Coaxial cable knowledge explanation

Coaxial cable refers to a cable with two concentric conductors, and the conductor and shielding layer share the same axis. The most common coaxial cable is composed of central conductor, insulating material layer, shielding layer composed of mesh fabric and external isolation material layer
Application of coaxial cable
Coaxial cable has a wide range of applications. Coaxial cable can transmit analog signals and digital signals in a low loss way, which is suitable for various applications, including TV broadcasting system, long-distance telephone transmission system, short-distance jumper between computer systems, local area network interconnection, etc.
Coaxial cables are divided into two types according to different impedance values, 50 Ω and 75 Ω, of which 50 Ω is used for digital signal transmission and 75 Ω signal is used for video signal transmission.
As a means of transmitting TV signals to thousands of households, coaxial cable has developed rapidly, which is cable TV network. A cable TV system can load dozens or even hundreds of TV channels, and its transmission range can reach dozens of kilometers.


Explanation of terms related to coaxial cable
Attenuation (insertion loss): attenuation refers to the part of energy lost in the transmission process, usually using “db/ length” as the calculation unit, for example: 31.0db/100inch. Attenuation will increase with the increase of frequency;
Bending radius: the minimum radius that the cable can reach after being bent without affecting any performance;
Central conductor: refers to the conductor at the center of the coaxial line, which can be single strand or multiple strands. The conductor diameter is measured by “AWG (American wire gauge)”;
Coaxial adapter: equipment used to convert connectors of different models or male and female connectors (such as BNC to SMA adapter);
Coaxial connector: the connecting device located at both ends of the coaxial cable assembly. The commonly used coaxial connectors have various types, such as BNC, SMA, SMB, f;


Insulating layer: insulating material, which separates the central conductor from the shielding layer;
Electromagnetic interference: electric energy or electromagnetic energy will cause damage to electronic signals;
Impedance: the resistance to current in an AC or converter circuit, in ohms. The two common impedance values in the coaxial line are 50 ohms and 75 ohms, which are used for data and video signal transmission respectively;
Insertion loss: the attenuation value measured before and after the system output is connected to the cable and equipment;
Radio frequency: the frequency range from 3MHz to 300GHZ is called “radio frequency”, which is mainly used for wireless and TV signal transmission;
Shielding layer: a conductive coating made of wire braid or metal foil, which wraps the insulating layer and central conductor. Common types of shielding layers and shielding effectiveness: 1 Single layer braided layer shielding (95% coverage) 2 Single layer braided layer shielding (60%) + aluminum foil winding (100%);
Propagation speed: usually expressed in percentage, it refers to the ratio of the propagation speed of electric energy to the speed of light in a certain length of cable;
VSWR: the ratio between the maximum and minimum measured effective voltage in the RF transmission line. The ratio usually increases with the increase of frequency. The larger the ratio, the greater the capacity loss and the lower the efficiency.
Performance of coaxial cable

  1. Shielding characteristics
    Shielding characteristic is an important parameter to measure the anti-interference ability of coaxial cable, and it is also an important parameter to measure the leakage prevention of coaxial cable. Poor shielding will affect the quality of the signal. Shielding performance mainly depends on the structure of the outer conductor of the cable, and solid-state shielding is the best, which is common in rigid or semi-rigid coaxial cables.
  2. Characteristic impedance
    Where: R is the resistance per unit length;
    L is the inductance per unit length;
    G is the conductivity per unit length;
    J is the imaginary part of a complex number;
    C is the capacitance per unit length.
    According to the above formula, the characteristic impedance changes with the change of frequency f. assuming that the inner and outer conductors are ideal conductors, R and G are ignored, so:
    That is, the characteristic impedance is independent of frequency and completely depends on the inductance and capacitance of the cable.
  3. The inductance and capacitance are related to the conductor material, the medium between the inner and outer conductors and the diameter of the inner and outer conductors, which is expressed by the formula:
    Where ε Is the relative dielectric constant of the insulator, which varies with the density of the material type. D is the inner diameter of the outer conductor, and D is the outer diameter of the outer conductor.
  4. In order to ensure the best transmission effect, the terminal load impedance should be equal to the characteristic impedance of the cable as far as possible.
  5. Attenuation characteristics
    The attenuation characteristic of coaxial cable is usually expressed by attenuation constant, that is, the decibel of signal attenuation of cable per unit length (such as 100m). The attenuation of signal transmission in coaxial cable is related to the size, dielectric constant and working frequency of coaxial cable.
    Where: F is the transmission signal frequency;
    Z is the characteristic impedance;
    K is a constant determined by the diameter, conductivity and shape of the inner and outer conductors;
    C is small and can be ignored.
    That is, the attenuation constant is proportional to the square root of the signal operating frequency. The higher the frequency, the greater the attenuation constant. The lower the frequency, the smaller the attenuation constant.
  6. Temperature coefficient
    The temperature coefficient indicates the influence of temperature change on cable characteristics. As the temperature increases, the cable loss increases, and as the temperature decreases, the cable loss decreases
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