*Question E9A04:
Which of the following factors affect the feed point impedance of an antenna?*  

*Correct Answer is  B: Antenna height*  

### AI Response: (may contain errors)  

The correct answer to the question about factors affecting the feed point impedance of an antenna is **B: Antenna height**. Here's why:

### Explanation of the Correct Answer (B: Antenna height)
- **Antenna height** significantly influences the feed point impedance because it affects the antenna's radiation pattern and the surrounding environment. As the height increases, the impedance can change due to variations in ground reflection and the antenna's interaction with the surrounding space. Higher antennas typically have lower feed point impedance due to better radiation efficiency and reduced losses.

### Explanation of Incorrect Answers
- **A: Transmission line length**  
  While the length of the transmission line can affect the impedance seen at the transmitter end, it does not directly change the feed point impedance of the antenna itself. The feed point impedance is determined by the antenna's physical characteristics and its environment, not the transmission line.

- **C: The settings of an antenna tuner at the transmitter**  
  An antenna tuner is used to match the impedance of the antenna system to the transmitter. However, it does not affect the actual feed point impedance of the antenna. The tuner can adjust the impedance seen by the transmitter but does not change the inherent characteristics of the antenna.

- **D: The input power level**  
  The input power level does not affect the feed point impedance of the antenna. The impedance is a function of the antenna's design and environment, not the power being fed into it. However, higher power levels can lead to non-linear effects in some cases, but this is not a direct factor in determining feed point impedance.

### Useful Formulas or Rules of Thumb
- The feed point impedance of a dipole antenna can be approximated using the formula:
  
  $$Z_{in} = 73 + j42.5 \, \Omega$$

  for a center-fed half-wave dipole in free space. This value can vary based on height and surrounding conditions.

- A general rule of thumb is that as the height of a dipole antenna increases above ground, the feed point impedance tends to decrease, often approaching the theoretical value of 73 ohms in free space.

Understanding these factors will help you grasp how antenna characteristics influence performance and impedance, which is crucial for the Extra Class exam.