*Question G9B03:
Which of the following best describes the radiation pattern of a quarter-wave ground-plane vertical antenna?*  

*Correct Answer is  D: Omnidirectional in azimuth*  

### AI Response: (may contain errors)  

The correct answer to the question about the radiation pattern of a quarter-wave ground-plane vertical antenna is:

**D: Omnidirectional in azimuth**

### Explanation of the Correct Answer:
- A quarter-wave ground-plane vertical antenna typically has a radiation pattern that is **omnidirectional in the horizontal plane (azimuth)**. This means it radiates equally in all directions around the vertical axis, making it effective for communication in all horizontal directions.

### Why the Other Choices are Incorrect:
- **A: Bi-directional in azimuth**  
  - This would imply that the antenna radiates more strongly in two opposite directions, which is not the case for a quarter-wave ground-plane antenna. It radiates uniformly around the vertical axis.

- **B: Isotropic**  
  - An isotropic radiator is a theoretical antenna that radiates equally in all directions in three-dimensional space. While the quarter-wave ground-plane antenna is omnidirectional in azimuth, it does not radiate equally in all directions vertically, as it has a null in the direction of the ground.

- **C: Hemispherical**  
  - A hemispherical radiation pattern would suggest that the antenna radiates in a half-sphere above it, which is not accurate for a quarter-wave ground-plane antenna. While it does radiate above the ground, it is not limited to a hemispherical pattern; it is more accurately described as omnidirectional in the horizontal plane.

### Useful Rules of Thumb:
- **Antenna Types**: Vertical antennas, like the quarter-wave ground-plane, are often used for their omnidirectional properties in the azimuth plane.
- **Ground Plane Effect**: The ground plane acts as a reflector, enhancing the radiation pattern and allowing for effective communication in all horizontal directions.

This understanding of the radiation pattern is crucial for effective antenna selection and deployment in amateur radio operations.