MXPA99007249A - Aerially installed communications cable - Google Patents

Abstract

A communications cable is provided having a support strand (32) and at least one coaxial cable (34) helically wound around the support strand. The coaxial cable includes an inner conductor (40), a dielectric (42) surrounding the inner conductor, an outer tubular metallic sheath (44) surrounding the dielectric, and optionally a protective jacket (46) surrounding the metallic sheath. The communications cable is manufactured such that the ratio of the length of coaxial cable to the length of support strand is between about 1.005 and 1.015. The communications cable is also preferably wound around the support strand using a varying lay length thereby minimizing signal loss due to cable damage. The communications cable of the invention is especially suitable for aerial installation and may be installed in one pass, without the need to form expansion loops in the coaxial cable.

Description

CABLE OF COMMUNICATIONS INSTALLED OF AERIAL FORM FIELD OF THE INVENTION The present invention relates broadly to communication cables and more particularly to the aerial installation of communication cables suitable for the transmission of RF signals.

BACKGROUND OF THE INVENTION The coaxial cables currently commonly used for the transmission of FR signals include an inner conductor, a metal cover surrounding the inner conductor and serving as an outer conductor, and optionally a protective sheath surrounding the metal cover. A dielectric surrounds the inner conductor and isolates it electrically from the metallic cover that surrounds it. An illustrative cable construction uses an expanded foam dielectric to surround the inner conductor and fill the space between the inner conductor and the surrounding metal shell. In an alternative construction, an air dielectric coupled with disk-shaped polymer spacers is used to support the center conductor in spaced relation of the outer conductor.

A common use for these types of coaxial cable is as trunk and distribution cable for voice, data and video transmissions. Frequently, the coaxial junction and distribution cable is installed overhead, that is, hanging between service poles. A concern in the installation of the coaxial cable is the generally limited bending properties that are characteristic of the coaxial cable. Specifically, when installing the coaxial cable, care must be taken to avoid causing curls or bends in the coaxial cable because such curls or bends will adversely affect the signal propagation properties of the cable. Curls or folds can also serve as sites for cable structural failures after repeated cycles of thermal expansion and contraction due to daily and seasonal temperature changes. As illustrated in Figures 1-4, the conventional method for installing coaxial cable overhead is generally a time-consuming procedure. Typically, as shown in Figure 1, a supporting rope 10 or "messenger" is first installed by fixing the rope to a service pole 12 and directing it along pulleys 14 or by other means to successive service posts 16. As illustrated in Figure 2, the coaxial cable 18 is then installed by pulling the coaxial cable along the length of the support rope 10 using pulleys 20 hung from the support rope 10 or by other means. The coaxial cable 18 is then attached to the support rope 10 by attaching or connecting the coaxial cable to the support rope as shown in Figures 3 and 4. In spaced locations, the coaxial cable 18 is formed in expansion loops 24 as shown in Figure 4 to accommodate the expansion and contraction of the coaxial cable during daily and seasonal temperature changes. In the conventional installation method, numerous steps must be taken to install the communications cable overhead.

An alternative is to provide the coaxial cable and the support rope or messenger in the same protective case and hang the support rope 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 take thermal expansion into account.

Although expansion loops may sufficiently address the problem of thermal expansion and contraction, there are several problems associated with the use of expansion loops. For example, the expansion loops flex many times during the life of the cable. As a result, the voltage located on the expansion loop can lead to cable failure in the loop, thereby affecting a portion if not all of the signal propagated. The tendency of the expansion loop to fail therefore requires a premature replacement of the cable. Additionally, the need to manually form the expansion loops during installation provides an opportunity to create unwanted curls or bends in the cable. Additionally, as described above, the formation of expansion loops in the coaxial cable during its installation is time consuming.

BRIEF DESCRIPTION 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 communication cable of the invention is provided as a single unit thus allowing the communication cable to be installed quickly in a single step. In particular, the present invention provides a communication cable having a support rope and at least one coaxial cable wound around the support rope such that the coaxial cable can accommodate dimensional changes resulting from thermal expansion and contraction. More particularly, the coaxial cable is wound in the form of a helix or "braid" around the support or messenger cord using a specific ratio of coaxial cable length in excess to length of supporting rope. The coaxial cable includes an inner conductor, a dielectric surrounding the inner conductor, and an outer tubular metal shell 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 cover that surrounds the metal cover.

Coaxial cables installed in aerial form 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 can extend for hundreds or thousands of meters between the connectors. Consequently, the thermal expansion and contraction can generate very high tensile forces in the electrical connectors, which can degrade the signal propagation properties of the cable or even cause the coaxial cable to be disconnected from the connector, interrupting the cable system. It has been discovered, however, that by arranging the coaxial cable in a configuration wound helically around the supporting rope, and controlling the ratio of the length of the coaxial cable to the length of the supporting rope within prescribed parameters of between 1.005 and 1,015, the cable can effectively withstand shrinkage and severe thermal expansion without the need for expansion loops. In a preferred embodiment, this relationship is maintained between 1,006 and 1,010. The coaxial cable is also preferably wound around the support rope using variable lengths of lay, thus limiting the introduction of structural return losses (PRE) or periodic impedance inequalities which negatively affect the transmitted signal.

Cables using a braided configuration of conductors and messenger cord have been proposed here above for use in certain applications. For example, U.S. Patent No. 2,473,965 to Morrison et al., Shows a braided cable arrangement used for the transmission of electrical current. Small diameter coaxial cables braided with an insulated support have also been produced for certain specialized applications of low bandwidth, such as radio transmission. However, these prior art applications do not encounter the severe levels of tensile force during thermal contraction encountered by the larger diameter coaxial cables of the present invention. Furthermore, these prior art applications have not recognized the importance of adequately controlling the ratio of the length of the cable to the length of the support cord as taught by the present invention.

In addition to providing a communication cable as described above, the present invention includes a method for forming a communication cable. The method generally consists of advancing a tensioned support rope and advancing at least one coaxial cable consisting of an internal conductor, a dielectric surrounding the inner conduit, and an outer tubular metal cover that surrounds the dielectric. The advancing coaxial cable is helically guided around the supporting rope that advances along the length of the supporting rope while the ratio of the length of the coaxial cable to the length of the supporting rope is controlled between 1.005 and 1,015. As described above, the coaxial cable is also wound preferably helically around the support rope using variable lay lengths. The communication cable of the invention can be installed relatively quickly in a passage between service posts, without the need to form expansion loops in the coaxial cable. Therefore, the coaxial cable is not generally subject to the localized voltages that occur in the expansion loops. In addition, because the coaxial cable is wound helically around the support string with the ratio of the length of coaxial cable to the length of controlled support filament to between 1.005 and 1.015, the coaxial cable can suffer thermal expansion and contraction without disconnect from the connectors. Additionally, by varying the length of the coaxial cable laying around the support rope, any degradation of the transmitted FR signals resulting from the periodic damage to the coaxial cable is minimized. These and other features of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description which describes the preferred embodiments of the invention in the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figures 1-4 are schematic views showing a prior art method for aerially installing a communication cable by sequentially hanging a support rope, hanging a coaxial cable, fixing the coaxial cable to the support rope and forming expansion loops in the coaxial cable. Figure 5 is a perspective view of a communications cable installed in aerial form according to the invention.

Figure 6 is a cross-sectional view of the communication cable of Figure 5 taken along lines 6-6 of Figure 5 illustrating the support rope and the coaxial cable. Figure 7 is a schematic view and a method for forming a communication cable according to the invention. Figure 8 is a cross-sectional view taken along lines 8-8 of Figure 7 and showing how the coaxial cable is wound around the support rope. Figure 9 is a schematic view of a method for aerially installing a one-step communication cable in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Referring now to Figure 5, a communication cable 30 according to the invention is shown consisting of a support rope 32 and coaxial cable 34. The coaxial cable 34 is wound helically around the support rope 32 along of the length of the support rope. Although only one coaxial cable 34 is illustrated in Figure 5, one or more additional coaxial cables oriented parallel to the coaxial cable 34 may also be wound helically around the support cord 32. Additionally, other types of cables may also be wound helically around of the supporting rope 32 parallel to the coaxial cable 34. The communication cable 30 is typically installed in aerial form and hung between two predetermined locations, at least one of which is preferably raised. For example, the communication cable 30 may be hung between a service post 36 and a second location, typically a second service post. The communication cable 30 is fixed to the service pole 36 normally attaching the support rope 32 to the pole by any suitable means such as brackets 38. The communication cable 30, and particularly, the coaxial cable 34, is typically used for the transmission of FR signals for broadband telecommunications applications such as data applications, voices, and video applications. The support rope 32 used in the communication cable 30 of the invention is preferably relatively strong to support the weight of the coaxial cable 34 or cables wound helically around the support rope. A material especially suitable for the support rope 32 is a galvanized steel cable. The support rope 32 may be additionally surrounded by a protective cover (not shown) if desired. Suitable materials for the protective cover include thermoplastic coating materials such as polyethylene, polyvinyl chloride, polyurethane and various gums.

The coaxial cable 34 used in the invention is generally of the type that transmits RF signals, such as for broadband applications having a bandwidth of up to 1 GHz, ie applications for voice, data and video. In particular, the trunk and communications cable commonly used for these applications can be used. As shown in Figure 6, the coaxial cable 34 consists of an inner conductor 40, a dielectric 42 surrounding the inner conduit, and a metal cover 44 which acts as an outer conductor surrounding the dielectric. The coaxial cable 34 may additionally include a protective cover 46 surrounding the metal cover as illustrated in Figures 5 and 6. In the coaxial cable 34, the inner conductor 40 is formed of a suitable electrically conducting material such as copper or aluminum. Preferably, the inner conductor 40 is solid copper, copper tube or copper-coated aluminum. In the illustrated embodiment, only a single internal conductor 32 is shown, since this is the most common arrangement for coaxial cables of the type used to transmit RF signals. The inner conductor 40 is surrounded by a dielectric 42 as air or a polymeric material. Typically, when the air is used as the dielectric material, longitudinally spaced polymer disks are used as spacers between the inner conductor 40 and the metal shell 44. Preferably, however, the dielectric 42 is a solid continuous polymeric material and may be adhesively adhered to inner conductor 40 using a suitable adhesive such as an ethylene-acrylic acid copolymer. Exemplary polymers for the dielectric 42 include polyethylene, polypropylene, and polystyrene. Preferably, in order to reduce the density of the dielectric and thereby reduce the dielectric constant, the dielectric must be a closed cell expanded foam dielectric. Preferably, the foam dielectric is high density polyethylene or a mixture of high density and low density polyethylene. Typically, the foam dielectric has a density of less than 0.28 g / cc. Surrounding the dielectric 42 closely is a metal cover 44 tubular exterior. Preferably, the cover 44 is adhesively bonded to the dielectric 42 using a suitable adhesive such as an ethylene-acrylic acid copolymer to support the cover during the bending of the coaxial cable 34. The cover 44 is also preferably characterized to be mechanically and electrically continuous. This allows the cover 44 to effectively serve to mechanically and electrically seal the cable against external influences as well as to seal the cable against RF radiation leakage. The metal cover 44 can be formed of several electrically conductive metals such as copper or aluminum. For voice, data and video applications, the outer diameter of the metal cover is typically between 1.25 and 2.54 cm. As stated above, the outer surface of the metal cover 44 may be surrounded by a protective sheath 46. Suitable compositions for the outer protective sheath 46 include thermoplastic coating materials such as polyethylene, polyvinyl chloride, polyurethane and various gums. Typically, the protective sheath 46 is adhesively adhered to the cover 44 with a suitable adhesive such as an ethylene-acrylic acid copolymer. Preferably, the coaxial cable 34 used in the present invention is designed to have good flexibility, i.e. improved bending characteristics, thereby allowing the coaxial cable to be easily formed in a helical configuration around the supporting rope 32 without cause curls, bends or other defects in the coaxial cable.

Desirably, the coaxial cable used in the present invention should have a minimum bend radius of less than 10 cable diameters. The minimum bending radius is determined by progressively bending the cable over small and increasingly smaller spindles of uniform radius. After each bend, the wire is examined for any sign of rippling or curling. The smallest radius spindle in which the first wavy signs occur is defined as the minimum bend radius. In order to provide a coaxial cable 34 having the desired flexibility and bending characteristics, a relatively thin metal cover 44 is preferably used. The preferred coaxial cable for use in the present invention has a tubular metal cover 44 with a wall thickness selected to maintain a G / D ratio (ratio of wall thickness to outside diameter) of less than 2.5%. In addition, the adhesive bond of the cover 44 to the foam dielectric 42 increases the flexibility of the coaxial cable 34 by supporting the cover 44 in the fold to prevent damage to the coaxial cable. Additionally, the increased stiffness of the core (the inner conductor 40 and dielectric 42) in relation to the rigidity of the cover 44 is beneficial for the bending characteristics of the coaxial cable 34.

Specifically, the coaxial cables 34 used in the invention preferably have a core to shell stiffness ratio of at least . A preferred cable having adequate flexibility for use in the invention is QR cable available from CommScope, Inc. in Hickory, North Carolina. The core to deck stiffness ratio described above is determined by independently evaluating the compressive stiffness of the core (inner conductor 40 and dielectric 42) and outer conductor 44 as would be observed from its side. A sample of fixed length (2.54 cm) of core and outer conductor is placed in a compressive loading facility (universal tester) and deflected by a defined amount. For the core and the outer conductor, this deviation is defined as 12% of its respective diameter. The stiffness ratio is then expressed as a ratio of the recorded loads to the defined deviation. As will be readily understood by one skilled in the art, coaxial cable 34, used in the invention, and specifically, the conductors used in the coaxial cable 34 are subjected to thermal expansion and contraction due to daily and seasonal temperature changes which can cause flexing in the cable and possibly damage to the cable. Advantageously, because the coaxial cable 34 is wound helically around the supporting rope 32 without the existence of expansion loops, the thermal expansion (contraction) of the coaxial cable is distributed through the cable and does not result in voltage localized to the cable . Additionally, the coaxial cable 34 is preferably wound helically around the support rope 32 using variable run lengths L thereby limiting the introduction of structural return loss (SRL) or periodic impedance inequalities that adversely affect the transmitted FR signal. . The coaxial cable 34 used in the invention is preferably wound tightly around the support rope 32 such that the coaxial cable 34 contacts the support rope along the majority of the length of the support rope. Therefore, the coaxial cable 34 is supported by the support rope 32 without the need to tie or tie the coaxial cable to the support rope. The tightening of the coaxial cable 34 around the support rope 32 can be described as a ratio of the length of coaxial cable used in the communication cable 30 to the length of the support rope 32 used in the communication cable. For example, the excess length of the coaxial cable 34 per 30.48 meters of support rope 32 is between 15.24 and 45.72 cm and typically between 20.32 and 30.48 cm. therefore, the ratio of the length of the coaxial cable 34 straight to the length of the support rope 32 is between 1.005 and 1.015 and is typically between 1.006 and 1.010. As will be understood, the excess length, and therefore the above relationship, can also be determined by straightening the coaxial cable 34 used in the communication cable.

It has been found that the ratio of the coaxial cable length 34 straight to the length of the support rope 32 is critical for the effective fabrication and installation of the braided communications cables of the invention. Specifically, in braided communications cables having a ratio below 1.005, the coaxial cable is subjected to high voltage forces during periods of thermal contraction which can cause the coaxial cable to be disconnected from the connectors. Also, if this ratio is larger than 1015, the coaxial cable is too loosely wound around the support rope and the manufacture of the braided cables becomes difficult to control. In this case, the coaxial cable may collide with the processing machinery during manufacture resulting in damage to the coaxial cable. In addition, the coaxial cable tends to form large loops during manufacture that make it difficult to assemble the cable on reels and can make cable installation on service poles extremely difficult. This relationship has been particularly critical for the manufacture and installation of large diameter cables, ie, cables wherein the diameter of the tubular metal cover 44 is larger than 1.25 cm. The communication cable 30 of the invention is constructed before installation. The method for forming the communication cable 30 consists of advancing the support rope 32 and the coaxial cable 34 from supply reels 50 and 52, respectively, by unwinding the support rope and the coaxial cable from the reels. The support rope 32 and the coaxial cable 34 are preferably stretched with a predetermined amount of tension from the reels 50 and 52, by tensioners 54. The support rope 32 and the coaxial cable 34 are subsequently directed into a stringer 56 that aligns the support rope and the coaxial cable in a parallel orientation. The support rope 32 and the coaxial cable 34 then advance into an end effector 58 which contains diverting means for bending the coaxial cable in a helix configuration tightly around the support rope. Preferably, a series of rollers 59 deflect and guide the coaxial cable 34 around the support rope 32.

As shown in FIG. 8, the path P of the coaxial cable 34 around the support rope 32 is generally circular and can be either clockwise or counter-clockwise. As stated above, the coaxial cable 34 is preferably flexible to allow the coaxial cable to be wound helically around the support rope 32 without causing damage to the coaxial cable. In addition, one or more additional cables (for example coaxial cables) may be aligned parallel to the coaxial cable 34 and wound helically around the supporting rope 32. As described above, the coaxial cable of the invention is manufactured in such a way that the ratio of the length of the coaxial cable 32 straight to the length of the support rope 32 is between 1,005 and 1,015 and typically between 1,006 and 1,010. In addition, the coaxial cable 34 is wound around the support 32 using lengths L of variable lay. As illustrated in FIG. 5 and for objects of the present, the run length L is defined as the distance between the points at which the center 64 of the coaxial cable 34 crosses directly on the center 66 of the support rope 32. Preferably, for the coaxial cable 34 typically used in the present invention, the length L of the array ranges within a predetermined range of between 60.96 and 81.28 cm. For example, the length L of laying can range between 63.5 and 68.58 cm or between 66.04 and 76.20 cm. The length L of varied lining prevents periodic structural damage to the coaxial cable 34 and therefore limits the formation of structural return loss (PRE) or periodic impedance inequalities that negatively affect the transmitted FR signal, as dissipating the signal corresponding to a certain frequency scale. As shown in Figure 7, the coaxial cable 34 leaves the end effector 58 wound helically around the support rope 32 to form the communication cable 30. The communication cable 30 is continuously stretched by the localized stretching means 54 down the end effector 58 and can be assembled on a suitable container, such as a reel 60 for storage and shipping. The communication cable 30 of the invention is especially suitable for aerial installation in which at least one of the locations on which the communication cable is fixed is raised from the floor. As illustrated in Figure 9, a predetermined length of communications cable 30 is provided by unwinding the communication cable from a suitable container such as a reel 60. The communications cable goes to a first aerial location as a first service post 62 and a first location on the support rope 32 is fixed to the service pole by means suitable as clamps 18 (Figure 1). A length of the communication cable 30 is then directed from the first service pole 62 to a second aerial location as a second service pole 64. The communications cable may be directed aerially by suitable means such as a pulley 66. Once the communications cable 30 has been directed to the second service post 64, a second location on the support rope 34 is then fixed to the second service post 64 and the support rope 34 is generally tensioned until it is substantially tensioned. The communication cable 30 can then be installed in other aerial locations in the same way.

The installation of the communication cable 30 is a one-step procedure and does not require separate installation of the support rope 32 and the coaxial cable 34. Therefore, the installation can be achieved relatively quickly. In addition, due to the helical winding of the coaxial cable 34 around the support rope 32, the expansion loops in the coaxial cable are not necessary to prevent damage by thermal expansion. Additionally, the communication cable 30 of the invention is not subjected to localized voltage by the thermal expansion because the expansion is distributed along the length of the coaxial cable 34. As described above, the length L of cable laying coaxial 34 around the support rope 32 is varied. As a result, if periodic damage occurs to the coaxial cable 34, the degradation of the FR signals transmitted by the coaxial cable will be minimized.

It is understood from the reading of the above description of the present invention, that one skilled in the art could make changes and variations thereto. These changes and variations are included in the spirit and scope of the following appended claims.

Claims (9)

NOVELTY OF THE INVENTION CLAIMS

1. - A communication cable for aerial installation comprising a support rope and at least one coaxial cable wound helically around the support rope so that the ratio of the length of coaxial cable to the length of supporting rope is between 1.005 and 1.015, said coaxial cable consisting of an inner conductor, a dielectric surrounding the inner conductor, and an outer tubular metal shell surrounding the dielectric.

2 - . 2 - The communication cable according to claim 1 further characterized in that the ratio of the length of coaxial cable to the length of supporting rope is between 1,006 and 1,010.

3. - The communication cable according to any of the preceding claims further characterized in that the tubular cover has a diameter between 1.27 and 2.54 cm.

4. - The communication cable according to any of the preceding claims further characterized in that the tubular cover has a diameter of more than 1.27 cm.

5. - The communication cable according to any of the preceding claims further characterized in that the coaxial cable is wound helically around the support rope using a variable length of laying.

6. - The communication cable according to any of the preceding claims further characterized in that the coaxial cable is wound helically around the support rope using a length of lay that ranges within a predetermined range of between 60.96 and 81.28 cm.

7. - The communication cable according to any of the preceding claims further characterized in that at least one coaxial cable consists of two or more coaxial cables aligned parallel to one another.

8. - The communication cable according to any of the preceding claims further characterized in that the coaxial cable additionally consists of a protective sheath surrounding the metal cover.

9. - A method for forming a communication cable as described in any of the preceding claims, the method consists of the steps of: advancing a tensioned support rope; advancing at least one coaxial cable consisting of an inner conductor, a dielectric surrounding the inner conductor, and an outer tubular metal shell surrounding the dielectric; and simultaneously driving the coaxial cable that advances helicely around the supporting rope that advances along the length of the supporting rope and controlling the ratio of the length of the coaxial cable to the length of the supporting rope between 1.005 and 1.015