Estimate the lowest-lobe takeoff (elevation) angle of a horizontal antenna from its height above ground and the frequency.
A horizontal antenna over ground has its lowest radiation lobe at an elevation angle of arcsin(wavelength / (4 times height)). Higher antennas give a lower angle, which is better for DX. Below a quarter wavelength high there is no low lobe; the antenna fires mostly upward (NVIS), good for regional contacts.
It is the elevation angle of the strongest radiation lobe. A low takeoff angle (say 10 to 20 degrees) favours long-distance DX via the ionosphere; a high angle favours nearby regional contacts. For a horizontal antenna it depends mainly on height over ground.
The lowest lobe is at arcsin(wavelength / (4 times height)). Doubling the height roughly halves the angle. An antenna one wavelength up has a lowest lobe near 14 degrees, good for DX; at a quarter wave up it fires almost straight up.
Near Vertical Incidence Skywave is high-angle radiation that goes nearly straight up and rains back down over a few hundred kilometres, ideal for regional coverage. A low horizontal antenna, under about a quarter wavelength high, is naturally an NVIS antenna.