This tool calculates the VSWR from S-parameter measurements.
Enter S11 also known as the input reflection coefficient.
Use either a
- dB value less than 0
- linear value between -1 and +1
VSWR to S11 Formula
S11 (dB) = 20*Log10(S11)
VSWR = (1+|S11|)/(1-|S11|)
the absolute value of the input reflection coefficient,|S11| < 1
Background
S11 is the forward reflection coefficient (at port 1) with the opposite port (2) terminated in 50 ohm.
VSWR is a measure of the impedance match between a feedline’s characteristic impedance and the attached load. The load can be an antenna or any RF module such as a filter or amplifier. The greater the degree of mismatch the less efficient the design and implementation.
VSWR is often expressed as a ratio to 1. For example, if the VSWR is 1.9 then it is written as 1.9:1.
Within the context of antennas a good value of VSWR is 1.5 or lower. In an ideal situation the load is perfectly matched to the feedline characteristic impedance and S11 (linear) =0. In this case, no signal energy is reflected and all of it is transmitted forward.
How to Calculate VSWR from S Parameters
A network analyzer can be used to make S parameter measurements. Measure S11 across the desired frequency range. Use the S11 value in the calculator above to find the equivalent VSWR. The graph below shows input return loss for an amplifier.
At 1000 MHz, the S11 (dB) = -RL (dB) = -15 dB. The equivalent VSWR is 1.43.
What is the relationship between S11 and Input Return Loss (RL)?
S11 expressed in dB is given by
S11 (dB) = -RL (dB)
Essentially RL > 0. Note that the plot above shows the input return loss of the amplifier and it’s a positive number greater than 6 dB across the measurement range.
Return loss can be converted to VSWR by first using this simple relationship and then the calculator on this page. Alternatively use the return loss to VSWR calculator.