Coaxial cable

Abstract

To provide a coaxial cable having improved high frequency property in particular due to reduction of electrostatic capacity between a central conductor and an outer conductor.SOLUTION: A coaxial cable 1 comprises: a central conductor 2 which is formed by stranding first to third conductive wires 21 to 23; a foamed insulator 3 in which a cavity is formed so as to surround the central conductor 2; and an outer conductor 4 provided around the foamed insulator 3. A part of the foamed insulator 3 is a protruding part 31 extending from an inner surface 30a of a cavity 30 facing the central conductor 2 via a gap toward the central conductor 2, the protruding part 31 contacts an outer peripheral edge 2a of the central conductor 2 to support the central conductor 2 within the cavity 30, and approximately the entire part of the outer peripheral edge 2a of the central conductor 2 faces the inner surface 30a of the cavity 30 via a gap except for areas contacting the protruding part 31.SELECTED DRAWING: Figure 3A

Description

[Technical Field]

[0001] The present invention relates to a coaxial cable. [Background technology]

[0002] Conventionally, some coaxial cables have an insulator disposed between a central conductor and an outer conductor arranged on the same axis, and have been designed to reduce the capacitance between the central conductor and the outer conductor by forming a gap between the insulator and the central conductor or the outer conductor, thereby improving the electrical characteristics (see, for example, Patent Documents 1 and 2).

[0003] The coaxial cable described in Patent Document 1 increases the cross-sectional area of ​​the gap between the central conductor and the insulator by adjusting the extrusion pressure and draw-down rate when forming the insulator by extruding resin around the central conductor. Patent Document 1 also describes how the insulator is extruded by adjusting the extrusion pressure and draw-down rate so that the adhesion between the central conductor and the insulator is one-third or less of the breaking strength of the central conductor, thereby forming an appropriate-sized gap between the central conductor and the insulator without using a foamed insulator, thereby achieving both electrical and mechanical properties.

[0004] The coaxial cable described in Patent Document 2 has a central conductor, an extruded coating layer provided on the outer periphery of the central conductor, and a tubular coating layer provided directly on the extruded coating layer. Melt fracture (wavy surface roughness) occurs on the outer surface of the extruded coating layer during extrusion coating, and voids are formed in the valleys of this melt fracture. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-198973 [Patent Document 2] International Publication No. 2017 / 013765 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, with the increase in communication speeds, there has been a demand for coaxial cables that have a smaller capacitance between the center conductor and the outer conductor than ever before, have excellent high-frequency characteristics, and are suitable for high-speed signal transmission. Although increasing the air gap is an effective way to reduce the capacitance, the coaxial cable structures described in Patent Documents 1 and 2 have limitations on the size of the air gap that can be formed, and have not necessarily been able to fully meet this demand.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a coaxial cable having improved high frequency characteristics, in particular, by reducing the electrostatic capacitance between the central conductor and the outer conductor. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the present invention provides a coaxial cable comprising a central conductor formed by twisting together a plurality of conducting wires, a foamed insulation having a cavity formed to surround the central conductor, and an outer conductor provided around the foamed insulation, wherein a part of the foamed insulation forms a protrusion extending from the inner surface of the cavity facing the central conductor across a gap toward the central conductor, the protrusion contacting the outer peripheral edge of the central conductor to support the central conductor within the cavity, and the outer peripheral edge of the central conductor, except for the part contacting the protrusion, faces the inner surface of the cavity across a gap. [Effects of the Invention]

[0009] According to the present invention, the high frequency characteristics of the coaxial cable are improved by reducing the capacitance between the central conductor and the outer conductor. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a side view showing an example of the configuration of a coaxial cable according to an embodiment of the present invention; [Figure 2] (a) to (d) are cross-sectional photographs of the coaxial cable at multiple locations. [Figure 3A] 2(b) is a schematic diagram showing each cross section shown in FIG. 2(a). [Figure 3B] 2(b) is a schematic diagram showing each cross section shown in FIG. 2(b). [Figure 3C] 2(c) is a schematic diagram showing each cross section shown in FIG. [Figure 3D] 2(d) is a schematic diagram showing each cross section shown in FIG. [Figure 4] FIG. 1 is a schematic diagram showing the general configuration of a manufacturing apparatus. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Embodiment Mode] Fig. 1 is a side view showing an example of the configuration of a coaxial cable according to an embodiment of the present invention. Figs. 2(a) to 2(d) are cross-sectional photographs of the coaxial cable at multiple locations in the longitudinal direction. Figs. 3A, 3B, 3C, and 3D are schematic diagrams showing the cross sections shown in Figs. 2(a) to 2(d).

[0012] The coaxial cable 1 includes a central conductor 2 formed by twisting together a plurality of conducting wires 21 to 23, a foamed insulation 3 provided around the central conductor 2, an outer conductor 4 provided around the foamed insulation 3, a skin layer 5 provided between the foamed insulation 3 and the outer conductor 4, and a jacket 6 provided around the outer conductor 4. This coaxial cable 1 is used, for example, as a medical cable, and transmits electrical signals by the potential difference between the central conductor 2 and the outer conductor 4. More specific examples of medical cables include probe cables, catheter cables, and endoscope cables.

[0013] The central conductor 2 is a stranded wire formed by twisting together three conductive wires 21-23. The three conductive wires 21-23 are silver-plated copper alloy wires with a circular cross section, and are twisted spirally around the central wire C of the coaxial cable 1. As shown in FIG. 3A, the outer diameters of the three conductive wires 21-23 are D1, D2, and D3, respectively, and D1, D2, and D3 are equal to each other and are each 0.1 mm or less. As an example, the central conductor 2 is 42 AWG (American Wire Gauge) to 48 AWG, and D1, D2, and D3 are each 0.020 mm or more and 0.038 mm or less. Hereinafter, the three conductive wires 21-23 will be referred to as the first conductive wire 21, the second conductive wire 22, and the third conductive wire 23, respectively.

[0014] The outer conductor 4 is formed by spirally winding a plurality of elemental wires 41. In this embodiment, 32 elemental wires 41 are spirally wound around the outer periphery of the skin layer 5. The spiral winding direction of the elemental wires 41 in the outer conductor 4 is opposite to the spiral winding direction of the first to third conducting wires 21 to 23 in the central conductor 2. The elemental wires 41 are wire rods made of a copper alloy and have a circular cross section, and are formed to have an outer diameter smaller than the outer diameters D1, D2, D3 of the first to third conducting wires 21 to 23.

[0015] The wires 41 are made of hard copper or tin-plated copper alloy. Each of the wires 41 has an outer diameter D4 (see FIG. 3A) of 0.05 mm or less and an electrical conductivity (electrical conductivity) of 70% IACS or more. The outer diameter D4 of the wire 41 is more preferably in the range of 0.032 mm or less, and the electrical conductivity of the wire 41 is more preferably in the range of 80% IACS or more. More specifically, the outer diameter D4 of the wire 41 is, for example, 0.022 mm or more and 0.0254 mm or less. Here, IACS (International Annealed Copper Standard) refers to the volume resistivity of annealed standard soft copper (volume resistivity: 1.7241×10 -2 The electrical conductivity (IACS) is an index showing low electrical resistance, with a conductivity of 100% IACS being 1 μΩm. The area ratio of the central conductor 2 to the outer conductor 4 (the value obtained by dividing the cross-sectional area of ​​the outer conductor 4 by the cross-sectional area of ​​the central conductor 2) is, for example, 4.91 or more and 8.87 or less.

[0016] The skin layer 5 is a non-foamed insulator interposed between the foamed insulation 3 and the outer conductor 4. In this embodiment, as shown in FIG. 1, the skin layer 5 is formed by spirally winding a strip-shaped insulating tape 50 having an adhesive layer 52 formed on one side of an electrically insulating base material 51 made of resin. The insulating tape 50 is wound around the outer periphery of the foamed insulation 3 with the adhesive layer 52 facing inward so that the tape and the base material overlap in the width direction. Suitable materials for the base material 51 include, for example, PET (polyethylene terephthalate), PEI (polyetherimide), and PI (polyimide). The adhesive layer 52 is made of, for example, a hot-melt adhesive that can be bonded by heat and pressure bonding. The insulating tape 50 has a thickness of, for example, 0.01 mm.

[0017] Alternatively, the skin layer 5 may be formed by thin-wall extrusion of a fluororesin, for example. As the fluororesin, ETFE (tetrafluoroethylene-ethylene copolymer) or FEP (tetrafluoroethylene-hexafluoropropylene copolymer) may be used, but PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) is particularly suitable. When formed by thin-wall extrusion of a resin, the thickness of the skin layer 5 is, for example, 0.005 mm.

[0018] The jacket 6 is formed by wrapping a resin tape 61 made of, for example, PET (polyethylene terephthalate) or PI (polyimide). Alternatively, the jacket 6 may be formed by extrusion coating with a resin such as PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) or ETFE (tetrafluoroethylene-ethylene copolymer). The thickness of the jacket 6 is, for example, 0.015 mm or more and 0.02 mm or less, and the outer diameter of the jacket 6 is, for example, 0.21 mm or more and 0.34 mm or less.

[0019] The foamed insulation 3 has an outer diameter of, for example, 0.129 mm or more and 0.23 mm or less. A cavity 30 formed by foaming is provided in the center of the foamed insulation 3. The cavity 30 extends throughout the entire coaxial cable 1 in the longitudinal direction of the coaxial cable 1. A part of the foamed insulation 3 forms a protrusion 31 that extends from an inner surface 30a of the cavity 30, which faces the central conductor 2 across a gap, toward the central conductor 2. The foamed insulation 3 has a large number of protrusions 31, and each protrusion 31 is formed in a columnar or wall shape facing the central conductor 2.

[0020] The protrusion 31 contacts the outer peripheral edge 2a of the central conductor 2 to support the central conductor 2 within the cavity 30. In Figures 3A to 3D, this outer peripheral edge 2a is indicated by a solid line, and portions of the outer peripheral surfaces of the first to third conducting wires 21 to 23 that are not included in the outer peripheral edge 2a are indicated by dashed lines. In Figures 3A to 3D, the points where the first to third conducting wires 21 to 23 are closest to each other or where they come into contact are indicated as closest points P1 to P3. The outer peripheral edge 2a is a portion of the outer peripheral surface of each of the first to third conducting wires 21 to 23 that faces the inner surface 30a of the cavity 30 between the closest points P1 to P3.

[0021] Except for the portion in contact with the protrusion 31, the outer peripheral edge 2a of the central conductor 2 faces the inner surface 30a of the cavity 30 across a gap. Specifically, 90% or more of the outer peripheral edge 2a of the central conductor 2 faces the inner surface 30a of the cavity 30 across a gap. In other words, less than 10% of the outer peripheral edge 2a of the central conductor 2 faces the inner surface 30a of the cavity 30 across a gap. More preferably, 95% or more of the outer peripheral edge 2a of the central conductor 2 faces the inner surface 30a of the cavity 30 across a gap.

[0022] Among multiple cross sections of the coaxial cable 1 perpendicular to the longitudinal direction, in some cross sections the center conductor 2 is supported by one or more protrusions 31, and in other cross sections the center conductor 2 is not supported by any protrusions 31. Figures 2(a) and 3A show cross sections in which the center conductor 2 is supported by two protrusions 31, while Figures 2(b), (c), 3B, and 3C show cross sections in which the center conductor 2 is supported by one protrusion 31. Figures 2(d) and 3D show cross sections in which the center conductor 2 is not supported by any protrusions 31.

[0023] The shape of the inner surface 30a of the cavity 30 of the foamed insulation 3 in a cross section perpendicular to the longitudinal direction of the coaxial cable 1 is a substantially triangular shape corresponding to the shape of the outer periphery 2a of the central conductor 2. More specifically, as shown in Figures 3A to 3D, when the center points of the first to third conducting wires 21 to 23 are C1 to C3, a line segment L starting from the center line C, passing through the center points C1 to C3, and reaching the inner surface 30a of the cavity 30 is 11 ~L 13 The average value of the length of the line segment L that starts from the center line C, passes through the closest points P1 to P3, and reaches the inner surface 30a of the cavity 30 21 ~L 23 longer than the average length of

[0024] Note that the line segment L does not necessarily extend to all points on the cross section of the coaxial cable 1. 11 ~L 13 The average length of the line segment L 21 ~L 23 In other words, in cross sections at a plurality of locations (for example, 10 locations) in the longitudinal direction of the coaxial cable 1, the length of the line segment L 11 ~L 13 The average length of the line segment L 21 ~L 23 It is sufficient if the length is longer than the average of the average lengths of the

[0025] In addition to the cavities 30, the foamed insulation 3 has numerous bubbles 300 of various sizes and shapes. The foamed insulation 3 has a foaming degree of 40% or more and 70% or less. The foaming degree is calculated using the formula: 100 - (specific gravity after foaming / specific gravity before foaming) x 100. Furthermore, the foamed outer surface 3a has an uneven shape due to foaming. That is, as shown in FIGS. 1 and 3A to 3D, the foamed insulation 3 has numerous recesses 32 and numerous protrusions 33 formed on the outer surface 3a. The numerous protrusions 33 of the foamed insulation 3 contact the skin layer 5 made of the insulating tape 50, while the recesses 32 between the protrusions 33 do not contact the skin layer 5.

[0026] In this embodiment, the proportion of the area of ​​the outer peripheral surface 3a of the foamed insulation 3 that is in contact with the skin layer 5 is 50% or less. In the portions of the outer peripheral surface 3a of the foamed insulation 3 that are not in contact with the skin layer 5, a gap 10 is formed between the foamed insulation 3 and the skin layer 5. Note that if the foamed insulation 3 is tightened too much, the convex portions 33 will be crushed and will also come into contact with the concave portions 32, so the insulating tape 50 is wrapped around the outer periphery of the foamed insulation 3 with tension that does not tighten the foamed insulation 3 too much.

[0027] Next, a manufacturing apparatus and a manufacturing method for forming the foamed insulation 3 around the central conductor 2 will be described with reference to FIG.

[0028] 4 is a schematic diagram showing the general configuration of this manufacturing apparatus 7. The manufacturing apparatus 7 includes a feeder 71 that feeds out the central conductor 2, a preheater 72 that heats the central conductor 2 fed from the feeder 71, a foaming extruder 73 that extrudes a foaming resin material around the heated central conductor 2, a sizing die 74 that adjusts the outer diameter of the extruded foaming resin material, and a cooling water tank 75 that cools the foaming resin material. A foamed electric wire 8 consisting of the cooled foamed insulation 3 and the central conductor 2 is taken up by a winder 81. Thereafter, a skin layer 5 is formed around the foamed electric wire 8 taken up by the winder 81 by winding an insulating tape 50 around the skin layer 5 or by extrusion molding of a resin, a plurality of wires 41 are spirally wound around the skin layer 5 to form an outer conductor 4, and a jacket 6 is further formed around the outer conductor 4, thereby obtaining a coaxial cable 1.

[0029] The foamed resin material contains a foaming agent, which forms cavities 30, numerous bubbles 300, and protrusions 31. The cavities 30 are formed when numerous bubbles generated by the foaming agent come into contact with any of the first to third conductive wires 21 to 23 and unite around the central conductor 2. The protrusions 31 are formed when a portion of the foamed resin material in contact with the outer periphery 2a of the central conductor 2 is stretched by the bubbles that have gathered around the central conductor 2. The size of the cavities 30 can be increased or decreased by adjusting the amount of foaming agent, the extrusion pressure of the foaming extruder 73, the cooling temperature of the foamed resin material in the cooling water tank 75, etc.

[0030] (Actions and Effects of the Embodiments) According to the embodiment described above, the following actions and effects can be obtained.

[0031] (1) The central conductor 2 is supported within the cavity 30 by the protrusions 31, and the outer peripheral edge 2a of the central conductor 2 faces the inner surface 30a of the cavity 30 across a gap, except for the portion in contact with the protrusions 31. This reduces the capacitance between the central conductor 2 and the outer conductor 4, thereby improving high-frequency characteristics in particular. That is, since the dielectric constant of the air in the cavity 30 is lower than the dielectric constant of the material of the foamed insulation 3, surrounding the central conductor 2 almost entirely within the cavity 30 reduces the capacitance between the central conductor 2 and the outer conductor 4, thereby improving high-frequency characteristics. According to this embodiment, the capacitance between the central conductor 2 and the outer conductor 4 can be reduced to, for example, 58 pF / m or less.

[0032] (2) Because the central conductor 2 is formed by twisting the first to third conducting wires 21 to 23, the numerous bubbles generated by the foaming agent tend to come into contact with any of the first to third conducting wires 21 to 23 and unite around the central conductor 2. That is, if the central conductor were formed by twisting, for example, seven conducting wires, the shape of the outer periphery of the central conductor would be close to a circle, making it difficult for the numerous bubbles to unite around the central conductor during molding of the foamed insulation, and making it impossible to form a large cavity. However, in this embodiment, the shape of the outer periphery 2a of the central conductor 2 is trefoil-shaped, making it possible to form a relatively large cavity 30 around the central conductor 2. Furthermore, costs can be reduced compared to when the central conductor is twisted seven wires. If the central conductor were a single solid wire, it would be difficult to form a protrusion on the foamed insulation, and the central conductor would move longitudinally within the cavity. However, by forming the central conductor 2 by twisting together the first to third conducting wires 21 to 23 as in this embodiment, an appropriate amount of protrusion 31 can be formed, and the central conductor 2 can be prevented from moving longitudinally relative to the foamed insulation 3.

[0033] (3) The shape of the inner surface 30a of the cavity 30 in a cross section perpendicular to the longitudinal direction of the coaxial cable 1 is approximately triangular, corresponding to the shape of the outer edge 2a of the center conductor 2. Therefore, the effect of reducing the capacitance between the center conductor 2 and the outer conductor 4 is more reliably achieved while supporting the center conductor 2 at the center of the cavity 30.

[0034] (4) Since the outer diameters D1, D2, and D3 of the first to third conductors 21 to 23 are each 0.1 mm or less, the coaxial cable 1 can be suitably used as a medical cable. Furthermore, in a cross section perpendicular to the longitudinal direction of the coaxial cable 1, a cavity 30 having a size sufficient compared to the cross-sectional area of ​​the first to third conductors 21 to 23 can be formed.

[0035] (5) The area of ​​the outer surface 3a of the foamed insulation 3 that is in contact with the skin layer 5 is 50% or less, and in the part where the outer surface 3a of the foamed insulation 3 is not in contact with the skin layer 5, a gap 10 is formed between the foamed insulation 3 and the skin layer 5, so that this gap 10 can also reduce the electrostatic capacitance between the central conductor 2 and the outer conductor 4.

[0036] (6) The outer conductor 4 is formed by helically winding the plurality of wires 41, which increases the flexibility of the coaxial cable 1. Furthermore, the outer diameter of the plurality of wires 41 in the outer conductor 4 is 0.032 mm or less, and the conductivity of the plurality of wires 41 is 80% or more. Therefore, the diameter of the coaxial cable 1 can be reduced while still ensuring sufficient current capacity, making the coaxial cable suitable for use in, for example, medical cables.

[0037] (Summary of the embodiment) Next, the technical ideas grasped from the above-described embodiments will be described by using the reference numerals and the like in the embodiments. However, the reference numerals in the following description do not limit the components in the claims to the members and the like specifically shown in the embodiments.

[0038] [1] A coaxial cable (1) comprising: a central conductor (2) formed by twisting together a plurality of conducting wires (21-23); a foamed insulation (3) having a cavity formed to surround the central conductor (2); and an outer conductor (4) provided around the foamed insulation (3), wherein a part of the foamed insulation (3) forms a protrusion (31) extending from an inner surface (30a) of the cavity (30) facing the central conductor (2) across a gap toward the central conductor (2), the protrusion (31) contacts an outer peripheral edge (2a) of the central conductor (2) to support the central conductor (2) within the cavity (30), and the outer peripheral edge (2a) of the central conductor (2) faces the inner surface (30a) of the cavity (30) across a gap over substantially the entirety, except for the part that contacts the protrusion (31), of the central conductor (2).

[0039] [2] The coaxial cable (1) according to the above [1], wherein the central conductor (2) has three of the plurality of conductive wires (21 to 23).

[0040] [3] The coaxial cable (1) according to [2] above, wherein the shape of the inner surface (30a) of the cavity (30) in a cross section perpendicular to the longitudinal direction is a substantially triangular shape corresponding to the shape of the outer periphery (2a) of the central conductor (2).

[0041] [4] The coaxial cable (1) according to any one of the above [1] to [3], wherein the outer diameters (D1, D2, D3) of the plurality of conductors (21 to 23) are each 0.1 mm or less.

[0042] [5] A coaxial cable (1) according to any one of the above [1] to [4], wherein a skin layer (5) formed by a spirally wound insulating tape (50) or extrusion molding of a resin is formed between the foamed insulation (3) and the outer conductor (4), and the proportion of the area of ​​the outer surface (3a) of the foamed insulation (3) that is in contact with the skin layer (5) is 50% or less.

[0043] [6] The coaxial cable (1) according to any one of the above [1] to [5], wherein the outer conductor (4) is formed by spirally winding a plurality of wires (41).

[0044] [7] The coaxial cable (1) according to any one of [1] to [6] above, wherein the outer diameter (D4) of the plurality of strands (41) of the outer conductor (4) is 0.05 mm or less, and the conductivity of the plurality of strands (41) is 70% IACS or more.

[0045] [8] The coaxial cable (1) according to any one of [1] to [6] above, wherein the outer diameter (D4) of the plurality of strands (41) of the outer conductor (4) is 0.032 mm or less, and the conductivity of the plurality of strands (41) is 80% IACS or more.

[0046] Although the embodiments of the present invention have been described above, the invention according to the claims is not limited to the embodiments described above. It should be noted that not all of the combinations of features described in the embodiments are necessarily essential to the means for solving the problems of the invention. [Explanation of symbols]

[0047] 1... Coaxial cable 2... Center conductor 21-23...First to third conducting wires 2a...Outer periphery 3...Foam insulation 30...Cavity 30a...Inner surface 31...Protrusion 3a...Outer surface 4...Outer conductor 41...Element wire 5...Skin layer 50...Insulating tape 6...Jacket

Claims

1. A cable comprising a central conductor formed by twisting together three conducting wires, a foamed insulator having a cavity formed so as to surround the central conductor, and an outer conductor provided around the foamed insulator, The foaming degree of the foamed insulation is 40% or more and 70% or less, a part of the foamed insulation is a protrusion extending from an inner surface of the cavity facing the central conductor across a gap toward the central conductor, the protrusion contacting an outer periphery of the central conductor to support the central conductor within the cavity; the outer periphery of the central conductor, except for the portion in contact with the protrusion, faces the inner surface of the cavity across a gap; Coaxial cable.

2. The portion of the outer peripheral edge of the central conductor that is in contact with the protrusion is less than 10%.

2. The coaxial cable according to claim 1.

3. the shape of the inner surface of the cavity in a cross section perpendicular to the longitudinal direction is a substantially triangular shape corresponding to the shape of the outer periphery of the central conductor; 3. The coaxial cable according to claim 1 or 2.

4. The outer diameter of each of the three conductors is 0.1 mm or less.

4. The coaxial cable according to claim 1.

5. a skin layer formed by a spirally wound insulating tape or by extrusion molding of a resin between the foamed insulation and the outer conductor; The ratio of the area of ​​the outer surface of the foamed insulation that is in contact with the skin layer is 50% or less.

5. The coaxial cable according to claim 1.

6. The outer conductor is formed by spirally winding a plurality of wires.

6. A coaxial cable according to claim 1.

7. The outer conductor has an outer diameter of the plurality of strands of 0.05 mm or less and a conductivity of the plurality of strands of 70% IACS or more.

7. The coaxial cable according to claim 6.

8. The outer conductor has an outer diameter of the plurality of strands of 0.032 mm or less and a conductivity of the plurality of strands of 80% IACS or more.

8. The coaxial cable according to claim 6 or 7.

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