eld (dB) at a station as a function of k PRE-PLANNING IN TRANSMISSION NETWORK results in variation of the k-factor. This is called k-fading. An object which could be con- sidered as clear from the first Fresnel zone may well start acting as an obstacle with changing atmospheric conditions i.e. with changing k-factor. For this reason, transmis- sion/microwave planning engineers should plan the link for clearance not only for standard conditions (i.e. k = 1.33), but also for non-standard conditions such as k = 0.6, etc. The variation of the received signal power (with diversity antenna) versus the k-value is shown in Figure 3.15. Diffraction Fading Diffraction occurs when a microwave signal grazes an object and in consequence loses (disperses) energy. This is known as diffraction fading. The amount of energy lost is depen- dent on the type of surface or object. Objects may act as obstacles (i.e. interfere in the first Fresnel zone under standard atmospheric conditions when k = 1.33), or may act as obstacles under changing atmospheric conditions (i.e. during k-fading). Fading due to diffraction is primarily of two types smooth-sphere or knife edge (see Figure 3.16): Earth Surface Earth Surface Smooth Sphere diffraction Knife-edge diffraction Figure 3.16 Types of diffraction Smooth-sphere diffraction: This takes place when the microwave signal passes over the surface of the Earth (e.g. over water). Microwave links having path lengths into tens of kilometres, passing over vast plain land or a water body, may display this when there are changes in the atmospheric conditions (i.e. k-fading). The losses due to smooth-sphere diffraction may be as high as 12-15 dB. Knife-edge diffraction: The losses that occur due to grazing of the microwave signal over bushes, trees or forests are called knife-edge diffraction. This may happen during normal functioning of the links and be due to variation in atmospheric conditions (k-fading). There are a few methods calculating the losses due to this kind of diffraction, the major ones be