*Question T9B09:
What can cause erratic changes in SWR?*  

*Correct Answer is  B: Loose connection in the antenna or feed line*  

### AI Response: (may contain errors)  

To understand what can cause erratic changes in SWR (Standing Wave Ratio), let's analyze the correct answer and the other options.

### Correct Answer: B - Loose connection in the antenna or feed line

- **Explanation**: A loose connection in the antenna or feed line can lead to intermittent contact, which causes variations in the impedance seen by the transmitter. This results in fluctuating SWR readings because the power reflected back to the transmitter changes as the connection intermittently makes and breaks contact. 

### Why the Other Choices are Incorrect:

- **A: Local thunderstorm**  
  - **Explanation**: While a thunderstorm can introduce noise and interference, it does not directly affect the SWR. SWR is primarily a function of the impedance matching between the antenna and the feed line, which is not influenced by atmospheric conditions.

- **C: Over-modulation**  
  - **Explanation**: Over-modulation affects the quality of the transmitted signal and can lead to distortion, but it does not cause changes in SWR. SWR is related to the impedance mismatch, not the modulation of the signal.

- **D: Overload from a strong local station**  
  - **Explanation**: Overloading from a strong local station can cause receiver issues, such as desensitization or intermodulation distortion, but it does not directly affect the SWR. SWR is determined by the antenna system's impedance characteristics.

### Useful Concepts:

- **SWR Definition**: SWR is defined as the ratio of the maximum to minimum voltage along the transmission line. It indicates how well the antenna is matched to the transmission line.

- **SWR Formula**: The relationship can be expressed as:
  
  $$SWR = \frac{1 + \Gamma}{1 - \Gamma}$$

  where $\Gamma$ (Gamma) is the reflection coefficient, which quantifies how much power is reflected due to impedance mismatch.

- **Rule of Thumb**: An SWR of 1:1 indicates perfect matching, while higher values indicate increasing levels of mismatch, which can lead to power loss and potential damage to the transmitter.

In summary, a loose connection is the primary cause of erratic SWR changes, while the other options do not directly influence SWR readings. Understanding these concepts will help you in your exam preparation.