Coaxial cable

Abstract

To provide a coaxial cable which can favorably maintain electric characteristics even when used at extremely low temperature.SOLUTION: A coaxial cable 200 is provided, comprising a center conductor 210, a dielectric layer 220, an outer conductor layer 230, and an outer coating 240. The outer conductor layer is formed by winding a plurality of element wires transversely, and each of the element wires has a circular cross section and differs at least in outer diameter. Each of the plurality of element wires has a circular cross-section and at least has a different diameter, comprising an element wire A with the largest outer diameter and an element wire B with an outer diameter at least 30% smaller than that of the element wire A. The plurality of element wires are also composed of a plurality of element wires A and at least one element wire B. For each of the distances LCA(1-n) from the center point of the center conductor to the center point of element wire A, and each of the distances LCB(1-n) from the center point of the center conductor to the center point of the element wire B, LCA<LCB in 60% or more of the total number of wires.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a coaxial cable. In particular, the present invention relates to a coaxial cable capable of avoiding changes in electrical characteristics due to thermal contraction of a resin layer caused by cooling.

Background Art

[0002] As a coaxial cable, a coaxial cable disclosed in Document 1 is already known. This coaxial cable includes, in order from the center in a cross-sectional view, a center conductor, a dielectric layer, an outer conductor layer, and an outer sheath. The dielectric layer and the outer sheath of this coaxial cable are made of resin. Further, the outer conductor layer of this coaxial cable covers the dielectric layer by winding a plurality of metal strands of the same diameter around it horizontally.

[0003]

Patent Document 1

Disclosure of the Invention

Problems to be Solved by the Invention

[0004] However, when a coaxial cable as described above is used in a temperature environment of an extremely low temperature state close to absolute zero (for example, -196°C or lower to below absolute zero), the following problems may occur due to shrinkage caused by temperature changes in the dielectric layer and the outer sheath.

[0005] That is, for example, when used in an extremely low temperature state near absolute zero by cooling, such as a coaxial cable used in a quantum computer in recent years, the inventors have found that due to shrinkage of the dielectric layer and the outer sheath, the shielding characteristics by the outer conductor deteriorate, and as a result, the desired electrical characteristics required for the coaxial cable cannot be obtained. Specifically, a gap is generated between the dielectric layer and the outer conductor layer due to shrinkage of the resin, and the outer conductor is no longer supported by the surface of the dielectric layer. As a result, each strand of the horizontally wound shield arranged in parallel has a place to escape from the tension applied to each other, and the parallel arrangement collapses. As a result, the shielding density is significantly reduced, such as a gap being generated between the strands, from the state shielded by the plurality of strands arranged in parallel, leading to a deterioration of the cable characteristics.

[0006] The present invention has been made in view of the above circumstances, and an object thereof is to provide a coaxial cable capable of maintaining stable electrical characteristics even in a low-temperature environment.

Means for Solving the Problems

[0007] The coaxial cable of the present invention includes a center conductor, a dielectric layer covering the center conductor, an outer conductor layer covering the dielectric layer, and an outer sheath covering the outer conductor layer. The outer conductor layer is formed by winding a plurality of strands horizontally. Each of the plurality of strands has a circular cross-section, and has at least a strand A having the largest outer diameter with different outer diameters, and a strand B having an outer diameter at least 30% smaller than the outer diameter of the strand A. And the plurality of strands are composed of a plurality of strand As and at least one strand B. The respective distances LCA(1~n) from the center point of the center conductor to the center point of the strand A and the respective distances LCB(1~n) from the center point of the center conductor to the center point of the strand B are such that, in a ratio of 60% or more of the number of strands, LCA < LCB.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Mode for Carrying Out the Invention

[0009] Hereinafter, the constituent elements of the superconducting cable according to the present invention will be described in detail. First, the prior art and the technical idea of the present invention will be described. FIGS. 1 to 4 are diagrams for explaining the problems of the prior art. FIG. 1 is a diagram schematically showing a cross section of a coaxial cable of the prior art. As shown in the figure, a conventional coaxial cable 100 extends in the longitudinal direction and includes, in order from the center toward the outside in the radial direction, a center conductor 110, a dielectric layer 120, an outer conductor layer 130, and an outer sheath 140. The dielectric layer 120 and the outer sheath 140 are made of resin, and the outer conductor layer 130 is formed by winding a plurality of metal wires of the same diameter around. At room temperature (for example, 25°C), each strand of the outer conductor 130 is in contact with the dielectric layer 120 on the inner side in the radial direction and in contact with the outer sheath 140 on the outer side in the radial direction.

[0010] However, when this coaxial cable is placed in a state from room temperature to an extremely low temperature state (for example, -196°C), as shown in FIG. 2, the dielectric layer 120 contracts in the radial direction (although not shown, the outer sheath 140 also contracts similarly). Due to the contraction of the dielectric layer and the like, as shown in FIG. 3, a gap G1 is generated between the outer conductor 130 and the dielectric layer 120 (the hatched portion in the figure).

[0011] The void G1 generated in this way, and the external conductor 130 is compressed from the outside due to the shrinkage of the jacket 140 separately, as shown in FIG. 4, the parallel arrangement of each strand of the external conductor 130 is disturbed, and gaps and variations in the distance from the center conductor occur between the strands. The gaps between the strands of the external conductor 130 and the variations in the distance from the center conductor cause a decrease in the shielding density, which in turn leads to a deterioration of the electrical characteristics of the coaxial cable.

[0012] Next, the technical idea of the present invention for solving the above problems will be described with reference to FIGS. 5 to 7. FIG. 5 is a diagram schematically showing a cross-section of a coaxial cable for explaining the technical idea of the present invention. As shown in the figure, the coaxial cable 200 of the present invention extends in the longitudinal direction (the depth and front-back direction in the figure), and in order from the center toward the outside in the radial direction, it includes a center conductor 210, a dielectric layer 220, an external conductor layer 230, and a jacket 240. The dielectric layer 220 and the jacket 240 are made of resin, and the external conductor layer 230 is formed by horizontally winding a plurality of strands 230a, 230b having two or more outer diameters in the same direction. Among the plurality of strands constituting the external conductor layer 230, at least more than half of the strands 230A having a large outer diameter are arranged such that the distance (LCA) from the center of the center conductor to the center of the strand A is smaller than the distance (LCB) from the center of the center conductor to the center of the strand 230B having a small outer diameter. That is, the center of the strand 230B having a small outer diameter is arranged outside the circle (the broken-line circle in the figure) passing through the center of the strand 230A.

[0013] The external conductor 230 arranged in this way is, as described above, placed in a cryogenic state (e.g., -196°C) from the normal temperature state. As the dielectric layer 220 and the jacket 240 contract radially, even if a void G2 is formed as shown in FIG. 6, as shown in FIG. 7, as the thick strands 230A with a larger outer diameter approach each other, the thin strands 230B with a smaller outer diameter are pushed outward in the radial direction. As a result, from the state where the thick strands 230A, the thin strands 230B, and the thick strands 230A are arranged in parallel in that order, the thick strands 230A can approach each other by the size of the thin strands 230B that were sandwiched between the thick strands 230A. Thus, even if a void G2 is generated between the external conductor layer 230 due to the reduction in the outer diameter of the dielectric layer 220, the inner diameter of the external conductor layer 230 also decreases accordingly, so that the shielding density of the external conductor layer 230 does not decrease, and it is possible to avoid degrading the electrical characteristics of the cable.

[0014] On the other hand, if, in the majority of the thin strands 230B, the distance (LCA) from the center of the center conductor 210 to the center of the thick strand 230A is arranged to be greater than the distance (LCB) from the center of the center conductor to the center of the thin strand 230B with a smaller outer diameter, the effects of the present invention cannot be obtained.

[0015] This is because the thin strands 230B cannot escape outward as the dielectric layer 230 contracts, and the thick strands 230A cannot approach each other. Thus, not only can the external conductor not follow the contraction of the dielectric layer, but it also exacerbates the variation in each strand, resulting in a decrease in the shielding density similar to the prior art and degrading the electrical characteristics of the cable.

[0016] The present invention can be modified in various ways and can have various forms, and embodiments will be described in detail in the text. However, this is not intended to limit the present invention to a specific disclosed form, and it should be understood to include all modifications, equivalents, and alternatives included in the technical idea and scope of the present invention. Similar reference numerals are used for similar components while explaining each drawing. Terms such as first, second, etc. can be used to explain various components, but the said components should not be limited by the said terms. The said terms are used only for the purpose of distinguishing one component from another. The terms used in the present application are merely used to explain a specific embodiment and are not intended to limit the present invention. Singular expressions include plural expressions unless clearly different in context. In the present application, terms such as "including" or "made" are intended to specify the existence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and should be understood not to preclude in advance the existence or addition possibility of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0017] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning in the context of the related art, and should not be interpreted in an ideal or overly formal sense unless clearly defined in the present application.

[0018] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The same member numbers refer to the same components regardless of the drawings. Redundant explanations in the embodiments will be omitted. Also, embodiments and their examples are shown below, but the present invention is not limited thereto, and any invention based on the above technical idea is acceptable.

[0019] Next, embodiments based on the technical idea of the present invention described above will be described with reference to the drawings. The embodiments described below do not limit the invention according to the claims, and not all combinations of the features described in the embodiments are essential for the implementation of the present invention.

[0020] FIG. 8 shows the configuration of the first embodiment of the present invention. As shown in FIG. 8, the coaxial cable 300 according to the present invention has a dielectric layer 320 covering the periphery of a center conductor 310, an outer conductor layer 330 around the dielectric layer 320, and an outer sheath 340 covering the periphery of the outer conductor. The outer conductor layer 330 is composed of a plurality of strands having at least two or more outer diameters, namely, the outer diameter of a strand (thick strand A) 330A having the maximum outer diameter and the outer diameter of a strand (thin strand B) 330B having the minimum outer diameter. The strand 330B is composed of strands having an outer diameter at least 50% smaller than that of the strand 330A. Note that the outer diameter sizes of the strands of the outer conductor layer 330 may be two types or a mixture of three or more types.

[0021] The center conductor 310 of the coaxial cable 300 according to the present invention is composed of a single wire or a stranded metal wire. The center conductor 310 may be formed not only of a copper alloy wire but also of a copper wire or a copper alloy wire such as tin plating, silver plating, nickel plating, or niobium titanium plating, or crude copper. Using a stranded wire for the center conductor 310 is preferable because it has better flexibility and is less likely to break compared to a single wire, especially when using thin strands. Also, a single wire may be used for the center conductor 310. When using a single wire with the same conductor cross-sectional area, the outer diameter can be made smaller compared to a stranded wire.

[0022] For the center conductor 310 of the coaxial cable 300 according to the present invention, a thin conductor of AWG (American Wire Gauge) 24 or more is used. For example, when using a single wire of an AWG29 silver-plated copper alloy wire for the center conductor 21 of the coaxial cable 300, a single wire of a silver-plated copper alloy wire with an outer diameter of 0.287 mm is used.

[0023] The dielectric layer 320 of the coaxial cable 300 according to the present invention is formed of a tetrafluoroethylene - hexafluoropropylene copolymer (FEP). Further, instead of FEP, the insulator layer 22 may be formed of polyethylene or polytetrafluoroethylene (PTFE), tetrafluoroethylene - perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene - ethylene copolymer (ETFE), or the like. The dielectric layer 320 is sized to have a predetermined impedance from the outer diameter of the center conductor 310, and in this embodiment, it is coated with a thickness of 0.3315 mm.

[0024] The outer conductor 330 is composed of a plurality of metal strands, and strands with an outer diameter that differs by at least 60% or more are mixed and used. Silver-plated soft copper wire can be used for the thick strands 330A, and silver-plated hard copper wire can be used for the thin strands 330B. Further, the outer conductor 330 composed of a plurality of strands is wound horizontally in the same direction along the outer peripheral surface of the dielectric layer 320. For example, 34 silver-plated soft copper wires with an outer diameter of 0.08 mm and 11 silver-plated hard copper wires with an outer diameter of 0.05 mm are mixed and wound horizontally in the same direction around the outer periphery of the dielectric layer 320 with an outer diameter of 0.95 mm. The strands of the outer conductor 330 can be configured in various combinations such as copper wires or copper alloy wires with tin plating, silver plating, nickel plating, etc., and strands of the same size can be configured with different materials, other than the above.

[0025] In this embodiment, strands 330A and 330B with an outer diameter that differs by 62.5% are used as the plurality of strands that make up the outer conductor 330. However, in order to obtain the effects of the invention described later, preferably 80 to 90% or more, more preferably 70 to 80% or more, and particularly preferably 30 to 70% different strands can be used. When using different strands with a size diameter of 29% or less, due to the shrinkage of the resin layer such as the dielectric, the thinner strand B moves to the outside. As a result, although adjacent strands A can approach each other by the interval of strand B from the state where they were arranged in the order of strand A, strand B, and strand A, the closer the curvature of the cross-section of the strands becomes, the greater the friction, and it becomes difficult for strand B to move to the outside. Also, even if strand B moves to the outside, if the outer diameter of strand B does not differ much from that of strand A, it will be pushed back by the outer sheath 340, and the desired effect cannot be obtained. On the other hand, when the outer diameters of strand A and strand B are too different, although a certain effect can be obtained, even if strand B moves to the outside, the distance by which adjacent strands A can approach each other becomes shorter, and the range that can follow the shrinkage of the dielectric layer becomes limited.

[0026] Also, in FIG. 8, the distance LCA from the center point of the center conductor 310 to the center point of the strand 330A and the distance LCB from the center point of the center conductor 310 to the center point of the strand 330B were described as being configured such that LCA < LCB. Here, in the relationship between the strand 330A and the strand 330B, not all need to have the above configuration (LCA < LCB). As shown in FIG. 9, it is sufficient that 70% or more of the proportion of the number of strands 330B satisfies LCA < LCB, preferably 80% or more, and particularly preferably 90% or more.

[0027] Also, for the strand 330A and the strand 330B, it is sufficient that at least the strand 330A has a number of strands that is 2 times or more that of the strand 330B, preferably 3 times or more, and particularly preferably 5 times or more. According to the above configuration, the effects of the present invention described above can be obtained.

[0028] Next, the second embodiment of the present invention will be described with reference to FIG. 10. FIG. 10 is a diagram schematically showing the second embodiment. In the description of the second embodiment, only the parts different from the first embodiment will be described.

[0029] In the coaxial cable 400 of the second embodiment, the positional relationship of the plurality of strands of the outer conductor 430 is different from that of the first embodiment. The coaxial cable 400 of the second embodiment includes a plurality of strands 430A having a large outer diameter (13 strands in this embodiment) and a plurality of strands 430B having an outer diameter smaller than that of the strands 430A (3 strands in this embodiment), and the outer diameter of the strands 430B is 50% or less of the outer diameter of the strands 430A. Similar to the first embodiment, for the strands 430B having such an outer diameter, the distance (LCB) from the center point of the center conductor 410 to the center point of the strands 430B is greater than the distance (LCA) from the center point of the center conductor 410 to the center point of the strands 430A (LCA < LCB). Also, as shown in FIG. 10, for the strands B having such an outer diameter, the distance (LOB) from the center point of the center conductor 410 to the point intersecting the outermost portion of the cross-sectional contour of the strands 430B through the center of the strands 430B is smaller than the distance (LOA) from the center point of the center conductor 410 to the point intersecting the outermost portion of the cross-sectional contour of the strands 430A through the center of the strands 430A, and more than 70% of the total number of the strands B is smaller in terms of the number ratio (LOA > LOB).

[0030] The coaxial cable 400 configured in this way can not only obtain the effects of the coaxial cable of the first embodiment described above, but also obtain the above-described effects when the thinner strands B move outward due to the shrinkage of the resin layer such as the dielectric. In the normal temperature state, the strands 430A are in contact with the outer sheath 440, while the strands 430B are spaced apart from the outer sheath 440 without contact. Therefore, when placed at a low temperature, not only is there a space ensured in advance for the strands 430B to move outward even if the resin layer such as the dielectric shrinks, but also it is less affected by the shrinkage from the outside of the outer sheath 440. Thus, it is possible to avoid the disturbance of the arrangement of the strands and maintain the shielding density, and thereby avoid the deterioration of the electrical characteristics of the coaxial cable.

[0031] In the present embodiment, in the relationship between the individual strands 430A and 430B, a configuration in which LOA > LOB for all the individual strands has been described first. However, in the total number ratio of the individual strands, it is sufficient that LOA > LOB in a number ratio of 70% or more, preferably 80% or more, and more preferably 90% or more.

[0032] In the coaxial cable according to the first and second embodiments described above, in the thin-diameter individual strands B that constitute the outer conductor, a configuration in which not all the individual strands B are continuously arranged with each other has been described. However, the present invention is not limited to this, and it is sufficient that 80% or more are not continuously arranged with each other. This is because when the ratio is less than 80%, when the individual strands B move outward as the resin layer shrinks, the thin-diameter individual strands B tend to move inward and outward, making it difficult to obtain the desired effect.

[0033] Further, when comparing the shrinkage rates of the resin layers (dielectric layer and jacket) of the coaxial cable according to the above-described embodiment at -196°C under liquid nitrogen, it is more preferable that the shrinkage rate S2 of the jacket is greater than the shrinkage rate S1 of the dielectric layer (S1 < S2). This is because by configuring the shrinkage rate S2 of the jacket to be larger, not only does the larger-diameter jacket tend to adhere closely to the outer periphery of the outer conductor as the resin layer shrinks, but also the jacket presses the adjacent individual strands A against the dielectric layer due to the outward movement of the individual strands B, making it easier to guide the individual strands A and B to the desired positions.

[0034] Also, in the above-described embodiment, a configuration in which the individual strands A and B are made of the same material has been described. However, more preferably, when comparing the tension T1 of the individual strands A and the tension T2 of the individual strands B, it is more preferable that T2 < T1. Thereby, when the individual strands of the outer conductor follow the dielectric layer due to the shrinkage of the resin layer, the individual strands B can be more easily pushed outward, and a more stable effect can be obtained.

[0035] In addition, the strands of the outer conductor are preferably formed by horizontally winding around the insulator layer at an angle of 18° to 40°, and the winding direction can be either right-handed or left-handed. For example, after horizontally winding around the outer peripheral surface of the insulator layer at an angle of 25°, the outer conductor is compressed by passing it through a die. Since strands with an outer diameter difference of 10% or more are mixed, it becomes possible to efficiently compress the strands, and due to the contact between the strands changing from line contact to surface contact, a coaxial cable with excellent shielding characteristics can be obtained.

[0036] The outer conductor of the coaxial cable according to the present invention can be made into a coaxial cable with excellent shielding characteristics by forming it so that the horizontal winding density is 1.0 or more. The horizontal winding density is represented by the ratio of the conductor shielding area to the horizontal winding surface area. The symbol D in FIG. 4 indicates the average horizontal winding diameter, and it can be obtained by the sum of the outer diameter of the insulator and the outer diameter d of the horizontally wound strand. w The horizontal winding surface area of a coaxial cable with a length P is represented by P×πD. The conductor shielding area refers to the area covered by the horizontally wound strands among the horizontal winding surface area, and can be obtained by the following formula using the number n of horizontally wound strands and the outer diameter d of the horizontally wound strands. w Therefore, the horizontal winding density can be obtained by the following formula.

Equation

Equation

[0037] For the coaxial cable according to the present invention, a jacket layer of PFA can be provided around the outer conductor. The jacket layer may be formed of polyethylene, polyester, polyimide, or tetrafluoroethylene (PTFE), tetrafluoroethylene - hexafluoropropylene copolymer (FEP), tetrafluoroethylene - ethylene copolymer (ETFE), etc.

[0038] For the coaxial cable according to the present invention, a plurality of coaxial cables can be twisted together to form a multi-core cable. A jacket layer is formed on the outer periphery of the twisted plurality of coaxial cables to form a transmission cable.

[0039] Next, the invention will be described in more detail with the following examples. The following examples illustrate the invention, and the content of the present invention is not limited by the following examples.

[0040] Next, examples and comparative examples are shown. The examples and comparative examples shown in FIG. 11 are configured based on the difference in the configuration of the outer conductor after aligning the configuration conditions of the center conductor, dielectric layer, and outer covering.

[0041] These examples and comparative examples are carried out by connecting the coaxial cable (test length 500 mm) to be tested to a lead cable (MWX322 manufactured by Jungong Co., Ltd.) connected to a VNA (P5024A manufactured by Keysight Technologies) and submerging the coaxial cable to be tested under liquid nitrogen. In this test, after evaluating the initial characteristics at room temperature and the characteristics at low temperature, it is repeated to return to room temperature, and a maximum of 3 low-temperature evaluations are carried out.

Claims

1. A central conductor, a dielectric layer covering the central conductor, an outer conductor layer covering the dielectric layer, and a jacket covering the outer conductor layer, and the outer conductor layer is formed by spirally winding a plurality of element wires, each of the plurality of element wires has a circular cross-section, and has at least an element wire A having the largest outer diameter with different outer diameters, and an element wire B having an outer diameter at least 30% smaller than the outer diameter of the element wire A, and the plurality of element wires are composed of a plurality of element wires A and at least one element wire B, each distance LCA(1-n) from the center point of the central conductor to the center point of the element wire A and each distance LCB(1-n) from the center point of the central conductor to the center point of the element wire B satisfy LCA < LCB at a ratio of 60% or more of the number of wires. The coaxial cable is characterized by this.

2. The coaxial cable according to claim 1, wherein the ratio of the number of element wires A and element wires B among the plurality of element wires is at least 2:1 or more.

3. The coaxial cable according to claim 1, wherein each distance LOA and LOB from the center of the central conductor to the farthest part to the element wire A and the element wire B satisfy LOA > LOB at a ratio of 70% or more of the number of wires.

4. The coaxial cable according to claim 1, wherein the element wires B are not arranged in parallel continuously with each other.

5. The dielectric layer and the jacket are each made of resin, and the shrinkage rate S1 of the dielectric layer and the shrinkage rate S2 of the jacket satisfy S2 > S1 at -196°C. The coaxial cable according to claim 1 is characterized by this.

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