GB2227600A - Stranded transmission line - Google Patents

Abstract

A transmission line comprises a pair of twisted conductors wherein each conductor has a plurality of twisted strands within an insulating sheath, the arrangement being such that the strands in each conductor are twisted in the same sense as the twist of the conductors. Preferably the lay length of the strands and conductors are identical. This arrangement allows current to flow along the same wire strands at high frequency without incurring contact resistance and thus an increase in signal attenuation.

Description

Stranding Lr?D! The invention relates to stranded electrical conductors and in particular to stranded conductors arranged as twisted pairs.

Stranded conductors are carrnonly used in cables which are required to be flexible and to withstand bend fatigue. Stranded conductors are often formed by laying 6 component wires around a central wire with some lay angle chosen to give a desired level of mechanical flexibilty. The stranded conductor is then coated with an insulating material before being subject to a further stranding operation with another conductor to form a twisted pair.

For some cable api?lications four conductors may be stranded together to form a quad consisting of two separate pairs. The lay angle of the twisted pair or quad may be selected to give mechanical flexibilty and to minimize crosstalk effects when the pair or quad is strand into a multi-pair cable centre.

Although stranded conductors can withstand repeated bending, and are essential in many cable applications, normal stranded construction arrangents cause an increase in the high frequency resistance wiich reduces transmission performance.

The object of the present invention is to provide a stranded conductor twisted pair with transmission characteristics substantially the same as a solid wire conductor twisted pair.

The invention provides a transmission line comprising a pair of twisted conductors wherein each conductor has a plurality of twisted strands within an insulating sheath, the arransement being such that the strands in each conductor are twisted in the same sense as the twist of the conductors.

Preferably the lay length of the strand twist is identical to the lay length of the conductor pair twist.

The inventor has discovered that in the above arrangement the small individual solid wires which make up one stranded conductor are in the same positions relative to the individual solid wires which make up the other conductor in the pair. It is postulated that in this arrangenent the high frequency current flows along the sane individual wires wilich make up the conductor and is not forced by skin and proxir;iity effects to cross fray wire to wire incurring a contact resistance and an increase in signal attenuation.

The invention will now be described by way of exa wle only with reference to the accampanying Drawings of which: Figure 1 illustrates a stranded conductor; Figure 2 illustrates the skin effect of current flow in an isolated conductor pair at high frequency; Figure 3 illustrates the influence of the proximity effect on the Figure 2 skin effect; Figure 4 shows the high frequency current flow along a conventional twisted r;ulti-strand conductor and Figure 5 is a cross-section through quadded pairs to illustrate the invention.

A nulti-strand conductor 10, is shown in cross-section in Figure 1.

The cable comprises six individual wires 11, laid at some helix angle around a central wire 12. Conductor stranding arrangements other than a six round one construction are used and the invention will apply equally well to these cases. Stranded conductors can withstand bend fatigue effects which would otherwise cause failure in a single wire of larger cross-section. At low frequencies the attenuation of stranded and solid conductors with the same total cross-sectional area of copper is equal. As the frequency increases the AC resistance of both solid and stranded conductors increases, however, the AC resistance increase is found to be greater in stranded areas.

Figure 2 illustrates the current flow in a pair of isolated conductors at high frequencies. The e current flows along the outsides 21,22 of the conductors 23,24 and the higher the frequency the thinner the skin within which the current flows. This well known skin effect causes the increase in AC resistance observed at high frequencies.

If the conductors which make up the pair are in close proximity because they have been formed into a cable there is also a proximity effect at high frequencies which distorts the current flow in the conductors and this further increases the AC resistance. This is illustrated in Figure 3 which shows that the attraction of opposite currents in the two conductors 31, 32, causes the skin current to flowprecom,inantly in the adjacent surface regions 33, 34.

Figure 4 illustrates the high frequency current flow 40 in a conventional nulti-strand conductor 41 as postulated by the inventor.

Because of the combination of the skin and proximity effects the current flow follows the lines 40 on the surface adjacent to the return current cable (not shown). Thus the current continuously flows from one strand wire to another encountering inter-wire contact resistance between adjacent wires.

Measurements of AC resistance, DC resistance, and attenuation in dBs/km are given in the Table for a solid conductor pair and a standard stranded conductor pair for various frequencies from 700 kHz to 1.6 I7EIz. In this particular case botn the solid and stranded conductor pairs were twisted or stranded into a quad as the measurements were taken as part of an investigation into the performance of pairs arranged in quads. As can be seen from the Table, the AC/DC resistance ratio of the stranded conductor pair in a standard quad is significantly higher than the resistance ratio measured in a solid conductor pair. Tllis nay be explained by the inter-wire contact resistance effect.

SQLID CONDUCTOR MATCHED LAY PAIR ST:DA PAIR PAIR FREQUENCY kHz AC RESIST ZEl AC RESIST ATTEN AC RESIST ATTEN 700 377.7 13.6 445.1 15.4 1000 435.1 16.0 458.3 16.7 567.7 20.4 1300 505.9 19.0 536.9 20.3 679.4 24.9 1600 584.3 22.5 617.7 24.0 785.0 29.5 DC RESIST 108.6 118.0 116.9 AC/DC RESIST AT 1600 k}Iz 5.38 5.23 6.72 I I I Inter-wire contact resistance effects on the A: : resistance of stranded conductors can be largely overcone by matched lay stranding, where the lay length of the conductor strand is equal to the lay length of the helix in which the conductors are twisted together to form a pair and in the same sense. In this arrangement the component wires of one conductor rer.lain in the same positions relative to the component wires of the other conductor along the length of the twisted pair. Two conductor pairs in quadded construction 51-54 are shown in Figure 5 in a cross-section plane. le same strand wires 55,56 of conductors 51,52 which form the first pair in the quad are always nearest each other and similarly for the second pair, strand wires 57,58 are always in closest proximity in conductors 53,54.This arrangement of match lay stranding applies equally to single conductor pairs as to the two pairs of conductors laid up as shown in Figure 5.

7ne Table above gives measureDents of AC resistance in a stranded conductor pair in a matched lay quad. These may be cowpare with the results obtained from a standard quad arrangetnt made up fron stranded conductors where there is no matching between the lay of the conductor strand and the lay of the quad. All the quads had the sane spacing between the conductor pairs and the same overall dimensions. As can be seen from the Table tile matched lay quad has an AC to DC resistance ratio significantly less than that of a pair in standard quad and equal to that of the solid conductor case.If thematcneG lay quad conductors had the same quantity of copper per unit length as the solid conductors the DC resistances would be the same and therefore the AC resistance would also be the Same across the frequency range.

TL neasurements in the Table were nave on stranded conuctors which were compacted. If non-compacted conductors had been used, the resistance increase in the standard quad caused by stranding would have been greater.

5atched lay stranding will therefore give a relatively greater reduction in resistance in the case of non-coppacted stranded conductors. The improvement offered by matched lay stranding will also increase with frequency as the skin and proximity effects became pronounced.

By matching the stranding of conductor wires and the subsequent stranding of these conductors into twisted pairs or quad formation in a cable as described above: a) the AC resistance of the twisted pair is equal to that of a twisted pair with solid copper conductors with the same cross section of copper and with the same spacing between the conductors.

b) by using matched stranding, a cable manufacturer may produce twisted pairs or quads which have good mechanical properties anG which offer the same electrical transmission perfonrance as solid conductors.

Claims (2)

Claims

1. A transmission line comprising a pair of twisted conductors wherein each conductor has a plurality of twisted strands within an insulating sheath, the arrangement being such that the strands in each conductor are twisted in the same sense as the twist of the conductors.

2. A transmission line as claimed in claim 1 wherein the lay length of the strand twist is identical to the lay length of the conducted pair twist.