Return loss is a measure of how well the components of an RF system are matched in terms of their
impedance. Impedance is the opposition to the flow of alternating current in a circuit, and it depends
on the frequency, resistance, capacitance, and inductance of the components. When the impedance
of the source, the transmission line, and the load are not equal, some of the power is reflected back
to the source, causing a loss of forward power. This loss is expressed in decibels (dB) as return loss.
The higher the return loss, the lower the reflection and the better the impedance matching.
Conversely, the lower the return loss, the higher the reflection and the worse the impedance
matching.
VSWR (Voltage Standing Wave Ratio) is another way of expressing the same concept. It is the ratio of
the maximum voltage to the minimum voltage along a transmission line due to the interference of
the incident and reflected waves. A VSWR of 1:1 means that there is no reflection and perfect
impedance matching. A VSWR higher than 1:1 means that there is some reflection and impedance
mismatch. The higher the VSWR, the higher the reflection and the lower the return loss.
Therefore, a significant impedance mismatch between components in an RF system will cause high
reflection, high VSWR, and low return loss.
