*Question G3B06:
What usually happens to radio waves with frequencies below the LUF?*  

*Correct Answer is  C: They are attenuated before reaching the destination*  

### AI Response: (may contain errors)  

To understand what happens to radio waves with frequencies below the Lowest Usable Frequency (LUF), let's break down the correct answer and the other options.

### Correct Answer: C - They are attenuated before reaching the destination

- **Explanation**: The LUF is the lowest frequency at which radio waves can be effectively transmitted and received via the ionosphere. Frequencies below the LUF typically do not have enough energy to be reflected by the ionosphere and instead get absorbed by the atmosphere, leading to significant attenuation. This means that the signal weakens and may not reach the intended destination.

### Why the Other Choices are Incorrect:

- **A: They are refracted back to Earth**  
  - **Explanation**: Refraction occurs when radio waves are bent as they pass through different layers of the atmosphere. However, frequencies below the LUF do not get refracted back to Earth; they are absorbed instead.

- **B: They pass through the ionosphere**  
  - **Explanation**: While some lower frequencies can pass through the ionosphere, those below the LUF are not effectively transmitted. Instead, they are absorbed, leading to attenuation rather than successful transmission.

- **D: They are refracted and trapped in the ionosphere to circle Earth**  
  - **Explanation**: This statement suggests that lower frequencies can be trapped and refracted in the ionosphere, which is not true for frequencies below the LUF. Such frequencies do not have the necessary characteristics to be refracted and trapped; they are instead absorbed.

### Summary

- Frequencies below the LUF are primarily **attenuated** due to absorption by the atmosphere.
- The LUF is critical for determining the effective transmission of radio waves via the ionosphere.

This understanding is essential for the Extra Class exam, as it highlights the relationship between frequency and ionospheric behavior.