n, Dri minus D is an odd multiple of the wavelength, multipath fading will occur. Thus, the difference between the two diversity (at the receiving site) antennas should be such that Drl - Dr is an odd multiple of half wavelength (Figure 3.12). Figure 3.12 Multipath fading TRANSMISSION NETWORK PLANNING AND OPTIMISATION Fading of the signal level can be broadly classified into two kinds: flat fading and frequency-selective fading. These and other forms of fading are now discussed. Flat Fading Flat fading is 'flat' across the transmitted spectrum, which means that, in the absence of any other source that would reduce the signal level, the fading is a function of thermal noise only. There are two forms of flat fading: ducting, and fading due to rain attenuation. Earth Surface Figure 3.13 Ducting Ducting Effect In certain conditions the atmosphere acts as a duct, and thereby becomes capable of trapping the signal within itself. The signal departs from its original direction and moves in the direction of the duct, as shown in Figure 3.13. Once the signal is trapped in the layer, it propagates as if travelling inside an optical fibre cable and going in the direction of the waveguide. The signal gets trapped only when the critical angle is exceeded, SO the signal should enter the duct at an angle less than the critical angle. The ducting phenomenon also depends upon the intensity of the duct, dimensions, and the frequency of operation. Ducts do not need to travel upwards as in the figure, but may also travel downwards towards the surface of the Earth. Rain Attenuation Rainfall attenuates a microwave signal. The level of attenuation is dependent upon factors such as the frequency of operation, the polarisation of the signal, the path length, and the rainfall rate. Other rain factors that influence attenuation include drop size, shape and distribution, terminal velocity and temperature. Rain starts having an impact at frequencies above 10 GHz, and the attenuation becomes critical beyond 15 GHz. The higher the frequency of 