*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, the key is to ensure that the antenna feed point impedance is matched to the characteristic impedance of the feed line. This is because standing waves are caused by reflections that occur when there is an impedance mismatch.

### Correct Answer: D
- **Explanation**: When the antenna feed point impedance matches the characteristic impedance of the feed line, all the power is transferred from the feed line to the antenna without any reflections. This condition is known as a "matched" condition, and it minimizes standing waves.
- **Formula**: The reflection coefficient $\Gamma$ 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

  When $Z_L = Z_0$, $\Gamma = 0$, meaning no reflections occur.

### Why Other Choices Are Incorrect:
- **Choice A**: "The antenna feed point must be at DC ground potential"
  - **Explanation**: DC grounding does not affect the impedance matching or standing waves. It is more related to safety and lightning protection.

- **Choice B**: "The feed line must be an odd number of electrical quarter wavelengths long"
  - **Explanation**: The length of the feed line does not inherently prevent standing waves. While certain lengths can transform impedances, they do not eliminate reflections caused by mismatches.

- **Choice C**: "The feed line must be an even number of physical half wavelengths long"
  - **Explanation**: Similar to choice B, the length of the feed line does not prevent standing waves. An even number of half wavelengths can repeat the impedance at the feed point, but it does not solve mismatch issues.

In summary, matching the antenna feed point impedance to the feed line's characteristic impedance is crucial to prevent standing waves and ensure efficient power transfer.