MICROWAVES ABSORPTION BY OXYGEN AND . WATER VAPOUR AT . TROPICAL LATITUDES

Abstract

The total atmospheric absorption due to oxygen and water vapour in the frequency range, 1O-350GHz was evaluated employing Liebe's empirical model. The basic data include monthly mean meteorological parameters of pressure, temperature and relative humidity obtained from four tropical sites, namely Akure (Lat.07°1TN,Long.05°14'E),Kano(Lat.03°12'N, Long.08° 32'E), Lagos (Lat. 06° N, Long.03° 21 'E) and Minna (Lat. 09° 5TN, Long.06° 32'E). All the four stations are in Nigeria. The specific attenuations by oxygen and water vapour are presented together with the graphical illustrations of the results for the two principal seasons in Nigeria, dry and wet seasons The wet season has a greater attenuation than the dry season. An exception to this takes place at Akure site in 1991 when the attenuation during the dry season exceeds that of wet season. In addition to this seasonal variation, the specific attenuation also exhibits geographical variations. Lagos, a coastal station, has the highest attenuation; Kano a far inland station, has the least attenuation value. The water vapour three resonant absorption lines are obtained at 22,184 and 326GHz. The weakest spectrum line being at 22GHz,while the strongest is at 326GHz. Oxygen has an isolated resonant absorption at 118GHz and a series of close lines at 54-66GHz, the peak of this is at 60GHz. Using typical International Telecommunication Union (lTU) temperate climatic parameters of pressure, temperature and water vapour pressure, the specific attenuation for a temperate region is computed which, in turn, is compared with those from tropical sites. Attenuation is higher in the tropical sites than in the temperate zone. The tropical sites attenuation at 326GHz for wet season is about two times the attenuation for the temperate region.