Impedance (also called alternative current (AC) resistance) is an important property of the transmission medium in radio frequency applications. While 50 Ω is the target impedance for coaxial transmission media such as an antenna cable (75 Ω for TV or audio applications), 100 Ω is the standard for differential transmission media such as digital cables. Here there are some exceptions, for example, the common USB interface, which specifies a differential impedance of 90 Ω.

All transmitters and receivers, as well as transmission lines and connectors are designed to match a particular impedance. Therefore, one speaks of impedance matching when designing components. The better the components are matched with respect to the specified impedance, the less losses (reflections) occur in the entire transmission path. Every change in diameter and interface along the transmission path causes mismatches in impedance. Therefore, interfaces should be designed to be as homogeneous and compact as possible to avoid unnecessary reflections.