Return loss is the professional way to describe an antenna match, and it confuses beginners because bigger is better. Where SWR runs from 1.0 upward with lower being better, return loss runs in decibels with higher being better: a return loss of 20 decibels means a better match than 10 decibels. This SWR return loss calculator translates between the two systems in both directions, so you can read analyzer screens, data sheets, and magazine articles without converting in your head.
The maths starts, as always, with the reflection coefficient gamma, which is SWR minus one over SWR plus one. Return loss in decibels is minus 20 times the base-10 logarithm of gamma. At SWR 1.5, gamma is 0.2, and return loss is about 14 decibels. At SWR 2.0, gamma is one third, and return loss is about 9.5 decibels. At SWR 3.0, gamma is one half, and return loss is about 6 decibels. A perfect match at SWR 1.0 would give infinite return loss, so the tool caps the display at 99 decibels, which is already far beyond measurable.
The reverse direction is just as useful. Given return loss, gamma is 10 raised to minus return loss over 20, and SWR is one plus gamma over one minus gamma. So a data sheet that promises 20 decibels of return loss means gamma of 0.1 and SWR of about 1.22. A filter specified at 14 decibels return loss means SWR near 1.5. The tool accepts either input: enter SWR and read return loss, or enter return loss and read the equivalent SWR, with reflection coefficient and reflected percentage shown in both cases.
Here is a worked comparison across common values. SWR 1.2 gives about 21 decibels return loss and 0.8 percent reflected power. SWR 1.5 gives about 14 decibels and 4 percent reflected. SWR 2.0 gives about 9.5 decibels and 11 percent reflected. SWR 3.0 gives about 6 decibels and 25 percent reflected. Notice how return loss compresses the scale: the gap from SWR 1.2 to 1.5 looks small as a ratio but spans 7 decibels, while the gap from 2.0 to 3.0 spans only 3.5 decibels. Decibels reward good matches generously.
Return loss appears everywhere in radio data sheets, which is why the translation matters. Antenna makers often specify return loss rather than SWR, cable makers quote return loss per length, and analyzer screens can display either unit. A specification of 10 decibels return loss sounds impressive until you convert it: it is SWR 1.92, an ordinary match losing about 10 percent of power. A specification of 20 decibels is genuinely good, near SWR 1.22. Knowing the conversion stops marketing numbers from misleading you.
Measurement location matters, just as with SWR. Return loss measured at the radio includes cable loss twice, once going out and once coming back, so a long lossy cable makes the antenna look better than it is. Measure at the antenna feed point for the true figure, or measure at the radio and treat the result as a system figure that includes the cable. Also measure across the band: an antenna can show 20 decibels at the centre frequency and 8 decibels at the edges, and the edges are where reflected power starts to matter.
What return loss should you accept? For most amateur transmitting work, 14 decibels or better, meaning SWR 1.5 or better, is a comfortable target. Between 10 and 14 decibels the system works but deserves attention, and below 10 decibels, meaning SWR above 1.92, you should fix the antenna or check for faults before running full power. For receiving only, return loss barely matters: a poor match loses a little signal, but the noise floor usually matters more, and almost any wire hears something.
In short, this tool converts SWR to return loss through the reflection coefficient: return loss equals minus 20 times the log of gamma, and the reverse conversion runs the same path backwards. Use it to read data sheets honestly, to compare analyzer readings in either unit, and to keep perspective: 20 decibels is excellent, 14 is good, 10 is ordinary, and below that it is time to fix the antenna. Bigger decibels mean a better match, and now you can prove it in either language.