Dielectric characterization of fog in the terahertz regime

Electromagnetic radiation at millimeter and sub-millimeter (terahertz) wavelengths are being considered for various applications, including remote sensing, wireless communications, and radars. However, wireless links implemented in millimeter wavelengths above 30 GHz suffer from absorption and dispersion effects in air, which emerge mainly due to oxygen molecules, humidity, and water droplets. Such frequency dependent atmospheric propagation effects become more severe as the frequency is increased to the terahertz regime. Moreover, weather conditions like haze, fog, and rain cause a further decrease in the overall link-budget leading to a degradation in the channel performance. In the current paper, the physical properties of the atmosphere and their effect on the electromagnetic radiation within the sub-millimeter wavelengths are studied theoretically and experimentally. Expressions for the attenuation and group delay are presented in terms of the electric susceptibility of the atmospheric medium in the presence of suspended water droplets. The analytical estimations are demonstrated experimentally in a controlled water fog chamber.