Ground waves are radio waves propagating parallel to and adjacent to the surface of the Earth, following the curvature of the Earth beyond the visible horizon. This radiation is known as Norton surface wave, or more properly Norton ground wave, because ground waves in radio propagation are not confined to the surface.
The normal line-of-sight distance to the horizon is about 80 to 100 kilometers (50 to 60 miles) at best, for an antenna on a tall tower. But signals at medium or low frequency are observed to travel several hundred miles beyond the horizon, up to several thousand kilometers in some cases. This "hugging the ground" effect is called ground wave, as opposed to direct wave or sky wave. This effect does not occur for signals in the VHF or UHF frequencies, which typically propagate only about 100 km (60 miles) to the horizon.
AM radio stations rely on ground wave propagation to cover their listening areas, which are typically several hundred miles radius.
Lower frequency radio waves, below 3 MHz, travel efficiently as ground waves. In ITU nomenclature, this includes (in order): medium frequency (MF), low frequency (LF), very low frequency (VLF), ultra low frequency (ULF), super low frequency (SLF), extremely low frequency (ELF) waves.
Ground propagation works because lower-frequency waves are more strongly diffracted around obstacles due to their long wavelengths, allowing them to repeatedly bend downward to follow the Earth's curvature. See Knife Edge Effect
Do not confuse with refraction. Refraction also causes radio waves to bend toward the ground, slightly extending propagation distance, but refraction alone generally only extends the radio horizon by about 15% (4/3), not enough to explain the much longer distances that ground waves are observed to propagate. Refraction decreases at lower frequencies, whereas diffraction increases at lower frequencies. Medium-wave and Long-wave signals are of such long wavelength that mountain ridges and low hills act as a knife edge, diffracting signals downward into the valleys behind such obstacles. The horizon itself may act as the obstacle around which long waves diffract, as when low-frequency waves are observed to travel thousands of kilometers across open oceans, where no sharp-edged obstacles exist.
Conductivity of the surface affects the propagation of ground waves, with more conductive surfaces such as sea water providing better propagation. Increasing the conductivity in a surface results in less dissipation. The refractive indices are subject to spatial and temporal changes. Since the ground is not a perfect electrical conductor, ground waves are attenuated as they follow the earth's surface. The wavefronts initially are vertical, but the ground, acting as a lossy dielectric, causes the wave to tilt forward as it travels. This directs some of the energy into the earth where it is dissipated, so that the signal decreases exponentially.
Most long-distance LF "longwave" radio communication (between 30 kHz and 300 kHz) is a result of groundwave propagation. Mediumwave radio transmissions (frequencies between 300 kHz and 3000 kHz), including AM broadcast band, travel both as groundwaves and, for longer distances at night, as skywaves. Ground losses become lower at lower frequencies, greatly increasing the coverage of AM stations using the lower end of the band. The VLF and LF frequencies are mostly used for military communications, especially with ships and submarines. The lower the frequency the better the waves penetrate sea water. ELF waves (below 3 kHz) have even been used to communicate with deeply submerged submarines.
Ground waves have been used in over-the-horizon radar, which operates mainly at frequencies between 2–20 MHz over the sea, which has a sufficiently high conductivity to convey them to and from a reasonable distance (up to 100 km or more; over-horizon radar also uses skywave propagation at much greater distances). 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 only to authorized frequencies in the range.
Mediumwave and shortwave reflect off the ionosphere at night, which is known as skywave. During daylight hours, the lower D layer of the ionosphere forms and absorbs lower frequency energy. This prevents skywave propagation from being very effective on mediumwave frequencies in daylight hours. At night, when the D layer dissipates, mediumwave transmissions travel better by skywave. Ground waves do not include ionospheric and tropospheric waves.
The propagation of sound waves through the ground taking advantage of the Earth's ability to more efficiently transmit low frequency is known as audio ground wave (AGW).