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Jin Wanbang coaxial cable structure and use of materials The most important electrical performance of coaxial cable is low attenuation, uniform impedance and high return loss. The key point for leakage cable is its optimal coupling loss. The main role of coaxial cable is to transmit signals. Therefore, it is very important to ensure that the cable structure and material have good transmission characteristics throughout the life of the cable.
1. Inner conductor of coaxial cable Copper is the main material of the inner conductor and can be in the following forms: Annealed copper wire, annealed copper tube, copper clad aluminum wire. In general, the small cable inner conductor is a copper wire or copper clad aluminum wire, while the large cable uses a copper tube to reduce the cable weight and cost. Emboss the large cable outer conductor so that good enough bending performance is obtained.
The inner conductor has a great influence on the signal transmission because the attenuation is mainly caused by the loss of the inner conductor resistance. The conductivity, especially the surface conductivity, should be as high as possible. The general requirement is 58MS/m (+20°C), because at high frequencies, the current is only transmitted in a thin layer on the conductor surface. This phenomenon is called Skin effect, the effective thickness of the current layer is called the skin depth. Table 1 shows skin depth values ​​at a specific frequency when copper pipes and copper clad aluminum wires are used as inner conductors.
The quality of the copper used for the inner conductor is very high. The copper should be free from impurities and the surface should be clean, smooth and smooth. The diameter of the inner conductor should be stable and the tolerance is small. Any change in diameter will reduce the impedance uniformity and return loss, so the manufacturing process should be precisely controlled.
2. The outer conductor of the outer conductor of the coaxial cable has two basic functions: the first is the effect of the return conductor and the second is the shielding effect. Leakage of the outer conductor of the cable also determines its leakage performance. The outer conductors of the coaxial feeder cable and the super flexible cable are welded by embossed copper tubes. The outer conductors of these cables are completely closed, and the cables are not allowed to have any radiation.
The outer conductor is usually covered by a copper tape longitudinally. On the outer conductor layer, longitudinal or transverse slots or holes are opened.
Slotting of the outer conductor is more common in crimped cables. The grooving is formed by equidistant cutting of the ridge peaks in the axial direction. The proportion of cut-offs is small, and the slot spacing is much smaller than the transmitted electromagnetic wavelength.
Obviously, leakage cables can be made by processing non-leakage cables in the following manner: The peaks of the outer conductors of common corrugated cables, which are common in non-leaking cables, are cut at an angle of 120 degrees to obtain a suitable set of slot structures. The shape, width, and slot structure of the leaky cable determine its performance.
The copper used for the outer conductor should also be of good quality, high conductivity and free of impurities. Outer conductor dimensions should be tightly controlled within tolerances to ensure uniform characteristic impedance and high return loss.
3, coaxial cable dielectric RF coaxial cable media is far more than just from the insulation, the final transmission performance is mainly determined after the insulation, so the choice of dielectric materials and its structure is very important. All important properties, such as attenuation, impedance, and return loss, are very much related to insulation. The most important requirements for insulation are: low relative dielectric constant, small dielectric loss factor, to ensure low attenuation, consistent structure, to ensure uniform impedance, large return loss, stable mechanical properties to ensure long life.
Waterproof and moisture-proof physical high foam insulation can meet all the above requirements. With advanced extrusion and gas injection processes and special materials, the degree of foaming can reach more than 80%. Such electrical performance is close to air-insulated cables. In the gas injection method, nitrogen gas is directly injected into the medium material in the extruder, and this process is also called a physical foaming method. In contrast to the chemical foaming method, the foaming degree can only reach about 50%, and the dielectric loss is large. The foaming structure obtained by the gas injection method is uniform, which means that the impedance is uniform and the return loss is large.
The coaxial cable produced by Anhui Wanbang has very good electrical performance due to its small dielectric loss angle and high foaming degree. The characteristics of the foaming medium are more important at high frequencies. It is this special foaming structure that determines the very low attenuation performance of the cable at high frequencies.
The unique multi-layer insulation (internal thin layer-foamed layer-outer thin layer) co-extrusion process can obtain a uniform and closed foamed structure, featuring stable mechanical properties, high strength, and good moisture resistance. In order to maintain good electrical performance of the cable in a wet environment, we have specifically designed a cable that adds a thin layer of solid PE to the outer surface of the foam insulation. This outer thin layer can effectively prevent moisture intrusion and protect the electrical performance of the cable from the beginning of production. This design is particularly important for leaky cables with open holes in the outer conductor. In addition, the insulating layer is tightly wrapped on the inner conductor through the inner thin layer, further improving the mechanical stability of the cable. Moreover, the thin layer contains a special stabilizer, which can ensure compatibility with copper, but also to ensure the long-term service life of our cable. Choosing the right internal thin layer material can completely achieve satisfactory performance, such as: moisture, adhesion and stability.
This multi-layer insulation design (inner thin layer - foamed layer - outer thin layer) can simultaneously achieve excellent electrical performance and stable mechanical properties, thereby increasing the long-term service life of our coaxial cable.
4. Coaxial cable sheathing The most commonly used sheathing material for outdoor cables is black linear low density polyethylene, which is similar in density to LDPE but comparable in strength to HDPE. In contrast, in some cases, we prefer to use HDPE, which can provide better mechanical properties for the sheath and resistance to friction, chemicals, moisture, and different environmental conditions.
UV-resistant black HDPE can withstand weather stress caused by extremely high temperatures and extreme UV rays. When emphasis is placed on the fire safety of cables, low smoke, halogen-free flame retardant materials should be used. In the leaky cable, in order to reduce the spread of fire, a fire retardant tape may be used between the outer conductor and the sheath to keep the easily meltable insulating layer in the cable. Let the cable work properly.
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