This tool calculates the Return loss in dB. It uses the Voltage Standing Wave Ratio VSWR to first calculate the reflection coefficient Γ.
Γ is then used to calculate the return loss.
Γ = (VSWR – 1)/(VSWR + 1)
Return Loss (dB) = -20*Log10(|Γ|)
Return loss is a measure of how much power is reflected back at a transition in an RF circuit. A transition can be at the input to an antenna, amplifier, filter, mixer or any other RF building block.
Many times a manufacturer will specify the VSWR only and this calculator helps calculate the return loss.
For example, the following plot shows the VSWR as a function of frequency for a mixer.
A good value of VSWR is 1.5, where 96% of the power is transmitted forward and only 4% is reflected back (using the mismatch loss calculator). In this case, the VSWR is always less than 1.5.
Using the calculator, for a VSWR of 1.5, the return loss is calculated as 14 dB. As the VSWR increases, the numerical value of return loss decreases.
The way to look at this intuitively is: As the value of return loss (RL) decreases, the ratio between the input signal and the return signal is smaller. For a VSWR of 50 for instance, the return loss is only 0.35 dB.
At RL = 0 dB, all the input power is reflected back.
Ideally you want RL to be as large as possible. That maximizes the amount of power transmitted forward into the RF circuit – whether it’s an amplifier or a filter.