A low-loss leaky coaxial cable and its manufacturing method

Abstract

The present invention belongs to the field of wireless communication, and more specifically, relates to a low-loss leaky coaxial cable and a manufacturing method thereof. The leaky coaxial cable includes an inner conductor, a composite insulation layer, an outer conductor, and a sheath layer coaxially arranged in sequence, and each layer forms a concentric cylindrical structure centered on the axis. By roughening the outer surface of the inner conductor and the outer surface of the outer conductor of the leaky coaxial cable respectively, the contact between the inner conductor and the insulation layer and the contact between the outer conductor and the sheath layer are improved. At the same time, the solid inner skin layer originally used for bonding the inner conductor and the insulation layer is omitted, which not only improves the signal transmission performance but also reduces the transmission loss. Compared with existing products, the leaky coaxial cable with the inner and outer conductor surfaces treated in the present invention optimizes the attenuation index by 2.6% to 5.8% in the multi-frequency band from 900 MHz to 2700 MHz on the premise of meeting the requirements of the adhesion between the insulation layer and the inner conductor, the standing wave, and the insulation dielectric strength test.

Description

Technical Field

[0001] The present invention belongs to the field of wireless communication, and more specifically, relates to a low-loss leaky coaxial cable and a manufacturing method thereof. Background Art

[0002] A leaky coaxial cable radiates to the outside world and receives electromagnetic wave signals from the outside world by providing openings on its outer conductor, thus having the dual functions of a signal transmission line and a transceiver antenna. It is mainly used in occasions where radio signal propagation is poor, such as tunnels, mine shafts, subways, underground buildings, mountainous areas and other environments.

[0003] Existing leaky coaxial cables are usually composed of an inner conductor, an insulating layer, an outer conductor and a sheath layer. The inner conductor is usually made of high-purity copper or silver and is responsible for transmitting radio frequency signals; the insulating layer is located between the inner conductor and the outer conductor and is used to isolate the two and support the structure. The outer conductor is used to transmit signals and shield external signals, and openings for signal leakage are provided on the outer conductor.

[0004] The product structure of conventional leaky coaxial cables complies with the industry standard YD / T 2491 "Communication Cable - Radiating Leaky Coaxial Cable with Physical Foamed Polyethylene Insulation and Longitudinally Wrapped Copper Tape Outer Conductor". Its insulating layer is usually divided into three layers. The solid thin insulating layers near the inner and outer conductors of the cable are mainly made of low-density polyethylene (LDPE) or high-density polyethylene (HDPE), and ethylene-vinyl acetate (EVA) resin with a bonding effect is incorporated; the middle foam insulating layer is thicker, and its material composition is mainly a mixture of LDPE and HDPE with a nucleating agent incorporated, and is made by a physical foaming continuous extrusion process. This kind of insulating structure can better balance the requirements in terms of attenuation, cost and production efficiency. However, there are problems of a large dielectric constant of the solid insulating layer and too high cable transmission attenuation in the existing leaky coaxial cables. The commonly used insulating layer between the inner and outer conductors is a polyolefin physical foaming structure, and its dielectric constant is about 1.25. However, the physical foaming layer cannot bond well with the surface of the inner conductor, which will cause the insulating layer to separate from the inner conductor, resulting in a change in the cable structure and affecting normal transmission. The outer conductor and the sheath layer also have poor bonding due to being two completely different materials; in addition, when the cable is in a long-term energized state, poor heat dissipation performance will also affect its service life.

[0005] To solve the above problems, the usual method is to first extrude a solid inner skin layer on the surface of the inner conductor, and then extrude a foaming layer on the inner skin layer. The function of the solid inner skin layer is to bond the inner conductor and the foamed polyethylene layer. However, since the inner skin layer is generally a mixed material of solid polyethylene and glue EVA, the comprehensive dielectric constant is about 2.4 (data at 1 MHz). Compared with the dielectric constant of 1.25 of the physical foaming layer, the presence of the inner skin layer greatly increases the transmission loss of the cable.

[0006] Therefore, there is an urgent need for an improved leaky coaxial cable structure to solve the above problems. SUMMARY OF THE INVENTION

[0007] Aiming at the defects of the prior art, the purpose of the present invention is to provide a low-loss leaky coaxial cable and its manufacturing method. By moderately roughening the surfaces of the inner conductor and the outer conductor respectively, the contact between the inner conductor and the insulating layer, and between the outer conductor and the sheath layer is improved, and the solid inner skin layer is omitted, thereby reducing the cable loss and enhancing the signal transmission performance, and solving the technical problems such as poor contact between the inner conductor and the insulating layer, and between the outer conductor and the sheath layer, and high transmission loss in the prior art.

[0008] To achieve the above purpose, the present invention provides a low-loss leaky coaxial cable, which includes an inner conductor, a composite insulating layer, an outer conductor, and a sheath layer coaxially arranged in sequence, and each layer forms a concentric cylindrical structure centered on the axis;

[0009] The composite insulating layer includes a foamed resin layer and a solid outer skin layer; wherein, the outer surface of the inner conductor is in direct contact with the foamed resin layer, and the solid outer skin layer is in direct contact with the inner surface of the outer conductor;

[0010] The outer surface of the inner conductor in direct contact with the foamed resin layer has a microscopic rough structure formed by roughening treatment, and its roughness is 0.5 μm to 5.0 μm;

[0011] The outer conductor is provided with a plurality of openings for allowing signal leakage; and the outer surface of the outer conductor also has a microscopic rough structure formed by roughening treatment, and its roughness is 5 μm to 20 μm.

[0012] Preferably, the roughness of the outer surface of the inner conductor is 1 μm to 3 μm.

[0013] Preferably, the surface roughening treatment methods of the inner conductor and the outer conductor are independently laser roughening, chemical etching, mechanical polishing or electrochemical treatment.

[0014] According to another aspect of the present invention, a method for preparing the leaky coaxial cable is provided, including the following steps:

[0015] (1) Perform surface roughening treatment on the outer surface of the inner conductor to control its roughness within the range of 0.5 μm to 5.0 μm, and obtain the roughened inner conductor;

[0016] (2) Prepare the foamed resin layer and the solid outer skin layer on the outer surface of the roughened inner conductor by a double-layer co-extrusion process;

[0017] (3) The outer surface of the outer conductor is subjected to surface roughening treatment so that its roughness is controlled within the range of 5 μm to 20 μm, and the roughened outer conductor is obtained;

[0018] (4) The roughened outer conductor is longitudinally wrapped outside the solid outer skin layer;

[0019] (5) The sheath layer is prepared on the outer surface of the outer conductor by an extrusion process.

[0020] Generally speaking, compared with the prior art, the above technical solutions conceived by the present invention mainly have the following technical advantages:

[0021] (1) By roughening the outer surface of the inner conductor of the leaky coaxial cable, the contact between the inner conductor and the insulating layer is significantly improved. At the same time, the solid inner skin layer originally used for bonding the inner conductor and the insulating layer material is omitted, reducing the transmission loss of the cable. At the same time, the outer surface of the outer conductor is also roughened, which not only enhances the adhesion between the outer conductor and the sheath layer, but also helps the cable to dissipate heat. Experimental tests show that under the same other process conditions, when the surfaces of the inner conductor and the outer conductor of the leaky coaxial cable are moderately roughened and the inner skin layer is omitted, the overall attenuation index is reduced by 2.6% - 5.8% compared with the prior art without roughening and with an inner skin layer.

[0022] (2) The optimized cable structure has better signal transmission performance and anti-interference ability, and is suitable for high-frequency and high-speed communication scenarios.

[0023] (3) The surface roughness of the inner conductor is controlled within the range of 0.5 μm to 5.0 μm, and the surface roughness of the outer conductor is controlled between 5 μm and 20 μm, which can not only ensure the contact, but also avoid signal reflection and increased loss, and can also improve its heat dissipation performance.

[0024] (4) The improved manufacturing method of the leaky coaxial cable of the present invention is simple and easy to implement, and is suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic end face structure diagram of the leaky coaxial cable of Comparative Example 1 of the present invention.

[0026] Figure 2 It is a schematic side structure diagram of the leaky coaxial cable of Comparative Example 1 of the present invention.

[0027] Figure 3 It is a schematic end face structure diagram of the leaky coaxial cable of Embodiment 1 of the present invention.

[0028] Figure 4 It is a schematic side structure diagram of the leaky coaxial cable of Embodiment 1 of the present invention.

[0029] In all the drawings, the same reference numerals are used to denote the same elements or structures, where: 1 - inner conductor; 2 - composite insulating layer; 3 - outer conductor; 4 - sheath layer; 5 - opening; 6 - identification structure; 21 - solid inner skin layer; 22 - foamed resin layer; 23 - solid outer skin layer. Detailed implementation manners

[0030] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0031] The embodiments of the present invention are implemented on the premise of the technical solutions of the present invention, and detailed implementation manners and processes are given. However, the protection scope of the present invention is not limited to the following embodiments. For the process parameters without specific conditions noted in the following embodiments, they are usually in accordance with conventional conditions.

[0032] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0033] The "inner", "outer", "inner layer", "outer layer" mentioned in the present invention are defined relative to the central axis of the leaky coaxial cable, where "inner" refers to the side close to the central axis, and "outer" refers to the side far from the central axis.

[0034] The leaky coaxial cables of the prior art, such as Figure 1 and Figure 2As shown, it includes an inner conductor 1, a composite insulation layer 2, an outer conductor 3, and a sheath layer 4 that are coaxially arranged in sequence. Each layer forms a concentric cylindrical structure centered on the axis. The composite insulation layer is composed of a solid inner skin layer 21, a foamed resin layer 22, and a solid outer skin layer 23; the solid inner skin layer 21 and the solid outer skin layer 23 are usually solid polyethylene layers. Usually, a resin with a bonding effect is incorporated into the solid inner skin layer for bonding the metal inner conductor and the thermoplastic foamed polyolefin resin insulation layer. However, incorporating the bonding resin often results in a high dielectric constant of the solid inner skin layer, which increases the transmission loss of the cable. Moreover, directly bonding the metal surface of the inner conductor to the thermoplastic resin results in poor contact between the two completely different materials, which is also likely to cause a decline in the electrical performance of the cable. To address these technical defects, the present invention provides a low-loss leaky coaxial cable, such as Figure 3 and Figure 4 As shown, it includes an inner conductor 1, a composite insulation layer 2, an outer conductor 3, and a sheath layer 4 that are coaxially arranged in sequence. Each layer forms a concentric cylindrical structure centered on the axis. The composite insulation layer is composed of a foamed resin layer 22 and a solid outer skin layer 23. Compared with the above prior art, its solid inner skin layer 21 is omitted; the outer surface of the inner conductor 1 is in direct contact with the foamed resin layer 22, and the solid outer skin layer 23 is in direct contact with the inner surface of the outer conductor 3. The outer surface of the inner conductor in direct contact with the foamed resin layer has a microscopic rough structure formed by roughening treatment, and its roughness (Ra) ranges from 0.5 μm to 5.0 μm. The outer conductor 3 is provided with several openings 5 for allowing signal leakage. Correspondingly, an identification structure 6 is provided on the sheath layer 4 on the other side of the side where the openings are located, facilitating the identification of the position where the outer conductor openings 5 are located during installation. Usually, the identification structure 6 is an outer conductor slot identification line arranged along the length direction of the sheath layer, and the center of a single slot identification line is at the 180° position of the slot center, and the center of a double-slot identification line is at the 180° position of the slot center. Moreover, the outer surface of the outer conductor also has a microscopic rough structure formed by roughening treatment, and its roughness is from 5 μm to 20 μm.

[0035] The present invention provides a leaky coaxial cable that improves the contact between the inner conductor and the insulating layer, the contact between the outer conductor and the sheath layer, and omits the inner skin layer by improving the surface structures of the inner conductor and the outer conductor. When the leaky coaxial cable is operating, signals are transmitted between the outer surface of the inner conductor and the inner surface of the outer conductor. Therefore, traditional leaky coaxial cables require the surfaces of the inner conductor and the outer conductor to be smooth to prevent reflections caused by surface defects of the conductors and affect the use. However, experiments have found that when the outer surface of the inner conductor is moderately roughened, it actually does not affect the electrical performance of the product within the standard range. Moreover, the roughened outer surface of the inner conductor also improves its adhesion to the foamed polyolefin layer insulating layer. At the same time, since the contact between the inner conductor and the foamed insulating layer is increased by roughening the outer surface of the inner conductor, the original solid inner skin layer structure can be directly omitted. Omitting this structure simultaneously avoids the transmission loss of the original inner skin layer to the cable, so the transmission loss of the coaxial cable can be further reduced. The present invention also roughens the outer surface of the outer conductor at the same time. Experiments have proved that the roughened outer surface of the outer conductor does not affect the transmission of signals, and its roughness can be greater than that of the outer surface of the inner conductor. The purpose of roughening the outer surface of the outer conductor in the present invention is to improve its adhesion to the sheath layer while improving the heat dissipation of the cable, and further improve the service life of the cable.

[0036] The roughness (Ra) of the inner conductor surface ranges from 0.5 μm to 3.0 μm, and a better range is from 1.0 μm to 3.0 μm. This roughness range can ensure good contact between the inner conductor and the insulating layer, and avoid signal reflection and increased loss caused by excessive roughness. Experiments have found that compared with the inner conductor, the surface roughness of the outer conductor has basically no effect on the electrical performance of the leaky coaxial cable within the range of 5μm - 20μm. The outer surface of the outer conductor is not involved in signal transmission, and its roughness range can be controlled within the range of 5 μm to 20 μm.

[0037] The surface roughening treatment method of the inner conductor and the outer conductor of the present invention can adopt laser roughening, chemical etching, mechanical polishing or electrochemical treatment to form a uniform concave-convex structure on the surfaces of the inner conductor and the outer conductor, increasing the contact area with the insulating layer / sheath layer. However, it is necessary to strictly control the roughening degree of the outer surface of the inner conductor to avoid affecting signal transmission due to size fluctuations on the conductor surface caused by excessive roughening, or causing electric breakdown due to the occurrence of electric field concentration effects caused by sharp metal edges generated by excessive roughening.

[0038] The outer surface of the inner conductor and the outer surface of the outer conductor of the present invention are both roughened to form a microscopic rough structure. It can be understood that the present application does not limit the shape or material type of the inner conductor and the outer conductor itself, and the types of inner conductors and outer conductors reported in the prior art are all applicable to the present application. In some embodiments, the above-mentioned inner conductor can be copper-clad aluminum wire, spiral corrugated copper tube, smooth copper tube, etc., and in some other embodiments, high-purity silver material can also be used. The above-mentioned outer conductor can be a spiral corrugated metal outer conductor, an annular corrugated metal outer conductor or a wire braided outer conductor. In some embodiments, the above-mentioned metal inner conductor and outer conductor are both copper conductors.

[0039] The insulating layer between the inner conductor and the outer conductor of the leaky coaxial cable of the present invention omits the original solid inner skin layer that plays a bonding role for the inner conductor and the foamed insulating layer compared with the prior art. The remaining foamed resin layer and solid outer skin layer can also adopt the insulating layer materials reported in the prior art. For example, the foamed resin layer can be foamed polyolefin, such as foamed polyethylene, etc., with a foaming degree of 40% - 85% and a thickness of 3mm - 15mm. The solid outer skin layer is solid polyolefin, such as low-density polyethylene LDPE (density range is 0.910 g / cm³ - 0.930 g / cm³), or high-density polyethylene HDPE (density range is 0.940 g / cm³ - 0.976 g / cm³); the thickness of the solid outer skin layer is the same as that of the solid outer skin layer in the prior art, generally 0.05mm - 0.15mm.

[0040] In some embodiments, the openings on the outer conductor are linear holes, oval holes, U-shaped holes, L-shaped holes or combined holes.

[0041] In some embodiments, the above-mentioned sheath layer is a polyolefin sheath, such as polyethylene resin, polypropylene resin, polystyrene resin, etc., and the thickness of the sheath layer is 0.3mm - 4mm.

[0042] The improved leaky coaxial cable of the present invention has a composite insulating layer composed of a foamed resin layer and a solid outer skin layer, omitting the traditional solid inner skin layer. Through the surface roughening treatment of the inner conductor and the outer conductor, the inner conductor is in direct contact with the foamed resin layer, significantly improving the contact property and reducing the signal transmission loss; the sheath layer is in direct contact with the roughened outer conductor surface. Understandably, it can also optimize the contact property to a certain extent and can significantly improve the heat dissipation performance of the outer conductor.

[0043] The present invention also provides a manufacturing method of a leaky coaxial cable, including the following steps:

[0044] (1) Perform surface roughening treatment on the outer surface of the inner conductor to control its roughness within the range of 0.5 μm to 5.0 μm to obtain the roughened inner conductor;

[0045] (2) A foamed resin layer and a solid outer skin layer are prepared by double-layer co-extrusion on the outer surface of the roughened inner conductor;

[0046] (3) The outer surface of the outer conductor is subjected to surface roughening treatment to control its roughness within the range of 5 μm to 20 μm, obtaining a roughened outer conductor;

[0047] (4) The roughened outer conductor is longitudinally wrapped outside the solid outer skin layer;

[0048] (5) The sheath layer is prepared on the outer surface of the outer conductor by an extrusion process.

[0049] The roughening treatments in steps (1) and (3) of the present invention are not limited to specific roughening methods. For example, they can each independently be laser roughening, chemical etching, mechanical polishing or electrochemical treatment. The preferred roughening method is the laser surface roughening process. Laser roughening can ensure a high degree of consistency in the treatment process, and is easy to perform online synchronous treatment, facilitating popularization. Different roughness inner conductor and outer conductor surfaces can be obtained by adjusting the roughening process parameters.

[0050] In some embodiments, the outer surface of the inner conductor in step (1) is roughened by chemical etching. The etching solution is a dilute nitric acid solution with a concentration of 0.1 mol / L - 2 mol / L, and the etching time is 1 minute - 30 minutes, forming a uniform micro-rough structure with the roughness controlled within the range of 0.5 μm to 5.0 μm. In other embodiments, the outer surface of the outer conductor in step (3) is roughened by chemical etching. The etching solution is a dilute nitric acid solution with a concentration of 0.1 mol / L - 2 mol / L, and the etching time is 5 minutes - 60 minutes, forming a uniform micro-rough structure with the roughness controlled within the range of 5.0 μm to 20.0 μm.

[0051] In some other embodiments, a laser is used to process the surface of the inner conductor in step (1). Specifically, the laser power is controlled between 10 W and 300 W, the scanning speed is between 600 mm / s and 1200 mm / s, the scanning pitch is between 0.05 mm and 0.12 mm, and the roughness is preferably controlled within the range of 1.0 μm to 3.0 μm. A laser can also be used to roughen the outer surface of the outer conductor in step (3). Specifically, the laser power is controlled between 100 W and 450 W, the scanning speed is between 600 mm / s and 1200 mm / s, the scanning pitch is between 0.05 mm and 0.25 mm, and the roughness is preferably controlled within the range of 5.0 μm to 20.0 μm. The laser treatment can use common lasers, such as solid-state lasers (365 nm solid-state lasers or 1064 nm solid-state lasers), fiber lasers, gas lasers (carbon dioxide lasers or copper vapor lasers), or semiconductor lasers. Preferably, a fiber laser is used.

[0052] In some embodiments, the inner conductor after surface roughening is a preheated inner conductor. Generally, the surface preheating temperature is controlled between 80 °C and 120 °C according to different products, so that the foamed layer can better adhere to the conductor surface.

[0053] In some embodiments, step (2) is specifically as follows: The molten polyolefin and the foaming gas are mixed evenly to form an aerosol, and the aerosol and the molten polyolefin for preparing the solid outer skin, such as polyethylene, are co-extruded on the surface of the roughened inner conductor to form a foamed resin layer and a solid outer skin covering the outside of the foamed resin layer. The extrusion temperature is 130 °C to 210 °C to ensure that the foamed polyolefin material is tightly combined with the inner conductor to form a foamed resin layer.

[0054] The outer surface of the outer conductor of the present invention is subjected to surface roughening treatment to control its roughness within the range of 5 μm to 20 μm to obtain the roughened outer conductor; then the roughened outer conductor is longitudinally wrapped outside the solid outer skin; finally, a sheath layer is extruded on the surface of the outer conductor to complete the manufacture of the cable.

[0055] The following are examples and comparative examples:

[0056] Comparative Example 1

[0057] The leaky coaxial cable provided in this comparative example is applied to a traditional cable with the model HLRCTYZ-50-32 in the YD / T 2491 standard (the cable specification is -32), and its schematic end face structure diagram is as Figure 1 shown, and the schematic side structure diagram is as Figure 2As shown. From the inside to the outside, an inner conductor 1, a composite insulation layer 2, an outer conductor 3 and a sheath layer 4 are coaxially nested in sequence, wherein the composite insulation layer 2 includes a solid inner skin layer 21, a foamed resin layer 22 and a solid outer skin layer 23 from the inside to the outside, wherein the inner conductor 1 is a solid copper-clad aluminum wire, the solid inner skin layer 21 and the solid outer skin layer 23 are low-density polyethylene, the foamed resin layer 22 is foamed polyethylene, the outer conductor 3 is a copper strip welded corrugated tube, the sheath layer 4 is polyethylene, the outer conductor 3 is provided with an opening 5 for allowing signal leakage, the sheath layer 4 is provided with an identification structure 6, the identification structure 6 is two identification lines arranged along the length direction of the sheath layer, and the center of the two identification lines is 180° at the center of the opening. The preparation method of the leaky coaxial cable of this comparative example is carried out according to the following steps:

[0058] (1) Low-density polyethylene and glue (EVA resin) are mixed in a ratio of 3:1 and extruded at 130-220°C, and evenly coated on the surface of the inner conductor of a smooth copper tube with a diameter of 13.1 mm to form a bonded solid inner skin layer with a thickness of 0.15 mm.

[0059] (2) High-density polyethylene, low-density polyethylene and nucleating agent (a mixture of cyclodextrin nanosponge (NS) and vinyl triethoxysilane (VTES), mass ratio 1:1) were mixed in a mass ratio of 70:29:1, and after high-temperature plasticization at 130-195°C, high-pressure gas (carbon dioxide) was injected into the molten state. After being fully mixed in the screw, they were extruded at high pressure and attached to the inner skin layer for foaming and cooling to form an insulating foamed cable core with a diameter of 32.3 mm and a foaming degree of 80%.

[0060] (3) At the same time as the foaming layer in the previous step, a low-density polyethylene solid outer layer with a thickness of about 0.1 mm is extruded, which is evenly coated on the periphery of the foaming layer, so that the insulation layer has better sealing properties.

[0061] (4) Use a copper strip with a thickness of 0.1 mm and pre-grooved holes to longitudinally wrap the copper tube through a forming mold. At the same time, insert the cable core into the longitudinally wrapped copper tube and directly enter the sheath machine head. A layer of low-smoke halogen-free flame-retardant sheath material with a thickness of 2 mm is evenly extruded on the outside of the copper tube to form a finished sheath. The outer diameter of the sheath is 37.1 mm.

[0062] Comparative Example 2

[0063] The other aspects are the same as those of Comparative Example 1, except that the outer surface of the outer conductor of Comparative Example 1 is subjected to surface laser roughening treatment in this comparative example, and the specific steps are as follows:

[0064] Steps (1), (2) and (3) are the same as those in Comparative Example 1.

[0065] Step (4): The outer surface of a copper strip with a thickness of 0.1 mm that has been pre-etched with slots is subjected to laser roughening treatment using a fiber laser. The laser power is 320 W, the scanning speed is 800 mm / s, the scanning pitch is 0.08 mm, and the surface roughness is controlled at 10 μm to obtain an outer conductor with a roughened outer surface.

[0066] (5) The outer conductor with a roughened outer surface is longitudinally wrapped around a copper tube through a forming die. At the same time, a cable core is inserted into the longitudinally wrapped copper tube and then directly enters the sheath head. A layer of low-smoke, halogen-free, flame-retardant sheath material with a thickness of 2 mm is evenly extruded outside the copper tube to form a finished sheath, and the outer diameter of the sheath is 37.1 mm.

[0067] Comparative Example 3

[0068] Other conditions are the same as those in Comparative Example 2, except that the laser roughening process parameters are controlled so that the roughness of the outer surface of the outer conductor after laser roughening is 20 μm.

[0069] Example 1

[0070] The leaky coaxial cable provided in this example is applied to a traditional cable with the model HLRCTYZ-50-32 in the YD / T 2491 standard (the cable specification is -32). The schematic diagram of its end face structure is as Figure 3 shown, and the schematic diagram of its side structure is as Figure 4 shown. From the inside to the outside, there are coaxially nested an inner conductor 1, a composite insulation layer 2, an outer conductor 3, and a sheath layer 4 in sequence. Among them, the composite insulation layer 2 includes a foamed resin layer 22 and a solid outer skin layer 23 from the inside to the outside. The outer surface of the inner conductor 1 is in direct contact with the inner surface of the foamed resin layer, and the outer surface of the solid outer skin layer 23 is in direct contact with the inner surface of the outer conductor 3. The inner conductor 1 is a solid copper-clad aluminum wire, the foamed resin layer 22 is foamed polyethylene, the solid outer skin layer 23 is low-density polyethylene, the outer conductor 3 is a copper strip welded corrugated tube, the sheath layer 4 is polyethylene, there are openings 5 on the outer conductor 3 for allowing signal leakage, and there is a marking structure 6 on the sheath layer 4. The marking structure 6 is two marking lines arranged along the length direction of the sheath layer, and the centers of the two marking lines are at the 180° position of the opening center. The preparation method of the leaky coaxial cable in this example is carried out according to the following steps:

[0071] (1) The outer surface of a smooth copper tube with a diameter of 13.1 mm is subjected to laser roughening treatment using a fiber laser. The laser power is 60 W, the scanning speed is 1000 mm / s, the scanning pitch is 0.08 mm, and the surface roughness is controlled at 2 μm. Then, a preheating treatment is carried out, and the temperature before entering the foaming head is 80 - 120 °C.

[0072] (2) High-density polyethylene, low-density polyethylene and nucleating agent (a mixture of cyclodextrin nanosponge (NS) and vinyl triethoxysilane (VTES), mass ratio 1:1) were mixed in a mass ratio of 70:29:1, and after high-temperature plasticization at 130-195°C, high-pressure gas (carbon dioxide) was injected into the molten state. After being fully mixed in the screw, the mixture was directly attached to the inner conductor with roughened surface by high-pressure extrusion for foaming and cooling to form an insulating foamed cable core with a diameter of 32.3 mm and a foaming degree of 80%.

[0073] (3) At the same time as the foaming layer in the previous step, a low-density polyethylene solid outer layer with a thickness of about 0.1 mm is extruded, which is evenly coated on the periphery of the foaming layer, so that the insulation layer has better sealing properties.

[0074] (4) The outer surface of the copper strip with a thickness of 0.1 mm and grooves engraved in advance was roughened by a fiber laser. The laser power was 320 W, the scanning speed was 800 mm / s, the scanning interval was 0.08 mm, and the surface roughness was controlled at 10 μm to obtain an outer conductor with a roughened outer surface.

[0075] (5) The outer conductor with roughened outer surface is longitudinally wrapped in a copper tube through a forming mold. At the same time, the cable core is inserted into the longitudinally wrapped copper tube and directly enters the sheath machine head. A layer of low-smoke halogen-free flame-retardant sheath material with a thickness of 2 mm is evenly extruded on the outside of the copper tube to form a finished sheath. The outer diameter of the sheath is 37.1 mm.

[0076] Example 2

[0077] The other aspects are the same as those of Example 1, except that the laser roughening process parameters are controlled so that the roughness of the inner conductor surface after roughening treatment is 3 μm.

[0078] Example 3

[0079] The other aspects are the same as those of Example 1, except that the laser roughening process parameters are controlled so that the roughness of the inner conductor surface after laser roughening is 4 μm.

[0080] Example 4

[0081] The rest is the same as in Example 1, except that the laser roughening process parameters are controlled so that the roughness of the inner conductor surface after laser roughening is 5 μm.

[0082] Comparative Example 4

[0083] The other aspects are the same as those of Example 1, except that the laser roughening process parameters are controlled so that the roughness of the inner conductor surface after laser roughening is 0.3 μm.

[0084] Comparative Example 5

[0085] Other aspects are the same as in Example 1, except that the laser roughening process parameters are controlled so that the roughness of the inner conductor surface after laser roughening is 6 μm.

[0086] Comparative Example 6

[0087] Other aspects are the same as in Example 1, except that the laser roughening process parameters are controlled so that the roughness of the inner conductor surface after laser roughening is 8 μm.

[0088] The leaky coaxial cables of the above comparative examples and examples were subjected to verification tests according to the test methods in the YD / T 2491 standard. The test items were: a. Adhesion test between the insulation layer and the inner conductor (considering the influence of the roughening process on the structural stability of the product); b. Standing wave test (considering the influence of the roughening process on the standing wave index); c. Insulation dielectric strength test (considering whether the surface roughness after inner conductor roughening will produce metal sharp corners resulting in local electric field concentration, thus causing abnormal insulation withstand voltage index); d. Loss / attenuation test (considering the influence of the roughening process on the cable attenuation after removing the inner skin layer). The measured results are shown in Table 1 (" / " in Table 1 indicates that no roughening treatment was carried out).

[0089] Table 1

[0090]

[0091] Based on Comparative Example 1, in Comparative Example 2 and Comparative Example 3, only the outer conductor surface was roughened with roughnesses of 10 μm and 20 μm respectively. From the data in Table 1, it can be seen that only roughening the outer surface of the outer conductor, the electrical properties of the leaky coaxial cable are almost the same as those of Comparative Example 1 without roughening, indicating that roughening the outer conductor surface of the leaky coaxial cable does not affect the electrical properties of the cable. Understandably, roughening the outer conductor can improve its adhesion to the sheath layer and contribute to the heat dissipation of the outer conductor.

[0092] From the test data of test item d in Table 1, it can be seen that compared with Comparative Example 1, in Examples 1 to 4, with the same other process, the outer surface of the outer conductor was roughened to the same degree (roughness of 10 μm), and at the same time, the inner conductor surface was roughened to different degrees (roughness from 2 μm to 5 μm), and after removing the inner skin layer, the attenuation index was overall optimized (decreased) by 2.6% - 5.8%.

[0093] As can be seen from the test data of test item a in Table 1, the index of the adhesion between the insulating layer and the inner conductor increases with the increase of the surface roughness of the inner conductor. When the surface roughness of the inner conductor drops to 0.3 μm (Comparative Example 4), the adhesion between the conductor and the insulating layer cannot meet the normal requirements. Moreover, when the adhesion index is unqualified, it may indirectly lead to the unqualified standing wave index of test item b due to the unstable structure between the insulating layer and the inner conductor.

[0094] As can be seen from the test data of test item c in Table 1, when the surface roughness of the inner conductor exceeds 5 μm (Comparative Example 5 and Comparative Example 6), the electric field concentration effect caused by the sharp corners on the metal surface may cause the failure of the insulation dielectric strength test. At the same time, according to the experimental data, when the roughening degree of the outer surface of the inner conductor is too high, the insulation dielectric strength test fails while the standing wave test meets the standard; the insulation dielectric strength index has problems prior to the standing wave index. Therefore, the influence of the standing wave is not considered as a priority, which also indicates that as long as the surface roughness of the inner conductor is controlled within a suitable range, the standing wave test is usually qualified.

[0095] From the above test data, it can be known that the feasible surface roughness after roughening the outer surface of the inner conductor of the leaky coaxial cable is 0.5 μm to 5 μm, and the better roughness range is 1 μm to 3 μm; the surface roughness of the outer conductor can be controlled within 5 μm - 20 μm, and preferably 10 μm - 20 μm.

[0096] It is easy for those skilled in the art to understand that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A low-loss leaky coaxial cable, characterized in that: The cable comprises an inner conductor, a composite insulating layer, an outer conductor and a sheath layer which are coaxially arranged in sequence, and each layer forms a concentric cylindrical structure with the axis as the center; The composite insulating layer is composed of a foamed resin layer and a solid outer skin layer; wherein the outer surface of the inner conductor is in direct contact with the foamed resin layer, and the solid outer skin layer is in direct contact with the inner surface of the outer conductor; The material of the foamed resin layer is foamed polyolefin; the foaming degree of the foamed polyolefin is 40%-85%; the thickness of the foamed resin layer is 3mm-15mm; The solid outer skin layer is solid polyolefin; the thickness of the solid outer skin layer is 0.05mm~0.15mm; The outer surface of the inner conductor directly in contact with the foamed resin layer has a microscopic rough structure formed by roughening treatment, and the roughness thereof is 0.5 μm to 5.0 μm; The inner conductor is a copper-clad aluminum wire or a smooth copper tube; The outer conductor is provided with a plurality of openings for allowing signal leakage; and the outer surface of the outer conductor also has a microscopic rough structure formed by roughening treatment, and the roughness thereof is 5 μm to 20 μm.

2. The leaky coaxial cable according to claim 1, characterized in that The roughness of the outer surface of the inner conductor is 1 μm to 3 μm.

3. The leaky coaxial cable according to claim 1, characterized in that: The surface roughening treatment methods of the inner conductor and the outer conductor are independently laser roughening, chemical etching, mechanical grinding or electrochemical treatment.

4. The leaky coaxial cable according to claim 1, characterized in that: The opening on the outer conductor is a straight hole, an elliptical hole, a U-shaped hole, an L-shaped hole or a combined hole.

5. A method for preparing a leaky coaxial cable according to any one of claims 1 to 4, characterized in that: The steps include: (1) performing a surface roughening treatment on the outer surface of the inner conductor so that the roughness thereof is controlled within a range of 0.5 μm to 5.0 μm, thereby obtaining a roughened inner conductor; (2) preparing the foamed resin layer and the solid outer skin layer on the outer surface of the inner conductor after the roughening treatment by a double-layer co-extrusion process; (3) performing a surface roughening treatment on the outer surface of the outer conductor so that the roughness thereof is controlled within a range of 5 μm to 20 μm, thereby obtaining a roughened outer conductor; (4) longitudinally wrapping the roughened outer conductor outside the solid outer skin layer; (5) The sheath layer is prepared on the outer surface of the outer conductor by an extrusion process.

6. The method according to claim 5, characterized in that The roughening treatments in step (1) and step (3) are independently laser roughening, chemical etching, mechanical grinding or electrochemical treatment.

7. The method according to claim 6, characterized in that When step (1) uses laser roughening to perform the outer surface roughening treatment of the inner conductor, the laser power is 10W-300W, the scanning speed is between 600mm / s and 1200mm / s, and the scanning interval is between 0.05mm and 0.12mm; and / or, When step (3) uses laser roughening to perform the outer surface roughening treatment of the outer conductor, the laser power is 100W-450W, the scanning speed is between 600mm / s and 1200mm / s, and the scanning interval is between 0.05mm and 0.25mm.

8. The method according to claim 5, characterized in that Step (2) specifically comprises: uniformly mixing molten polyolefin and foaming gas to form an aerosol, and performing double-layer co-extrusion of the aerosol and the molten polyolefin material for preparing the solid outer skin layer on the outer surface of the inner conductor after the roughening treatment to form the foamed resin layer and the solid outer skin layer coated on the outside of the foamed resin layer.

9. The method according to claim 5, characterized in that The inner conductor in step (1) is an inner conductor that has been preheated at a temperature of 80°C-120°C.

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