Propagation properties of sub-millimeter waves in foggy conditions

The propagation of millimeter (MMW) and submillimeter (terahertz, THz) waves in the atmosphere is subject to absorptive and dispersive effects. The resulting attenuation and temporal group delay increase in cloudy and hazy weather. In this work, the effects of water droplets suspended in the air on the propagation of electromagnetic radiation in submillimeter waves are studied theoretically and experimentally. A comparison is made between the link budget in THz frequencies and the expected attenuation for free space optics (FSO) links. Using the modified millimeter-wave propagation model, the frequency-dependent attenuation and group delay are expressed in terms of the complex refractivity of the atmospheric medium. The theory is employed to study the effect of fog and clouds on the accuracy of a frequency-modulated continuous-wave high-resolution radar operating at 330 GHz. In an experiment, the propagation of MMW was studied in a controlled fog chamber for various ranges of visibility, even below 1 m. The resulting attenuation and group delay of submillimeter waves were measured, while the properties of fog (optical visibility distance and water content) were monitored using FSO techniques. Apart from attenuation, the incremental group delay caused by fog also affected the accuracy of the radar. The experimental results were compared with those of an analytical model and were in good agreement even for very low visibility in very foggy conditions. Dispersive effects should be considered in the design of remote sensing radars operating in the MMW and THz regimes.