US2711439A - Electric cables - Google Patents

Description

June 21, 1955 Filed April 11, 1949 United States Patent 0 ELECTRIC CABLES Esmond Wassell Smith, Beech croft, Chislehurst, England, assignor to Canada Wire & Cable Company Limited, Leaside, Ontario, Canada, a company of Canada Application April 11, 1949, Serial No. 86,714 Claims priority, application Great Britain April 13, 1948 7 Claims. (Cl. 174--102) This invention relates to electric cables.

More particularly the invention is concerned with communication cables and especially with a construction of conductor suitable for the transmission of high frequencies and primarily intended to form central conductors in coaxial cables which are insulated partly or wholly with solid dielectric material.

Two classes of construction of the conductor of a high frequency cable are known. In the first class, which is used for small conductors up to between 0.1 and 0.2 inch in diameter, either a single solid wire, or a strand of wires and/or tapes may be used. The best electrical efficiency is obtained with a single solid wire, but in many cases where a high mechanical safety factor is essential the use of a single wire is inadmissible. In such cases it is common to lay up a number of thin copper tapes, say 3 or 4, round a small solid wire.

The tapes are applied with a relatively long lay and are rolled so that they fit the surface of the solid wire and form a substantially closed cylinder encircling it. The electrical efficiency of this structure is reasonably good, but when the overall size of the conductor increases it begins to fall appreciably short of the ideal which is obtainable with a conductor having a smooth unbroken or uninterrupted cylindrical outer surface.

The second type of construction is used for conductors of larger size having a diameter greater than about inch, and in it the conductor takes the form of a shell of copper tapes assembled upon a core which may be of non-conductive material and which contributes but a small proportion of the mechanical strength of the whole structure. The shell is preferably formed in two layers, as described in British Patent No. 623,748, the inner of which comprises a tape or tapes wound on the core with a relatively short lay. This layer is resistant to the bending which must be applied to the cable during manufacture and installation, and its purpose is to bridge any discontinuities which may develop in the outer layer which immediately surrounds it. The outer layer preferall ably takes the form of a single tape folded longitudinally around the inner layer. electrical efficiency of the outer layer, upon the outer surface of which the high frequency currents are conveyed, is high, but that its resistance to bending is weak. Hence the necessity of combining it with an inner layer, as described above, is apparent.

In the past the gap between the two diameter ranges mentioned above has been difilcult to fill where both flexibility and high electrical efiiciency are required, and little success has been achieved in making a compromise with the object of obtaining both a good degree of flexibility and that electrical efiiciency which is essential for a high frequency conductor.

According to the present invention a conductor for use in an electric communication cable for operation at high frequencies comprises a single copper tape folded longitudinally around a circular core built up of multiple It will be appreciated that the 2,711,439 Patented June 21, 1955 strands of conductive material and preferably formed of at least 7 circular copper wires.

The copper tape may completely encircle the core without overlap, its butting edges extending parallel with or substantially parallel with the longitudinal axis of the cable, or alternatively in some cases we may employ a. conductive tape of such width that when folded around the centre strand its edges overlap. Such overlap may vary from a relatively small one up to as much as 360 degrees, in which case a continuous double layer is produced.

In some cases it may be desirable to produce a closed tubular structure, for example by soldering together the butting edges or by soldering the overlapping edge onto the underlying turn or even by soldering the entire overlapping surfaces together. To facilitate such soldering the tape used may be previously coated with tin.

The invention is especially applicable to high frequency signalling conductors of diameter between approximately 0.2 inch and approximately 0.35 inch in diameter. As an example a conductor of overall diameter 0.255 inch is made by longitudinally folding a soft annealed copper tape 0.775 inchx0.0l5 inch around a strand of 7 wires each 0.075 inch in diameter. I have found that this combination is simple to manufacture and that the longitudinal tape, once applied to the strand in such a manner as to enclose it tightly, does not tend to open up during subsequent handling provided that the radii of the drums on which it is wound are not less than about 2 feeta condition which can be conveniently satisfied. The thickness of the longitudinal tape should be between 0.010 and 0.020 inch for the best combination of mechanical properties. I have found the high frequency conductivity of the composite conductor to be only about 1% below that of an ideal tube, and that its mechanical strength is ample. Further, having a resistance to direct current of only about 1.2 ohms per nautical mile of 2029 yards it is particularly suitable for submarine cables incorporating submerged repeaters which are supplied with direct current power from the cable terminals.

Another advantage arising from the use of a conductor in accordance with this invention is its suitability for reliable jointing and in accordance with a further feature of this invention a joint is formed by intertwining or marrying the wires of the strand in such a way that the ends of the strand of one conductor overlie the longitudinal tape of the other and are soldered to it, whereby two lengths of the composite conductor are effectively united, both mechanically and electrically.

The invention is illustrated in the accompanying drawings in which:

Figure l is an elevational view of a high frequency coaxial communication cable with the outer conductor omitted and with insulation and part of the inner conductor cut away so as more clearly to show the internal construction;

Figure 2 is a cross-sectional view of the cable shown in Figure l but showing additionally the outer conductor;

Figure 3 is a perspective view showing one stage of the formation of a joint between two cable ends, the outer conductor being omitted;

Figure 4 is a view similar to Figure 3 but showing the formation of the joint at a later stage; and

Figure 5 is a view similar to Figure 2 but showing a conducting tape forming part of the inner conductor folded with an overlap of approximately 360 degrees.

Referring to Figs. 1 and 2, 1 is a stranded soft copper central conductor core composed of six circular wires laid round a circular centre wire, all the wires being of the same diameter, for example 0.075 inch, so as to form a single composite strand having an overall diameter of 3 0.225 inch. Over this strand a soft copper tape 2 having a thickness of between 0.010 and 0.020 inch is folded longitudinally, the width of this tape being such as to encircle the strand completely without overlap. The tape is folded circumferentially around the composite core with zero wrapping lead longitudinally of the core so as to provide a tubular conductor free from joints extending around the core. Advantageously the tape is 0.015 inch thick, and for such a thickness the tape is made 0.775 inch wide so that after the stretching, to which it is subjected when being folded round the central strand, its edges meet to form a butt joint. In order to provide optimum conditions for the conveyance of high frequency energy the tape is applied so that its butting edges 3 are parallel to the axis of the composite conductor. 4 indicates a covering of solid insulating material, for example polythene, extruded over the conductor. The outer conductor of the coaxial cable is indicated at 8, the usual outer insulation and armoring not being shown.

The interstices of the central conductor core, both within the strand and between it and the enfolding tape, may be filled with compound to exclude air, in accordance with known practice.

Referring now to Figs. 3 and 4 there are illustrated two stages in the jointing of two lengths of the flexible inner conductor shown in Figs. 1 and 2. At the end of each conductor the thin longitudinally folded tape 2 is cut away for a distance of some six inches and the outer wires of the strand 1 are opened up as indicated in Fig. 3. The centre wires of the strand are then jointed together by soldering or welding, as indicated at 5. The opened wires of one length of strand are then intertwined with those of the other length as shown in Fig. 3 and twisted around the opposite conductor so as to overlie the longitudinal tape 3 thereof as indicated at 6 in Fig. 4. When the married joint has been completed in this way the whole joint is preferably soldered solid so that the two lengths of conductor are efiectively united, both mechanically and electrically.

Figure 5 shows a cable comprising a composite core 1 made up of a plurality of twisted strands, a tube 2' formed by a tape wrapped with an overlap of 360 degrees and a surrounding sheath 4' of insulating material. The outer conductor is shown at 8. As in the form shown in Figures 1 to 4, inclusive, the tape is folded circumferentially around the composite core with zero wrapping lead longitudinally of the core so as to provide a tubular conductor free from joints extending around the core.

Other forms of conductor joint may, of course be employed. For example, the individual wires of the stranded core may be brazed or welded together, after removing a suflicient length of the longitiudinal tape to permit this operation, and the length of tape may be replaced subsequently and soldered into position to form a substantially uniform diameter throughout the jointed conductors.

I claim:

1. A high frequency coaxial communication cable comprising a flexible inner conductor constituted by a central conductor core comprising a plurality of bare Wires of a conductive metal twisted together to form a composite strand of comparatively high tensile strength, and a single thin flexible metal conductive tape folded circumferentially around said core substantially throughout its length with zero lapping lead longitudinally of the core and providing a flexible tubular high frequency conductor of comparatively low tensile strength free from joints extending around the core; an outer conductor; and insulation between said outer and inner conductors.

2. A cable as claimed in claim 1 wherein the metal tape is applied to the core with its edges overlapping.

3. A cable as claimed in claim 1 in which the tape is applied to the core with its edges overlapping and the width of the tape is such as to provide an overlap of 360 so as to form a double layer.

4. A cable as claimed in claim 1 wherein portions of the metal tape constituting the outer conductive layer are united.

5. A cable as claimed in claim 1 wherein the metal conductive tape has electrical and physical properties approximately the same as copper.

6. A cable as claimed in claim 1 wherein the core and metal conductive tape are of the same metal.

7. A cable as claimed in claim 1 wherein said core and metal conductive tape are of copper and the outer surface of the circumferentially folded copper tape is smooth.

References Cited in the file of this patent UNITED STATES PATENTS 1,231,568 Clark July 3, 1917 1,821,908 Fowle Sept. 1, 1931 2,149,771 Hunter et al Mar. 7, 1939 2,217,284 Lunt Oct. 8, 1940 2,319,744 Mougey May 18, 1943 2,379,318 Saiford June 26, 1945 FOREIGN PATENTS 407,944 France Jan. 12, 1910 1,301 Great Britain June 9, 1910 272,487 Great Britain Aug. 11, 1927 489,000 Germany Jan. 30, 1930 438,530 Great Britain Nov. 19, 1935 887,667 France Aug. 16, 1943 573,261 Great Britain Nov. 13, 1945

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