Radio frequency coaxial cable working at super cut-off frequency and preparation method thereof

Abstract

The invention discloses a radio-frequency coaxial cable working at an ultra-cut-off frequency and a preparation method of the radio-frequency coaxial cable. The coaxial cable provided by the invention sequentially comprises an inner conductor, a foaming insulating layer and an outer conductor from inside to outside, the inner conductor and the outer conductor are coaxially nested; the outer conductor is an embossed copper pipe; the foaming insulating layer is made of foaming polyethylene; the foamed polyethylene is manufactured by physical foaming by adopting hexafluorocyclobutane as a foaming agent, a closed-cell structure with the closed-cell rate larger than 90% and the pore size of 10-100 microns is formed, and the outer conductor and the foamed insulating layer are subjected to heat treatment at the temperature of 80-125 DEG C to induce secondary pore growth. According to the invention, by utilizing the principle of secondary growth of bubbles, the phenomena of stress accumulation and nonuniform air holes caused by the fact that bubble tissues of the foaming insulating layer are pressed in the embossing process of the outer conductor are improved, scattering and reflection of signals are reduced, and the cable can still keep excellent standing-wave ratio performance in an adjacent wave band exceeding a cut-off frequency; therefore, the 5G communication requirement of the large-size coaxial cable with the wave band of 2.7 G to 3.3G is met.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communications, and more particularly relates to a radio frequency coaxial cable operating above the cutoff frequency and a preparation method thereof. Background Art

[0002] As a core component of the antenna system, RF coaxial cable is widely used in the field of mobile communications, especially in the transmission of RF signals between base stations and antennas, covering bands such as meter waves, decimeter waves, and centimeter waves. In existing technologies, the operating frequency of this type of cable is limited by its cutoff frequency. The cutoff frequency of coaxial cable with traditional specifications is usually around 2.7GHz. When the operating frequency exceeds this limit, the high-order harmonic effect will cause the standing wave ratio and transmission loss performance to deteriorate sharply, making the cable unable to be effectively used in the high frequency band. This limitation has seriously restricted the development of modern mobile communications, especially in the context of the growing demand for high-frequency signal transmission in 5G networks. Current solutions mainly rely on small-sized cables to increase the operating frequency, but the small size design introduces the new problem of increased transmission loss, affecting communication coverage and signal quality.

[0003] Early RF coaxial cable designs focused on optimizing conductor materials and structures to improve basic performance, such as improving signal integrity by adjusting the thickness of the foam insulation layer or the conductor layout. As communication frequency bands expand to higher ranges, the industry has attempted to reduce cable size to break through the cutoff frequency limit, which has, to a certain extent, increased the upper limit of the operating frequency. However, small-sized cables increase resistance due to the reduction in conductor cross-sectional area, resulting in a significant increase in transmission loss. Although this evolution solved the frequency problem, it sacrificed loss performance, forming an irreconcilable contradiction between size, frequency, and loss. During the development process, although improvements in material science and process have alleviated some defects, they have failed to fundamentally balance the high-frequency demand and low-loss requirements, highlighting the technical bottleneck.

[0004] The root of the problem lies in the inability of existing cable designs to achieve the triple goals of large size, high frequency, and low loss. Large cables are limited by their cutoff frequency and cannot support the high frequency bands required by 5G. Small cables, while capable of higher frequencies, suffer from increased transmission losses, which weaken signal strength and coverage. This contradiction is exacerbated by the rapid development of modern mobile communications. 5G networks require cables to operate beyond the cutoff frequency while maintaining low standing wave and low loss at larger sizes. The limitations of existing foam insulation layer structures and foaming processes, such as uneven pores or insufficient closed-cell ratios, further amplify signal scattering and reflection issues, leading to degraded performance. Therefore, innovative solutions are urgently needed to overcome these inherent shortcomings and meet the stringent requirements of high-frequency communications. Summary of the Invention

[0005] In response to the above defects or improvement needs of the prior art, the present invention provides a radio frequency coaxial cable operating above the cutoff frequency and a preparation method thereof, the purpose of which is to use hexafluorocyclobutane, which has good solubility in polyethylene melt, as a foaming agent for primary foaming to form pores with high closed-cell rate and high dimensional consistency. At the same time, hexafluorocyclobutane is adsorbed and fixed on the pore walls to form corrugation of the outer conductor, and then the pores are allowed to grow again through heat treatment to eliminate the uneven pores and stress accumulation caused by the corrugation process of the outer conductor, reduce signal scattering and reflection, and still operate normally in the near bands exceeding the cutoff frequency, thereby solving the technical problem of existing coaxial cables, especially large-size coaxial cables, in which the transmission loss in the high-frequency band exceeds the cutoff frequency and is unable to meet the requirements of 5G communications.

[0006] To achieve the above object, according to one aspect of the present invention, there is provided a radio frequency coaxial cable operating above the cutoff frequency, comprising, from the inside out, an inner conductor, a foamed insulating layer, and an outer conductor; the inner conductor and the outer conductor are coaxially nested;

[0007] The outer conductor is a corrugated copper tube;

[0008] The foam insulation layer is made of foamed polyethylene;

[0009] The foamed polyethylene is made by physical foaming using hexafluorocyclobutane as a foaming agent to form a closed-cell structure with a closed-cell ratio greater than 90% and a pore size of 10-100 μm, and the outer conductor and the foamed insulating layer are heat-treated at 80° C. to 125° C. to induce secondary pore growth.

[0010] Preferably, in the radio frequency coaxial cable operating at super-cutoff frequency, the hexafluorocyclobutane configuration is 1,1,2,2,3,4-hexafluorocyclobutane.

[0011] Preferably, in the RF coaxial cable operating at super-cutoff frequency, talcum powder is added as a nucleating agent during the foaming process, with an addition amount of 0.3-1wt%, so that the pore size is stabilized at 80-100 μm and the foaming degree is between 78% and 85%.

[0012] Preferably, the radio frequency coaxial cable operating at super-cutoff frequency has a specification of 15 / 8 inches and above, and the outer conductor is a corrugated copper tube with a corrugation depth of 1-3 mm.

[0013] Preferably, the operating frequency range of the super-cutoff frequency radio frequency coaxial cable covers 2.7 GHz to 3.3 GHz.

[0014] According to another aspect of the present invention, there is provided a method for preparing the super-cutoff frequency radio frequency coaxial cable, which comprises the following steps:

[0015] (1) Hexafluorocyclobutane is used as a foaming agent and mixed with polyethylene melt for physical foaming to form an insulating extrudate;

[0016] (2) Under a protective atmosphere, the insulating extrudate obtained in step (1) is extruded onto the outside of the inner conductor to form a pre-foamed layer having a closed-cell structure with a closed-cell ratio greater than 90% and a pore size of 10-100 μm;

[0017] (3) The copper strips are longitudinally wrapped on the outside of the pre-foamed layer and welded to form a copper tube and corrugated on the outside of the copper tube to make an outer conductor corrugated copper tube;

[0018] (4) heat-treating the corrugated intermediate at 80°C to 125°C for 10-20 minutes to allow the pre-foamed layer to undergo secondary foaming to form a foamed insulation layer;

[0019] (5) Extruding the sheath material on the outside of the outer conductor corrugated copper tube to form an outer sheath.

[0020] Preferably, in the method for preparing the radio frequency coaxial cable operating above the cutoff frequency, the polyethylene melt in step (1) is a mixture of HDPE and LDPE in a ratio of 7:3, wherein the polyethylene melt contains 0.3-1wt% of a nucleating agent, and the nucleating agent is a mixture of talc and polyvinyl acetate in a mass ratio of 1:1.

[0021] Preferably, in the method for preparing the radio frequency coaxial cable operating above the cutoff frequency, the protective atmosphere in step (2) is nitrogen; the extrusion pressure of the insulating extrudate is 350-400 Bar, and the temperature is controlled at 180°C-240°C.

[0022] Preferably, in the method for preparing the radio frequency coaxial cable operating above the cutoff frequency, step (2) comprises extruding the insulating extrudate in a vertical state and cooling it to room temperature.

[0023] Preferably, in the method for preparing the radio frequency coaxial cable operating above the cutoff frequency, the corrugation depth in step (3) is between 2 and 4 mm.

[0024] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects compared with the prior art:

[0025] The present invention provides a radio frequency coaxial cable operating at a super-cutoff frequency and a preparation method thereof. Hexafluorocyclobutane is used as a foaming agent. Since hexafluorocyclobutane has good solubility in polyethylene melt, it physically penetrates the polyethylene melt, activates C-F bonds and reacts with free radicals, effectively increasing the closed-cell ratio of the foamed polyethylene to over 90%. In addition, small hexafluorocyclobutane molecules (with a diameter of approximately 0.5 nm) can penetrate into the interchain spaces of the polyethylene, so that the hexafluorocyclobutane is not only present in the foamed pores but also adsorbed and fixed in the polyethylene pore walls. When the foamed polyethylene material is heated again, the hexafluorocyclobutane adsorbed and fixed by the polyethylene volatilizes, and the pore structure of the foamed polyethylene is again changed in the closed and softened bubbles, resulting in secondary bubble growth. The present invention utilizes the principle of secondary growth of bubbles to improve the stress accumulation and uneven pores caused by the pressure on the bubble tissue of the foamed insulation layer during the corrugation of the outer conductor, reduce signal scattering and reflection, and enable the cable to maintain excellent standing wave ratio performance in adjacent bands exceeding the cutoff frequency, thereby meeting the 5G communication requirements of large-size coaxial cables in the 2.7G to 3.3G bands. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a voltage standing wave ratio test diagram of the radio frequency coaxial cable operating above the cutoff frequency provided in Example 1 at frequencies above the cutoff frequency;

[0027] Figure 2 This is a voltage standing wave ratio test diagram of the radio frequency coaxial cable operating above the cutoff frequency provided in Example 2 at frequencies above the cutoff frequency;

[0028] Figure 3 This is a voltage standing wave ratio test diagram of the radio frequency coaxial cable operating above the cutoff frequency provided in Example 3 at frequencies above the cutoff frequency. DETAILED DESCRIPTION

[0029] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0030] The radio frequency coaxial cable provided by the present invention operates at a frequency exceeding the cutoff frequency, and comprises, from the inside out, an inner conductor, a foamed insulation layer, and an outer conductor; the inner conductor and the outer conductor are coaxially nested; the present invention is aimed at coaxial cables of 15 / 8 inches and above (larger sizes), and by improving the closed-cell rate and uniformity of the foamed material of the foamed insulation layer, the transmission loss of high-frequency signals below the cutoff wavelength is reduced, thereby covering the operating frequency range of 2.7 GHz to 3.3 GHz and meeting the 5G communication requirements of large-size coaxial cables.

[0031] The outer conductor is a corrugated copper tube; the corrugation depth of the outer conductor is 1-3mm. The corrugation of the outer conductor is an important reason for the deterioration of the foaming uniformity of the foamed insulation layer. During production, the foaming material of the foamed insulation layer is relatively uniform. During the corrugation process of the outer conductor after cooling, axial non-uniform stress is introduced into the foamed insulation layer, resulting in an insurmountable deterioration of the uniformity of the foamed insulation layer in subsequent processing. The present invention creatively adopts a secondary pore growth process to redistribute the stress of the foamed insulation layer after corrugating the outer conductor, thereby improving the uniformity of the foamed insulation layer, reducing signal scattering and reflection, and reducing transmission loss.

[0032] The foamed insulating layer is made of foamed polyethylene; the foamed polyethylene is physically foamed using hexafluorocyclobutane as a foaming agent, and the configuration of the hexafluorocyclobutane is 1,1,2,2,3,4-hexafluorocyclobutane.

[0033] The foamed polyethylene forms a closed-cell structure with a closed-cell ratio greater than 90% and a pore size of 10-100 μm, and the outer conductor and foamed insulation layer are heat-treated at 80°C to 125°C to induce secondary pore growth. Talc is added as a nucleating agent during the foaming process in an amount of 0.3-1wt% to stabilize the pore size at 80-100 μm. The amount of nucleating agent added is usually 1-3wt%. Since the foaming agent uses hexafluorocyclobutane with better solubility, the amount of nucleating agent is reduced to maintain the foaming degree between 78% and 85%, keeping the pores stable and facilitating the growth of secondary pores.

[0034] Compared with traditional nitrogen or carbon dioxide foaming agents, the present invention adopts hexafluorocyclobutane as a foaming agent, which has better compatibility with polyethylene matrix materials. Polyethylene is a common material for the insulation layer of coaxial cables, and its chemical structure and physical properties have a crucial impact on the foaming effect. The unique molecular structure of hexafluorocyclobutane enables it to form a closer interaction with the polyethylene matrix material at the molecular level. This good compatibility lays the foundation for the success of the foaming process. By using hexafluorocyclobutane as a foaming agent, during the growth process of foaming pores, the formation and growth of pores can be completed without relying on higher temperature and pressure conditions, and the obtained foaming pores are fine, uniform, and closed. Hexafluorocyclobutane is used for foaming, and the closed-cell rate reaches 90%. Part of the hexafluorocyclobutane is dissolved in the closed pore walls. When the pore walls are heated and softened, the dissolved hexafluorocyclobutane is released into the closed pores, achieving secondary pore growth. This reduces the stress accumulation in the insulation layer caused by corrugation of the outer conductor, improves the uniformity of the pores, and ultimately alleviates the problem of abruptly enhanced signal scattering and reflection exceeding the cutoff frequency band, providing communication capabilities in the super-cutoff frequency band.

[0035] The method for preparing a radio frequency coaxial cable operating above the cutoff frequency provided by the present invention comprises the following steps:

[0036] (1) Hexafluorocyclobutane is used as a foaming agent and is mixed with polyethylene melt for physical foaming to form an insulating extrudate; the polyethylene melt is a mixture of HDPE and LDPE in a ratio of 7:3, which contains 1-3wt% of a nucleating agent, and the nucleating agent is a mixture of talc and polyvinyl acetate in a mass ratio of 1:1.

[0037] The foaming agent is specifically: 1,1,2,2,3,4-hexafluorocyclobutane.

[0038] (2) Under a protective atmosphere, the insulating extrudate obtained in step (1) is extruded onto the outside of the inner conductor to form a pre-foamed layer having a closed-cell structure with a closed-cell ratio greater than 90% and a pore size of 10-100 μm. The protective atmosphere is preferably nitrogen. The extrusion pressure of the insulating extrudate is 350-400 bar, and the temperature is controlled at 180°C-240°C. The insulating extrudate is extruded in a vertical state and cooled to room temperature.

[0039] High temperature and high pressure promote the dissolution of hexafluorocyclobutane in molten PE, intensify the movement of PE molecular chains, expand the amorphous region, and allow small hexafluorocyclobutane molecules (about 0.5 nm in diameter) to penetrate into the gaps between PE chains, so that hexafluorocyclobutane not only exists in the pores after foaming, but is also adsorbed and fixed on the PE pore walls.

[0040] (3) The copper strips are longitudinally wrapped on the outside of the pre-foamed layer and welded to form a copper tube and corrugated on the outside of the copper tube to make an outer conductor corrugated copper tube; the corrugation depth is between 2-4 mm;

[0041] (4) heat-treating the corrugated intermediate at 80°C to 125°C for 10-20 minutes to allow the pre-foamed layer to undergo secondary foaming to form a foamed insulation layer;

[0042] (5) Extruding the sheath material on the outside of the outer conductor corrugated copper tube to form an outer sheath.

[0043] This solution heat treats the intermediate formed by the outer conductor being coated with a foam layer and corrugated. In the manufacturing process of coaxial cables, corrugating the outer conductor after coating it with a foam layer is a common process step, but the corrugation process will inevitably cause the pores of the foam layer to deform, thereby affecting its structure and performance. Through subsequent heat treatment, at the microscopic level, on the one hand, the hexafluorocyclobutane dissolved in the pore wall is released under the action of heat, providing a continuous source of gas for the growth of the pores; on the other hand, the pores grow driven by temperature, allowing the pores that were originally deformed by corrugation to be readjusted and optimized. In this process, the structure of the foam layer is further adjusted and improved, thereby effectively reducing the possible delamination, slippage, dislocation and other undesirable phenomena between the outer conductor and the foam layer, enhancing the bonding strength and integrity between the two, and improving the mechanical stability and reliability of the coaxial cable.

[0044] The following are examples:

[0045] The super-cutoff frequency RF coaxial cable provided in this embodiment is a 50Ω corrugated copper tube outer conductor RF coaxial cable for wireless communication, with specification code 32, and is a 15 / 8-inch coaxial cable. The structure, from the inside out, includes an inner conductor, a foam insulation layer, and an outer conductor outer sheath. The inner and outer conductors are coaxially nested, and the design parameters are as follows:

[0046] The inner conductor is a smooth copper tube with an outer diameter of 13.10 mm; the outer conductor is an annular corrugated copper tube with an outer diameter of 35.80 mm and a corrugation depth of 1.8 mm.

[0047] The foamed insulating layer is made of foamed polyethylene and is produced by physical foaming using hexafluorocyclobutane as a foaming agent. It has a closed-cell structure with a closed-cell rate greater than 90% and a pore size of 80-100 μm.

[0048] The manufacturing method of the radio frequency coaxial cable operating above the cutoff frequency provided in this embodiment is as follows:

[0049] (1) 1,1,2,2,3,4-hexafluorocyclobutane is used as a foaming agent and is added to a polyethylene melt for physical foaming to form an insulating extrudate; the polyethylene melt is a mixture of HDPE and LDPE in a ratio of 7:3, which contains 0.3-1wt% of a nucleating agent, and the nucleating agent is a mixture of talc and polyvinyl acetate in a mass ratio of 1:1.

[0050] (2) Under a protective atmosphere, extrude the insulating extrudate obtained in step (1) onto the outside of the inner conductor to form a closed-cell ratio greater than 90%. The protective atmosphere is nitrogen. The extrusion pressure of the insulating extrudate is 350-400 bar, and the temperature is controlled at 180°C-240°C. Extrude the insulating extrudate in a vertical state and cool it to room temperature.

[0051] (3) The copper strips are longitudinally wrapped on the outside of the pre-foamed layer and welded to form a copper tube. The outer side of the copper tube is corrugated to make an outer conductor corrugated copper tube. The corrugation depth is 3 mm.

[0052] (4) heat-treating the corrugated intermediate at 80°C to 125°C for 10-20 minutes to allow the pre-foamed layer to undergo secondary foaming to form a foamed insulation layer;

[0053] (5) Extruding the sheath material on the outside of the outer conductor corrugated copper tube to form an outer sheath.

[0054] The specific parameters are as follows

[0055] Example 1 Example 2 Example 3 Nucleating agent addition amount 0.3wt% 0.5wt% 1wt% Extrusion pressure 400Bar 400Bar 350 Bar Extrusion temperature 220℃ 200℃ 200℃ Closed-cell rate 91% 93% 93% Pore ​​size 98μm 85μm 82μm Heat treatment temperature 80℃ 105℃ 125℃ Heat treatment time 20min 15min 10min Foaming degree 85% 83% 78%

[0056] After heat treatment, the outer conductor bellows is stretched and the corrugation depth is deeper than when it was formed. Therefore, this part of the shrinkage is reserved during the early corrugation.

[0057] The coaxial cables made in Examples 1 to 3 were tested for standing wave ratio at 2.7-3.3 GHz. The results are as follows: Figures 1-3 As shown, the standing wave ratio of the coaxial cables produced in Examples 1 to 3 is maintained at a low level in the band near the cutoff frequency, meeting the 5G communication requirements.

[0058] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A radio frequency coaxial cable operating above the cutoff frequency, characterized in that: From the inside out, it includes an inner conductor, a foamed insulating layer and an outer conductor; the inner conductor and the outer conductor are coaxially nested; The outer conductor is a corrugated copper tube; The foam insulation layer is made of foamed polyethylene; The foamed polyethylene is made by physical foaming using hexafluorocyclobutane as a foaming agent to form a closed-cell structure with a closed-cell ratio greater than 90% and a pore size of 10-100 μm, and the outer conductor and the foamed insulating layer are heat-treated at 80° C. to 125° C. to induce secondary pore growth.

2. The radio frequency coaxial cable operating above the cutoff frequency as claimed in claim 1, characterized in that: The configuration of the hexafluorocyclobutane is 1,1,2,2,3,4-hexafluorocyclobutane.

3. The radio frequency coaxial cable operating above the cutoff frequency as claimed in claim 1, characterized in that: Talc is added as a nucleating agent during the foaming process, with an addition amount of 0.3-1wt%, to stabilize the pore size at 80-100μm and the foaming degree between 78% and 85%.

4. The radio frequency coaxial cable operating above the cutoff frequency as claimed in claim 1, characterized in that: The coaxial cable has a specification of 15 / 8 inches or above, and the outer conductor is a corrugated copper tube with a corrugation depth of 1-3 mm.

5. The radio frequency coaxial cable operating above the cutoff frequency as claimed in claim 4, characterized in that: The operating frequency range covers 2.7GHz~3.3GHz.

6. The method for preparing a radio frequency coaxial cable operating above the cutoff frequency according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) Hexafluorocyclobutane is used as a foaming agent and mixed with polyethylene melt for physical foaming to form an insulating extrudate; (2) Under a protective atmosphere, the insulating extrudate obtained in step (1) is extruded onto the outside of the inner conductor to form a pre-foamed layer having a closed-cell structure with a closed-cell ratio greater than 90% and a pore size of 10-100 μm; (3) The copper strips are longitudinally wrapped on the outside of the pre-foamed layer and welded to form a copper tube and corrugated on the outside of the copper tube to make an outer conductor corrugated copper tube; (4) heat-treating the corrugated intermediate at 80°C to 125°C for 10-20 minutes to allow the pre-foamed layer to undergo secondary foaming to form a foamed insulation layer; (5) Extruding sheath material on the outside of the outer conductor corrugated copper tube to form an outer sheath.

7. The method for preparing a radio frequency coaxial cable operating above the cutoff frequency according to claim 6, wherein: The polyethylene melt in step (1) is a mixture of HDPE and LDPE in a ratio of 7:3, wherein the polyethylene melt contains 0.3-1 wt% of a nucleating agent, and the nucleating agent is a mixture of talc and polyvinyl acetate in a mass ratio of 1:

1.

8. The method for preparing a radio frequency coaxial cable operating above the cutoff frequency according to claim 6, wherein: The protective atmosphere of step (2) is nitrogen; the extrusion pressure of the insulating extrudate is 350~400Bar, and the temperature is controlled at 180℃~240℃.

9. The method for preparing a radio frequency coaxial cable operating above the cutoff frequency according to claim 6, wherein: Step (2) extrude the insulating extrudate in a vertical state and cool it to room temperature.

10. The method for preparing a radio frequency coaxial cable operating above the cutoff frequency according to claim 6, wherein: Step (3) The embossing depth is between 2-4 mm.

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