VSWR to Return Loss Conversion Table & Formulas

It is often necessary to convert between VSWR and return loss - this can be done using simple formulas or a table of values


VSWR & Transmission Line Theory Tutorial Includes:
What is VSWR?     Reflection Coefficient     VSWR formulas & calculations     How to measure VSWR     How to use a VSWR meter     Simple SWR bridge circuit     What is return loss     VSWR / Return Loss Table    


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.

Return Loss   = - 20 log 10 ( VSWR   -   1 VSWR   +   1 ) 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.

VSWR   =   10 Return Loss (db) 20 + 1 10 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.



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