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?
Why NVIS works by firing energy almost straight up so the ionosphere reflects it back down over a circle roughly 0 to 300 miles across, filling in the skip zone. A horizontal dipole mounted low, about 1/10 to 1/4 wavelength above ground, has its main lobe pointing nearly straight up because ground reflection reinforces the straight-up direction. On 40 meters by day that means roughly 10 to 30 feet up, which is easy to put up and gives reliable regional coverage.
Watch out A horizontal dipole at 1/2 wavelength is the classic DX height: the pattern splits into lower-angle lobes with a null overhead, which is exactly what you do not want for short skip. Verticals are worse still, radiating at very low angles toward the horizon.
NVIS = Never Very-high Install Skyward: keep the horizontal wire low, 1/10 to 1/4 wavelength, to shoot straight up.