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Introduction of coaxial cable

Aug 07, 2020

The definition of coaxial cable

Coaxial Cable is sometimes called RF Cable. Coaxial RF Cable is the most commonly used structure.Due to the concentric position of the internal and external conductors, the electromagnetic energy is limited to the propagation in the medium between the internal and external conductors, so it has significant advantages such as small attenuation, high shielding performance, wide frequency band and stable performance.Usually used to transmit rf energy from 500 kilohertz to 18 gigahertz.Coaxial cable has two concentric conductors, and the conductor and shielding layer share the same shaft.The most common coaxial cable consists of a copper conductor insulated by an insulating material, with another ring conductor and its insulator on the outside of the inner layer, and then the whole cable is wrapped in a polyvinyl chloride or teflon sheath.Coaxial cable of the points from the purpose can be divided into 50 Ω baseband cable and coaxial cable of the radio frequency of 75 Ω broadband cable.The baseband cable is divided into fine coaxial cable and coarse coaxial cable.Baseband cables are only used for digital transmission with data rates up to 10Mbps.50 Ω characteristic impedance rf coaxial cable is mainly used for baseband signal transmission, the transmission bandwidth of 1 ~ 20 MHZ, 75 Ω characteristic impedance rf coaxial cable used in CATV network, so called CATV cable, transmission bandwidth can reach 1 GHZ, current CATV cable transmission bandwidth of 750 MHZ.

The structure and material of coaxial cable 

The most important electrical properties of cables are low attenuation, uniform impedance and high return loss, and the best coupling loss is also very important for leakage cables.The main function of a cable is to transmit signals, so it is important to ensure that the cable structure and materials have good transmission characteristics throughout the life of the cable.

1. Inner conductor

Copper, the main material of the inner conductor, may be in the following forms: annealed copper wire, annealed copper tube, copper-coated aluminum wire.Usually, the inner conductor of small cables is copper wire or copper-clad aluminum wire, while large cables are made of copper tubes to reduce the weight and cost of the cable.The outer conductor of large cable is furrowed so that good enough bending properties can be obtained.

The inner conductor has great influence on signal transmission because the attenuation is mainly caused by the resistance loss of the inner conductor.Its conductivity, especially the surface conductivity, should be as high as possible, the general requirement is 58MS/m (+20℃), 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 skin depth.Table 1 shows the skin depth values at specific frequencies when copper tubes and copper-clad aluminum wires are used as inner conductors.

The inner conductor copper material quality requirement is very high, the requirement copper material should be free of impurity, the surface is clean, level off, smooth.The inner conductor diameter should be stable with small tolerance.Any change in diameter reduces impedance uniformity and return loss, so the manufacturing process should be controlled precisely.

2. Outer conductor

The outer conductor has two basic functions: the first is the function of the loop conductor, and the second is the shielding function.The outer conductor of the leakage cable also determines its leakage performance.The outer conductor of coaxial feeders and superflexible cables is welded with rolled copper tubes. The outer conductor of these cables is completely enclosed and does not allow any radiation from the cables.

The outer conductor is usually covered longitudinally by copper tape.The outer conductor layer is provided with a longitudinal or transverse notch or aperture.

Grooving of outer conductor is common in rolled - type cable.The rolling crest is formed by equidistance cutting and grooving along the axial direction.The truncated part accounts for a very small proportion, and the slot spacing is far less than the length of the transmitted electromagnetic wave.

It is obvious that the leakage cable can be made by processing the non-leakage cable in the following way: cutting the outer conductor crest of the common wrinkle-type cable at an Angle of 120 degrees to obtain a set of suitable slot structure.The shape, width and slot structure of the leakage cable determine its performance.

The outer conductor of copper material should also be of good quality, high conductivity, no impurities.The outer conductor size should be strictly controlled within the tolerance range to ensure uniform characteristic impedance and high return loss.

3. Insulating medium

The medium of rf coaxial cable is not only used for insulation, the final transmission performance is determined after insulation, so the choice of medium material and its structure is very important.All important properties, such as attenuation, impedance, and return loss, are strongly related to insulation.The most important requirements for insulation are:

The dielectric constant is low and the dielectric loss Angle factor is small to ensure the attenuation is small.

Consistent structure to ensure uniform impedance and large return loss;

Mechanical stability to ensure a long life;

Waterproof and moistureproof.

Physical high foam insulation can meet all the above requirements.With advanced extrusion and gas injection technology and special materials, the foaming degree can reach more than 80%, which is similar to the electrical performance of air insulated cable.In the gas injection method, nitrogen is directly injected into the medium material in the extruder, which is also known as the physical foaming method.With this opposite chemical foaming method, its foaming degree can only reach about 50%, the medium loss is large.The foam structure obtained by gas injection method is consistent, which means its impedance is uniform and return loss is large.

Our RF cables have very good electrical properties due to small dielectric loss Angle and large foaming degree of insulating materials.Foaming media characteristics are more important at high frequency. It is this special foaming structure that determines the very low attenuation performance at high frequency.

The unique multi-layer insulation (inner thin layer - foaming layer - outer thin layer) co-extrusion process can obtain uniform, closed foaming structure, with stable mechanical properties, high strength and good moisture-proof properties.In order to make the cable maintain good electrical performance in a humid environment, we specially designed a cable: add a thin layer of solid core PE on the outer surface of the foam insulation layer.This thin outer layer can effectively prevent moisture intrusion and protect the electrical properties of the cable from the beginning of production. This design is particularly important for leaky cables with holes in the outer conductor.In addition, the insulation layer is tightly wrapped on the inner conductor through the inner thin layer, which further improves the mechanical stability of the cable.Moreover, the thin layer contains special stabilizer to ensure the compatibility with copper and the long service life of our cable.Satisfactory properties, such as moisture resistance, adhesion and stability, can be obtained by selecting suitable inner thin layer materials.

This multi-layer insulation design (inner thin layer - foaming layer - outer thin layer) provides both excellent electrical and stable mechanical properties, thus improving the long service life and reliability of our RF cables.

4, Outer jacket

The most commonly used sheath material for outdoor cables is black linear low-density polyethylene, which has a density similar to LDPE but a strength comparable to HDPE.Instead, in some cases, we prefer HDPE, which provides better mechanical properties and resistance to friction, chemistry, moisture and different environmental conditions.

The uV-resistant black HDPE can withstand climate stresses such as extremely high temperatures and extremely high uv levels.When emphasizing the fire safety of cables, low-smoke halogen-free flame retardant materials should be used.In leakage cables, to reduce the spread of fire, a fireproof band can be used between the outer conductor and sheath, so that the insulation layer which is easy to melt can be retained in the cable.