*Question E9D02:
How can two linearly polarized Yagi antennas be used to produce circular polarization?*  

*Correct Answer is  C: Arrange two Yagis on the same axis and perpendicular to each other with the driven elements at the same point on the boom and fed 90 degrees out of phase*  

### AI Response: (may contain errors)  

To produce circular polarization using two linearly polarized Yagi antennas, the correct method is to arrange them on the same axis and perpendicular to each other, with the driven elements at the same point on the boom and fed 90 degrees out of phase. 

### Explanation of the Correct Answer (C):
- **Circular Polarization**: This occurs when two linearly polarized waves are combined in such a way that their electric field vectors rotate in a circular motion. 
- **Configuration**: By placing two Yagi antennas perpendicular to each other (one horizontal and one vertical) and feeding them 90 degrees out of phase, the resultant fields combine to create a rotating electric field, which is characteristic of circular polarization.

### Why the Other Choices are Incorrect:
- **Choice A**: Stack two Yagis to form an array with the respective elements in parallel planes fed 90 degrees out of phase.
  - **Incorrect**: While feeding 90 degrees out of phase is correct, having the antennas in parallel planes does not achieve the necessary perpendicular orientation to create circular polarization.

- **Choice B**: Stack two Yagis to form an array with the respective elements in parallel planes fed in phase.
  - **Incorrect**: Feeding in phase does not create the necessary phase difference to achieve circular polarization, and being in parallel planes does not help in achieving the required orientation.

- **Choice D**: Arrange two Yagis collinear to each other with the driven elements fed 180 degrees out of phase.
  - **Incorrect**: Feeding the antennas 180 degrees out of phase would cancel each other out rather than produce circular polarization. Additionally, being collinear does not provide the necessary perpendicular orientation.

### Summary:
To achieve circular polarization with two Yagi antennas:
- **Orientation**: Perpendicular to each other
- **Phase Difference**: 90 degrees out of phase

This configuration allows the electric fields to combine effectively, resulting in circular polarization.