Return loss is the ratio, in dB, between the power sent to an antenna and the power reflected back from it. A higher return loss means less power is reflected, so the antenna is better matched to its cable and radio.

Return loss and VSWR describe the same match on different scales. A return loss of 10 dB means 10% of the power is reflected, equal to a VSWR of about 1.9:1. A return loss of 14 dB equals 1.5:1 with 4% reflected, and 20 dB equals about 1.2:1 with 1% reflected.

Return loss is sometimes printed as a negative number, such as -10 dB. The negative figure is the reflection coefficient, often labelled S11, which is the same measurement written the other way round: a return loss of 10 dB and an S11 of -10 dB describe an identical match. On an S11 plot, a curve that dips further below zero shows a better match at that frequency.

Return loss plots show where an antenna is well matched across frequency. The plot dips at the frequencies the antenna is designed for, and the width of each dip shows the usable bandwidth on that band. For a cellular antenna, check that the plot is good across every band the local networks use, not only at its best point.