Question
Which of the following antenna types will be most effective as a near vertical incidence skywave (NVIS) antenna for short-skip communications on 40 meters during the day?
Answer Options
- A) A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground
- B) A vertical antenna placed between 1/4 and 1/2 wavelength above the ground
- C) A horizontal dipole placed at approximately 1/2 wavelength above the ground
- D) A vertical dipole placed at approximately 1/2 wavelength above the ground
Correct Answer: A
Explanation
Near Vertical Incidence Skywave (NVIS) propagation is necessary for reliable short-skip communication (up to 400 miles) that covers the area between ground-wave and traditional skywave coverage (the skip zone). NVIS requires a high-angle radiation lobe (close to 90^\circ straight up).
The antenna best suited for this is a horizontal dipole placed between \frac{1}{10} and \frac{1}{4} wavelength above the ground. This low height causes the signal to be reflected by the ground into a large, high-angle lobe, directing the signal straight up to the ionosphere for reflection back down over the local region.
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