FR2753561A1 - Transmission line for high-frequency signals - Google Patents

Abstract

The conductors, made up in twisted pair or quad construction cables, employ solid, i.e. unstranded, aluminium or aluminium alloy cores (PC) bonded to thin outer claddings (RV) of copper or copper alloy. Due to skin effect, at frequencies rising above 1 MHz, increasing proportions of the current carried flow in the thin copper cladding. The overall conductor diameter lies typically between 0.4 and 0.8 mm, the thickness of cladding between 5 and 100 micrometres. The composite conductors are drawn down to the required diameter using well established techniques developed for conventional conductors, passing through successive dies, then annealed and covered with extruded plastic insulation (G), before being made up into cables with tape or sheath overall protection.

Description

Transmission line for high frequency signals
The present invention relates to a telecommunication for high-frequency signals of the order of several hundreds of Hz to several hundred MHz, and more particularly conductive cores son insulated conductors of a transmission line consisting of two twisted son. Such a transmission line is also called "pair" and is distinguished from a coaxial line not belonging to the field of the invention, by two separate insulated wires, and by the small diameter of the conductive cores, less than one millimeter. When the line has three or four twisted wires, it is called "third or" quarter.

 An insulated conductor wire comprises a conductive copper core wire most often surrounded by an insulating sheath of a thermoplastic material.

Generally, the conductive copper core has a diameter of between 0.45 and 0.7 mm, for example 0.51 mm, or 0.60 mm, or 0.64 mm. The insulating sheath is made of polyolefin, polyethylene for example, and can be solid or cellular. The outer diameter of the insulated wire is typically between 0.9 and 1.4 mm.

 Several insulated conductors are paired (2-wire) or joined in quads (4-wire), and then bundled into bundles typically 4 pairs or 2 quads, such as 4 pairs, or 2 times 4 pairs, or 3 times 4 pairs for cables horizontal distribution, called capillary cables, and 24, 32, 64 or 128 pairs for vertical distribution cables, called ring cables.

 The evolution of LAN cables over the last ten years is characterized by a steady increase in bit rates, some MHZ in the mid-1980s, 100 MHz today, 150 MHz in the near future and 300 or 600 MHz in the medium term, and by a continuous decrease in prices despite the increase in flows.

 The conductive core of the insulated conductor wires for local networks is made from annealed solid copper, with the exception of flexible insulated conductor wires for cords consisting of divided copper core cores. In current practice, a wire worm of larger diameter is drawn to obtain a reduction in the nominal diameter of the core. This drawing is followed by annealing and then insulation of the wire by the insulating sheath.

These three steps are generally performed continuously in succession ("tandem" operation).

 Copper, silver or gold have much higher electrical conductivities than other common metals. Since copper has a lower manufacturing cost and is more abundant than silver, it has been chosen for most telecommunication cables. However, copper still contributes to the relatively high cost of the cables.

 The present invention aims to provide a conductor for transmission line for high frequency signals that reduces the cost of manufacturing insulated conductors, but without changing the traditional steps of drawing, heat treatment and insulation.

 To this end, a transmission line for high frequency signals comprising at least two insulated conductor wires is characterized in that each of the insulated conductor wires comprises a conductive core composed of a central portion made of a first conductive material, and a coating surrounding the central portion, said coating being a second conductive material having an electrical conductivity greater than that of the central portion.

 In a preferred embodiment, the second conductive material on the outside of the wire is copper or low-alloy copper, and the first conductive material inside the wire is aluminum or low-alloy aluminum.

 The electrical behavior of the conductive core is similar to the traditional all-copper one for the transmission of high frequency signals, since the current lines are almost concentrated in the outer layer, preferably in copper because of the effect of skin.

Due to its predominantly center aluminum, the composite core is less expensive and lighter than solid copper for making LAN cables. The drawing, annealing and insulating steps to obtain an insulated conductor wire according to the invention are implemented without modifying the equipment or the manufacturing procedure for the traditional insulated wires with a 100% copper core.

 The invention also relates to a method of manufacturing an insulated conductive wire for a transmission line which is characterized by drawing a wire blank having a central portion of a first conductive material, such as aluminum, surrounded by a coating material. a second conductive material, such as copper, in said conductive core, the diameter of the blank being greater than that of the core of the conductor, and the thickness of the coating of the blank being greater than that of the coating of the soul.

 When the ratio of diameters between blank and core is too large, the drawing comprises several roughing passes, continuously or in separate operations, in several drawing machines so as to gradually reduce the diameter of the blank.

 The drawing of the blank, the coating of which can be performed by electrolytic deposition, uses the same (s) wire drawing machine as for a conventional solid copper core.

Other features and advantages of the present invention will emerge more clearly on reading the following description of several preferred embodiments of the invention with reference to the corresponding appended drawings in which
- Figure 1 is a schematic longitudinal view of a transmission line with two twisted son;
FIG. 2 is a cross-sectional view on a larger scale of one of the insulated conductor wires in the transmission line according to the invention; and
- Figure 3 shows schematically a continuous chain of insulated wire manufacturing according to the invention.

 A transmission line comprises two twisted insulated conductor wires F1 and F2 each comprising a wired conductive core A surrounded by a sheath of insulating material G, as shown in FIG. 1. For example, the line F1-F2 constitutes a pair of wires twisted with other pairs in a cable.

The linear attenuation a of the transmission line for high frequency signals, for example in the frequency range between 1 and 600
MHz, is proportional to the linear resistance R / 2 of the conducting wires F1 and F2 according to the relation
R 2Z
c
where Zc denotes the characteristic impedance of the line F1-F2.

At a high frequency above about 1 MHz, the line resistance R increases as a result of the skin effect, so-called skin effect. This increase in resistance is due to the increasingly marked concentration of the current lines to the external surface of the conductive core A in AC when the frequency increases. At a constant frequency, the current density thus increases from the center of the wired conductive core to its periphery. The depth of penetration 5 is written

 with Kl the permeability of the metal of the conductive core, f the frequency, and the electrical conductivity of the metal. It is as if the useful section of the conductor decreases from nu2 / 4 to 2sD6, where D is the diameter of the wired conductive core.

est ainsi constant pour un métal donné, par exemple égal à 66 pour le cuivre et 84,5 pour l'aluminium.In the case of transmission lines for high-frequency signals, the metal of the conductive cores is non-magnetic, so that the permeability of the metal is that of the vacuum: A = pO = 4 It. . The product

is thus constant for a given metal, for example equal to 66 for copper and 84.5 for aluminum.

The depth of penetration is even lower than the frequency f is high and the conductivity u is high and therefore the metal is more conductive. The linear resistance in Q / m is written

 and increases when the conductivity of the metal decreases.

The best electrically conductive materials readily available are by order
6 decreasing conductivity expressed in 106 S / m silver with a = 61, copper with o = 58, gold with o = 40.6 and aluminum with a = 35.4. Gold and silver are not used for LAN cables because of their cost. Aluminum is in particular slightly less good conductor than copper but much lighter and less expensive and has never been used in transmission lines twisted pair including LAN. The following table shows the respective penetration depths for these two metals at different frequencies.

<Tb>

<SEP> DEPTH <SEP> FROM <SEP> PENETRATION <SEP> (ym) <SEP>
<tb> FREQUENCY <SEP> (MHz) <SEP> COPPER <SEP> ALUMINUM
<tb><SEP> 1 <SEP> 66 <SEP> 84.5
<tb><SEP> 10 <SEP> 20.87 <SEP> 26.72
<tb><SEP> 100 <SEP> 6.66 <SEP> 8.45
<tb><SEP> 300 <SEP> 3.81 <SEQ> 4.87
<tb><SEP> 600 <SEP> 2.69 <SE> 3.45
<Tb>
At high frequency, the penetration depths in copper and aluminum are substantially equal. Above 10 MHz in particular, penetration depths are low.

 According to the invention, any first "good conductor" material is used in the core portion PC of the core A of the wires in the transmission line as shown in FIG. 2. The electrical resistance of the first material even slightly higher than that the second material of an RV coating covering the first material provides suitable attenuations. In particular, the higher the frequency, the more the resistance of the core is close to that of the outer coating RV surrounding the central cylindrical solid part PC.

For reasons of transmission, the coating
RV is a layer of a second highly conductive material having a thickness of a few tens of ssm, such that 0.05 <2e / D <0.5, where D is the outside diameter of the composite core A in the range 0, 4 and 0.8 mm; in practice the thickness e is preferably between 5 and 100 ssm, and chosen greater than the penetration depth indicated in the previous table for the lower limit of the band of the useful frequency of the transmission line.

 The wires in the transmission lines and consequently in the cables containing them, made with composite conductive cores according to the invention, are less expensive than the copper core according to the prior art if one chooses a set of first and second conductive materials whose price per unit volume is lower than that of copper, and are lighter if a second internal conductive material less dense than copper is chosen.

 According to a preferred embodiment, the central portion PC is made of aluminum and is coated on the outside with a copper RV layer, the copper having an electrical conductivity of 58 greater than that of aluminum equal to 35.4.

 The term "aluminum" within the meaning of the invention includes both aluminum metal and alloys in which the aluminum is weakly alloyed with at least one other metal; similarly, the term "copper" in the sense of the invention includes both copper metal and cuprous alloys where the copper is weakly alloyed with at least one other metal. In these alloys of aluminum and cuprous are for example included with a content of 0.1 to 5%, at least one of the following metals: beryllium, cadmium, chromium, cobalt, tin, magnesium, manganese, nickel, zirconium. The addition of at least one of these metals particularly in the central portion PC contributes to increasing the mechanical strength of the conductive core A, especially as the wire core is thin. The ductility of the yarn is thus increased especially for drawing operations, or more simply drawing.

 As shown in FIG. 3, the wire drawing starts from a wire blank E having a larger diameter of between 2 and 8 mm and having a cylindrical central portion PCE of diameter DE which has been coated with a thick copper coating RVE. eE electrolytically, with DE> D and eE> e. The blank E is drawn in a die FR of a drawing machine, or in successive drawing die sectors, along the arrow FL, in the conductive core according to the invention.

In practice, the ratio of the homothety [(DE / eE) / (D / e)) of the drawing between the first conductive material, such as aluminum, for central parts PCE and PC roughing and core and the second Conductive material, such as copper, for RVE and RV roughing and core coatings is between 0.8 and 1.2.

Then, the drawn composite core is annealed in an FO oven so as to make the soul more flexible.

The annealed composite core is then sheathed by passing through the head of an extruder BO from which emerges the conductor wire F1, F2 isolated with the extruded sheath G.

The invention makes it possible to manufacture insulated wires having transmission properties similar to those of the current conductive wires but with a less expensive and lighter wire. Depending on the thickness of copper or copper alloy chosen, the attenuation which is related to the electrical resistance may be of a slightly higher level between 1 and some
MHz. Other transmission properties are not affected.

 Some examples illustrate below the object of the invention.

- Example 1
Roughing E in copper-plated aluminum having an external diameter DE = 1.63 mm and a copper thickness eE = 40 μm
drawing of the blank to the outer diameter of D = 0.51 mm with a copper coating thickness e of about 15 Cim.

 Isolated insulated conductors F1, F2 with a conductive core A according to Example 1 are twisted in pairs and assembled into a bundle of 4 pairs which is placed under a ribbon or a thin plastic holding sheath, itself surrounded by a shielding metal screen laid lengthwise. The assembly is then sheathed to obtain the cable.

- Example 2
Coppered aluminum blank having an external diameter DE = 8 mm and a copper thickness eE = 800 μm;
reducing this first blank to a second blank having an outer diameter of 2.5 mm and a copper thickness of 255 ssm by a first pass in a drawing machine;
drawing of the second blank in a core A having an outside diameter D of 0.52 mm and a copper coating thickness e of 50 μm per pass in a second drawing machine, and continuous insulation of the core by a sheath G.

 According to this second example, the continuous production line thus comprises first and second drawers, an annealing furnace and an extruder.

 Then pairs of conductive wires with cores according to Example 2 are twisted and assembled into a bundle of 4 pairs.

 Each beam is placed under a holding sheath surrounded by a metal shield screen placed lengthwise. The assembly is then sheathed to obtain the cable.

- Example 3
Roughing E copper-plated aluminum having an outer diameter of DE = 2.5 mm and a copper thickness eE = 65 Zm;
drawing of the blank and in-line insulation to the outer diameter of D = 0.64 mm with a copper coating thickness e of 17 ssm, and continuous insulation of the core A thus obtained by a sheath G.

 Then isolated conductor son according to Example 3 are twisted in quads. Two fourths are put under a common support sheath which is surrounded by a shield screen placed lengthwise. The assembly is then sheathed to obtain the cable.

Claims (9)

 1 - Transmission line for high frequency signals comprising at least two insulated conductor wires (F1, F2), characterized in that each of the insulated conductor wires comprises a conductive core (A) composed of a central portion (PC) in one first conductive material, and a coating (RV) surrounding the central portion, said coating being a second conductive material having an electrical conductivity greater than that of the central portion.  2 - transmission line according to claim 1, wherein the first conductive material is aluminum or low-alloy aluminum, and the second conductive material is copper or low-alloy copper.  3 - transmission line according to claim 1 or 2, wherein the outer diameter (D) of the core (A) is between 0.4 and 0.8 mm, and the thickness (e) of the coating ( RV) is between 5 and 100 Hm.  4 - transmission line according to claim 1 or 2, wherein the ratio between twice the thickness (e) of the coating (RV) and the diameter (D) of the core (A) is between 0, 05 and 0.5.  5 - A method of manufacturing an insulated conductive wire for transmission line according to any one of claims 1 to 4, characterized by the drawing (TR) of a wire blank (E) having a central portion (PCE) in a first conductive material surrounded by a coating (RVE) in a second conductive material in said conductive core (PC, RV), the diameter (DE) of the blank being greater than that (D) of the core, and the the thickness (eE) of the coating (RVE) of the blank being greater than that (e) of the coating (RV) of the core.  The method of claim 5, wherein said drawing comprises a plurality of roughing passes in drawers.  7 - Process according to claim 5 or 6, wherein the ratio of homothety [(DE.e) / (eE.D)] of the drawing between the first conductive material for central parts (PCE, PC) roughing and core and the second conductive material for coatings (RVE, RV) roughing and core is between 0.8 and 1.2.  8 - Process according to any one of claims 5 to 7, comprising following the drawing, an annealing (FO) of the drawn core, and a cladding (BO) of the core annealed by an insulating sheath (G ).  9 - Process according to any one of claims 5 to 8, wherein the first conductive material is aluminum or low-alloy aluminum, and the second conductive material is copper or low-alloy copper.