Question
How can two linearly polarized Yagi antennas be used to produce circular polarization?
Answer Options
- A) Stack two Yagis to form an array with the respective elements in parallel planes fed 90 degrees out of phase
- B) Stack two Yagis to form an array with the respective elements in parallel planes fed in phase
- 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
- D) Arrange two Yagis collinear to each other with the driven elements fed 180 degrees out of phase
Correct Answer: C
Explanation
Circular polarization (CP) is an ideal wave polarization for satellite or EME communication, as it eliminates fading due to Faraday rotation or satellite spin. CP is synthesized by combining two linearly polarized signals that are orthogonal (perpendicular) to each other and exactly 90^\circ out of phase in time.
Two linearly polarized Yagi antennas can produce circular polarization by being physically arranged so that two Yagis are on the same axis and perpendicular to each other (\text{H} and \text{V} components) with the driven elements at the same point on the boom. Most critically, they must be fed 90 \text{ degrees} out of phase. This creates the necessary rotating E-field vector that defines circular polarization.
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