*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 radio wave propagation method that is particularly effective for short-distance communication, typically within a range of 300 miles (about 500 kilometers). It utilizes high elevation angles to reflect radio waves off the ionosphere, allowing for effective communication over relatively short distances.

### Correct Answer Explanation:
**B: Short distance MF or HF propagation at high elevation angles**  
- NVIS is characterized by the use of high-angle radiation (typically between 60° and 90°) to achieve short-distance communication. This is particularly useful for emergency communications and local operations, as it allows signals to be reflected back to the ground over short distances.

### Incorrect Answer Explanations:
- **A: Propagation near the MUF**  
  - The Maximum Usable Frequency (MUF) refers to the highest frequency that can be used for reliable communication via skywave propagation. While NVIS can occur below the MUF, it is not specifically defined by proximity to it.

- **C: Long path HF propagation at sunrise and sunset**  
  - This describes a different propagation phenomenon known as "long path" propagation, which occurs over longer distances and is influenced by the time of day. NVIS is not focused on long distances but rather on short-range communication.

- **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 propagation typically involves signals bouncing off the ionosphere multiple times, which is not characteristic of NVIS, as NVIS relies on a single high-angle reflection.

### Useful Rules of Thumb:
- NVIS is most effective on frequencies between 3 MHz and 30 MHz, particularly in the HF band.
- For optimal NVIS performance, use antennas designed for high-angle radiation, such as dipoles or inverted V antennas, positioned close to the ground.

Understanding NVIS propagation is crucial for effective short-range communication, especially in emergency or localized scenarios.