Return Loss and Mismatch Loss: What SWR Really Costs Your Signal

Ask a ham how well an antenna is working and the answer is almost always an SWR number. Professionals and the screens of analyzers often give a different number: the return loss, in decibels. The two describe the same thing, a measure of how much of the transmitted wave bounces back from the antenna, but each answers a slightly different question. This guide explains how return loss and mismatch loss relate to SWR, gives a conversion table, shows with a few calculations how little power is wasted by a modest mismatch, and explains why a long run of coaxial cable makes a bad antenna look better than it is.

Three Ways to Describe the Same Mismatch

When a transmission line feeds a load whose impedance differs from that of the line, part of the forward wave is reflected. The ratio of the reflected voltage to the forward voltage is called the reflection coefficient, written with the Greek letter gamma. It ranges from 0, a perfect match, to 1, a total reflection. The Wikipedia article on standing wave ratio defines the SWR as the ratio of the maximum to the minimum voltage along the line and relates it directly to the reflection coefficient. Return loss and mismatch loss are two other ways of expressing the same quantity.

• SWR is a ratio from 1:1 upward. It is easy to read on a meter and is the common language of amateurs.
• Return loss is the difference in decibels between the forward and the reflected power. A high return loss means a good match: 20 dB means that only one percent of the power comes back.
• Mismatch loss is the power that fails to reach the load, expressed in decibels. It answers the practical question of how much signal you lose, and it is always small for modest SWR values.

The relationship between them is as follows. The reflection coefficient is the SWR minus one, divided by the SWR plus one. The return loss in decibels is minus twenty times the logarithm of the reflection coefficient. The fraction of power that is reflected is the square of the reflection coefficient, and the mismatch loss is minus ten times the logarithm of one minus that fraction. The Wikipedia article on return loss gives the definition in terms of incident and reflected power and notes that a high return loss indicates a good match.

Conversion Table

The table has the numbers that you will use most often. All values are calculated from the formulas above for a lossless line.

One decibel is a very small change that most listeners can hardly hear, and one S-unit is typically taken to be 6 dB. The table shows that an SWR of 2:1 costs about half a decibel, and even 3:1 costs only about one and a quarter decibels. That is far less than most operators think. The reason the SWR still matters is not the loss of the mismatch itself but what comes with it: transmitters that fold back their power at a high SWR, extra loss in the feed line and a risk of heating components. Our article on what an SWR meter tells you and what it hides makes the same point from the other side.

Return Loss Is Not Power Lost

The names invite confusion. Return loss is not the loss of power in the system. It is the loss that the reflected wave suffers relative to the incident wave, and a larger number is better. A return loss of 20 dB is a good match, and a return loss of 6 dB is a poor one. Mismatch loss, on the other hand, is a real loss of the power that reaches the load. When you read an analyzer plot, the return loss axis is usually drawn so that a deeper dip is a better match; a NanoVNA, for example, shows the S11 parameter in decibels, which is the same quantity with a negative sign. Our guide to NanoVNA calibration and sweeps shows how to read it.

A rule of thumb for amateur use: a return loss of 14 dB or more corresponds to an SWR below 1.5:1, while 10 dB is about 1.9:1 and 20 dB is about 1.2:1. If you remember these three anchor points, you can translate between the two languages without a table.

Why Cable Loss Hides a Mismatch

The reflected wave has to travel back along the cable to the transmitter, so it passes through the cable loss twice, once on the way to the antenna and once on the way back. For a lossy cable the reflection measured at the transmitter is therefore smaller than the reflection at the antenna. In decibels the return loss at the transmitter equals the return loss at the antenna plus twice the cable loss. An antenna with an SWR of 3:1 has a return loss of 6 dB; if the cable has a matched loss of 1 dB at that frequency, the return loss at the shack is 8 dB and the SWR reads about 2.3:1. With 2 dB of loss, an antenna with an SWR of 2:1 reads about 1.5:1 at the transmitter.

This effect explains why a bad antenna on a long run of thin coax can look acceptable on the meter, and why an antenna that is measured at the end of a short test lead often shows an SWR higher than the one in the shack. It also explains why the coaxial cable you choose matters at VHF and above, where loss is higher; our comparison of RG-58, RG-8X and LMR-400 gives the numbers. The cable does not fix the mismatch, it only hides it, and the power that you lose in the cable is lost for good. If you need the truth about the antenna, measure at the antenna or use a cable that is short and low in loss.

Where Return Loss Is Useful

Return loss is handy in several situations. Filters, amplifiers and preamplifiers are specified in decibels of return loss at their input and output ports, so comparing devices from a data sheet is easier in this form. A good duplexer or a band-pass filter in a repeater system has a return loss of 20 dB or more in its pass band, which corresponds to an SWR of about 1.2. Directional couplers, bridges and noise bridges measure return loss directly; a return loss bridge on a spectrum analyzer is the classic way to check an antenna in a professional setting.

It is also the natural quantity when you sweep a wide range of frequencies, because the plot has a sensible scale: the deepest points are the best matches, and the width of the dip at a chosen level, such as 10 dB or 14 dB, gives you the usable bandwidth of the antenna. Our Smith chart guide shows how the same data can be turned into impedance, and explains why a matching network can change the return loss without changing the antenna.

Measuring Return Loss at Home

You do not need laboratory equipment to measure return loss. A NanoVNA or any antenna analyzer that shows S11 gives it directly across a band, and a simple SWR meter gives you the equivalent after a conversion. The steps are simple. Calibrate the instrument at the end of the cable you use, connect the antenna through the shortest practical length of feed line, sweep the band of interest, and read the number at your operating frequency. Then repeat with the antenna replaced by a known load, such as a 50 ohm dummy load, to confirm that the instrument reads a very high return loss, typically better than 30 dB, on a good load. A reading that is much worse than that on a known good load tells you that the cable, the connector or the calibration is the problem, not the antenna.

Take the measurement at the same place every time. The result changes with the height of the antenna, with rain on the wires and with the position of the cable, so a log of readings with the date and the weather gives you a record that you can compare after a storm or a change in the installation. If a number falls by several decibels between two measurements without any change you made, look for a damaged connector or water in the cable before you start modifying the antenna.

Worked Example

Suppose that you measure an antenna with an analyzer and find a return loss of 12 dB at the operating frequency. The reflection coefficient is ten to the power of minus twelve over twenty, about 0.25. The SWR is one plus 0.25 divided by one minus 0.25, which is about 1.67:1. The power reflected is the square, about 6.3 percent, so the power that reaches the antenna is about 93.7 percent. The mismatch loss is minus ten times the logarithm of 0.937, about 0.28 dB. Your 100 watt transmitter delivers about 94 watts to the antenna, and the change in signal at the other end is far too small to notice.

Now check the feed line. If your coax has a loss of 3 dB, half of the power is already gone before it reaches the antenna, and the 0.28 dB from the mismatch is a minor detail compared with it. This is a common situation on the upper bands with cheap cable, and the right action is to replace the coax, not to fiddle with a tuner to flatten the SWR.

Common Misunderstandings

• Reflected power is not heat in the transmitter. In a lossless system the reflected power returns to the source and is reflected again; it is not lost, though it can stress the amplifier.
• A tuner does not remove the mismatch loss on the line. A tuner at the transmitter makes the radio happy but leaves the standing waves on the cable, so the cable loss with a high SWR stays higher than with a matched load.
• A low SWR does not mean a good antenna. A dummy load has a perfect SWR and radiates nothing.
• One meter is not a measurement lab. Cheap meters can be off by a considerable margin at the extremes of their range, and the reading depends on the cable and the connectors.

The Bottom Line

Return loss and mismatch loss are different views of the same reflection that SWR describes. Return loss in decibels tells you how well the load is matched, with higher numbers being better, such as 14 dB for 1.5:1 and 20 dB for 1.2:1. Mismatch loss tells you how much power fails to arrive, and for modest mismatches it is tiny: about 0.5 dB at 2:1 and about 1.25 dB at 3:1. Cable loss shrinks the reflection that you measure at the transmitter by twice its value, so a long cable can hide a poor antenna. Use the numbers to decide what is worth fixing, and spend your effort on loss, radiation and noise before chasing a perfect 1.0.

Frequently Asked Questions

What is return loss?

The difference in decibels between the power sent toward a load and the power reflected back from it. A higher number means a better match.

How do I convert SWR to return loss?

Calculate the reflection coefficient as (SWR minus 1) divided by (SWR plus 1), then take minus 20 times the base-10 logarithm of it. An SWR of 2:1 gives 9.5 dB.

How much power is lost at 2:1 SWR?

About 11 percent of the power is reflected, and the mismatch loss on a lossless line is about 0.5 dB.

Is a return loss of 10 dB good enough?

It corresponds to an SWR of about 1.9:1, which most transmitters tolerate and which costs less than 0.5 dB, although a transmitter with foldback may reduce power.

Why does my SWR look better at the shack than at the antenna?

Because the reflected wave passes through the cable loss twice. Lossy or long cable reduces the reflection that you measure at the transmitter.

Is S11 the same as return loss?

S11 is the reflection coefficient expressed in decibels, which is the negative of the return loss. A VNA that shows minus 20 dB at a frequency is showing a return loss of 20 dB.