Attenuation by Atmospheric Gases

The transmission attenuation caused by atmospheric gases results from the molecular resonance of oxygen and water vapour.

An oxygen molecule has a single permanent magnetic moment. At certain frequencies, its coupling with the magnetic field of an incident electromagnetic wave causes resonance absorption. In particular, at frequencies around 60 GHz a coupling occurs between the intrinsic moment of the electron, its spin, and the rotational energy of the molecule, generating a series of absorption lines quite close to each other in the spectrum. These absorption lines come to merge, thus forming a single and broad absorption band. Fig. 6.5 represents the specific attenuation due to oxygen at different altitudes in the 50-70 GHz frequency range: the lower the pressure, the higher the resolution of the bands. Fig. 6.6 represents the specific attenuation coefficient due to atmospheric gases.

Liquid Water Absorption Spectrum GhzAttenuation Atmospheric Gases

Frequency (GHe)

Fig. 6.6. Specific attenuation due to atmospheric gases (ITU-R P.676)

Frequency (GHe)

Fig. 6.6. Specific attenuation due to atmospheric gases (ITU-R P.676)

A water vapour molecule behaves like an electric dipole. The interaction of such a molecule with an incident wave disorientates the molecule by generating an additional internal potential energy. The attenuation maximum reached around the 22 GHz frequency is due to the resonance of the water molecule which starts to rotate while absorbing a high proportion of the incident electromagnetic energy.

The most accurate method for evaluating the attenuation due to atmospheric gases is by taking into account the contribution of all the absorption lines of oxygen and water vapour and the continuous spectrum of the absorption due to water and ice. Several models can be found in the literature (Liebe 1993; ITU-R 1999 Rec. ITU-R P.476-4; Salonen 1990; Gibbins 1986; Konefal et al. 1999).

The reference model, developed by Liebe et al., is known as the MPM93 model (Liebe 1993). This model allows determining the refractive index related to atmospheric oxygen and water vapour as well as the attenuation related to each of these components for frequencies up to 1000 GHz (Liebe 1981, 1985, 1989, 1993). The input parameters of this model are the pressure, the temperature, the relative humidity observed over a vertical profile of the Earth's atmosphere and the frequency (COST255 1999; ITU-R P.676-4).

As a numerical application of this model, the specific attenuation due to atmospheric gases in the case of an average atmosphere (7.5 g/m3) was found to be equal to approximately 0.2 dB/km and 15 dB/km at 20 and 60 GHz respectively.

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