Question
What is near vertical incidence skywave (NVIS) propagation?
Answer Options
- A) Propagation near the MUF
- B) Short distance MF or HF propagation at high elevation angles
- C) Long path HF propagation at sunrise and sunset
- D) Double hop propagation near the LUF
Correct Answer: B
Explanation
Near Vertical Incidence Skywave (NVIS) is a specialized technique used for reliable, regional communication (typically 0 to 400 miles) that requires filling the skip zone. NVIS utilizes MF or HF propagation at very high elevation angles (close to 90 degrees, or straight up).
By transmitting the signal almost straight up, the ionosphere (usually the F layer) refracts the signal back down over a wide, circular area surrounding the transmitter. This ensures coverage in the entire area that would otherwise be the skip zone. NVIS is generally performed on the lower HF bands (40m, 60m, 80m, 160m) and is critical for emergency communications.
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