JP4198002B2 - coaxial cable - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a coaxial cable for high-frequency signal transmission used for high-frequency equipment and mobile communication devices.
[0002]
[Prior art]
Conventionally, a fluororesin has been used for a dielectric layer in a coaxial cable, and among them, polytetrafluoroethylene (hereinafter referred to as “PTFE”) having a low dielectric constant has been widely used. However, since the required insertion loss and reflection loss cannot still be satisfied, there has been a device for lowering the dielectric constant by providing a void by foaming in the dielectric layer surrounding the inner conductor. Examples of these dielectric layers include those obtained by adding an agent (for example, a foaming agent) that forms voids to PTFE resin, or PTFE stretched tape that has been previously made porous by stretching.
However, the measure to add a foaming agent to PTFE resin is to use an extra thing, so it is wasteful in terms of cost, and it is difficult to control the size and interval of the gap, and the void ratio is accurately set. There is a disadvantage that it cannot be controlled. Furthermore, as shown in FIG. 3, the gap near the surface of the foamed resin forms a dent on the surface depending on the location. This leads to a variation in outer diameter, and therefore has a fatal defect that reflection loss increases.
On the other hand, since the PTFE stretched tape is wound while being spirally stacked on the conductor or insulator surface at a constant interval, irregularities are inevitably generated on the outer surface. This also leads to variations in the outer diameter, which also increases the reflection loss. Further, when bending and stretching are repeated, there are disadvantages such as the gap between the tapes is shifted and the dielectric constant varies in the longitudinal direction of the cable.
Further, a proposal has been made to secure strength by additionally coating a solid layer of tetrafluoroethylene / hexafluoropropylene (hereinafter referred to as “FEP”) on the outer periphery of the PTFE stretched tape in a separate process (for example, , See Patent Document 1). However, in this case, the interface with the FEP solid layer does not become smooth due to the surface unevenness of the PTFE stretched tape, so that the effect of suppressing reflection loss cannot be expected.
[0003]
Japanese Patent Laid-Open No. 2002-358841
[Problems to be solved by the invention]
An object of the present invention is to provide a coaxial cable that has a required mechanical strength and low reflection loss while realizing low insertion loss in a high frequency band and is excellent in total balance.
[0005]
[Means for Solving the Problems]
The inventors of the present invention can easily solve the conventional problems by forming the dielectric layer into a two-layer structure in which the outer layer is a solid skin layer and the inner layer is a non-foaming porous core layer. It came.
[0006]
Thus, according to the present invention, in the coaxial cable including the inner conductor, the dielectric layer surrounding and bending the inner conductor, and the outer conductor, the dielectric layer is formed of a skin-core type integrally extruded two made of a low dielectric resin. A layer structure in which the core layer is non-foaming porous and functions as a dielectric layer having a lower dielectric constant and mechanical strength than the skin layer, while the skin layer is a layer of the core layer. A coaxial cable is provided that functions as a strength protective layer.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a longitudinal sectional view showing an example of the two-layer coaxial cable of the present invention.
FIG. 2 is a cross-sectional view of FIG.
FIG. 3 is a side view of a conventional dielectric layer having voids due to foaming.
FIG. 4 is a side view of a conventional dielectric layer using a stretched tape.
1-2, (1) is an inner conductor, (2) is a dielectric layer, a non-foaming porous core layer (2a) surrounding the inner conductor (1) and the core layer (2a). And a solid skin layer (2b). (3) is an outer conductor layer surrounding the dielectric layer (2), and (4) is a protective jacket layer covering the outer conductor layer (3).
[0008]
As is apparent from FIGS. 1 and 2, the dielectric layer (2) has two layers. In this case, the outer layer is a solid skin layer (2b) and the inner layer is non-layered. The foaming porous core layer (2a) is formed.
This produces the following advantages.
a. Since the skin layer has a relatively high strength, the shape can be maintained when bent.
b. Since the skin layer surface is smooth, reflection loss is reduced.
c. Since the core layer is non-foaming porous, a uniform foaming rate can be obtained.
d. Since the skin layer and the core layer are formed by integral extrusion, compared to the case of covering the skin layer after forming the core layer in advance, there is no influence of the surface irregularities and the outer diameter variation of the core layer, Therefore, the adhesive interface between the two is formed on a very stable smooth surface, thereby realizing a stable low reflection loss.
[0009]
The coaxial cable of the present invention can be easily manufactured by extrusion molding, and one embodiment thereof will be described below.
In the present invention, a preform is formed by adding an extrusion aid in advance to a low dielectric resin such as a fluororesin, particularly PTFE fine powder, to make a preform. At that time, use at least two types of resins with different flow properties during extrusion, and use the resin with the lower flowability (grade with relatively poor flowability) as the inner layer and the higher flowability (grade with good flowability). ) Resin as an outer layer (bamboo ring shape). Next, the preform having the two-layer structure is coated (enclosed) on the outer peripheral surface of the inner conductor (1) by paste extrusion. At this time, since the outer side of the preform having the two-layer structure has good flowability, the fibrils of the resin are densely packed in the extrusion process and no voids are formed. Therefore, the hard (solid) skin layer (2b ). On the other hand, since the resin on the inner side does not have good flowability, the fibrils of the resin are cut, and as a result, a non-foaming porous core layer (2a) is formed.
In this manner, the solid skin layer (2b) and the non-foaming porous core layer (2a) are integrally formed continuously on the outer peripheral surface of the inner conductor (1), and then passed through a drying furnace and a firing furnace. Rolled up. At this time, the porosity of the non-foaming porous core layer (2a) may be in the range of 5 to 70%, but 20 to 50% is particularly preferable. This upper limit value is set to ensure the required strength (hardness), and the lower limit value is set to suppress an increase in dielectric constant. Further, the thickness of the core layer (2a) may be in the range of 0.1 to 5.0 mm, while the thickness of the skin layer (2b) may be in the range of 0.01 to 1.5 mm. 0.1-1 mm is a preferable range. This lower limit value is set to ensure the necessary strength, and the upper limit value is set to suppress an increase in dielectric constant.
Subsequently, a shield such as a metal braid is applied to the outer periphery of the dielectric layer (2) to obtain the outer conductor layer (3). At this time, the gaps in the braid of the metal braid shield may be filled with the metal plating by putting it in the molten metal plating layer.
Finally, the outer circumference of the outer conductor layer (3) is covered with a protective jacket (4) to complete the coaxial cable of the present invention.
In the above-described embodiment, naphtha is generally used as the extrusion aid added to the fluororesin, and a typical example thereof is “Isopar” (trade name, manufactured by Exxon Chemical Co., Ltd.). What is necessary is just to add this adjuvant suitably in the range of 5.0-20.0 weight% with respect to resin.
Furthermore, as the inner conductor (1), a silver-plated annealed copper wire, a silver-plated hard copper wire, a silver-plated copper-coated steel wire or the like having a strand diameter of 0.2 to 1.8 mm is employed. The outer conductor (3) is preferably braided. In this case, a silver-plated annealed copper wire, a silver-plated hard copper wire, a silver-plated copper-coated steel wire, a tin-plated annealed copper wire, etc. Of these, a braid obtained by braiding the wire with 8, 12, 16, 24, or 32 is particularly preferred. On the other hand, for the protective jacket layer (4), a known structure such as extrusion coating of fluorine, olefin or vinyl resin may be applied.
[0010]
【Example】
The following is an example of the coaxial cable shown in FIGS.
First, PTFE resin fine powder (trade name: “6-J”, manufactured by Mitsui DuPont Fluorochemical Co., Ltd.) is used as a fluororesin having relatively poor flowability, and an extrusion aid (product) is added to 100 parts thereof. Name: “Isopar”, Exxon Chemical Co., Ltd. (18 parts) was added to prepare a resin that forms a non-foaming porous core layer (2a). At the same time, PTFE resin fine powder (trade name: “6C-J”, manufactured by Mitsui DuPont Fluorochemical Co., Ltd.) is used as a flowable fluororesin, and 100 parts of this is used as an extrusion aid (trade name: 18 parts of “Isopar” (manufactured by Exxon Chemical Co., Ltd.) was added to prepare a resin for forming a solid skin layer (2b). Then, after aging each for about 8 hours, preforming was performed to make a two-layered preform (bamboo ring shape) having an outer layer portion thickness of 4 mm and an inner layer portion thickness of 30 mm. Next, the two-layer preform was put into an extruder, paste was extruded onto the outer peripheral surface of a silver-plated copper-coated steel wire (1) as an internal conductor at a linear speed of 10 m / min, and then set to about 200 ° C. A drying treatment was performed for about 3 minutes while passing through a drying furnace, and further, a heating treatment was performed for about 5 minutes while passing through a heating furnace set at about 400 ° C., followed by natural cooling and winding. At this time, the non-foaming porous core layer (2a) forming the dielectric layer (2) has a thickness of 1.5 mm and a porosity of 35%, and the skin layer (2b) has a thickness of 0.00. A solid structure of 2 mm was exhibited. Subsequently, the outer perimeter of the dielectric layer (2) was subjected to a metal braid shield of 24 strokes with a tin-plated annealed copper wire having a strand diameter of 0.1 to obtain an external conductor (3). Finally, FEP resin was extruded and coated on the outer periphery of the outer conductor layer (3) to form a protective jacket layer (4), thereby obtaining the coaxial cable of the present invention.
【The invention's effect】
According to the present invention, as a result of skillfully utilizing the flow characteristics during simultaneous extrusion of two types of resins having relatively different flow properties, the outer layer is a solid skin layer, the inner layer is a non-foaming porous material, and A stable dielectric layer in which the interfaces between the two are integrally joined can be obtained. Therefore, in the coaxial cable of the present invention, the required characteristics of mechanical strength, low insertion loss, and low reflection loss are realized in a trinity.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing an example of a coaxial cable of the present invention.
FIG. 2 is a cross-sectional view of FIG.
FIG. 3 is a side view of a conventional dielectric layer having voids due to foaming.
FIG. 4 is a side view of a conventional dielectric layer using a stretched tape.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Inner conductor 2 Dielectric layer 2a Non-foaming porous core layer 2b Solid skin layer 3 Outer conductor 4 Protective jacket layer

Claims (5)

In a coaxial cable including an inner conductor, a dielectric layer surrounding the inner conductor, and an outer conductor, the dielectric layer is a skin-core type integrally extruded two-layer structure made of a low dielectric resin. The core layer is non-foaming porous and functions as a dielectric layer having a lower dielectric constant and mechanical strength than the skin layer, while the skin layer functions as a strength protective layer for the core layer. A featured coaxial cable. The coaxial cable according to claim 1, wherein the dielectric layer is made of polytetrafluoroethylene resin. The coaxial cable according to claim 1, wherein the core layer has a porosity of 5 to 70%. The coaxial cable according to any one of claims 1 to 3, wherein a thickness of the core layer is in a range of 0.1 to 5.0 mm. The coaxial cable according to any one of claims 1 to 4, wherein the skin layer has a thickness of 0.01 to 1.5 mm.

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