Ham Radio General Class Practice Test

How much must the power output of a transmitter be raised to change the S-meter reading on a distant receiver from S8 to S9?

1. Approximately 1.5 times

2. Approximately 2 times

3. Approximately 4 times

4. Approximately 8 times

The correct answer is: Approximately 4 times

To understand why raising the power output to change the S-meter reading from S8 to S9 requires approximately four times the power, it’s important to consider how S-meter readings correlate with signal strength and power levels. Signal strength indicated on an S-meter generally follows a logarithmic scale, which means that each S-unit is a marker of a significant increase in signal strength. Specifically, an increase of 1 S-unit corresponds to a change of about 6 dB. Therefore, moving from S8 to S9 represents an increase of 6 dB. Since decibels represent a ratio of power levels, an increase of 6 dB equates to a power increase of a factor of 4. This is because a ratio of power levels can be calculated using the formula for decibels: \[ \text{dB} = 10 \times \log_{10}\left(\frac{P2}{P1}\right) \] By rearranging this for a 6dB increase, we find: \[ 6 = 10 \times \log_{10}\left(\frac{P2}{P1}\right) \] \[ \log_{10}\left(\frac{P2}{P