452,772

PATENT SPECIFICATION

Application Date: Feb. 25, 1935. No. 6038/35.

Complete Specification Left: Nov. 13, 1935.

Complete Specification Accepted: Aug. 25, 1936.

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PROVISIONAL SPECIFICATION

Improvements in Cables Suitable for the Transmission of High Frequency Electric Currents

We, ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, and EDWARD CECIL CORK, a British Subject of 50A, Kenilworth Road, Ealing, London, W.5, do hereby declare the nature of this invention to be as follows:-

The present invention relates to cables suitable for the transmission of high frequency electric currents.

It has been proposed in co-pending Application No. 35102/34 (Serial No. 452,713) to provide an electric cable comprising an insulating sheath having a conductor disposed within it, the conductor being held clear of the sheath over the greater part of its length by virtue of its shape or configuration. An outer conducting sheath, such as metal braiding, may be provided upon the insulating sheath.

The cable with which this prior Application was particularly concerned was one having a high characteristic impedance suitable for operating over a wide ban of frequencies and was in the form of a zig-zag conductor enclosed within an insulating sheath.

The cable described in detail was one containing a very thin conductor in order to obtain the highest possible characteristic impedance consistent with reasonably low attenuation. Such a cable is useful for runs of several hundred feet for modulation frequencies of television, e.g. up to one or two megacycles per second.

It has now been found that this type of cable construction is also applicable to feeders for short waves (for example, 40 megacycles per second or more), and to circuits for medium waves or carrier frequencies which are generally required to cover long distances with minimum attenuation, but where obtaining a high characteristic impedance is of no particular importance. For such purposes the attenuation at high frequencies is of importance, and it is necessary to employ larger conductors (with reference to the diameter of the external concentric return) than those described in the earlier specification.

The present invention is accordingly concerned with certain improvements in or modifications of the invention set forth in the prior Application above referred to, including the application of the earlier invention to cables having a relatively low characteristic impedance.

In carrying the invention into effect we may proceed as follows:-

A wire, which may be about 0.06 inch diameter is bent into zig-zag form, the apices being rounded. This wire is then drawn into a tube of insulating material such as rubber, the internal and external diameters of which are ¼ inch and 3/8 inch respectively. The outer surface of the insulating tube is covered with metal braid, metal tape, lead sheath or the like, to form a return conductor.

A cable of this type may be used for a feeder connecting an aerial to a receiver or transmitter. If the operating frequency lies within the band of normal broadcast frequencies, then the attenuation will be very low and long lengths of feeder can be employed without any serious loss of energy. At higher frequencies the attenuation will be somewhat greater but the cable can be used with reasonable efficiency at frequencies of 40 megacycles per second and over. The use of an internal conductor of relatively large diameter, having regard to the diameter of the sheath return, is necessary in order to keep the attenuation of the feeder at a low value at high frequencies. A cable of this type has a low characteristic impedance compared with one employing a very thin internal conductor, but for carrier frequency transmission lines this is not, in general, a disadvantage.

If the cable is liable to be subjected to continuous sharp bending it is preferable to arrange the zig-zag to have a slope of about 45°, that is to say each substantially straight part of the wire is inclined at an angle of about 45° to the axis of the insulating tube. If the cable is not liable to be subjected to continuous sharp bending the pitch of the zig-zag may be increased so that the wire makes a maximum angle of about 5° to 15° with the axis of the tube. Further if it is arranged that the inside conductor is slightly stretched when inserted in the insulation, it can be arranged that the zig-zag formation causes only a slight increase of resistance above that obtained from a straight conductor.

A cable constructed in this way has the advantage of permitting stretching of the rubber tube and braiding when this is employed without breaking the central conductor. Further, the conductor is held out of contact with the inner walls of the rubber tube throughout the greater part of its length on account of its zig-zag configuration and it is therefore substantially air spaced.

The increased length of the central conductor due to the bending thereof increases the capacity but also increases the inductance.

Shapes of crinkling of the internal conductor or wire other than the above described zig-zag form, may be employed. For example, whereas a rather sinusoidal shape is desirable for a thin conductor subject to considerable bending, with a thicker conductor a more pointed or saw-tooth shape may be employed, to give the minimum distance of contact between the conductor and the insulating wall. Alternatively the shape of the crinkles may be definitely "peaky" and similar in appearance to the wave-form obtained from a sine wave with a strong third harmonic component, causing a "peaky" wave. As a further alternative, the general run of the wire may be straight and located along the axis of the tube with small kinks, disposed at intervals along its length, these kinks extending from the central position of the wire out to the wall, so that the wire is held by these small kinks clear of the wall. The kinks may, for example, be of semicircular shape and of suitable radius and they may be formed in the wire at suitable distances apart. The semicircles may all lie in one plane alternate semicircles being located on opposite sides of the axis of the tube. In another construction of this type each kink may be approximately in the form of one complete cycle of a sine wave, the maximum displacements in opposite directions from the axis being such that they bear on opposite internal surfaces of the tube and hold the wire in its axial position. Double kinks other than sinusoidal ones may be employed. For example they may be more rounded as in the case of two semicircular kinks, one being displaced along the axis of the tube with respect to the other by a distance equal to its diameter; on the other hand they may be square topped or pointed.

It has been found, in cables of the above described types, that if the cable be made up in a flexible form, and is subjected to flexure, the high frequency losses are increased, du to the inner conductor lying over to one side of the cable. It has been found that this effect can be prevented by providing crinkles in two axes at right angles to the length of the cable.

For example, in the types of crinkling described above in which all the crinkling is in one plate, the crinkling limits the movement of the wire in the plan but does not prevent movement of the wire normal to this plane. By introducing additional crinkles extending above and below this plane, the wire can be centrally located and prevented from moving normal to this plane.

The same effect may be obtained by rotating the plane of the crinkling at each successive crinkle. For example, in the case in which single semicircular kinks or crinkles are formed along the wire, the crinkles may, instead of extending in opposite directions, extend successively in directions differing by say 120°, so that three crinkles serve to locate the wire completely.

In general, crinkling may be arranged to extend from the axis of the cable in any required direction towards the insulating wall supporting the wire.

The insulating material used to prevent the wire from touching the outer concentric return may be of any suitable type, but is preferably formed of material having low dielectric losses for high frequency currents. The cable may, if desired, be sealed at both ends to prevent the entry and deposition of moisture in the inner walls of the insulating tube. As another alternative the crinkled wire may be formed of stranded wire so as to reduce its high frequency resistance.

The present invention of course includes many modifications of the construction described above by way of example. Thus the gauge and cross sectional shape of the central conductor may be chose to suit requirements. The wire may be tinned, if desired, and the sheath may be formed of a rubber tube having a lining of pure unvulcanised rubber to reduce corrosion of the wire. Any suitable form of metallic return may be used.

The central conductor may also be inserted into the sheath in a continuous process, the sheath material, such as rubber, being extruded around the zig-zag shaped or crinkled wire. The wire may be given the zig-zag or crinkle form by passing it between two suitably toothed wheels meshing with one another. The extent of the corrugation of the wire to give it its zig-zag or crinkled from is not critical but should preferably be so chose that the wire is placed under slight stress after it is introduced into the insulating sheath.

The cable described may be used to form a larger cable containing more than one high frequency conductor, each within its own sheath, and if desired other circuits for power supply etc. the outer insulating sheath of the composite cable is preferably covered with a metal shield such as braiding and this shield may conveniently be connected to the outer conductors of the individual inner cables. Alternatively the outer conductors of the individual inner cables may be insulated and may be used as insulated returns in two wire circuits in which the potentials of both wires may differ from earth potential.

Dated this 25th day of February, 1935.

REDDIE & GROSE,

Agents for the Applicants,

6, Bream’s Buildings, London, E.C.4.

COMPLETE SPECIFICATION

Improvements in Cables Suitable for the Transmission of High Frequency Electric Currents

We, ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, and EDWARD CECIL CORK, a British Subject, of 50A Kenilworth Road, Ealing, London, W.5, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-

The present invention relates to cables suitable for the transmission of high frequency electric currents.

The invention has for its principal object to provide a new or improved high frequency electric cable which is simple and relatively inexpensive and which has satisfactory electrical constants over a wide range of frequencies.

The present invention is concerned with cables of the type which comprises a hollow cylindrical sheath having a conductor supported within it at supporting regions spaced apart from one another along the length of the conductor, the parts of the conductor between adjacent supporting regions, constituting the greater parts of the length of the conductor, being wholly or substantially wholly air spaced from the inner surface of the sheath. The sheath is usually of insulating material and may be surrounded by a conducting coating. In one know form of cable of this type an insulator is provided at each of the supporting regions to hold the conductor spaced from the sheath. In another know form of cable of the same type the conductor is deformed in such a way that it bears against the inner surface of the sheath at each of the supporting regions. The deformation is however such that the conductor bears only upon a single small localised area of the sheath surface at each of the supporting regions so that there is no positive location of the conductor in any one of the regions.

According to the present invention there is provided an electric cable of the type above set forth having the characteristic feature that the supporting of the conductor is effected by giving to the conductor at each of the said regions a shape such that the conductor bears upon the sheath at a plurality of points so situated that the conductor is positively located in the sheath at each of the supporting regions.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawing, wherein

Fig. 1 shows a part sectional longitudinal view of one form of concentric cable according to the invention,

Fig. 2 shows a section and Fig. 3 shows a part cut away longitudinal view of another cable according to the invention,

Figs. 4 and 5 show alternative forms which the central conductor of a cable according to the invention may have, and

Fig, 6 shows a section of a multiple cable according to the invention.

Referring to Fig. 1 of the drawing a wire 1, which may be of No. 40 S.W.G. and which is preferably of springy material such as cadmium copper, a hard-drawn alloy which usually comprises copper with about 1% cadmium, is deformed at supporting regions 7 spaced apart thrilling. Co-pending Application No. 35102/34 (Serial No. 452,713) is concerned with a cable having its central conductor of spring material and no claim is made to this feature broadly in the present application. Each deformation is approximately in the form of a complete cycle of a sine wave. Double kinks other than sinusoidal ones may be employed at the supporting regions. For example they may be more rounded as in the case of two semi-circular kinks, one being displaced along the axis of the tube with respect to the other by a distance equal to its diameter; on the other hand they may be square topped or pointed. This wire is then drawn into a rubber tube 3 having an internal diameter of about 1/8 inch and an external diameter of about 3/8 inch. The amplitudes of the kinks are such that the crests thereof bear against the inner walls of the rubber tube 3. The outer surface of the rubber tube 3 is bound with tape 4 which is covered with two layers 5, 6 of braiding of a metal such as copper either before of after the central conductor 1 is inserted in the tube 3. Because the conductor 1 bears upon two nearly opposite points upon the inner surface of the tubular sheath 3 at each supporting region, the conductor is positively supported at each of the regions.

A cable constructed in this way has the advantage of permitting stretching of the rubber tube 3 and braiding 5, 6 without breaking the central conductor 1. Further, the supporting regions 7 occupy a relatively small part of the length of the conductor 1 and the conductor is therefore held out to contact with the inner walls of the rubber tube 3, and is accordingly wholly air-spaced therefrom, throughout the greater parts of its length. If the central conductor is itself coated with insulating material, the greater part of the surfaces of the central conductor and sheath are still substantially wholly air-spaced from one another. In this case the sheath may be either of insulating or of conducting material. If the whole of the insulation is put adjacent to the central conductor, the dielectric losses will not be as low for a given insulation as those obtained with the same thickness of insulation adjacent to the sheath. However, if the voltages are low, so that thin insulation can be used, a satisfactory working condition can be obtained. Furthermore, a working cable is obtained, for example, by covering the inner conductor with a thin insulation of enamel, but placing the main dielectric, such as paper, adjacent to the outer return conductor. In the case where the rubber tube is replaced by paper wrapping, as will be described later, if the paper wrapping becomes displaced so as to leave a gap, the enamel on the central conductor will prevent a short circuit, provided that the working voltages are low. Such a thin coating of enamel will not unduly increase the dielectric losses, since the main dielectric will be air.

The increased length of the central conductor due to the bending thereof increases the capacity but also increases the inductance.

With one sample constructed as described above, having a length of 100 feet a characteristic impedance of 185 ohms was obtained. Used as a transmission line working from an impedance of 170 ohms into an open circuit, such as the grid circuit of a thermionic valve, a substantially flat characteristic both for amplitude and relative delay for frequencies up to two megacycles per second or over may be obtained with the 100 foot length of cable above mentioned.

The cable described above contains a very thin conductor in order to obtain the highest possible characteristic impedance consistent with reasonably low attenuation. Such a cable is useful for runs of several hundred feet for modulation frequencies of television, e.g. up to one or two megacycles per second.

It has been found that this type of cable construction is also applicable to feeders for short waves (for example, 40 megacycles per second or more), and to circuits for medium waves or carrier frequencies which are generally required to cover long distances with minimum attenuation, but where obtaining a high characteristic impedance is of no particular importance. For such purposes the attenuation at high frequencies is of importance, and it is necessary to employ a larger central conductor (with reference to the diameter of the external concentric return) than that described above. In one cable of this type the insulating tube 3 of Fig. 1 may have the dimensions given above and the inner conductor 1 may be bent to the same shape, but instead of being 40 S.W.G. (0.0048 inch diameter) may be 0.06 inch diameter. A cable with this comparatively thick inner conductor may be used for a feeder connecting an aerial to a receiver or transmitter. If the operating frequency lies within the band of normal broadcast frequencies, then the attenuation will be very low and long lengths of feeder can be employed without any serious loss of energy. At higher frequencies the attenuation will be somewhat greater but the cable can be used with reasonable efficiency at frequencies of 40 megacycles per second and over. The use of an internal conductor of relatively large diameter, having regard to the diameter of the shat return, is necessary in order to keep the attenuation of the feeder at a low value of high frequencies. A cable of this type has a low characteristic impedance compared with one employing a very thin internal conductor, but for the carrier frequency transmission lines this is not, in general, a disadvantage.

In the example described, the deformation is such that the conductor lies in a plane. If desired, however additional deformations of similar shape to the described but extending above and blow the plane of the deformations already described may be provided at each supporting region.

Alternatively the plane of the deformation may be rotated at each successive supporting region. For example, the planes of adjacent deformations may make angles of say 120° with one another.

The insulating material used, for example as what has been called the sheath, to prevent the wire from touching the outer concentric return sheath 5, 6 maybe of any suitable type, but is preferably formed of material having low dielectric losses for high frequency currents. Thus this insulation may be produced by winding strips of paper to form a tube enclosing the central conductor, the outer conductor being formed as a metal braid or lead sheath outside the paper tube. The cable may, if desired, be sealed at both ends to prevent the entry and deposition of moisture in the inner walls of the insulating tube. As another alternative the central conductor 1 may be formed of stranded wire so as to reduce its high frequency resistance.

The present invention of course includes many modifications of the construction described above by way of example. Thus the gauge and cross sectional shape of the central conductor may be chose to suit requirements. The wire may be tinned, if desired, and the sheath may be formed of a rubber tube having a lining of pure unvulcanised rubber to reduce corrosion of the wire. Any suitable form of metallic return may be used.

Since high frequency currents only flow in the inner surface of the outer conductor, it is advantageous, in cases where a lead sheath or other poor conducting sheath is employed, to insert a sheath of high conductivity foil or wire between the insulating sheath and the conducting sheath. This high conductivity sheath may comprise copper tapes of the order of 0.001 inch thickness laid around the insulating sheath and running almost parallel with the length of the cable. Such tapes may be held in position by a metal binding having a short lay.

A cable incorporating high conductivity tapes or strips is shown in Figs. 2 and 3. In these Figures the central conductor 1 is a copper wire about 0.1 inch diameter. This conductor 1 has deformations of kinks 8 formed at intervals of about 3½ inches along its length. Each kink comprises two portions 81 extending in opposite directions from the axis of the wire. An exactly similar set of kinks, 9 each comprising two portions 91 is formed intermediate the first set, the second set being formed in a plane perpendicular to the plane of the first set. The central conductor thus has kinks at intervals of 1¾ inches along its length. A strip 10 of paper about 1¼ inches wide is wound helically around the central conductor 1 to form a tube of about ¼ inch internal diameter, the pitch of the helix being about 3 inches. Around this tube are wound three further strips 11, 12, 13 of paper, each strip being about 11/16 inch wide and being wound with a pitch of 1¼ inches. The external diameter of the paper tube so formed is about 3/16 inch. On the outer surface of this tube and parallel with the axis thereof are placed four strips 14 of copper foil each about 9/32 inch in width. These constitute a copper coating on the surface of the paper tube and they are held in position by a fifth copper strip 15 which is wound helically around the four longitudinal strips, the pitch of the helix being about 2 inches. This is surrounded by a lead sheath of 16 of external diameter about ½ inch.

The central conductor may also be inserted into the sheath in a continuous process, the sheath material, such as rubber, being extruded around the deformed wire. The wire may be given the desired deformation by passing it between two suitable toothed wheel meshing with one another. The external of the deformation of the wire is not critical.

In the cables described above, the central conductor is in the form of a wire of circular cross-section, suitable kinks being formed along its length. In some cases other forms of central conductor may be preferable. For example, a wire may be deformed in the manner shown in Fig. 3 and then flattened in the plane of the deformations by a rolling or like process. The resulting strip may then be given a continuous twist of 90° per supporting region. A construction of this type has the advantages over types employing central conductors of circular cross-section that the flexibility of the central conductor is greater (this is of considerable importance if the central conductor is 0.1 inch diameter or more), the copper loss per pound of copper is reduced and the excess inductance at low frequencies due to current flowing through the body of the conductor is reduced and the cable will therefore require less correction for this effect. In another arrangement a flat tape is deformed in a plane perpendicular to the plane of the tape. This tape may be used with or without subsequent twisting. In yet another arrangement, a flat strip 17, Fig. 4, has projects 18 formed on its edges. The projects 18 may be produced by deforming the strip at each projection. Alternatively small pieces of metal 19, Fig. 5 may be spot-welded to the strip 17 at points 20. The arrangements of Figs. 4 and 5 are equally applicable to cables employing a central conductor of circular cross-section. Thus the projections 18 of Fig. 4 may be produced by deforming a circular wire to produce suitable ears or fingers.

The cable described may be used to form a larger cable containing more than one high frequency conductor, each within its own sheath, and if desired other circuits for power supply etc. The outer insulating sheath of the composite cable is preferably covered with a metal shield such as braiding and this shield may conveniently be connected to the outer conductors of the individual inner cables. Alternatively the outer conductors of the individual inner cables may be insulated and may be used as insulated returns in two wire circuits in which the potentials of both wires may differ from earth potential.

A twin cable according to the invention is shown in Fig. 5. The central conductors 21, 22 are similar to that shown in Figs. 2 and 3 and are enclosed in paper tubes 23, 24. On the outer surfaces of these tubes there are formed conducting sheaths 25, 26 which in their turn are surrounded by insulating material 27, 28. These two cables are arranged to form a twin cable by enclosing them in an outer sheath comprising and insulating binding 29 surrounded by a lead sheath 30. Clearly a multiple cable containing more than two conductors may be formed in this way.

Throughout this specification the term "air-spaced" is used in a wide sense. It is intended to cover any case where the space surrounding the air-spaced member is filled with an insulating fluid. Thus a cable may be filled with dry nitrogen or other suitable fluid.

Having now particularly ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-

  1. An electric cable comprising a hollow cylindrical sheath having a conductor supported within it at supporting regions spaced apart from one another along the length of the conductor, the parts of the conductor between adjacent supporting regions, constituting the grater parts of the length of the conductor, being wholly or substantially wholly air spaced from the inner surface of the sheath, characterised in that the supporting of the conductor is effected by giving to the conductor at each of the said regions a shape such that the conductor bears upon the sheath at a plurality of points so situated that the conductor is positively located in the sheath at each of the supporting regions.
  2. An electric cable according to claim 1, wherein said sheath is of insulating material.
  3. An electric cable according to claim 1, wherein said sheath is of conducting material and wherein the conductor bears, through an insulating coating provided thereupon, upon the inside of the sheath.
  4. An electric cable according to claim 1, 2 or 3, wherein at the said supporting regions the conductor is provided with two projections extending in opposite directions therefrom.
  5. An electric cable according to claim 4, wherein the projects of each of said regions are constituted by two kinks formed in the conductor.
  6. An electric cable according to any of the preceding claims, wherein the conductor between the said supporting regions is substantially straight and is arranged to lie substantially centrally within the sheath.
  7. An electric cable according to any of the preceding claims, wherein the cable comprises an outer sheath of material of high conductivity surrounded by material of a lower conductivity.
  8. An electric cable substantially as described with reference to any of the figures of the accompanying drawing.

Dated this 13th day of November, 1935.

REDDIE & GROSE,

Agents for the Applicants,

6, Bream’s Buildings, London, E.C.4.

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Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. - 1936.