*Question E9D08:
What happens as the Q of an antenna increases?*  

*Correct Answer is  B: SWR bandwidth decreases*  

### AI Response: (may contain errors)  

As the Q of an antenna increases, the SWR (Standing Wave Ratio) bandwidth decreases. Let's break down the reasoning:

### Correct Answer: B - SWR bandwidth decreases

- **Explanation**: 
  - The Q factor (quality factor) of an antenna is a measure of its selectivity or sharpness of resonance. A higher Q indicates a narrower bandwidth around the resonant frequency. 
  - This means that as the Q increases, the range of frequencies over which the antenna can operate effectively (i.e., maintain a low SWR) becomes smaller. Thus, the SWR bandwidth decreases.

### Why the Other Choices are Incorrect:

- **A: SWR bandwidth increases**  
  - This is incorrect because a higher Q indicates a narrower bandwidth, not a wider one. A lower Q would result in a broader bandwidth.

- **C: Gain is reduced**  
  - This is not necessarily true. While a higher Q can lead to increased gain at the resonant frequency due to better energy storage, it does not inherently reduce gain. Gain is more related to the design and efficiency of the antenna rather than the Q factor alone.

- **D: More common-mode current is present on the feed line**  
  - This statement is not directly related to the Q factor. Common-mode currents can be influenced by various factors, including feed line design and grounding, but an increase in Q does not inherently increase common-mode currents.

### Useful Formulas or Rules of Thumb:

- The relationship between Q, bandwidth ($BW$), and resonant frequency ($f_0$) can be expressed as:

$$
Q = \frac{f_0}{BW}
$$

- From this formula, you can see that as Q increases, the bandwidth ($BW$) must decrease for a constant resonant frequency ($f_0$).

In summary, a higher Q results in a narrower SWR bandwidth, which is why the correct answer is B. Understanding the implications of Q on bandwidth is crucial for antenna design and operation in amateur radio.