A local amateur radio operator reports receiving your 100-watt 2-metre simplex transmission with an S-meter reading of "30 dB over S9." What power could you use to reduce that reading to S9?

To reduce a received signal by a specific decibel amount, you must decrease your transmitter power by that exact same decibel amount. For power calculations, a 10 dB change corresponds to a factor of 10. A 30 dB reduction is equal to three successive 10 dB drops. This means you must divide your original power by 10, then by 10 again, and by 10 a third time, which is equivalent to dividing by 1,000. Starting at 100 watts and dividing by 1,000 yields 0.1 watts. Options like 10 watts or 1 watt are incorrect because they only account for power reductions of 10 dB or 20 dB, respectively.