Cable impedance

When radio frequency signals are transmitted via coaxial cable or ribbon cable, the impedance of the cable is significant in determining the load placed on the source and the efficiency of the transmission.

Provided the internal impedance of the source and the impedance of the load or termination of the cable are equal, it is possible to manufacture a cable which will present a constant impedance at both ends, regardless of the length of the cable segment. This impedance is called the characteristic impedance of the cable.

Thus, a 50 ohm coaxial cable, terminated in a non-inductive 50 ohm resistor, will present a 50 ohm impedance to whatever equipment is connected to its other end, regardless of the cable length.

The most common impedances of coaxial cable are 75 ohm, used for unbalanced connection of television antennass, and 50 ohm, used for computer networks and many other purposes. The only common impedance of ribbon cable is 300 ohms, used for balanced connection of television antennas. Thus, the baluns used in converting signals from a television antenna between balanced and unbalanced incorporate a 4:1 transformer for impedance matching purposes.