This tool calculates the **S11** from the Voltage Standing Wave Ratio (**VSWR**).

Enter **VSWR** (a value greater than or equal to 1)

## Formula

**S11 = (VSWR-1)/(VSWR+1)**

**S11 (dB) = 20*Log _{10}(S11)**

The input reflection coefficient, **S11** **< 1**

**Example Calculation**

For a VSWR of **1.5**, the corresponding value of S11 is **0.2**.

**As the value of VSWR increases, S11 increases as well**

**and the quality of the match decreases**.

**Background**

Many RF components such as filters and amplifiers can be modeled as 2-port networks with their scattering or S-parameters.

In a 2 port network, **S11** is the forward reflection coefficient (at port 1) when port (2) terminated in 50 ohm.

For the 2 port network shown below, **S11 = b1/a1**

**VSWR** is a measure of the match between the characteristic impedance of the source 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 greater the amount of power reflected back.

This calculator provides the VSWR when the source and load impedances are specified. It can for instance be used to calculate the VSWR when the source and load are 50 Ω and 75 Ω, respectively.

VSWR is often expressed as a ratio to 1. For example, if the VSWR is **2.5** then it is written as **2.5:1**.

**What’s a good value of VSWR?**

**A good value of VSWR is 1.5 or lower**. In an ideal situation, the VSWR=1. In this case, the load is perfectly matched to the feedline characteristic impedance and **S11 (linear) = 0** and **S11 (dB) = -∞**. In this situation, no signal energy is reflected and all of it is transmitted forward.