*Question T3A09:
Which of the following results from the fact that signals propagated by the ionosphere are elliptically polarized?*  

*Correct Answer is  B: Either vertically or horizontally polarized antennas may be used for transmission or reception*  

### AI Response: (may contain errors)  

The correct answer to the question is:

**B: Either vertically or horizontally polarized antennas may be used for transmission or reception.**

### Explanation of the Correct Answer:
- **Elliptical Polarization**: Signals propagated by the ionosphere are often elliptically polarized, which means that the electric field vector of the radio wave traces out an ellipse as it propagates. This characteristic allows for flexibility in the orientation of receiving antennas.
- **Antenna Orientation**: Because the signal can be received regardless of whether the antenna is oriented vertically or horizontally, both types of antennas can effectively receive the signal. This is particularly useful in amateur radio, where conditions can vary.

### Explanation of the Incorrect Choices:
- **A: Digital modes are unusable**: This is incorrect because digital modes can be used effectively over ionospheric paths. The polarization does not inherently make digital modes unusable.
  
- **C: FM voice is unusable**: This is also incorrect. FM voice can be transmitted and received over ionospheric paths. The polarization does not prevent FM signals from being usable.
  
- **D: Both the transmitting and receiving antennas must be of the same polarization**: This is incorrect because, with elliptical polarization, the receiving antenna can be either vertically or horizontally polarized. It does not require matching polarization for effective communication.

### Summary:
- **Key Point**: The flexibility in antenna polarization due to elliptical polarization allows for effective communication using either vertical or horizontal antennas.
- **Rule of Thumb**: When dealing with signals that are elliptically polarized, you can use antennas of different polarizations without significant loss in signal quality.

This understanding is crucial for the Extra Class exam and practical amateur radio operations.