*Question G9A04:
What causes reflected power at an antenna’s feed point?*  

*Correct Answer is  C: A difference between feed line impedance and antenna feed point impedance*  

### AI Response: (may contain errors)  

Reflected power at an antenna's feed point occurs when there is an impedance mismatch between the feed line and the antenna. This mismatch causes some of the power to be reflected back toward the transmitter instead of being radiated by the antenna.

### Correct Answer:  
**C: A difference between feed line impedance and antenna feed point impedance**

1. **Explanation:**
   - When the impedance of the feed line does not match the impedance of the antenna, not all the power is transferred efficiently. This mismatch causes a portion of the power to be reflected back, leading to reflected power.
   - The reflection coefficient ($\Gamma$) can be calculated using the formula:  
     $$\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}$$  
     where:
     - $Z_L$ = load impedance (antenna)
     - $Z_0$ = characteristic impedance of the feed line
   - A non-zero reflection coefficient indicates reflected power.

### Incorrect Answers:

- **A: Operating an antenna at its resonant frequency**
  - At resonance, an antenna is typically well-matched to the feed line, minimizing reflected power. Therefore, this does not cause reflected power.

- **B: Using more transmitter power than the antenna can handle**
  - This might damage the antenna but does not inherently cause reflected power. Reflected power is related to impedance mismatch, not power levels.

- **D: Feeding the antenna with unbalanced feed line**
  - While using an unbalanced feed line with a balanced antenna can cause other issues (like common-mode currents), it does not directly cause reflected power unless there is an impedance mismatch.

Understanding impedance matching is crucial for minimizing reflected power and ensuring efficient power transfer from the transmitter to the antenna.