*Question E9C02:
What type of radiation pattern is created by two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart and fed 90 degrees out of phase?*  

*Correct Answer is  A: Cardioid*  

### AI Response: (may contain errors)  

When two 1/4-wavelength vertical antennas are spaced 1/4-wavelength apart and fed 90 degrees out of phase, they create a cardioid radiation pattern. Here's why:

1. **Correct Answer: Cardioid (A)**
   - **Explanation**: A cardioid pattern is characterized by a strong directional lobe in one direction and a null in the opposite direction. This pattern occurs because the 90-degree phase difference between the antennas causes constructive interference in one direction and destructive interference in the opposite direction. The spacing of 1/4-wavelength helps achieve this specific pattern by ensuring the phase difference results in the desired interference pattern.

2. **Why Other Choices Are Incorrect**:
   - **B: A figure-eight end-fire along the axis of the array**: 
     - A figure-eight end-fire pattern typically results from antennas fed in phase or 180 degrees out of phase, not 90 degrees. The end-fire pattern would have lobes along the axis of the array, which is not the case here.
   - **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 appropriately, usually 1/2-wavelength apart. The 90-degree phase difference and 1/4-wavelength spacing do not support this pattern.
   - **D: Omni-directional**: 
     - An omni-directional pattern radiates equally in all directions, which is not possible with the given setup. The phase difference and spacing create directional characteristics, not uniform radiation.

3. **Useful Concepts**:
   - **Phase Difference**: The 90-degree phase difference is crucial for creating the cardioid pattern, as it determines the direction of constructive and destructive interference.
   - **Antenna Spacing**: The 1/4-wavelength spacing is important for achieving the desired interference pattern, as it affects the phase relationship between the signals from each antenna.

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