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Elektromagnetische Felder

High-frequency electromagnetic fields

  • In mobile communications, high-frequency electromagnetic fields are used for wireless transmission of voice and data.
  • In the electromagnetic spectrum, high-frequency fields fall within the frequency range between about 100 kilohertz and 300 gigahertz.
  • The intensity or strength of the fields is either given in the form of

    Power density is the product of electric and magnetic field strength.

In mobile communications, high-frequency electromagnetic fields are used for wireless transmission of voice and data. In free space they propagate as waves at the speed of light while being able to transmit energy and information over long distances.

Frequency and wavelength

In the electromagnetic spectrum, high-frequency fields fall within the frequency range between about 100 kilohertz (abbreviated "kHz"; 1 kHz = 1,000 hertz) and 300 gigahertz (abbreviated "GHz"; 1 GHz = 1,000,000,000 hertz). Hertz (abbreviated "Hz") is the unit of measurement for frequency, that is, for the number of oscillations per second.

Frequency and wavelength are interlinked by the velocity of propagation and describe the wave character of the fields. At high frequencies the wavelengths are small and at low frequencies they are large. For typical mobile radio frequencies between 400 megahertz (abbreviated "MHz"; 1 MHz = 1,000,000 Hz) and 3 gigahertz, the wavelengths are 75 to 10 centimetres, for example.

Strength of the fields

The intensity or strength of the fields is either given in the form of

Power density is the product of electric and magnetic field strength.

Propagation of the fields Propagation of the fields of mobile communicationsThe high-frequency fields at a certain measurement point depend - among other things - on the distance to the site, on the transmit power of the transmitters at the site, on the type, orientation and downtilt of the transmitter antennas as well as on the signal attenuation by vegetation and buildings.

Propagation of the fields

With increasing distance from the source - in the case of mobile communications from the antenna of a mobile phone base station or a mobile terminal - the field strengths and power densities drop off rapidly. In free space, for example, power density decreases with the square of the distance along the propagation direction, that is, with double the distance it decreases to one-fourth.

In the real environment, conditions of propagation are often significantly more complicated:

  • High-frequency electromagnetic fields can be reflected, deflected or also completely or partially absorbed by objects within their propagation paths. To what extent these effects influence the propagation depends - among other things - on the shape, size and material of the objects.
  • The antennas of mobile phone base stations do not radiate equally in all directions but have preferential directions. This contributes to the reason why the field strengths in the area around a transmitter can vary even at identical distances to the source.

Statements on the field intensity at a certain point can only be made when they are based on measurements or simulation calculations which take into account the relevant conditions of propagation. The field strengths can usually not be inferred from the distance to a transmitter alone.

That field intensity drops off rapidly with increasing distance from the antenna generally also applies to mobiles and smart phones. This is the reason why, for example, a user's head is exposed to significantly lower field strengths when hands free headsets are used, than during phone calls while holding the mobile or smartphone directly to a user's ear. This effect can not only be seen with permanently installed hands free units, for example in motor vehicles, but also when using wired and wireless headsets.

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State of 2018.11.21

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