*Question T7C06:
What does an SWR reading of 4:1 indicate?*  

*Correct Answer is  D: Impedance mismatch*  

### AI Response: (may contain errors)  

An SWR (Standing Wave Ratio) reading of 4:1 indicates an impedance mismatch. Here's a breakdown of why this is the correct answer and why the other choices are incorrect:

### Correct Answer: Impedance mismatch
- An SWR of 4:1 means that the voltage standing wave ratio is four times higher in one direction than the other. This indicates that a significant portion of the power is being reflected back to the transmitter rather than being radiated by the antenna. 
- A good impedance match typically has an SWR of 1:1, meaning nearly all the power is being transmitted effectively.

### Why the other choices are incorrect:
- **A: Loss of -4 dB**  
  - This choice is incorrect because an SWR of 4:1 does not directly translate to a specific dB loss. The relationship between SWR and dB loss is not linear and requires specific calculations. A higher SWR indicates more reflected power, not a straightforward loss value.

- **B: Good impedance match**  
  - This choice is incorrect because a good impedance match is represented by an SWR of 1:1. An SWR of 4:1 indicates a poor match, leading to significant power reflection.

- **C: Gain of +4 dB**  
  - This choice is incorrect because SWR does not indicate gain. Instead, it reflects how well the antenna system is matched to the transmission line. A gain of +4 dB would imply an increase in power, which is not related to the SWR reading.

### Useful Formulas and Rules of Thumb:
- The relationship between SWR and return loss (in dB) can be approximated using the formula:
  
  $$ 
  RL = 20 \log_{10} \left( \frac{SWR + 1}{SWR - 1} \right) 
  $$

- For an SWR of 4:1:
  
  $$ 
  RL = 20 \log_{10} \left( \frac{4 + 1}{4 - 1} \right) = 20 \log_{10} \left( \frac{5}{3} \right) \approx 20 \times 0.2218 \approx 4.44 \text{ dB} 
  $$

This shows that a higher SWR correlates with a lower return loss, indicating a poor match. Understanding these concepts is crucial for effective antenna system performance.