The characteristic impedance, the main body of a coaxial cable is composed of two inner and outer conductors. There is resistance and inductance for the current flowing in the conductor, and conductance and capacitance for the voltage between the conductors. These characteristics are distributed along the line. For the distribution constant, due to the dimensional accuracy and the non-uniformity of the dielectric material in manufacturing, although the characteristic impedance of the coaxial cable used in the cable television system is 75Ω, the characteristic impedance of the coaxial cable actually used is usually (75 ± 5) Ω. Therefore, in order to prevent signal energy reflection and achieve the best transmission effect, the terminal load impedance should also be as equal to the characteristic impedance of the cable as much as possible.
Attenuation characteristics. The attenuation characteristics of coaxial cables are usually expressed by the attenuation constant, that is, the decibels of signal attenuation per unit length (such as 100m). The attenuation constant is proportional to the average square root of the operating frequency F of the signal, that is, the higher the frequency, the larger the attenuation constant, the lower the frequency, and the smaller the attenuation constant. Generally speaking, for a 5mm thick home, 100m is not a problem.
The life span of a cable is that any cable has a certain life. After the cable is used for a period of time, due to the aging of the material, the conductor resistance increases, and the leakage current of the insulating medium increases. When the attenuation constant of the cable increases by 10 [%] When -15 [%], the cable should be renewed. The life of the general cable is 7-20 years depending on the quality and the use occasion.
Temperature coefficient, the temperature coefficient indicates the degree of influence of temperature changes on the characteristics of the cable. As the temperature increases, the cable loss increases, the temperature decreases, and the cable loss decreases. The temperature change of the cable attenuation value is about 0.2 [%] dB / ℃, which indicates that the cable attenuation changes by 0.2 [%] on the original basis. If the temperature change is ± 25 ℃, the cable attenuation value changes by ± 5 [%] dB. Assume that a certain type of cable is 1500m long at 20 ° C. When wearing single clothes, the 550MHZ signal is α = 7.9dB / 100m. Let the temperature coefficient be 0.2 [%] / ° C. The attenuation is 1500m × 7.9dB / 100m = 118.5dB. When the temperature changes by 40 ° C, it feels a little hot when you feel it, the change of attenuation is: 118.5dB × 0.2 [%] / ° C × 40 ° C = 9.48dB. In addition, the attenuation of the coaxial cable varies with the frequency, and the change in temperature will not only cause the change in the attenuation, but also the change in the slope. In actual work, the measures to eliminate the influence of temperature changes on the system are the use of temperature-compensated amplifiers, automatic gain control amplifiers, and automatic slope control amplifiers. For home use, this aspect need not be considered.
Shielding characteristics. Shielding characteristics are a parameter for measuring the anti-interference ability of coaxial cables and an important parameter for measuring the leakage resistance of coaxial cables. If the cable is not well shielded, the transmission signal will not only be affected by the crosstalk of external clutter, which will affect the quality of the cable TV signal, but also leak out and interfere with other signals. Neighbors' wired signals are the reason.