Patent Publication Number: US-11037702-B2

Title: High frequency cable comprising a center conductor having a first wire stranded by plural second wires that provide corners free of gaps

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on Japanese patent application No. 2018-122821 filed on Jun. 28, 2018, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high frequency cable. 
     2. Description of the Related Art 
     As a cable for high frequency signal transmission, there is, e.g., a flexible coaxial cable with a center conductor configured as a stranded member formed by stranding a plurality of conductor wires together and compressed so that voids between the center conductor wire and the surrounding conductor wires are substantially filled with a material for the conductor wires (See JP-561-45512 A). 
     [Patent Document 1] 
     SUMMARY OF THE INVENTION 
     In the cable described in JP-561-45512 A, however, gap formation (hereinafter, also referred to as “depression formation”) occurs on an outer peripheral surface of the center conductor between the adjacent stranded wires, which may lead to a degradation in electrical properties of the cable. In the high frequency cable used in high frequency signal transmission, this electrical property degradation resulting from the depression formation then becomes much more pronounced. 
     Accordingly, it is an object of the present invention to provide a high frequency cable with improved electrical property degradation in high frequency signal transmission. 
     For the purpose of solving the above-described problem, the present invention provides high frequency cables defined by [1], [2], [3], and [4] below. 
     [1] A high frequency cable, including a center conductor comprising one first wire, which is located at the center of the center conductor, and a plurality of second wires, which are located around that one first wire, the one first wire and the plurality of second wires being stranded together, in which respective outer peripheral surfaces of the plurality of second wires constitute a substantially continuous circular peripheral surface as an outer peripheral surface of the center conductor. 
     [2] The high frequency cable as defined in [1] above, wherein the one first wire has a substantially hexagonal shape cross section, in which the plurality of second wires are configured as six second wires each having a substantially fan-shaped cross section surrounded by one circular arc, one base, and two lateral sides joining the one circular arc and the one base at their respective ends thereof, in which, in a transverse cross section view thereof, the bases of the substantially fan-shaped cross sections of the six second wires are contiguous with respective sides, of the substantially hexagonal shape cross section of the one first wire, while the lateral sides of the substantially fan-shaped cross sections of the six second wires are contiguous with respective lateral sides of the substantially fan-shaped cross sections of adjacent second wires, with the circular arcs of the substantially fan-shaped cross sections of the six second wires constituting the substantially continuous circular peripheral surface as the outer peripheral surface of the center conductor. 
     [3] The high frequency cable as defined in [1] or [2] above, wherein the center conductor is elongated by of 10% or more. 
     [4] The high frequency cable as defined in any one of [1] to [3] above, wherein, of the plurality of second wires, the adjacent second wires in a circumferential direction of the center conductor are separately in contact with each other. 
     Points of the Invention 
     According to the present invention, it is possible to provide the high frequency cables with improved electrical property degradation in high frequency signal transmission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing one example of a structure of a high frequency cable according to an embodiment of the present invention; 
         FIG. 2  is a table showing one example of test results on electrical properties for an example of the present invention and a conventional example; 
         FIG. 3  is a diagram showing the results on attenuation shown in  FIG. 2 ; and 
         FIG. 4  is a table showing one example of test results on durability against external forces for the example of the present invention and the conventional example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiment 
       FIG. 1  is a transverse cross section view showing one example of a structure of a high frequency cable according to an embodiment of the present invention. As one example of the high frequency cable, a coaxial cable with each constituent layer thereof in a coaxial arrangement will be described below. As shown in  FIG. 1 , the high frequency cable is configured to include a center conductor  11 , an insulating layer  12 , which is provided around an outer periphery of the center conductor  11 , outer conductors  13 , which are provided around an outer periphery of the insulating layer  12 , and an outermost sheath layer  14 , which is provided around an outer side of the outer conductors  13 . 
     (Center Conductor  11 ) 
     The center conductor  11  is configured to include a stranded wire formed by stranding a plurality of wires  110  together. The number of wires  110  to be stranded together is not particularly limited, but is preferably seven, or nineteen, or thirty seven, for example. Further, the plurality of wires  110  are more preferably configured to be concentrically stranded together with one of the plurality of wires  110  being located at the center of the center conductor  11 , and the other wires being arranged in circumferentially equally divided positions, respectively, of the center conductor  11 . Note that in  FIG. 1 , there is shown the configuration example with seven wires  110  being stranded together. 
     For the wires  110 , e.g. a soft copper wire may be used. The soft copper wire may be subjected to a plating such as silver (Ag) plating or the like. Specifically, for the wires  110 , e.g., a copper wire such as a HiFC™ (registered trademark) conductor or the like may be used. 
     The wires  110  are preferably configured to be small in diameter, and specifically, the wires  110  are preferably configured to have a diameter of 0.065 to 0.070 mm. Further, the stranded wire of the center conductor  11  is configured to be able to have a pitch length of e.g. about 8.7±0.5 mm. Furthermore, the wires  110  are configured to elongate by 10% or more in a longitudinal direction of the wires  110 . 
     The center conductor  11  is configured to include one wire  110  (hereinafter, also referred to as “core  110 A”), which is located at the center of the center conductor  11  and a plurality of other wires  110  (hereinafter, also referred to as “surrounding wires  110 B”), which are located around that core  110 A. Further, respective outer peripheral surfaces  110 Ba of the plurality of surrounding wires  110 B on a side of the insulative layer  12  constitute an outer peripheral surface  11   a  of the center conductor  11 . Note that in  FIG. 1 , there is shown the configuration example with the number of surrounding wires  110 B being set at six, as one example. Here, the core  110 A is shown as one example of a first wire. Further, the surrounding wires  110 B are shown as one example of second wires. 
     The core  110 A is configured to have a substantially hexagonal shape cross section. That is, the core  110 A is configured to have a substantially hexagonal column shape. 
     Further, the surrounding wires  110 B are each configured to have a substantially fan-shaped cross section surrounded by one circular arc, one base, which is located in a side of a core  110 A of the center conductor  11  relative to that one circular are and opposite that one circular arc, and two lateral sides, which are joining the one circular arc and the one base at respective ends thereof. That is, the surrounding wires  110 B are each configured to have a columnar shape surrounded by one outer peripheral surface  110 Ba, which is located in a side of an insulating layer  12  of the center conductor  11  and formed of a circular peripheral shape curved surface, one bottom surface  110 Bb, which is located in a side of a core  110 A of the center conductor  11  and formed of a planar surface, and two lateral surfaces  110 Bc, which are joining the one outer peripheral surface  110 Ba and the one bottom surface  110 Bb at respective ends thereof in a peripheral direction of the center conductor  11 . 
     The six surrounding wires  110 B are each being provided in such a manner as to be in surface contact with the core  110 A. Specifically, the respective bottom surfaces  110 Bb of the six surrounding wires  110 B are provided in such a manner as to be in surface contact with the side surfaces  110 Aa, respectively, of the substantially hexagonal column shape core  110 A. In other words, in the transverse cross section view shown in  FIG. 1 , the respective constituent bases of the substantially fan-shaped cross sections of the six surrounding wires  110 B are provided in such a manner as to be contiguous with the constituent sides, respectively, of the substantially hexagonal shape cross section of the core  110 A. 
     The adjacent surrounding wires  110 B in a circumferential direction of the center conductor  11  are provided in such a manner as to be separately in surface contact with each other. Here, the term “separately” means that the adjacent surrounding wires  110 B in the circumferential direction of the center conductor  11  are not being joined to each other. 
     Specifically, the respective lateral surfaces  110 Bc of adjacent ones of the surrounding wires  110 B in the circumferential direction of the center conductor  11  are provided in such a manner as to be in surface contact with each other. In other words, in the transverse cross section view shown in  FIG. 1 , the constituent lateral sides of the substantially fan-shaped cross sections of the six surrounding wires  110 B are provided in such a manner as to be contiguous with respective constituent lateral sides of the substantially fan-shaped cross sections of adjacent surrounding wires  110 B. Such a configuration of the surrounding wires  110 B results in preventing the occurrence of specified size gap formation (hereinafter, also referred to as “depression formation”) at corners in a side of the insulating layer  12  between the adjacent surrounding wires  110 B in the circumferential direction of the center conductor  11 . 
     By being configured in the above described manner, as shown in  FIG. 1 , the surrounding wires  110 B are constituting the substantially continuous circular peripheral surface as an outer peripheral surface  11   a  of the center conductor  11 . That is, the center conductor  11  is configured to have a substantially circular columnar shape like one single-wire conductor. In other words, in the transverse cross section view shown in  FIG. 1 , the outer peripheral edge of the center conductor  11  is configured to have an irregularity-free substantially circular shape. Note that the term “irregularity-free” does not mean “no irregularity,” but means that the size of the irregularity is suppressed to be less than a specified micro size. Such a shape of the outer peripheral edge of the center conductor  11  allows the distances between the outer peripheral surface  11   a  of the center conductor  11  and the outer conductors  13  in radial directions of the high frequency cable  1  to be held substantially constant regardless of peripheral directions of the center conductor  11 . 
     Further, the center conductor  11  is configured to have an elongation of 10% or more in its longitudinal direction. 
     (Insulating Layer  12 ) 
     The insulating layer  12  is configured as a layer formed of an insulating material. The insulating layer  12  is formed of, for example, a fluorine resin. For the fluorine resin, for example, a tetrafluoroethylene/ethylene copolymer (ETFE), a tetrafluoroethylene/hexafluoropropylene copolymer (FEP), or a tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer (PFA) is suitable. The insulating layer  12  is preferably configured to have a thickness of 0.20 to 0.22 mm 
     (Outer Conductor  13 ) 
     The outer conductor  13  is configured as, e.g., a tin-plated (Sn-plated) soft copper wire, a tin-plated copper wire, a tin-plated copper alloy wire, a silver-plated (Ag-plated) copper wire, or a silver-plated copper alloy wire. A large number (e.g., 30 to 60) of the outer conductors  13  are wrapped in a helical arrangement at a specified pitch (for example, 9.7±1.0 mm) around the outer periphery of the insulating layer  12 . The outer conductors  13  may be spirally wrapped (wrapped in a side by side arrangement), or in a meshed arrangement (also called “braided arrangement”) around the outer periphery of the insulating layer  12 . The outer conductors  13  are preferably configured to have an outer diameter of 0.70 to 0.73 mm 
     (Sheath Layer  14 ) 
     The sheath layer  14  is formed by using a material such as, but not specially limited to, PVC (polyvinyl chloride), PE (polyethylene), FEP (e.g., a polymer such as TEFLON™), or the like. The sheath layer  14  may be configured as a single layer, or as multiple layers. Further, the sheath layer  14  may be provided with a separator, a braid, etc., if desired. The sheath layer  14  is preferably configured to have a thickness of 0.055 to 0.065 mm. 
     [Center Conductor  11  Producing Method] 
     Next, a center conductor  11  producing method will be described. The center conductor  11  producing method includes the steps of: forming a stranded wire by stranding a plurality of wires  110  together; compressing the stranded wires  110  to such a central direction that the stranded wire has a circular shape transverse cross section; and heating the compressed stranded wire. 
     The stranded wire compressing step results in deforming the transverse cross section of the one core  110 A into a substantially hexagonal shape, while deforming the remaining six surrounding wires  110 B into substantially fan shapes, respectively, as described previously. Further, this stranded wire compressing step results in bringing the six surrounding wires  110 B into surface contact with each other, thereby preventing the occurrence of depression formation at corners in a side of the insulating layer  12  between the adjacent surrounding wires  110 B in the circumferential direction of the center conductor  11 . In other words, the stranded wire compressing step results in the six surrounding wires  110 B forming the substantially circular columnar shape center conductor  11 . Note that the wires  110  are strengthened by an increase in work hardening rate in the compression, but then subjected to the occurrence of compressive strains. 
     The compressed stranded wire heating step is performed in order to release the compressive strain energy caused in the stranded wire by the above-mentioned stranded wire compressing step. As the compressive strain energy stored in the stranded wire increases, the electrical properties of the stranded wire degrade. The heating step is performed to release this compressive strain energy and thereby recover the electrical properties of the stranded wire. 
     The compressed stranded wire heating step is performed by using, for example, a heating furnace (not shown) and the like. The compressed stranded wire (wires  110 ) may be subjected to thermal annealing at a specified temperature using an annealing furnace (not shown). The heating step results in recovering the electrical properties of the stranded wire (wires  110 ) up to about 98% of the electrical properties of a soft copper wire. 
     (Experimental Results 1) 
     The inventors conducted an experiment to compare the electrical properties for the high frequency cable  1  according to the above-described embodiment of the present invention (hereinafter also referred to as “the high frequency cable  1  according to the Example”) and a high frequency cable according to a conventional example (hereinafter also referred to as “the high frequency cable according to the comparative example”). This experiment will be described below with reference to  FIGS. 2 and 3 . 
       FIG. 2  is a table showing one example of test results on electrical properties for the high frequency cable  1  according to the Example and the high frequency cable according to the comparative example. The inventors measured characteristic impedance, conductor resistance at 20° C., electrostatic capacitance at 1 KHz, and attenuation between 100 MHz and 40 GHz, as one example of indices for indicating the electrical properties. In these measurements, for the Example, the high frequency cable  1  including a center conductor having a circular columnar shape having a substantially continuous circular peripheral surface resulting from the compression in the above-described stranded wire compressing step was used. On the other hand, for the comparative example, the high frequency cable including a center conductor with depression formation occurring in the outer peripheral surface of the center conductor due to being not compressed was used. Note that the detailed conditions of the high frequency cable  1  used in the measurements are shown in  FIG. 2  of the high frequency cable  1  with respect to a comparative example. 
       FIG. 3  is a diagram showing the results based on attenuation as shown in  FIG. 2  for the high frequency cable  1  according to the Example and the high frequency cable according to the comparative example. The horizontal axis shows the frequency (GHz). The vertical axis shows the attenuation (dB/m). Here, the attenuation refers to the attenuation of a signal which occurred for a period of time for which that signal was input to one end of a unit length of the high frequency cable  1  and output from the other end thereof. Further, a graph A (solid line) shows the attenuation in the high frequency cable  1  according to the Example, while a graph B (broken line) shows the attenuation in the high frequency cable according to the comparative example. 
     As shown in  FIG. 3 , it was verified that the attenuation in the high frequency cable  1  according to the Example was smaller than the attenuation in the high frequency cable according to the comparative example, in a high frequency region (e.g., 3 GHz or higher). 
     (Experimental Results 2) 
     In addition, the inventors conducted an experiment to compare the durability against external forces, for the high frequency cable  1  according to the Example and the high frequency cable according to the comparative example. This experiment will be described below with reference to  FIG. 4 . 
       FIG. 4  is a table showing one example of test results on durability against external forces for the high frequency cable  1  according to the Example and the high frequency cable according to the comparative example. The results of a test (hereinafter, also referred to as “electrical continuity test”) for checking the presence or absence of electrical continuity of the high frequency cable  1  when subjected to a specified number of torsions will be described below, as one example of indices for indicating the durability of the high frequency cable  1  against external forces. Note that the presence or absence of electrical continuity was checked by measuring the electrical resistance of the high frequency cable  1 . 
     In the electrical continuity test, the high frequency cable  1  having a length of 20 mm and a weight of 50 g was subjected to alternate repetitions of 180 degree clockwise and counterclockwise torsions around a central shaft in its longitudinal direction. In addition, the torsions were performed at 30 cycles per minute. Note that the checking of the presence or absence of electrical continuity was performed by measuring the electrical resistance of the high frequency cable  1  immediately after performing the following specified numbers of torsions: 1,000, 2,000, 3,000, 4,000, 5,000 and 10,000. 
     As shown in  FIG. 4 , in the same manner as in the above-described electrical property testing, for the Example, the above-described high frequency cable  1  including a center conductor having a circular columnar shape having a substantially continuous circular peripheral surface was used, while, for the comparative example, the high frequency cable including a center conductor with depression formation occurring in its outer peripheral surface due to being not compressed was used. Note that, as shown in  FIG. 4 , the essential conditions other than the condition of the presence or absence of the stranded wire compressing step, specifically, the numbers of wires  110  constituting the center conductors  11 , the materials for the center conductors  11 , the materials for the insulating layers  12 , the materials for the outer conductors  13 , the materials for the sheath layers  14  and the like, as shown in  FIG. 1 , were the same in the Example and the comparative example. 
     As shown in  FIG. 4 , based on test results on durability against external forces for an example of the present invention and a comparative example having the same materials for the (i.e., Ag-plated) center conductor (i.e., 7-strand wires), insulating layer (i.e., FEP), outer conductor (i.e., Sn-plated soft copier), and sheath layer (i.e., PFA), it was verified by the results of the electrical continuity testing that the high frequency cable according to the comparative example had no electrical continuity (see “Absent” in  FIG. 4 ) due to being subjected to the numbers of torsions of more than 5,000, while on the other hand, the high frequency cable  1  according to the Example had an electrical continuity (see “Present” in  FIG. 4 ) even after being subjected to the numbers of torsions of at least 10,000. 
     (Applications) 
     The high frequency cable  1  according to the embodiment of the present invention described above is suitable for a cable to be mounted on a communication device such as a wireless device and the like, for example. Further, although the above embodiment has been described by using the coaxial cable as one example, the high frequency cable  1  may be applied to a multicore cable for a LAN (Local Area Network) and the like. 
     Operations and Advantageous Effects of the Embodiment 
     According to the embodiment of the present invention described above with respect to  FIG. 1 , since the respective outer peripheral surfaces  110 Ba of the plurality of wires form the substantially continuous circular peripheral shape outer peripheral surface  11   a  of the center conductor  11 , it is possible to provide the high frequency cable with improved electrical property degradation in high frequency signal transmission. In addition, since the high frequency cable includes the center conductor  11  formed by stranding the plurality of wires  110  together, it is possible to provide the high frequency cable excellent in the durability against external forces as well. 
     The reason for the enhancement in the electrical properties is considered to be that since the outer peripheral surfaces  110 Ba of the plurality of surrounding wires  110 B form the substantially continuous circular peripheral shape outer peripheral surface  11   a  of the center conductor  11 , that is, the center conductor  11  has the circular columnar shape, the distances between the outer peripheral surface  11   a  of the center conductor  11  and the outer conductors  13  in the radial directions of the high frequency cable  1  are held substantially constant regardless of the peripheral directions of the center conductor  11  as in a single-wire conductor, resulting in good symmetric properties of an electric field and a magnetic field to be produced between the center conductor  11  and the outer conductors  13 . 
     Although the embodiments of the present invention have been described above, the above described embodiments are not to be construed as limiting the inventions according to the claims. It should also be noted that not all combinations of the features described in the embodiments are indispensable to the means for solving the problem of the invention. 
     Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.