S11 to dB Converter

Use this tool to convert Linear to dB values of S11.

The forward reflection coefficient S11 is a measure of impedance mismatch at a transition in an RF circuit.

Enter either one of the following

  • Linear value of S11 (greater than 0 and less than 1)
  • dB value of S11 (less than 0)




What is S11?

For a two port network, S11 is the input reflection coefficient (also known as Γ or Gamma) with a value between 0 and 1.

S11 = b1/a1 (when port 2 is terminated – usually in 50 Ω) where a1 is the incident power on port 1 and b1 is the power reflected from port 1.

Two Port Network


if S11 is expressed as a linear quantity,

S11dB = 20*Log10(S11linear)

On the other hand, if S11 is expressed in dB

S11linear = 10(S11dB/20)

???? S11 when expressed in dB is always a negative number

Example Calculations

If the reflected signal b1 = 0, then all of the input signal a1 is transmitted forward and S11linear = 0; S11dB = -∞

If all of the input signal is reflected back, then b1=a1 and none of it is transmitted forward. This is the poorest match possible and the reflection coefficient S11linear = 1; S11dB = 0

The calculator can also be used to convert between linear and dB values of S21, S22 and S12.

S11 for different RF components

The following table shows linear S11 values for various Radio Frequency modules

ModuleIdeal S11
Lossless Transmission Line0
Attenuator (any dB loss)0
Amplifier (any dB gain)0
Filter in-band (LPF, BPF, HPF)0
Filter out-of-band (LPF, BPF, HPF)1

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