When it comes to the voltage standing wave ratio (VSWR), we’re often told that we should struggle to keep our antennas tuned and with a VSWR below 2:1. This advice is specially useful if you don’t want to fry your rig with powerful reflected signals due to an impedance mismatch. But if your VSWR is within the safe limit, by no means you should worry about how much these reflective losses are affecting your transmission.

A signal’s power level in dB is denoted by power \(p\) compared to a reference power \(p_0\) (both in W, for instance). Therefore,

\[ P = 10 \log{10}{\left ( \frac{p}{p_0} \right )} \]

Given that 1 S-unit is roughly 6 dB, our transmitter would need to transmit at 25% of the original power for the receiver’s S-meter to drop one bar. This could make a difference if we need this extra bar to arrive above the receiver’s noise floor. But if we’re already S9, halving the power twice won’t affect your receiver’s ability to make you out. This why QRP (operating with 5 W at most) works if the conditions are right.

Now let’s look at how the VSWR impacts the signal power.

So, to get a drop of one bar in the S-meter (-6 dB), you’d have get a VSWR of 14:1. You could never reach this high a VSWR on modern radios because they have a safety measure that blocks any transmission attempt at excessively higher VSWR values. This is true for the Xiegu G90, for instance, which only works if the VSWR is 3:1 at most (a loss of approximately 1 dB or 0.17 S-units)