Abstract:
This internally shielded telephone cable provides more efficient shielding for meeting near-end crosstalk requirements in carrier systems having expanded channel capacity and increase in equivalent frequency from 772 kHz to 1.576 MHz or higher. The improved shielding uses tapes bent into configurations that obtain greater shielding area from a single tape, and constructions that are less expensive than prior art and produce a mechanically stronger and more unitary cable structure.

Description:
RELATED PATENT 
     This invention is an improvement on the construction disclosed in U.S. Pat. No. 3,803,340, issued Apr. 9, 1974. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Continued expansion of channel capacity increases frequency bandwidth and thus necessitates improvement in shielding efficiency. The recent development of 48 channel PCM carrier system (Tl-C) and consequently increase in the equivalent frequency from 772 kHz to 1.576 MHz put additional demand on shielding efficiency of the internal shield. The near-end crosstalk requirements were tentatively established at 80dB worst case power sun, and our design per U.S. Pat. No. 3,803,340 was found to be marginal in performance, especiall in small pair count cables. 
     The improved shields require the use of more metal for shielding and necessitate changes in the method by which the cables are manufactured, but by novel configuarations of the shields, some of the cost of added material can be offset by more efficient use of the shield areas and the mechanical strength of the cables can be increased to withstand better the rigors of installation. 
     Internally shielded cables should be competitive costwise with the two separate cable approach; that is, each standard cable used for a different direction of transmission. The preferred embodiments of this invention are more economical than the use of different cables for each direction of transmission of signals. Shielded telephone cables are normally buried, and consequently their overall shields are used for lightning protection. 
     Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views: 
     FIG. 1 is a diagrammatic cross-sectional view of a shielded cable such as shown in our patent referred to above; 
     FIG. 2 is a diagrammatic cross-section of a cable having more shielding and constructed to meet the more exacting requirements of present telephone transmission practices; 
     FIGS. 3 and 4 show modifications of the construction shown in FIG. 2 and illustrate the preferred embodiments of this invention; 
     FIG. 5 is a greatly enlarged, fragmentary sectional view showing the metal shielding with corrosion protecting coating on both sides; and 
     FIG. 6 is an enlarged fragmentary sectional view taken on the lines 6--6 of FIG. 4. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows the prior art in which a cable 10 has two groups of conductors for carrying the messages in opposite directions. One group is designated by the reference character 12, and the other group by the reference character 14. The group 12 is enclosed in a metal shield or screen 16 having a diametral portion 18 which shields the group of conductors 12 from the conductors 14 of the other group. A core wrap 20 passes around both groups 12 and 14 to hold them together and to serve as a dielectric spacer between the conductors 12 and 14 and an outer shield 22. 
     This core wrap 20 also spaces the shield 16 from the outer shield 22. This construction of FIG. 1 represents the prior art, such as that in our U.S. Pat. No. 3,803,340, previously referred to. 
     FIG. 2 shows a telephone cable 26 which has conductors 28 for transmitting messages in one direction, and conductors 30 for transmitting signals in the other direction. These groups of conductors 28 and 30 are each of generally semi-circular cross-section, and the conductors of each group are bound together by a plastic core wrap 32 around the conductors 28, and a separate core wrap 34 around the conductors 30. These core wraps are preferably plastic tapes. 
     Beyond the core wraps 32 and 34, there are metal shields 36 and 38, respectively, which are preferably corrugated and in contact with the core wraps 32 and 34, respectively. 
     This shielding 36 and 38 serves the dual purpose of improved isolation between directions of transmission as well as lightning protection. Its conductivity is made equivalent to standard 8 mil thick overall aluminum shielding by suitable selection of thickness and length of overlaps. Both shields 36 and 38 are of the type having the aluminum surface coated on both sides with acrylic acid copolymer polyethylene, and are made to adhere to each other along the portions of the shields 36 and 38 which extend across the diameter of the cable in contact with one another. This adhesion is obtained from a separate laminating operation or by the heat which is present when a plastic jacket 40 is extruded over the entire circumferential surfaces of the shields 36 and 38. The outer jacket 40 also adheres to the outside surfaces of the shields 36 and 38. 
     The adhesion between the confronting diametral portions of the shields 36 and 38 is very important in providing the cable with mechanical strength, since it secures the opposite halves of the cable core together and results in a unibody construction of the cable core. This prevents possible distortion and damage of the cable during installation. 
     Another advantage in bonding the confronting faces of the shields 36 and 38 to one another is that it eliminates a possible water channel between the shields. 
     FIG. 3 shows a modified construction and parts in FIG. 3 which correspond to those in FIG. 2 are indicated by the same reference character with a prime appended. Instead of two shields 36 and 38, each with its own lap seam 42 and 44, respectively, the construction shown in FIG. 3 has the shields around the different groups of conductors formed from a single tape. 
     This tape shield, designated by the reference character 46, extends from a location near the top of the group of conductors 30&#39;, and around the circumferential portion of the group of conductors 28&#39; to a location at the bottom of the group 28&#39;. At this location, designated by the reference character 48, the tape 46 is bent so as to extend upwardly between the confronting, generally flat faces of the groups of conductors 28&#39; and 30&#39;. As the tape shield 46 extends upwardly to the upper end of the group of conductors 30&#39;, it approaches its starting location and then bends to the right in FIG. 3 and extends for a short distance in contact with the beginning of this tape 46. 
     After passing beyond the beginning of the tape 46, it extends circumferentially around the arcuate part of the group of conductors 30&#39; until it overlaps for a short distance in contact with the circumferential portion of its width, which extends downward around the left-hand side of the group of conductors 28&#39;. 
     Both ends of the tape 46 are preferably bonded to the portions of its length which the ends overlap so as to brace the tape shield 46 against distortion, particularly at its longitudinal edges and at its flat portion which extends between the groups of conductors 28&#39; and 30&#39;. In order to further strengthen this flat portion of the tape shield 46, a flat reinforcing tape 50 may be optionally bonded to the flat portion of the tape 46, so as to give the tape 46, in effect, increased thickness and greater stiffness over the flat area. If the tape 50 is of the same thickness as the tape 46, then the straight flat portion of shielding between the groups of conductors 28&#39; and 30&#39; is twice as thick as the curved portions of the tape shield 46; and this provides greatly increased crosstalk isolations as well as strength in a diametral plane without impairing the flexibility of the cable in directions transverse to this plane. The confronting faces of the tape 50 and the flat portion of the tape 46 are bonded together, as in FIG. 2. 
     FIG. 4 shows another modified construction, in which two tapes are used and they are bonded to one another at locations other than the confronting flat faces between the two groups of conductors. Parts in FIG. 4 corresponding to those in FIG. 2 are indicated by the same reference characters with a letter &#34;a&#34; appended. A shielding tape 54 starts at the upper end of the group of conductors 28a and extends around the cylindrical portion of the group of conductors 30a, and then upwardly between the two groups of conductors to form a portion 56 of the shield 54; and the upper end of this portion 56 does not connect with the rest of the tape 54. 
     Another tape shield 58 has one end bonded to the tape 54 at a location 60 near the bottom of the cable. This tape shield 58 then extends to the left, in FIG. 4, around the curved side of the group of conductors 28a to the top of the cable where the shield 58 is bonded to the shield 54 at a location 62. By bonding the tapes 54 and 58 to one another across the entire areas of the portions 56 and 66; and by bonding the edge of each tape to a portion of the other tape across their lapped areas, the opposite halves of the cable core are firmly secured together to provide a unitary structure, and the procedure for manufacturing the cable is facilitated as compared with the construction shown in FIG. 3. 
     The shielding tape of FIGS. 3 and 4 can consist of a suitable thickness of metal of a single tape and this approach eliminates the necessity of laminating two tapes along the diameter portion of the cable assembly. The industry requirement for a shielding tape, which extends circumferentially around a communication cable, is that the shield be 8 mils in thickness. This invention uses wider tape which has a total width greater than the circumference of the cable core; and the same total cross-section can be obtained with thinner tape. In other words, the thickness of the tape of this invention is chosen so that the total cross-section of the wider tape is equal to the cross-section of an 8 mil circumferential tape. 
     FIG. 5 is a detail sectional view showing the shield 36 with the corrosion protecting coating 70 adhered to the opposite faces of the tape shield 36. This is the preferred construction of the shields in all of the other figures. 
     FIG. 6 shows the circumferentially corrugated shield 58 with its inner lobes in contact with the core wrap 32a, and its outer surface in contact with the extruded jacket 40a. 
     A number of constructions have been shown and described. Those shown in FIGS. 3 and 4 are the preferred embodiment of the invention. Changes and modifications can be made in the illustrated construction without departing from the invention as defined in the claims.