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
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