US2637783A - Communication transmission line conductor - Google Patents

Description

May 5, 1953 L G. ERICKSON 'E1-AL 2,637,783

comuNIcATroN TRANSMISSION LINE CONDUCTOR Filed Jan. 2, 1951 I IN V EN 2:0125 8( lem; arf f'nbolz HLE APP erf "a6-,uga

A TTOR/VE YS Patented May 5, 1953 COMIHUNICATlON TRANSMISSION LINE CONDUCTOR Lennart G. Erickson, Hillsborough, and Kurt E. Appert, Atherton Heights, Calif., assignors to Lenkurt Electric Co., Inc., San Carlos, Calif., a corporation of Delaware vApplication January 2, 1951, serial No. 203,985

This invention relates to lines for the transmission of communications and to conductors for use in such lines. Specifically it relates to the conductors for telephone transmission at voice frequencies and what would now be considered lower carrier frequencies, but there is no sharp line of demarcation which limits its usefulness to any definite band of frequencies.

It is well known that the ideal or distortionless transmission line is one Whose constants meet the equation LG-RC, where L is the inductance perunit length of line, G is the leakage admittance between the conductors, R is the resistance and C the effective capacitance, all considered over the same unit length. In any line which does not meetthe conditions of this equation distortion is introduced by two different but related effects; first, different frequencies are attenuated to different degrees, and second, different frequencies are propagated along the line at different velocities. In almost all practical lines the product CR is much larger than the product LG, and where this is the case the higher frequencies are attenuated to a greater degree than the low ones. Where wide bands of frequencies must be transmitted this becomes very important and several means have been adopted to compensate for it. Various such methods are known; in most conductor cables, and, formerly, in open wire lines, loading coils are introduced at intervals to increase the effect of inductance of the line. This improves the characteristics of the line up to the frequency Where the separation of the coils is approximately equal to 1A, wavelength, but above this frequency the line ceases to transmit, the attenuation becoming very high. A more recent development is the use of the coaxial cable; as the term is understood today such cables are expensive, and while they may pass an enormous band of frequencies their attenuation is not uniform and they do cut off at some high value of frequency.

Continuously loaded transmission lines or cables have been built, wherein the entire length of the conductor constituting the cable is wrapped with tape of a high permeability alloy such as Permalloy." Such cables are highly effective, but are also expensive to construct.

The distortion produced by transmission lines which are not of ideal character is not serious where the lines are short. Where lines are very long, such as those used in transcontinental circuits, or where a large amount of trame has to be carried, as between large cities several 5 Claims. (Cl. 178-45) hundred miles apart, the use of the expensive constructions and the multiple repeaters required for coaxiallines are justified and will pay for themselves within reasonable length of time. The greatly increasing use of the telephone in rural districts, the use of carrier currents on medium haul lines, and the constantly increasing cost of pole line and underground construction have made it desirable to improve the propagation characteristics of lines in situations where the high costs and high accuracies required for coaxial cable or like lines would not be warranted. The raising of the ratio of inductance to capacity-times-resistance in even a moderate degree has highly beneficial effects upon the quality of the communication that can be maintained, the number of messages which can be transmitted over a single pair of wires, the distance to which such messages can be transmitted or all of' these combined. Furthermore, if this can be done without introducing a definite cutoff frequency into the system the entire installation becomes more fiexible and an overall improvement in system performance can be attained.

Among the objects of this invention, therefore, are to provide a uniformly loaded transmission line which can be constructed at relatively low cost; provide a type of coaxial conductor which, for moderate carrier frequencies at least, does not require the accuracy of construction or the number of repeaters required for conventional coaxial lines; to provide a type of conductor which will give marked improvement in transmission characteristics, whether used in open wire lines or coaxial cable, and to provide the above at a cost so low as to make these advantages available in locations where the trafiic is moderate or light.

Broadly considered, the invention comprises the use of a conductor having a core, preferably of greater permeability than air, having a conductor disposed helically about it. This helical arrangement of the conductor, of course, increases itsl resistance, however, the over-all performance is improved by the loading effect of the skin inductance, that of the higher frequencies, in particular, becoming less. The composite conductor thus formed may be covered with an insulating layer and the whole encased in a conducting sheath, therefore forming an improved coaxial cable.

In coaxial conductors of the ordinary type great care must be taken to maintain the central conductor accurately along the axis, since if this is not done a large portion of the advantages is 3' lost. The necessity for such accuracy is one factor which makes the cost of such cables very high. Where the cables are formed in accordance with this invention, however, inequalities tend to average out and ordinary methods oi construction are sufficiently accurate.

In one preferred forrn of the invention, however, there are particular advantages from the point of View of economy of initial installation. In this form the central core .of the conductor is a single Wire of iron or mild steel, having a coating of material of much higher conductivity, usually copper, bonded thereto. `Such conductors are well known in the art; they are formed either by casting a cylinder of copper around a steel billet and then forming the whole into wire form, or else they are formed by electroplating a :sheath of copper around the central core. ln accordance with `this invention a helical thread `is rolled or otherwise formed about a composite conductor of this type. This is done in much the same manner in which threads are rolled upon small screws, the metal flowing under the prese sure of the rollers into the helical forni. In contradistinction to what happens when the composite bar isdrawn, the flow of metal in the rolling process is almost entirely limited to the copper sheath and the rolling process is con-- tinued until any remaining of copper which connects adjacent turns of the helix thus form-ed is `very thin; in some cases a small increase in inductance is all that is needed and may be btained by .causing a small proportion or the total energy to travel helically down the center conductor.

In the drawings:

l is a representation oi a length of the one construction ci conductor embodying the invention;

Fig. 2 is a longitudinal section oi cable utiliz ing a central conductor as illustrated in l;

Fig. 3 is a similar longitudinal section of a cable utilizing standard materials for the central conductor `Considering first Fig. 1, the drawing shows a short length of conductor in accordance with this invention before and after it been sub jected to the rolling process which gives the con duct-or its helical form. In the ligure there shown a central core l, preferably of mild steel when used with this forni of device. Surrounding the .core is a jacket of copper 3.

Conductors of this character, as thus far riescribed, Well known in the art. When formed by the process oi casting or otherwise bonding .a cylinder of copper around a steel billet and then rolling and drawing the compound billet into wire, both core and jacket being proportion ally reduced in the process, the product is well known and has been marketed under the trade name of Copper-weld. Similar wire has also been made by electroplating a copper jacket upon a `pre-dravm. wire, with or without a final drawing process, but this method is not as common commercially. It is available, however, for use in accordance with this invention.

The copper jacket 3 is then formed, preferably by rolling, into helical conlguration as shown t in Fig. l. The saine eii'ect could be secured, of course, by cutting away the more highly conductive metal between the turns of the helix; this is a more expensive process, however, not only in the tooling itself but also because off the loss of the copper, for although the chips formed by so cutting cou-1d be remelted andrecovered in part there is always a considerable loss in such processes. The rolling process is usually preferred. So used, the copper flows under the pressure of the rolls, that which originally lay in the position occupied by the grooves between the turns of the `helix being squeezed out into the turns themselves.

It is to be noted that it is not necessary that all of the more highly conducting metal be removed from the grooves in order to accomplish the-desired ellec't. The impedance between turns of the helix is made relatively high even though a film or thin layer of the copper continues to exist. This is because of the large reduction in cross-section of conductor. A portion of the cur- "rent follows the helical path Which is increased in inductance both by increase in length and by increase of ux linkage with the adjacent turns.

A conductor of this character may be used as the axial conductor in a coaxial cable as illustrated in Fig. 2. Under these circumstances the conductor 'is embedded in an insulating material 5, preferably one having a low dielectric constant and low dielectric losses, such as polyethylene. Over Jhe insulating coating is a shield or sheath l, preferably ci copper, which forrns the return conductor. -utside Iof this, again, there may be a protective jacket il, which may be Aof plastic such as thoseknown by the trade names Vinylite or nylon A cable thus formed has rather interesting characteristics. Its characteristic impedance, Z, follows `su-bstantially the coaxial cable formula rig By increasing the quantity L the impedance is raised, and with proper choice of materials and dimensions the rise may be to a value several times as great .as that with a vsimple round conductor. Other quantities remaining the same, the losses in a cable ci' this character vary inversely with the characteristic impedance. Either the total length of line or the length between repeaters may be .correspondingly increased.

Other types of coaxial cable than those using solid dielectrics are, of course, well known. The saine advantages as those accruing in the solid dielectric type are also gained if air or oil dielectric is used.

The primary advantage of the particular WD@ of conductor that has `heen described is that it coinnines improved electrical performance with high tensile strength. Where this latter is not a requirement and particularly where low initial cost is of less importance, the principles involved in the invention may be employed to even greater advantage. A modiiied form of the invention is illustrated in Fig. 3 which embodies such advantages. In this case the cor-e ll will be of a ferromagnetic material chosen for its magnetic characteristics rather than for its combination of cheapness and tensile strength, the core being either a single strand yor a number of strands of the material chosen, which may, for example, be one of the nichel-cobalt alloys ofthe Permalloy class. The actual conductor lil is' then Wound helically around the core, and may be either insulated or uninsulated. The surrounding layers, comprising the copper return conductor 'l and protective jacket il maybe the same as before. So, also, may he the insulation, which may be solid if desired or beads may be employed at intervals to separate the central and shielding conductors, using air as the primary dielectric.

Other modifications of the invention will be apparent to those skilled in the art. It is well known that the inductance of a given length of wire is greatly increased by forming it into a coil, irrespective of the use of a ferromagnetic core. With such a construction the inductance rises much more rapidly than does the resistance and therefore an appreciable gain is possible simply by winding the internal conductor about a core of any kind, although to obtain appreciable advantages the pitch of the helix will have to be less Where a non-magnetic core is used than where a magnetic type is employed. Since this is a matter of degree only, and since, vexcept for the pitch of the helix, the diagrams would otherwise be identical, this latter modication is not specifically shown. In this latter case the core could be of natural or synthetic fibre.

In the past, conductors have been made em- -ploying braided or twisted strands of copper or aluminum around a steel core for supplying tensile strength. So far as is known, however, where such structures have been used every effort has been made to secure intimate contact between adjacent strands of the more highly conductive metal, in order to simulate, most closely, a conductor having a solid conductive jacket. Where braided conductors are used they may be considered as interlaced helices having opposite directions of twist, which would neutralize their inductive effect. Where twisted structures have been employed the twist has been in one direction only if a single layer of high conductive material has been used, but where two layers have been employed one layer has usually been made with the left hand and the other with a right hand twist. Such structures give none of the advantages of that here described. The subdivision into a multiplicity of strands has added flexibility with some reduction in eddy current losses. These may be important gains, but they have no relation to the improvement in transmission characteristics of the structure here described. No claim is made to structures of the character mentioned which do not tend to increase the inductance of the composite conductor produced, the desire being that the protection granted be restricted within the limitations set forth in the following claims but not otherwise.

Having now described the invention what is claimed is:

l.. A conductor for use in communication transmission lines comprising a core of relatively low conductivity and metal of relatively high conductivity disposed about said core and connected thereto by a fused bond, a substantial y portion of said metal being, at all points along said conductor, lumped on one side of said core in the form of a continuous helix the turns whereof are sufficiently separated so that a portion of the electrical energy follows the helical path.

2. A conductor in accordance with claim 1 wherein said core is ferromagnetic.

3. A transmission-line cable comprising a central conductor in accordance with claim 1, and a tubular conductor surrounding and insulated from said central conductor and substantially coaxial with the core thereof.

4. A transmission-line cable in accordance with claim 3 wherein said core is ferromagnetic.

5. A coaxial cable comprising a central core, a helical conductor disposed about said core in electrical contact therewith and connected thereto by a fused bond, adjacent turns of said conductor being disposed so as to confine a major portion of current flowing in said cable into a unidirectionally rotating helical path, and a tubular conductor surrounding and insulated from said helical conductor and substantially coaxial with the core thereof.

LENNART G. ERICKSON. KURT E. APPERT.

References Cited in the ille of this patent UNITED STATES PATENTS Number Name Date 759,120 McGeorge May 3, 1904 796,100 Wilson Aug. 1, 1905 1,119,246 Cuntz Dec. 1, 1914 1,711,832 Cooper May 7, 1929 2,008,227 Reilley July 16, 1935 2,250,907 Edwards July 29, 1941 2,321,021 Dyer June 8, 1943

Images