This tool uses the Voltage Standing Wave Ratio (VSWR) to calculate the Return Loss (RL) in dB.
Return Loss (dB) = -20*Log10((VSWR – 1)/(VSWR + 1))
Return loss (RL) is a measure of how much power is reflected back at a transition in an RF circuit or subsystem. 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. However many RF engineers prefer to work with Return Loss. This tool will help with that conversion.
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 VSWR to Reflected Power calculator). In the case of the mixer above, 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. For example, VSWR = 10 gives RL = 1.74 dB.
If Pi is the incident power and Pr is the reflected power, the return loss is given by
RL = 10*Log10(Pi/Pr)
As the reflected power increases, the value of return loss (RL) decreases. Conversely when Pr decreases, the ratio between the input signal and the return signal is higher.
For a VSWR = 50 for instance, RL = 0.35 dB.
At RL = 0 dB, all the input power is reflected back.
Ideally Pr is minimized by design. This minimizes the power reflected back and maximizes the amount of power transmitted forward into the RF circuit – whether it’s an amplifier or a filter.
A higher numerical value of VSWR represents a poor match. Conversely a lower VSWR value represents a good match.
The ideal value of VSWR is 1 where none of the power is reflected back. It represents an excellent match. This is also the minimum value of VSWR i.e. VSWR cannot be less than 1. There is no maximum value of VSWR and so it is designated as infinity ∞ .
Mismatch Loss – this is a comprehensive calculator that uses the source and load impedance values to compute:
- Reflection Coefficient (Γ)
- Return Loss
- Mismatch Loss
- Forward and Reverse Power