Ground wave

Ground waves are

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.

Overview

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-

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.

Ground waves propagate in

Conductivity of the surface affects the propagation of ground waves, with more conductive surfaces such as sea water providing better propagation.[1] Increasing the conductivity in a surface results in less dissipation.^[2] 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,^[3] so that the signal decreases exponentially.

Applications

Most long-distance LF "

• Skywave

References