Small-diameter coaxial cable and multi-core cable

Abstract

To provide a small-diameter coaxial cable and a multi-core cable, which can inhibit degradation in visual appearance and electrical characteristics caused by collapse of a shape of an inner conductor.SOLUTION: A small-diameter coaxial cable 1 includes: an inner conductor 2 that is formed by twisting four metal strands 21; an insulator 3 that covers the periphery of the inner conductor 2; a shield layer 4 that covers the periphery of the insulator 3; and a jacket layer 5 that covers the periphery of the shield layer 4. Assuming a twist pitch of the inner conductor 2 is defined as P and a layer center diameter of the inner conductor 2 is defined as Pd, a ratio P / Pd is 25 or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a small-diameter coaxial cable and a multi-core cable.

Background Art

[0002] In a multi-core cable used as a medical cable such as a probe cable for ultrasonic diagnosis, a very thin small-diameter coaxial cable with an outer diameter of 0.25 mm or less is used. In such a small-diameter coaxial cable, a stranded conductor obtained by twisting metal strands is used as the inner conductor. The number of metal strands used for the inner conductor needs to be determined so that when a desired cross-sectional area is obtained, the distance between the inner conductor and the shield layer (outer conductor) can be sufficiently ensured in consideration of the cable outer diameter, and the cost does not become too high.

[0003] For example, when the number of metal strands is three, the metal strands may become thick and the distance between the inner conductor and the shield layer may not be sufficiently ensured. Also, for example, when a seven-strand twist is formed by twisting six metal strands around one metal strand, although the distance between the inner conductor and the shield layer can be ensured, the outer diameter of the metal strand becomes too small and the cost becomes very high. And in the case of a five-strand or six-strand twist, since the twist shape is difficult to stabilize, in a small-diameter coaxial cable, it may be desirable to set the number of metal strands to four in consideration of the balance between performance and cost (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, when the number of metal strands is four, there is a problem that displacement is likely to occur in the relative positions of the four metal strands in a cross-sectional view. For example, between two metal strands arranged opposite to each other, two metal strands arranged opposite to each other may get caught, etc., and there is a risk that the shape of the inner conductor will collapse. If the shape of the inner conductor collapses, abnormalities may occur in the appearance of the small-diameter coaxial cable, or thin portions may occur in the insulator in a part of the circumferential direction, reducing the distance between the inner conductor and the shield layer and deteriorating the electrical characteristics.

[0006] Therefore, an object of the present invention is to provide a small-diameter coaxial cable and a multi-core cable capable of suppressing deterioration of appearance and electrical characteristics due to collapse of the shape of the inner conductor.

Means for Solving the Problems

[0007] The present invention aims to solve the above problems, and includes an inner conductor formed by twisting four metal strands, an insulator covering the periphery of the inner conductor, a shield layer covering the periphery of the insulator, and a jacket layer covering the periphery of the shield layer. When the twist pitch of the inner conductor is P and the layer center diameter of the inner conductor is Pd, a small-diameter coaxial cable is provided in which P / Pd is 25 or less.

[0008] Further, the present invention aims to solve the above problems, and provides a multi-core cable including a cable core having a plurality of the small-diameter coaxial cables and a sheath covering the periphery of the cable core all at once.

Effects of the Invention

[0009] According to the present invention, it is possible to provide a small-diameter coaxial cable and a multi-core cable capable of suppressing deterioration of appearance and electrical characteristics due to collapse of the shape of the inner conductor.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

[0011] [Embodiment] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0012] FIG. 1 is a diagram showing a small-diameter coaxial cable 1 according to an embodiment of the present invention, (a) is a cross-sectional view showing a cross-section perpendicular to the longitudinal direction, and (b) is a diagram for explaining the layer core diameter. As shown in FIG. 1 (a), the small-diameter coaxial cable 1 includes an inner conductor 2, an insulator 3 that covers the periphery of the inner conductor 2, a shield layer 4 that covers the periphery of the insulator 3, and a jacket layer 5 that covers the periphery of the shield layer 4.

[0013] (Inner conductor 2) The inner conductor 2 is formed by twisting four metal strands 21 together. By forming the inner conductor 2 with four metal strands 21, compared with the case where the inner conductor 2 is formed by three metal strands 21, when the inner conductor 2 has the same conductor cross-sectional area and the insulator 3 has the same outer diameter, the distance between the inner conductor 2 and the shield layer 4 can be reduced, and good electrical characteristics can be maintained. Further, by forming the inner conductor 2 with four metal strands 21, compared with the case where the inner conductor 2 is formed by seven metal strands 21 (when six metal strands 21 are twisted around one metal strand 21), the outer diameter of the metal strand 21 can be increased, and cost reduction is possible.

[0014] The metallic element wire 21 is made of copper or a copper alloy. The surface of the metallic element wire 21 may be plated with silver, tin, or the like. In this embodiment, a metallic element wire 21 made of a silver-plated copper alloy is used to increase the conductivity and mechanical strength. From the perspective of cost, the outer diameter of the metallic element wire 21 is preferably 0.015 mm or more, and from the perspective of preventing the insulator 3 from becoming too thin in some parts, it is preferably 0.20 mm or less. In this embodiment, a metallic element wire 21 with an outer diameter of 0.018 mm is used. The outer diameter when four metallic element wires 21 are twisted together is 19.6 mm.

[0015] In the small-diameter coaxial cable 1 according to this embodiment, when the twist pitch of the inner conductor 2 is P and the layer core diameter of the inner conductor 2 is Pd, P / Pd is set to 25 or less. The twist pitch P of the inner conductor 2 is the interval along the longitudinal direction at a position where the circumferential positions of any metallic element wire 21 are the same. Also, as shown in Fig. 1(b), the layer core diameter Pd of the inner conductor 2 is the diameter of a circle passing through the centers of the four metallic element wires 21 in a cross-section perpendicular to the longitudinal direction. If the outer diameter of the entire inner conductor 2 is D and the outer diameter of the metallic element wire 21 is d, then Pd = (D - d).

[0016] As shown in Fig. 2, in the four-strand twisted inner conductor 2, when twisted, a gap 2a is formed at the center, and some metallic element wires 21 may enter the gap 2a, etc., easily causing the shape of the inner conductor 2 to collapse. In particular, a pair of opposing metallic element wires 21 (the left and right metallic element wires 21 in Fig. 2) may enter the gap 2a, and the shape is likely to collapse such that the other pair of opposing metallic element wires 21 (the upper and lower metallic element wires 21 in Fig. 2) are pushed outward in the radial direction. Such a shape collapse is likely to occur when the twist pitch P of the inner conductor 2 is relatively large and the twist is loose. By setting P / Pd of the inner conductor 2 to 25 or less as in this embodiment, it becomes possible to suppress the shape collapse of the inner conductor 2 due to the loosening of the twist.

[0017] As shown in FIG. 2, when the shape of the internal conductor 2 collapses, in a cross-sectional view (a cross-sectional view perpendicular to the longitudinal direction), the shape formed by connecting the centers of the four metal strands 21 is no longer square, but becomes a substantially parallelogram shape, and the inner angle θ deviates from 90°. The greater the deviation of this inner angle θ from 90°, the greater the degree of appearance defects and deterioration of electrical characteristics. Therefore, the inner angle θ when connecting the centers of the four metal strands 21 should satisfy the following formula |θ - 90°| ≤ 10° and more preferably, it should satisfy the following formula |θ - 90°| ≤ 5° is good.

[0018] If the twist pitch P of the internal conductor 2 is too small, disconnection is likely to occur when twisted, so the P / Pd of the internal conductor 2 is preferably 16 or more. That is, the P / Pd of the internal conductor 2 is preferably 16 or more and 25 or less. In this embodiment, the twist pitch P of the internal conductor 2 is 0.5 mm, and the P / Pd of the internal conductor 2 is 19.6.

[0019] (Insulator 3) The insulator 3 is formed to cover the periphery of the internal conductor 2. In this embodiment, the insulator 3 has a foamed layer 31 made of a foamed resin that covers the periphery of the internal conductor 2, and a non-foamed skin layer 32 that covers the periphery of the foamed layer 31. By having the foamed layer 31, the dielectric constant of the insulator 3 can be increased, and in particular, the electrical characteristics when transmitting high-frequency signals can be improved. Since the foamed layer 31 has air bubbles, by covering its periphery with the non-foamed skin layer 32, insulation between the internal conductor 2 and the shield layer 4 is ensured.

[0020] In this embodiment, as the foamed layer 31, one made of foamed PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer) is used. The thickness of the foamed layer 31 is 0.043 mm, and the outer diameter is 0.129 mm. The foamed layer 31 is extrusion-molded around the internal conductor 2 by full extrusion.

[0021] The skin layer 32 is formed by winding a resin tape around the foam layer 31. In this embodiment, as the resin tape, one having an adhesive layer made of a thermosetting resin provided on one surface of a resin layer is used. With the adhesive layer on the inner side (the foam layer 31 side), it is spirally wound so that a part in the width direction of the resin tape overlaps. Then, after winding the resin tape, heat is applied to cure the adhesive layer and integrate the skin layer 32 and the foam layer 31.

[0022] As the resin tape used for the skin layer 32, PET (polyethylene terephthalate), PI (polyimide), PEEK (polyetheretherketone), PEI (polyetherimide), etc. can be used. In order to withstand heat during soldering etc., it is more desirable to use a resin tape with as high heat resistance as possible for the skin layer 32. More specifically, it is more desirable to use a resin tape made of a resin with a softening temperature higher than that of PET. As the resin tape used for the skin layer 32, it is more desirable to use one made of PI, PEEK, or PEI. Here, the thickness of the skin layer 32 was set to 0.005 mm.

[0023] (Shield layer 4) The shield layer 4 is composed of a horizontally wound shield in which a plurality of metal wires 41 are spirally wound around the insulator 3. The metal wire 41 is made of copper or a copper alloy. The metal wire 41 may have a plating made of silver, tin, etc. on its surface. In this embodiment, similar to the metal wire 41 of the inner conductor 2, a metal wire 41 made of a silver-plated copper alloy was used to increase the conductivity and mechanical strength. The outer diameter of the metal wire 41, that is, the thickness of the shield layer 4, was set to 0.020 mm.

[0024] To make the small-diameter coaxial cable 1 easier to bend and improve its flex resistance, it is preferable that the twisting direction of the shield layer 4 and the twisting direction of the inner conductor 2 are the same. Note that the twisting direction of the shield layer 4 or the inner conductor 2 is the direction in which the metal wires 21, 41 rotate from one end to the other end when the small-diameter coaxial cable 1 is viewed from one end.

[0025] (Jacket layer 5) The jacket layer 5 is provided to cover the periphery of the shield layer 4. In the present embodiment, the jacket layer 5 is formed by winding a resin tape. In the present embodiment, the jacket layer 5 has a two-layer structure. In the first layer, a non-adhesive resin tape is spirally wound such that a part in the width direction overlaps. Then, in the second layer, an adhesive resin tape provided with an adhesive layer made of a thermosetting resin on one surface of a resin layer is used, and the resin tape is spirally wound such that a part in the width direction overlaps with the adhesive layer on the inner side. Then, it is heated to cure the adhesive layer to form the jacket layer 5. As the resin constituting the resin tape, PET (polyethylene terephthalate), PI (polyimide), PEEK (polyetheretherketone), PEI (polyetherimide), etc. can be used. The thickness of the jacket layer 5 is 0.015 mm. The outer diameter of the jacket layer 5, that is, the outer diameter of the small-diameter coaxial cable 1, is 0.250 mm or less, more preferably 0.220 mm or less. In the present embodiment, the outer diameter of the jacket layer 5, that is, the outer diameter of the small-diameter coaxial cable 1, is 0.209 mm.

[0026] By forming the jacket layer 5 with a two-layer structure of a non-adhesive resin tape and an adhesive resin tape, it is possible to suppress the jacket layer 5 from being adhered to the shield layer 4, and improve the bendability and flex resistance. Note that it is desirable that the winding direction of the inner non-adhesive resin tape is the same as that of the shield layer 4 to make it difficult to unwind the twist of the shield layer 4. And the winding direction of the outer adhesive resin tape is preferably different from the winding direction of the inner non-adhesive resin tape. This is because if both the inner layer and the outer layer have the same winding direction, the small-diameter coaxial cable 1 may be given a bend habit.

[0027] (Multi-core cable 10) Next, the multi-core cable 10 using the small-diameter coaxial cable 1 will be described. FIG. 3 is a diagram showing the multi-core cable 10 according to the present embodiment, (a) is a cross-sectional view showing a cross-section perpendicular to the longitudinal direction, and (b) is a cross-sectional view showing a cross-section perpendicular to the longitudinal direction of the sub-twisted wire 11.

[0028] As shown in FIGS. 3(a) and 3(b), the multi-core cable 10 includes a cable core 12 having a plurality of small-diameter coaxial cables 1, and a sheath 16 that collectively covers the periphery of the cable core 12. In the present embodiment, a binding tape 14 is wound around the cable core 12, a collective shielding layer 15 is provided so as to cover the periphery of the binding tape 14, and a sheath 16 is provided so as to cover the periphery of the collective shielding layer 15.

[0029] The cable core 12 is formed by twisting a plurality of sub-twisted wires 11 obtained by twisting a plurality of small-diameter coaxial cables 1. In the illustrated example, 16 small-diameter coaxial cables 1 are twisted to form the sub-twisted wire 11, and 12 sub-twisted wires 11 are twisted to form the cable core 12. The cable core 12 is configured using a total of 192 small-diameter coaxial cables 1.

[0030] The sub-twisted wire 11 is formed by twisting 5 small-diameter coaxial cables 1 and then twisting 11 small-diameter coaxial cables 1 around the periphery thereof. The twisting direction in each layer of the sub-twisted wire 11 is the same. Hereinafter, the twisting direction in each layer of the sub-twisted wire 11 (the direction in which the small-diameter coaxial cable 1 rotates from one end to the other end when viewed from one end of the sub-twisted wire 11) is referred to as the twisting direction of the sub-twisted wire 11.

[0031] The cable core 12 is formed by twisting 3 sub-twisted wires 11 and then twisting 9 sub-twisted wires 11 around the periphery thereof. The twisting direction in each layer of the cable core 12 is the same. Hereinafter, the twisting direction in each layer of the cable core 12 (the direction in which the sub-twisted wire 11 rotates from one end to the other end when viewed from one end of the cable core 12) is referred to as the twisting direction of the cable core 12.

[0032] The twisting direction of the cable core 12 and the twisting direction of the sub-twisted wire 11 are the same. This makes the multi-core cable 10 easier to bend and improves its flex resistance. The outer diameter of the cable core 12 is 3.7 mm.

[0033] The binding tape 14 is spirally wound around the cable core 12 such that a part of its width direction overlaps. In order to make the small-diameter coaxial cable 1 move easily within the binding tape 14 during bending of the multi-core cable 10 or the like and improve the bendability and flex resistance, it is desirable that the binding tape 14 be made of a material that is as slippery as possible. More specifically, as the binding tape 14, one made of a fluororesin such as PTFE (polytetrafluoroethylene) can be used. However, since a tape made of PTFE is expensive, it is more desirable to use a foamed tape made of foamed polypropylene that is lower in cost and slippery as the binding tape 14. In particular, when using the collective shield layer 15 made of a braided shield as in this embodiment, by using the binding tape 14 made of foamed polypropylene, the binding tape 14 serves as a buffer layer, relaxing the clamping of the cable core 12 by the collective shield layer 15 and further improving the flex resistance.

[0034] Also, the winding direction of the binding tape 14 is set to be different from the twisting direction of the cable core 12. Thereby, it becomes possible to suppress the collapse of the shape of the cable core 12, and a multi-core cable 10 with an outer shape closer to a circular shape can be obtained. Note that the winding direction of the binding tape 14 is the direction in which the binding tape 14 rotates from one end to the other end when the multi-core cable 10 is viewed from one end.

[0035] The collective shield layer 15 is composed of a braided shield in which a plurality of strands are braided. In this embodiment, tinned copper foil yarn is used as the strand for the collective shield layer 15. The tinned copper foil yarn is formed by spirally winding a copper foil with tin plating around the yarn. Here, a monofilament with a diameter of 0.08 mm is used as the central yarn, and by making the strands thin and light, the multi-core cable 10 is made thinner and lighter.

[0036] The sheath 16 is provided to cover the periphery of the integrated shield layer 15. In the present embodiment, a sheath 16 made of PVC (polyvinyl chloride) with a thickness of 0.6 mm is used. The sheath 16 is preferably formed by tube extrusion so that the resin constituting the sheath 16 does not enter between the strands of the integrated shield layer 15. The outer diameter of the sheath 16, that is, the outer diameter of the multi-core cable 10, is 5.4 mm.

[0037] (Operations and Effects of the Embodiment) As described above, in the small-diameter coaxial cable 1 according to the present embodiment, the P / Pd of the inner conductor 2 is set to 25 or less. Thereby, it is possible to suppress the deformation of the shape of the inner conductor 2 and suppress the deterioration of the appearance and electrical characteristics.

[0038] (Summary of the Embodiment) Next, the technical idea grasped from the embodiment described above will be described by referring to the reference numerals and the like in the embodiment. However, each reference numeral and the like in the following description are not limited to the members and the like that specifically show the components in the claims in the embodiment.

[0039] [1] A small-diameter coaxial cable (1) comprising an inner conductor (2) formed by twisting four metal strands (21), an insulator (3) covering the periphery of the inner conductor (2), a shield layer (4) covering the periphery of the insulator (3), and a jacket layer (5) covering the periphery of the shield layer (4), wherein when the twist pitch of the inner conductor (2) is P and the core diameter of the inner conductor (2) is Pd, P / Pd is 25 or less.

[0040] [2] The small-diameter coaxial cable (1) according to [1], wherein the P / Pd of the inner conductor (2) is 16 or more and 25 or less.

[0041] [3] The small-diameter coaxial cable (1) according to [1], wherein the outer diameter of the jacket layer (5) is 0.250 mm or less.

[0042] [4] The insulator (3) has a foam layer (31) made of a foamed resin that covers the periphery of the internal conductor (2), and a skin layer (32) formed by winding a resin tape around the foam layer (31). The resin tape is made of a resin having a softening temperature higher than that of PET. The coaxial cable (1) according to [1].

[0043] [5] The resin tape is made of polyimide, polyether ether ketone, or polyether imide. The coaxial cable (1) according to [4].

[0044] [6] A cable core (12) having a plurality of the coaxial cables (1) according to any one of [1] to [5], and a sheath (16) that collectively covers the periphery of the cable core (12). The multi-core cable (10).

[0045] [7] The cable core (12) is formed by twisting a plurality of sub-twisted wires (11) obtained by twisting a plurality of the coaxial cables (1). The multi-core cable (10) according to [6].

[0046] [8] The twisting direction of the sub-twisted wire (11) and the twisting direction of the cable core (12) are the same direction. The multi-core cable (10) according to [7].

[0047] [9] It includes a binding tape (14) wound around the cable core (12), and the winding direction of the binding tape (14) is different from the twisting direction of the cable core (12). The multi-core cable (10) according to [8].

[0048]

[10] It includes a binding tape (14) wound around the cable core (12), a collective shield layer (15) composed of a braided shield that covers the periphery of the binding tape (14), and the sheath (16) that covers the periphery of the shield layer (15). The binding tape (14) is made of foamed polypropylene. The multi-core cable (10) according to [6].

[0049] (Supplementary Note) The embodiments of the present invention have been described above. However, the embodiments described above do not limit the invention according to the claims. Also, it should be noted that not all combinations of the features described in the embodiments are essential means for solving the problems of the invention. Further, the present invention can be implemented with appropriate modifications without departing from its gist.

Explanation of Reference Numerals

[0050] 1…Small-diameter coaxial cable 2…Inner conductor 21…Metal strand 3…Insulator 31…Foam layer 32…Skin layer 4…Shield layer 41…Metal strand 5…Jacket layer 10…Multi-core cable 11…Sub-twisted wire 12…Cable core 14…Binding tape 15…Overall shield layer 16…Sheath

Claims

1. An inner conductor formed by twisting four metal strands, an insulator covering the periphery of the inner conductor, a shield layer covering the periphery of the insulator, and a jacket layer covering the periphery of the shield layer, wherein when the twist pitch of the inner conductor is P and the core diameter of the inner conductor layer is Pd, P / Pd is 25 or less, a small-diameter coaxial cable.

2. The small-diameter coaxial cable according to Claim 1, wherein P / Pd of the inner conductor is 16 or more and 25 or less.

3. The small-diameter coaxial cable according to Claim 1, wherein the outer diameter of the jacket layer is 0.250 mm or less.

4. The insulator has a foamed layer made of a foamed resin covering the periphery of the inner conductor and a skin layer formed by winding a resin tape around the foamed layer, wherein the resin tape is made of a resin having a softening temperature higher than that of PET, The small-diameter coaxial cable according to Claim 1.

5. The small-diameter coaxial cable according to Claim 4, wherein the resin tape is made of polyimide, polyetheretherketone, or polyetherimide.

6. A cable core having a plurality of the small-diameter coaxial cables according to any one of Claims 1 to 5, and a sheath covering the periphery of the cable core in a unified manner, a multi-core cable.

7. The multi-core cable according to Claim 6, wherein the cable core is formed by twisting a plurality of sub-twisted wires obtained by twisting a plurality of the small-diameter coaxial cables.

8. The multi-core cable according to Claim 7, wherein the twisting direction of the sub-twisted wire and the twisting direction of the cable core are the same.

9. The multi-core cable according to Claim 8, further comprising a binding tape wound around the periphery of the cable core, wherein the winding direction of the binding tape is different from the twisting direction of the cable core.

10. A unified shield layer including a binding tape wound around the periphery of the cable core and a braided shield covering the periphery of the binding tape, and the sheath covering the periphery of the shield layer, wherein the binding tape is made of foamed polypropylene, The multi-core cable according to Claim 6. ​ ​ ​ ​ ​ ​ ​

Images