*Question G3B11:
What happens to HF propagation when the LUF exceeds the MUF?*  

*Correct Answer is  A: Propagation via ordinary skywave communications is not possible over that path*  

### AI Response: (may contain errors)  

When considering HF propagation, it's important to understand the concepts of the Lowest Usable Frequency (LUF) and the Maximum Usable Frequency (MUF).

### Correct Answer Explanation:
- **Correct Answer: A: Propagation via ordinary skywave communications is not possible over that path.**
  
  When the LUF exceeds the MUF, it indicates that the frequencies available for communication are not suitable for skywave propagation. The MUF is the highest frequency that can be used for skywave propagation over a specific path, while the LUF is the lowest frequency that can be used. If the LUF is higher than the MUF, it means that all usable frequencies for that path are above the maximum frequency that can be effectively reflected by the ionosphere, making ordinary skywave communications impossible.

### Incorrect Answer Explanations:
- **B: HF communications over the path are enhanced.**
  
  This is incorrect because if the LUF exceeds the MUF, it means that no frequencies are available for effective communication, thus not enhancing HF communications.

- **C: Double-hop propagation along the path is more common.**
  
  This is also incorrect. Double-hop propagation requires frequencies to be within the range of the MUF. If the LUF is above the MUF, double-hop propagation cannot occur.

- **D: Propagation over the path on all HF frequencies is enhanced.**
  
  This choice is incorrect as well. If the LUF exceeds the MUF, it means that no frequencies are available for propagation, so there cannot be any enhancement in propagation.

### Summary:
- **LUF**: Lowest Usable Frequency for communication.
- **MUF**: Maximum Usable Frequency for communication.

When $LUF > MUF$, ordinary skywave communications are not possible, confirming that choice A is correct. 

### Rule of Thumb:
- Always remember: For effective HF skywave communication, the frequency must be between the LUF and MUF. If the LUF is higher than the MUF, communication is not feasible.