the trajectory is kept constant (i.e. a straight line) and variation is made to the surface of the Earth. Variation in the values of k and subsequent variation in the Earth's curvature is shown in Figure 3.10. The condition k = 1.33 is for the standard atmosphere variability (dNIdh) of --40 N/km. Under this condition, the Fresnel zone should be 100% clear. When the variability increases beyond -40 N/km, then the path will experience sub-refractive conditions. Owing to this increase and the subsequent variation in the k-factor, the Earth's surface would look to be flat, thereby taking the signal closer to the surface of the Earth. This would mean that the obstacles that were 'clear' before the occurrence of this condition now start to obstruct the Fresnel zone. This condition is shown as k = 0.8. When the variability reduces substantially from the -40 N/km value, then the path experiences super-refractive conditions which means that the Earth's curvature 'increases' substantially. If the value of dNIdh can be reduced to - 157 N/km, then the k-factor tends to infinity. This would mean that the Earth would look to be flat, and so the microwave signal also. Further reduction would mean a negative value of k, thereby increasing the probability of ducting taking place in the microwave link. Multipath Propagation When the microwave signal leaves the transmitting antenna, it can take different paths to reach the receiving antenna (see Figure 3.11). One of the paths taken is a straight line or the Fresnel zone. Other paths may include reflections from the atmosphere or the surface of the Earth. This is called multipath propagation and results in more than one signal reaching the receiving antenna. This may result in reduction of the signal level if the phase difference is an odd multiple of half a wavelength. The severity of the reduction of the signal value will depend upon the reflected signal level. Hence, the signal level arriving at the receiving antenna is the sum of all the signals arriving at that antenna. PRE-PLA