Atmospheric absorption and dispersion

If electromagnetic waves are propagating in media of low loss the effect of the medium can be characterized with a complex permittivity, imaginary part of which is significantly smaller than its real part. Loss due to the electrically neutral molecules of the air can be characterized like that and is usually negligible. Electrically asymmetrical molecules cause higher loss; it is reasonable to characterize them more precisely.

Of this type is the molecule of water vapor and also of several gases of lower density being of little significance. Water vapor has three resonance lines in the millimeter band, about 22.3 GHz, 183.3 GHz and 323.8 GHz; these are slightly depending on the temperature. Molecule of oxygen is paramagnetic, its magnetic moment causes similar resonance phenomena. It has an isolated resonance line at 118.74 GHz and a relatively broad absorption band at 50-to-70 GHz, composed of several resonance lines. A method to compute the attenuation due to gasous absorption is given in Recommendation 676-2 of ITU-R.

A few figures: attenuation above 20 GHz, depending on air moisture is nowhere lower than .05-.1 dB/km. About 60 GHz it is always higher than 10 dB/km.

Dispersion is a second potential consequence of molecular interaction. Attenuation and group delay of the medium is frequency dependent causing linear distortion. Dispersion is significant in the absorption band of 60 GHz, however bandwidth of the medium is in the order of GHz; therefore the distortion is non-negligible only in the transmission of signals in the order of Gbit/sec. Present-day systems are of much lower bitrate.

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