*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)  

To understand the radiation pattern created by two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed in phase, we need to analyze how the antennas interact with each other.

### Correct Answer: C - A figure-eight broadside to the axis of the array

1. **Why the correct answer is correct:**
   - When two antennas are fed in phase and spaced 1/2-wavelength apart, they create constructive interference in the direction perpendicular to the line connecting them (broadside). This results in a radiation pattern that resembles a figure-eight shape, with maximum radiation in the broadside direction and nulls along the axis of the array.

2. **Why the other choices are incorrect:**
   - **A: Omni-directional**: An omni-directional pattern radiates equally in all horizontal directions. The configuration described does not produce this pattern due to the phase relationship and spacing.
   - **B: Cardioid**: A cardioid pattern has a single lobe with a null directly opposite the direction of maximum radiation. This configuration does not create a cardioid pattern.
   - **D: A figure-eight end-fire along the axis of the array**: An end-fire pattern occurs when antennas are spaced and fed in such a way that maximum radiation is directed along the line of the array. This is not the case here, as the maximum radiation is broadside.

### Useful Concepts:
- **Constructive Interference**: Occurs when the waves from both antennas add together in phase, enhancing the signal in certain directions.
- **Destructive Interference**: Occurs when the waves are out of phase, leading to nulls in the radiation pattern.

### Summary:
The configuration of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed in phase results in a figure-eight radiation pattern broadside to the axis of the array due to constructive interference in that direction.