The Impact of SWR and Loss in Audio and RF Systems
For audio there is a 3dB rule states that a change of 3 decibels in sound level corresponds to a doubling or halving of the power or intensity of the sound. However, for RF there is a 6dB rule states that a change of 1 S-Unit corresponds to 6 decibels, equivalent to a voltage ratio of two, or a power ratio of four.
From a single reference antenna, 100 watts ERP at an SWR of 1.0:1 becomes 88.9 watts when your SWR is 2.0:1. Remember that to add 1 S-Unit to your transmitted signal you need to increase your power by 400%. Reference the chart below, and you will see that a difference of 1 SWR unit from 1.0:1 to 2.0:1 puts losses right at about 2.78% of an S-Unit, which is pretty much undetectable on the receiving side.
* ERP = Percentage of Effective Radiated Power
SWR and Loss
| SWR READING | % OF LOSS | ERP* | S-Unit % Difference (Receiving Side) |
| 1.0:1 | 0.00% | 100.00% | 0.00% |
| 1.1:1 | 0.20% | 99.80% | 0.05% |
| 1.2:1 | 0.80% | 99.20% | 0.20% |
| 1.3:1 | 1.70% | 98.30% | 0.43% |
| 1.4:1 | 2.80% | 97.20% | 0.70% |
| 1.5:1 | 4.00% | 96.00% | 1.00% |
| 1.6:1 | 5.30% | 94.70% | 1.33% |
| 1.7:1 | 6.70% | 93.30% | 1.68% |
| 1.8:1 | 8.20% | 91.80% | 2.05% |
| 1.9:1 | 9.60% | 90.40% | 2.40% |
| 2.0:1 | 11.10% | 88.90% | 2.78% |
| 2.1:1 | 12.60% | 87.40% | 3.15% |
| 2.2:1 | 14.10% | 85.90% | 3.53% |
| 2.3:1 | 15.50% | 84.50% | 3.88% |
| 2.4:1 | 17.00% | 83.00% | 4.25% |
| 2.5:1 | 18.40% | 81.60% | 4.60% |
| 2.6:1 | 19.80% | 80.20% | 4.95% |
| 2.7:1 | 21.10% | 78.90% | 5.28% |
| 2.8:1 | 22.40% | 77.60% | 5.60% |
| 2.9:1 | 23.70% | 76.30% | 5.93% |
| 3.0:1 | 25.00% | 75.00% | 6.25% |
| 4.0:1 | 36.00% | 64.00% | 9.00% |
| 5.0:1 | 44.40% | 55.60% | 11.10% |
| 6.0:1 | 51.00% | 49.00% | 12.75% |
| 7.0:1 | 56.30% | 43.80% | 14.05% |
| 8.0:1 | 60.50% | 39.50% | 15.13% |
| 9.0:1 | 64.00% | 36.00% | 16.00% |
| 10.0:1 | 66.90% | 33.10% | 16.73% |
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