Bump

A bump is understood to be radio waves from a transmitting antenna that propagate along the earth's surface and follow its curvature. In contrast, the sky wave propagates in a straight line like light from a transmission mast and is shielded by mountains, for example.

Radiation of a ground wave near the surface and a sky wave reflected by the ionosphere.

The range of a bump depends on the one hand on the frequency and on the other hand on the nature of the ground.

Medium wave or long wave

The bump in the ground is very important for medium and long wave transmitters . Long-wave transmitters have a surface wave range with an average surface conductivity of up to 1000 kilometers, medium-wave transmitters one of 250 kilometers, whereby the exact value of the electrical conductivity of the earth (a floor with high electrical conductivity such as seawater results in a greater surface wave range than one with poor electrical conductivity like desert sand), the transmission power and the type of transmitting antenna. In these frequency ranges, the sky wave is absorbed by the absorbing layers of the ionosphere during the day and does not reach the reflective layers of the ionosphere. At night, the attenuation is lower, which is why the range increases considerably. For this reason, you can receive more stations in the long and medium wave range in the evening than during the day. However, distortions can also occur if ground and sky waves arrive at the receiver with the same intensity. This is known as near shrinkage .

Shortwave

In the shortwave range, the bump has a very short range and only plays a significant role for local calls in CB radio (11 m band). The sky wave, on the other hand, can be reflected by the ionosphere if the frequency is chosen correctly and then migrate back to the earth's surface. For this reason, shortwave transmitters have a very large range and can even be received worldwide.

VHF

In the VHF range, the range of the ground wave is negligible, which is why one speaks of a direct wave . Their range is heavily dependent on the height of the antenna above ground, the close vicinity ( Fresnel zone ) and the frequency. In commercial radio frequency planning assumes a range of 15 km. Theoretically, 30 km would be possible, beyond that the curvature of the earth is noticeable, there is no line of sight. Better diffraction of longer wavelengths (low frequencies) results in z. B. for the 4 m band a much more favorable spread than in the 2 m or 70 cm range. Obstacles such as trees or mountains do not create sharp radio shadows, they can be "flowed around".

Diffraction effects on the ground or inhomogeneities of the atmosphere near the ground, e.g. B. Inversion weather conditions in autumn, allow significantly larger ranges by the hour. Of course, the path attenuation increases sharply, but this can be compensated for within certain limits by higher transmission powers and directional antennas. Some high-altitude VHF radio stations can thus achieve reliable ranges of up to 200 km. The usable reception areas of the Deutschlandfunk - Hornisgrinde transmitter in the northern Black Forest and Ochsenkopf transmitter in the Fichtelgebirge touch each other . Both transmitters work with transmission powers of around 100 kW, which are unusually high for FM radio transmitters.

Atmospheric effects are by their nature highly variable. They are undesirable in commercial use because they do not increase the distance that can be reliably bridged and, in extreme cases, lead to interference between different networks on the same frequency. In amateur radio, however, they are desirable because they can enable extremely long connections.

But in the FM band also occurs reflections of the bump of objects, and this the more likely the higher the frequency. Small metal parts on high-rise buildings can serve as a mirror for the radio waves. In general, one can say that because of the better diffraction, VHF frequencies are preferred in rural areas. In urban areas, on the other hand, UHF frequencies are advantageous because of the better “illumination”, as these can be more easily reflected by other buildings.

literature

Helmut Röder, Heinz Ruckriegel, Heinz Häberle: Electronics 3rd part, communications electronics. 5th edition, Verlag Europa-Lehrmittel, Wuppertal, 1980, ISBN 3-8085-3225-4
Stratis Karamanolis: All About CB. A manual for the CB radio operator. 2nd Edition. Karamanolis, Putzbrunn, 1977.
Jürgen Detlefsen, Uwe Siart: Basics of high frequency technology. 2nd, expanded edition. Oldenbourg, Munich / Vienna 2006, ISBN 3-486-57866-9 .