Coaxial cable

[57] ABSTRACT

A coaxial cable has an outer conductor bonded to the cable jacket and an overlapped longitudinal seam. The seam is sealed with a layer of adhesive and has a bare stripe, free from adhesive, to allow metal-to-metal electric contact.

Claims, 4 Drawing Figures

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PATENTED MAY 919/2

3,662,090

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COAXIAL CABLE

BACKGROUND OF THE INVENTION

This invention comprises an improvement in coaxial cables of the type of which FIG. 1 of Tomlinson’s U.S. Pat. No. ⁵3,315,025, dated Apr. 18, 1967, illustrates a commercially successful embodiment. Such cables have application in the transmission of high frequency signals, particularly television signals, and are characterized by the fact that a metal tape, constituting the outer conductor is coated with a layer of ad- ¹⁰hesive that bonds it to the cable jacket and also bonds together an overlapping longitudinal seam in the tape itself. Cables of this type have been commercially and operationally successful. It is however apparent that where the outer conductor is overlapped some redundancy results, and that a reduction in the overlapped area would afford an economy. The required area of the overlapping portion is determined, in part, by the “window” in the shielding which results from the adhesive coating, even though the coating is very thin. Actually, any 20 thickness of coating at all prevents the complete enclosure of the cable core by an electrically continuous cylinder, and, at the very high frequencies for which the subject cables are intended, extra overlapping of the outer conductor is required to prevent any substantial shielding leakage. 25

SUMMARY

I have found that all the advantages of known cables can be retained and the amount of overlapping of the shielding reduced by overlapping the shielding with metal-to-metal contact. To this end I have invented a coaxial television cable that comprises an inner conductor, a cylindrical wall of insulation surrounding the conductor, and a metal tape surrounding the wall of insulation. The tape comprises a longitudinal seam ,, which is formed from a lengthwise strip along the edge of the tape and this strip overlaps a portion of the tape. My cable also comprises a solid, abrasion-resistant, synthetic-resin jacket surrounding the tape and a layer of adhesive which bonds the entire outer surface of the tape, which is not covered by the strip, to the jacket and also bonds the tape to the overlying area of the strip. The outer surface of the tape has a longitudinal bare stripe, free from adhesive, under the strip and in electrical contact with it. This stripe may comprise an edge of the tape or, in other embodiments of my invention, be 45 removed from the edge with the edge being bonded to the strip. Instead of a bare stripe my cable may comprise a lengthwise fold in the tape under the strip. This fold is such that it brings two adhesive-coated portions of the tape together and, at the same time, makes electrical contact with 50 the strip. The thickness of the tape may be advantageously reduced at the fold, below the thickness of the remainder of the tape. In preferred embodiments of my invention the jacket comprises a polyolefin and the adhesive comprises a copolymer of ethylene and acrylic acid.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a section of a cable comprising one embodiment of my invention.

FIG. 2 shows a section of a cable comprising another embodiment of my invention.

FIG. 3 shows a section of a cable comprising still another embodiment of my invention.

FIG. 4 shows a section of a cable comprising a modification of the embodiment of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED

EMBODIMENTS

Referring first to FIG. 1, my Cable, indicated generally by 70 the numeral 10 comprises a central conductor such as a copper conductor 11, a cylindrical wall 12 of insulation such as the expanded polyolefin insulation described in U.S. Pat. No. 3,315,025 and a metal tape 13 surrounding the wall 12 to comprise the outer shielding conductor of the cable 10. The 75 tape 13 is coated on its outer surface with a layer 14 of adhesive and the cable is covered overall with a solid, abrasion-resistant, synthetic-resin jacket 16 which bonds the tape 13 to the jacket 16. As so far described the cable 10 corresponds to the cable described in U.S. Pat. No. 3,315,025 hereinabove mentioned. A strip 17 of the tape 13 overlaps a portion 18 of the tape and is bonded to this portion by the adhesive on the surface of the tape at the edge 19 and also at 21. However, in the present improvement a stripe 22 has been left bare on the surface of the tape 13 under the strip 17 where electrical contact is made between the tape and the overlying strip, which is free from adhesive on its underside. As described in U.S. Pat. No. 3,315,025, the disclosure of which are incorporated in the description of the present application, the bonding effect of the adhesive is actuated during the hot extrusion of the jacket 16, and the compression which takes place during this extrusion brings the under surface of the strip 17 down firmly against the upper surface of the stripe 22 to complete the electrical contact and close any shielding “window” in the cable.

In the embodiment of FIG. 2 a bare stripe 23 in the surface of the tape 13 includes the edge of the tape. This has the advantage that it reduces the required overlap and thus the amount of metal required in the outer conductor of the cable. Another advantage of the embodiment of FIG. 2 is the better electrical contact afforded by a comer 24 of the tape being pressed into the overlying strip 17. In FIG. 3 the tape 13 has been bent back upon itself in a fold 26. As shown in the drawing the extent of this fold is exaggerated for clarity, but a wide fold is not necessary to practice my invention. Indeed a short fold so long as it exposes a bare area 27 of the tape 13 to the bare underside of the strip 17 is sufficient and a shorter fold will have the benefit of greater economy. In FIG. 4 the fold 26 has been compressed, reducing the thickness of the tape at the portions 28 and 29 and the double adhesive layer within the fold. Compression of the fold is readily accomplished by passing the tape 13 through driven rollers at the time it is folded around the wall 12. The embodiments of FIGS. 3 and 4 have the advantage that they can utilize tapes that have been slit from wide sheets of adhesively coated metal.

I have described embodiments of my invention which are exemplary rather than definitive and for which I desire an award of Letters Patent as defined in the appended claims.