Coaxial cables are often used for connections between devices, or used in bus-type network topologies. The central axis of the coaxial cable is a copper wire, and the outer layer is an insulating material. The outer side of the insulating material is wrapped by a hollow cylindrical mesh copper conductor, and the outermost layer is an insulating layer. Compared with twisted pair cable, coaxial cable has strong anti-interference ability, good shielding performance, stable data transmission, and low price. Moreover, it can be used without connecting to a hub or switch. Coaxial cables specification can be divided into baseband coaxial cables and broadband coaxial cables in terms of usage. Coaxial cables are divided into 50Ω baseband cables and 75Ω broadband cables. Baseband cables are divided into thin coaxial cables and thick coaxial cables. The baseband cable is only used for digital transmission, and the data rate can reach 10Mbps.
A coaxial cable is composed of two different concentric conductors separated by insulating materials. The inner conductor is copper-clad aluminum or copper core wire, and the outer conductor is an aluminum tube or metal braided mesh sheath. If the outer conductor is grounded, it will act as a shielding layer for the central conductor, which has the effect of resisting external interference. In the theory of coaxial cable transmission, it can ensure radio frequency transmission inside the coaxial cable. Due to the short distance between the inner and outer conductors, there will be certain cables between each other. The inductance and capacitance are evenly distributed along the transmission line. So the characteristic impedance of the transmission line depends on the inductance and capacitance of the line, and they depend on the size of the line.
The attenuation constant of a coaxial cable reflects the loss of electromagnetic energy during transmission along the cable and is one of the main transmission characteristics of a coaxial cable. The attenuation of the signal in the transmission line is caused by the distributed capacitance and distributed inductance in the transmission line. The attenuation in the coaxial cable is related to the frequency. As the frequency increases, the inductance of the series inductance increases, and the capacitive reactance of the parallel capacitor decreases. When the signal is transmitted in the cable, after a certain distance, the signal will become weaker and weaker, and the weakening of the signal will become attenuation. The higher the frequency, the greater the loss.
The environmental temperature change is the main reason for the signal level fluctuation in the TV coaxial cable. Because the characteristics of the coaxial cable are closely related to the change of the environmental temperature, especially the long trunk line of the large system is more prominent, the transmission performance of the system will vary with The temperature changes. When the ambient temperature increases, the corresponding cable loss also increases. The environmental temperature change is one of the most important factors that make the signal level change in the TV coaxial cable system. Temperature mainly affects the DC resistance and dielectric loss of the coaxial cable. When the ambient temperature rises, as the frequency increases, the loss of the conductor and medium in the cable will increase; on the contrary, when the temperature decreases, the loss of the conductor and medium in the cable will also decrease. As the temperature has a greater influence on cable loss, special attention should be paid to maintenance personnel.
Coaxial cables are usually used to transmit cable television signals, video signals, digital signals, and various other high-frequency signals. According to different purposes, the standards for selecting cables are also different.
Good quality cables are compact and firm in appearance, and the outer sheath is smooth and flexible. The woven mesh is thick and dense. Except for the number of braided wires, the braid angle of the shielding layer is less than 450. However, some coaxial cable manufacturers, to save materials, the braid angle of the cable is greater than 700, which will make the shielding characteristics of the cable worse. The central conductor or insulating part of the cable with poor technology can be pulled out. The aluminum foil of the outer conductor of the four-shielded cable is divided into two types: bonding and overlapping. The overlap is to wrap a layer of aluminum foil on the coaxial cable physical foam insulator, and overlap a part of the joint, generally 3mm. Bonding is the bonding between aluminum foil and physical foam insulator. Bonding is better than overlapping shielding, and the best four-shielded cables are bonded.
The coaxial cables used in cable television systems and high-frequency broadband monitoring systems are mostly high-frequency physical foam cables. Due to the poor low-frequency anti-interference features of the coaxial cable, the external interference in the low-frequency band is strong and the interference frequency is high, so the frequency band below 5MHz is intentionally avoided in use. But the frequency bandwidth of the above-mentioned system work, therefore, when choosing the cable, should pay special attention to the high-frequency attenuation characteristic and reflection loss. When the cable television reversely transmits the signal, the noise of all terminals will converge to the front end. Therefore, in order to minimize the interference of the cable from the outside world, 4-layer shielded or aluminum tube cables are usually used.
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