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.
💡 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)
The reflection coefficient is a dimensionless number between 0 and 1, so its value is identical whether you describe your line with metric (SI) units such as ohms and metres or with American customary references such as the 75 Ω video standard — the ratio cancels the units out.
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:
- Select the Reflection coefficient (Γ) → VSWR mode.
- Enter the value 0.2 for the reflection coefficient (Γ) into the VSWR calculator.
- Tada! We'll see that the VSWR calculator gives us a value of 1.5 : 1 for VSWR!
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:
- Enter the value of VSWR.
- Find the corresponding value of the reflection coefficient (Γ).
- Use the value of Γ to find the return loss by applying the formula:
Return loss = −20 × log10(|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
The fraction of the incident power that is reflected back toward the source is the square of the reflection coefficient magnitude. Expressed as a percentage:
Reflected power (%) = |Γ|² × 100
The remaining power is what actually reaches the load: transmitted power (%) = (1 − |Γ|²) × 100. For Γ = 0.5, the reflected power is 0.5² × 100 = 25%, so 75% of the power is delivered to the antenna and a quarter is reflected back.
VSWR to mismatch loss
Mismatch loss is the amount of power lost (in dB) because of the impedance mismatch — that is, the power that never makes it to the load relative to a perfectly matched case. It is calculated as:
Mismatch loss = −10 × log10(1 − |Γ|²)
For Γ = 0.5, the mismatch loss is −10 × log₁₀(1 − 0.25) = −10 × log₁₀(0.75) ≈ 1.25 dB. A perfectly matched system (Γ = 0, VSWR = 1 : 1) has zero mismatch loss and an infinite return loss.
Quick reference: common VSWR values
| VSWR | |Γ| | Return loss | Reflected power | Mismatch loss |
|---|---|---|---|---|
| 1.0 : 1 | 0.000 | ∞ | 0.0% | 0.00 dB |
| 1.5 : 1 | 0.200 | 13.98 dB | 4.0% | 0.18 dB |
| 2.0 : 1 | 0.333 | 9.54 dB | 11.1% | 0.51 dB |
| 3.0 : 1 | 0.500 | 6.02 dB | 25.0% | 1.25 dB |
| 5.0 : 1 | 0.667 | 3.52 dB | 44.4% | 2.55 dB |
FAQs
What is a good VSWR value?
A VSWR of 1 : 1 is ideal (perfect impedance match). In practice, a VSWR below 1.5 : 1 is considered excellent, up to 2 : 1 is good for most antennas, and values above 3 : 1 usually indicate a significant mismatch that wastes power and may stress the transmitter.
What is the relationship between VSWR and the reflection coefficient?
They are two ways of describing the same mismatch. The reflection coefficient |Γ| is the ratio of reflected to incident voltage amplitude, while VSWR = (1 + |Γ|)/(1 − |Γ|). As Γ goes from 0 to 1, VSWR goes from 1 to infinity.
Why is return loss infinite when VSWR is 1 : 1?
Because return loss = −20 × log₁₀(|Γ|), and at a perfect match Γ = 0. The logarithm of zero is negative infinity, so the return loss is infinite — meaning no power is reflected at all.
Do the units of measurement matter?
No. VSWR, the reflection coefficient, reflected power percentage, return loss and mismatch loss are all dimensionless or expressed in decibels. When entering impedances, you can use metric/SI ohms or American reference impedances (such as 50 Ω for RF and 75 Ω for video) — the resulting VSWR is exactly the same, because Γ is a ratio of impedances.
Is there any money involved in a VSWR calculation?
No — VSWR is a purely electromagnetic quantity with no monetary component, so no currency conversion is required for this calculator.