How does a signal that reads 20 dB over S9 compare to one that reads S9 on a receiver, assuming a properly calibrated S meter?
Why S meter readings above S9 are given in decibels, and 20 dB is a power ratio of 10^(20/10) = 10^2 = 100. So a signal 20 dB over S9 delivers 100 times the power of an S9 signal (it is also 10 times the voltage, since voltage ratio is 10^(20/20)).
Watch out The choice saying 20 times more powerful confuses the decibel number with the power ratio itself; decibels are logarithmic, so 20 dB is not 20 times. Ten times power would be 10 dB, not 20 dB.
Every 10 dB multiplies power by 10: 10 dB = 10x, 20 dB = 100x, 30 dB = 1000x.