Interference Controlled Communications Cable

Abstract

A communication cable having a plurality of pairs or triplets of parallel conductors, wherein each pair or triplets of conductors are imbedded in a first insulation material to form a core, and a plurality of cores are imbedded in a second insulation material, or jacket. The conductors extend in planar form through an axial cross-section of the cable. The first and second insulations are of different dielectrics. FIELD OF THE INVENTION Electrical communication cables of the type having a plurality of paired conductors whereby a number of electrical signals can be transmitted simultaneously. DESCRIPTION OF THE PRIOR ART Signal cables generally are formed of twisted pairs. To communicate more data faster, it is necessary to use faster rise time pulses and higher frequencies. Under these circumstances twisted pairs and triplets start to become inadequate, mainly because crosstalk between signals grows excessive. Presently the industry, in a highly costly effort involving the multitude of combinations of the length and lay of the twists, is trying to establish adequate control. The new cable of the invention solves the crosstalk problem of the twisted pairs. In twisted pairs, the electromagnetic field is not confined within the boundaries of the solid insulation material; rather a large segment of it extends into the surrounding air. It is this phenomenon which causes crosstalk. The problem is inherent in the basic design features of the twisted pairs. The characteristic impedance of a two wire signal transmission line depends basically on the relationship between the centerdistance of the two wires and the diameter of the wires. In twisted pairs, the centerdistance is determined by the outer diameter of the insulation because the wires are twisted together; therefore, the uniformity of the characteristic impedance also will depend on this. With a multitude of lengths and lays of the twists, it is difficult to maintain a uniform centerdistance. Additionally, the problem of crosstalk in twisted pairs is compounded to a great extent by the air effect between the circular shaped insulators. In order to eliminate this air, the whole structure of twisted pairs must be changed. SUMMARY OF PRESENT INVENTION A pair of parallel wires are imbedded together in a body of first insulation material to form a core. The insulation has a rectangular or ellipsoid or circular cross-section; the width of the insulation is about twice the centerdistance of the wires. The height of the insulation is greater than, but not necessarily more than twice, the centerdistance. The insulation has a low dielectric constant possibly not higher than 2.4 or less and a low dissipation factor in the range of magnitude of 10.sup..sup.-4. A core of a pair of wires with such a cross-section will include the majority of the electromagnetic field and particularly in the area where the density of this field is the greatest, namely, between the two wires. A multitude of such cores are imbedded in a second insulation, or jacket, material having a dielectric different from the first insulation, extending in cross-section in planar form. Every individual pair is completely surrounded by this jacket material. The jacket material has a higher dielectric constant than the first insulation material, of the core, immediately surrounding the wires. It is believed that the substantial reduction in crosstalk is achieved in the present cable for the following reasons: Electrical wavelength in an insulated transmission line is shorter by the square root of the dielectric constant of the insulation than in air, both in the direction of the longitudinal axis, as well as in cross-section of the line. A low loss dielectric material with the correct geometry surrounding the conductors confines a high percentage of the electro-magnetic field. The lossy dielectric material of a higher dielectric constant with the correct geometry surrounds the pair imbedded in the first dielectric material and reduces the electromagnetic interference effect to or from any adjacent pairs. For space, weight and material saving, the thickness of the low loss material can be smaller but not smaller than D and still include more than 80 percent of the electromagnetic field in the low loss basic insulation material. The small but far extending part of the electromagnetic field will be confined by propagating in the higher dielectric constant and higher dissipation factor material and will keep the crosstalk below critical levels. The characteristic impedance of such design may be expressed by the formula: Z.sub.d = Z.sub.0 1.sqroot..sup.-1 z.sub.o = 120 cosh.sup..sup.-1 D/d where Z.sub.d characteristic impedance in cable Z.sub.o characteristic impedance in air dielectric .epsilon. relative dielectric constant D centerdistance of wires d wire diameter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial cross-sectional view of the cable of the invention showing a cable having three pair of conductors.

FIG. 2 is an axial cross-sectional view of one of the cores shown in the cable of FIG. 1, indicating dimensions.

FIG. 3 is a partial axial cross-sectional view of an alternative embodiment of the invention showing a cable with ground conductors between pairs.

FIGS. 4a, 4b and 4c show axial cross-sectional views of steps involved in one method of forming the cable of the invention.

FIG. 4a is an axial cross-sectional view of a plurality of cores before separation.

FIG. 4b shows an axial cross-sectional view of the cores of FIG. 4a after being separated.

FIG. 4c shows the cores of FIG. 4b imbedded in a jacket.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown therein a cross-section of a cable constructed in accordance with the invention.

Conductors 20 and 21 are imbedded in a first insulation material 22 to form a core 23. A plurality of cores 23 extend in planar form as viewed in axial cross-section and are imbedded in a second insulation, or jacket, material 25. Although three cores are shown in the embodiment of FIG. 1, it should be understood that any number of cores may be used.

The core 23, as seen in detail in FIG. 2, has parallel extending conductors 20 and 21 imbedded in insulation 22 of rectangular or ellipsoid or circular cross-section; the width of the insulation (W) is about twice the centerdistance (D) of the wires. The height (t) of the insulation 22 is greater than but not necessarily more than twice, the centerdistance (D). The insulation 22 has a low dielectric constant, relative to air, or 2.4 or less, and a low dissipation factor in the range of magnitude of 10.sup..sup.-4 (e.g. polyethylene or foam).

A core of such cross-section will include the majority of the electromagnetic field and particularly in the area where the density of this field is the greatest; namely, between the two wires. The air pocket valleys, characteristic of conventional twisted pairs, or triplets, will be eliminated by the new shape of the primary low loss insulation material 22.

A jacket 25 of a lossy dielectric material (higher dielectric constant and higher dissipation factor, e.g. vinyl) tightly surrounds the cores 23. Jacket 25 excludes air from the immediate perimeter of the core 23 where some minor part of the electromagnetic field still exists. The elimination or containment of this circumferential air effect prevents this excess crosstalk. The wall thickness of the jacket is preferably at least one-half the centerdistance (D) of the wires.

FIG. 3 shows an embodiment of the invention wherein cable 27 has a ground potential wire 28 imbedded between cores 23 in jacket 25. This arrangement provides for greater crosstalk control.

FIGS. 4a, 4b and 4c show one method of forming the cable of the invention. As seen in FIG. 4a, an extrusion 32 is formed of core insulation material 22 around a plurality of parallel conductor wires 20 and 21. Such extrusion is of a dimension sufficient to form cores 33, 34 and 35 (as seen in FIG. 4b) upon being slit along proposed cut lines 36. Cores 33, 34 and 35, and then continuously imbedded in extruded jacket 37 to form the cable of the invention as seen in FIG. 4c. The steps shown in FIGS. 4a, 4b and 4c, may occur in sequence along the path where the cable is continuously being formed with conductors 20 and 21 drawn from storage reels (not shown) at one end and the finished cable assembled at the other end.

In the alternative, pairs or triplets may be individually embedded into separately extruded cores, as shown in FIG. 2, with a plurality of cores then extruded into a jacket, as shown in FIG. 4c.

Claims

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A crosstalk controlled communications cable comprising

a. a plurality of pairs or triplets of parallel extending conductors;

b. a plurality of cores, each of said cores formed of a first insulation material having imbedded therein one of said pairs or triplets of conductors; and

c. a jacket of a second insulation material surrounding each of said cores and having imbedded therein said cores; wherein

a. the cores

1. have an axial cross-section wherein the least outside dimension is at least as great as the centerdistance between adjacent wires imbedded in the core;

2. extend longitudinally parallel to one another throughout the jacket;

3. extend, in axial cross-section through the jacket, in planar relationship;

b. the jacket is of generally rectangular cross section; and

c. the first insulation of the core and the second insulation of the jacket are of different dielectric properties; wherein the first insulation of the core has a dielectric constant and dissipation factor lower than those of the second insulation material of the jacket.

2. The cable of claim 1 wherein the jacket has imbedded therein a ground wire extending longitudinally between cores.