What is Return Loss

Return loss is the loss of power in the signal returned/reflected by a discontinuity in a transmission line or optical fibre.

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Return loss is a quantity often used within RF circuits where impedance matching is important. The return loss is the proportion of a signal that is reflected as a result of an impedance mismatch.

The return loss approach is akin to VSWR, voltage standing wave ratio, but it is widely used in applications where feeders are not used, or they are very short in comparison with a wavelength and hence the concept of standing waves is not applicable.

Typically return loss is used in circuit applications, whereas VSWR is used in association with feeders / transmission lines.

Return loss basics

The definition of return loss is that it is the loss of power in the signal returned / reflected by a discontinuity in a transmission line or optical fibre. This is normally expressed in decibels.

In other words if all the power was transferred to the load, then there would be an infinite return loss. Conversely if there is an open or short circuit termination, then all the power will be returned and there will be no return loss.

The return loss is normally calculated as follows:

R = 10 \log_{10} (\frac{P_{i}}{P_{r}})

Then as the reflection coefficient Γ is the ratio of the forward and reflected voltages, and power is proportional

R (dB) = -20 \log_{10} (Γ)

Return loss is a figure which is widely used for assessing items like the input characteristics of an RF component, or when measuring the characteristics of a network using a vector network analyzer. As such the return loss is an important chancteristic.