*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 (quality factor) of an antenna increases, the SWR (Standing Wave Ratio) bandwidth decreases. Here's why:

1. **Correct Answer: SWR bandwidth decreases (B)**
   - The quality factor, $Q$, of an antenna is a measure of how narrow or wide the bandwidth is relative to its center frequency. It is defined as:
     $$ Q = \frac{f_0}{BW} $$
     where:
     - $f_0$ = resonant frequency
     - $BW$ = bandwidth
   - A higher $Q$ means a narrower bandwidth. Therefore, as $Q$ increases, the range of frequencies over which the antenna maintains a low SWR becomes smaller, leading to a decrease in SWR bandwidth.

2. **Why the other choices are incorrect:**
   - **A: SWR bandwidth increases**
     - This is incorrect because a higher $Q$ results in a narrower bandwidth, not a wider one.
   - **C: Gain is reduced**
     - The gain of an antenna is not directly related to its $Q$. While $Q$ affects bandwidth, gain is more related to the antenna's design and efficiency.
   - **D: More common-mode current is present on the feed line**
     - Common-mode current is related to the balance of the feed line and antenna system, not the $Q$ of the antenna. Increasing $Q$ does not inherently increase common-mode currents.

Understanding the relationship between $Q$ and bandwidth is crucial for designing and using antennas effectively, especially in applications where bandwidth is a critical factor.