1. Characteristic impedance, the main body of the coaxial cable is composed of inner and outer conductors. There are resistance and inductance for the current flowing in the conductors, and conductance and capacitance for the voltage between the conductors. These characteristics are distributed along the line. , known as the distribution constant. Due to the influence of the uneven dimensional accuracy and the purity of the dielectric material in manufacturing, in the cable TV system, although the characteristic impedance of the coaxial cable required to be used is 75Ω, the characteristic impedance of the coaxial cable usually used is (75±5)Ω. Therefore, in order to prevent the reflection of signal energy and achieve the best transmission effect, the terminal load impedance should also be equal to the characteristic impedance of the cable as much as possible.
2. Attenuation characteristics, the attenuation characteristics of coaxial cables are usually expressed by attenuation constants. The attenuation constant is proportional to the mean square root of the operating frequency of the signal, that is, the higher the frequency, the greater the attenuation constant, and the lower the frequency, the smaller the attenuation constant.
3. The service life of the cable is limited. Any cable has a certain lifespan. After the cable is used for a period of time, due to the aging of the material, the resistance of the conductor increases, and the leakage current of the insulating medium increases. When the attenuation constant of the cable increases compared with the nominal value, The cable should be updated. Generally, the life of the cable is 7-20 years depending on the quality and application.
4. Temperature coefficient, the temperature coefficient indicates the degree of influence of temperature changes on the characteristics of the cable, the temperature increases, the loss of the cable increases, and the temperature decreases, the loss of the cable decreases. In practical work, the measures to eliminate the influence of temperature change on the system are to use temperature-compensated amplifiers, automatic gain control amplifiers and automatic slope control amplifiers.
5. Shielding characteristics, shielding characteristics are a parameter to measure the anti-interference ability of coaxial cables, and also an important parameter to measure the leakage resistance of coaxial cables. If the cable shielding is not good, the transmission signal will not only be crosstalked by external clutter, affecting the quality of the cable TV signal, but also leak out to interfere with other signals, which will be received by non-cable TV users, which will seriously affect the normal access of cable TV.
