Electricity: conductors and insulators – Conduits – cables or conductors – Conductor structure
Reexamination Certificate
1995-06-27
2001-04-17
Kincaid, Kristine (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Conductor structure
C174S036000
Reexamination Certificate
active
06218624
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a coaxial cable comprising a central conductor and an outer conductor which are separated from each other by an electrically insulating layer, said outer conductor being provided, if necessary, with at least a protective coating. The invention also relates to a method of manufacturing a coaxial cable having this structure.
BACKGROUND
Coaxial cables are known per se, for example, from U.S. Pat. No. 4,368,576, filed by Applicants. The known coaxial cables usually comprise an elongated, central conductor of metal which is concentrically situated in an elongated, tubular outer conductor of metal. Said central conductor is usually composed of a solid copper wire which is circular in section. Copper-clad wires of aluminum or steel are also known to be used for this purpose. Central conductors composed of a bundle of stranded or wound wires, so-called litzes, are also known.
The outer conductor of a coaxial cable is often composed of a layer of fine, stranded or wound metal wires or a wound metal foil. Aluminum or copper, which latter material may be tin-plated or not, is usually used as the material for these wires and foils. An important property of stranded outer conductors is that they provide the coaxial cable with a high degree of flexibility.
The central conductor and the outer conductor are generally separated from each other by a layer of an electrically insulating material, preferably a solid or foamed synthetic material. Coaxial cables in which air is used as the electrically insulating material between the conductors (so-called “semi-air spaced cables”) are also known.
If necessary, one or more additional protective coatings of an electrically insulating material, preferably a synthetic resin, can be provided on the outer conductor. Dependent upon the usage of the coaxial cable, these coatings are provided with reinforcing elements, for example in the form of wires of metal or synthetic resin which are wound in the same direction. The presence of such protective coatings, however, is not absolutely necessary. For example, it is known to use bundles of coaxial cables without a protective coating in transmission cables of ultrasound equipment.
The known coaxial cable has disadvantages. It has been found that when this type of cable is miniaturized, the provision of the outer conductor becomes problematic. This applies both to a stranded outer conductor and to an outer conductor of metal foil. For example, the metal wires used for a stranded outer conductor must have a minimum thickness. The use of wire thicknesses below 25 micrometers results in an unsatisfactory stranding process. In addition, also when larger wire thicknesses are used, the stranding process proceeds very slowly. When relatively thin coaxial cables with a stranded or wound outer conductor are used, the rates typically are of the order of 10-30 cm per minute. Also, when coaxial cables comprising an outer conductor which is made of a foil are miniaturized, production-technical problems occur when the foil is provided. In practice it has been found that it is impossible to wind the foil when the diameter of the cables is less than 1.5 mm. However, when thicker cables are used, the provision process is very laborious and time-consuming.
SUMMARY
It is an object of the invention to provide a coaxial cable which does not have the above-mentioned disadvantages. The invention more particularly aims at a coaxial cable having a relatively thin outer conductor. The inventive cable should also exhibit a relatively high electromagnetic shielding. The coaxial cable in accordance with the invention must further be reliable and its manufacture should be simple and take little time. This relates, in particular, to the rate of providing the outer conductor on the electrically insulating layer.
These and other objects are achieved by means of a coaxial cable of the type mentioned in the opening paragraph, which is characterized according to the invention in that the outer conductor comprises an electroconductive lacquer layer.
It has been found that such electroconductive lacquers can be provided in very thin layers. Layer thicknesses below 200 micrometers, even below 100 micrometers, can be provided on an electrically insulating layer without any problem. Consequently, the invention enables relatively thin coaxial cables to be manufactured. Such thin coaxial cables can be very successfully used as connection wire in ICs. Experiments leading to the invention have shown that electroconductive lacquer layers having a thickness in the range from 5-30 micrometers are still satisfactory. It is noted that the expression “electroconductive lacquer layer” is to be understood to mean herein a layer comprising electroconductive particles which are embedded in a polymeric matrix. An example of such a layer is a lacquer layer comprising electroconductive soot particles in a thermoplastic resin.
Surprisingly, it has been found that such electrocondcutive lacquers are sufficiently elastic to preclude the formation of detrimental hair cracks in the outer conductor upon bending of the coaxial cable. This applies in particular when the thickness of the lacquer layer is below 50 micrometers. It has been found that these lacquers adhere to a large number of insulating synthetic resins, such as polyolefins, foamed or non-foamed polyethylene, polypropylene or mixtures thereof, and also on polyvinyl chloride (PVC) and fluorine-containing polymers. By virtue of the uniform structure of the outer conductor thus provided, the electromagnetic shielding of the inventive cable is better than that of cables provided with stranded outer conductors. It is further noted that the lacquer layers can be rapidly applied in a simple manner. Application rates of many tens of meters per minute can be realized without any problem. Thus, the rate of application is much higher than in the case of stranded outer conductors or outer conductors of wound metal foil.
A preferred embodiment of the coaxial cable is characterized in accordance with the invention in that the lacquer layer comprises electroconductive particles of metal, preferably silver or copper. Electroconductive lacquer layers comprising metal particles exhibit a relatively high conductivity. This applies in particular to silver or copper particles. If the outer conductor of the inventive cable must exhibit a specific conductivity, the use of conductive lacquers on the basis of metal particles allows a thinner layer to be applied than when a lacquer on the basis of conductive soot particles would be used. The exact quantity of conductive particles in the lacquer, the resin to be used, the exact layer thickness, the exact conductivity of the lacquer layer etc. can be routinely determined by those skilled in the art.
A very suitable embodiment of the coaxial cable is characterized in accordance with the invention in that a thin metal layer is present on the electroconductive lacquer layer. This embodiment is particularly suitable for those inventive coaxial cables whose electroconductive lacquer layer exhibits too low of an electric conductivity for a specific application. In addition, this measure results in a further improvement of the electromagnetic shielding of the cable.
The metal layer can be applied in various ways, for example, by means of vacuum deposition or sputtering. However, the outer metal layer is preferably provided in an electrochemical process, for example electroless nickel-plating, or from a metal bath, for example by hot tinning. For reasons relating to costs and production-technical aspects, the metal layer can most suitably be applied by means of electrodeposition.
The invention also relates to a method of manufacturing a coaxial cable. This method is characterized in accordance with the invention in that a central conductor which is provided with an electrically insulating layer, is passed through a solution of an electroconductive lacquer, whereafter the cable is dried and, if necessary, said lacquer layer is provided with at least a
de Boer Hans
Hanssen Hans
van Oorschot Jos
Belden Wire & Cable Company
Conte Robert E. I.
Kincaid Kristine
Lee Mann Smith McWilliams Sweeney & Ohlson
Nguyen Chau N.
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