FR2785715A1 - HIGH FREQUENCY PAIR OR QUARTE TRANSMISSION CABLE - Google Patents

Abstract

The cable comprises two or four insulated conductor wires (11, 12) having a diameter d and symmetrical with respect to a longitudinal axis (XX). In order to remedy the crosstalk between pairs or quads of wires while giving low dielectric losses, a dielectric means (41-42) is contained in an axial dummy cylinder (C1) having a diameter greater than 2 d and tangent to the dielectric cylinder following at most a number of tangency lines (T1-T2) equal to the number of threads. The dielectric means maintains the wires (11, 12) inside the dummy cylinder with an air gap (I1, I2) between each wire and the cylinder. For example, the dielectric means comprises as many dielectric rods (41, 42) as there are wires (11, 12); each ring is tangent to two son along two lines of tangency and to the fictitious cylinder along a line of tangency (T1, T2).

Description

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  The present invention relates to a transmission cable for high frequency signals, telephone and / or data processing and video (voice data, images). The cable comprises at least a pair or a quarter of individually insulated electrical conductor wires which can be surrounded by a

holding envelope.

  In a high frequency transmission cable of this type, the insulated conductor wires forming a pair or a fourth star are arranged perfectly symmetrically with respect to a longitudinal axis of cable. Any lack of symmetry in the pair or the quadrant leads to parasitic couplings and therefore transmission errors which can only be corrected depending on the data rate

  transmitted in the pair or the quarter.

  The higher the data rate transmitted in the cable and the higher the frequency bandwidth, the more difficult it is to maintain good transmission quality in the cable. In particular, crosstalk phenomena between pairs

  or between quartes quickly become unacceptable.

  When the envelope surrounding the pair or quarter is an individual shielding screen, this helps to avoid crosstalk between the pair or quarter and others. Furthermore, to protect transmission cables against electromagnetic interference or coupling (EMC), it is known to collectively coat a set of

  pairs or quads with a continuous metallic screen.

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  However, an individual or collective screen generates crosstalk phenomena by the third circuit thus created. Depending on the distance between screen and pair or quarter, the characteristics of each pair or quarter, such as characteristic impedance and

  linear loss, can be changed.

  The invention aims to remedy the drawbacks mentioned above, and more particularly to separate the pairs or quads from each other in a cable comprising several pairs or quads, or to separate from the pair or quarter

  the screen surrounding the pair or quarter.

  To this end, a high frequency transmission cable comprising a predetermined number of individually insulated electrical conductor wires having a predetermined diameter d and symmetrical with respect to a longitudinal axis, is characterized in that it comprises dielectric means contained in a cylinder fictitious having a diameter greater than 2d, centered on the longitudinal axis and tangent to the fictitious cylinder following at most a number of lines of tangency equal to the number of wires to keep the electrically conductive wires isolated inside the fictitious cylinder with a gap air between each

wire and the dummy cylinder.

  Thanks to the dielectric means, the electrically conductive wires isolated from a pair or quarter are kept at a relative distance from the electrically conductive wires isolated from another neighboring pair or quarter in a cable with several pairs or quads; between neighboring pairs or quads is created by the invention an air gap having a sufficient width, preferably greater than d, which allows

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  to reduce couplings, in particular to get rid of crosstalk phenomena between pairs

or quartes.

  In addition, thanks to this air gap, the high frequency transmission cable according to the invention is a cable with low losses. In fact, the linear capacity of the cable being inversely proportional to the ratio of the diameter of the fictitious cylinder to the diameter d of the insulated electrical conductor wires, the linear capacity decreases when the diameter of the fictitious cylinder coincides with the internal surface of a holding envelope such that a metallic screen increases, or when the permittivity (dielectric constant) of the medium surrounding the wires decreases. As the linear loss of the transmission cable is proportional to the square root of the linear capacity, the cable insertion losses decrease when the cylinder diameter

fictitious increases.

  According to preferred inexpensive embodiments, the dielectric means comprises a number of dielectric rods equal to the number of insulated dielectric wires. Each rod is tangent to two wires along two respective tangent lines and to the cylinder

  fictitious along a respective line of tangency.

  When the number of wires is two, that is to say when the cable comprises a pair, two rods are joined to each other along the longitudinal axis, and each insulated electrical conductor wire is housed in a substantially V-shaped groove formed between the two adjoining rods, the diameter of the rods being greater than 1.5 d; or the two insulated electrical conductors are joined to each other along the axis

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  longitudinal, and each of the two rods is housed in a substantially V-shaped groove formed between the two insulated electrical conductor wires, the diameter of the rods being greater than 0.66 d; or else the two insulated electrical conductor wires are symmetrically spaced from one another along the longitudinal axis, and the two rods are symmetrically spaced from each other along the longitudinal axis, the rod diameter

being greater than d.

  When the number of threads is four, that is

  say when the cable comprises a quarter, that is to say each of the four rods is joined longitudinally to two other rods, and each insulated electrical conductor wire is housed in a substantially V-shaped groove formed between two respective joined rods, the diameter of the rods being greater than 2 , 4 d; either each insulated electrical conductor wire is joined longitudinally to two other wires, and each of the four dielectric rods is applied in a substantially V-shaped groove formed between two insulated insulated electrical conductor wires, the

  diameter of the rods being greater than 0.41 d.

  In the first variants above for pair or quarter, the distance of the metallic conductors of the same pair (quarter) decreases the "proximity" effect which plays a harmful role on the attenuation. The latter is thus still

improved.

  The insulated electrical conductors and the dielectric rods are preferably twisted together around the longitudinal axis so as to

  naturally hold them together.

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  At least one of the dielectric rods can be tubular. The introduction of tubular rods into the cable increases the volume of air between two or four insulated electrical conductors forming a pair or quarter in the cable and another pair or neighboring quarter, which decreases the permittivity of the cable and

  consequently the linear weakening of this one.

  At least one of the dielectric rods can

  include at least one optical fiber.

  The air gap between each wire and the fictitious cylinder can be at least greater

substantially at d / 2.

  According to another embodiment of the invention which is less bulky but more expensive, the insulated electrical conductor wires are joined longitudinally two by two, and the dielectric means comprises a dielectric strand wound helically around

  all of the insulated electrical conductors.

  Preferably, the dielectric strand has a cross section diameter substantially greater than d / 2, and the pitch of the helical strand is less than the pitch of

  twisted insulated electrical conductors.

  The cable may include a cylindrical envelope whose inner surface coincides with the fictitious cylinder. The envelope may be a shielding screen with at least one metal face, or

  well a dielectric holding sheath.

  Other characteristics and advantages of the present invention will appear more clearly on

  reading the following description of several

  preferred embodiments of the invention with reference to the corresponding appended drawings in which:

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  - Figure 1 is a cross section of a transmission cable with two insulated electrical conductors spaced apart by two solid dielectric rods joined according to a first embodiment of the invention; - Figure 2 is similar to Figure 1, but with two hollow dielectric rods; - Figure 3 is a cross section of a transmission cable with two insulated insulated electrical conductor wires and two solid dielectric rods spaced apart according to a second embodiment of the invention; - Figure 4 is similar to Figure 3, but with two hollow dielectric rods; - Figure 5 is a cross section of a cable comprising four elementary cables such as that shown in Figure 1; - Figure 6 is a cross section of a transmission cable with two insulated electrical conductor wires spaced apart by two solid dielectric rods spaced apart according to a variant of the first and second embodiments; - Figure 7 is a cross section of a transmission cable with four insulated electrical conductors spaced apart by four solid dielectric rods according to a third embodiment of the invention; - Figure 8 is similar to Figure 7 but with four hollow dielectric rods; - Figure 9 is a cross section of a transmission cable with four insulated insulated electrical conductor wires and four solid dielectric rods according to a fourth embodiment of the invention;

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  - Figure 10 is similar to Figure 9 but with hollow dielectric rods; - Figure 11 is a perspective view of a section of transmission cable comprising a pair of insulated electrical conductor wires, surrounded by a helical dielectric strand according to a fifth embodiment of the invention; - Figure 12 is a cross section of the cable section shown in Figure 11; and - Figure 13 is a cross section similar to Figure 12 relating to a cable comprising a quarter of insulated electrical conductor son. With reference to FIGS. 1 and 3, high frequency transmission cables CAl and CA2 according to first and second embodiments of the invention each comprise a pair of individually insulated electrical conductors 11 and 12. Each insulated electrical conductor wire consists of a solid conductor or a strand of fine annealed copper wires 2 and an individual insulating sheath 3 surrounding the solid conductor 2. For example, the sheath 3 has an outside diameter d of the order of 0.5 to 1.6 mm and is in solid polyethylene or polypropylene, or cellular, or cellular and massive. The cable CAl is axially symmetrical, that is to say the insulated electrical conductor wires 1 and 2 are symmetrical with respect to a longitudinal axis of

  XX symmetry and twisted around it.

  The transmission cable CAl, CA2 also comprises a dielectric means in the form of two

  cylindrical stuffing rods 41 and 42, 51 and 52.

  The rods are made of low-thermoplastic material

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  permittivity (dielectric constant), for example in extruded thermoplastic polymer, such as polyethylene or polypropylene or fluorinated, solid or cellular polymer. The rods 41 and 42, 51 and 52 are twisted helically together with the insulated conducting wires 11 and 12 around the longitudinal axis XX. As shown in Figures 1 and 3, the rods 41 and 42, 51 and 52 symmetrical with respect to the axis XX are tangent inside a fictitious cylinder Cl, C2 of predetermined diameter Dl, D2 along two lines helical and parallel tangency T1 and T2 diametrically opposite with respect to the longitudinal axis XX of the cable. In cross section of the cable, the diameter passing through the axes of the rods is perpendicular to the diameter passing through the axes of the

insulated conductors 11 and 12.

  According to the first embodiment shown in Figure 1, the rods 41 and 42 are tangent to each other along the longitudinal axis XX, while being twisted around this axis, and have a diameter DJ1 = Dl / 2. The insulated electrical conductor wires 11 and 12 are held in diametrically opposite helical V-shaped grooves formed between the rods 41 and 42 by twisting both the rods and the wires. Each insulated conductor wire 11, 12 is applied along two lines of

  respective helical tangency of rods 41 and 42.

  In order for the conductive wires thus applied to be contained in the fictitious cylinder Cl, it is necessary

  that the DJ1 diameter of the rods is greater than 1.5 d.

  The diameter DJ1 of the rods 41 and 42 is greater than approximately 2d so that an air gap Il, I2 between each insulated conductor wire 11, 12 and the interior of the fictitious cylinder Cl has a radial width LI

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  greater than approximately d / 2, i.e. greater than DJ1 / 4 = approximately D1 / 8. This gap width is the distance between each insulated conductor wire 11, 12 and the inner surface of a cylindrical holding envelope 61 merged with the fictitious cylinder C1, as in the cable CAla shown in FIG. 2 according to a variant of the first achievement. In practice, when two elementary cables CAla are joined longitudinally, the minimum distance between a conductive wire isolated from one pair and a conductive wire isolated from the other pair is greater than 2LI and is sufficient for signals in one of the pairs

  do not disturb signals in the other pair.

  For example, as shown in FIG. 5, a cable comprises four elementary cables CA1 according to the invention, that is to say four pairs of insulated conductive wires 11-12 twisted each with two dielectric rods 41 and 42. The four elementary cables are maintained and protected in a collective shielding screen 7, possibly surrounding an insulating holding tape. The screen 7 has at least one metallized face, or is completely metallic. An outer protective sheath 8 in insulating material such as PVC, or in halogen-free flame retardant material surrounds the screen 7. Tear wires, not shown, inside the screen and between the screen 7 and the sheath 8 and an electrical continuity wire, not shown, between the screen 7 and the sheath 8 are provided. As a variant, the metal screen 7 is replaced by a sheath made of halogen-free flame-retardant thermoplastic material with a thickness of a few tenths of a millimeter, surrounded by a thin complex aluminum polyester screen of a few

tens of micrometer.

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  If we take care to longitudinally offset the helix pitch of the twists formed by the four pairs 11-12 to further reduce the electromagnetic coupling between pairs, as shown in Figure 5, the distance between two wires of two different pairs in four pair cable is fine

  greater than the preferred minimum distance 2LI = d.

  Referring to Figure 3 showing the second embodiment, the rods 51 and 52 in the cable CA2 are not tangent to each other, but on the other hand the insulated conductive wires 51 and 52 are tangent to each other along the longitudinal axis of the cable XX. Against diametrically opposite V-shaped grooves formed between the insulated conductive wires 11 and 12, the rods 51 and 52 are applied respectively by twisting around both the rods 51 and 52 and the wires 11 and 12. Each rod 51, 52 is in mechanical contact along two respective helical tangent lines with the conductive wires

isolated 11 and 12.

  In this second embodiment, the gap Il, I2 between each insulated conductor wire 11, 12 and the interior of the fictitious cylinder C2 is created when the diameter DJ2 of the rods 51 and 52 is greater than 0.66 d. Preferably, the width LI of the interstices Il and I2 is greater than approximately d / 2, which corresponds to a minimum width of interstice of approximately D2 / 6. The diameter of the rods 51 and 52 is preferably at least substantially equal to approximately d. The second embodiment of cable CA2 is thus less bulky than the first embodiment, with equal diameter d of wire

insulated conductor.

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  By analogy with FIG. 2, a variant of cable CA2a of the second embodiment comprises a cylindrical holding envelope 62 whose inner surface coincides with the fictitious cylinder C2, that is to say tangent with each of the rods 51 and 52 and separated by a distance LI from the conducting wires

isolated 11 and 12.

  The cylindrical holding envelope 61, 62 in FIG. 2, 4 is a metallic shielding screen in the form of a tube having at least one metallic face or is completely metallic. According to another variant, the casing 61, 62 is a dielectric tubular retaining sheath containing the pair

11-12.

  According to other variants of the first and second embodiments, the solid rods 41 and 42, 51 and 52 are replaced by stuffing tubes 41a and 42a, 51a and 52a made of thermoplastic material with low permittivity, of the same outside diameter DJ1, DJ2, as shown in figures 2 and 4. These variants help to reduce the linear weakening of the cable thanks to the additional air trapped in

  the stuffing tubes 41a and 42a, 51a and 52a.

  In the multi-pair cable shown in FIG. 5, the elementary cables CAl can be partially or totally by cables CA2 such as that shown in FIG. 3, and / or by cables

  CAla and / or CA2b shown in Figures 2 and 4.

  According to a variant of the first and second embodiments shown in FIG. 6, a transmission cable CA2b has two insulated electrical conductors 11 and 12 which are symmetrically spaced from the longitudinal axis of the cable, and two solid or hollow dielectric rods, 51b and 52b

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  which are also symmetrically spaced from the longitudinal axis of the cable. In cross section, in a manner substantially analogous to a quarter of conductors, the centers of rods 5lb and 52b of diameter dj are located at two opposite vertices of a square with side dj, and the centers of conductors 11 and 12 are located substantially in said square under the other two opposite vertices of the square. The diameter d of the conductors 11 and 12 is less than the rod diameter dj. Thin interstices of width LI are thus provided between the insulated conductors 11 and 12 and the internal periphery C2b of a cylindrical holding envelope 62b which is tangent only to the dielectric rods 51b and 52b. The internal diameter of

  the envelope 62b is substantially equal to 2.41 dj.

  The paired elementary cable structures shown in FIGS. 1 to 4 correspond to similar structures for star quads.

shown in Figures 7-10.

  According to a third embodiment, in the cable CA3, CA3a shown in FIG. 7, 8, four solid or tubular stuffing rods 43 to 46, 43a to 46a are twisted together with two pairs of insulated conducting wires 11-13 and 12-14 around the longitudinal axis of cable XX. Each wire 11 to 14 has two tangent points in cross section with

  two respective adjacent rods 43-44, 531-54 to 46-

  43, 56-53 which have in common a line of tangency which, in cross section, is aligned radially with the cable axis XX and the axis of the wire 11 to 14. Viewed in cross section, the conductive wires 11 to 14 are centered on the vertices of a square whose major axes coincide with the diagonals of a square with a smaller side, at the vertices of which are T

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  centered the rods 43 to 46, 43a to 46a. The cable CA3, CA3a comprises a first pair of insulated conducting wires 11-13 housed in two external "grooves" in diametrically opposite ves between two adjacent rods 43 and 44 and between the two other adjacent rods 45 and 46, and a second pair of insulated conductor wires 12-14 housed in two external diametrically opposite "grooves" between two adjacent rods 44 and 45 and between the two other adjacent rods 46 and

43, respectively.

  The diameter DJ3 of the rods 43 to 46, 43a to 46a is greater than 2.4 d so that the insulated conducting wires 11 to 14 in the cables CA3 and CA3a are contained in a fictitious cylinder C3 tangent to the four rods along four lines of tangency helical and parallel T3 to T6. The cylinder

  fictitious C3 has a diameter D3 = (1 + 2) DJ3 / 2.

  The air gap II to I4 between each insulated conductor wire 11 to 14 and the interior of the fictitious cylinder C3 has a radial width LI preferably greater than approximately d / 2, that is to say greater than 0, 2 DJ3 approximately. This radial width LI is the distance between each insulated conductor wire and the inner surface of a cylindrical envelope 63, made of metallic, plastic-metallic or plastic material, which is merged with the fictitious cylinder

  C3 in cable CA3a shown in Figure 8.

  According to a fourth embodiment, the cables CA4 and CA4a shown in FIGS. 9 and 10 each also include four rods 53 to 56, 53a to 56a twisted with insulated conductive wires 11-13 and 12-14 of two pairs. The conducting wires 11 to 14 are arranged in the center of the cable CA4, CA4a, in a similar manner to the rods 43 to 46, 43a to 46a in the cable CA3, CA3a. Seen in cross section, the wires

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  conductors 11 to 14 are centered on the vertices of a square with side d whose major axes are coincident with the diagonals of a square with a larger side, at the vertices of which the rods 53 to 56, 53a to 56a are centered. Each conducting wire 11 to 14 has two helical tangency lines around the longitudinal axis of cable XX, with two respective adjacent rods 53-54 to 56-53. The conducting wires 11 and 13, 12 and 14 of a pair are still diametrically

opposites.

  Each rod 53 or 53a, 54 or 54a, 55 or 55a, 56 or 56a is applied along two helical tangency lines in a respective external "groove" in vee, formed between a respective conducting wire 11 or 11a, 12 or 12a , 13 or 13a, 14 or 14a of one of the pairs and a respective conductive wire 14 or 14a, 11 or 11a, 12 or 12a, 13 or 13a of the other pair, these two wires being adjacent by a line of tangency helical. The diameter DJ4 of the four rods 53 to 56, 53a to 56a must be greater than 0.41 d, od is the diameter of the wires 11 to 14, so that the conducting wires are not in mechanical contact with a fictitious cylinder C4 tangent to four rods following four helical and parallel tangent lines T3 to T6 respectively. Air interstices II to I4 between the fictitious cylinder C4 such as that of the interior surface of a holding envelope 64 made of the same material as the envelope 61, 62, 63 defined above, and the insulated conducting wires 11 to 14 have a width LI preferably greater than approximately d / 2, that is to say approximately 0.6 DJ4.

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  Referring now to FIGS. 11 and 12, a transmission cable CA5 according to a fifth embodiment of the invention is of the type of cable CA2 according to the second embodiment, and comprises a pair of insulated electrical conductor wires 11 and 12 twisted one against the other. the other around the longitudinal cable axis. Instead of rods 51 and 52, a strand of helical dielectric stuffing 57 with circular section is wound around the pair.

of sons 11-12.

  The strand 57 is contained in a fictitious cylinder C5 and is tangent to it along a single line of helical tangency TH. The diameter of the internal helix of the strand 57 is at least equal to 2 d. The strand 57 is made of thermoplastic material with low permittivity and has a diameter DJ5 preferably at least of the order of d / 2 and a pitch greater than d and less than the pitch of the twists of the wires 11 and 12 in order to form a helical gap Relatively wide IH in the longitudinal direction between the turns of the tamping strand. The helical gap extends radially from the pair of wires 11-12 towards the

  fictitious cylinder C5 with diameter D5 = 2 (d + DJ5).

  According to FIG. 12, a holding envelope 65 having an interior surface merged with the

  fictitious cylinder C5 surrounds the helical strand 57.

  The casing 65 is a shielding screen with at least one metallic or completely metallic face, or else is a dielectric tubular retaining sheath

containing the pair 11-12.

  FIG. 13 shows as a variant a fourth star of two pairs of insulated electrical conductor wires 11-12 and 12-14 also surrounded by a thermoplastic helical strand 58 with low permittivity which is itself surrounded by an envelope of

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  holding 66 with a cylindrical inner surface C5 having a helical line of tangency TH with the strand 58. Like the strand 57, the strand 58 preferably has a section of diameter DJ6 greater than d / 2 and a pitch greater than d and less than the no twists of the wires 11 to 14. The internal helix diameter of the helical strand 57 is at least equal to d (1 + 2), and the diameter D6 of the internal surface C6 of the casing 66 is at least equal to d (1 + 4 /) + 2

DJ6, preferably d (2 +).

  In all of the embodiments of the invention, one or more hollow or solid rods may contain one or more optical fibers. The optical fibers FO are embedded in the dielectric material of the solid rods, as illustrated in FIGS. 1, 6 and 9, or are embedded in a filling and sealing material, such as gel, contained in rods

  hollow as illustrated in Figures 4 and 8.

Claims (13)

  1 - High frequency transmission cable comprising a predetermined number of individually insulated electrical conductor wires (11, 12; 11 to 14) having a predetermined diameter d and symmetrical with respect to a longitudinal axis (XX), characterized in that it includes a dielectric means (41-42; 51-52; 43-46; 53-56; 57-68) contained in a fictitious cylinder (Cl; C2; C3; C4; C5; C6) having a diameter greater than 2 d , centered on the longitudinal axis (XX) and tangent to the fictitious cylinder   along a number of tangent lines (T1-T2; T3-   T6; TH) at most equal to the number of wires to keep the insulated electrical conductors (11-12; 11-14) inside the fictitious cylinder with an air gap (I1-I2; I1-I4; IH) between each wire and the dummy cylinder.   2 - Cable according to claim 1, wherein the dielectric means comprises a number of dielectric rods (41, 42; 51, 52; 43 to 46; 53 to 56) equal to the number of insulated dielectric wires (11, 12; 11 to 14), each rod being tangent to two wires along two respective lines of tangency and to the fictitious cylinder along a line of tangency respective (T1, T2; T3 to T6).   3 - Cable according to claim 2, wherein the number of wires is two, the two rods (41, 42) are joined to one another along the longitudinal axis (XX), and each conductive wire electric insulated (11, 12) is housed in a substantially V-shaped groove formed between the two rods joined. 18 2785 715   4 - Cable according to claim 3, wherein the diameter (DJ1) of the rods (41, 42) is greater than 1.5 d.   - Cable according to claim 2, in which the number of wires is two, the two insulated electrical conductor wires (11, 12) are joined to one another along the longitudinal axis (XX), and each of the two rods (51, 52) is housed in a substantially V-shaped groove formed between the two wires insulated electrical conductors.   6 - Cable according to claim 5, wherein the diameter (DJ2) of the rods (51, 52) is greater than 0.66 d.   7 - Cable according to claim 2, in which the number of wires is two, the two insulated electrical conductor wires (11, 12) are symmetrically spaced from one another along the longitudinal axis (XX), and the two rods (51b, 52b) are symmetrically separated from each other along the longitudinal axis.   8 - Cable according to claim 7, wherein the diameter of the rods (51b, 52b) is greater than d. 9 - Cable according to claim 2, wherein the number of wires is four, each of the four rods (43 to 46) is joined longitudinally to two other rods, and each insulated electrical conductor wire (11 to 14) is housed in a groove 19 2785715   substantially in vee formed between two respective contiguous rods. - Cable according to claim 9, in which the diameter (DJ3) of the rods (43 to 46) is greater than 2.4 d.   11 - Cable according to claim 2, in which the number of wires is four, each insulated electrical conductor wire (11 to 14) is joined longitudinally to two other wires, and each of the four dielectric rods (53 to 56) is applied in a substantially V-shaped groove formed between two   insulated electrical conducting wires joined together.   12 - Cable according to claim 10, wherein the diameter (DJ4) of the rods (53 to 56) is greater than 0.41 d.   13 - Cable conforming to any of the   claims 2 to 12, wherein the wires   insulated electrical conductors (11, 12; 11 to 14) and the dielectric rods (41, 42; 51, 52; 43 to 46; 53 to 56) are twisted together around the axis longitudinal (XX).   14 - Cable conforming to any of   claims 2 to 13, wherein at least one of   dielectric rods (41a, 42a; 51a, 52a; 43a to 46a ; 53a to 56a) is tubular.   - Cable conforms to any of   claims 1 to 14, wherein at least one of   dielectric rods comprises at least one fiber optical (FO). 2785715   16 - Cable conforming to any of   claims 1 to 15, wherein the air gap   (I1-I2; I1-I4; IH) between each wire and the fictitious cylinder is at least substantially greater than d / 2. 17 - Cable according to claim 1, wherein the insulated electrical conductors (11, 12; 11 to 14) are joined longitudinally two by two, and the dielectric means comprises a dielectric strand (57; 58) wound helically around the 'set of insulated electrical conductors. 18 - Cable according to claim 17, wherein the dielectric strand has a section diameter significantly greater than d / 2.   19 - Cable according to claim 17 or 18, wherein the pitch of the helical strand (57; 58) is less than the pitch of twists of the conducting wires   electrical insulated (11-12; 11-14).   - Cable conforms to any of   Claims 1 to 19, comprising an envelope   cylindrical (61; 62; 63; 64; 65; 66) whose inner surface (Cl; C2; C3; C4; C5; C6) is   substantially confused with the fictitious cylinder.