Signal transmission cable

Abstract

To provide a signal transmission cable whose transmission characteristics are not easy reduced when bent.SOLUTION: A signal transmission cable 1 comprising a conductor 2, an insulator 3 covering the circumference of the conductor 2, a shield layer 4 covering the circumference of the insulator 3, and a sheath 5 covering the circumference of the shield layer 4, wherein between the insulator 3 and the shield layer 4, a plating base layer 6 is provided so as to cover the circumference of the insulator 3, the shield layer 4 has a plating layer 41 formed so as to be in contact with an outer peripheral surface of the plating base layer 6 and so as to cover the plating base layer 6, and the surface roughness of the outer peripheral surface of the plating layer 41 is smaller than the surface roughness of an inner peripheral surface of the plating layer 41.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Signal transmission cables for transmitting high-frequency signals are used as internal wiring in electronic devices such as imaging devices used for autonomous driving, smartphones, and tablet terminals, or as wiring in machine tools such as industrial robots. As such signal transmission cables, for example, coaxial cables are used.

[0003] 2. Description of the Related Art Known conventional coaxial cables include those in which a shield layer is formed by spirally winding a tape member, such as a copper tape having copper foil provided on a resin layer, around an insulator (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2000-285747 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the conventional coaxial cable described above, when the coaxial cable is bent, gaps may occur in the overlapping portions of the tape members, or wrinkles may occur in the tape members. In such cases, the change in insertion loss (S21) or characteristic impedance between the bent portion and other portions (straight portions that are not bent) may become large, and the transmission characteristics may deteriorate.

[0006] SUMMARY OF THE PRESENT DISCLOSURE An object of the present invention is to provide a signal transmission cable whose transmission characteristics are less likely to deteriorate when bent. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, the present invention provides a signal transmission cable comprising a conductor, an insulator covering the conductor, a shielding layer covering the insulator, and a sheath covering the shielding layer, wherein a plating base layer is provided between the insulator and the shielding layer, covering the insulator and being movable relative to the insulator in the longitudinal direction of the cable, and the shielding layer has a plating layer formed so as to contact the outer peripheral surface of the plating base layer and to cover the plating base layer. Effect of the Invention

[0008] According to the present invention, it is possible to provide a signal transmission cable whose transmission characteristics are less likely to deteriorate when bent. [Brief description of the drawings]

[0009] [Figure 1] 1A and 1B are diagrams showing a signal transmission cable according to one embodiment of the present invention, in which (a) is a cross-sectional view showing a cross section perpendicular to the longitudinal direction of the cable, and (b) is an enlarged photograph of the cross section. [Diagram 2] FIG. 2 is a diagram illustrating the formation of a plating layer. [Diagram 3] FIG. 2 is a diagram showing a blast treatment device, where (a) is a perspective view and (b) is a plan view seen from the front in the conveying direction. [Figure 4] FIG. 4 is a graph showing the measurement results of the surface roughness of the outer peripheral surface of the plating base layer after blasting. [Diagram 5] 11 is a graph showing measurement results of insertion loss S21, in which (a) shows the measurement result when the signal transmission cable is straight and not bent, and (b) shows the measurement result when the signal transmission cable is bent with a bending radius of 1 mm. [Figure 6] 1A is a graph showing the measurement results of the change in characteristic impedance due to bending, and FIG. 1B is a graph showing the insertion loss S21 and the characteristic impedance of the bent portion relative to the bending radius. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [Embodiment Mode] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0011] FIG. 1 shows a signal transmission cable according to the present embodiment, where (a) is a cross-sectional view showing a cross section perpendicular to the longitudinal direction of the cable, and (b) is an enlarged photograph of the cross section.

[0012] 1(a), the signal transmission cable 1 includes a conductor 2 disposed at the center of the cable, an insulator 3 covering the conductor 2, a shield layer 4 covering the insulator 3, and a sheath 5 covering the shield layer 4. In other words, the signal transmission cable 1 is a coaxial cable including the conductor 2 as an inner conductor and the shield layer 4 as an outer conductor.

[0013] The signal transmission cable 1 is used, for example, as a fixed cable for connecting a robot and a control device in a factory or the like, and has a length of, for example, about 25 m to 100 m. When the signal transmission cable 1 is wired inside an electronic device, its length is, for example, about 5 mm to 200 mm. Note that "covering" also includes the case where it is arranged via another layer. For example, another layer may be arranged between the conductor 2 and the insulator 3, or between the shield layer 4 and the sheath 5.

[0014] (Conductor 2) In the signal transmission cable 1 according to the present embodiment, the conductor 2 is made of a compressed stranded conductor obtained by twisting together a plurality of strands 2a and compressing the stranded conductor so that its cross-sectional shape perpendicular to the cable longitudinal direction is a predetermined shape such as a circular shape. In the present embodiment, the stranded conductor obtained by twisting seven strands 2a concentrically is passed through a die having a smaller diameter than the stranded conductor and a circular outlet, thereby forming the conductor 2 having a circular cross-sectional shape as shown in FIG. 1(a). The strand 2a disposed at the center is substantially hexagonal in cross-sectional view, and the six strands 2a disposed around the center are substantially sector-shaped in cross-sectional view. In addition, the adjacent strands 2a are preferably in contact (surface contact) with each other so that no gap is generated between the strands 2a. In addition, the outer surface of the compressed stranded conductor is preferably smooth in the cable circumferential direction and the cable longitudinal direction. 1 according to the present embodiment, the conductor 2 is made of a compressed stranded conductor having a circular cross-sectional shape, but the conductor 2 may be made of a compressed stranded conductor compressed into a cross-sectional shape other than a circle (for example, a polygonal shape such as a square). Since the conductor 2 is a compressed stranded conductor having a circular cross-sectional shape, the signal transmission cable 1 can be easily bent in any direction, and therefore can be easily routed by bending.

[0015] Although a normal stranded conductor that is not compressed is more flexible and easier to bend than a solid wire conductor, there are many gaps between the strands, and therefore the conductor resistance is higher and the conductivity is lower than that of a solid wire conductor of the same outer diameter. By using a compressed stranded conductor as the conductor 2 as in the present embodiment, the strands 2a are in close contact with each other and there are no gaps between the strands 2a. Therefore, the conductor 2 using the compressed stranded conductor can have a lower conductor resistance than a normal stranded conductor of the same outer diameter. As a result, by using a compressed stranded conductor as the conductor 2, the conductivity is improved and good attenuation characteristics are obtained. Furthermore, the current that transmits a high-frequency signal (also simply referred to as current) mainly passes through the outer periphery of the conductor 2 due to the skin effect. When the conductor 2 is made of a stranded conductor in which multiple strands 2a are twisted together, the curvature of the strands is smaller than that of a solid wire conductor having the same outer diameter as the stranded conductor, and therefore the cross-sectional area of ​​the portion through which the current passes is smaller than that of a solid wire conductor having the same outer diameter as the stranded conductor. In contrast, in the present embodiment, a compressed stranded conductor made by compressing a stranded conductor is used as the conductor 2, so that the wires 2a are in close contact with each other and the outer periphery of the conductor 2 has a concentric shape similar to that of a solid wire conductor. As a result, in the conductor 2 made of a compressed stranded conductor, the cross-sectional area of ​​the portion through which the current passes is larger than that of a stranded conductor having the same outer diameter, and good attenuation characteristics are obtained.

[0016] In order to obtain good attenuation characteristics, it is desirable that the electrical conductivity of the compressed stranded conductor used as the conductor 2 is 99% IACS or more. In this embodiment, in order to achieve high electrical conductivity, soft copper wires made of pure copper plated with silver are used as the strands 2a of the conductor 2. Note that soft copper wires that are not silver plated may also be used as the strands 2a. Furthermore, by compressing the strands 2a through a die, distortion is imparted to the strands 2a, decreasing the electrical conductivity, but by subsequently carrying out a heat treatment (annealing treatment), the distortion can be removed to achieve a conductivity of 99% IACS or more.

[0017] (Insulator 3) It is desirable to use an insulator 3 having as low a dielectric constant as possible in order to improve the transmission characteristics of high-frequency signals (more specifically, to make it difficult for high-frequency signals in the band of 10 MHz to 50 GHz to be attenuated when transmitted). In this embodiment, an insulator made of fluororesin is used for the insulator 3. As the fluororesin used for the insulator 3, FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (tetrafluoroethylene-perfluoroalkylvinylether copolymer), or the like, may be used. The thickness of the insulator 3 is desirably 0.2 mm or more and 2.0 mm or less.

[0018] (Sheath 5) A plating base layer 6, a shield layer 4, and a sheath 5 are sequentially provided around the insulator 3. The plating base layer 6 and the shield layer 4 will be described in detail later.

[0019] The sheath 5 is made of an insulating resin composition such as fluororesin, PVC (polyvinyl chloride), urethane, or polyolefin. In this embodiment, the sheath 5 is made of PFA, which is a fluororesin. The fluororesin used for the sheath 5 may be FEP.

[0020] The sheath 5 is formed by extrusion molding. However, if solid molding is performed, the resin constituting the sheath 5 may get into the spaces between the metal wires of the outer shield layer 42 described below, which may cause the signal transmission cable 1 to become hard and difficult to bend. Therefore, in this embodiment, the sheath 5 is molded by tube extrusion. This prevents the resin constituting the sheath 5 from getting into the spaces between the wires of the outer shield layer 42, and separates the sheath 5 from the outer shield layer 42. That is, in this embodiment, the sheath 5 and the outer shield layer 42 are not bonded together, and the outer shield layer 42 can move relatively freely within the sheath 5. This makes it easier to bend the signal transmission cable 1.

[0021] (Plating Undercoat Layer 6) The signal transmission cable 1 according to the present embodiment includes a plating base layer 6 provided between the insulator 3 and the shield layer 4 so as to cover the periphery of the insulator 3. The plating base layer 6 is a layer that serves as a base when forming a plating layer 41, which will be described later, and is particularly a layer that provides a predetermined surface roughness Ra for the inner surface of the plating layer 41. In the present embodiment, a fluororesin is used as the insulator 3, and since it is difficult to directly form the plating layer 41 on the fluororesin, the plating base layer 6 that serves as a base for the plating layer 41 is provided so as to cover the insulator 3 made of fluororesin.

[0022] The plating base layer 6 is preferably made of an insulating resin capable of forming the plating layer 41 on its outer peripheral surface. In the present embodiment, the plating base layer 6 made of PE (polyethylene) is used, but the plating base layer 6 made of PP (polypropylene) may be used. It is preferable that the plating base layer 6 is formed thin in order to suppress the influence on the transmission characteristics, and the thickness of the plating base layer 6 is preferably thinner than the thickness of the insulator 3. More specifically, the thickness of the plating base layer 6 is preferably 0.5 times or less the thickness of the insulator 3, for example, 0.10 mm or more and 0.20 mm or less. When the thickness of the plating base layer 6 is 0.10 mm or more, the mechanical strength of the plating base layer 6 is increased, and therefore, it is easy to suppress the breakage of the plating base layer 6 due to bending. Furthermore, when the thickness of the plating base layer 6 is 0.20 mm or less, the stress applied to the plating layer 41 when the signal transmission cable 1 is bent, for example (the stress applied to the plating layer 41 as the plating base layer 6 bends in accordance with the bending of the signal transmission cable 1) is small, making it easier to prevent cracks from occurring in the plating layer 41.

[0023] Since any gap between the plating base layer 6 and the insulator 3 adversely affects the transmission characteristics, it is preferable that the plating base layer 6 is provided in contact with the outer surface of the insulator 3 without any gaps. Whether the plating base layer 6 is in contact with the outer surface of the insulator 3 without any gaps can be observed using, for example, an optical microscope or an electron microscope.

[0024] Furthermore, it is more preferable that the plating base layer 6 is provided so as to be able to move in the cable longitudinal direction relative to the bending of the insulator 3 when the signal transmission cable 1 is bent (slide in the cable longitudinal direction relative to the insulator 3). This makes it possible to suppress the occurrence of cracks in the plating layer 41 when the signal transmission cable 1 is bent, as the plating base layer 6 moves in the cable longitudinal direction relative to the bending of the insulator 3. Note that the "crack" referred to here refers to a fissure in the plating layer 41 that occurs in the range from the outer surface of the plating layer 41 to the inner surface of the plating layer 41 (the surface in contact with the insulator 3).

[0025] When a crack occurs in the plating layer 41, a phenomenon called co-crack may occur. However, in this embodiment, the plating layer 41 is formed via the plating base layer 6, which is a separate member from the insulator 3. Therefore, even if a crack occurs in the plating layer 41, there is no risk of co-crack occurring in the insulator 3, and it is possible to suppress defects such as poor insulation.

[0026] In addition, it is preferable that the plating base layer 6 is not joined to the insulator 3 and is provided in a state in which it can be peeled off from the insulator 3. This makes it possible to easily peel off the plating layer 41 from the insulator 3 to expose the insulator 3 during terminal processing of the signal transmission cable 1, thereby improving the workability of terminal processing.

[0027] The outer surface of the plating underlayer 6 is subjected to a predetermined treatment in order to form a plating layer 41. The details of this treatment will be described later.

[0028] (Shield layer 4) The shield layer 4 has a plating layer (inner shield layer) 41 formed so as to cover the plating base layer 6, and an outer shield layer 42 provided so as to cover the plating layer 41. The outer shield layer 42 does not necessarily have to be provided.

[0029] The outer shield layer 42 is made of metal wires, and is configured by braiding or horizontally winding the metal wires. In this embodiment, the outer shield layer 42 is configured by a braided shield made of braided metal wires. The metal wires may be, for example, soft copper wires or hard copper wires made of copper or a copper alloy. They may also be metal wires made of aluminum or an aluminum alloy. The metal wires may be plated on their outer surfaces. In this embodiment, the outer shield layer 42 is configured as a single layer, but the outer shield layer 42 may be configured as a multiple layer. The metal wires that configure the outer shield layer 42 may have lubricity on their surfaces. For example, lubricity may be imparted by applying a lubricant such as talc powder to the surface of the metal wires.

[0030] The provision of the outer shield layer 42 can prevent the shield layer 4 from being electrically insulated even if the plating layer 41 is broken due to some unexpected damage. Furthermore, the provision of the outer shield layer 42 can further reduce loss of low-frequency signals due to the thickness of the outer shield layer 42, even if the plating layer 41 is thin.

[0031] The plating layer 41, together with the outer shield layer 42, constitutes an outer conductor, and is formed so as to be in direct contact with the outer peripheral surface of the plating base layer 6. As described above, the outer shield layer 42 is formed by braiding or horizontally winding metal wires, but with only the outer shield layer 42, internal signals may be radiated to the outside through gaps between the metal wires, resulting in a high attenuation. By providing the plating layer 41, the gaps between the metal wires of the outer shield layer 42 are filled, further reducing the attenuation. The plating layer 41 and the outer shield layer 42 are in contact with each other and are electrically connected.

[0032] The plating layer 41 is preferably made of a metal having a conductivity of 99% or more (99% IACS or more), and may be made of, for example, copper or silver.

[0033] The thickness of the plating layer 41 is preferably 2 μm or more and 5 μm or less. When the thickness of the plating layer 41 is 2 μm or more, cracks are unlikely to occur in the plating layer 41 even if the outer shield layer 42 and the plating layer 41 come into contact when bending is applied. Furthermore, when the thickness of the plating layer 41 is 5 μm or less, it is possible to prevent the plating layer 41 from becoming hard and making it difficult for the signal transmission cable 1 to bend.

[0034] As shown in FIG. 1(b), in the signal transmission cable 1 according to the present embodiment, the surface roughness of the outer peripheral surface of the plating layer 41 is smaller than the surface roughness of the inner peripheral surface of the plating layer 41. The outer peripheral surface of the plating layer 41 is the surface located radially outward of the plating layer 41 and is the surface in contact with the outer shield layer 42. The inner peripheral surface of the plating layer 41 is the surface located radially inward of the plating layer 41 and is the surface in contact with the plating base layer 6. As shown in FIG. 1(b), the plating layer 41 is in contact with the plating base layer 6 without any gaps, so the surface roughness of the inner peripheral surface of the plating layer 41 is the same as the surface roughness of the outer peripheral surface of the plating base layer 6. FIG. 1(b) is an enlarged photograph of a cross section of the prototype signal transmission cable 1.

[0035] By increasing the surface roughness of the inner circumferential surface of the plating layer 41, an anchor effect makes it difficult for the plating layer 41 to peel off from the plating base layer 6. In this embodiment, the surface roughness of the inner circumferential surface of the plating layer 41 (i.e., the surface roughness of the outer circumferential surface of the plating base layer 6) is increased by intentionally roughening the plating base layer 6. To suppress peeling of the plating layer 41 from the plating base layer 6, it is preferable that the arithmetic mean roughness Ra of the inner surface of the plating layer 41 be 2 μm or more.

[0036] Furthermore, by reducing the surface roughness of the outer peripheral surface of plating layer 41, wear of plating layer 41 and outer shield layer 42 is suppressed when outer shield layer 42 rubs against plating layer 41, for example, when signal transmission cable 1 is bent, and damage (cracks) due to wear of plating layer 41 can be suppressed. The arithmetic mean roughness Ra of the outer peripheral surface of plating layer 41 is preferably smaller than the arithmetic mean roughness Ra of the inner surface of plating layer 41, and is preferably less than 2 μm.

[0037] In this way, by making the surface roughness of the outer surface of plating layer 41 smaller than the surface roughness of the inner surface of plating layer 41, it is possible to prevent cracks in plating layer 41 due to wear with outer shielding layer 42. Even if cracks do occur in plating layer 41, plating layer 41 is less likely to peel off from plating base layer 6, and the transmission characteristics are less likely to deteriorate when signal transmission cable 1 is bent.

[0038] Furthermore, in this embodiment, since the plating layer 41 is formed on the plating base layer 6 made of resin, even if the signal transmission cable 1 is bent appropriately according to the wiring layout, the plating base layer 6 can slide relative to the insulator 3 while maintaining a state of contact with the outer surface of the insulator 3 without any gap, and the distance between the conductor 2 and the plating layer 41 (the distance between the inner conductor and the outer conductor) can be kept approximately constant. For example, if a metal tape with a metal layer formed on one surface of the resin layer is vertically wrapped instead of the plating layer 41 and the plating base layer 6, the bending may cause wrinkles or folds in the metal tape, causing a gap between the insulator and the metal tape, causing a local change in the characteristic impedance, and the return loss due to the mismatch of the characteristic impedance may increase. In contrast, in the signal transmission cable 1 according to this embodiment, the plating base layer 6 flexibly deforms in response to bending, so that the distance between the conductor 2 and the plating layer 41 can be kept approximately constant, and the characteristic impedance can be kept approximately constant in the cable longitudinal direction of the signal transmission cable 1, thereby suppressing the return loss and obtaining good attenuation characteristics.

[0039] (Method of forming plating layer 41) 2 is a diagram illustrating the formation of the plating layer 41. When forming the plating layer 41, first, the first cable base 1a is sent out from the delivery drum 10a and subjected to a surface modification treatment. The first cable base 1a is formed by sequentially forming an insulator 3 and a plating base layer 6 around a conductor 2.

[0040] In the surface modification treatment, a blasting treatment device 11 is used to spray powder onto the outer peripheral surface of the plating base layer 6, thereby performing a blasting treatment to roughen the outer peripheral surface of the plating base layer 6 to a predetermined surface roughness, and then a corona discharge treatment is performed by a corona discharge device 12 to modify (make hydrophilic) the surface of the plating base layer 6.

[0041] As shown in Fig. 3(a) and (b), the blasting device 11 has a plurality of nozzles 11a-11d (four in this example), and is configured to spray powder from different directions in the circumferential direction of the first cable base 1a using the plurality of nozzles 11a-11d, thereby making the outer peripheral surface of the plating base layer 6 uniform in surface roughness. Here, the four nozzles 11a-11d are configured to spray powder onto the first cable base 1a from directions that differ by 90° in the circumferential direction, but the number and arrangement of the plurality of nozzles 11a-11d are not limited thereto as long as the blasting process can be performed so that the entire circumference of the first cable base 1a has a surface roughness to be described later and no gap occurs between the insulator 3 and the plating base layer 6. For example, when N nozzles are arranged in the circumferential direction of the first cable base 1a, the N nozzles are arranged so as to be shifted from each other by an equal angle (360° / N nozzles) along the circumferential direction. The amount of powder sprayed from each of the nozzles and the air pressure at which the powder is sprayed may be changed according to the shape of the first cable base 1a. For example, when the shape of the first cable base 1a is circular, the amount of powder sprayed from each of the nozzles and the air pressure may be the same. As a result, the surface of the plating base layer 6 of the first cable base 1a is roughened to a predetermined surface roughness Ra (for example, surface roughness Ra is 2.0 μm or more) without generating a gap between the insulator 3 and the plating base layer 6. Since the surface of the plating base layer 6 has the predetermined surface roughness Ra, the inner surface of the plating layer 41 formed through a pretreatment or the like described later can be made to have a surface roughness equal to the surface roughness of the surface of the plating base layer 6.

[0042] Here, dry ice is used as the powder used in the blasting device 11. However, the powder is not limited to this, and for example, powder made of metal particles, carbon particles, oxide particles, carbide particles, nitride particles, etc. may also be used.

[0043] Returning to FIG. 2, after the surface modification treatment, electroless plating pretreatment is performed. The electroless plating pretreatment is a pretreatment for film formation by electroless plating, and here, a Pd-Sn catalyst treatment for adsorbing palladium (Pd)-tin (Sn) colloid to the outer peripheral surface of the plating base layer 6, a Pd activation treatment for removing Sn from the adsorbed Pd-Sn colloid, and a Pd ion liquid immersion treatment for enhancing the amount of Pd adsorption are sequentially performed by the pretreatment device 13. Note that in this embodiment, Pd is adsorbed to the outer peripheral surface of the plating base layer 6 in the electroless plating pretreatment, but the metal to be adsorbed is not limited to Pd, and for example, Pt or Au can also be used.

[0044] Then, electroless plating is performed by the electroless plating device 14. In the electroless plating, a copper film is formed using the Pd adsorbed in the pretreatment as a seed. Then, electrolytic plating is performed by the electrolytic plating device 15. In the electrolytic plating, the copper film formed by the electroless plating is thickened. As a result, a plating layer 41 is formed. The second cable base body 1b on which the plating layer 41 is formed is wound up on the winding drum 10b. Then, an outer shield layer 42 and a sheath 5 are sequentially provided around the plating layer 41 to manufacture the signal transmission cable 1.

[0045] FIG. 4 shows the measurement results of the surface roughness of the outer peripheral surface of the plating base layer 6 after the blasting treatment. The surface roughness was measured using a laser microscope (VK8510, manufactured by Keyence Corporation) with a measurement area of ​​200 μm × 100 μm, and the arithmetic mean roughness Ra was measured at five points spaced 10 mm apart along the cable longitudinal direction, and the average value of the five measured values ​​was calculated. In FIG. 4, the average value is indicated by a black circle, and the variation of the measured values ​​at the five points is indicated by an I-shaped bar. As shown in FIG. 4, it can be seen that the arithmetic mean roughness Ra of the outer peripheral surface of the plating base layer 6 is 2 μm or more at any position in the circumferential direction, and the average value is 3 μm or more. Since the plating layer 41 is formed on the outer peripheral surface of the plating base layer 6, the surface roughness of the outer peripheral surface of the plating base layer 6 is equal to the surface roughness of the inner peripheral surface of the plating layer 41.

[0046] Depending on the conditions of the blasting treatment, a gap may be generated between the insulator 3 and the plating base layer 6. Therefore, it is preferable to perform the blasting treatment under conditions in which no gap is generated between the insulator 3 and the plating base layer 6. The inventors have found that when the linear speed (transport speed) of the first cable base 1a is 2 m / min, no gap is generated between the insulator 3 and the plating base layer 6 when the air pressure of the blasting treatment is 0.5 MPa, and a gap is generated between the insulator 3 and the plating base layer 6 when the air pressure is 0.6 MPa. Therefore, in this case, it is desirable to set the air pressure of the blasting treatment to less than 0.6 MPa, more preferably 0.5 MPa or less.

[0047] (Transmission characteristics of signal transmission cable 1) A sample of the signal transmission cable 1 in Fig. 1 without the outer shield layer 42 and sheath 5 was fabricated, and the transmission characteristics were measured. First, the transmission loss (insertion loss) S21 was measured. The S21 was measured when the signal transmission cable 1 was in a straight state (without bending) and when the signal transmission cable 1 was bent with a bending radius R of 1 mm. The measurement results are shown in Fig. 5(a) and (b), respectively.

[0048] As shown in Figures 5(a) and (b), when the prototype sample is bent with a bending radius R = 1 mm, S21 is almost the same as when it is straight. More specifically, the change in S21 when bent with a bending radius R = 1 mm compared to S21 when it is straight was small, less than 0.4 dB at a frequency of 28 GHz. Although not shown, S21 was measured by changing the bending radius R from 40 mm to 1 mm, but the change in S21 when bent at a bending radius other than R = 1 mm (change compared to S21 when it is straight) was small and was almost the same as the change in S21 when bent with a bending radius R = 1 mm.

[0049] Next, the change in characteristic impedance was measured for the straight state and when the bending radius R was changed from 40 mm to 1 mm. The results are summarized in Figure 6(a). As shown in Figure 6(a), when the bending radius R is 2.5 mm or less, it can be seen that the characteristic impedance changes slightly at the bent part.

[0050] Figure 6(b) shows the S21 at 28 GHz (S21@28 GHz) and the characteristic impedance of the bent portion. As shown in Figure 6(b), when the bending radius R is reduced to 2.5 mm or less, there is a slight change in S21 and the characteristic impedance, but the change is small. Also, when the bending radius R is 5 mm or more, there is almost no change in S21 and the characteristic impedance compared to the straight state. From the above, it was confirmed that a signal transmission cable 1 has been created whose transmission characteristics are less likely to deteriorate when bent.

[0051] (Functions and Effects of the Embodiments) As described above, the signal transmission cable 1 of this embodiment includes a plating base layer 6 provided between the insulator 3 and the shielding layer 4 so as to cover the periphery of the insulator 3, and the shielding layer 4 has a plating layer 41 formed so as to contact the outer peripheral surface of the plating base layer 6 and to cover the plating base layer 6, and the surface roughness of the outer peripheral surface of the plating layer 41 is smaller than the surface roughness of the inner peripheral surface of the plating layer 41.

[0052] This configuration prevents the shield layer from wrinkling when bent, as occurs when a tape member is used for the shield layer as in the conventional technology, and the transmission characteristics are less likely to deteriorate when bent. Furthermore, even when the outer shield layer 42 is provided, it is possible to prevent cracks in the plating layer 41 caused by wear with the outer shield layer 42, and even if cracks occur in the plating layer 41, the plating layer 41 is less likely to peel off from the plating base layer 6. As a result, a signal transmission cable 1 whose transmission characteristics are less likely to deteriorate when bent can be realized.

[0053] (Summary of the embodiment) Next, the technical ideas grasped from the above-described embodiment will be described by using the reference numerals and the like in the embodiment. However, the reference numerals and the like in the following description do not limit the components in the claims to the members and the like specifically shown in the embodiment.

[0054] [1] A signal transmission cable (1) comprising a conductor (2), an insulator (3) covering the conductor (2), a shielding layer (4) covering the insulator (3), and a sheath (5) covering the shielding layer (4), wherein a plating base layer (6) is provided between the insulator (3) and the shielding layer (4) so ​​as to cover the insulator (3), the shielding layer (4) has a plating layer (41) formed so as to be in contact with the outer peripheral surface of the plating base layer (6) and to cover the plating base layer (6), and the surface roughness of the outer peripheral surface of the plating layer (41) is smaller than the surface roughness of the inner peripheral surface of the plating layer (41).

[0055] [2] The signal transmission cable (1) according to [1], wherein the thickness of the plating underlayer (6) is thinner than the thickness of the insulator (3).

[0056] [3] The signal transmission cable (1) according to [1] or [2], wherein the arithmetic mean roughness Ra of the inner surface of the plating layer (41) is 2 μm or more.

[0057] [4] The signal transmission cable (1) according to any one of [1] to [3], wherein the insulator (3) is made of a fluororesin and the plating underlayer (6) is made of polyethylene or polypropylene.

[0058] Although the embodiment of the present invention has been described above, the invention according to the claims is not limited to the embodiment described above. It should be noted that not all of the combinations of features described in the embodiment are essential to the means for solving the problems of the invention. The present invention can be modified appropriately without departing from the spirit of the invention. [Explanation of symbols]

[0059] 1...Signal transmission cable 2...Conductor 3...Insulator 4…Shield layer 41…Plating layer 42…Outer shield layer 5…Sheath 6…Plating base layer

Claims

1. A conductor; An insulator that covers the conductor; a shield layer covering the insulator; a sheath covering the shielding layer, a plating base layer is provided between the insulator and the shield layer so as to cover the periphery of the insulator and to be movable relative to the insulator in the cable longitudinal direction; The shield layer has a plating layer formed so as to contact an outer peripheral surface of the plating base layer and to cover the plating base layer. Signal transmission cable.

2. The conductor is a compressed stranded conductor obtained by twisting together a plurality of wires and compressing the conductor so that the cross-sectional shape perpendicular to the cable longitudinal direction is circular.

2. The signal transmission cable according to claim 1.

3. the surface roughness of the outer circumferential surface of the plating layer is smaller than the surface roughness of the inner circumferential surface of the plating layer; The arithmetic mean roughness Ra of the inner circumferential surface of the plating layer is 2 μm or more; The arithmetic average roughness Ra of the outer peripheral surface of the plating layer is less than 2 μm; 3. A signal transmission cable according to claim 1 or 2.

4. The plating underlayer is made of polyethylene or polypropylene.

4. A signal transmission cable according to claim 1.

5. The insulator is made of a fluororesin.

5. A signal transmission cable according to claim 1.

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