*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, you need to combine their signals in such a way that the resulting electromagnetic wave rotates in a circular manner. Here's how the correct setup works:

### Correct Answer: 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**

1. **Perpendicular Arrangement**: By placing the Yagis perpendicular to each other, you ensure that one antenna is horizontally polarized and the other is vertically polarized.
2. **90 Degrees Out of Phase**: Feeding the antennas 90 degrees out of phase means that the signal from one antenna leads or lags the other by a quarter of a wavelength. This phase difference is crucial for creating the rotating electric field characteristic of circular polarization.

### Why 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"
  - **Parallel Planes**: If the antennas are in parallel planes, they will not produce the orthogonal fields needed for circular polarization. The fields will simply add linearly, not circularly.

- **Choice B**: "Stack two Yagis to form an array with the respective elements in parallel planes fed in phase"
  - **In Phase**: Feeding the antennas in phase will result in linear polarization, not circular. The signals will reinforce each other in the same direction.

- **Choice D**: "Arrange two Yagis collinear to each other with the driven elements fed 180 degrees out of phase"
  - **Collinear Arrangement**: This setup will not create the necessary orthogonal fields. Feeding them 180 degrees out of phase will result in cancellation or reinforcement along the same axis, not circular polarization.

### Key Concept:
- **Circular Polarization**: Achieved by combining two orthogonal (perpendicular) linear polarizations with a 90-degree phase difference. This creates a rotating electric field vector, which is the hallmark of circular polarization.

Understanding these principles will help you correctly identify how to achieve circular polarization with Yagi antennas.