Electricity: conductors and insulators – Conduits – cables or conductors – Insulated
Reexamination Certificate
1999-06-04
2001-12-04
Nguyen, Chau N. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Insulated
Reexamination Certificate
active
06326551
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a coaxial cable, and more particularly to an improved low-loss coaxial cable having good moisture-absorbing, bending, handling and electrical properties.
BACKGROUND OF THE INVENTION
The coaxial cables commonly used today for the transmission of RF signals, such as cable television signals and cellular telephone broadcast signals, include a core containing an inner conductor, a metallic sheath surrounding the core and serving as an outer conductor, and in some instances a protective jacket that surrounds the metallic sheath. A dielectric surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. In many known coaxial cable constructions, an expanded foam dielectric surrounds the inner conductor and fills the space between the inner conductor and the surrounding metallic sheath.
The design of coaxial cables has traditionally been a balance between the electrical properties (e.g., high signal propagation, and low attenuation) and the mechanical or bending properties of the cable. For example, in some coaxial cable constructions, air and plastic spacers are used between the inner conductor and the outer conductor to reduce attenuation and increase signal propagation of the cable. Nevertheless, the plastic spacers that are placed between the inner and outer conductors do not adequately support the outer conductor in bending. Thus, the outer conductor is subject to buckling, flattening or collapsing during bending which can render the cable unusable. One alternative has been to use a foam dielectric between the inner and outer conductors as described above. However, although the bending properties are improved, the rate at which the signals are propagated is typically reduced.
One recent advance in the coaxial cable industry for RF cables has been the construction of larger diameter cables. Larger diameter cables generally possess a greater average power rating and reduced attenuation over relatively smaller diameter cables. Unfortunately, however, because these cables have large diameters, they are typically not as flexible as their smaller diameter counterparts. As a result, there is a greater level of difficulty in installing these cables.
Another problem with larger diameter cables and cables generally is that moisture in the cable can corrode the conductors thus negatively affecting the electrical and mechanical properties of the cable. In particular, during installation of the cable, moisture can enter the cable at the connectors. This moisture can also travel within the cable through the foam or air dielectric or along interfaces in the cable, e.g., between a foam dielectric and a metallic sheath.
Several methods have been proposed to prevent moisture from entering the cable and being transported through the cable. For example, hydrophobic, adhesive compositions have been applied at interfaces in the cable to prevent moisture from moving along these interfaces. Water-blocking compositions have also been used at other locations in the cable to limit water transport in the cable. In addition, hydrophilic, moisture-absorbent materials have been used in cables to act as water-blocking materials. These hydrophilic materials not only water-block the cable but also remove moisture that is present in the cable.
Copending U.S. patent application Ser. No. 08/911,538 to Moe et al., filed on Aug. 14, 1997, which has been incorporated herein in its entirety by reference, proposes a new cable construction using an inner conductive tube thus reducing the cost of the cable and providing a cable with good flexibility. Although this new construction provides numerous benefits, using an inner conductive tube creates the possibility of moisture moving through the cable. In particular, moisture can enter the tube during connectorization especially in moist conditions. This moisture can be transported within the inner conductive tube thereby causing corrosion of the inner conductive tube. Therefore, there is a need in the art to avoid moisture transmission in this region of the cable and the attendant potential effects upon the electrical and mechanical properties of the cable.
SUMMARY OF THE INVENTION
The present invention provides a coaxial cable having good moisture-absorbing and water-blocking properties thereby limiting the negative effects of moisture on the electrical and mechanical properties of the cable and allowing the cable to maintain its signal transmission performance over time. Furthermore, the moisture-absorbent materials used in the cable typically bind the moisture away from the electrically conducting surfaces of the cable to prevent corrosion of these surfaces. The cables of the invention have excellent electrical properties, flexibility and bending properties. In addition, these cables avoid buckling, flattening or collapsing in bending, even in larger diameter cable designs. The coaxial cable of the invention can also be easily connectorized and can be produced at low cost.
The present invention provides a coaxial cable comprising an inner conductive tube, a moisture-absorbent material within the inner conductive tube, a dielectric surrounding the inner conductive tube, and a tubular metallic outer sheath surrounding the dielectric. The inner conductive tube can be supported with a plastic rod adjacent an inner surface of the inner conductive tube and the moisture-absorbent material is preferably provided between the plastic rod and the inner conductive tube or within the plastic rod. The plastic rod preferably comprises a foamed polymeric material but can be a solid polymeric material. In addition, the plastic rod can include a foamed polymeric layer and a solid polymeric layer surrounding the foamed polymeric layer with the moisture-absorbent material present either in the foamed polymeric layer or between the foamed polymeric layer and solid polymeric layer. The plastic rod can also be adhesively bonded to the inner conductive tube to provide water blocking at the interface between the plastic rod and the inner conductive tube.
In one embodiment of the invention, the plastic rod is a hollow plastic rod that includes a central structural member adjacent an inner surface of the plastic rod and that supports the plastic rod. In this embodiment, the moisture-absorbent material is preferably provided within the central structural member or on an interface between the central structural member and the inner surface of the plastic rod. The central structural member is preferably a fiber reinforced plastic rod or a metal wire. The central structural member can also include more than one moisture-absorbent material with each moisture-absorbent material having different characteristics or rates of moisture absorbency.
In another embodiment of the invention, a moisture-absorbent material layer is present adjacent the inner surface of the inner conductive tube and preferably in direct contact with the adhesive composition that contacts the inner surface of the inner conductive tube. Alternatively, a textile material such as a wick of woven cotton yarn can be provided within the inner conductive tube. This textile material could also include other moisture-absorbent materials that remove moisture in the cable.
The present invention also provides a method of making a coaxial cable. An inner conductive tube that includes a moisture-absorbent material adjacent an inner surface of the inner conductive tube is advanced along a predetermined path of travel. A polymer composition is applied onto the inner conductive tube and a tubular metallic outer sheath is formed onto and encircles the polymer composition to define a dielectric between the inner conductive tube and the outer sheath. The inner conductive tube can be provided by advancing a plastic rod comprising a moisture-absorbent material along a predetermined path of travel and forming the inner conductive tube onto the plastic rod. In this embodiment, the moisture-absorbent material can be applied to an outer surface of the plastic rod or t
Alston & Bird LLP
CommScope Properties LLC
Nguyen Chau N.
LandOfFree
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