*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)  

Radio waves with frequencies below the Lowest Usable Frequency (LUF) are typically attenuated before reaching their destination. Here's why:

1. **Correct Answer (C): They are attenuated before reaching the destination.**
   - The LUF is the lowest frequency at which a radio wave can propagate over a given path under specific ionospheric conditions. Frequencies below the LUF experience significant absorption in the ionosphere, particularly in the D-layer, which leads to attenuation. This means the signal weakens and may not reach the intended destination.

2. **Why the other choices are incorrect:**
   - **A: They are refracted back to Earth**
     - While higher frequencies can be refracted back to Earth by the ionosphere, frequencies below the LUF are absorbed rather than refracted.
   - **B: They pass through the ionosphere**
     - Frequencies below the LUF do not pass through the ionosphere; instead, they are absorbed and attenuated.
   - **D: They are refracted and trapped in the ionosphere to circle Earth**
     - This describes a phenomenon known as ducting, which is not applicable to frequencies below the LUF. Such frequencies are absorbed rather than trapped.

3. **Useful Concepts:**
   - **LUF (Lowest Usable Frequency):** The minimum frequency that can be used for reliable communication over a specific path.
   - **Ionospheric Absorption:** Primarily occurs in the D-layer, affecting lower frequencies more significantly.

Understanding the behavior of radio waves in relation to the LUF is crucial for effective communication planning in amateur radio operations.