COAXIAL CABLE, MULTICORE CABLE, AND ELECTRONICS USING THEM

Abstract: The invention is to make the outer conductor of a coaxial cable easily removable in making a terminal treatment, provide the coaxial cable with an excellent insulating characteristic, and suppress the occurrence of noise even if the coaxial cable is subjected to bending or rotation. For this purpose, a coaxial cable is produced by using one or a plurality of coaxial element wires each thereof comprising an outer conductor formed by having a ribbon-shaped conductor of a virtually rectangular cross section with its four corners smoothed and helically wrapped around the insulation layer at a wrapping angle of 45 degrees or more. Further, an electronic apparatus is provided by having such a coaxial cable disposed at a portion where it is subjected to rotation or bending.

Description: TECHNICAL FIELD The present invention relates to single-core coaxial element wires or coaxial cables, or multicore coaxial cables which are used for the connection of a liquid crystal display within a notebook computer, or for sensor cables within a medical-purpose ultrasonic wave diagnostic apparatus, and the like, and further, relates to electronic apparatuses using the same. BACKGROUND ART Coaxial cables comprising a coaxial element wire, made up of a center conductor, an insulation layer, and an outer conductor, and a jacket disposed over the coaxial element wire have been in use thus far. Included among the types of coaxial cables are a single-core cable formed by providing a single coaxial element wire with a jacket, a multicore cable formed by providing a plurality of single-core cables with a common jacket, and a multicore cable formed by providing a plurality of coaxial element wires with a common jacket. Included among the types of arrangements of coaxial element wires or single-core cables in a multicore cable are a flat-type multicore cable obtained by arranging coaxial element wires or coaxial cables on a plane and a twisted-layer multicore cable obtained by twisting them together. There are cases where the same type of cables are combined in such a single-core or multicore coaxial cable and where different types such as communication wires, power wires, and the like are compounded therein to provide a compound cable. In the conventional coaxial cables, a metallic tape or a laminate tape obtained by laminating a metallic tape and an insulating film of polyester or the like is generally used as the outer conductor (shield). A braided structure of metallic tapes as disclosed in Japanese Laid-open Utility Model No. Hei 2-47726 (JP-2 047 726 U) and No. Hei 2-47728 (JP-2 047 728 U) is known. The advantage of the outer conductor when it is formed of braided metal tapes is that it does not become loose. On the other hand, its disadvantage is that removal of the outer conductor is troublesome when, for example, making a terminal treatment. Figure 4 is a side view showing a conventional coaxial cable employing braided metallic tapes. Referring to FIG. 4, reference numeral 11 denotes a center conductor, 12 denotes an insulation layer, 13 denotes an outer conductor formed by braiding metallic tapes, and 14 denotes a jacket. Metallic tapes obtained by slitting a wide metallic tape are normally used. However, at the time of slitting the metallic tape, sharp edges such as burrs are produced on the cut surface and such edge portions can injure the insulation layer or cause a voltage concentration on that portion thereby decreasing the dielectric strength of the insulation layer. This problem becomes serious especially when a small-diameter coaxial cable whose insulation layer thickness is as small as 0.15 mm or less is used. Further, when a conventional coaxial cable is used for connecting devices within an electronic apparatus, especially when it is used in a notebook computer at the rotating portion where the monitor portion and the main body portion are connected or it is disposed at the moving portion of the diagnostic sensor cable which moves when changing examined parts of body, there arises a problem of electrostatic noises produced by friction between the insulation layer and the outer conductor of the moving coaxial cable. JP-4019923 discloses a coaxial cable with an outer conductor formed by a metal tape wound helically around an insulated center conductor. DISCLOSURE OF THE INVENTION The present invention was made as a result of the various investigations which the inventors had conducted on the above described problems and it can be applied to coaxial cables of various types as described above. The inventors have found that a coaxial element wire and a coaxial cable being flexible, producing minimal noise when making a mechanical movement, having good mechanical durability, and being small in outer diameter can be obtained by helically wrapping, around the insulation layer, a ribbon-shaped conductor obtained by rolling and flattening a copper or copper alloy wire and thereby constructing an outer conductor. The invention can be defined as a method of forming a coaxial element wire comprising a center conductor, an insulating layer, and an outer conductor wherein one or a plurality of ribbon-shaped conductors is spirally wrapped around the insulation layer to form the outer conductor, characterized by forming said insulation layer with a thickness where the thickness is smallest of not greater than 0.15 mm, and pressing a copper or copper alloy round wire into a form of virtually rectangular cross-section with its four corners smoothed to obtain said one or plurality of ribbon-shaped conductors used for wrapping. The invention can additionally be defined as a coaxial element wire comprising a center conductor, an insulation layer, and an outer conductor formed as one or a plurality of ribbon-shaped conductors helically wrapped around said insulation layer, the coaxial element wire being characterized in that the thickness of said insulation layer where the thickness is smallest is not greater than 0.15 mm and the or each said ribbon-shaped conductor has a virtually rectangular cross-section with its four corners smoothed. The wrapping angle of the ribbon-shaped conductor with respect to the axis of the coaxial element wire may be 45 degrees or more. When the center conductor is such that is provided by twisting a plurality of conductor wires together, the thickness of the insulation layer is given by the thickness at the portion where the smallest value is obtained in the measurement of the insulation layer thickness in the circumferential direction. Preferably, the ribbon-shaped conductor is made of a metal including copper and the ribbon-shaped conductor is wrapped around the insulation layer under a tension of 30% or more of the tensile strength of the ribbon-shaped conductor. Further, the above described coaxial element wire may be provided with a jacket so as to be formed into a single-core coaxial cable. Further, a plurality of the above described coaxial element wires may be combined and provided with a common jacket so as to be formed into a multicore cable. Further, the aforesaid single-core coaxial cables may be provided with a common jacket to be formed into a multicore cable. An electronic apparatus may be characterized in that the above described coaxial element wire, coaxial cable, or multicore cable is disposed therein at a place where the wire or cable is subjected to mechanical rotation or bending of the electronic apparatus. The ribbon-shaped conductor used here, of a virtually rectangular cross section having its four corners smoothed, can be manufactured with ease and at low cost by rolling and flattening a round wire of copper or a copper alloy. In the invention, the ribbon-shaped conductor has no edge that forms an acute angle at the circumference of the cross-section, and therefore, when the same is helically mounted as the outer conductor, harm to the insulation layer or voltage concentration does not occur. Further, such a ribbon-shaped conductor of a virtually rectangular cross section has high mechanical strength and, because it is not braided, it can be removed with no trouble when, for example, making a terminal treatment. Further, through investigation by the inventors, it was found that the noise occurring in a coaxial cable due to rotation or bending at the portion where it is disposed in an electronic apparatus is an electrostatic noise caused by friction between the insulation layer and the outer conductor. Since the outer conductor of the present invention is helically mounted with one long side of the virtually rectangular form of the ribbon-shaped conductor facing the insulation layer, the area of the contact face between the ribbon-shaped conductor and the insulation layer is sufficiently large to increase friction therebetween and, hence impedes the phenomenon of sliding movement of the ribbon-shaped conductor and the insulation layer along each other, thereby suppressing the occurrence of electrostatic noise. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view showing a single-core coaxial cable employing a typical coaxial element wire embodying the present invention. Figure 2 is a schematic view showing the manner of wrapping of a ribbon-shaped conductor. Figure 3 is a schematic diagram showing a cross-sectional view of a flat cable as an example of a multicore cable. Figure 4 is a side view showing a conventional coaxial cable employing braided metallic tapes. Figure 5 is a diagram showing a cross-sectional view of a ribbon-shaped conductor compared with a round wire before being pressed. Figure 6 is a diagram explanatory of a bending test of a coaxial element wire. Figure 7 is a diagram explanatory of a torsion test of a coaxial element wire. Examples of the invention will now be described with reference to the accompanying drawings. The coaxial element wire constituting the coaxial cable embodying the present invention basically has an insulation layer with a thickness of 0.15 mm or less, and hence, the coaxial element wire can be made smaller in diameter. Accordingly, positive effects are exhibited especially when it is applied to a coaxial cable or a thin flat type multicore cable for use in wiring in an electronic apparatus which has a small space for wiring and hence requires decrease in the volume of wires and cables occupying the space. Further, the coaxial element wire is constructed by using as the outer conductor a ribbon-shaped conductor obtained by pressing and flattening a copper or copper alloy round wire and helically wrapping the ribbon-shaped conductor around the insulation layer. Figure 1 is a perspective view schematically showing a single-core coaxial cable employing a typical coaxial element wire embodying the present invention. Referring to FIG. 1, reference numeral 1 denotes a center conductor of copper, copper alloy, or the like, 2 denotes an insulation layer made of PFA, polyester, polyimide film, or the like, and 3 denotes an outer conductor formed of a ribbon-shaped conductor whose cross-section is virtually a rectangle having its four corners smoothed. The ribbon-shaped conductor can be produced by such a method as chamfering four corners of a rectangular conductor. It can also be manufactured by pressing and flattening a copper or copper alloy round wire, which is advantageous in terms of production cost. The ribbon-shaped conductor is helically wrapped around the insulation layer 2 to provide the outer conductor 3. (1) Thickness of the insulation layer: Since the setting position or angle of electronic apparatuses such as notebook computers and sensors for medical purposes is manually changed, there are increasing demands for further downsized and light weight apparatuses. Hence, narrower coaxial cables are being demanded. When a coaxial cable is deformed by rotation or bending of a portion of a device in which it is disposed, strain is imposed on the coaxial cable, especially on its outer conductor, and such strain becomes greater, accompanied by an increase in produced noises, with the increase of the outer diameter. Therefore, the insulation layer of the coaxial element wire and the coaxial element wire constituting embodiments of the invention is required to have a thickness of as thin as 0.15 mm or less. While it is preferred that the insulation layer thickness be as small as possible, since it is subjected to deformation by repeated bending or torsion during the service period, it is desired that it be given a thickness of, for example, 0.03 mm or more, which is considered to be the minimum value when mechanical strength and flexibility are taken into account. (2) Outer conductor: The ribbon-shaped conductor, which is formed by pressing and flattening a round wire made of a metal, such as copper, copper alloy, or the like, is helically wrapped around the insulation layer to form the outer conductor. Since such a ribbon-shaped conductor is obtained by pressing a round wire, the cross section thereof has a smooth form at the four corners and it takes on virtually a rectangular form not having any acute edge all along the circumference. The outer conductor is constructed by wrapping the ribbon-shaped conductor around the insulation layer with one long side of the virtually rectangular form facing the insulation layer. Because the ribbon-shaped conductor has such a form, it can be provided free from an acute edge as was produced in the slit tape in the conventional art and, therefore, injury to the insulation layer or localization of voltage rarely occurs so that a stabilized insulating withstand-voltage characteristic can be obtained. Further, since a round wire made of copper or copper alloy is pressed and flattened to be used as the ribbon-shaped conductor without annealing, a merit can be obtained such that the ribbon-shaped conductor can be wrapped up so as not to become loose, without the need for braiding as was practiced in the method of the conventional art. When wrapping the ribbon-shaped conductor, it must be kept under a tension not impairing the characteristic of the insulation layer, while enabling the wrapped up ribbon-shaped conductor to constantly fasten the insulation layer, and under such a tension that will not cause the coaxial element wire or the coaxial cable to be damaged when the same is bent or twisted. It is preferred that the tension be not smaller than 30% and not greater than 80% of the tensile strength of the ribbon-shaped conductor. Further, a layer obtained by depositing a metal on a thin tape may be disposed under the outer conductor. Then, both an improvement in the shielding effect and an increase in the insulating withstand-voltage of the insulation layer can be attained.The wrapping angle of the ribbon-shaped conductor is preferably 45 degrees or more for providing flexibility. While, it is more preferably 60 degrees or more, if it is increased close to 90 degrees, the productivity is greatly decreased and it is undesirable. Therefore, the maximum limit is approximately 80 degrees. As to the size of the outer conductor, it is desired that the thickness be 0.03 mm or less in order to reduce the outer diameter of the coaxial element wire and the coaxial cable and, in view of the mechanical strength, it is desired that it be not smaller than 0.01 mm. From the viewpoint of maintaining the characteristics which the outer conductor should have, it is better for the ribbon-shaped conductor to have a large width, preferably 0.1 mm or more. However, from the point of view of the operability of the wrapping work and the cost of production, one having a width of 0.3 mm or less is preferable because that of a small width is economical in material costs and allows the wrapping work to be made free of wrinkle formation. Especially from the point of view of electrical characteristics, mechanical characteristics, and workability, a tape-shaped conductor 0.025 mm thick and 0.20 mm wide manufactured by pressing a round wire of 0.08 mm in outer diameter or a tape-shaped conductor 0.012 mm thick and 0.18 mm wide manufactured by pressing a round wire of 0.05 mm in outer diameter has excellent characteristics as the outer conductor. (3) Multicore cable: Especially in the case of the multicore cable of the present invention, regardless of whether manufactured by having coaxial element wires assembled and provided with a common jacket or by having single-core coaxial cables assembled and provided with a common jacket, there is no danger of the insulation layer being injured by the outer conductor even if the coaxial element wires are subjected to a force applied from the side, i.e., a lateral pressure, when they are twisted for assembling work or the like, since the outer conductor of the coaxial element wires has a smooth surface free from an acute edge along its circumference as a result of manufacture from a round wire by pressing. Hence, a risk such that the dielectric strength of the insulation layer is deteriorated can be avoided and, thus, thinner-walled and smaller-diametered multicore cable, having the mechanical durability and electrical characteristic required of the multicore cable maintained, can be realized. Embodiments of the invention will be concretely described in the following examples. (Example 1) For use as the outer conductor, a tin-plated round wire of a copper alloy of 0.05 mm in outer diameter having a cross section as shown in FIG. 5(A) was pressed and thereby a long ribbon-shaped conductor 0.012 mm thick and 0.18 mm wide having a cross section as shown in FIG. 5(B) was manufactured. As the insulation layer, PFA (tetrafluoroethelene-perfluoroalkylvinylether copolymer) resin was extruded to cover the periphery of a center conductor of 0.09 mm in outer diameter (seven tin-plated copper-alloy wires of 30 µm in outer diameter being stranded ) by a known extruding and covering method so that a circular profile of 0.23 mm in outer diameter is formed. Then, the above described tape-shaped conductor was helically wrapped around the same, so as to form an angle of 68 degrees with respect to the axis of the coaxial element wire, by open wrapping as shown in FIG. 2(A), spaced apart at a pitch of 0.29 mm, under a tension of 0.59N (60 gf) per piece, and thereby a coaxial element wire was manufactured. On the coaxial element wire, a withstand voltage test for the basic characteristics, as well as bending and torsion tests and an electrostatic noise test for the insulation characteristics in a case where it is used in a rotating or bending portion, were performed. At this time, since a coaxial cable is manufactured by combining the coaxial element wires in various ways, the evaluation was carried out on the coaxial element wire per se, to evaluate the same with the effect of the jacket eliminated. Withstand voltage test: Using a coaxial element wire of 300 m length, a DC voltage of 1000 V was applied between the center conductor and the outer conductor for one minute and the occurrence of any dielectric breakdown was checked for. As a result, there was no fault observed, such as to break down the insulation layer, with respect to the withstand voltage. Thus, it has been confirmed that the coaxial element wire has good characteristics as a coaxial cable. Mandrel bending test: The testing method is schematically shown in FIG. 6. Having a coaxial element wire 20 held, at its center portion, between two metallic bars 22 of 5 mm in outer diameter and having a load 21 of 0.49N (50 gf) attached to its lower end, the upper end portion was bent so as to be wrapped around the metallic bar on one side at 90 degrees, then straightened, and then wrapped around the metallic bar on the other side at 90 degrees. Counting a set of bending to one side and the other side as one cycle, 1000 cycles of the bending operation was carried out at a rate of 30 cycles/minute. Thereafter, the withstand voltage test as described above was carried out on the article, in which no inferiority in withstand voltage was observed. Thus, it has been confirmed that the coaxial element wire has excellent characteristics against repeated bending. Torsion test: The testing method is schematically shown in FIG. 7. A coaxial element wire 20 of a length of 20 cm was vertically hanged down having the upper end thereof fixed to an upper end fixing point 24 and having a load 23 of 0.49N (50 gf) attached to the lower end thereof. The load 23 was caused to alternately turn 180 degrees around the center axis of the coaxial cable clockwise and counterclockwise. Counting a set of twisting clockwise and counterclockwise as one cycle, 1000 cycles of the twisting operation were carried out at the rate of 30 cycles/minute. Thereafter, the withstand voltage test as described above was carried out on the coaxial element wire, in which no inferiority in withstand voltage was observed. Thus, it has been confirmed that the coaxial element wire has excellent characteristics against repeated twisting. Electrostatic noise characteristic: In order to further evaluate the value of the electrostatic noise produced at the time when an abrupt deformation is caused to a coaxial element wire, a coaxial element wire of a length of 50 cm was horizontally stretched, a cotton wire of a length of 20 cm was attached to the center thereof, and a load of 0.20N (20 gf) was attached to the other end of the cotton wire. While the voltage between the center conductor and the outer conductor of the coaxial element wire was measured with a voltmeter, the weight was allowed to fall by its own weight from the altitude of the coaxial element wire, and the electrostatic noise characteristic was measured as the maximum value of the voltage variation. As a result of the measurements performed ten times in the same manner, a maximum of 2.5 mV was obtained as the maximum voltage variation. Meanwhile, a similar evaluation was made on a coaxial element wire having an outer conductor made of the conventional braided type shown in Fig. 4. At this time, the maximum value of the voltage variation was as high as 100 mV. From this result, it has been confirmed that substatial improvement in attenuating the electrostatic noise can be obtained by utilizing the present invention. Then, as shown in FIG. 3, 10 pieces of coaxial element wires were arranged in parallel and they were wrapped up by an adhesive-coated polyester tape, as a jacket 6, so as to be formed into a flat type multicore cable. Further, a coaxial element wire was provided with a jacket so as to be formed into a single-core coaxial cable and 30 pieces of such single-core coaxial cables were twisted together and provided with a common jacket on the outside. Thereby, a multicore cable being small in diameter while having flexibility and mechanical durability required of a multicore cable was obtained. Also, excellent insulating and other characteristics have been confirmed with the multicore cables thus obtained. (Example 2) In Example 1, a coaxial element wire was produced by helical wrapping of a ribbon-shaped conductor under a tension of 0.54N (55 gf) per piece, at a pitch of 0.18 mm, at an angle of 75 degrees, and in a butt-joined manner as shown in FIG. 2(B). This coaxial element wire was excellent in all of the withstand voltage characteristics, bending characteristics, torsion characteristics, and electrostatic noise characteristics. Using this coaxial element wire, a single-core coaxial cable, a flat type multicore cable, and a multicore cable were produced in the same manner as in Example 1. It was confirmed also with the thus obtained coaxial cable and multicore cables that their insulating characteristics and other characteristics are good. (Example 3) In Example 1, a coaxial element wire was produced by helical wrapping of ribbon-shaped conductors under a tension of 0.64N (65 gf) per piece, at a pitch of 0.29 mm, and at an angle of 68 degrees (double sheets were wrapped, each in open wrapping, in the same direction) as shown in FIG. 2(C). The coaxial element wire shown in FIG. 2(D) was also produced by wrapping ribbon-shaped conductors at a pitch of 0.29 mm and at an angle of 68 degrees, with the second one wrapped in the opposite direction. These coaxial element wires had excellent withstand voltage characteristics, bending characteristics, torsion characteristics, and electrostatic noise characteristics and especially excellent shielding characteristics of the outer conductor layer. Also by the use of these coaxial element wires, a single-core coaxial cable, a flat type multicore cable, and a multicore cable were produced in the same manner as in Example 1. Excellent insulating characteristics and other characteristics were also confirmed with the thus obtained coaxial cable and multicore cables. INDUSTRIAL APPLICABILITY Since, as described above, a coaxial element wire is produced by using a ribbon-shaped conductor of a virtually rectangular cross-section with four corners thereof smoothed as the outer conductor and wrapping the ribbon-shaped conductor around the insulation layer to provide the outer conductor, a small-diameter coaxial cable being flexible and excellent in mechanical durability can be provided by the use of the coaxial element wire. By combining a plurality of such coaxial element wires or coaxial cables and covering the same with a jacket, it is also made possible to use the product as a multicore cable. Further, by disposing the thus obtained coaxial cable or multicore cable in a rotating portion or a bending portion of an electronic apparatus, an electronic apparatus maintaining excellent insulating characteristics for a long time and producing little electrostatic noise can be obtained and high quality and high speed intra-apparatus signal transmission can be achieved.

Claims: A coaxial element wire comprising a center conductor (1), an insulation layer (2), and an outer conductor (3) formed as one or a plurality of ribbon-shaped conductors (3) helically wrapped around said insulation layer (2), the coaxial element wire being characterized in that the thickness of said insulation layer where the thickness is smallest is not greater than 0.15 mm and the or each said ribbon-shaped conductor (3) has a virtually rectangular cross-section with its four corners smoothed. A coaxial element wire according to Claim 1, wherein the wrapping angle of said ribbon-shaped conductor (3) with respect to the axis of said coaxial element wire is 45 degrees or more. A coaxial element wire according to Claim 1 or Claim 2, wherein said ribbon-shaped conductor (3) is made of a metal including copper and is wrapped around said insulation layer (2) under a tension of 30% or more of the tensile strength of said ribbon-shaped conductor. A coaxial element wire according to any of Claims 1 to 3, wherein said ribbon-shaped conductor (3) is formed from a copper or copper alloy round wire pressed into a flat form having a virtually rectangular cross-section with its forms corners smoothed. A coaxial element wire according to any of Claims 1 to 4, wherein said thickness of said insulation layer is 0.03 mm or more. A coaxial cable comprising one piece of said coaxial element wire set forth in any of Claims 1 to 5 provided with a jacket (4). A multicore cable comprising an assembly of a plurality of said coaxial cables set forth in Claim 6 provided with a common jacket. A multicore cable comprising an assembly of a plurality of said coaxial element wires set forth in any of Claims 1 to 5 provided with a common jacket. An electronic apparatus characterized in that it has at least one of said coaxial cables set forth in Claim 6 or said multicore cables set forth in Claim 7 or Claim 8 disposed at a portion where said at least one coaxial cable or multicore cable is subjected to mechanical rotation or bending of said electronic apparatus. A method of forming a coaxial element wire comprising a center conductor (1), an insulating layer (2), and an outer conductor (3) wherein one or a plurality of ribbon-shaped conductors (3) is spirally wrapped around the insulation layer (2) to form the outer conductor, characterized by forming said insulation layer with a thickness where the thickness is smallest not greater than 0.15 mm, and pressing a copper or copper alloy round wire into a form of virtually rectangular cross-section with its four corners smoothed to obtain said one or plurality of ribbon-shaped conductors used for wrapping. A method according to Claim 10, wherein the ribbon-shaped conductor (3) is wrapped around the insulation layer (2) at a wrapping angle of 45 degrees or more. A method according to Claim 10 or Claim 11, wherein said ribbon-shaped conductor (3) is wrapped under a tension of 30% or more of the tensile strength of said ribbon-shaped conductor. A method according to any of Claims 10 to 12, wherein said thickness of said insulation layer is 0.03 mm or more.

Inventors: ENDO SEIJI, MORI AKINORI, YAMAMOTO AKIRA, YOKOI KIYONORI, ENDO, SEIJI, MORI, AKINORI, YAMAMOTO, AKIRA, YOKOI, KIYONORI, ENDO, Seiji, Kantoh Works, MORI, Akinori, Kantoh Works, YAMAMOTO, Akira, Kantoh Works, YOKOI, Kiyonori, Kantoh Works
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1999
Main classification: H01B11
IPC codes: H01B11