The VSWR calculator above is a converter for a measurement you already have. You point an antenna analyser, a directional wattmeter or a network analyser at a feedline, it gives you one number, and this page gives you every other number that describes the same reflection: standing wave ratio, return loss, reflection coefficient magnitude, mismatch loss in decibels, and the actual watts delivered and reflected. Nothing here is a design tool. It reads the mismatch you have.
That boundary is deliberate, and it is worth stating up front because Arb Digital publishes a second RF page that starts from the other end. The impedance matching calculator begins with a source impedance and a load impedance and designs the two-element L-network that removes the mismatch; standing wave ratio appears there as the headline figure describing the problem before the network is fitted. This page begins with a reading off an instrument and never asks what the load impedance is, because in the field you usually do not know it. One page starts from impedances and produces a network. This one starts from a measurement and produces the rest of the measurement.
Transmitting Is a Licensed Activity
Standing wave ratio is measured on a feedline, and feedlines usually end in an antenna. Deliberately radiating radio-frequency energy requires a licence almost everywhere. Amateur, commercial, maritime and broadcast transmission all sit inside allocated bands with power limits and technical conditions, and the rules differ by country. The ARRL guide to getting licensed is the usual entry point for the amateur service in the United States, and every other country has a national regulator that publishes the equivalent. Because limits are usually written as radiated power rather than transmitter output, the EIRP calculator is the page that tells you where you stand once feeder loss and antenna gain are counted.
What This VSWR Calculator Does
Every quantity on this page is a restatement of one physical thing: the fraction of an incident wave that comes back from a discontinuity. The reflection coefficient Γ is the ratio of the reflected voltage wave to the incident voltage wave. Standing wave ratio is the ratio of the maximum to the minimum voltage in the standing wave pattern that the incident and reflected waves produce when they add along the line. Return loss expresses the same reflection in decibels. Mismatch loss expresses how much of the incident power never reaches the load.
Enter any one of them and the calculator produces the others. The hero figure is standing wave ratio because that is the number people quote, the grid gives the reflection coefficient, return loss, mismatch loss and the power that actually arrives at the load, and the bars split the forward power you entered into a delivered part and a reflected part. The note underneath gives the two purely resistive load values that would produce your ratio on a line of the characteristic impedance you set.
How to Use It
- Choose the form you measured. An analogue SWR meter gives a ratio. A vector network analyser gives S11 in decibels. A directional wattmeter gives forward and reflected watts. All five inputs are equivalent.
- Enter the reading. If your instrument shows S11 as a negative decibel figure, enter its magnitude in the return loss box; return loss is conventionally quoted as a positive number.
- Set the forward power. This does not change any of the ratios. It only converts them into watts so you can see what is coming back down the line.
- Set the line impedance. Only the two candidate resistive load values depend on it. Leave it at 50 Ω unless you are working on a 75 Ω system.
- Read the mismatch loss, not the ratio. Mismatch loss is the figure that tells you how much power you actually lost, and it is almost always far smaller than the ratio makes people expect.
The Formula: Five Ways of Writing One Number
Start from the reflection coefficient magnitude |Γ|, a voltage ratio between 0 and 1. Standing wave ratio follows from it directly:
SWR = (1 + |Γ|) ÷ (1 − |Γ|), and inverting it, |Γ| = (SWR − 1) ÷ (SWR + 1).
Return loss is the same magnitude on a decibel scale: RL = −20 log10|Γ|, with the factor of 20 because Γ is a voltage ratio. Reflected power is the square of the voltage ratio, so the reflected fraction is |Γ|² and the mismatch loss — the decibel shortfall in power delivered to the load — is ML = −10 log10(1 − |Γ|²). When you have a forward and reflected power pair from a wattmeter, |Γ| = √(Preflected ÷ Pforward).
Work the default through by hand. An SWR of 1.5 gives |Γ| = 0.5 ÷ 2.5 = 0.2000. Return loss is −20 log10(0.2) = 13.98 dB. The reflected power fraction is 0.2² = 0.04, so 4.00 per cent comes back and 96.00 per cent gets through, which is a mismatch loss of −10 log10(0.96) = 0.177 dB. With 100 W forward, 4.00 W is reflected and 96.00 W reaches the load. Check it from the other direction: a return loss of 14 dB gives |Γ| = 10−0.7 = 0.1995 and an SWR of 1.1995 ÷ 0.8005 = 1.498, which is the same measurement inside rounding. The definitions behind all of this are set out in the ARRL transmission lines reference.
Why 2:1 Sounds Terrible and Costs Almost Nothing
A standing wave ratio of 2:1 has a reputation as a fault. In power terms it is not one. At 2:1 the reflection coefficient magnitude is exactly one third, so one ninth of the incident power reflects and eight ninths reach the load. That is a mismatch loss of 0.51 dB. On a receive path it is inaudible. On a transmit path it costs about eleven per cent of your power, which is a fifth of an S-unit at the far end and will never be the reason a contact fails.
Even 3:1, a figure many operators refuse to transmit into, reflects a quarter of the power for a mismatch loss of 1.25 dB. The reasons to care about a rising ratio are real, but they are not the reflected power itself. Solid-state power amplifiers fold their output back or shut down entirely when the reflection climbs, because the reflected wave presents the output devices with load impedances they were never designed for. The standing wave pattern raises voltage at some points along the line and current at others, stressing insulation, connectors and capacitors well beyond what the average power suggests. And on a lossy feeder, the reflected wave travels the whole length of the cable a second time, so cable loss multiplies with mismatch instead of adding to it.
That last effect is the one that decides whether a mismatch matters. On five metres of good coax at HF it is negligible. On thirty metres of thin cable at UHF it can dominate the whole link budget. The free space path loss calculator covers what happens once the signal has left the antenna; the loss inside the feeder happens before that and is easy to forget.
The Reading Cannot Tell You Which Side of Z0 the Load Is On
A magnitude-only instrument measures how much comes back, not what it came back from. For a purely resistive load there are always two resistances that produce the same standing wave ratio on a given line: Z0 multiplied by the ratio, and Z0 divided by it. On 50 Ω cable, a reading of 2:1 is consistent with a 100 Ω load and equally consistent with a 25 Ω load. The tool shows both values in the notes because the measurement genuinely does not distinguish them.
In practice a load is rarely purely resistive anyway, and then the family of impedances producing a given ratio becomes an entire circle on the Smith chart rather than two points. This is exactly why a vector instrument that reports phase as well as magnitude is worth having: it tells you whether the load is inductive or capacitive, which tells you which way to move an element. If you need to know the actual complex impedance in order to design a network around it, that is the point at which the impedance matching calculator and the RLC impedance calculator take over from this page.
Where the Meter Sits Changes What It Reads
Standing wave ratio is a property of a mismatch, but the number an instrument reports is a property of where the instrument is standing. Put a meter at the antenna feedpoint and it reads the mismatch of the antenna. Put the same meter at the transmitter, at the far end of a lossy run of cable, and it reads a lower ratio — not because the antenna improved, but because the cable attenuated the reflected wave on its way back. A badly mismatched antenna on a long, lossy feeder can present a comfortably flat reading at the shack end while almost none of the power is actually reaching the air.
This is the single most misread measurement in amateur radio. A flat reading at the transmitter is not evidence of a good antenna; it is evidence of a good match presented to the transmitter, which is a different claim. An antenna tuner makes that claim true by construction and changes nothing at the antenna. If a reading at the shack looks suspiciously perfect on a long feeder, measure at the feedpoint and compare. The line geometry that sets the cable impedance and its loss is the subject of the cable impedance calculator.
Reading Directional Wattmeters Without Being Fooled
A directional wattmeter shows forward and reflected power as two separate readings, and the natural instinct is to subtract them to get delivered power. That instinct is right, and the arithmetic in this tool does exactly that, but two things trip people up. The first is that a cheap meter is calibrated for one line impedance, usually 50 Ω, and reading it on 75 Ω cable produces a systematic error in both readings. The second is that many meters are frequency-limited and their directivity degrades outside the band they were built for, which shows up as a reflected reading that never quite falls to zero even into a good dummy load.
A useful sanity check is to measure a known good 50 Ω dummy load. Whatever reflected power the meter reports there is your instrument's noise floor, and any reading below roughly twice that figure should be treated as flat rather than as a real number. Converting decibel figures back and forth while you do that is the job of the decibel calculator.
Arb Digital builds free tools like this one because genuinely useful pages earn attention. If you want calculators, tools or technical content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Treating standing wave ratio as a power loss — 2:1 costs about half a decibel, and the real consequences are amplifier foldback, voltage stress and multiplied feeder loss, not the reflected watts.
- Mixing voltage and power ratios — return loss uses 20 log10 because Γ is a voltage ratio, while mismatch loss uses 10 log10 because it describes power. Using the wrong factor doubles or halves the answer.
- Trusting a flat reading taken at the transmitter — feeder loss attenuates the reflected wave and flatters the measurement, so a long lossy run can hide a badly mismatched antenna.
- Assuming a tuner fixed the antenna — it presents a good match to the transmitter and leaves the standing wave on the feeder, along with the extra loss that goes with it.
- Reading a single value of load impedance out of a ratio — a magnitude-only measurement is consistent with two resistive values, and with a whole circle of complex ones.
Related Free Tools From Arb Digital
When you want to remove the mismatch rather than measure it, the impedance matching calculator designs the network. The cable impedance calculator derives the characteristic impedance from the cable geometry, the reactance calculator converts component values into ohms at your frequency, and the wavelength calculator sets the physical dimensions that antenna work depends on. For the link itself, pair the EIRP calculator with the free space path loss calculator, and use the dipole antenna calculator for a centre-fed half-wave element. Everything Arb Digital publishes sits on the free online tools hub. The underlying guided-wave theory is covered in MIT OpenCourseWare's 6.013 Electromagnetics and Applications.
Frequently Asked Questions
That page starts from a source impedance and a load impedance and designs an L-network or transformer ratio to match them, showing standing wave ratio as the headline description of the problem it is solving. This page starts from a measurement taken off an instrument, with no knowledge of the load impedance at all, and converts it between standing wave ratio, return loss, reflection coefficient, mismatch loss and watts. One designs a fix, the other reads the symptom.
There is no universal threshold, because the number that matters is what your equipment tolerates rather than the ratio itself. Most solid-state transmitters begin folding their output back somewhere between 1.5:1 and 2:1 to protect the output devices, which is usually the practical limit. In pure power terms 2:1 costs about half a decibel, so on a receive-only path a considerably worse ratio is often perfectly usable.
About 0.51 decibels, or roughly 11 per cent of the incident power. The reflection coefficient magnitude at 2:1 is one third, so one ninth of the power reflects. That is a small direct loss, and the practical problems with a high ratio come from amplifier protection circuits, component voltage stress and multiplied loss on a lossy feeder rather than from the reflected power itself.
Return loss describes how much power comes back, on a decibel scale, and a large return loss means a good match. Mismatch loss describes how much power fails to reach the load, and a small mismatch loss means a good match. They are not the same quantity and they move in opposite directions: a 14 decibel return loss corresponds to a mismatch loss of only about 0.18 decibels.
Not uniquely. A magnitude-only measurement is consistent with two purely resistive load values, the line impedance multiplied by the ratio and the line impedance divided by it, and with an entire circle of complex impedances once reactance is allowed. You need a vector instrument that reports phase as well as magnitude to pin down a single value.
Because feeder loss attenuates the reflected wave on its way back down the line. A meter at the antenna feedpoint sees the true mismatch of the antenna, while the same meter at the transmitter end of a long lossy run sees a lower ratio. The antenna has not improved; the cable has simply absorbed part of the evidence, along with part of your power.
Yes, in magnitude. Network analysers display the reflection as S11 in decibels, which comes out negative because the reflected signal is weaker than the incident one. Return loss is conventionally quoted as the positive magnitude of that figure, so an S11 of minus 14 decibels is a return loss of 14 decibels. Enter the positive number in this tool.
Figures here describe an ideal lossless line and a magnitude-only measurement. Real feeders have loss, real instruments have finite directivity, and transmitting requires a licence in most jurisdictions.
