JP2003217364A - Foam insulated coaxial cable - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a foam insulated coaxial cable having favorable VSWR characteristics, foaming degree and damping characteristics without causing continuous foam cells in a groove part even when the foam is used as an electrically insulating layer of a coaxial cable of a diametrally large size wherein an inner conductor is a corrugated copper pipe or a corrugated aluminum pipe. <P>SOLUTION: The foam insulated coaxial cable is composed by coating polyethylene based on the insulating layer foamed and formed by using a nucleating agent and a physical foaming agent on an outer circumference of the corrugated inner conductor, and sequentially coating an outer conductor and a sheath layer on an outer circumference of the polyethylene based foam insulating layer. It is characterized in that the base resin of the polyethylene based foam insulating layer comprises polyethylene with density of 940-965 kg/m<SP>3</SP>and low density polyethylene with density of 910-930 kg/m<SP>3</SP>polymerized by a metallocene catalyst. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a foam insulated coaxial cable, and more particularly to a foam insulated coaxial cable using a corrugated inner conductor (corrugated metal tube) as an inner conductor.

[0002]

2. Description of the Related Art Conventionally, as a foaming composition used for manufacturing an antenna power supply line for mobile communication, an olefin resin such as polyethylene or polypropylene and 4,4'-oxybisbenzenesulfonylhydralide as a nucleating agent have been used. What consists of so-called chemical foaming agents such as SID (OBSH) and azodicarbonamide (ADCA), and as the foam, the above foaming composition is prepared by using various kinds of inert gas or carbon dioxide gas as a foaming agent. Each foam is known. It is also known to use a fluorine resin powder or a boron nitride powder as a nucleating agent in order to obtain a foam having higher electric characteristics.

However, recently, due to the downsizing of the feeder line for mobile communication antennas and the use of higher frequency bands, the conventional coaxial cable having an electrically insulating layer of the foam has insufficient electrical characteristics. However, there is a problem that the attenuation characteristics of the coaxial cable do not meet the required standard.

Conventional methods for improving the electrical properties of foams have focused mainly on nucleating agents and foaming agents.
The present inventor pays attention to polyethylene, which is a foamed object, and sincerely examines it, and discloses that a coaxial cable having a small diameter size (7/8 inch size) exhibits excellent attenuation characteristics in a high frequency band (for example, Japanese Patent Laid-Open No. 2004-242242). 2001-31792).

In recent years, it has become necessary to increase the diameter of the inner conductor as the diameter of the cable increases, so a corrugated inner conductor having good buckling resistance and cable bending characteristics is used. However, the foaming composition disclosed in Japanese Patent Application Laid-Open No. 2001-31792, which is used for a small-diameter coaxial cable, is
7/8 like 8 inch size, 13/8 inch size
When applied to a foam insulated coaxial cable having a corrugated inner conductor having a diameter larger than an inch size, open cells are apt to be generated in the groove portion of the corrugated inner conductor, and VSWR (Voltage)
There is a problem in that the manufacturing conditions for which the Standing Wave Ratio (voltage standing wave ratio) characteristics satisfy the required standard are extremely narrow. The inventor considers this phenomenon of continuous foaming as follows. Generally, high-density polyethylene has better dielectric properties than low-density polyethylene, but since high-density polyethylene alone has low moldability, it is necessary to add low-density polyethylene until molding is possible. Many high-density polyethylenes with excellent dielectric properties have low shear viscosity when dissolved,
Shear viscosity as a mixed polyethylene (mixture of high-density polyethylene and low-density polyethylene) is also low, and the resin pressure in the groove of the internal conductor decreases, causing the foaming agent dissolved in the resin to react sensitively and start foaming. I think that this may occur because the growth period of the foamed cells before cooling is longer than that at the site.

[0006]

DISCLOSURE OF THE INVENTION The present invention relates to a foamed insulated coaxial cable having a corrugated inner conductor, which has good VSWR characteristics, foaming degree, and attenuation characteristics without causing continuous foaming in the groove portion of the corrugated inner conductor. It was an object to provide a foam-insulated coaxial cable.

[0007]

Means for Solving the Problems The above-mentioned problems are as follows: (1) The outer periphery of a corrugated inner conductor is covered with a polyethylene foam insulating layer foam-molded using a nucleating agent and a physical foaming agent, A foamed insulating coaxial cable in which an outer conductor and a sheath layer are sequentially coated on the outer periphery of a foamed insulating layer, wherein the base resin of the polyethylene-based foamed insulating layer has a density of 940 kg / m polymerized by a metallocene catalyst.
^{3 to} 965 kg / m ³ polyethylene and density 910k
A foam-insulated coaxial cable, which is made of low-density polyethylene of g / m ^{3 to} 930 kg / m ³ . (2) The density polymerized by the metallocene catalyst is 940 kg.
/ M ^{3 to} 965 kg / m ³ of polyethylene has an MFR of 0.5 to 7, and the foam insulated coaxial cable according to (1). (3) The foam insulated coaxial cable as described in (1) or (2), wherein the nucleating agent is a fluororesin powder or a boron nitride powder. (4) Solid polyethylene is coated as an inner skin layer between the outer periphery of the corrugated inner conductor and the inner periphery of the polyethylene foam insulating layer (1) to (3).
The foam insulated coaxial cable according to any one of 1. (5) The foamed insulated coaxial cable according to any one of (1) to (4) is characterized in that the maximum outer diameter of the corrugated inner conductor is 30 mm or more.

[0008]

The present invention has a density of 940 kg obtained by polymerizing a polyethylene foam insulating layer with a metallocene catalyst as a base resin.
/ ^M 3 by polyethylene and density of ～965Kg / ^{m 3} uses a mixture of polyethylene of low density polyethylene 910 kg ^/ m 3 ～930Kg / ^{m 3,} not reactive with respect to under resin pressure drop, the copper tube with words wave Since no continuous bubbles are generated in the groove, VSWR characteristics, foaming degree, and damping characteristics are improved. Furthermore, by providing an inner skin layer between the corrugated outer conductor and the polyethylene foam insulating layer, the foaming agent dissolved in the foam insulating material is prevented from escaping from the corrugated inner conductor interface. It is possible to suppress open cells that tend to occur in the vicinity, and improve VSWR characteristics.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The corrugated inner conductor is used as an inner conductor of a foam insulated coaxial cable, and is made of corrugated conductive smooth aluminum pipe, smooth copper pipe, or the like. Good. By performing the corrugation processing, it is possible to increase the diameter of a cable having buckling resistance and good bending characteristics, as compared with a smooth tube that is not corrugated.
The wavy shape is a sine curve (y = B · sinx)
And a shape that is not smooth than the above-mentioned substantially sine curve. As for the form of the corrugated pitch, there are one in which one pitch is independent as in the ring shape (Fig. 1 (a)) and one in which the pitch is continuous as in the spiral shape (Fig. 1 (b)). is there. Among them, the ring shape is preferable to the spiral shape because even if rainwater or the like penetrates, the ring shape is more likely to stay at one pitch or one pitch without dripping over the entire cable. Maximum outer diameter of the corrugated inner conductor (distance A in FIG. 1)
Does not have to be specified, but it may be used for an inner conductor having a size of 30 mm or more which requires a larger diameter cable and improved buckling resistance and bending characteristics.

The polyethylene foam insulating layer is a base resin.
As a polyethylene polymerized with a metallocene catalyst (hereinafter,
Me-PE) and low density polyethylene (hereinafter
Below, also referred to as LDPE) is a mixed polyethylene
The Me-PE has an attenuation characteristic and a VSWR characteristic.
The density is 940 kg / m.^Three~ 965k
g / m^ThreeIf it is good. Preferably, the density is 940 kg
/ M^Three~ 965kg / m ^ThreeAnd MFR is 0.5 to 7
Are preferred. Density is 940kg / m^ThreeLess than
If this is not the case, the attenuation characteristics tend to deteriorate, and 965 kg /
m^ThreeLarger but more noticeable VSWR characteristics, foaming degree, damping
No improvement in characteristics can be obtained.

The "metallocene" of the metallocene catalyst used in the present invention is a non-electrolyte complex among bis (cyclopentadienyl) metal compounds, in which two cyclopentadienyl rings have a regular pentagonal structure. The metal atoms (eg, Zr, Ti, Vi, Cr, F) that are parallel to each other and are in the middle are
e, Co, Ni, Ru, Pd, etc., among which Zr is preferable from the viewpoint of polymerization uniformity), and is a generic term for molecules having a sandwich structure.

The metallocene catalyst, which is one of the catalysts for polymerizing polyethylene used in the present invention, has the sandwich structure as described above, and ethylene gas (raw material gas) is bonded to the 5-membered ring bonded to the metal. Since it cannot be contacted from the direction that is sterically blocked by the substituent, the metal and the monomer always contact from a fixed direction, and stereoregular polymerization occurs, so that the molecular weight and branched structure of the obtained polyethylene are Be uniform. Also,
It is superior to polyethylene that can be obtained by using other catalysts in that polyethylene with less impurities can be obtained. Furthermore, polyethylene polymerized with a metallocene catalyst has dielectric properties equal to or higher than those of high-density polyethylene,
In addition, since it has melting characteristics suitable for foam molding compared to high-density polyethylene, even when used as a foam insulating layer of a foam insulated coaxial cable, it is not necessary to increase the blending amount of LDPE and stable VSWR characteristics are achieved. It is possible to obtain a mixed polyethylene by ensuring the above-mentioned property and obtaining an excellent attenuation characteristic.

As the low density polyethylene, the density is
910kg / m^Three~ 930 kg / m ^ThreeUsed
It Density 910kg / m^ThreeLess than low density polyethylene
When used, the cells of the foam tend to be non-uniform
Yes, while 930 kg / m ^ThreeUse the larger one
When it does, a foam with a high degree of foaming tends not to be obtained
It is in. In addition, the SR ratio of the low density polyethylene
The SR ratio is usually
20-80, preferably 40-60, M
FR is usually about 0.1 to 10, preferably about 1 to 7,
In particular, a value of less than 5 is suitable.

Such low-density polyethylene is obtained by, for example, a low-pressure polyethylene produced by a conventional high-pressure method using oxygen or an organic peroxide as a polymerization initiator, a transition metal catalyst, and α.
-Linear low-density polyethylenes produced by a high-pressure method in which an olefin comonomer is added, a solution polymerization method, a slurry polymerization method, a gas phase polymerization method (all of which use a Phillips catalyst), or a standard catalyst is used. Low-pressure methods such as low-density polyethylene and linear low-density polyethylene produced by medium-pressure method such as solution polymerization method, solution polymerization method, slurry polymerization method, or gas phase polymerization method (all of which use Ziegler catalyst) It can be selectively obtained from low density polyethylenes and linear low density polyethylenes produced by. Further, low density polyethylenes or linear low density polyethylenes polymerized by using the above metallocene catalyst may be applied.

Regarding the composition ratio of Me-PE and LDPE in the mixed polyethylene used in the present invention, the blending amount of Me-PE in the total amount of both is 50% by weight or more, preferably 60% by weight or more, It is more preferably 70% by weight or more. The balance in the polyethylene mixture is low density polyethylene, and the amount of low density polyethylene in the total amount of both is 5% by weight or more, preferably 10% by weight.
The above is more preferably 15% by weight or more. 50% by weight of Me-PE in the polyethylene mixture
If it is less than this, the attenuation characteristic tends to deteriorate when the cable is used.

The base resin may contain other low-polarity polymers such as polypropylene and ethylene-propylene copolymer in addition to the above-mentioned mixed polyethylene. The amount of the polyethylene mixture in the base resin is 80% by weight. % Or more, preferably 90% by weight or more.

If necessary, the base resin of the material used for the polyethylene foam insulating layer of the present invention may further contain other compounding agents such as antioxidants, copper anti-corrosion agents, colorants or other additives. It may have been done. Since the above-mentioned other compounding agents generally deteriorate the electrical properties of the foam in general, the addition amount thereof is 0.05 to 2. per 100 parts by weight of the base resin in the total amount thereof.
It is about 0 parts by weight, preferably about 0.1 to 1.0 parts by weight.

In the present invention, the density of Me-PE and LDPE and MFR can be measured by the following methods. [Density]: The value at 20 ° C. is measured by the method specified in JIS K7112. [MFR]: Measured under the conditions of a temperature of 190 ° C. and a load of 2.16 kg using a melt indexer specified in JIS K 7210 and the method specified therein.

As the nucleating agent for foaming the base resin,
Fluororesin powder and / or boron nitride powder are used. At this time, the fluororesin may be any polymer such as a homopolymer or a copolymer of a fluorine-containing monomer, which can be powdered. Above all, the dielectric constant (20 ℃,
It is preferably a low polarity material having a 1 MHz) of 2.5 or less. Examples of the fluororesin include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene. Copolymer (FEP), tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride (PVdF), polychlorotrifluoroethylene (PCTFE), chlorotrifluoroethylene-ethylene copolymer (ECTFE)
Etc., among others PTFE, PFA, and ETFE
Is preferable, and PTFE is particularly preferable.

The particle size and the amount of the nucleating agent used may be those usually employed in the case of foaming polyethylene, the average particle size is about 0.05 to 50 μm, and the amount used is 100 weight of the base polymer. It is about 0.01 to 5 parts by weight per part. In order to further improve the fineness and uniformity of the foam cells, the average particle size of the fluororesin powder is 0.2 to
It is preferably about 10 μm, particularly about 0.2 to 5 μm, and in the case of boron nitride powder, the average particle size is preferably about 0.2 to 30 μm, particularly about 0.2 to 10 μm.

The average particle size of the powder can be measured by the following method. [Average particle size of powder]: A dispersion liquid obtained by introducing the powder to be measured into an organic liquid such as water or ethanol and dispersing for about 2 minutes while applying an ultrasonic wave of about 35 to 40 kHz. The amount of the particulates used in this case is such that the laser transmittance (ratio of the output light amount to the incident light amount) of the dispersion liquid is 70 to 95%, and then the dispersion liquid is used in a Microtrac particle size analyzer. The particle size (D1
, D2, D3 ···) and the number of existing particles for each particle size (N1
, N2, N3, ...) (the particle size (D) of each granular material is automatically measured by the Microtrac particle size analyzer for each granular material of various shapes. .). Then, the average particle size (μm) is calculated by the following formula (1) from the number (N) of individual particles existing in the visual field and each particle size (D). Average particle size (μm) = (ΣND ³ / ΣN) ^1/3 (1)

The material used for the polyethylene foam insulating layer of the present invention is the above-mentioned base resin, nucleating agent, and other compounding agents optionally used, such as an antioxidant, a copper damage inhibitor, and a coloring agent. Alternatively, other additives and the like are separately weighed and mixed so as to have a predetermined ratio to each other,
Then, the mixture is kneaded with a usual kneading machine such as a Banbury mixer and a hot roll. At that time, polymer components such as low-density polyethylene and high-density polyethylene are previously kneaded in a kneading machine and used as a polyethylene mixture in which they are uniformly mixed, and a nucleating agent and other compounding agents are also mixed and kneaded. Good.

The polyethylene foam insulating layer of the present invention comprises the above-mentioned base resin, nucleating agent, and other compounding agents optionally used, such as an antioxidant and a copper damage inhibitor.
Coloring agents and other additives are separately weighed and mixed in a predetermined ratio to each other, and then kneaded with a usual kneading machine such as a Banbury mixer or a heat roll, and then physical foaming is carried out according to a usual method. It can be produced by extruding from a high pressure inside the extruder in the presence of an agent into an atmosphere having a pressure lower than the high pressure. As the physical blowing agent, an inert gas, for example, dichlorodifluoromethane, dichloromonofluoromethane, monochlorodifluoromethane, trichloromonofluoromethane, monochloropentafluoroethane, halogenated hydrocarbons such as trichlorotrifluoroethane, nitrogen, carbonic acid. Gas, helium, argon, etc. are used. Among these foaming agents, H
CFC22, HCFC123, HCFC124, HCF
A hydrogen atom-containing chlorofluorocarbon such as C142b, a chlorine atom-free fluorocarbon, an inert gas such as nitrogen, carbon dioxide gas, helium, and argon. Among them, argon gives a foam having a uniform, fine, and high degree of foaming. Particularly preferred. Moreover, since these are non-destructive to the ozone layer, they are preferable in terms of environmental protection.

The amount of the physical foaming agent used is not particularly limited, but is usually 0.05 to 100 parts by weight per 100 parts by weight of the foaming composition.
It is about 1 part by weight, particularly about 0.05 to 0.5 part by weight. The physical foaming agent may be mixed with the organic polymer to be foamed in advance, or may be supplied into the extruder through a physical foaming agent supply port provided in the barrel of the extruder. It is preferable that the physical foaming agent has a high foaming degree obtained by foaming with an inert gas, for example, a foaming degree of 70% or more, particularly 75% or more. The degree of foaming, specific gravity S _S of the foam before the foam _composition, if the specific gravity of the foam and S _F, is calculated by the following equation (2). Specific gravity S _S and specific gravity S _F are J
It can be measured by the underwater displacement method (method A) specified in IS K 7112. Foaming degree _{(%) = (S S -S} F) × 100 / S S (2)

The outer periphery of the corrugated inner conductor and / or the outer conductor is coated with solid polyethylene as an inner skin layer on the outer periphery of the corrugated inner conductor before coating the foamed insulating layer. Covering the insulating layer is preferable because the VSWR characteristics are further improved. Here, solid polyethylene refers to polyethylene that has not been foamed, and among them, low density polyethylene is preferable from the viewpoint of improving the attenuation amount, and linear low density polyethylene is more preferable among them. Linear low-density polyethylene is a copolymer of ethylene and α-olefin produced by a known solution polymerization method, slurry polymerization method, gas phase polymerization method or the like using various catalysts such as Ziegler type catalysts and chromium type catalysts. It is a polymer. The low-density polyethylene or linear low-density polyethylene is granulated (granulated, granulated, having an average particle size of 2.0 mm to 7.0 mm) and containing no antioxidant or the like before being formed into a pellet (gas The average particle size before granulation produced by the phase polymerization method is 0.02 mm to 2.
It is sufficient to use a porous powder having a diameter of 0 mm. The thickness of the skin layer is preferably 0.05 mm to 1.0 mm, and if it is thinner than 0.05 mm, the VSWR characteristic does not contribute to improvement, and if it is thicker than 1.0 mm, the attenuation characteristic tends to deteriorate. As a coating method, a known coating method such as conventional extrusion molding or injection molding may be applied.

As the outer conductor, a publicly known metal material having conductivity may be used, and examples thereof include aluminum and copper, and those having a corrugated shape or a smooth shape may be used. In the case of the outer conductor, one having the same shape as the corrugated inner conductor described above can be used. As means for covering the outer periphery of the foam insulating layer with the outer conductor,
In the case of a smooth pipe, a step of covering the outer circumference of the foam insulating layer by a method of longitudinally winding an aluminum band or a copper band, or throwing an aluminum rough wire into a conclad machine to cover the outer circumference of the foam insulating layer. Examples of the corrugated processing include a known method of eccentric rotary die processing after the above steps.

The sheath layer covering the outer periphery of the outer conductor may be the one used for known electric wires,
Examples thereof include polyethylene and vinyl chloride. As a method of coating the outer circumference of the outer conductor, extrusion molding may be performed on the outer circumference of the outer conductor using a known extruder.

FIG. 2 is an explanatory view of an example of an apparatus for manufacturing a foam insulated coaxial cable of the present invention, in which 1 is a corrugated inner conductor supply section, 2 is an inner conductor preheater, and 3 is a physical foaming agent. A cylinder, 31 is a physical foaming agent injection nozzle installed in a barrel of a first extruder 4 described later, 32 is a pressure reducing valve, 4 is a first extruder, 41 is a hopper of the first extruder 4, and 42 is a first extrusion. Discharge port of the machine 4, 5 is a second extruder, 51 is a second extruder 5
Is a discharge port, 6 is a crosshead of the second extruder 5, and 7 is a cooling device. The first extruder 4 is connected to the second extruder 5 in a T-shape through its discharge port 42.

A material to be the polyethylene foam insulating layer (base resin, nucleating agent, and other compounding agents used as necessary, for example, antioxidant, copper damage inhibitor, colorant or other additive) is used. The hopper 4 of the first extruder 4 kneads the mixture in a Banbury mixer and then shreds it into pellets (granular) of about 2 mm square with a pelletizer.
1 and is melted in the first extruder 4. The physical foaming agent includes a cylinder 3, a pressure reducing valve 32, and a physical foaming agent injection nozzle 3.
It is pressed into the first extruder 4 via 1 and mixed with the melt. Then, the mixture of the physical foaming agent and the foaming composition mixed in the first extruder 4 is transferred to the second extruder 5 via the discharge port 42. The transferred mixture is
It is more sufficiently mixed in the second extruder 5 and transferred to the crosshead 6 via the discharge port 51.

The optimum set temperatures in the barrels of the first extruder 4 and the second extruder 5 are slightly different depending on the composition of the foaming composition and the type of the physical foaming agent, but generally the second extruder 5 is used. The temperature in the barrel of is preferably adjusted to be lower than that of the first extruder 4 and slightly higher than the melting point of the polyethylene mixture used. For example, when the melting point of the polyethylene mixture used is 132 ° C., the temperature and pressure inside the barrel of the first extruder 4 are 180 ° C. to 210 ° C.
The temperature and pressure inside the barrel of the second extruder 5 are about 130 ° C to 140 ° C and about 50 atm to 150 atm. Regarding the melting point of the polyethylene mixture, the melting point is the endothermic peak top measured by the differential scanning calorimetry at a temperature rising rate of 10 ° C./min and a weight of 10 mg.

Internal corrugated conductor 8 continuously supplied from the corrugated internal conductor supply unit 1 (corrugated inner conductor not covered with inner skin layer: 8a, corrugated inner conductor covered with inner skin layer) : 8b) passes through the preheater 2, the crosshead 6, and the cooling device 7 in sequence and travels. On the other hand, the mixture in the second extruder 5 is transferred to the crosshead 6 via the discharge port 51 and is supplied onto the corrugated internal conductor 8 that continuously runs, and the die installed in the discharge port of the crosshead 6 is supplied. After passing through (not shown), it is extruded into the atmosphere and foamed to form a foamed insulating layer on the outer periphery of the corrugated internal conductor 8. The foam insulation layer is cooled while passing through the cooling device 7, thus producing the foam insulation coaxial cable core 9 and winding it on the winding device 10. Thereafter, the outer periphery of the foam-insulated coaxial cable core 9 thus manufactured is covered with an outer conductor and a sheath, and a foam-insulated coaxial cable is manufactured. A longitudinal section of the produced foam insulated coaxial cable is shown in Fig. 3 (without inner skin layer).
It is shown in FIG. 4 (with inner skin layer). Also, in some cases, a foam-insulated coaxial cable is manufactured in which the outer skin layer is coated before coating the sheath, and then the sheath layer is coated.

[0032]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples, and comparative examples will also be shown to clarify the remarkable effects of the present invention.

A mixture having the composition shown in Table 1 was kneaded in a Banbury mixer at 160 ° C. and then chopped with a pelletizer to give pellets of about 2 mm square Example 1
(No inner skin layer), Example 2 (no inner skin layer), Example 3 (with inner skin layer), and Comparative Examples 1 to 6 (no inner skin layer) were obtained. Foam insulation with inner skin layer made of 0.5 mm thick linear low-density polyethylene (granulated (average particle size 0.5 mm) produced by vapor phase polymerization method) on the corrugated inner conductor A coaxial cable was manufactured (Example 3). In the table, those having no inner skin layer are marked as, for example, Example 1 (none), and those having an inner skin layer are marked as, for example, Example 3 (present).

[0034]

[Table 1]

Subsequently, the materials listed in Table 1 were used, and 65
Using a manufacturing apparatus having a two-stage extruder of mmφ-90 mmφ as shown in FIG. 2, an outer diameter of 1 is obtained as a corrugated inner conductor having a foam insulating layer formed of a foam of each material.
A foam insulating layer having an outer diameter of 44.5 mm was formed on the outer circumference of a corrugated copper tube having a diameter of 7.3 mm (13/8 inch size). At that time, the supply amount of argon gas was gradually increased to maximize the foaming degree of the foam insulating layer. An outer conductor having an outer diameter of 46.5 mm was coated on the foam insulation layer, and a polyethylene sheath having a thickness of 1.75 mm was further coated on the outer conductor to obtain a foam insulation coaxial cable. Further, in Example 3, the corrugated inner conductor has a linear low-density polyethylene of 0.5 mm in thickness (granular shape (average particle size of 0.
5 mm) was used), and a foam insulated coaxial cable was manufactured in the same process as described above.

The degree of foaming of the foamed insulation layer of each foamed insulated coaxial cable obtained, VSWR characteristics, attenuation characteristic of the cable,
Table 1 shows the pass / fail judgment results determined from the above. The degree of foaming was rated as ◯ when it was 70% or more, and as x when it was less than 70%. VSWR characteristics of 1.12 to 1.15 are ○, 1.1
When it was smaller than 2, it was marked with ⊚, and when it was larger than 1.15, it was marked with x. The foaming degree of the foamed insulated coaxial cable was calculated by the following formula (3), where S _{S is} the specific gravity before foaming and S _F is the specific gravity after foaming. Specific gravity S _S and specific gravity S _F are specified in JIS K711.
It was measured by the underwater substitution method (method A) specified in 2. Foaming degree _{(%) = (S S -S} F) × 100 / S S (3) VSWR characteristics of the foamed insulation coaxial cable, the maximum value _{V max} of the amplitude of the voltage standing wave, the minimum value _{V min} is measured, It was calculated by the following formula (4). VSWR = V _max / V _min (4) The attenuation characteristic of the foam insulation coaxial cable was measured by using a Wiltron 54111A, and 1
Attenuation amount at 2 GHz is 41 dB for 3/8 inch size
Those with less than / km were evaluated as ◯, and those with more than 41 dB / km were evaluated as x.

[0037]

In the foam-insulated coaxial cable according to the present invention, even when a corrugated copper tube, a corrugated aluminum tube or the like is used as the inner conductor, the polyethylene-based foam insulating layer is corrugated without suppressing the generation of open cells. Since it is possible to coat the outer circumference of the attached inner conductor, it is possible to provide a foamed insulated coaxial cable having excellent VSWR characteristics, foaming degree, and attenuation characteristics. Since a corrugated inner conductor can be used, it is possible to provide a flexible insulated foam insulated coaxial cable, which has excellent VSWR characteristics without increasing the cable attenuation as when simply increasing the content of low-density polyethylene. Can be realized. Further, by covering the outer periphery of the corrugated inner conductor with the inner skin layer, it is possible to further suppress the generation of open cells in the polyethylene foam insulating layer to be coated later, and to provide a foam insulating coaxial cable having more excellent VSWR characteristics. did it.

[Brief description of drawings]

FIG. 1 is a view showing a shape of a longitudinal section of a corrugated metal tube.

FIG. 2 is an explanatory diagram of an example of a manufacturing apparatus for a foam insulated coaxial cable.

FIG. 3 is a view showing a vertical section of a foam insulated coaxial cable (without an inner skin layer).

FIG. 4 is a view showing a vertical section of a foam insulated coaxial cable (with an inner skin layer).

[Explanation of symbols]

1 Corrugated inner conductor supply section 2 conductor preheater 3 Cylinder filled with physical foaming agent 31 Physical blowing agent injection nozzle 32 Pressure reducing valve 4 First extruder 41 Hopper of the first extruder 42 Discharge port of the first extruder 5 Second extruder 51 Discharge port of the second extruder 6 Crosshead of the second extruder 7 Cooling device 8 corrugated inner conductor 8a Corrugated inner conductor not covered by inner skin layer 8b Corrugated inner conductor covered with inner skin layer 9 Foam insulated coaxial cable core 10 Winding device 11 Foam insulation layer 12 outer conductor 13 Sheath layer 14 Foam insulated coaxial cable 15 Inner skin layer Maximum outer diameter of A corrugated inner conductor

Claims (5)

[Claims] 1. A corrugated inner conductor is covered with a polyethylene foam insulating layer foam-molded using a nucleating agent and a physical foaming agent, and an outer conductor and a sheath layer are formed on the outer periphery of the polyethylene foam insulating layer. A foam-insulated coaxial cable which is sequentially coated, wherein the base resin of the polyethylene-based foam insulating layer has a density of 940 kg polymerized by a metallocene catalyst.
/ M ^{3 to} 965 kg / m ³ of polyethylene and a density of 91
Foamed insulation coaxial cable, comprising the low density polyethylene ^{0kg / m 3 ~930kg / m 3} . 2. The density of the polymerized by the metallocene catalyst is 94.
0kg ^{/ m 3} ~965kg ^/ m 3 of polyethylene of MF
The foam insulated coaxial cable according to claim 1, wherein R is 0.5 to 7. 3. The foam insulated coaxial cable according to claim 1, wherein the nucleating agent is fluororesin powder or boron nitride powder. 4. A solid polyethylene is coated as an inner skin layer between the outer circumference of the corrugated inner conductor and the inner circumference of the polyethylene foam insulating layer.
~ The foam insulated coaxial cable according to claim 3. 5. The foam insulated coaxial cable according to claim 1, wherein the corrugated inner conductor has a maximum outer diameter of 30 mm or more.