It is often necessary to convert VSWR to return loss or return loss to VSWR. The conversion can be achieved quite easily using either simple formulas, or using a table of values.

Return Loss and VSWR are both parameters used to measure the match between a feeder and a load, although the two quantities are slightly different.

The VSWR is a measure of the standing waves set up in a feeder as a result of a mismatch, whereas the return loss looks at the amount of power absorbed by a load when power from a source is sent to it. The return loss being the difference between the incident power and the reflected.

In view of the overlap between the two quantities it is useful to be able to convert VSWR to return loss and return loss to VSWR.

## VSWR to return loss

Using the simple formula given below it is possible to convert a given value of VSWR to an equivalent return loss. Note that return loss is given as a ratio expressed in decibels.

$\mathrm{Return\; Loss}=-20{\mathrm{log}}_{10}\left(\frac{\mathrm{VSWR}-1}{\mathrm{VSWR}+1}\right)\mathrm{dB}$

Note that the return loss is given as a negative figure. Being a loss the returned power must be less than the forward power, and hence the return loss has a minus sign - negative figures of decibels represent a loss.

## Return loss to VSWR

It is also possible to convert from return loss to VSWR jsut as easily as the other way round.

Sometimes, for example whent he return loss is quited for an antenna, it might be useful to know what VSWR this would represent on the feeder.

$\mathrm{VSWR}=\frac{{10}^{\frac{\mathrm{Return\; Loss\; (db)}}{20}}+1}{{10}^{\frac{\mathrm{Return\; Loss\; (db)}}{20}}-1}$

Note: this is expressed as a ratio in the normal way for VSWR, i.e. x:1.

## VSWR to return loss & return loss to VSWR table

It is sometimes easier to undertake the conversions between return loss and VSWR using a table. Values of retrun loss to VSWR and VSWR to return loss can be read off for most useable values required within the table.

VSWR to Return Loss and Return Loss to VSWR | ||
---|---|---|

Return loss (db) | VSWR | Voltage Reflection Coefficient |

1 | 17.391 | 0.891 |

2 | 8.724 | 0.794 |

3 | 5.848 | 0.708 |

4 | 4.419 | 0.631 |

5 | 3.570 | 0.562 |

6 | 3.010 | 0.501 |

7 | 2.615 | 0.447 |

8 | 2.323 | 0.398 |

9 | 2.100 | 0.355 |

10 | 1.925 | 0.316 |

11 | 1.785 | 0.282 |

12 | 1.671 | 0.251 |

13 | 1.577 | 0.224 |

14 | 1.499 | 0.200 |

15 | 1.433 | 0.178 |

16 | 1.377 | 0.158 |

17 | 1.329 | 0.141 |

18 | 1.288 | 0.126 |

19 | 1.253 | 0.112 |

20 | 1.222 | 0.100 |

21 | 1.196 | 0.089 |

22 | 1.173 | 0.079 |

23 | 1.152 | 0.071 |

24 | 1.135 | 0.063 |

25 | 1.119 | 0.056 |

26 | 1.105 | 0.050 |

27 | 1.094 | 0.045 |

28 | 1.083 | 0.040 |

29 | 1.074 | 0.035 |

30 | 1.065 | 0.032 |

VSWR and return loss are both ways of expressing power reflected as a result of impedance mismatches. VSWR to return loss and return loss to VSWR conversions are often needed.

Converting from return loss to VSWR or vice versa can be achieved very easily using the basic formulas given above, or using the table provided.