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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