rther, the k-factor becomes less than unity, thus making the signal travel further away from the surface of the earth. When the variability of radio refractivity reduces much more than -40 N/km, the microwave signal makes an arc that is much smaller in radius (shown in Figure 3.46). This condition increases the clearance between the obstacles on the earth's surface and the microwave signal and is called the superrefractive condition. If the variability of refractivity increases to -157 N/km, the k-factor becomes infinity and the radius of the microwave signal becomes equal to that of the earth's surface. Graphically, this would mean both the microwave signal and the earth's surface behaving as straight lines. Under these conditions, the signal may not reach the surface of earth as the phenomenon of ducting takes place. In both the subrefractive and superrefractive conditions, the angle of transmission and angle of reception may change, depending upon the severity of change in atmospheric conditions. This leads to poor signal strength at the receiving antenna. Transmission Network Planning and Optimisation k = 1.33 k = 8 k = - -1 k = 0.8 Figure 3.45 k-fading Figure 3.46 Superrefraction Diffraction Fading Due to the variation in the atmospheric conditions that lead to variations in the k-factor (deviations from the standard atmospheric conditions), especially in the subrefractive domain, the signal bends in a way where the earth's surface starts to obstructing the direct path between the transmitter and receiver. The losses that happen due to this phenomenon depend upon the type of obstruction. There are basically two kinds of diffraction (shown in Figure 3.47): Smooth sphere diffraction Knife-edge diffraction Microwave Propagation Earth Surface Earth Surface Smooth Sphere diffraction Knife-edge diffraction Figure 3.47 Type of diffraction Smooth sphere diffraction takes place when the earth's surface (e.g. water) acts as a diffraction source itself. The losses are quite high in such cases, e.g. 15-20 dB. However, k