*Question E9C01:
What type of radiation pattern is created by two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase?*  

*Correct Answer is  D: A figure-eight oriented along the axis of the array*  

### AI Response: (may contain errors)  

When two 1/4-wavelength vertical antennas are spaced 1/2-wavelength apart and fed 180 degrees out of phase, they create a specific radiation pattern due to the interference of the radio waves they emit.

### Correct Answer: D: A figure-eight oriented along the axis of the array

1. **Explanation for the Correct Answer:**
   - **Figure-Eight Pattern:** When antennas are fed 180 degrees out of phase, the signals they emit interfere destructively in some directions and constructively in others. This results in a figure-eight pattern.
   - **Orientation Along the Axis:** The pattern is oriented along the axis of the array because the antennas are spaced 1/2-wavelength apart. This spacing causes the main lobes of the radiation pattern to be directed along the line connecting the antennas.

### Why Other Choices Are Incorrect:

2. **Choice A: Cardioid**
   - A cardioid pattern typically results from two antennas spaced 1/4-wavelength apart and fed with a 90-degree phase difference. This setup creates a directional pattern with a single main lobe and a null in the opposite direction, which is not the case here.

3. **Choice B: Omni-directional**
   - An omni-directional pattern radiates equally in all directions, which is characteristic of a single vertical antenna or multiple antennas fed in phase. The 180-degree phase difference in this setup prevents an omni-directional pattern.

4. **Choice C: A figure-eight broadside to the axis of the array**
   - A broadside figure-eight pattern occurs when antennas are fed in phase and spaced 1/2-wavelength apart. The 180-degree phase difference in this question causes the pattern to be along the axis, not broadside.

### Useful Concepts:

- **Phase Difference:** The phase difference between antennas affects the directionality of the radiation pattern. A 180-degree phase difference leads to a pattern oriented along the axis.
- **Spacing:** The distance between antennas (1/2-wavelength in this case) influences the shape and orientation of the pattern.

Understanding these principles helps predict the radiation pattern for different antenna configurations.