The feeder line is the signal line connecting the early TV and outdoor antenna, and its flat line is generally two-wire. It is also a term in the distribution network. It can refer to a branch connected to any distribution network node. It can be a feed-in branch or a feed-out branch.
- 1. What is the feeder
- 2. Types of feeders
1. What is the feeder
The feeder line is the signal line connecting the early TV and the outdoor antenna. The flat line is generally two-wire, and there is a wide distance between the two lines. The purpose is to reduce the attenuation of the weak signal of the TV by the distributed capacitance between the lines. The line body is made of insulating plastic. There is no external shielding layer, and the anti-interference ability is extremely poor. Its performance in outdoor use will also be affected by rainy weather. Because of the popularity of cable TV, the TV signal line is completely replaced by the coaxial cable.
Its main task is to effectively transmit signal energy. Therefore, it should be able to transmit the signal power from the transmitter to the input of the transmitting antenna with minimal loss, or transmit the signal received by the antenna to the receiver with minimal loss At the same time, it should not pick up or generate stray interference signals. In this way, the transmission line must be shielded. When the physical length of the feeder is equal to or greater than the wavelength of the transmitted signal, the transmission line is also called a long line.
2. Types of feeders
In the past, there were three main types of feeders: flat feeders, coaxial cables, and optical cables. Now there are only flat copper wires and coaxial cables. The most commonly used is coaxial cable. Used in cable television systems.
1. Coaxial cable
Commonly used coaxial cables have characteristic impedances of 75Ω and 50Ω. In urban cable television, the coaxial cable is directly connected to the signal source and the TV. Because of its low loss and strong anti-interference characteristics, it is an ideal TV signal transmission line. Commonly used coaxial cables are composed of an inner conductor, an insulating layer, a shielding layer, and an outer protective layer.
The outer protective layer of the coaxial cable is made of rubber, polyethylene and other materials, wrapped outside the shielding layer, and has the functions of mechanical protection, sealing, moisture-proof, and anti-corrosion. The inner conductor mainly plays the role of signal transmission in the cable, and solid copper wire is often used. In order to increase the mechanical strength of large-diameter cables, copper-clad steel is also used as the inner conductor.
The shielding layer is braided by copper wire, which plays a dual role of conduction and shielding. The metal shielding end should be grounded when in use. The insulator is located between the inner conductor and the metal shielding layer, and it is required to use an insulating medium with low high-frequency loss to make a structure similar to a lotus root core. Because the supporting effect of the insulator makes the conductor and the shielding layer concentric, it is called a coaxial cable.
2. Flat feeder
The flat feeder is also called parallel feeder. It is composed of two parallel wires. The two wires are fixed with insulating materials such as polyvinyl chloride or polyethylene. The main feeders produced in my country are SBVD and SBYD. The former has a harder texture. The texture is softer, but the former is less lossy than the latter. Since the two wires of the parallel feeder have the same capacitance to ground, they are also called balanced or symmetrical feeders.
The diameter of the wire of the parallel feeder is between 1mm and several mm, and the distance between the two wires is not more than 1/10 of the wavelength of the transmitted signal, generally about 13mm. The characteristic impedance of the parallel feeder is 300Ω, and the anti-interference ability is poor, but the price is cheap. Because of the large loss of the transmission signal, in the cable television system, the connection line between the components and some subscriber lines use flat feeders.
3. The role of feeder
The feeder plays the role of signal transmission in the cable television system, through which the signal received by the antenna is transmitted to the front-end system, and the signal output by the front-end is also transmitted by the cable to the TV of each user. Therefore, the quality and type of the feeder are important factors that directly affect the receiving effect and signal transmission quality of the cable TV system.