 5 is the gap g which results in copper layer 2 when aluminum portions 11, 12 a are welded together.

[0027] FIG. 6 shows a cross-sectional view of a coaxial cable resulting from the present invention with copper clad aluminum tubing for both the inner and outer conductors. Inner copper clad aluminum strip 3 may be either corrugated or smooth and functions as the inner conductor. Copper portion 2 of inner copper clad aluminum strip 3 is on the outside of strip 3. Dielectric 7 is shown disposed between the inner and outer copper clad aluminum strips 3. Dielectric 7 is typically physically foamed polyethylene (PE). Outer copper clad aluminum strip 3 may be either corrugated or smooth and functions as the outer conductor. Copper portion 2 of outer copper clad aluminum strip 3 is on the inside of strip 3. Both inner and outer copper clad aluminum strips 3 have a small gap g in copper portion 2 as shown in FIG. 6. The relation of these gaps to the circumference of strip 3 increases the attenuation of a signal conducted along the cable. The width of the gap also depends on the welding process, but according to the present invention, strip 3 can be manufactured for every welding process to give a minimum seam width and accordingly the least increase in attenuation.

[0028] FIG. 7 shows a graph plotting the attenuation of a coaxial cable using conductors according to the present invention and the attenuation of a prior art coaxial cable versus the width of weld seam 5 (which is the same as the width of the gap g). FIG. 7 shows the interdependence between the increase of the attenuation and the relation of the gap versus the circumference of the working diameter of the conductors 3.

[0029] The total attenuation A of the present invention copper clad aluminum/copper clad aluminum conductor cable is the sum of the attenuation of the internal conductor ai and the attenuation of the external conductor ae. A is shown in relation to Ao which is the attenuation of an analog coaxial cable with copper/copper conductors as shown in FIG. 7.

[0030] Though for any copper clad aluminum conductor cable, the increase of the attenuation does not exceed 1% in the worst case, FIG. 7 shows the possibility to reduce any increase of the attenuation to a minimum by producing a tailor made copper clad aluminum strip for each possible coaxial cable which has in both conductors the smallest possible gap.

[0031] One problem when welding prior art cladded strip into the form of a tube lies in the different melting points of the two clad metals. For example, if the strip is copper clad aluminum, and if one tries to weld the copper, the aluminum vaporizes. If one tries to weld the aluminum, the copper goes into solution and makes the seam brittle. The present invention solves this problem by producing a strip which has an aluminum area to be welded free from copper.

[0032] Although the present invention has been described in conjunction with preferred embodiments thereof, those of ordinary skill in the art will recognize that many modifications and variations may be made. The following claims are intended to cover all such modifications and variations.