*Question G9A07:
What must be done to prevent standing waves on a feed line connected to an antenna?*  

*Correct Answer is  D: The antenna feed point impedance must be matched to the characteristic impedance of the feed line*  

### AI Response: (may contain errors)  

To prevent standing waves on a feed line connected to an antenna, the correct approach is to ensure that the antenna feed point impedance matches the characteristic impedance of the feed line.

### Correct Answer Explanation:
- **D: The antenna feed point impedance must be matched to the characteristic impedance of the feed line.**  
  This is correct because when the impedance of the antenna (feed point) matches the characteristic impedance of the feed line, maximum power transfer occurs, and reflections are minimized. This prevents standing waves, which are caused by impedance mismatches leading to reflected signals.

### Incorrect Answer Explanations:
- **A: The antenna feed point must be at DC ground potential.**  
  This is incorrect because grounding the feed point does not directly address impedance matching. While grounding can help with safety and noise reduction, it does not prevent standing waves.

- **B: The feed line must be an odd number of electrical quarter wavelengths long.**  
  This is incorrect because using an odd number of quarter wavelengths can actually create a mismatch at the feed point, leading to standing waves. The length of the feed line should be considered in relation to the frequency and the impedance, but it does not inherently prevent standing waves.

- **C: The feed line must be an even number of physical half wavelengths long.**  
  This is also incorrect. While using an even number of half wavelengths can help in some cases, it does not guarantee that standing waves will be prevented. The key factor is still the impedance matching.

### Useful Formulas and Rules of Thumb:
- **Impedance Matching:**  
  To achieve impedance matching, use the formula:  
  $$Z_L = Z_0$$  
  where:
  - $Z_L$ = load impedance (antenna feed point)
  - $Z_0$ = characteristic impedance of the feed line (e.g., 50 ohms)

- **Standing Wave Ratio (SWR):**  
  The standing wave ratio can be calculated as:  
  $$SWR = \frac{1 + \Gamma}{1 - \Gamma}$$  
  where $\Gamma$ (the reflection coefficient) is defined as:  
  $$\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}$$  
  A lower SWR indicates better matching and fewer standing waves.

By ensuring proper impedance matching, you can effectively minimize standing waves on your feed line.