# Cable impedance

Cable impedance is a characteristic of cables used in electrical engineering, used to allow better communication through them.

## Principle

When transmitting radio frequency signals using coaxial fibers, the impedance of the cable itself is important, because it allows the designers to determine the load which will be placed on the source, and will also offer a number of hints about the efficiency of the transmission.

If the internal impedance of the source and termination are equal, manufacturers can produce cables with constant impedance at both ends, without being influenced by the length of the cable. This impedance provided by the manufacturer is called characteristic impedance. For example, a 75-ohm coaxial cable which is terminated in a non-inductive 75-ohm resistor will have a 75-ohm impedance, regardless of the cable's length or the device connected at the other end.

In order to calculate the characteristic impedance of a coaxial cable, the following formula may be used:

I = (138 / e ^ (1/2)) * log (D/d), where:

I is the characteristic impedance of the wire, e is the dielectric constant D and d are the diameter of the center conductor, and the inner diameter of the cable shield respectively log represents a base 10 logarithm.

## Common usage

Coaxial cable used in television for unbalanced connections to antennas has a characteristic impedance of 75 ohm. 50 ohm cables are used for communication purposes, including computer networks. Ribbon cables used for balanced antenna connections have a typical impedance of 300 ohm.s