*Question E9D06:
What happens to SWR bandwidth when one or more loading coils are used to resonate an electrically short antenna?*  

*Correct Answer is  B: It is decreased*  

### AI Response: (may contain errors)  

When one or more loading coils are used to resonate an electrically short antenna, the SWR (Standing Wave Ratio) bandwidth is decreased. Here's why:

1. **Why the correct answer is correct (B: It is decreased):**
   - Loading coils are used to make an electrically short antenna resonate at a desired frequency. However, these coils introduce additional inductance, which narrows the bandwidth over which the antenna can maintain a low SWR. This is because the reactance changes more rapidly with frequency when loading coils are present, leading to a narrower range of frequencies where the antenna is well-matched to the feed line.

2. **Why the other choices are incorrect:**
   - **A: It is increased**: This is incorrect because adding loading coils typically reduces the bandwidth, not increases it. The additional inductance from the coils causes the antenna to have a sharper resonance, thus decreasing the SWR bandwidth.
   - **C: It is unchanged if the loading coil is located at the feed point**: This is incorrect because the location of the loading coil does not prevent the decrease in bandwidth. The effect of the coil on bandwidth is due to its inductance, not its position.
   - **D: It is unchanged if the loading coil is located at a voltage maximum point**: This is also incorrect. While the position of the coil can affect the efficiency and radiation pattern of the antenna, it does not negate the fact that the bandwidth is decreased due to the added inductance.

3. **Useful formulas or rules of thumb:**
   - The bandwidth of an antenna is often related to its quality factor, $Q$, which is defined as:
     $$ Q = \frac{f_0}{\Delta f} $$
     where $f_0$ is the resonant frequency and $\Delta f$ is the bandwidth.
   - Adding loading coils increases the $Q$ of the antenna, which in turn decreases the bandwidth $\Delta f$.

In summary, using loading coils to resonate an electrically short antenna results in a decreased SWR bandwidth due to the increased inductance and quality factor.