US2447168A - High-frequency electric conductors and cables - Google Patents

Description

Aug. 17, 1948. J. N. DEAN ET AL 5 HIGH-FREQUENCY ELECTRIC CONDUCTORS AND CABLES Filed Sept. 16, 1943 2 Sheets-Sheet 1 Aug. 11, 1948. J. N. DEAN ETAL 2, 7, 6

HIGH-FREQUENCY ELECTRIC GONDUCTORS AND CABLES Filed Sept. 16, 1945 2 Sheets-Sheet 2 Patented Aug. 17, 1948 HIGH-FREQUENCY ELECTRIC CONDUCTORS AND CABLES John Norman Dean, Westerham, Esmond Wassell Smith, Beechcroft, Chislchurst, and Reginald Sear, London, England, assignors to Telegraph Construction & Maintenance Company, Limited, Greenwich, London, England Application September 16, 1943, Serial No. 502,678

' In Great Britain May 12, 1942 Claims. 1 This invention relates to high frequency cables, 1. e. cables which are operated at frequencies of the order of l megacycle per second or higher, and has for itsobject the reduction of resistance losses in the outer of two co-axial conductors or in the screen surrounding two identical balanced conductors.

In the production of flexible cables the outer conductor and/or screen above-mentioned are commonly made of a braid of tinned copper wires,

cycles or higher. In the description which fol lows attention is primarily paid to screened ba1- anced cables, although the invention covers both this and the co-axial type.

In a balanced cable with the above-mentioned conventional wire braid screen, the attenuation often rises rapidly while the cable is kept undisturbed under ordinary atmospheric conditions. This increase may amount to several hundred per cent, and although a reduction may'take place when the cable is installed or otherwisehandled, the attenuation soon rises again to an even higher value than before.

' According to this invention the screen and/or outer conductor in a high frequency cable is composed of a reinforced or duplicated, layer of thin metal of good electrical conductivity applied directly to the outer surface of the dielectric of the cable and in intimate contact therewith.

We have found that a single layer of paper tape backed with aluminium foil and wound helically over the cable dielectric forms a good screen until the cable 'is handled or bent, when the foil develops cracks, thereby rendering the screen imperfect and giving rise to increases in attenuation, comparable with those'mentioned above.

In one embodiment of the invention two such tapes are used, one wound over the other, the metal surfaces of both tapes being inward. Severe mechanical handling produces cracks in both layers of the tape but it has been found that the chances of their coinciding in position in both layers are very small, and that the second layer thus acts as the required reinforcement to same or of slightly less width, This composite tape is helically applied, metal surface inwards, each turn overlapping the last by about 0.1 inch. Any suitable reinforcing material will suit the purpose. 1

In a third embodiment a single paper-aluminium tape is first applied with overlapping turns as above, and the cable is then braided with the usual fine copper wire braid, this acting as the necessary reinforcement to the single tape. In any of the above cases one edge of the tape may be folded back so that a metal-to-metal contact is obtained between successive turns.

In any of the above embodiments, the metal may not necessarily be confined to aluminium any metal of good conductivity e. g. copper or silver, being included within the scope of the invention. Similarly, other backing materials than paper or linen, e. g. rubber, may be used.

In a fourth embodiment the thin metal is deposited direct on to the dielectric surface by known means and this covering is reinforced by a tape comprising thin metal, with or without a backing, or by a wire braid as in embodiments one and three above.

Cables constructed as above are normally finished off by a sheath of waterproof material.

In the case of co-axial cables longitudinal conductivity is of course essential and this may be ensured by omitting the non-conductive backing along an edge of the tape, or by folding the tape back along an edge as hereinbefore mentioned, or by omitting the non-conductive backing entirely in one or both of duplicated tapes.

The invention is illustrated by way of example in the accompanying drawings which show various ways in which the invention may be carried into effect. Figure 1 illustrates a balanced cable with layers comprising the cable structure partially removed to show the interior cable parts. Figures 2 to 6, inclusive, respectively show other balanced cable structure embodying the invention in modified form. Figures '7 to 10, inclusive,

are views similar to Figure 1 but showing differcomprise a paper backing carrying a layer 6 of thin metal of good electrical conductivity, such as aluminium foil. For the sake of clarity in the drawings the metal layer is shown as being slightly separated from the paper backing 4, but in actual practice the metal layer would be directly secured to the paper backing. The winding of the tapes in all the examples shown is such that, each turn of each tape overlaps the last turn by about 0.1 inch. The conductors are indicated at 6 and the outer sheath of waterproof material at l. The conductors are preferably twisted together.

In Figure 2 a single tape is used, this comprises a layer of metal foil such as aluminium directly affixed to a tape 3 of strong ton, whilst in Figure 3 a single metallized paper tape is used and this is reinforced by a fine copper wire braid 9.

Figure 4 shows a layer i0 of thin metal deposited direct on to the dielectric 3 and reinforced by an unbacked metal tape ii. If desired the metal tape H may be backed as shown at it in Figure 5 or a metal braid id as shown in Figure 6 may be used.

In Figure 7 the dielectric of the co-axial cable is wound with two tapes, one an unbacked metal tape Hi and the other a paper tape 15 carrying a metal layer 16, the central conductor being indicated at i! and the outer sheath of waterproof 'material at 68. If desired two unbacked metal tapes i9, 20 may be used as shown in Figure 8.

Figure 9 shows a method of using two metallized paper tapes 2 i, 22 with the edges of the tapes folded back at 23 so that a metal-to-metal contact is obtained between successive turns.

Figure shows the use of a metallized paper tape 24 in which the non-conductive backing is omitted along one edge 25, thereby obtaining metal-to-metal contact between successive turns and the tape isreinforced by a metal braid 26.

The benefit of the invention may be seen from the result of tests given below on cable having two conductors each of diameter 0.096"'insulated to a diameter of 0.23", the pair being twisted together and filled circular with dielectric material to a diameter of 0.475".

In the table, normal treatment" means winding the cable on a bobbin 6 inches diameter, then unwinding and rewinding it in the opposite direction, this cycle being repeated six times. Severe treatment means a repetition of the above in which a mandrel of diameter about 2 inches is used, the cable being also twisted. Storage means keeping the cable undisturbed under ordinary atmospheric conditions for a few weeks or longer.

Attenuation of cable decibels per 100 ft. at 200 male.

T pe of screen y After severe treatment After normal treatment or storage Initial One layer cotton-backed aluminium foil (see Figure 2) Two layers paper-backed aluminium foil (see Figure 1) One layer paper-backed aluminium foil and tinned copper wire braid (see Figure 3) a a 3. 4 4. s

linen or cot- 4 tially about 4.0 and rises to between 6 and 12 or higher within a few weeks after manufacture or installation. It will be seen that cables in accordance with the invention have not only substantially lower initial attenuations but greatly improved stability.

The table above illustrates the performance of the cables by quoting their attenuation, the measurement of which is a sensitive method of detecting changes in the state of the screen and/ or outer conductor. The stability of other characteristics, e. g. impedance and velocity of propagation, is improved in like manner to that of attenuation by the present invention.

What we claim is:

1. A high frequency signalling cable having two balanced conductors together surrounded by a dielectric and comprising a tape including a layer of non-conductive flexible material and a thin layer of metal wound upon the dielectric with overlapping turns with the thin layer of metal facing toward the inside of the cable and being in intimate contact with said dielectric, and a flexible conductive reinforcing layer immediately covering said tape.-

2. A high frequency signalling cable as claimed in. claim 1, in which the thin layer of metal is metal foil secured to the backing tape of non conductive material.

3. A high frequency signalling cable as claimed in claim l in which the flexible conductive reinforcing layer is a tape comprising imetal foil wound with overlapping turns.

4. A high frequency signalling cable as claimed in claim 1 in which the flexible conductive reinforcing layer isa tape comprising a backing of non-conductive material faced with metal and wound with overlapping turns.

5. A high frequency signalling cable having two balanced conductors together surrounded by a dielectric and comprising metal-faced paper tape wound with overlapping turns, metal face inwards, and inintimate contact with said dielectric, and a sheath of metal braid immediately covering said tape.

JOHN NORMAN DEAN. ESMOND WASSELL SMITH. REGINALD SEAR.

REFERENCES GllTlED The following references are of record in the v file of this patent:

UNITED STATES PATENTS Number Name Date 1,861,182 Hendey et al May 31, 1932 1,948,616 Fischer Feb. 27, 1934 2,019,297 Faucett Oct. 29, 1935 2,250,032 Osborne et a1 July 22, 1941 2,322,702 Peterson June 22, 1943 2,344,501 Bennett Mar. 21, 1944 FOREIGN PATENTS Number Country Date 385,146 Great Britain Dec. 22, 1932 559,518 Great Britain Feb. 23, 1944 650,556 France Sept. 24, 1928- 686,552 France Apr. 14, 1930 762,534 France Jan. 22, 1934

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