Electricity: conductors and insulators – Overhead – Ground clamps and cable clips
Reexamination Certificate
1998-02-05
2001-03-20
Kincaid, Kristine (Department: 2831)
Electricity: conductors and insulators
Overhead
Ground clamps and cable clips
C174S041000, C174S1130AS, C385S100000, C385S101000
Reexamination Certificate
active
06204445
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates broadly to communications cable and more particularly to the aerial installation of communications cable suitable for the transmission of RF signals.
BACKGROUND OF THE INVENTION
The coaxial cables commonly used today for the transmission of RF signals include an inner conductor, a metallic sheath surrounding the inner conductor and serving as an outer conductor, and optionally a protective jacket which surrounds the metallic sheath. A dielectric surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. One exemplary cable construction uses an expanded foam dielectric to surround the inner conductor and fill the space between the inner conductor and the surrounding metallic sheath. In an alternative construction, an air dielectric coupled with polymer spacers in the form of disks is used to support the center conductor in spaced relation from the outer conductor.
One common use for these types of coaxial cable is as trunk and distribution cable for voice, data and video transmissions. Often, the coaxial trunk and distribution cable is installed aerially, e.g., hung between utility poles. One concern in the installation of the coaxial cable is the generally limited bending properties which are characteristic of coaxial cable. Specifically, in installing the coaxial cable, care must be exercised to avoid causing crimps or bends in the coaxial cable because any such crimps or bends will adversely affect the signal propagation properties of the cable. The crimps or bends may also serve as sites for structural failure of the cable after repeated cycles of thermal expansion and contraction due to seasonal and daily temperature changes.
As illustrated in
FIGS. 1-4
, the conventional method of installing aerial coaxial cable is generally a time consuming process. Typically, as shown in
FIG. 1
, a support strand
10
or “messenger” is installed first by affixing the strand to a utility pole
12
and directing it along pulleys
14
or by other means to successive utility poles
16
. As illustrated in
FIG. 2
, the coaxial cable
18
is then installed by pulling the coaxial cable along the length of the support strand
10
using pulleys
20
hung from the support strand or other means. The coaxial cable
18
is then affixed to the support strand
10
by lashing or tying the coaxial cable to the support strand as shown in
FIGS. 3 and 4
. At spaced locations, the coaxial cable
18
is formed into expansion loops
24
as shown in
FIG. 4
to accommodate the expansion and contraction of the coaxial cable during seasonal and daily temperature changes. In the conventional installation method, numerous steps must be performed to aerially install the communications cable.
One alternative is to provide the coaxial cable and the support strand or messenger in the same protective jacket and to hang the support strand and the coaxial cable in the same step. However, this particular construction still requires the separate step of forming expansion loops in the coaxial cable to account for thermal expansion.
Although expansion loops may sufficiently address the problem of thermal expansion and contraction, there are various problems associated with the use of expansion loops. For example, the expansion loops are flexed many times during the life of the cable. As a result, localized stress on the expansion loop may lead to cable failure in the loop, therefore affecting a portion if not all of the propagated signal. The tendency of the expansion loop to fail thus necessitates early replacement of the cable. Additionally, the necessity of manually forming expansion loops during installation provides an opportunity for creating undesired crimps or bends in the cable. Furthermore, as described above, the formation of expansion loops in the coaxial cable during its installation is time-consuming.
SUMMARY OF THE INVENTION
The present invention provides a communications cable that can undergo thermal expansion and contraction without the need for expansion loops. In addition, the communications cable of the invention is provided as a single unit thereby allowing the communications cable to be installed quickly in one pass. In particular, the present invention provides a communications cable having a support strand and at least one coaxial cable wound about the support strand in such a way that the coaxial cable can accommodate dimensional changes resulting from thermal expansion and contraction. More particularly, the coaxial cable is helically wound or “stranded” about the support strand or messenger using a specified relationship of excess coaxial cable length to support strand length. The coaxial cable includes an inner conductor, a dielectric surrounding the inner conductor, and an outer tubular metallic sheath surrounding the dielectric. Preferably, the dielectric is an expanded foam dielectric such as a closed cell polyethylene foam. The coaxial cable may additionally include a protective jacket which surrounds the metallic sheath.
Aerially installed coaxial cables of the type to which the present invention is directed are connected at their ends to other components in the cable system by electrical connectors. The cable may extend for hundreds or thousands of feet between connectors. Consequently, thermal expansion and contraction can generate very high tensile forces at the electrical connectors, which can degrade the signal propagation properties of the cable or even cause the coaxial cable to pull out of the connector, disrupting the cable system. It has been discovered, however, that by disposing the coaxial cable in a helically wound configuration around the support strand, and by controlling the ratio of the length of the coaxial cable to the length of the support strand within prescribed parameters of between 1.005 and 1.015, the cable can effectively withstand severe thermal expansion and contraction without the need for expansion loops. In a preferred embodiment, this ratio is maintained at between 1.006 and 1.010. The coaxial cable is also preferably wound around the support strand using varying lay lengths thereby limiting the introduction of structural return loss (SRL) or periodic impedance mismatches which negatively affect the transmitted signal.
Cables using a stranded configuration of conductors and messenger strand have been proposed heretofore for use in certain applications. For example, U.S. Pat. No. 2,473,965 to Morrison et al. shows a stranded cable arrangement used for transmission of electrical power. Small diameter coaxial cables stranded with an insulated support have also been produced for certain specialized low bandwidth applications, such as radio transmission. However, these prior applications do not encounter the severe levels of tensile force during thermal contraction that are encountered by the larger diameter coaxial cables of the present invention. In addition, these prior applications have not recognized the importance of properly controlling the ratio of the cable length to support strand length as is taught by the present invention.
In addition to providing a communications cable as described above, the present invention includes a method of forming a communications cable. The method generally comprises advancing a tensioned support strand and advancing at least one coaxial cable comprising an inner conductor, a dielectric surrounding the inner conductor, and an outer tubular metallic sheath surrounding the dielectric. The advancing coaxial cable helically is guided around the advancing support strand along the length of the support strand while the ratio of the length of the coaxial cable to the length of the support strand is controlled at between 1.005 and 1.015. As described above, the coaxial cable is also preferably wrapped helically around the support strand using varying lay lengths.
The communications cable of the invention may be installed relatively quickly in one pass between utility poles, without the need to form expansion loops in the coaxial cable. Th
Bryant, Jr. Andy
Gialenios Michael Damon
Irvin David Jonathan
Alston & Bird LLP
CommScope Properties LLC
Kincaid Kristine
Walkenhorst W. David
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