VSWR Calculator - Voltage Standing Wave Ratio
Enter any one RF transmission parameter - VSWR, reflection coefficient, return loss, or a load/source impedance pair - and this calculator instantly computes the rest. You get VSWR, reflection coefficient, return loss, mismatch loss, reflected power percentage, and transmitted power percentage, along with a stepped explanation of every formula used. Switch the input mode using the drop-down to match whatever value your test equipment reports.
Formula
Worked example
A 50-ohm transmission line drives a 75-ohm load. Reflection coefficient: |75 - 50| / |75 + 50| = 25 / 125 = 0.200. VSWR: (1 + 0.200) / (1 - 0.200) = 1.200 / 0.800 = 1.500. Return loss: -20 log10(0.200) = 14.0 dB. Reflected power: 0.200^2 = 4.0%. Transmitted power: 96.0%. Mismatch loss: -10 log10(1 - 0.04) = 0.177 dB.
What is VSWR?
Voltage Standing Wave Ratio (VSWR) is a dimensionless number that describes how well a transmission line is matched to its load. When the load impedance differs from the line characteristic impedance, some of the forward-traveling wave is reflected back toward the source. These two waves add and subtract along the line, creating a standing-wave pattern with alternating voltage maxima and minima. VSWR is the ratio of the maximum to the minimum voltage in that pattern. A perfect match produces VSWR of exactly 1:1 (no standing wave, all power delivered). A short or open circuit produces infinite VSWR (all power reflected). In practice, any value below 2:1 is considered acceptable for most RF applications, and values below 1.5:1 are considered good.
How to use this calculator
Select the input mode that matches what your instrument reports: VSWR (most antenna analyzers), reflection coefficient (vector network analyzers), return loss in dB (many modern VNAs), or the raw impedance values of your source and load. Enter the known value and every other parameter is computed instantly. The Show your work panel walks through each formula step. The chart shows how reflected power varies with VSWR from 1:1 to 10:1, so you can see where your system sits on the curve. The reference table lists common VSWR values, their return loss equivalents, and the quality band each falls into, useful for specifying antennas and amplifiers.
VSWR, reflection coefficient, return loss, and mismatch loss - how they relate
All four quantities describe the same physical phenomenon - impedance mismatch - from different angles. The reflection coefficient (gamma, often written as capital gamma) is the most fundamental: it is the ratio of the reflected voltage to the incident voltage. VSWR is derived directly from gamma as (1 + gamma) / (1 - gamma). Return loss, expressed in dB, is -20 log10(gamma); a higher number always means a better match (less reflected power). Mismatch loss is the portion of incident power that is simply not delivered to the load because of the mismatch, calculated as -10 log10(1 - gamma squared). At VSWR 1.5:1 the mismatch loss is about 0.18 dB, negligible. At VSWR 3:1 it rises to about 1.25 dB, a meaningful loss in high-power or sensitive receiving systems.
Practical implications for RF system design
In a transmitter-to-antenna system, poor VSWR raises the standing-wave voltage peaks on the cable, which can exceed the cable or connector voltage rating. The reflected power also returns to the transmitter where, if not absorbed by a circulator or isolator, it may damage the final stage. In receiving systems, high VSWR at the antenna port degrades the noise figure of a low-noise amplifier by an amount equal to the mismatch loss. For this reason, many specifications call for antenna VSWR below 1.5:1 or a return loss above 14 dB across the operating band. Quarter-wave transformers, L-networks, and single or double stub tuners are the standard tools for improving VSWR at a fixed frequency, while broadband matching networks are used when the frequency range is wide.
VSWR quality ratings and typical applications
| VSWR | Return Loss (dB) | Reflected Power (%) | Match Quality | Typical Use Case |
|---|---|---|---|---|
| 1.00:1 | ∞ | 0.0 | Perfect | Theoretical ideal |
| 1.02:1 | 40.1 | 0.01 | Excellent | Precision lab work, aerospace |
| 1.10:1 | 26.4 | 0.23 | Very good | Professional broadcast antennas |
| 1.25:1 | 19.1 | 1.23 | Very good | Commercial base station antennas |
| 1.50:1 | 13.9 | 4.0 | Good | Most commercial RF equipment |
| 2.00:1 | 9.5 | 11.1 | Good | Acceptable for amateur radio |
| 2.50:1 | 7.4 | 18.4 | Acceptable | Lower performance systems |
| 3.00:1 | 6.0 | 25.0 | Acceptable | Borderline for most applications |
| 6.00:1 | 2.9 | 51.0 | Poor | Significant power waste |
| 10.0:1 | 1.7 | 66.9 | Unacceptable | Open/short circuit risk |
Industry-standard VSWR performance bands for RF transmission systems.
Frequently asked questions
What is a good VSWR for an antenna?
For most commercial and amateur radio applications, a VSWR below 2:1 is acceptable and below 1.5:1 is considered good. Professional broadcast and aerospace applications often require VSWR below 1.1:1 or even 1.05:1 across the full operating band. The corresponding return loss thresholds are roughly 9.5 dB (2:1) and 14 dB (1.5:1). These numbers translate directly to reflected power: 11% at 2:1 and 4% at 1.5:1.
What is the difference between return loss and VSWR?
Return loss and VSWR both measure impedance mismatch but use different scales. Return loss is expressed in decibels and increases as the match improves (higher is better, a counter-intuitive direction for some). VSWR is a dimensionless ratio and decreases toward 1:1 as the match improves (lower is better). They are interconvertible: VSWR 1.5:1 equals a return loss of 13.98 dB, and VSWR 2:1 equals 9.54 dB. Vector network analyzers typically report both.
What causes a high VSWR?
High VSWR results from any impedance mismatch between the transmission line and the load (antenna, amplifier input, or filter). Common causes include: antenna resonant frequency shifted away from the operating frequency; damaged or corroded connector causing a partial open or short; wrong cable impedance (using 75-ohm cable in a 50-ohm system); antenna detuning due to nearby metallic objects; or a faulty component in a matching network. A VSWR sweep across frequency quickly shows whether the problem is narrowband (resonance shift) or wideband (connector or cable fault).
How do I convert between VSWR and dB?
To convert VSWR to return loss: first compute gamma = (VSWR - 1) / (VSWR + 1), then RL = -20 log10(gamma). To go the other way: gamma = 10^(-RL / 20), then VSWR = (1 + gamma) / (1 - gamma). Quick rule of thumb: a 10 dB return loss is roughly a 2:1 VSWR; 14 dB is about 1.5:1; 20 dB is about 1.2:1; 26 dB is about 1.1:1.
Does VSWR affect received signal quality as well as transmit power?
Yes. On the receive side, a mismatch at the antenna input reflects some of the incoming signal energy rather than delivering it to the receiver. The mismatch loss (in dB) adds directly to the system noise figure because it appears before the first amplifier. For a low-noise amplifier (LNA) designed for very sensitive reception, even a 0.5 dB mismatch loss can be significant and is worth correcting with a matching network.