Application Date: Dec. 6, 1934. No. 35102/34.
Complete Specification Left: Nov. 13, 1935.
(Divided out of No. 452,772.)
Complete Specification Accepted: Aug. 31, 1936.
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 invention has for its principal object to provide a high frequency electric cable which is simple and relatively inexpensive and which has satisfactory electrical constants over a wide range of frequencies.
According to the present invention an electric cable comprises 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, is usually provided upon the insulating sheath.
In carrying the invention into effect, we may proceed as follows:-
A wire, which may be of No, 40 S.W.G. and which is preferably of springy material such as cadmium bronze, is bent into zig-zag form of about 45 degree slope, that is to say the angles at the apices are about 90°. The apices are rounded. This wire is then drawn into a rubber tube having an internal diameter of about 1/8 inch and an external diameter of about 3/8 inch. The lengths of the straight parts of the zig-zag are such that the apices bear against the inner walls of the rubber tube. The outer surface of the rubber tube is covered with a braiding of metal such as copper either before or after the central conductor is inserted.
A cable constructed in this way has the advantage of permitting stretching of the rubber tube and braiding 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.
With one sample constructed as described above, having a length of 100 feet a characteristic impedance of 185 ohms was obtained. For longer lengths of cable a thicker central conductor may be desirable to reduce attenuation but the characteristic impedance is not then so high.
Used as a transmission line working from an impedance of 170 ohms into an open circuit, such as the grid circuit of a valve, a substantially flat characteristic both for amplitude and relative delay was obtained with the 100 foot length of cable above mentioned.
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 and good results are obtainable in the construction above described with a wire of No. 34 S.W.G. 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 other than braiding 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 wire. The wire may be given the zig-zag form by passing it between two toothed wheels meshing with one another. The extent of the corrugation of the wire to give it its zig-zag form 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 6th day of December, 1934.
REDDIE & GROSE,
Agents for the Applicant,
6, Bream’s Buildings, London, E.C.4.
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 flexible high frequency electric cable which is simple and relatively inexpensive and which has satisfactory electrical constants over a wide range of frequencies. The term "flexible cable" is intended to include a bendable cable such as lead-sheathed cable which is liable to be bent in the process of laying.
The present invention is convened with cables of the type which comprises a tubular sheath of insulating material having within it a conductor which is held in spaced relation to the inner wall of the sheath over the greater part of its length by deformation of the conductor, the deformation being such that parts of the conductor run obliquely with respect to the axis of the sheath. In certain known constructions for example the conductor is given a zig-zag form, the angles at the apices of the zig-zag being large, that is to say substantially greater than 90°. The tubular insulating sheath is usually surrounded by a metallic sheath.
According to the present invention a flexible electric cable of the type above set forth has a conductor of springy material, that is to say of a material having a substantially high elastic limit than the copper commonly used in electrical conductors. Good results have been obtained with a hard drawn conductor formed from a copper alloy containing approximately 1% of cadmium.
According to a feature of the present invention the springy conductor of the cable has a form such that parts of the conductor run at angles of 45° or more to the axis of the sheath.
Certain of the features of the cables to be described form the subject of our co-pending Application No. 6038.35. (Serial No. 452,772). This specification relates to cables of the type 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 greater parts of the length of the conductor, being wholly or substantially wholly air spaced from the inner surface of the sheath. The particular form of cable described and claimed has 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. No broad claim is made in the present application to cables having this characteristic feature.
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 which embodies the present invention, and
Figs. 2 and 3 show modifications of Fig. 1.
Referring to Fig. 1 of the drawing a wire 1, which may be of No. 40 S.W.G. and which is of springy material such as cadmium copper, is bent into zig-zag form of about 45 degrees slop, that is to say the angles at the apices 2 are about 90°. The apices 2 are rounded. 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 lengths of the straight parts of the zig-zag are such that the apices 2 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 or after the central conductor 1 is inserted in the tube 3. This braiding constitutes the outer return conductor of the cable.
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 conductor is held out of contact with the inner walls of the rubber tube 3, and is therefore wholly air-spaced therefrom, throughout the greater part of its length on account of its zig-zag configuration. 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. Thus 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 here 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 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 zig-zag 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 of 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 insulting tube. If the cable is not liable to be subjected to continuous sharp bend the pitch of the zig-zag may be increased so that the wire makes an angle of 5° to 15° with the axis of the tube.
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.
In a modification which will be described with reference to Fig. 2, the general run of the central wire 1 is straight and is located along the axis of the tube 3. Small kinks 7 are formed in the wire 1, these kinks being disposed at intervals along its length and extending from the central position of the main part of the wire out to the inner wall of the tube 3. The wire is thereby held mainly clear of the wall. The kinks may be of substantially semi-circular form as shown, all the kinks being arranged in one plate and alternate kinks being located on opposite sides of the axis of the tube 3.
In the modification of the arrangement of Fig. 2 which is shown in Fig. 3, each kink 7 is approximately in the form of a 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 I 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 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.
It has been found, in cables of the above described types, that when the cable is subjected to flexure, the high frequency losses are increased, due 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 lying in two planes at right angles to one another and parallel to the length of the cable. For example, in the types of crinkling described above in which all the crinkling is in one plane, the crinkling limits the movement of the wire in this plane 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 semi-circular 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 crinkle 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 insulating between the central conductor and the sheath 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 insulting tube. As another alternative the crinkled wire may be formed of stranded wire so as to reduce its high frequency resistance, the strands being as before of springy material.
The present invention of course includes many modifications of the constructions described above by way of example. Thus the gauge and cross sectional shape of the central conductor may be chosen 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.
The central conductor may 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 crinkled from by passing it between two suitably toothed wheels meshing with one another. The extend of the corrugation of the wire to give it its zig-zag or crinkled form 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 crinkled in the manner shown in Fig. 3 and then flattened in the plane of the crinkles by a rolling or like process. The resulting crinkles strip may then be given a continuous twist of 90° per crinkle in which case the conductor will be self centering in a circular tube of suitable diameter. 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 loss per pound of conductor due to conductor resistance is reduced and the excess inductance at low frequencies due to current flowing though 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 crinkled in a plane perpendicular to the plane of the tape. This tape may be used with or without subsequent twisting.
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 conductor of the individual inner cables may be insulated and maybe used as insulated returns in two wire circuits in which the potentials of both wires may differ from earth potential./
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 insulting fluid. Thus a cable may be filled with dry nitrogen or other suitable fluid.
It will be noticed that in the specific examples of the invention above described and shown in the drawing, there is adopted the preferred arrangement in which the central conductor has portions which run at angles of 45° or more to the axis of the sheath.
Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-
Dated this 22nd day of June, 1936.
REDDIE & GROSE,
Agents for the Applicant,
6, Bream’s Buildings, London, E.C.4.
Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. - 1936.