Patent Application
20080035367
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Embodiments of the present invention are explained below by referring to the accompanying drawings. In the drawings, the same number refers to the same part to avoid duplicate explanation. The ratios of the dimensions in the drawings do not necessarily coincide with the explanation.
The inner conductor 12 is a single wire consisting of a copper wire, a copper alloy wire, or another electrically conductive filament body, or a stranded wire (for example, a stranded wire that has an outer diameter of 0.075 mm and is obtained by twisting together seven silver plating copper alloy wires, each having an outer diameter of 0.025 mm). The filling yarn 13 is formed from a fluorocarbon resin-based electrically insulating material and has about the same thickness as the inner conductor 12 (for example, an outer diameter of 0.075 mm). The inner conductor 12 and the filling yarn 13 are twisted together at a pitch of about 0.5 mm to about 10 mm.
The insulator 14 is formed by extrusion molding on the outside of the twisted body of the inner conductor 12 and the filling yarn 13 into a tube whose inside diameter is equal to the combined thickness of the inner conductor 12 and the filling yarn 13, and whose outer diameter is about 0.29 mm with a thickness of about 0.07 mm. The insulator 14 may have a cross-sectional shape other than that of a perfect circle, and may be oval as long as contact is established between the insulator 14 and both of the inner conductor 12 and the filling yarn 13 in an arbitrary cross section. An air layer 17, which has a low permittivity, is formed between the internal surface of the insulator 14 and the inner conductor 12.
A copper wire, copper alloy wire, or another electrically conductive filament body is spirally wound or braided around the external periphery of the insulator 14 to form an outer conductor 15. For example, a silver-plating copper alloy wire having an outer diameter of 0.03 mm may be spirally wound to form the outer conductor 15. Two layers of polyester tape, for example, having a thickness of about 0.004 mm may be overlap-wrapped on the outer periphery of the outer conductor 15 and fused together to form a sheath 16. As a result, an ultra fine coaxial cable 11 having an outer diameter of about 0.38 mm is obtained.
A portion of the inner conductor 12 of the coaxial cable 11 is in contact with the insulator 14, and the insulation layer between the inner conductor 12 and the outer conductor 15 as a whole largely comprises the air layer 17. As a result, the permittivity of the insulation layer, which is a mean of the permittivities of the air layer 17, the filling yarn 13, and the insulator 14, is about 1.3, is less than the permittivity (2.1) of an insulation layer composed of solid fluorocarbon resin. It is thereby possible to obtain a coaxial cable whose dielectric loss (attenuation) is as low as that of a coaxial cable using a foamed insulating resin or the conventional coaxial cable 1′. Furthermore, because the insulator 14 is formed by extrusion molding, the line speed for manufacturing and productivity can be increased as compared with the conventional coaxial cables using foamed fluorocarbon resin insulating tape. In addition, the same effect that is attained with an expensive foamed fluorocarbon resin can be obtained in the coaxial cable 11 by using less-expensive materials such as above-mentioned polyester tape.
The inside diameter of the insulator can be made smaller in the coaxial cable 11 as compared with that of the conventional cable 1′. Furthermore, because the inner conductor 12 and the filling yarn 13 both contact with the internal surface of the tubular insulator 14 at locations that are opposite to one another, a cylindrical thin-walled molding for the insulator 14 can be created and the occurrence of pinholes can be reduced. In the structure of the conventional coaxial cable 1′, the insulator comes into contact with the structural components (the yarn) inside the cable at only one location in the cross section perpendicular to the axis, and thus, the insulator tends to collapse at all other locations that are not contacting the structural components. It is believed that when the insulator collapses, the resin at the location where contact is made with the yarn flows into the surrounding areas and causes the resin to break and pinholes to form. In contrast, in the embodiment of the present invention, the insulator 14 comes into contact with the structural components (the inner conductor 12 and the filling yarn 13) at two locations in the cross section perpendicular to the axis of the cable 11. Therefore, the insulator 14 is less likely to collapse and the resin is less likely to break than in the conventional coaxial cable 1′. In the conventional coaxial cable 1′, the thickness of the insulator must be increased in order to prevent pinholes from forming. However, in the embodiment of the present invention, the thickness of the insulator 14 can be made smaller than in the conventional coaxial cable 1′ because pinholes are less likely to form. As a result, the coaxial cable 11 can be made as thin as a conventional coaxial cable formed by wrapping a foamed fluorocarbon resin tape.
Furthermore, because the inner conductor 12 and the filling yarn 13 are disposed within the insulator 14 in a twisted fashion, the inner conductor 12 is less likely to slip out. In addition, the heat resistance (about 250° C.) can be raised and resistance to soldering can be improved when the filling yarn 13 and the insulator 14 are both made of the same fluorocarbon resin material (for example, a tetrafluoroethylene/perfluoro alkyl vinyl ether copolymer (PFA)). The filling yarn 13 can be cut off at the same time as the insulator 14 by a laser-based terminal treatment, and workability is improved.
A multi-coaxial cable 20′ shown in
The advantages of the coaxial cables 21, 21′ include the ability to be used in the same manner as a flexible printed circuits (FPC) board by being flattened, particularly in wiring along a flat surface, and the ability to provide enhanced bending performance. Any of the multi-coaxial cables 20, 20′, 21, and 21′ can be shielded by the outer conductors 15 and can provide the desired impedance matching and EMI characteristics.
A ground bar 24 is then attached to the outer conductors 15 so as to be electrically connected. The bar may be, for example, soldered or bonded using an electrically conductive adhesive. The ends of the coaxial cables 11 are thereby arranged and kept together at prescribed intervals in a plane by the ground bar 24. Next, portions of the outer conductors 15 are left, and the outer conductors 15 further toward the end are removed to expose the insulators 14. Then, portions of the insulators 14 is left, and the insulators 14 further toward the end are removed to expose the inner conductors 12 and filling yarn 13. The filling yarn is cut and removed. The adhesive tape 22 may be peeled off if the electric connection terminals shown in
Next, the ground bar 24, the outer conductors 15, the insulators 14, the inner conductors 12, and part of the contacts 25 are covered by a connector id component. The outer conductors 15 are electrically connected to the grounding terminal of the connector at this time. If a metallic shell is used as the lid component, the shell can be used as the grounding terminal of the connector. In such cases, the ground bar 24 is electrically connected to the shell, and the shell is not electrically connected to the inner conductors 12. If the connector 23 is connected to a receptacle (not shown) on the side of the device, the contacts 25 are electrically connected to the signal circuit of the receptacle, and the ground bar 24 is electrically connected to the grounding circuit of the receptacle via the shell.
The product of the present invention (example), and a conventional product (comparative example) having the configurations shown in Table I were manufactured, and the electrical and mechanical characteristics were compared. Specifically, the foamed fluorocarbon resin tape used as the comparative example is a tape made of Poreflon® (registered trademark of Sumitomo Electric).
A foamed fluorocarbon resin tape and a binding polyester tape were used as the insulator in the comparative example, and the total outer diameter of the insulator was 0.25 mm. In contrast, a PFA filling yarn and an extrusion-molded PFA resin were used in the product of the present invention, and the outer diameter of the insulator was 0.29 mm. These electrical characteristics were measured and it was found that both the conventional product and the product of the present invention had about the same characteristics, namely, a conductor resistance of 5800 Ω/km, a characteristic impedance of 80Ω(10 MHz), and an attenuation of 430 dB/km (10 MHz).
It is apparent from the above results that the coaxial cable according to the present invention has excellent bending resistance characteristics and can be used in the bending parts of information-communicating devices, such as wirings that pass through hinges, and cables that are used in medical applications and are bent during handling. The conventional coaxial cable 1′ shown in
The entire disclosure of Japanese Patent Application No. 2005-014401 filed on Jan. 21, 2005 is hereby incorporated herein by reference.
