WO2008096941A1

WO2008096941A1 - Insulator for coaxial cable, method for preparing the same, and low loss large diameter coaxial cable using the same

Info

Publication number: WO2008096941A1
Application number: PCT/KR2007/003323
Authority: WO
Inventors: Chan-Yong Park; Bong-Kwon Cho; Gi-Joon Nam; Jung-Won Park; Dae-Sung Lee
Original assignee: Ls Cable, Ltd.
Priority date: 2007-02-08
Filing date: 2007-07-09
Publication date: 2008-08-14
Also published as: KR20080074382A

Abstract

The present invention relates to an insulator for a coaxial cable, a method for preparing the same, and a low loss large diameter coaxial cable using the same. The insulator for a coaxial cable according to the present invention has a degree of foaming of 80 % or more and includes foam cells with a closed-cell structure and a long radius of 10 to 500 D. The present invention can minimize attenuation while ensuring good signal characteristics in a ultrahigh frequency transmission, transmit a signal at a ultrahigh speed without signal interference, and allows for signal transmission of ultrahigh speed and large capacity due to an excellent propagation velocity.

Description

INSULATOR FOR COAXIAL CABLE, METHOD FOR

PREPARING THE SAME, AND LOW LOSS LARGE DIAMETER

COAXIAL CABLE USING THE SAME

Technical Field

[1] The present invention relates to an insulator for a coaxial cable, a method for preparing the same, and a low loss large diameter coaxial cable using the same, and in particular, to an insulator for a coaxial cable, which has a degree of foaming of 80 % or more and includes foam cells with a closed-cell structure and a long radius of 10 to 500 D, a method for preparing the same, and a low loss large diameter coaxial cable using the same. Background Art

[2] So far, a coaxial cable has been used for a long time and developed for various sizes and kinds of product groups. The coaxial cable comprises an inner conductor for signal transmission and an outer conductor arranged on a concentric axis of the inner conductor.

[3] The main development trends of the conventional coaxial cable were to reduce the loss of energy to be transmitted. For example, the development has been made to improve a structure design or dielectric characteristics between the inner conductor and the outer conductor or provide various functions outside the outer conductor.

[4] The coaxial cable has been mainly used for signal transmission of an antenna in the basement of a building or a cable TV. As a standard for evaluating the signal transmission characteristics, the characteristic impedance (Z) represented as the following Math Figure 1 is mainly reflected on structure design.

[5] MathFigure 1

[6] In the above Math Figure 1, ε is a dielectricity of an insulating material, d is a r diameter of an inner conductor, and D is an inside diameter of an outer conductor.

[7] The impedance matching is the most important parameter of the coaxial cable, and is calculated by the above Math Figure 1. The impedance of 33 Ω exhibits good power transmission characteristics of energy wave, impedance of 75 Ω exhibits distortion of signal waveform, and the intermediate impedance of 45 to 50 Ω is used as an international standard.

[8] The coaxial cable is disclosed in the prior art, for example U.S. Patent No. 6,130,385, U.S. Patent No. 4,965,412 and U.S. Laid-open Patent Publication No. 2003/0051897. The prior arts teach mainly a metal layer or metal-deposited film layer provided in or on an outer conductive shield layer to prevent an outward leakage of signal.

[9] Meanwhile, one of the main parameters of the coaxial cable, a propagation velocity

(V ) is calculated by the following Math Figure 3 after a dielectricity (ε ) is calculated f r by the following Math Figure 2. As a degree of foaming increases, the dielectricity decreases, which results in reduction of attenuation to improve the propagation velocity. [10] MathFigure 2

[11] In the above Math Figure 2, ε r,exp is a dielectricity after foam, ε r, solid is a dielectricity before foam, p exp is a density after foam, and p solid is a density before foam.

[12] MathFigure 3

[13] In the above Math Figure 3, V is a propagation velocity, and is calculated using the dielectricity (ε ) calculated through the above Math Figure 2. r

[14] Meanwhile, an expansion ratio (V ) of an insulator used in the coaxial cable is calculated by the following Math Figure 4, and a degree of foaming is calculated by the following Math Figure 5.

[15] MathFigure 4

f p

[16] In the above Math Figure 4, P is a polymer volume, and G is a gas volume (spatial volume). [17] MathFigure 5

[18] In the above Math Figure 5, a degree of foaming (D ) may be calculated directly from the expansion ratio (V ) represented as the above Math Figure 4, and its unit is indicated as percentage (%).

[19] Each of the properties represented as the above-mentioned Math Figures is directly or indirectly correlated with a signal transmission speed of the coaxial cable. According to the correlation between the above-mentioned properties and the signal transmission speed, it is important to select a resin for an insulating layer having a low dielectricity and excellent flow characteristics to ensure a high transmission speed and prevent attenuation during transmission. And, it is important to adjust a mixing ratio of the resin to increase a degree of foaming and decrease a dielectric constant value. In particular, with a rapid development of information communication technology, attempts have been made to use a high frequency of several hundred MHz to several GHz so as to transmit more amount of information in a shorter time. In the case that information is transmitted using a high frequency, it raises the demand for a technical improvement to minimize attenuation.

[20] Therefore, the related industry has attempted steadily to solve the problems, and the present invention was devised under this technical background. Disclosure of Invention Technical Problem

[21] Based on the above-mentioned conventional problems, an object of the present invention is to provide an insulator for a coaxial cable, which is made of such a material as to reduce dielectricity, minimize attenuation and maximize transmission speed of the coaxial cable, thereby ensuring a high efficiency transmission rate of the coaxial cable for high frequency transmission, a method for preparing the same, and a low loss large diameter coaxial cable using the same.

Technical Solution

[22] In order to achieve the object of the present invention, an insulator for a coaxial cable has a degree of foaming of 80 % or more, and includes foam cells with a closed- cell structure and a long radius of 10 to 500 D.

[23] In order to achieve the object of the present invention, a low loss large diameter coaxial cable comprises an inner conductor of a cylindrical shape; an inner skin layer surrounding the inner conductor; an insulator layer surrounding the inner skin layer in a layered design; an outer skin layer surrounding the insulator layer; an outer conductor provided along an outer periphery of the outer skin layer; and a coating layer surrounding the outer conductor completely, wherein the insulator layer has a degree of foaming of 80 % or more, and includes foam cells with a closed-cell structure and a long radius of 10 to 500 D.

[24] Preferably, the insulator layer is formed with a thickness corresponding to 60 to 170

% of an inside diameter of the inner conductor.

[25] In order to achieve the object of the present invention, a method for preparing an insulator layer of a low loss large diameter coaxial cable, which includes an inner conductor of a cylindrical shape, an inner skin layer surrounding the inner conductor, an insulator layer surrounding the inner skin layer in a layered design, an outer skin layer surrounding the insulator layer an outer conductor provided along an outer periphery of the outer skin layer, and a coating layer surrounding the outer conductor completely, comprises (Sl) preparing a resin, in which a high density polyethylene (HDPE) and a low density polyethylene (LDPE) are mixed; (S2) adding a chemical foaming agent to the resin mixture; (S3) injecting a gas for ensuring a low dielectricity into the resin mixture; (S4) foaming the prepared resin mixture; and (S5) extruding the foamed product to surround an upper surface of the inner skin layer in a layered design.

[26] Preferably, the resin mixture of the step (Sl) includes 45 to 75 weight% of the high density polyethylene (HDPE) and 25 to 55 weight% of the low density polyethylene (LDPE). Preferably, the chemical foaming agent of the step (S2) is any one selected from the group consisting of azodicarbonamide, sulfonylhydrazides and 5-phenyltetrazole, or mixtures thereof. At this time, preferably the chemical foaming agent is injected with a content of 150 to 750 ppm based on the whole weight of the resin mixture. Preferably, the gas for ensuring a low dielectricity of the step (S3) is carbon dioxide (CO ). Preferably, the foaming of the step (S4) is performed with a degree of foaming of 80 % or more. Preferably, the insulator layer formed through extrusion of the step (S5) has a thickness corresponding to 60 to 170 % of an inside diameter of the inner conductor. Preferably, the insulator layer formed through extrusion of the step (S5) includes foam cells with a closed-cell structure and a long radius of 10 to 500 D.

[27] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

[28] The insulator used in the coaxial cable of the present invention has a degree of foaming of 80 % or more, and includes foam cells with a closed-cell structure and a long radius of 10 to 500 D. The insulator is manufactured by (Sl) preparing a resin, in which a high density polyethylene (HDPE) and a low density polyethylene (LDPE) are mixed, (S2) adding a chemical foaming agent to the resin mixture, (S3) injecting a gas for ensuring a low dielectricity into the resin mixture, (S4) foaming the prepared resin mixture, and (S5) extruding the foamed product to surround an upper surface of the inner skin layer in a layered design.

[29] The resin mixture of the step (Sl) includes the high density polyethylene (HDPE) and the low density polyethylene (LDPE) to ensure a characteristic impedance and a low dielectricity for a high degree of foaming. In the case that a high density polyethylene only is used instead of the resin mixture, it is not preferable because transmission characteristics are improved in aspect of attenuation, but a degree of foaming does not reach 80 % or more. And, in the case that a low density polyethylene only is used instead of the resin mixture, it is not preferable because a degree of foaming is increased, but transmission characteristics are not ensured.

[30] Preferably, the resin mixture includes 45 to 75 weight% of the high density polyethylene and 25 to 55 weight% of the low density polyethylene. In the case that the mixing ratio is in the above-mentioned range, it is preferable because a degree of foaming is ensured to 80% on average and 87 % to the maximum. The degree of foaming is closely related with a melt strength, and in the case that the mixing ratio is not in the above-mentioned range, it is not preferable because a degree of foaming of 80 % is not obtained to the minimum.

[31] The chemical foaming agent (CFA) of the step (S2) is used to optimize cooling characteristics according to a large diameter.

[32] Generally, in the case that a large diameter coaxial cable was manufactured using a heat absorbing chemical foaming agent or a mixture of a heat generating chemical foaming agent and a heat absorbing chemical foaming agent, a degree of foaming of 80 % or more was not obtained due to thermal balance such as cooling, and a closed-cell structure for suppressing the flow of a liquid was not obtained. However, the present invention used only a heat generating chemical foaming agent, and thus cooling characteristics was optimized to obtain a degree of foaming of 80 % or more.

[33] The heat generating chemical foaming agent may be azodicarbonamide, sulfonyl- hydrazides or 5-phenyltetrazole.

[34] It is preferable to add the heat generating chemical foaming agent with a content of

150 to 750 ppm based on the whole weight of the resin mixture. In the case that the content of the heat generating chemical foaming agent meets the above-mentioned range, it is preferable because a desired degree of foaming is obtained and a proper reaction site is secured, so that proper cell size and degree of foaming are secured.

[35] The polymer insulator layer extruded on the inner skin layer has a thickness of 6 to

14 D, and its composition ratio is determined in consideration of a loss factor (tanδ).

[36] Preferably, the gas for ensuring a low dielectricity of the step (S3) is CO . At this time, preferably, pressure is applied up to a level of 150 to 350 bar so as to mix the gas for ensuring a low dielectricity to a supersaturated state. In the case that the pressure range meets the above-mentioned range, it is preferable because the resin mixture and the gas for ensuring a low dielectricity are mixed sufficiently and workability in the processing is obtained.

[37] The mixed resultant including the resin mixture, the heat generating chemical foaming agent, and the gas for ensuring a low dielectricity is foamed (S4). At this time, preferably, the foaming is performed with a degree of foaming of 80 % or more.

[38] And, the foamed product obtained through the step (S4) is extruded through the step

(S5) to surround an upper surface of the inner skin layer in a layered design, thereby forming an insulator layer.

[39] At this time, the insulator layer formed through extrusion of the step (S5) includes foam cells with a closed-cell structure and a long radius of 10 to 500 D. In the case that the long radius meets the above-mentioned range, it is preferable because a closed-cell structure is technically easily realized, and a proper interface dielectricity between the foam cells is secured to maintain a propagation velocity, minimize attenuation, and make an outer diameter of a product and an interval between the foam cells uniform.

[40] Preferably, the insulator layer formed through the step (S5) is extruded with a thickness corresponding to 60 to 170 % of an inside diameter of the inner conductor. In the case that the thickness meets the above-mentioned range, it is preferable because proper capacitance and impedance can be maintained.

[41] A large diameter coaxial cable including the insulator layer of the present invention, which has a degree of foaming of 80 % or more and includes foam cells with a closed- cell structure and a long radius of 10 to 500 D, comprises an inner conductor of a cylindrical shape, an inner skin layer surrounding the inner conductor, an insulator layer surrounding the inner skin layer in a layered design, an outer skin layer surrounding the insulator layer, an outer conductor provided along an outer periphery of the outer skin layer, and a coating layer surrounding the outer conductor completely. Brief Description of the Drawings

[42] FIG. 1 is a cross-sectional view illustrating a large diameter coaxial cable according to an embodiment of the present invention.

[43] FIG. 2 is an SEM photograph showing foam cross section of the coaxial cable ma nufactured according to an embodiment of the present invention.

[44] FIGs. 3 to 5 are SEM photographs showing foam cross sections of conventional coaxial cables manufactured according to comparative examples.

[45] FIG. 6 is a graph illustrating attenuation of the coaxial cable manufactured according to an embodiment of the present invention.

[46] FIG. 7 is a graph illustrating attenuation of the conventional coaxial cable man- ufactured according to the comparative example. Mode for the Invention

[47] Hereinafter, a large diameter coaxial cable of the present invention is described with reference to FIG. 1.

[48] FIG. 1 is a cross-sectional view illustrating the large diameter coaxial cable according to an embodiment of the present invention. As shown in FIG. 1, the large diameter coaxial cable according to an embodiment of the present invention comprises an inner conductor 10, an inner skin layer 20 surrounding the inner conductor 10, an insulator layer 30 surrounding the inner skin layer 20 in a layered design, an outer skin layer 40 surrounding the insulator layer 30, an outer conductor 50 provided along an outer periphery of the outer skin layer 40, and a coating layer 60 surrounding the outer conductor 50 completely. That is, when the large diameter coaxial cable according to an embodiment of the present invention is compared with a large diameter coaxial cable of the same structure, the present invention is characterized by the insulator layer 30 having a degree of foaming of 80 % or more and including foam cells with a closed-cell structure and a long radius of 10 to 500 D.

[49] The inner conductor 10 has a diameter of about 9 to 10 D, and is formed using copper or a copper alloy of about 0.5 D thickness. And, the inner conductor 10 is formed of a hollow cylindrical shape in consideration of characteristics of high frequency.

[50] The inner skin layer 20 is configured to surround the inner conductor 10, and may be made of polyethylene (PE) and so on.

[51] The insulator layer 30 is formed along the outer periphery of the inner skin layer 20 in a layered design for improvement of transmission characteristics. Preferably, the insulator layer 30 is formed with a thickness corresponding to 60 to 166 % of an inside diameter of the inner conductor 10. The description of the insulator layer 30 is made as mentioned above, and thus its detailed description is herein omitted.

[52] The outer skin layer 40 surrounding the insulator layer 30 may be made of polyethylene (PE) and so on.

[53] The outer conductor 50 provided along the outer periphery of the outer skin layer

40 is configured to prevent the loss of electromagnetic waves. The outer conductor 50 is formed in the shape of a metal tube made of copper or a copper alloy to prevent the loss of an electric signal. The outer conductor 50 is contacted with the outer skin layer 40 and the coating layer 60, and is formed of a flexible pipe having surface roughness to prevent changes in characteristics caused by repetitive bending and penetration of the outside moisture.

[54] The coating layer 60 surrounding completely the outer conductor 50 serves as a jacket for protecting the coaxial cable of the present invention.

[55] The coating layer 60 may be made of polyethylene (PE) and so on. [56] [57] Hereinafter, the present invent is described more specifically through a preferred embodiment for understanding of the present invention.

[58] [59] Example 1 [60] A resin mixture, in which 55 weight% of a high density polyethylene (HDPE) and 45 weight% of a low density polyethylene (LDPE) were mixed, was used as a resin for forming an insulator layer, and as a heat generating chemical foaming agent, 200 ppm of azodicarbonamide was added to the resin mixture. As a gas for ensuring a low di- electricity, CO was injected into the mixed resultant, and pressure was applied up to 250 bar.

[61] The mixed resultant was foamed to produce a foamed product, and the foamed product was extruded to form an insulator layer configured to surround in a layered design an upper surface of an inner skin layer surrounding an inner conductor. Next, an outer skin layer, an outer conductor and a coating layer were formed on the insulator layer using a typical method to manufacture a coaxial cable.

[62] [63] Example 2 and Comparative examples 1 and 2 [64] The example 2 and comparative examples 1 and 2 were carried out by the same method as the example 1 on conditions of the following Table 1.

[65] Table 1

[66] The degree of foaming and attenuation (D/D at 2 GHz) were measured using coaxial cables manufactured in the examples 1 and 2 and the comparative examples 1 and 2, and results are shown in the following Table 2.

[67] First, the degree of foaming was measured by measuring the volume of a foamed product, and the attenuation was measured using a network analyzer. [68] Table 2

[69] As shown in the above Table 2, it was found that, in comparison with the comparative examples 1 and 2, the examples 1 and 2 had a degree of foaming of 80 % or more and excellent characteristics in aspect of attenuation.

[70] And, FIGs. 2 and 3 are SEM photographs showing foam cross sections of a coaxial cable manufactured in the example 2 of the present invention and a coaxial cable manufactured in the comparative example 1 or 2, respectively.

[71] As shown in FIG. 2 showing foam cross section of the coaxial cable manufactured in the example 2 of the present invention, foam cells of a uniform closed-cell structure were formed in an insulator layer. It was also found that a coating layer of the same composition was formed at an interface of the foam cells, and the foam cells were continuously formed adjacent to each other.

[72] As shown in FIGs. 3 to 5 showing foam cross sections of the coaxial cables manufactured in the comparative examples, in the case of the comparative example 1, foam cells were formed sparsely and discontinuously (FIG. 3) and foam cells were collapsed due to poor cooling conditions (FIG. 4), and in the case of the comparative example 2, foam cells were extended due to combination of adjacent foam cells (FIG. 5).

[73] Through the above-mentioned results, it was found that the coaxial cables manufactured in the examples 1 and 2 of the present invention could secure a uniform volume of foaming, and unlike the comparative examples 1 and 2, reduce a partial difference in dielectricity caused by collapse or partial conglomeration of foaming to suppress signal deterioration by a high frequency transmission, thereby resulting in a ultrahigh speed transmission.

[74] And, attenuation of the coaxial cables manufactured in the example 2 and the comparative example 2 was measured using a network analyzer at an area of 30 MHz to 3 GHz, and results are shown in FIGs. 6 and 7.

[75] FIG. 6 shows attenuation of the coaxial cable manufactured in the example 2, and it was found that the coaxial cable of the example 2 had the remarkably improved dielectric characteristics by uniform foaming, and thus, an attenuation value was about 5.4 D at 2 GHz and about 6.9 D at 3 GHz.

[76] FIG. 7 shows attenuation of the coaxial cable manufactured in the comparative example 2, and it was found that, as frequency increases, attenuation increased considerably, for example about 6.15 D at 2 GHz and about 8.03 D at 3 GHz.

[77] Through the above-mentioned results, it was found that the attenuation of the coaxial cable of the example 2 was improved by about 10 % on average when compared with that of the comparative example 2.

[78] It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. Industrial Applicability

[79] An insulator of a low loss large diameter coaxial cable according to the present invention is made of an insulating material foamed to a high level, thereby minimizing attenuation and allowing a ultrahigh frequency transmission of GHz bandwidth without signal interference. And, the present invention can manufacture a coaxial cable of uniform foaming characteristics and extra-large size, and thus transmit a signal with ultrahigh speed and large capacity and suppress an abnormal combination of foam cells not to bring about a change in dielectric characteristics between foaming and unfoaming, thereby ensuring good signal characteristics without group delay.

Claims

[1] An insulator for a coaxial cable, wherein the insulator has a degree of foaming of

80 % or more, and includes foam cells with a closed-cell structure and a long radius of 10 to 500 D.

[2] A low loss large diameter coaxial cable, comprising: an inner conductor of a cylindrical shape; an inner skin layer surrounding the inner conductor; an insulator layer surrounding the inner skin layer in a layered design; an outer skin layer surrounding the insulator layer; an outer conductor provided along an outer periphery of the outer skin layer; and a coating layer surrounding the outer conductor completely, wherein the insulator layer has a degree of foaming of 80 % or more, and includes foam cells with a closed-cell structure and a long radius of 10 to 500 D.

[3] The low loss large diameter coaxial cable according to claim 2, wherein the insulator layer is formed with a thickness corresponding to 60 to 170 % of an inside diameter of the inner conductor.

[4] A method for preparing an insulator layer of a low loss large diameter coaxial cable, which includes an inner conductor of a cylindrical shape, an inner skin layer surrounding the inner conductor, an insulator layer surrounding the inner skin layer in a layered design, an outer skin layer surrounding the insulator layer, an outer conductor provided along an outer periphery of the outer skin layer, and a coating layer surrounding the outer conductor completely, the method comprising:

(51) preparing a resin, in which a high density polyethylene (HDPE) and a low density polyethylene (LDPE) are mixed;

(52) adding a chemical foaming agent to the resin mixture;

(53) injecting a gas for ensuring a low dielectricity into the resin mixture;

(54) foaming the prepared resin mixture; and

(S 5) extruding the foamed product to surround an upper surface of the inner skin layer in a layered design. [5] The method for preparing an insulator layer of a low loss large diameter coaxial cable according to claim 4, wherein the resin mixture of the step (Sl) includes 45 to 75 weight% of the high density polyethylene (HDPE) and 25 to 55 weight% of the low density polyethylene (LDPE). [6] The method for preparing an insulator layer of a low loss large diameter coaxial cable according to claim 4, wherein the chemical foaming agent of the step (S2) is any one selected from the group consisting of azodicarbonamide, sulfonylhydrazides and 5-phenyltetrazole, or mixtures thereof. [7] The method for preparing an insulator layer of a low loss large diameter coaxial cable according to claim 6, wherein the chemical foaming agent is added with a content of 150 to 750 ppm based on a whole weight of the resin mixture. [8] The method for preparing an insulator layer of a low loss large diameter coaxial cable according to claim 4, wherein the gas for ensuring a low dielectricity of the step (S3) is carbon dioxide

(CO₂). [9] The method for preparing an insulator layer of a low loss large diameter coaxial cable according to claim 4, wherein the foaming of the step (S4) is performed with a degree of foaming of 80

% or more. [10] The method for preparing an insulator layer of a low loss large diameter coaxial cable according to claim 4, wherein the insulator layer formed through extrusion of the step (S5) has a thickness corresponding to 60 to 170 % of an inside diameter of the inner conductor. [11] The method for preparing an insulator layer of a low loss large diameter coaxial cable according to claim 4, wherein the insulator layer formed through extrusion of the step (S5) includes foam cells with a closed-cell structure and a long radius of 10 to 500 D.