VSWR
(: 1)
Reflection coefficient (Γ)
Reflected power
%
Return loss
dB
Mismatch loss
dB

VSWR Calculator (Voltage Standing Wave Ratio Calculator)

By Madhu Raman
Last updated: May 14, 2021

This VSWR calculator allows you to calculate the reflection coefficient, reflected power, and mismatch loss for a given value of VSWR (voltage standing wave ratio). The calculator can also be used to find the value of VSWR using any of the other values. Here we will learn more about the meaning of VSWR and the associated VSWR formulas.

What is VSWR?

VSWR refers to the voltage standing wave ratio. The VSWR definition states that it is the ratio between the maximum voltage and the minimum voltage on a line (antenna). VSWR is a measure of the efficiency with which the power source transmits radio frequency (RF) through a transmission line.

Ideally, in a radio frequency circuit, the power source, transmission line, and the load at the other end have a characteristic impedance. These quantities - source impedance, line impedance and load impedance, respectively - must equal one another (impedance matching) to ensure the efficient transmission of power. In this ideal scenario, no power is lost, and the VSWR value is 1 : 1, which signifies 100% transmission of the input power from the source.

But, in practice, there may be a mismatch between the impedances, which would cause voltage fluctuations along the transmission line. This voltage fluctuation is measured through VSWR, which is the ratio of the maximum voltage to the minimum voltage along the line.

This voltage fluctuation also leads to a loss of power, which is reflected back along the transmission line. Hence, the higher the impedance mismatch, the higher the voltage fluctuation, resulting in a higher value of VSWR and a higher percentage of power reflected back without successful transmission.

RF circuit with forward and reflected power
RF circuit with forward and reflected power

VSWR can be expressed either as a **simplified number or as a ratio**, e.g., a VSWR of 2 can also be written as 2:1 to explicitly show the ratio form.

Additionally, the current flowing through the transmission line due to an applied voltage also results in an electromagnetic field and an associated electromagnetic force across the line.

VSWR formula

If the reflection coefficient (Γ) is known, then the VSWR (voltage standing wave ratio) can be calculated using the formula:

VSWR = (1 + |Γ|) / (1 - |Γ|)

Alternatively, if the VSWR is known, the reflection coefficient can be found using the alternative VSWR formula:

|Γ| = (VSWR - 1)/(VSWR + 1)

VSWR calculator example

For example, let's say we need to find the VSWR for a reflection coefficient of 0.2. Here's how we'd do it:

  1. Enter the value 0.2 for the reflection coefficient (Γ) into the VSWR calculator.
  2. Tada! We'll see that the VSWR calculator gives us a value of 1.5 : 1 for VSWR!
  3. This calculator works in both directions, so plugging in 1.5 : 1 for VSWR will give us the value of 0.2 for the reflection coefficient (Γ)!

VSWR to return loss

If we need to convert VSWR to return loss in dB, we'd need to:

  1. Enter the value of VSWR.
  2. Find the corresponding value of the reflection coefficient (Γ).
  3. Use the value of Γ to find the return loss by applying the formula:

Return loss = -20 * log(Γ)

For example, if the VSWR value is 3 : 1, we will first have to find the reflection coefficient's value using it, Γ = 0.5. We can then find the return loss as -20 * log(0.5), which will give us 6.02 dB

It's to be noted that this VSWR calculator can also be used in the reverse direction in order to find VSWR if the return loss value is known!

VSWR to reflected power percentage

Knowing the value of VSWR, we can also use it to find the reflected power percentage using the formula:

Reflected power (p) = 100 * Γ²

In order to use this formula, we must first find Γ from the value of VSWR and use it to find the reflected power as a percentage.

We can also extend this to find the through power percentage by subtracting the reflected power percentage from 100%.

Through power percentage = 100% - Reflected power percentage

Let's say that the VSWR is 2:1:

  1. Use the formula for Γ to get Γ = 1/3.
  2. Substitute this value into the formula for reflected power, which gives p = 11.11%.
  3. The through power percentage would be 100% - 11.11%, which will give us 88.89% (shown in the advanced mode)

What does this mean? It means that if VSWR is 2:1, then for 1000 watts of power transmitted through an antenna, about 889 watts of power goes through while 111 watts of power is reflected. The greater the reflected power percentage, the higher the value of VSWR!

It's also interesting to note that if we know the frequency of the radio wave that is to be transmitted through an antenna, we can use it to calculate the required dipole length.

VSWR to mismatch loss

The value of VSWR can also be used to find the mismatch loss in dB using the formula:

I = -10 * log(1 - Γ²)

  1. Enter the value of VSWR.
  2. Find Γ using the formula for the reflection coefficient.
  3. Using Γ, apply the formula for mismatch loss to get I.

Extending the same example we saw earlier, if VSWR = 2 : 1, then Γ will be equal to 1/3. Using this, we will get the value of mismatch loss as I = 0.5115 dB

This VSWR calculator also enables you to find VSWR if the value of mismatch loss is known.

We can also calculate power from voltage and current values, and convert the power value into different units using a power converter.

Extending on this topic, an alternating current (AC) circuit would have different types of power associated with it, which are related to each other by the power factor.

FAQ

What is reflection coefficient?

The reflection coefficient of an antenna is a measure of how much of the transmitted wave is reflected due to an impedance discontinuity.

What is mismatch loss?

The mismatch loss represents the loss in power at the output caused by impedance mismatches and/or reflections in signals.

What is the ideal value of VSWR?

The ideal VSWR value is 1 : 1, which shows that the reflected power is 0, signifying no power wastage. This is also its lowest value - VSWR can't be less than 1!

Madhu Raman
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