Under a ground wave is understood radio waves from a transmitting antenna, which propagate along the surface and follow its curvature. In contrast, the space wave propagates from a transmitter mast straight as light and, for example, shielded by mountains.
The range of a ground wave is dependent on the one hand on the frequency and on the other hand, the type of soil.
Medium wave or long wave
The bump is for medium and long-wave transmitters of great importance. Long wave transmitters have a ground wave range with average soil conductivity of up to 1000 kilometers, medium-wave transmitter such of 250 kilometers, where the exact value of the electrical conductivity of the soil ( a soil with high electrical conductivity as sea water results in a larger ground wave range than the one having poor electrical conductivity such as desert sand), the transmit power and the nature of the transmitting antenna depends. Space waves are absorbed during the day in these frequency areas of the ionosphere, and does not reach the reflecting ionosphere. At night, the attenuation is less, so the range increases considerably. For this reason, one can in the evenings more stations in the long and medium waveband received than during the day. However, it can also lead to distortions when ground and sky wave with the same intensity arriving at the receiver. This is called Nahschwund.
In the short- wave range the bump has a very short range and plays a significant role for local calls only in the CB (11 m band ). The sky wave can be reflected on the other hand, with proper frequency selection to the ionosphere and back hike back to the surface. For this reason, have short wave radio a very long range and can even be received around the world.
In the FM band, the range of the ground wave is negligible, so we speak of a direct wave. Their range depends on the height of the antenna above ground, the nearby area ( Fresnel zone ) and the frequency. In operation, radio frequency planning is based on a range of 15 km. Are theoretically possible 30 km, beyond the earth's curvature is noticeable, it lacks the sight. By better diffraction longer wavelengths ( low frequencies ) results, for example for the 4 m band a much better spread than in the 2- m or 70 - cm range. Obstacles such as trees or mountains produce no sharp radio shadow, they can " flow around ".
Diffraction effects on the ground or near-ground inhomogeneities of the atmosphere, eg Temperature inversions in the fall, allowing the hour also significantly greater ranges. Of course, the path loss increases greatly, which can be compensated within certain limits by higher transmission power and directional antennas, however. Some high-level FM radio stations reach as reliable ranges of up to 200 km. So touching the usable reception areas of the Germany radio transmitter Hornisgrinde in the northern Black Forest and transmitter ox head in the Fichtelgebirge. Both transmitters work with, for FM radio stations unusually high, transmit power of approximately 100 kW.
Atmospheric effects are highly variable in nature. You are in commercial use undesirable because they do not increase the distance to be bridged reliably and in extreme cases lead to interference between different networks at the same frequency. In amateur radio, however, they are desirable because they can allow extremely wide connections.
But in the FM band it comes to reflections of the ground wave of objects, and this is more, the higher the frequency the more. Small metal parts at high-rise buildings can thus serve as a reflection of radio waves. Generally one can say that because of better diffraction VHF frequencies are preferred more in rural areas. In the urban environment, however, the UHF frequencies are due to the better " illumination " advantageous because they can be easily reflected by other buildings.