of the time is given by Arain==Y/Rdeff (3.49)* As seen in the equations above, attenuation due to rain is dependent upon two frequency dependent constants k and a. Both of these constants are defined for spherical droplets. 3.5.2 Fast Fading In this type of fading, the time for which the signal fades ranges from a fraction of a second to a few minutes. The main cause of such fading is the multipath phenomenon. In a microwave link, ideally a signal takes one single path from the transmitting to the receiving antenna. However, it has been observed that a signal takes a few different paths from the transmitting to the receiving antenna (as shown in Figure 3.51). Thus the signal received by the receiving antenna consists of a direct signal and an indirect Microwave Propagation Figure 3.48 Rain charts for Asia Pacific (APAC) (Source: ITU-R PN 837-1. Reproduced with the kind permission of ITU.) Transmission Network Planning and Optimisation Figure 3.49 Rain charts for Europe and Africa (Source: ITU-R PN 837-1. Reproduced with the kind permission of ITU.) Microwave Propagation Figure 3.50 Rain charts for the Americas (Source: ITU-R PN 837-1. Reproduced with the kind per- mission of ITU.) Transmission Network Planning and Optimisation Table 3.12 Rain climatic zones showing the rainfall intensity exceeded [mm/h] (Source: ITU-R PN 837-1. Reproduced with the kind permission of ITU.) Percentage of time [%] 0.003 0.001 Table 3.13 Values k and a for vertical and horizontal polarisations (Source: ITU Recommendation 838. Reproduced with the kind permission of ITU.) Frequency (GHz) 0.0000387 0.0000352 0.9122 0.8801 0.0000868 0.0000784 0.9341 0.8905 0.0001543 0.0001388 0.9629 0.9230 0.0002416 0.0002169 0.9873 0.9594 0.0003504 0.0003145 1.0185 0.9927 0.0006479 0.0005807 1.1212 1.0749 0.001103 0.0009829 1.2338 1.1805 0.001813 0.001603 1.3068 1.2662 0.002915 0.002560 1.3334 1.3086 0.004567 0.003996 1.3275 1.3129 0.006916 0.006056 1.3044 1.2937 0.01006 0.008853 1.2747 1.2636 0.01882 0.01680 1.2168 1.1994 0.03689 0.03362 1.1549 1.1275 