*Question G4D06:
How much change in signal strength is typically represented by one S unit?*  

*Correct Answer is  A: 6 dB*  

### AI Response: (may contain errors)  

In amateur radio, the S meter is used to measure the strength of received signals. The S meter is typically calibrated such that a change of one S unit corresponds to a change in signal strength of 6 dB.

### Explanation:

1. **Correct Answer: 6 dB (Choice A)**
   - The standard convention for S meters is that a change of one S unit represents a 6 dB change in signal strength. This means that if a signal increases by one S unit, its power has increased by 6 dB, which is equivalent to a fourfold increase in power because:
     $$10 \log_{10}(4) \approx 6 \, \text{dB}$$

2. **Why the Other Choices are Incorrect:**
   - **12 dB (Choice B):** A 12 dB change would represent a much larger change in power, specifically a 16-fold increase, which is not the standard for one S unit.
   - **15 dB (Choice C):** A 15 dB change would represent an even larger change in power, specifically a 32-fold increase, which is not the standard for one S unit.
   - **18 dB (Choice D):** An 18 dB change would represent a 64-fold increase in power, which is far beyond the standard for one S unit.

### Useful Rule of Thumb:

- **6 dB Rule:** A 6 dB increase in signal strength means the power has increased by a factor of 4, and a 6 dB decrease means the power has decreased by a factor of 4. This is a useful rule of thumb for understanding changes in signal strength as measured by S units.