*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 primarily due to impedance mismatches. Let's break down the correct answer and the other choices:

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

- **Explanation**: When the impedance of the feed line does not match the impedance of the antenna at its feed point, some of the power sent from the transmitter is reflected back instead of being radiated. This mismatch can lead to standing waves on the feed line, which is a direct cause of reflected power.

### Why the Other Choices are Incorrect:

- **A: Operating an antenna at its resonant frequency**  
  - **Explanation**: Operating at the resonant frequency typically minimizes reflected power because the antenna is designed to match the feed line impedance at this frequency. Therefore, this choice does not cause reflected power.

- **B: Using more transmitter power than the antenna can handle**  
  - **Explanation**: While using excessive power can damage the antenna, it does not inherently cause reflected power. Reflected power is a result of impedance mismatch, not the amount of power used.

- **D: Feeding the antenna with unbalanced feed line**  
  - **Explanation**: While using an unbalanced feed line can lead to other issues, such as common mode currents, it does not directly cause reflected power unless there is also an impedance mismatch. The key factor for reflected power is the impedance difference.

### Useful Formulas and Rules of Thumb:

- **Reflection Coefficient ($\Gamma$)**: This is a measure of how much power is reflected due to impedance mismatch and is given by:
  
  $$\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}$$

  where:
  - $Z_L$ = load impedance (antenna feed point impedance)
  - $Z_0$ = characteristic impedance of the feed line

- **Standing Wave Ratio (SWR)**: This is a measure of the efficiency of power transfer and is related to the reflection coefficient:
  
  $$SWR = \frac{1 + |\Gamma|}{1 - |\Gamma|}$$

  A lower SWR indicates better matching and less reflected power.

Understanding these concepts will help you grasp why impedance matching is crucial in antenna systems and how it affects reflected power.