Ground wave

Ground wave is a mode of radio propagation that consists of currents traveling through the earth. Ground waves propagate parallel to and adjacent to the surface of the Earth, and are capable covering long distances by diffracting around the curvature of the Earth. This radiation is also known as the Norton surface wave, or more properly the Norton ground wave, because ground waves in radio propagation are not confined to the surface. This mode can be compared to line-of-sight propagation that requires no medium, and skywave propagation that reflects signals off the ionosphere.

Ground wave is an important propagation mode for radio signals below 30 MHz, but is generally insignificant at higher frequencies where line-of-sight propagation dominates. AM and longwave broadcasting, navigation systems such as LORAN, low-frequency time signals, non-directional beacons, and short-range HF communications all make use of it. Range depends on frequency and ground conductivity, with lower frequencies and higher ground conductivity permitting longer distances.^[1]