*Question E9C03:
What type of radiation pattern is created by two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed in phase?*  

*Correct Answer is  C: A figure-eight broadside to 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 in phase, they create a specific radiation pattern due to the constructive and destructive interference of the radio waves they emit.

### Explanation:

1. **Correct Answer: A figure-eight broadside to the axis of the array (Choice C)**
   - **Broadside Array**: When antennas are fed in phase and spaced 1/2-wavelength apart, the strongest radiation occurs perpendicular to the line connecting the antennas. This is known as a broadside pattern.
   - **Figure-Eight Pattern**: The radiation pattern resembles a figure-eight, with two main lobes of radiation occurring broadside to the axis of the array. This is due to the constructive interference in these directions.

2. **Why Other Choices Are Incorrect:**
   - **Choice A: Omni-directional**
     - An omni-directional pattern radiates equally in all directions, which is not the case here. The spacing and phasing create directional lobes, not uniform radiation.
   - **Choice B: Cardioid**
     - A cardioid pattern is typically produced by antennas with a specific phase difference, such as 90 degrees, not when fed in phase. It has a single main lobe and a null in the opposite direction.
   - **Choice D: A figure-eight end-fire along the axis of the array**
     - An end-fire pattern occurs when antennas are fed with a phase difference that causes the main lobes to align along the axis of the array. Feeding in phase with 1/2-wavelength spacing does not produce this pattern.

### Useful Concepts:

- **In-Phase Feeding**: When antennas are fed in phase, the waves they emit reinforce each other in certain directions, leading to strong radiation in those directions.
- **Spacing**: The 1/2-wavelength spacing is crucial for creating the broadside figure-eight pattern, as it ensures that the waves from each antenna constructively interfere broadside to the array.

Understanding these principles helps predict the radiation pattern based on antenna configuration and feeding.