*Question T3C01:
Why are simplex UHF signals rarely heard beyond their radio horizon?*  

*Correct Answer is  C: UHF signals are usually not propagated by the ionosphere*  

### AI Response: (may contain errors)  

To understand why simplex UHF signals are rarely heard beyond their radio horizon, let's analyze the correct answer and the other choices.

### Correct Answer: C
**UHF signals are usually not propagated by the ionosphere.**

- **Explanation**: UHF (Ultra High Frequency) signals, which typically range from 300 MHz to 3 GHz, have wavelengths that are too short to be effectively reflected by the ionosphere. This means that UHF signals primarily rely on line-of-sight propagation, which limits their range to the radio horizon. The radio horizon is determined by the height of the antenna and the curvature of the Earth.

### Why the Other Choices are Incorrect:

- **A: They are too weak to go very far**  
  - **Explanation**: While UHF signals can be weak, the primary reason they don't travel far is due to the lack of ionospheric propagation, not their inherent weakness. With sufficient power and a good antenna, UHF signals can be strong enough to reach further distances, but they still won't propagate beyond the radio horizon.

- **B: FCC regulations prohibit them from going more than 50 miles**  
  - **Explanation**: There are no specific FCC regulations that limit UHF signals to a distance of 50 miles. The range of a signal is primarily determined by its frequency and propagation characteristics, not by regulatory limits.

- **D: UHF signals are absorbed by the ionospheric D region**  
  - **Explanation**: While the D region of the ionosphere does absorb some radio frequencies, it primarily affects lower frequencies (like MF and HF). UHF signals are not significantly absorbed by the D region; rather, they simply do not reflect off the ionosphere, which is why they cannot travel beyond the radio horizon.

### Summary
- UHF signals are limited in range due to their inability to be reflected by the ionosphere, relying instead on line-of-sight propagation.
- Understanding the propagation characteristics of different frequency bands is crucial for effective communication in amateur radio.

This knowledge will help you answer similar questions on the Extra Class exam.