Electricity: conductors and insulators – Conduits – cables or conductors – Conductive armor or sheath
Reexamination Certificate
1998-05-01
2001-06-12
Reichard, Dean A. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Conductive armor or sheath
C174S109000, C174S1210AR
Reexamination Certificate
active
06246006
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a shielded cable and more particularly, to a non-braided drop cable for the transmission of RF signals.
BACKGROUND OF THE INVENTION
In the transmission of RF signals such as cable television signals, a drop cable is generally used as the final link in bringing the signals from a trunk and distribution cable directly into a subscriber's home. Conventional drop cables include an insulated center conductor that carries the signal and a conductive shield surrounding the center conductor to prevent signal leakage and interference from outside signals. In addition, the drop cable generally includes a protective outer jacket to prevent moisture from entering the cable. One common construction for drop cable includes an insulated center conductor, a laminated tape formed of metal and polymer layers surrounding the center conductor, a layer of braided metallic wires, and an outer protective jacket.
One problem with conventional braided drop cable is that it is difficult to attach to standard connectors. In particular, the braided shield is difficult to cut and attach to a standard connector and normally must be folded back over the cable jacket during connectorization of the cable. As a result, the metal braid increases installation time and costs. Furthermore, forming the metal braid is generally a time intensive process and limits the rate at which the cable can be produced. Therefore, there have been attempts in the industry to eliminate the braid from conventional drop cable.
For example, U.S Pat. Nos. 5,321,202; 5,414,213; and 5,521,331 to Hillburn teach replacing the outer braided shield of the conventional construction with a metallic foil shield or laminated metallic tape shield and adding a plastic layer between this shield and the inner shielding tape. Although this construction eliminates metal braids, it creates other connectorization problems. Specifically, when connectors are attached to these cables, a special coring or trimming tool is required to prepare the cable for the connector to be attached to the cable. This requires additional time during the connectorization of these cables.
Furthermore, the connector pull-off force of the braidless cable, i.e., the force needed to pull the connector off of the cable, is undesirably reduced as compared to braided cables.
SUMMARY OF THE INVENTION
The present invention provides a non-braided drop cable that can be easily attached to a connector and that can properly anchor a connector to prevent connector pull-off once the cable is connectorized. Furthermore, the present invention provides a drop cable with sufficient shielding to prevent signal leakage and interference from extraneous signals.
These features are provided by a non-braided shielded cable that includes a cable core comprising a center conductor and a dielectric layer surrounding the center conductor, a first electrically conductive shield surrounding the cable core and bonded thereto, a second electrically conductive shield surrounding the first shield, and a cable jacket surrounding the second shield and bonded thereto. According to the invention, an interstitial layer is located between the first and second shields and is composed of elongate strands disposed between said first and second shields so as to be freely displaceable axially while also serving to space the first and second shields apart from one another.
In a preferred embodiment of the invention, the first and second shields used in the cable are bonded metal-polymer-metal laminate tapes extending longitudinally of the cable and having overlapping longitudinal edges to produce 100% shielding coverage of the center conductor. Preferably, the first shielding tape is an aluminum-polyolefin-aluminum laminate tape and the second shielding tape is an aluminum-polyester-aluminum laminate tape. The strands of the interstitial layer are typically helically wound around the first shielding tape and are formed of metal wires and/or textile yarns. Preferably, these strands are metal wires covering less than 30 percent of the surface of the underlying first shielding tape. The metal wires can be provided as more than one layer having different orientations such as two layers have opposite helical orientations (e.g., counterclockwise and clockwise). The yarns for the interstitial layer typically cover less than 50 percent of the surface of the first shielding tape and are selected from the group consisting of polyester, cotton and aramid yarns and blends thereof. The interstitial layer can include both yarns and metal wires disposed alongside the yarns, and can also include a water blocking material.
The present invention also provides a method of making a shielded cable. In the manufacture of these cables, a cable core comprising a center conductor and a dielectric layer surrounding the center conductor is advanced and a first electrically conductive shielding tape is longitudinally wrapped or “cigarette-wrapped” around the cable core. The interstitial layer is applied to the first shielding cable typically by helically wrapping the strands around the first shielding tape. A second shielding tape is then longitudinally wrapped over the interstitial layer and a cable jacket extruded over the second shielding tape to produce the cable. Preferably, the method further comprises bonding the first shielding tape to the cable core and bonding the second shielding tape to the jacket. The shielding tapes are preferably bonded metal-polymer-metal laminate tapes having longitudinal edges that are positioned in an overlapping relationship. These laminate tapes also preferably include an adhesive on one surface thereof, with the first shielding tape including an adhesive on the inwardly facing surface adjacent the cable core and the second shielding tape including an adhesive on the outwardly facing surface over which the outer jacket is extruded to provide the desired bonds in the shielded cable.
The shielded cables of the invention are easy to attach to standard connectors. Specifically, because the shielded cable is not braided, the problems associated with braids are not experienced during connectorization of the shielded cable of the invention. In addition, the interstitial layer in the cable of the invention is composed of strands that are axially displaceable and thus do not require trimming prior to connectorization. Furthermore, these axially displaceable strands assist in anchoring the connector to the cable, thus increasing the pull-off resistance of the cable.
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Hardin Stanley D.
Story Christopher A.
Wessels Robert A.
Alston & Bird LLP
CommScope Properties LLC
Nguyen Chau N.
Reichard Dean A.
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