Low frequencies: the ground wave
Below about 1.6 MHz the signal follows the curvature of the Earth as a ground wave and reaches hundreds of kilometres, more over sea water than over dry ground. At night the lower ionosphere stops absorbing medium waves and distant stations can be heard from far away.
HF: the ionosphere does the work
From 1.6 to 30 MHz the F layer can reflect signals back to Earth, in one or more hops, up to thousands of kilometres. Whether a band works depends on the Sun, the hour, the season and the path: the usable window lies between the lowest frequency that survives absorption and the maximum usable frequency. Read the guides on HF propagation and see the result for your own station in the HF mode of the coverage map.
VHF, UHF and SHF: radio horizon and terrain
From 30 MHz upwards the ionosphere is almost transparent, so range is set by the radio horizon, by the terrain between the antennas and by the environment around the receiver. The higher the frequency, the more the signal behaves like light: shorter diffraction, more loss behind obstacles and a need for a clean Fresnel zone. In exchange, antennas become small and highly directional and bandwidth becomes large.
