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Why impedance of the coaxial cable is 50 or 75ohm

May 28, 2020

As the material and specific size are restricted..When the insulation material of low loss was applied to the flexible cable, the size and specification of the cable must be kept unchanging to match the existing equipment interface.The dielectric constant of PE is 2.3 and the impedance of the conductor with air (dielectric constant is 1) as the insulation layer is 77 ohms. If the insulation space is filled with PE, the impedance will be reduced to 51 ohms.The exact standard is 50 ohms.

Coaxial cable is composed of inner and outer conductors, for there is electric current flows through a conductor resistance and inductance, the voltage between conductors conductance and capacitance exist, these features are distributed along the route, that is called the distributed constant, Due to the size precision in the manufacture and the influence of uneven medium material purity, Cable TV system requires the use of the coaxial cable of the characteristic impedance of 75 ohms.

Different parameters correspond to an optimal impedance.When the diameter ratio of inner and outer conductor is 1.65, the conductor has the maximum power rate transmission capacity, corresponding to an impedance of 30 ohms (note: LG1.65 *138=30 ohms, to use air as the dielectric medium, because at this time the dielectric constant is minimum, if using solid polyethylene with dielectric constant of 2.3, the impedance is less than 20 ohms).

When the diameter ratio of the most suitable voltage osmosis is 2.7 and the corresponding impedance is about 6 ohms, the osmotic current will be very high.The attenuation is only due to the loss of the conductor, where the attenuation is approximately 50% higher than that at the minimum attenuation impedance (diameter ratio 3.5911) of 77 ohms, at which rate (D/ D = 3.5911), the maximum power is limited to half of the maximum power of a 30 ohm cable. 

The 50 ohm coaxial cable is probably the most common cable in use, typically in radio transmitters and receivers, laboratory equipment, ether, etc. 

Another common type of cable is the 75 ohm coaxial cable, commonly used for video transmission, limited TELEVISION networks, antenna feeders, long distance telecommunications applications, etc.

The bare parallel wires used for telegraph and telephone are also typically of an impedance of 600 ohms.A pair of twisted-pair wires of standard size 22, using an appropriate insulator, having an average impedance of about 120 ohms due to machining restrictions, another transmission line with its own characteristic impedance.Some antenna systems use a 300 ohm double lead to match the reduced half-wave band impedance in free space.