The term “conductor” is used herein to mean a bare conductor that may however optionally be provided with conductive outer plating, e.g. plating with sliver or the like. The conductor may be a single-strand or a multi-strand conductor.
The term “covering based on PTFE” is used herein to mean a covering that is made up of a majority of PTFE and that may also contain other ingredients or fillers, such as dyes, etc.
The electric wire of the invention is a wire designed for transmitting a signal, in particular at high frequency or at very high frequency. The insulator plays a very large part in the electrical performance of an electric cable, and in particular in the field of high-frequency signal transmission. The propagation speed of electromagnetic waves depends on the dielectric constant ∈ of the insulator, and is inversely proportional to the square root of said dielectric constant. In addition, the attenuation of the transmitted signals is a function involving the frequency and the dielectric constant of the insulating medium. Materials are thus being sought that have dielectric constants that are as low as possible for use in insulation, so as to make low-loss cables. Fluorine-containing polymers, in particular PTFE, are often chosen for that use because they have dielectric constants ∈ that are very low, in the range 2.0 to 2.1.
In order to reduce the dielectric constant still further, compared with the dielectric constants of those materials, a first solution consists in choosing an insulating material that also includes porous material. That brings a considerable improvement in performance because the dielectric constant of air is only 1. The presence of air in the insulator thus makes it possible to lower the dielectric constant. For example, cables insulated with expanded PTFE can have dielectric constants lying in the range 1.7 to 1.3, depending on the porosity of the PTFE.
Another solution for lowering the dielectric constant consists in providing the insulator with continuous longitudinal air-filled channels or cells.
Patent WO 2005/066979 to E.I. Dupont de Nemours thus describes a method of manufacturing electric wires having one or more conductors, protected by a covering of PTFE, the covering having continuous longitudinal cells.
In the method of manufacturing electric wires that is disclosed by that document, the resulting cells in the electric wire are separated from the conductor by a layer of PTFE.
A cross-section through such an electric wire is shown in FIG. 1. That electric wire 10 has a central conductor 12, and a covering 14. The conductor 12 is a single-strand copper conductor of circular cross-section. The covering 14 is a hollow covering that is of substantially cylindrical shape (over a segment of wire that is substantially rectilinear), inside which covering four continuous longitudinal cells 16 are formed. The cells 16 are separated from one another in pairs by separator partitions or spacers 18. The conductor is received in a sheath 20 that is directly in contact with its outside surface, and the radially innermost ends of the spacers 18 bear against the sheath. The presence of the sheath 20 results in the cells 16 being at a certain distance from the conductor 12, thereby limiting their ability to reduce the dielectric constant of the electric wire.