*Question G3C10:
What is near vertical incidence skywave (NVIS) propagation?*  

*Correct Answer is  B: Short distance MF or HF propagation at high elevation angles*  

### AI Response: (may contain errors)  

Near Vertical Incidence Skywave (NVIS) propagation is a technique used to achieve short-distance communication using medium frequency (MF) or high frequency (HF) bands. Here's why the correct answer is B:

1. **Correct Answer Explanation (B):**
   - NVIS propagation involves transmitting radio waves at high elevation angles, typically between 60 and 90 degrees. These waves are reflected back to Earth by the ionosphere, allowing communication over short distances, typically 30 to 400 miles.
   - This method is particularly useful in areas with challenging terrain, such as mountains or dense forests, where line-of-sight communication is difficult.

2. **Why Other Choices Are Incorrect:**
   - **A: Propagation near the MUF**  
     - The Maximum Usable Frequency (MUF) is the highest frequency that can be used for skywave propagation between two points. NVIS is not specifically related to operating near the MUF; it focuses on high-angle propagation for short distances.
   - **C: Long path HF propagation at sunrise and sunset**  
     - Long path propagation involves signals traveling the long way around the Earth, often used for long-distance communication. NVIS is for short distances and does not rely on specific times like sunrise or sunset.
   - **D: Double hop propagation near the LUF**  
     - The Lowest Usable Frequency (LUF) is the lowest frequency that can be used for reliable communication. Double hop refers to signals reflecting twice off the ionosphere for longer distances, which is not the focus of NVIS.

3. **Useful Concepts:**
   - **Elevation Angle:** NVIS uses high elevation angles to ensure the signal is reflected back to the ground over a short distance.
   - **Ionospheric Reflection:** The ionosphere acts as a mirror for radio waves, allowing them to return to Earth and cover areas that are not reachable by direct line-of-sight.

Understanding NVIS is crucial for effective communication in regions where traditional methods are impractical.