Patent Application
20080189940
The present invention relates to communication cables with shielded twisted pair conductors and more specifically to the use of a asymmetrically copper clad tape with an alloy steel inner layer as the shield of the cable.
Buried cables for non-gopher resistant applications traditionally utilize a bronze shield. The bronze is historically alloy C220 and contains approximately 90% copper. The shield is the predominate contributor to the cost of the buried cable. As the cost of copper rises, the cost of such buried cables also increases.
Another type of buried cable instead has a shielding of copper clad steel (CCS). This CCS shielded cable is typically used in applications requiring gopher resistance. The CCS tapes contain less copper than the bronze shields and are, therefore, less expensive. These CCS tapes are also known as bimetallic tapes or bimetallic shields.
The bronze shields and the CCS shields are useful for providing gopher resistance as well as for protection from lightning surges and electromagnetic interference.
CCS bimetallic shields utilize a construction of two copper layers on either side of an inner steel layer. Traditionally, the two copper layers are of equal thickness. Capacitive coupling between the conductors in the core of the cable and the steel layer of the bimetallic shield can be problematic. This capacitive coupling increases the mutual capacitance between the conductors as well as signal attenuation along the conductors. Generally, gopher resistant cables have an inner jacket separating the conductors in the core of the cable from the bimetallic shielding. This inner jacket provides a spacing between the conductors and the shielding. This spacing decreases the capacitive coupling between the conductors and the shield and also decreases the negative effects associated with the capacitive coupling.
However, non-gopher resistant cables do not generally require an inner jacket. Thus, the use of a bimetallic CCS shielding for non-gopher resistant applications (such as cost reduction of the shielding) can involve a cable with a CCS bimetallic shielding and no inner jacket. In such a cable, the bimetallic shield is much closer to the conductors. The proximity of the steel layer within the CCS shielding to the conductors can increase the capacitive coupling to unacceptable levels. This can be mitigated by various means, such as utilizing an inner jacket or significantly increasing the insulation thickness over the conductors. However, these approaches add substantially to the cost of the cable.
Accordingly, there is a need in the art for a buried cable with a bimetallic CCS shield that does not suffer from excessive capacitive coupling between the shield and the conductors when the conductors are positioned adjacent to the bimetallic shield.
The present invention supports a buried cable comprising a bimetallic shield (also called a bimetallic tape) where the thickness of the copper is increased on one side of the tape and decreased on the other side of the tape. Such a tape can be referred to as asymmetrically clad alloy steel (ACAS) tape. The side of the tape having the thicker copper can be placed adjacent to the conductors of the cable. The thicker copper between the steel layer of the shield and the conductors can act to substantially reduce the capacitive coupling.
In one aspect of the ACAS shielding, the reduced capacitive coupling can enhance the performance of the cable with respect to mutual capacitance and attenuation without resorting to the use of an inner jacket or significantly increasing the conductor insulation diameter.
Another aspect of the ACAS shielding is that overall protection from lightning surges and electromagnetic interference is maintained. Additionally, the thin layer of copper on the outside of the shield protects the inner layer of steel from corrosion. These beneficial aspects are realized while achieving the cost benefits of reducing the total amount of copper in the shield.
The discussion of asymmetrically clad alloy steel tapes for use as shielding in buried communication cables presented in this summary is for illustrative purposes only. Various aspects of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the drawings and the claims that follow. Moreover, other aspects, systems, methods, features, advantages, and objects of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such aspects, systems, methods, features, advantages, and objects are to be included within this description, are to be within the scope of the present invention, and are to be protected by the accompanying claims.
Many aspects of the invention can be better understood with reference to the above drawings. The elements and features shown in the drawings are not to scale, emphasis instead being placed upon clearly illustrating the principles of exemplary embodiments of the present invention. Moreover, certain dimensions may be exaggerated to help visually convey such principles. In the drawings, reference numerals designate like or corresponding, but not necessarily identical, elements throughout the several views.
The present invention supports a cable used to communicate, voice, data or other information. The cable can comprise multiple pairs of twisted conductors and an outer jacket that extends along the outside surface of the cable defining a longitudinal core, internal to the cable. The conductor pairs can be disposed in the core of the cable along with a shielding tape. The shielding tape can be positioned around the conductors but within the outer jacket.
The shielding may be an asymmetrically clad alloy steel (ACAS) tape. The asymmetrical cladding comprises copper that is thicker on a first side of the tape and thinner on a second side. When the tape is positioned between the conductors and the jacket, the thicker copper layer can be positioned adjacent to the conductors. The thicker copper layer can reduce the capacitive coupling between the conductors and the steel layer of the shielding tape without the added expense of an inner jacket or increased insulation thickness between the conductors and the shielding tape. The tape may also reduce lightning noise and other electromagnetic interference. Additionally, the copper layers of the tape can help prevent corrosion of the steel layer.
Exemplary cables comprising an ACAS shielding tape will now be described more fully hereinafter with reference to
The invention can 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 having ordinary skill in the art. Furthermore, all “examples” or “exemplary embodiments” given herein are intended to be non-limiting, and among others supported by representations of the present invention.
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One exemplary dimensioning of the bimetallic ACAS tape 100 can comprise an overall thickness of 3.75 mils with a layering ratio (by volume) of 5% copper, 55% steel, and 40% copper. This example would result in the three layers having thicknesses of 0.19 mils of copper, 2.06 mils of steel and 1.5 mils of copper. Another exemplary embodiment may have layers of 0.2 mils of copper, 2 mils of steel, and 1.5 mils of copper. These are mere examples and are not intended to limit the invention. Various other thickness ratios can be employed where a layer of steel is positioned between two copper layers and one of the copper layers is thicker than the other copper layer. These other thickness ratios do not depart from the spirit or scope of the invention.
Turning now to
The outer jacket 210 can seal the cable 200 from the environment and provide strength and structural support. The outer jacket 210 can be characterized as an outer sheath, a jacket, a casing, or a shell. The outer jacket 210 can be extruded or pultruded and can be formed of plastic, rubber, PVC, polymer, polyolefin, polyethylene, modified ethylene-CTFE (under the trademark VATAR), acrylic, polyamide (nylon), silicone, urethane, or other insulator, for example.
The core 230 of the cable 200 can contain air, a gas, paper, a filler, a cross filler, an asymmetrical cross filler, a foam filler matrix, or any other filler material. The cable 200 may be either an air-core design or the cable 200 may contain water-blocking material. An inner jacket may also be positioned within the core 230 of the cable 200
The insulated conductors 250 can be copper, aluminum, gold, silver, an alloy, or any other conductive material covered by an insulator formed of plastic, paper, rubber, PVC, polymer, polyolefin, polyethylene, polypropylene, flouropolymer, modified ethylene-CTFE (under the trademark VATAR), acrylic, silicone, urethane, or any other insulating material, for example. The insulation of the conductors 250 may be solid, or cellular, or a combination thereof, for examples.
The conductors 250 can be grouped in sets of two as twisted pairs. Alternatively, the conductors 250 can be ungrouped or grouped in sets of three, four, five, six, seven, eight, or more than eight conductors, for example. Also, there can be one, two, there, four, five, six, seven, eight, 16, 48, 50, 100, or any other number of total conductors 250 within the cable 200. One or more of the conductors 250 can also be optical fibers. One or more of the conductors 250 can also be coaxial assemblies.
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In Step 510, a test shielding tape comprising multiple metallic layers can be fabricated. This tape can formed by plating, electroplating, depositing, soldering, welding, or otherwise affixing the layers of metal together or onto one another. The tape can also be formed by stacking the metals together and then rolling them into a single tape. The forming of the tape may also include steps of heating, melting, partially melting, annealing, trimming, or applying pressure. The test shielding tape can include a bimetallic tape comprising a steel alloy plated, or clad, by copper. The copper cladding may be symmetric or asymmetric. That is, one side of the tape may have a thicker layer of copper than the other layer of tape.
In Step 520, a test cable can be fabricated by wrapping the test shielding tape around a plurality of conductors. The test shielding tape can be the one from Step 510. Also, an outer cable jacket can be applied around the test shielding. The tape and jacket can be applied in substantially the same step or in separate steps. A jacketing or sheathing machine can be used. The outer jacket can be extruded.
In Step 530, a test signal can be transmitted through at least one conductor or pair of conductors within the test cable. This test cable is the one formed in Step 520. The test signal can span a range of frequencies for the intended operation of the cable. For example DC (direct current) up to 100 GHz or more. The test signal can have various waveforms such as square, random, triangular, chirped, sinusoidal, periodic, or continuous, for examples. The test signal can have various spectrums such as band limited, white, pink, Gaussian, base-band, pass-band, enveloped or random, for examples.
In Step 540, the test signal transmitted in Step 530 can be monitored for interaction with the test shielding tape. This interaction may imply performance characteristics of the test cable formed in Step 520. The interaction may be monitored by oscilloscope, BERT (bit error rate tester), network analyzer, cable tester, spectrum analyzer, cross-talk tester, capacitance meter, signal loss tester, signal attenuation tester or other electronic testing or monitoring equipment, for examples.
In Step 550, the thickness of at least one of the metallic layers of the test shielding (from Step 510) may be specified in response to any interactions between the test shielding and the test signal as monitored in Step 540. In this step, at least one thickness parameter of the shielding can be specified that may define a shielding that favors a desired performance characteristic of the cable. The thickness may be specified in mils thickness, microns thickness, or other units. The thickness may be specified as a percentage of total thickness or mass. The thickness may be specified as relative ratios of absolute thickness or percent thickness, or in any other absolute or relative form or measurement. The thickness may be specified in ranges.
In Step 560 a production shielding tape can be fabricated. The production shielding tape can have multiple metallic layers. One or more parameters of the production shielding tape may be specified by the thicknesses determined in Step 550. This tape can formed by plating, electroplating, depositing, soldering, welding, or otherwise affixing the layers of metal together or onto one another. The tape can also be formed by stacking the metals together and then rolling them into a single tape. The forming of the tape may also include steps of heating, melting, partially melting, annealing, trimming, applying pressure, applying electromagnetic radiation, or applying vibration. The tape manufactured can include a bimetallic tape comprising a steel alloy plated, or clad, by copper. The copper cladding may be symmetric or asymmetric. That is, one side of the tape may have a thicker layer of copper than the other layer of tape.
In Step 570, a plurality of conductors can be protected by wrapping the production shielding tape around the conductors. The wrapping can include placing an asymmetrically copper clad tape around the conductors so that the thicker layer of copper is closer to the conductors and the thinner layer of copper faces the outside of the cable. The wrapping can include forming the tape longitudinally around the conductors or also forming the tape helically around the conductors, for examples. The wrapping can be automated within a cable forming system or machine. The system may be known as a sheathing machine or a sheathing line comprising multiple machines. A filler structure, filling material, or water blocking material may be added among the conductors within the core of the cable.
In Step 580, an outer jacket can be extruded around the production shielding tape. Since the tape may be wrapped around the conductors, the outer jacket may be around both conductors and the production shielding tape. This extrusion of an outer jacket can be considered to form a completed shielded cable. The jacketing can be automated within a cable forming system or machine. The system may be known as a sheathing machine or a sheathing line comprising multiple machines.
In Step 590, the cable formed in Step 580 can be rolled up onto a take-up reel. The take-up of the cable can also be onto or into some other form provided to contain or support the fabricated cable. After the cable is formed and rolled up, it may be tested for conductivity, cross-talk, attenuation, shorts, bandwidth capacity, transfer function, transfer spectrum, shielding efficacy, other electrical properties, and/or any manner of physical properties. The process 500, while possibly run continuously, may be considered complete after Step 590.
From the foregoing, it will be appreciated that an embodiment of the present invention overcomes the limitations of the prior art. Those skilled in the art will appreciate that the present invention is not limited to any specifically discussed application and that the embodiments described herein are illustrative and not restrictive. From the description of the exemplary embodiments, equivalents of the elements shown therein will suggest themselves to those skilled in the art, and ways of constructing other embodiments of the present invention will suggest themselves to practitioners of the art. Therefore, the scope of the present invention is to be limited only by the claims that follow.
