Coaxial cable is composed of single-strand or multistrand wire, around which there is provided an insulating layer, which in turn is coated by a tubular conductor layer.

Around this structure, there is applied at least one more insulating layer. The number of the shielding and insulating layers depends on the target of usage and on the need for shielding. Inner conductors can be made of copper or copper- coated light aluminum wire. In that case the aluminum wire serves as a support for the copper wire. In the cable, the conductor serves as a signal path, whereas the insulation prevents the access of interference in the conductor and insulates the inner conductor and the outermost conductor layer from each other.

Coaxial cables are mainly used as signal and data transmission cables. The properties of the cables affect the frequency range of the cable needed in data transmission. With lower frequencies, there is utilized the whole transversal area of the conductor, whereas with higher frequencies, only the conductor surface affects the electric properties of the cable. For example a coarse and hard copper wire surface causes more interference in signal transmission than a smooth and soft-annealed copper wire. The reason for this is that with high frequencies, the signal proceeds on the surface of the conductor.

Generally in small coaxial cables, with an outer diameter of 1/2 inches (= roughly 12.7 millimeters) or less, it is known to employ copper wire or copper-coated aluminum wire, i. e. CCA wire, as the inner conductor.

For example from the publication US 3,800, 405 there is known a method for making copper-coated aluminum wire, in which method the aluminum wire is

sheathed by copper band, so that the copper band is welded, in the lengthwise direction of the wire, to close around the aluminum wire.

However, the use of copper-coated aluminum wire as inner conductor may cause problems, for instance because of the complicated and expensive production process. When using CCA wire, the aluminum core may at some places extrude through the surrounding copper, resulting in electric problems in the cable, such as intermodulation. The using of a solid copper wire as the inner conductor makes the cable heavy, and may turn out to be expensive.

The object of the present invention is to introduce a new type of cable structure. A particular object of the invention is to introduce a cable structure where the employed inner conductor is copper tube.

The invention is characterized by what is set forth in the characterizing parts of the independent claims. Other preferred embodiments of the invention are characterized by what is set forth in the other claims.

Remarkable advantages are achieved by using a cable structure according to the invention. The invention relates to a cable structure comprising at least two conductor layers ; an inner conductor and an outer conductor, as well as at least two insulation layers, the outer diameter of said cable structure being not larger than 13 millimeters, in which case the inner conductor is a copper tube. By employing a thin-walled copper tube as the inner conductor of a coaxial cable, there is achieved a light-weight and electrically high-quality cable structure. In addition, the interconnectability of different cable structures is enhanced.

According to a preferred embodiment of the invention, the copper tube is seamless. In that case the surface of the copper tube is smooth, which improves the electric properties of the cable.

According to a preferred embodiment of the invention, a copper tube serving as an inner conductor is made of unoxidized copper. According to a preferred embodiment, phosphorus is alloyed in the copper tube serving as the inner

conductor. According to a preferred embodiment of the invention, the outer diameter of the. inner conductor is preferably 1.5-5 millimeters. According to a preferred embodiment of the invention, the thickness of the inner conductor wall is preferably 0.3-1 millimeters. According to a preferred embodiment of the invention, the electroconductivity of the inner conductor is over 100% IACS.

According to a preferred embodiment of the invention, the inner surface of the outer conductor and the outer surface of the inner conductor are essentially smooth. The insulation layer of a cable structure according to the invention is for instance polythene foam. By means of the above described properties, there is achieved a cable structure that has excellent electric properties.

According to a preferred embodiment of the invention, the copper tube serving as the inner conductor is made by extruding and drawing at least in one step.

According to another preferred embodiment of the invention, the copper tube serving as the inner conductor is made by rolling and drawing at least in one step.

A further object of the invention is the use of a seamless copper tube made of oxygen-free copper and alloyed with phosphorus, as the inner conductor of a coaxial cable with an outer diameter not larger than 13 millimeters. Another object of the invention is the use of a copper tube, with an outer diameter preferably 1.5 - 5 millimeters and a wall thickness preferably 0.3-1 millimeters, as the inner conductor of a coaxial cable having an outer diameter not larger than 13 millimeters. Yet another object of the invention is the use of a copper tube made by extruding and drawing, as the inner conductor of a coaxial cable with an outer diameter not larger than 13 millimeters. Yet another object of the invention is the use of a copper tube made by rolling and drawing, as the inner conductor of a coaxial cable with an outer diameter not larger than 13 millimeters.

Advantageously by avoiding the use of aluminum in the cable structure, the recirculation of the cable is made easier. When using copper tube, there is advantageously avoided the occurrence of intermodulation in the cable, and by

employing as the inner conductor a copper tube with a good surface quality, the electric properties of the cable structure are improved.

The invention is described in more detail below with reference to the appended drawing.

Figure 1 A cable structure according to the invention Figure 1 illustrates by way of example a cross-section of a cable structure 1 according to the invention; there can be distinguished a copper tube serving as the inner conductor 2, an insulation layer 3 sheathing said copper tube, an outer conductor 4 as well as an insulation layer 5 sheathing the outer conductor.

According to this example, the employed material of the insulation layer is polythene foam. The outer diameter 6 of a cable structure according to the example is 9.5 millimeters, i. e. 3/8 inches. In measures, the copper tube serving as the inner conductor is designed to fit in the above described cable structure, in which case its outer diameter 7 is preferably 1.5-5 millimeters, and its wall thickness 8 is preferably 0.3-1 millimeters. The inner surface 9 of the outer conductor 4 of the cable structure and the outer surface 10 of the inner conductor 2 are essentially smooth, which affects the electric properties of the cable. The copper tube serving as the inner conductor is made of oxygen-free copper alloyed with phosphorus, in which case the electroconductivity of the copper tube is over 100% IACS.

According to the example, the copper tube serving as the inner conductor 2 is made by extruding cast copper billet into a tube, and by thereafter drawing the tube to the desired degree in the presence of a drawing ring and a drawing mandrel, in which case both the tube diameter and wall can be reduced simultaneously. In the final drawing, there can be utilized flat drawing in order to achieve small measures for the tube. A copper tube can also be made by rolling a hollow, continuously cast tube billet, whereafter the tube is drawn to the final size.

By means of the above described production methods, there is achieved an advantageously seamless copper tube.

For a man skilled in the art, it is obvious that the various preferred embodiments of the invention are not restricted to the above described examples only, but may vary within the scope of the appended claims.