Ham Radio General Class Practice Test

How does an S meter reading of 20 dB over S-9 compare to an S-9 signal, assuming a properly calibrated S meter?

1. It is 10 times weaker

2. It is 20 times weaker

3. It is 20 times stronger

4. It is 100 times stronger

The correct answer is: It is 100 times stronger

An S meter reading of 20 dB over S-9 indicates the strength of the received signal compared to a baseline of S-9 (which is defined as a signal strength of 9 S-units above the noise floor). To understand this, it's important to know how decibels (dB) work in relation to signal strength. The S meter scale is logarithmic, and a change of 10 dB represents a tenfold change in power. Therefore, an S-9 signal is the reference point, which is considered a certain power level. Each additional S-unit typically represents about 6 dB of increase in signal strength. When you have a signal that reads S-9 plus 20 dB, this means you have 20 dB more than S-9. Since every 10 dB increase represents a tenfold increase in power, an increase of 20 dB is equivalent to 10 times 10, or 100 times more power than the S-9 signal. This is why an S meter reading of 20 dB over S-9 is described as 100 times stronger than the reference S-9 level.