If a signal transmitted with a power of 150 watts is received with an S-meter reading of "20 dB over S9," what would be the new reading if power was reduced to 15 watts?

Dropping the transmitter power from 150 watts to 15 watts is a 10-fold reduction in power (150 divided by 10 equals 15). According to the rules of decibels, a factor of 10 decrease in power always equals exactly a 10 dB reduction in signal strength. The initial S-meter reading was "20 dB over S9". To find the new reading, simply subtract the 10 dB signal loss from the starting value. Subtracting 10 dB from "S9 plus 20 dB" leaves a new reading of "S9 plus 10 dB".