SWR Calculator: Reflected Power, Return Loss and Mismatch Loss

The short answer

At 2 to 1 SWR, 11 percent of the forward power is reflected and the mismatch loss is 0.51 dB, which is inaudible on the air. At 3 to 1 the figures are 25 percent and 1.25 dB. The practical reason to care about SWR is that most solid state transmitters fold back power above about 2 to 1 to protect the output stage, not that the reflected power is lost.

1.5:1 SWR: 4 percent reflected
2:1 SWR: 11 percent reflected
3:1 SWR: 25 percent reflected
Foldback starts: About 2:1

Standing wave ratio is the most watched and least understood number in amateur radio. Operators chase it to 1.05 to 1 and lose nothing but time, while the genuinely damaging problems, common-mode current and feedline loss, sit unnoticed behind a perfect reading. This page gives you the arithmetic and then an honest account of when the number matters.

Calculator

SWR, reflected power and mismatch loss

Enter SWR directly, or enter a load impedance and let the calculator derive it against a 50 ohm line.

Input method

SWR As read on the meter, expressed against 1.

Transmitter power for the watt figures

SWR

2.00 : 1

Power reflected

11.1 percent

Power forward

88.9 percent

Return loss

9.5 dB

Mismatch loss

0.51 dB

Reflection coefficient

0.333

Equivalent load

100 or 25 ohms

Comfortable. Nearly every transmitter runs full power here and the loss is inaudible.

SWR = (1 + ρ) / (1 - ρ) | ρ = (SWR - 1) / (SWR + 1) | Reflected power = ρ²

What SWR actually is

When a transmission line is terminated in an impedance that does not match its characteristic impedance, part of the incident wave reflects back towards the source. The forward and reflected waves add along the line, reinforcing at some points and cancelling at others, producing a standing pattern of voltage maxima and minima.

Standing wave ratio is simply the ratio of that maximum voltage to that minimum voltage. A perfectly matched line has no reflected wave, so there is no pattern, and the ratio is 1. A line terminated in an open circuit or a dead short reflects everything, so the minimum is zero and the ratio is infinite.

On a 50 ohm line terminated in a purely resistive load, SWR is just the ratio of the larger number to the smaller: 100 ohms gives 2 to 1, and so does 25 ohms. An SWR meter cannot tell those two cases apart, which is the first limitation worth knowing. It also cannot tell you whether the load is reactive, which is the second, and it is why an antenna analyser that shows resistance and reactance separately is a genuinely different instrument rather than an expensive SWR meter.

The table people should look at instead of the meter

SWR converted into everything it actually means
SWR	Reflection coeff	Power reflected	Power forward	Return loss	Mismatch loss	Load on 50 ohms
1.0 : 1	0.000	0.0 percent	100.0 percent	Infinite dB	0.00 dB	50 ohms
1.1 : 1	0.048	0.2 percent	99.8 percent	26.4 dB	0.01 dB	55 ohms
1.2 : 1	0.091	0.8 percent	99.2 percent	20.8 dB	0.04 dB	60 ohms
1.5 : 1	0.200	4.0 percent	96.0 percent	14.0 dB	0.18 dB	75 ohms
1.7 : 1	0.259	6.7 percent	93.3 percent	11.7 dB	0.30 dB	85 ohms
2.0 : 1	0.333	11.1 percent	88.9 percent	9.5 dB	0.51 dB	100 ohms
2.5 : 1	0.429	18.4 percent	81.6 percent	7.4 dB	0.88 dB	125 ohms
3.0 : 1	0.500	25.0 percent	75.0 percent	6.0 dB	1.25 dB	150 ohms
4.0 : 1	0.600	36.0 percent	64.0 percent	4.4 dB	1.94 dB	200 ohms
5.0 : 1	0.667	44.4 percent	55.6 percent	3.5 dB	2.55 dB	250 ohms
7.0 : 1	0.750	56.3 percent	43.8 percent	2.5 dB	3.59 dB	350 ohms
10.0 : 1	0.818	66.9 percent	33.1 percent	1.7 dB	4.81 dB	500 ohms

The column that matters is mismatch loss, and its values are the point of this page. At 2 to 1 the loss is half a decibel. At 3 to 1 it is 1.25 dB. Even at 5 to 1 it is 2.55 dB, which is under half an S unit. Reflected power is not destroyed; it travels back to the transmitter, is re-reflected by the source, and most of it eventually reaches the antenna. Only what the feedline absorbs on the extra journey is genuinely lost.

Why 2 to 1 is the number everyone quotes

It is not a physics threshold. It is where most solid state transmitters begin folding back output power to protect the final transistors, which have no tolerance for the voltage and current excursions a mismatch produces. Below 2 to 1 you get full power; above it, the radio quietly gives you less. That behaviour, and not radiated loss, is the practical reason the number is watched.

When SWR genuinely matters

When the feedline is lossy. The additional loss from a mismatch scales with the matched loss of the line, so 3 to 1 on 100 feet of RG-213 at 7 MHz costs almost nothing while the same 3 to 1 on 100 feet of RG-58 at 440 MHz is severe. This is the single most important qualifier and it is worked through on the coax loss calculator.
When you are driving an amplifier. Amplifier protection circuits vary and the consequences of tripping them mid-transmission range from an annoying fault light to a failed device. Keep an amplifier's load under 2 to 1 as a working rule.
When the SWR is changing. A reading that moves with weather, with frequency in an unexpected way, or with where you stand is telling you something is wrong. A high but stable reading is a mismatch; a moving one is a fault.
When it suddenly gets better. This is the counterintuitive one. A feedline that develops water ingress becomes lossier, and a lossier line masks the antenna's true SWR. An antenna that has always read 1.8 to 1 and now reads 1.2 to 1 is usually a warning rather than an improvement.

When SWR does not matter

Chasing 1.05 to 1. The difference between 1.5 to 1 and 1.05 to 1 is 0.16 dB. Nobody has ever heard that difference on the air.
Assuming low SWR means an efficient antenna. A 50 ohm dummy load has a perfect SWR and radiates nothing. So, nearly, does a vertical with a bad radial field, because ground loss appears as resistance and pushes the feedpoint towards 50 ohms. Covered on the vertical calculator.
Reading it at the radio and calling it the antenna's SWR. Feedline loss flatters the reading. Measure at the feedpoint with a NanoVNA H4 vector network analyzer$89.90 if you want to know what the antenna is doing.

Reading the shape, not the number

A single SWR reading at one frequency tells you very little. A sweep across the band tells you nearly everything, which is the real argument for an analyser over a meter.

What the shape of an SWR sweep is telling you
What you see	What it means	What to do
A clean dip below the band	The antenna is too long	Trim both legs equally. One percent of length moves resonance about one percent.
A clean dip above the band	The antenna is too short	Add wire, or add capacity at the ends. Cutting further makes it worse.
A shallow dip that never gets low	Feedpoint impedance is far from 50 ohms, or there is loss	Check height, check the ground system, check for a lossy feedline.
Very broad, low everywhere	Something lossy is dominating, often water in the coax	Suspect the feedline first. A genuinely broadband antenna is the exception.
Ripples across the sweep	Common-mode current, or a connector fault	Add a choke at the feedpoint. Then check every connector.
Reading changes when you touch the coax	The feedline is part of the antenna	Choke it. Twelve turns on a ferrite core at the feedpoint.
Infinite or near infinite	Open or short circuit	A connector, usually the one you made last. Check with an ohmmeter, not a transmitter.

The full diagnostic procedure, in the order that finds faults fastest, is on reading SWR and fixing a bad match.

What to measure with

An in-line meter such as the Daiwa CN-501H cross-needle SWR and power meter$151.99 stays permanently in the line and shows forward, reflected and SWR at once on a cross-needle display, which reveals a changing match faster than any digital readout. For VHF and UHF, the Surecom SW-102 digital VHF/UHF SWR meter$57.99 is small enough to leave in a mobile install permanently.
A vector network analyser such as the NanoVNA H4 vector network analyzer$89.90 sweeps a whole band, shows resistance and reactance separately, plots a Smith chart and does time-domain reflectometry to find where a fault is along the cable. Under a hundred dollars for what used to be laboratory equipment.
A ruggedised analyser such as the RigExpert AA-650 Zoom antenna analyzer$746.73 is what you carry up a mast when a NanoVNA is too fragile and a dropped instrument means a second climb.
A dummy load such as the XRDS-RF 100 W dummy load, 50 ohm$45.49 is how you verify that the meter and the radio are both honest before blaming the antenna. Every diagnosis should start here.

Safety

RF exposure: you must evaluate your own station

The FCC requires every amateur station to perform a routine RF exposure evaluation and to keep the result. The categorical exemption that used to excuse amateurs by power level alone was removed, so there is no wattage below which you can simply assume you comply.

Radio frequency energy at close range is a real hazard, not a paperwork exercise. Transmitting at power into an antenna a few feet from a person, especially on the bands between 30 and 300 MHz where the exposure limit is at its strictest, can exceed the maximum permissible exposure for an uncontrolled environment.

Nothing on this page evaluates your station. The answer depends on your frequency, your power at the antenna, your antenna gain, your duty cycle and how close people actually get. Run the numbers for your own installation.

Testing into a real antenna radiates. Tune up, check power and verify meters into a dummy load rather than on the air, and remember that any exposure evaluation you have done assumes your normal operating pattern rather than an afternoon of continuous carrier while you chase a match.

How to run an RF exposure evaluation

The tuner question

An antenna tuner does not tune the antenna. It is an impedance matching network placed between the transmitter and the feedline, and it presents the transmitter with a 50 ohm load regardless of what the antenna is doing. The transmitter delivers full power. The meter reads 1 to 1. Nothing whatsoever has changed on the far side of the tuner.

That is genuinely useful. A tuner lets one wire antenna work on bands where it is nowhere near resonant, which is why every multiband wire station has one. What it does not do is reduce the standing wave on the feedline or the loss that standing wave causes, so on a long UHF run with a bad antenna the tuner solves the wrong problem completely. Full treatment on antenna tuner basics and the honest comparison on tuner versus resonant antenna.

Common questions

Questions people ask about this

What SWR is acceptable for ham radio?

Under 2 to 1 is comfortable and under 1.5 to 1 is excellent, but the honest answer is that the number matters far less than people believe. At 2 to 1 only 11 percent of the forward power is reflected, and the mismatch loss is 0.51 dB, which is inaudible. Most solid state transmitters begin reducing power above about 2 to 1 to protect the output stage, and that power foldback, rather than any radiated loss, is the real reason to care.

Does high SWR damage a transmitter?

Modern solid state radios protect themselves by folding back power as SWR rises, so damage from a mismatch is unusual. What genuinely causes damage is transmitting into an open or shorted connector at full power on an older radio without protection, or running an amplifier into a mismatch, since amplifiers are less forgiving and their protection circuits vary. Tube amplifiers tolerate mismatch better than solid state ones.

What does 1.5 to 1 SWR actually mean?

It means the standing wave on the feedline has a voltage maximum 1.5 times its minimum, which corresponds to a load of about 75 ohms or about 33 ohms on a 50 ohm line. Four percent of the forward power is reflected back to the transmitter and 96 percent goes on to the antenna. The mismatch loss is 0.18 dB, which is one twentieth of an S unit and entirely undetectable on the air.

Why does my SWR change when I touch the coax?

Because common-mode current is flowing on the outside of the coax shield, which has made the feedline part of the antenna. Your hand changes the antenna. This is a real fault rather than a curiosity: it puts RF in the shack, distorts the radiation pattern and makes every measurement unreliable. The fix is a common-mode choke at the feedpoint, and often a second one partway down the run.

Does an antenna tuner reduce SWR on the feedline?

No, and this is the single most misunderstood point in amateur radio. A tuner in the shack presents a 50 ohm load to the transmitter, so the radio is happy and the meter reads 1 to 1. The standing wave on the coax between the tuner and the antenna is completely unchanged, and so is the additional loss it causes. A tuner solves the transmitter problem, not the feedline problem.

How do I measure SWR properly?

Measure at the antenna feedpoint with an antenna analyser or a vector network analyser, not at the radio through a length of coax. Feedline loss makes SWR look better the further you get from the antenna, so a badly matched antenna on 100 feet of lossy cable can read a comfortable 1.5 to 1 in the shack while the feedpoint is at 4 to 1 and the difference is heating the cable.

Keep going

Related on this site

Keeping your own cut lengths, SWR sweeps and exposure record? The Station Build & Antenna Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.

Disclaimer Researched guidance for planning purposes, not professional engineering, electrical or legal advice. Antenna work, RF exposure, mast and tower work, grounding and battery handling all carry real risk that depends on your specific site and installation. Verify anything safety-critical against current FCC rules, the National Electrical Code and the manufacturer's own documentation, and hire a professional where the job calls for one. We research equipment from published specifications, regulatory documents and verified owner reviews rather than claiming hands-on testing we have not done.