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Ground wave is a mode of radio propagation that consists of Electric current traveling through the Soil. Ground waves propagate parallel to and adjacent to the surface of the Earth, and are capable of covering long distances by Diffraction around the Earth's curvature. 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. Groundwave contrasts with line-of-sight propagation that requires no medium, and skywave via the ionosphere.
Ground wave is important for radio signals below 30 MHz, but is generally insignificant at higher frequencies where line-of-sight propagation dominates. AM broadcasting and longwave broadcasting, navigation systems such as Loran-C, low-frequency Time signal, non-directional beacons, and short-range High frequency communications all make use of it. Range depends on frequency and ground conductivity, with lower frequencies and higher ground conductivity permitting longer distances.
As the distance increases, ground waves spread out according to the inverse-square law. The imperfect conductivity of the ground tilts the waves forward, dissipating energy into the ground. The long wavelengths of these signals allow them to diffract over the horizon, but this leads to further losses. Signal strength tends to fall exponentially with distance once the Earth's curvature is significant. Above about 10 kHz, atmospheric refraction helps bend waves downward. Only vertically polarized waves travel well; horizontally polarized signals are heavily attenuated.
Groundwave signals are relatively immune to fading but changes in the ground can cause variation in signal strength. Attenuation over land is lowest in the winter in temperate climates and higher over water when seas are rough. Hills, mountains, urban areas, and forests can create areas of reduced signal strength. The Skin effect of ground waves varies, reaching tens of meters at medium frequencies over dry ground and even more at lower frequencies. Propagation predictions thus require knowing the electrical properties of subsurface layers, which are best measured from groundwave attenuation.
In the development of radio, ground waves were used extensively. Early commercial and professional radio services relied exclusively on long wave, low frequencies and ground-wave propagation. To prevent interference with these services, amateur and experimental transmitters were restricted to the high frequencies (HF), felt to be useless since their ground-wave range was limited. Upon discovery of the other propagation modes possible at medium wave and short wave frequencies, the advantages of HF for commercial and military purposes became apparent. Amateur experimentation was then confined to only authorized frequencies in the range.
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