Atmospheric propagation in the frequency range 100-1000 GHz

The influence of the atmosphere on propagation in the frequency range 100–1000 GHz is discussed in terms of molecular absorption, effects due to particles or droplets, and refractive effects. Compared with propagation at lower frequencies, molecular absorption shows a substantial increase in effect, mainly due to absorption by water vapour molecules and this aspect is given particular attention. Molecular absorption may be modelled using published line parameters, but attention must also be given to significant absorption caused by double molecules of water vapour (dimers), which at present are not included in the line tabulations. The total predicted absorption accounts for the main aspects of molecular absorption observed in the field, but at frequencies with minimum attenuation, away from the strong resonance absorption regions, field measurements have shown significant differences which are not understood. Refractive effects and the influence of particles, involving both absorption and scattering, can be modelled, but prediction is limited by the need for appropriate meteorological data which has a particular relevance to propagation in this frequency range. Comparison with prediction is made both for horizontal transmission paths and slant paths through the atmosphere.