US1861851A - Submarine signaling cable - Google Patents

Description

' Patented June :7, 1 932 UNI ED STATES PA E OFFICE JoEN J. GILBERT, or roar WASHINGTON, NEW YORK, AssIeNoa T WESTERN L C- TBIC COMPANY, INCORPORATED, 0E NEW YORK, N. Y., A CORPORATION OF NEW Yo x SUIBMARINE smNALiNG CABLE Application filed September 1, 1922. Serial No. 585,619.

' This invention relates to submarine signaling cables, and particularly to such cables for high frequency transmission.

An object of the invention is to provide a high frequency submarine cable having large inductance and small alternating current resistance but without inductive loading distortion or intermodulation of co-existing signals.

Another object of the invention is to provide a novel type of submarine cable for the transmission, with comparatively low attenuation, ofcurrents of frequencies used in carrier telegraphy and telephony. v

The most usual type Ofsubmarine cableconsists of a core comprising a copper conductor insulated with a layer of dielectric,

which is surrounded by a covering of jute and.

a sheath of steel armor wires. The attenuation constant pernautical mile at the frequency f may be calculated J from. the formula:

where R, G, L and C are, respectively, the

resistance, the leakance, the inductance and the capacity per nauticalmile of the cable The Equation (1) simplifies, when 27rfL is small compared to unity, to:

where d and D are, respectively, the diameters of the conductor and of the core, and k is the dielectric constant of the insulating material provementin of the core. It is evident that the capacity can be diminished by increasing the thickness of the insulating layer or by decreasing the dielectric constant, and a certain amount of imtransmission can be obtained in this manner, the limit being set by mechanical and cost considerations. 7.

Included in the cable circuit is the return conductor, consisting of the steel armor wires and the surrounding sea water in parallel; and it is of importance to determine how much this conductor contributes to the re-' sistance and inductance of the circuit. This can best be done by considering the more general electrical system comprising a central insulated conductor surrounded by a return conductor, comprising a number of co-axial conducting cylinders connected in parallel. It can be shown in this case that at high frequericies the return; current has a very decided tendency to crowd in close to the central coiiductor, and if the innermost of the return group of conductors if of low impedance compared. with the others, practically all return current is carried by this conductor. The inductance per unit length of the system is then where D is the internal diameter of the innermost conductor. The resistance of the return conductormay be computed by a rigorous but rather involved analytic process; see, for instance, the paper by J. R. Carson and J. J. Gilbert, in the Journal of the Franklin Institute, December, 1921, entitled Transmission Characteristics of the Submarine Cable. A very close approximation may be obtained, however, from the formula:

most return conductor. It is evident, therefore, that L may be increased and R, decreased by placing thereturn conductor as far from the core as mechanical limitations will permitfand that B may be further decreased by constructing the return conductor of non-magnetic, highly conductive matew Equation (5) resistance return for rial. The effect upon R of varying D in may be understood from a consideration of the fact that, at the frequencies under. consideration, the current penetrates only to a limited depth below the surface of a conductor and the superficial area of the conductor is, therefore, an important factor in determining its resistance. The same considerations hold when we use for the innermost return conductor a layer of wires instead of the solid cylindrical sheath, and in place of Formula (5) we can use an equivalent approximation:

. whereA is the effective superficial area of all the wires.

In the ordinary type of cable the innermost return conductor is the sheath of armor wires, and, for the frequencies in which we are interested, practically all the return current is carried by these wires. Due to the mutual action'of the currents in theseveral wires constituting this conductor, and to the high permeability of the material of which they are ordinarily made, the current is confined to a com aratively thin layer at the surface of the wires. Consequently theresistance of the return conductor is large, amounting at high frequencies to several times the resistance of the central conductor. This'results in a large increase in attenuation and diminished efficiency of transmission.

In the design of the recently laid Habana- Key West carrier current submarine cable it was sought to eliminate this increase in attenuation by increasing the inductance of the cable by loading, and decreasing the resistance of the sea return by surrounding the core with a copperv sheath serving as a low the current. A description of this cable appears in a paper by W. H.

' Martin, G. A. Anderegg and B. W. Kendall in the Journal of the American Institute of Electrical Engineers, March, 1922. Loading,

eitherby means of coils, or by means of a layer of magnetic material surrounding the conductor, is attended by losses which contribute to the effective resistance of the conductor, and hence increase the attenuation constant of the cable. This added resistance, which is due mainly to eddy currents in the loading material, is proportional to the square of the frequency, and in the frequency range under consideration it becomes so large that it may nullify the effect of the added inductance. Additional disturbances are introduced in the loaded cable on account of the non-linear magnetic characteristics of the loading material. These disturbances become apparent in the interaction between currents of the various frequencies which are being transmitted over the cable or in the intro- 66 duction of the harmonics of these frequencies,

, pensing with the loading and they are important factors in limiting the efliciency of transmission of the cable.

According to the present invention it is proposed to provide a cable which will have at high frequencies a low effective resistance and a high inductance. This is done by dismaterial in the core and removing the conducting tape from the region adjacent the core to one more remote, and preferably to that of the armor wires. Further, it is preferred to employ for the core conductor a wire which is stranded to a degree suflicient to keep the increment in resistance due to skin effect to a low value, and to utilize for the return circuit a ring of wires or tapes having high conductivity for the return current, which may either take the place of the usual ring or steel armor Wires or be located within the latter ring sufficiently far from the core to insure that the inductance obtained is helpful in reducing attenuation and that the return conductor will have a large superficial area and consequently a small effective resistance. In the claims the use of the word wire or wiresas applied to the conductors for the return current is intended to include tape or tapes.

In the following detailed specification reference is made to the accompanying drawing,

in which Figs. 1 to 4 are cross-sectional views of four different forms of a cable in accordance with this invention. Fig. 5 is a graphic showing of the relationship of attenuation constant and frequency for the type of cable shown in Fig.4 and for a continuously loaded cable of about the same weight and overall dimensions and designed in accordance with current practice for high frequency cables.

Figs. 6 and 7 are sectional views of alternative arrangements of a modified form of conductor.

. Referring to Fig. 1, the central conductor 11 is surrounded with a layer 12 of gutta percha or rubber insulation, which in turn is surrounded with the usual layer 13 of jute yarn. This jute forms a bedding for armor wires 14 laid on in the. usual manner, but the material of these wires is copper or other non-magnetic, highly conductive metal or alloy, rather than iron or steel, the material usually employed. There is preferably applied overthe outside of the armor wires wrappings 17 of tarred jute yarn, or of a heavy burlap-like fabric known as Hessian band, in accordance with the usual practice.

are: the absence of magnetic material, the

providing of a return conductor of high conduetivity in a region remote from the central core to permit an increase in the superl the invention is 35 The wires may 9 by keeping 50 wires ficial area of the conductor and an increase in inductance of the cable, and the absence of all highly conductive material between. the return conductor and the main signaling conductor. By mentioning these requirein a manner not contemplated in the prior art, one or more of the principles upon which based. It is to be understood, therefore, that the invention in its various aspects is fined only by Wherever in the appended claims the expression insulating material is usedwith reference to the jute material 13 or the wrapthe appended claims.

20 pings 17, it is to be inteiipfeted in the light 2 of such materials unless inconsistent with the context. 0 a In the modification shown m,F1g. 2,-1n

place of the copper armor wires. of'Fig. 1,

cop

e comparable with the depth "of current penetration into the conductor so that thesteel corev will havea very small effect uponthe impedance of the wires. be individually wrapped with fabric tape 21 impregnated with a preservative compound in the usual manner. This tape serves the double purpose of protecting the wire and making the cablemore flexible the wiresseparated. This wrapping may be employed also in the modifications shown in Figs. 1, 3 and 4.

Fig. 3 discloses afurther modification, in which the return circuit wires of low im- 45 pedance are interspersed among the steel armor wires.

The modification in Fig. 4 is characterized within the steel armor. The ring of copper in this case is embedded in the inner jute layer and preferably placed as close to the armor ring as mechanical conditions Wlll permit. The copper wires may be replaced 6 by copper clad iron or steel wires.

The arrangements of Figs. 3 and 4 enjoy the advantage of wide separation of the components of the return current which results indiminished mutual reactions among them and lower return resistance.

In place of the wires used in the arrange ments just described it is possible to use tape #of copper or otherhighly conductive material.

When a conductor of steelcovered with a layer of material of high conductivity is not limited 'to the particularform or forms chosen for illustration, but is dequency,

r clad iron or steel armor wires are used. The thicknesebof the copper covering in this case-should the return conductor being placed entirely meability as said used it may be desirable to have'as much as possible of the highly conductive portion on the side nearest the cable core, where the eatest current concentration is found. -onducto'rs of this type are shown in cross section in Figs. 6 and 7. Inthe' former figure the core'60 of iron or steel is surrounded by a coating or layer 61 of material of high conductivity which is of varying thickness, as shown. In Fig. 7 an iron orsteel tape is similarly covered with a coating or layer 71 of material of high conductivity.

- In Fig. 5 are shown the values of the attenuation constants, plotted against frefor the type of cable shown in Fig. 4 and for a continuously loaded cable of about the same weight and overall dimensions, designed in accordance with current practice for. high frequency transmission. On account of losses in the loading material, the

attenuation constant of the latter cable increases very rapidly with frequency, for frequencies greater than 3000 cycles per second,

thereby greatly limiting the length of cable over which such frequencies can be transmitted. On the proposed type of cable, a frequency of 10,000 cycles per second could be transmitted four times as far as on the loaded cable, with the same overall attenuation; and

'much greater relative gain in distance could be obtained for still 1111 her frequencies. 7 The invention claime is:

1. A submarine signaling cable for high frequency transmission comprising a signaling conductor, insulation surrounding said conductor,a return circuit path comprising a wire having an external layer of material of high conductivity,

said wire being in the region of the usual steel or iron armor wires and spaced from said of conducting material having a greater thickness .at the portion nearest the central signaling conductor and insulating material filling the space between said return circuit wires and said insulation.

2. A submarine cable comprising a signaling conductor, insulation around said conductor, a plurality of'bi-metallic wires surrounding said insulation and spaced therefrom and having much larger aggregate tenaling conductor,

sile strength than saidsi a portion at least of sai ,wires having approximately the same conductivity and persignaling conductor, and insulatin material filling the space between .said con uctor and said wires.

3. A submarine signaling cable for high frequency transmission comprising a signaling conductor, insulation surrounding said conductor, a return circuit comprising wires outside said insulation and taking the place of a portion at least of the usualsteel or iron insulation, said layer 1 armor wires, superficial portions of said return circuit wires being of material having higher conductivity than iron and the central ing therefor imw portions being of iron and insulating ma-' terial filling the space between said return circuit wires and said insulation characterized in this that said portions of high conductivity have their greatest thickness at points nearest to the signaling conductor. I

4. A signaling cable comprising a signaling conductor, insulation surrounding said conductor, and a return circuit path comprising conductive material having a portion of lower resistance than other portions, the greater part of said portion of low resistance being nearer said signaling conductor than said other portions.

5. A signaling cable comprising a signale ing conductor, a return circuit path spaced from said signaling conductor comprising conductive material having one portion of lower resistance than other portions, the greater part of said portion of low resistance being nearest said signaling conductor, and insulating material filling the space between said signaling conductor and said return circ'uit path.

6. A signaling cable for high frequency currents comprising two signaling conductors spaced apart and insulation between said conductors, one at least of said conductors having greater conductivity in the region toward said other conductor than the more remote portions.

.7. Submarine signaling cable armored with wire havin a central core and a coverich the. central core is made of steel and the covering is composedprincipally of copper.

In Witness whereof, I hereunto subscribe my name this 31st day of August A. D. 1922.

- JOHN J. GiLBERT.

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