*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 discussing HF propagation, two important terms are the Lowest Usable Frequency (LUF) and the Maximum Usable Frequency (MUF).

1. **Correct Answer Explanation:**

   - **Correct Answer: A**  
     When the LUF exceeds the MUF, it means that the lowest frequency that can be used for reliable communication is higher than the maximum frequency that can be supported by the ionosphere for skywave propagation. This results in ordinary skywave communications not being possible over that path because no suitable frequency exists between the LUF and MUF.

2. **Why Other Choices Are Incorrect:**

   - **Choice B: HF communications over the path are enhanced**  
     This is incorrect because when the LUF exceeds the MUF, it indicates poor propagation conditions, not enhanced ones. There is no frequency available for effective skywave communication.

   - **Choice C: Double-hop propagation along the path is more common**  
     Double-hop propagation requires suitable frequencies to reflect off the ionosphere multiple times. If the LUF is greater than the MUF, no frequencies are available for even single-hop propagation, let alone double-hop.

   - **Choice D: Propagation over the path on all HF frequencies is enhanced**  
     This is incorrect because, with the LUF exceeding the MUF, no HF frequencies can propagate via skywave, so enhancement is not possible.

3. **Useful Formulas or Rules of Thumb:**

   - **LUF and MUF Relationship:**  
     For effective skywave communication, the frequency used should be between the LUF and MUF:
     $$ \text{LUF} < \text{Frequency} < \text{MUF} $$

   - **Propagation Condition:**  
     If $\text{LUF} > \text{MUF}$, then no suitable frequency exists for skywave propagation.

Understanding these concepts helps in predicting and analyzing HF propagation conditions, which is crucial for effective amateur radio communication.