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 cable for signal transmission.

Background Art

[0002] A cable for signal transmission for transmitting a high-frequency signal is used for internal wiring of imaging devices used for automatic driving or the like, electronic devices such as smartphones and tablet terminals, or wiring of machine tools such as industrial robots. As this signal transmission cable, for example, a coaxial cable is used.

[0003] As a conventional coaxial cable, there is known one in which a tape member such as a copper tape provided with a copper foil on a resin layer is spirally wound around an insulator to form a shield layer (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the above-described conventional coaxial cable, 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 a case, the insertion loss (S21) and the change in characteristic impedance between the bent portion and other portions (straight portions that are not bent) may increase, and there is a risk that the transmission characteristics may deteriorate.

[0006] Therefore, an object of the present invention is to provide a cable for signal transmission whose transmission characteristics are less likely to deteriorate when bent.

Means for Solving the Problems

[0007] The present invention aims to solve the above problems and provides a signal transmission cable comprising a conductor, an insulator covering the conductor, a shield layer covering the insulator, and a sheath covering the shield layer, wherein a plating underlayer is provided between the insulator and the shield layer, covering the periphery of the insulator and movable relative to the insulator in the longitudinal direction of the cable, and the shield layer has a plating layer formed to contact the outer surface of the plating underlayer and to cover the plating underlayer. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a signal transmission cable that does not easily degrade in transmission characteristics when bent. [Brief explanation of the drawing]

[0009] [Figure 1] This figure shows a signal transmission cable according to one embodiment of the present invention, where (a) is a cross-sectional view showing a cross section perpendicular to the longitudinal direction of the cable, and (b) is a photograph of a magnified view of the cross section. [Figure 2] This is a diagram illustrating the formation of the plating layer. [Figure 3] This diagram shows a blast processing device; (a) is a perspective view, and (b) is a plan view taken from the front in the direction of transport. [Figure 4] This graph shows the measurement results of the surface roughness of the outer surface of the plating underlayer after blast treatment. [Figure 5] This graph shows the measurement results for the insertion loss S21. (a) shows the measurement results for the signal transmission cable in a straight state without bending, and (b) shows the measurement results for the signal transmission cable bent with a bending radius of 1 mm. [Figure 6] (a) is a graph showing the measurement results of the change in characteristic impedance due to bending, and (a) is a graph showing the insertion loss S21 and the characteristic impedance of the bent portion with respect to the bending radius. [Modes for carrying out the invention]

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

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

[0012] As shown in Figure 1(a), the signal transmission cable 1 comprises a conductor 2 located in the center of the cable, an insulator 3 surrounding the conductor 2, a shield layer 4 surrounding the insulator 3, and a sheath 5 surrounding the shield layer 4. In other words, the signal transmission cable 1 is a coaxial cable comprising a conductor 2 which is the inner conductor and a shield layer 4 which is the outer conductor.

[0013] The signal transmission cable 1 is used, for example, as a fixed cable connecting a robot and control equipment in a factory, and its length is, for example, about 25m to 100m. Also, when the signal transmission cable 1 is wired inside electronic equipment, its length is, for example, about 5mm to 200mm. Note that "covering" includes cases where other layers are placed in between. For example, other layers may be placed 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 this embodiment, the conductor 2 consists of a compressed stranded conductor formed by twisting together a plurality of strands 2a and compressing them so that the cross-sectional shape perpendicular to the longitudinal direction of the cable is a predetermined shape such as a circle. In this embodiment, a stranded conductor formed by concentrically twisting seven strands 2a is compressed by passing it through a die having a smaller diameter than the stranded conductor and a circular outlet, thereby forming a conductor 2 with a circular cross-sectional shape as shown in Figure 1(a). The strand 2a located in the center has a roughly hexagonal shape in cross-section, and the six strands 2a located around it have a roughly fan shape in cross-section. Furthermore, it is preferable that adjacent strands 2a are in contact (surface contact) so that there are no gaps between each strand 2a. In addition, it is preferable that the outer surface of the compressed stranded conductor be a smooth surface in the circumferential direction of the cable and in the longitudinal direction of the cable. In the signal transmission cable 1 according to this embodiment shown in Figure 1, the conductor 2 is made of a compressed stranded conductor with a circular cross-section. However, the conductor 2 may also be made of a compressed stranded conductor that has been compressed to a shape other than a circle (for example, a polygonal shape such as a square). Because the conductor 2 is a compressed stranded conductor with a circular cross-section, the signal transmission cable 1 can be easily bent in any direction, making it easy to route by bending.

[0015] While conventional stranded conductors that have not undergone compression processing are more flexible and easier to bend than single-strand conductors, they have more gaps between the strands, resulting in higher conductor resistance and lower conductivity compared to single-strand conductors of the same outer diameter. In this embodiment, by using a compressed stranded conductor as conductor 2, the strands 2a are tightly packed together, eliminating the gaps between them. Therefore, conductor 2 using a compressed stranded conductor can have lower conductor resistance compared to a conventional stranded conductor of the same outer diameter. As a result, by using a compressed stranded conductor as conductor 2, conductivity is improved and good attenuation characteristics are obtained. Furthermore, the current (also simply called current) transmitting high-frequency signals mainly passes through the outer circumference of conductor 2 due to the skin effect. When conductor 2 is constructed using a stranded conductor made by twisting together multiple strands 2a, the curvature of the strands is smaller than that of a single-strand conductor with the same outer diameter as the stranded conductor. Therefore, the cross-sectional area of ​​the part through which the current passes is smaller than that of a single-strand conductor with the same outer diameter as the stranded conductor. In contrast, in this embodiment, by using a compressed stranded conductor as the conductor 2, the individual strands 2a are in close contact with each other, and the outer circumference of the conductor 2 becomes concentric, similar to that of a single-wire conductor. As a result, in a conductor 2 made of a compressed stranded conductor, the cross-sectional area of ​​the part through which the current passes is larger compared to a stranded conductor having the same outer diameter, thus providing good attenuation.

[0016] To obtain good attenuation characteristics, it is desirable that the conductivity of the compressed stranded conductor used as conductor 2 be 99% IACS or higher. In this embodiment, in order to achieve high conductivity, soft copper wire made of silver-plated pure copper was used as the strand 2a of conductor 2. Alternatively, soft copper wire without silver plating may be used as the strand 2a. Furthermore, although compression through the die imparts strain to the strand 2a and reduces its conductivity, the strain can be removed by subsequent heat treatment (annealing) to achieve conductivity of 99% IACS or higher.

[0017] (Insulator 3) As the insulator 3, in order to improve the transmission characteristics of high-frequency signals (more specifically, for example, to make it less likely to attenuate when transmitting high-frequency signals in the band of 10 MHz to 50 GHz), it is desirable to use a material with as low a dielectric constant as possible. In this embodiment, an insulator 3 made of a fluororesin is used. As the fluororesin used for the insulator 3, FEP (tetrafluoroethylene - hexafluoropropylene copolymer), PFA (tetrafluoroethylene - perfluoroalkyl vinyl ether copolymer), etc. may be used. The thickness of the insulator 3 is preferably 0.2 mm or more and 2.0 mm or less.

[0018] (Sheath 5) Around the insulator 3, an electroplating base layer 6, a shield layer 4, and a sheath 5 are sequentially provided. Details of the electroplating base layer 6 and the shield layer 4 will be described later.

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

[0020] The sheath 5 is formed by extrusion molding. However, if solid molding is performed, the resin constituting the sheath 5 may enter between the metal strands of the outer shield layer 42 described later, and the signal transmission cable 1 may become hard and difficult to bend. Therefore, in this embodiment, the sheath 5 is formed by tube extrusion. As a result, the resin constituting the sheath 5 is suppressed from entering between the strands of the outer shield layer 42, and the sheath 5 and the outer shield layer 42 are separated. That is, in this embodiment, the sheath 5 and the outer shield layer 42 are not adhered, and the outer shield layer 42 can move relatively freely within the sheath 5. Thereby, the signal transmission cable 1 becomes easier to bend.

[0021] (Electroplating base layer 6) In the signal transmission cable 1 according to this embodiment, a plating underlayer 6 is provided between the insulator 3 and the shield layer 4, covering the periphery of the insulator 3. The plating underlayer 6 is a layer that serves as a base when forming the plating layer 41, which will be described later, and in particular, it is a layer that gives the inner surface of the plating layer 41 a predetermined surface roughness Ra. In this embodiment, a fluororesin is used as the insulator 3, and since it is difficult to directly form the plating layer 41 on the fluororesin, a plating underlayer 6 is provided to cover the insulator 3 made of fluororesin, which serves as a base for the plating layer 41.

[0022] The plating underlayer 6 is preferably made of an insulating resin on which the plating layer 41 can be formed on its outer surface. In this embodiment, the plating underlayer 6 made of PE (polyethylene) was used, but the plating underlayer 6 made of PP (polypropylene) may also be used. The plating underlayer 6 is preferably made thin in order to minimize the impact on transmission characteristics, and the thickness of the plating underlayer 6 is preferably thinner than the thickness of the insulator 3. More specifically, the thickness of the plating underlayer 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. If the thickness of the plating underlayer 6 is 0.10 mm or more, the mechanical strength of the plating underlayer 6 is increased, making it easier to suppress the fracture of the plating underlayer 6 due to bending. Furthermore, if the thickness of the plating underlayer 6 is 0.20 mm or less, the stress on the plating layer 41 when the signal transmission cable 1 is bent (the stress on the plating layer 41 due to the plating underlayer 6 bending in accordance with the bending of the signal transmission cable 1) is reduced, making it easier to prevent cracks from forming in the plating layer 41.

[0023] If there is a gap between the plating underlayer 6 and the insulator 3, it will adversely affect the transmission characteristics. Therefore, it is preferable that the plating underlayer 6 is provided in contact with the outer surface of the insulator 3 without any gaps. The fact that the plating underlayer 6 is in gap-free contact with the outer surface of the insulator 3 can be observed, for example, using an optical microscope or an electron microscope.

[0024] Furthermore, it is more desirable that the plating underlayer 6 be provided such that it can move relative to the bending of the insulator 3 in the longitudinal direction of the cable when the signal transmission cable 1 is bent (it can slide relative to the insulator 3 in the longitudinal direction of the cable). This makes it possible to suppress the occurrence of cracks in the plating layer 41 by causing the plating underlayer 6 to bend while moving relative to the bending of the insulator 3 in the longitudinal direction of the cable when the signal transmission cable 1 is bent. Here, "crack" 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 that contacts the insulator 3).

[0025] Furthermore, if cracks occur in the plating layer 41, a phenomenon called co-cracking may occur. However, in this embodiment, since the plating layer 41 is formed via a plating underlayer 6, which is a separate component from the insulator 3, even if cracks occur in the plating layer 41, there is no risk of co-cracking occurring in the insulator 3, and it is possible to suppress defects such as insulation failure.

[0026] Furthermore, the plating underlayer 6 is not bonded to the insulator 3, and is provided in a state that allows it to be peeled off from the insulator 3. This makes it possible to easily peel the plating layer 41 off the insulator 3 and expose the insulator 3 when terminating the signal transmission cable 1, thereby improving the workability of the terminating process.

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

[0028] (Shield layer 4) The shield layer 4 includes a plating layer (inner shield layer) 41 formed to cover the plating underlayer 6, and an outer shield layer 42 provided to cover the plating layer 41. The outer shield layer 42 is optional.

[0029] The outer shield layer 42 is made of metal wires, which are braided or wound horizontally. In this embodiment, the outer shield layer 42 is made of a braided shield made of braided metal wires. Examples of metal wires include soft copper wire and hard copper wire made of copper or a copper alloy. Alternatively, metal wires made of aluminum or an aluminum alloy may be used. The outer surface of the metal wires may be plated. In this embodiment, the outer shield layer 42 is made of one layer, but the outer shield layer 42 may be made of multiple layers. Furthermore, the metal wires constituting the outer shield layer 42 may have lubricating properties on their surface. For example, lubrication may be provided by applying a lubricant such as talc powder to the surface of the metal wires.

[0030] By providing the outer shielding layer 42, even if the plating layer 41 is damaged due to some unexpected damage, the shielding layer 4 will not be electrically insulated. Furthermore, by providing the outer shielding layer 42, even if the plating layer 41 is thin, the thickness of the outer shielding layer 42 can further reduce the loss of low-frequency signals.

[0031] The plating layer 41, together with the outer shield layer 42, constitutes the outer conductor and is formed to be in direct contact with the outer surface of the plating underlayer 6. As described above, the outer shield layer 42 is formed by braiding or winding metal wires, but with only the outer shield layer 42, the internal signal may be radiated to the outside through the gaps between the metal wires, potentially resulting in significant attenuation. By providing the plating layer 41, the gaps between the metal wires in the outer shield layer 42 are filled, further reducing the attenuation. The plating layer 41 and the outer shield layer 42 are in contact and electrically connected.

[0032] The plating layer 41 is preferably made of a metal with an electrical conductivity of 99% or more (99% IACS or more), for example, a metal made of copper or silver can be used.

[0033] The thickness of the plating layer 41 is preferably between 2 μm and 5 μm. If the thickness of the plating layer 41 is 2 μm or more, cracks are less likely to occur in the plating layer 41 even when it comes into contact with the outer shield layer 42 when bending is applied. Also, if the thickness of the plating layer 41 is 5 μm or less, it is possible to suppress the hardening of the plating layer 41, which would make the signal transmission cable 1 less flexible.

[0034] As shown in Figure 1(b), in the signal transmission cable 1 according to this embodiment, the surface roughness of the outer circumferential surface of the plating layer 41 is smaller than the surface roughness of the inner circumferential surface of the plating layer 41. The outer circumferential surface of the plating layer 41 is the surface located radially outward of the plating layer 41 and is the surface that contacts the outer shield layer 42. The inner circumferential surface of the plating layer 41 is the surface located radially inward of the plating layer 41 and is the surface that contacts the plating underlayer 6. As shown in Figure 1(b), since the plating layer 41 is in gapless contact with the plating underlayer 6, the surface roughness of the inner circumferential surface of the plating layer 41 is equivalent to the surface roughness of the outer circumferential surface of the plating underlayer 6. Figure 1(b) is an enlarged photograph of the 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, the plating layer 41 becomes less likely to peel off from the plating underlayer 6 due to the anchoring effect. 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 underlayer 6) is increased by intentionally roughening the plating underlayer 6. To suppress the peeling of the plating layer 41 from the plating underlayer 6, the arithmetic mean roughness Ra of the inner surface of the plating layer 41 should be 2 μm or more.

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

[0037] In this way, by making the surface roughness of the outer surface of the plating layer 41 smaller than the surface roughness of the inner surface of the plating layer 41, it is possible to suppress cracking of the plating layer 41 due to abrasion with the outer shield layer 42. Even if cracking occurs in the plating layer 41, the plating layer 41 will be less likely to peel off from the plating underlayer 6, and the transmission characteristics will not deteriorate as much when the signal transmission cable 1 is bent.

[0038] Furthermore, in this embodiment, since the plating layer 41 is formed on the plating underlayer 6 made of resin, even when the signal transmission cable 1 is bent appropriately according to the routing layout, the plating underlayer 6 can slide against the insulator 3 while maintaining gap-free contact with the outer surface of the insulator 3, making it possible to keep the distance between the conductor 2 and the plating layer 41 (the distance between the inner conductor and the outer conductor) substantially constant. For example, if a metal tape with a metal layer formed on one side of the resin layer is wound longitudinally instead of the plating layer 41 and the plating underlayer 6, bending can cause wrinkles and folds in the metal tape, creating gaps between the insulator and the metal tape, which can locally change the characteristic impedance and increase the return loss due to the mismatch in characteristic impedance. In contrast, in the signal transmission cable 1 according to this embodiment, since the plating underlayer 6 deforms flexibly in response to bending, the distance between the conductor 2 and the plating layer 41 can be kept substantially constant, making it possible to keep the characteristic impedance substantially constant in the longitudinal direction of the signal transmission cable 1, thereby suppressing return loss and obtaining good attenuation characteristics.

[0039] (Method for forming the plating layer 41) Figure 2 illustrates the formation of the plating layer 41. When forming the plating layer 41, first, the first cable base 1a is fed from the feed drum 10a and subjected to surface modification treatment. The first cable base 1a has an insulator 3 and a plating underlayer 6 sequentially formed around the conductor 2.

[0040] In the surface modification treatment, a blast treatment device 11 is used to spray powder onto the outer surface of the plating underlayer 6, roughening the outer surface of the plating underlayer 6 to a predetermined surface roughness. Subsequently, a corona discharge treatment is performed using a corona discharge device 12 to modify (hydrophilize) the surface of the plating underlayer 6.

[0041] As shown in Figures 3(a) and (b), the blasting apparatus 11 has a plurality of (in this case, four) nozzles 11a to 11d, and is configured to make the outer surface of the plating underlayer 6 uniform in surface roughness by using these plurality of nozzles 11a to 11d to spray powder from different directions in the circumferential direction of the first cable base 1a. Here, the four nozzles 11a to 11d are configured to spray powder onto the first cable base 1a from directions that are 90° apart in the circumferential direction, but the number and arrangement of the plurality of nozzles 11a to 11d are not limited to this, as long as the blasting treatment can be performed so that the surface roughness described later is achieved over the entire circumference of the first cable base 1a and no gaps are created between the insulator 3 and the plating underlayer 6. For example, when arranging N nozzles in the circumferential direction of the first cable base 1a, each of the N nozzles is arranged to be offset by an equal angle (360° / N nozzles) along the circumferential direction. Furthermore, the amount of powder blown out from each of the multiple nozzles and the air pressure used for blowing should be changed according to the shape of the first cable base 1a. For example, if the shape of the first cable base 1a is circular, the amount of powder blown out from each of the multiple nozzles and the air pressure should be the same. As a result, the surface of the plating base layer 6 is roughened to a predetermined surface roughness Ra (for example, a surface roughness Ra of 2.0 μm or more) without any gap being formed between the insulator 3 and the plating base layer 6. By having the surface of the plating base layer 6 have a predetermined surface roughness Ra, the inner circumferential surface of the plating layer 41 formed after the pretreatment described later can be made to have the same surface roughness as the surface of the plating base layer 6.

[0042] In this example, dry ice was used as the powder for the blasting apparatus 11. However, the powder is not limited to dry ice; for example, powders consisting of metal particles, carbon particles, oxide particles, carbide particles, nitride particles, etc., can also be used.

[0043] Returning to Figure 2, after the surface modification treatment, electroless plating pretreatment is performed. Electroless plating pretreatment is a pretreatment for film formation by electroless plating. Here, the pretreatment device 13 sequentially performs the following steps: Pd-Sn catalyst treatment to adsorb palladium (Pd)-tin (Sn) colloid onto the outer surface of the plating substrate 6; Pd activation treatment to remove Sn from the adsorbed Pd-Sn colloid; and Pd ion solution immersion treatment to enhance the amount of adsorbed Pd. In this embodiment, Pd was adsorbed onto the outer surface of the plating substrate 6 during the electroless plating pretreatment, but the metal to be adsorbed is not limited to Pd; for example, Pt or Au can also be used.

[0044] Subsequently, electroless plating is performed using the electroless plating apparatus 14. In electroless plating, a copper film is formed using Pd adsorbed by the pretreatment as a seed. Then, electrolytic plating is performed using the electrolytic plating apparatus 15. In electrolytic plating, the copper film formed by electroless plating is thickened. This forms the plating layer 41. The second cable base 1b with the plating layer 41 is wound onto the winding drum 10b. Then, the signal transmission cable 1 is manufactured by sequentially applying the outer shield layer 42 and the sheath 5 around the plating layer 41.

[0045] Figure 4 shows the measurement results of the surface roughness of the outer surface of the plating underlayer 6 after blasting. Surface roughness was measured using a laser microscope (VK8510, manufactured by Keyence Corporation) with a measurement area of ​​200 μm × 100 μm. The arithmetic mean roughness Ra was measured at five locations at 10 mm intervals along the longitudinal direction of the cable, and the average of the five measurements was calculated. In Figure 4, the average value is shown by ●, and the variation of the five measurements is shown by I-shaped bars. As shown in Figure 4, the arithmetic mean roughness Ra of the outer surface of the plating underlayer 6 is 2 μm or more at all positions in the circumferential direction, and its average value is 3 μm or more. Since the plating layer 41 is formed on the outer surface of the plating underlayer 6, the surface roughness of the outer surface of the plating underlayer 6 becomes equal to the surface roughness of the inner surface of the plating layer 41.

[0046] Furthermore, depending on the conditions during blasting, a gap may form between the insulator 3 and the plating underlayer 6. Therefore, it is preferable to perform the blasting under conditions that prevent a gap from forming between the insulator 3 and the plating underlayer 6. The inventors' investigations revealed that when the linear velocity (transport speed) of the first cable base 1a was 2 m / min, no gap formed between the insulator 3 and the plating underlayer 6 when the air pressure for blasting was 0.5 MPa, but a gap formed between the insulator 3 and the plating underlayer 6 when the air pressure was 0.6 MPa. Therefore, in this case, it is desirable to set the air pressure for blasting 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 shown in Figure 1 was fabricated without the outer shield layer 42 and sheath 5, and its transmission characteristics were measured. First, the transmission loss (insertion loss) S21 was measured. S21 was measured for the signal transmission cable 1 in a straight state (unbent) and for the signal transmission cable 1 bent with a bending radius R=1mm. The measurement results are shown in Figures 5(a) and (b), respectively.

[0048] As shown in Figures 5(a) and (b), when the prototype sample described above was bent at a bending radius R=1mm, S21 was found to be almost the same as that of the straight sample. More specifically, the change in S21 when bent at a bending radius R=1mm compared to S21 in the straight sample was negligible, less than 0.4dB at a frequency of 28GHz. Although not shown in the figures, S21 was measured while varying the bending radius R from 40mm to 1mm, but the change in S21 when bent at bending radii other than R=1mm (the change compared to S21 in the straight sample) was negligible and almost the same as the change in S21 when bent at a bending radius R=1mm.

[0049] Next, the change in characteristic impedance was measured for the straight state and for each case where 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), it can be seen that when the bending radius R is 2.5 mm or less, the characteristic impedance changes slightly at the bent portion.

[0050] Figure 6(b) shows the S21 (S21@28GHz) at 28GHz and the characteristic impedance of the bent section. As shown in Figure 6(b), when the bending radius R is small, such as 2.5mm or less, there is a slight change in S21 and characteristic impedance, but the change is small. Furthermore, when the bending radius R is 5mm or more, S21 and characteristic impedance remain almost unchanged compared to the straight state. From the above, it has been confirmed that a signal transmission cable 1 that does not easily degrade in transmission characteristics when bent has been realized.

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

[0052] This configuration suppresses the occurrence of wrinkles in the shield layer when bending, as is the case with conventional technology where tape material is used for the shield layer, thus reducing the degradation of transmission characteristics when bent. Furthermore, even when an outer shield layer 42 is provided, it is possible to suppress cracking of the plating layer 41 due to abrasion with the outer shield layer 42, and even if cracking occurs in the plating layer 41, the plating layer 41 is less likely to peel off from the plating underlayer 6. As a result, a signal transmission cable 1 can be realized in which the transmission characteristics do not degrade when bent.

[0053] (Summary of the embodiments) Next, the technical concept understood from the embodiments described above will be described using the reference numerals and other symbols from the embodiments. However, the reference numerals and other symbols in the following description are not limited to the components in the claims that are specifically shown in the embodiments.

[0054] [1] A signal transmission cable (1) comprising a conductor (2), an insulator (3) covering the conductor (2), a shield layer (4) covering the insulator (3), and a sheath (5) covering the shield layer (4), wherein a plating underlayer (6) is provided between the insulator (3) and the shield layer (4) so ​​as to cover the periphery of the insulator (3), and the shield layer (4) has a plating layer (41) formed so as to contact the outer surface of the plating underlayer (6) and to cover the plating underlayer (6), and the surface roughness of the outer surface of the plating layer (41) is smaller than the surface roughness of the inner 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] A signal transmission cable (1) according to any one of [1] to [3], wherein the insulator (3) is made of fluororesin and the plating underlayer (6) is made of polyethylene or polypropylene.

[0058] Although embodiments of the present invention have been described above, the embodiments described above do not limit the invention as defined in the claims. Furthermore, it should be noted that not all combinations of features described in the embodiments are necessarily essential for solving the problem of the invention. In addition, the present invention can be implemented with appropriate modifications without departing from its spirit. [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 underlayer

Claims

1. A conductor and An insulator covering the conductor, A shielding layer covering the periphery of the insulator, A signal transmission cable comprising a sheath covering the periphery of the shield layer, Between the insulator and the shielding layer, there is a plating underlayer provided so as to cover the periphery of the insulator and so as to be movable relative to the insulator in the longitudinal direction of the cable, The shield layer has a plating layer formed so as to contact the outer surface of the plating underlayer and to cover the plating underlayer. Cable for signal transmission.

2. The conductor consists of a compressed stranded conductor formed by twisting together multiple strands and compressing them so that the cross-sectional shape perpendicular to the longitudinal direction of the cable is circular. 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 surface of the plating layer is 2 μm or more. The arithmetic mean roughness Ra of the outer surface of the aforementioned plating layer is less than 2 μm. A signal transmission cable according to claim 1 or 2.

4. The aforementioned plating underlayer is made of polyethylene or polypropylene. A signal transmission cable according to any one of claims 1 to 3.

5. The insulator is made of fluororesin. A signal transmission cable according to any one of claims 1 to 4.

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