Input Impedance and Return Loss

For an efficient transfer of energy, the impedance of the radio, antenna, and transmission cable connecting them must be the same. Transceivers and their transmission lines are typically designed for 50Ω impedance. If the antenna has an impedance different than 50Ω , then there is a mismatch and an impedance matching circuit is required. When any of these components are mismatched, transmission efficiency suffers.

Return loss is another way of expressing mismatch. It is a logarithmic ratio measured in dB that compares the power reflected by the antenna to the power that is fed into the antenna from the transmission line. The relationship between SWR and return loss is the following:

While some energy will always be reflected back into the system, a high return loss will yield unacceptable antenna performance.