*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)  

A quarter-wave ground-plane vertical antenna is a common type of antenna used in amateur radio. Let's break down the radiation pattern and why the correct answer is "Omnidirectional in azimuth."

### Correct Answer: Omnidirectional in azimuth

1. **Why it's correct:**
   - A quarter-wave ground-plane vertical antenna radiates equally in all horizontal directions. This means that if you were to look at the radiation pattern from above, it would form a circle, indicating that the signal strength is the same in every direction around the antenna. This is what "omnidirectional in azimuth" means.

### Why the Other Choices are Incorrect

2. **Choice A: Bi-directional in azimuth**
   - A bi-directional pattern would mean the antenna radiates more strongly in two opposite directions, which is not the case for a quarter-wave vertical antenna. This pattern is more typical of a dipole antenna.

3. **Choice B: Isotropic**
   - An isotropic antenna is a theoretical point source that radiates equally in all directions, both horizontally and vertically, forming a perfect sphere. Real antennas, including the quarter-wave vertical, cannot achieve this pattern.

4. **Choice C: Hemispherical**
   - A hemispherical pattern would imply that the antenna radiates in a half-sphere, which would include significant vertical radiation. The quarter-wave vertical primarily radiates horizontally, not in a hemispherical pattern.

### Useful Concepts

- **Radiation Pattern:** The graphical representation of the strength of the radio waves emitted by the antenna in different directions.
- **Omnidirectional:** Equal radiation in all directions in a given plane, typically the horizontal plane for vertical antennas.

Understanding these concepts will help you identify the radiation patterns of different antennas, which is a key part of the Extra Class exam.