Enter **S11** the input reflection coefficient, either as a linear number or dB. The tool will calculate the reflected power as a percentage of the incident power.

**Formula**

**Γ = S11** (linear)

or

**Γ = 10 ^{(S11dB/20)}**

**Reflected Power (%) = 100*(|Γ| ^{2})**

where **Γ** is the reflection coefficient and **0 < Γ** **< 1**

**Background**

**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. This is shown in the picture below.

**Formula**

if S11 is expressed as a linear quantity,

**S11 _{dB} = 20*Log_{10}(S11_{linear})**

On the other hand, if S11 is expressed in dB

**S11 _{linear} = 10^{(S11dB/20)}**

💡 S11 when expressed in dB is always a negative number

**What is Reflected Power?**

This refers to the power that’s reflected back into the source due to an impedance mismatch*. Ideally all the power is transmitted forward and none of it is reflected back. However, in practical systems, a certain percentage of the input power is reflected.

To minimize this reflection in a 50 ohm system for example, it’s important that the input and output impedance of every component is either 50 Ω or matched to it.

*To calculate the mismatch loss from S11

- Use the S11 to calculate the Voltage Standing Wave Ratio (VSWR)
- VSWR can be used to calculate mismatch loss

**Can S11 be converted to S21?**

No it cannot. S21 represents the forward transmission.

Referring to the picture above, **S21 = b2/a1** which has no relation to **S11= b1/a1**.

**Example Calculation**

If S11 is 0.2, 4% of the incident power is reflected.

S11 varies between 0 and 1.

- If S11 is 0, 0% of the power is reflected.
- If S11 is 1, 100% of the incident power is reflected.