SPATIAL DISTRIBUTION AND FREQUENCY OF OCCURRENCE OF ANOMALOUS PROPAGATION OF TERRESTRIAL MICROWAVE RADIO SIGNALS OVER NIGERIA

Abstract

The variation of refractivity with height is of major importance in determining the path of propagation of radio wave in the troposphere. The impact of anomalous propagation such as sub refraction, super refraction and ducting on terrestrial paths is of great importance and these are studied in this research work. The investigation of anomalous terrestrial propagation of radio waves is carried out with the aim of improving communication services in Nigeria. Measurement of climatological parameters such as pressure, temperature and relative humidity among others, at the ground surface and at the first 100 m altitude over five locations across Nigeria (Akure, Minna, Enugu, Jos and Sokoto) were taken for a period of 3 years (2010 – 2012). The device used for the measurement is the Davis wireless Vantage Pro 2 instrument equipped with integrated sensor suit, a solar panel and wireless console in all the stations. The fixed measuring method using a high tower was employed in this study, with ISS positioned at the aforementioned height. The data obtained were used to compute the radio refractivity, the refractivity gradient, the effective earth radius factor (k-factor), the geoclimatic factor, K and the fading due to atmospheric multipath associated with abnormal refractive layers at Ultra high frequency, (UHF), Very high frequency, (VHF) and Extremely high frequency, (EHF) bands at each of the locations. The results of these computations were used to investigate propagation conditions of the locations considered in the study. From the study, the radio refractivity, N values for the wet months are higher than for the dry months and also decrease with altitude. Also, the k-factor values are lower in the dry months when compared with the wet months. The average statistical distribution of clear air effects over the study locations show a prevalence of sub-refraction in the arid region and super-refraction in the coastal region. The implication of the prevalent sub-refraction is that radio signals will have reduced horizon and consequently outage may be experienced at the receiving end in the region. However, when super-refraction and ducting are prevalent, stronger signals are received but interference from distant stations may be received at the location. The overall result obtained shows that the geoclimatic factor which accounts for variability of the distribution of multipath fading varies with the months, seasons and year. Based on the link geometrical parameters used, the fade depth exceeded for 0.01% of time were estimated by logarithmic fits with very high coefficient of determination. Finally, the reduced values of radio refractivity seen in Southern part of Nigeria may be as a result of gas flaring activities from oil exploration in the region ,which brings with it emission of carbon dioxide and Sulphur output into the atmosphere at a height close to 100 m