What is Return Loss and How to Measure It?

Return loss is something similar to SWR while considering the impedance matching of your antenna and feedline. Return loss is related to the reflected signal when there is an impedance mismatch. Return loss is considered in many more applications than in just the antenna and feedline. It is also used in RF circuits where an antenna and feedline are not involved. In those situations, as the signal paths are very short in comparison with the wavelength, standing waves are not applicable. If the whole power output is transferred to the load, there is no reflected power and the return loss is considered to be infinite. But when the antenna is disconnected or the RF circuit is open, all the power will be returned and there will be no return loss! That is an interesting concept and a little bit difficult to comprehend as it looks like a misnomer at one look!

Return loss is expressed in decibels and calculated from the incident power and the reflected power. Return loss in dB will be ten times the logarithm of the ratio of incident and reflected powers. Return loss is inversely related to the SWR. Higher return loss would mean lower SWR. In some literature return loss may be expressed with a negative sign. In that case it will be the inverse of the return loss defined initially. Then the formula will be the ratio between reflected power and incident power.

Another term used in this context is the reflection coefficient Γ, which is the ratio of the forward and reflected voltages. Reflection coefficient and return loss are related to each other. As the power is proportional to the square of the voltage, return loss in dB will be 20 times the logarithm of the absolute value of reflection coefficient.

If both incident and reflected power are expressed in dBm (decibels relative to a milliWatt), then return loss will be simply the difference between the two. That is because all the figures become logarithmic in value. You may recall that if you take logarithm, division is just subtraction of the values. Return loss can be converted to SWR and vice versa using a more complex formula. Alternatively, there are charts which show return loss along with SWR and reflection coefficient. From these charts, if one is available, the other one can be found out.

For measuring return loss with NanoVNA, the tracing is set to LOGMAG, meaning logarithmic magnitude in decibels. In NanoVNA and other devices with two ports, S11 is the Input Reflection Coefficient. That is the ratio of reflected wave to the incident wave at the input port or port 1. Just for comparison, S22 is the output reflection coefficient which quantifies the ratio of the reflected wave to the incident wave at the output port or port 2.