Structure and Transmission Characteristics of RF Coaxial Cable
structure
The RF coaxial cable consists of four parts: an inner conductor, an insulator, an outer conductor, and a sheath. The insulator insulates the inner and outer conductors and keeps their axes aligned, which is called a coaxial cable. The inner and outer conductors are separated by dielectric materials (insulation materials), which largely determine the transmission speed and loss characteristics of coaxial cables. The commonly used insulation materials are mixtures of dry air, polyethylene, polypropylene, polyvinyl chloride, and other materials. Physical foam cables are preferred for applications due to their low loss, good frequency characteristics, and low water ingress.
Transmission characteristics
(1) Electromagnetic Field Distribution in Coaxial Cables
The electric field intensity is distributed in a sinusoidal manner, and the electric waves transmitted in coaxial cables will not leak outside the cable. In applications, the outer conductor is usually grounded, so it has a good shielding effect. The transmitted television signal is not affected by external noise, and the signal inside will not radiate.
(2) Skin effect
When the current of high-frequency signal flows through the cable, the current is concentrated on the conductor surface, which reduces the effective cross-sectional area of the conductor and increases the resistance value. This phenomenon is called the skin effect. Because of the skin effect, the current in the coaxial cable flows only along the outside of the inner conductor and the inside of the outer conductor. Therefore, many properties of the cable depend on the outer diameter of the inner conductor and the inner diameter of the outer conductor, and the electromagnetic fields inside and outside the cable do not interfere with each other. The skin depth is inversely proportional to the square root of frequency f (MHz), therefore, the conductor loss of coaxial cables is directly proportional to the square root of frequency.

Coaxial cable performance

(1) Characteristic impedance

The characteristic impedance Z c is defined as the ratio of voltage to current at any point on the cable when the coaxial cable terminals are matched. The characteristic impedance of a coaxial cable is determined by the diameter of the conductor and the medium between the conductors, and is independent of the cable length. In CATV system, the characteristic impedance of coaxial cable is 75 Ω.

(2) Attenuation constant β Temperature coefficient

The attenuation of RF signals during transmission in coaxial cables is related to the size, dielectric constant, and operating frequency of the cable. The degree of attenuation of coaxial cable signals, in terms of attenuation constant（ β ） Represents the number of dB of signal attenuation per unit length (such as 100 meters) of a cable. The attenuation constant is proportional to the square root of the signal frequency, that is, in the same section of cable, the higher the signal frequency, the greater the attenuation constant; The lower the signal frequency, the smaller the attenuation constant. The temperature coefficient represents the impact of temperature changes on the cable loss value. As the temperature increases, the cable loss value increases; As the temperature decreases, the loss value of the cable decreases. The temperature coefficient is defined as the percentage of increase or decrease in signal attenuation caused by a cable when the temperature increases or decreases by 1 ℃.

The attenuation constant is closely related to the frequency of the signal and also to the temperature coefficient. In the practical application of CATV networks, with the increase of time, coaxial cables will age, and the attenuation characteristics of the cables will change greatly. After 3 years, the attenuation of the cables will increase by about 15%, and after 6 years, it will increase by about 40%.

2. CATV RF Transmission Trunk Design Technology

The function of RF transmission trunk is to use coaxial cables as the transmission medium, and use trunk amplifiers to compensate and balance the attenuation and temperature characteristics of the cables, so as to maintain the appropriate level value of the transmitted signal. The attenuation of RF signals transmitted in cables increases with the extension of transmission distance. The attenuation characteristics of RF coaxial cables cause signals of different frequencies to attenuate to varying degrees in the cables. Therefore, in RF transmission trunk lines, signal level compensation and equalization should be carried out. Therefore, cables with good transmission characteristics and trunk amplifiers with excellent performance should be selected. Due to the use of active device amplifiers, the system's cutoff noise ratio should be adjusted The impact of cross talk has been reduced to * small.

Composition Technology of Transmission Mains

The RF transmission trunk consists of RF coaxial cables, trunk amplifiers, attenuators, equalizers, and directional couplers. The radio frequency (RF) transmission trunk is transmitted by the CATV front-end or by the front-end through optical cables to the optical receiver to output RF signals, providing radio frequency television signals for transmission.