 similar size to the incident wavelength, the process is described using the Mie scattering theory (> 10 GHz). Because of rainfall, wet radomes may cause additional losses that need to be accounted for in the link budget calculations. These losses can be caused by reflection, absorption and/or scattering. If the antenna (and its radome) is quite old, the losses may be even greater than a couple of decibels. Thus, attenuation due to rain can be summarized as follows: As the distance increases, the attenuation increases for a given rainfall rate. With a constant distance, the attenuation the microwave link increases with the rainfall rate. TRANSMISSION NETWORK PLANNING AND OPTIMISATION Vertically polarized links experience less degradation in signal level than do horizontally polarised links. Extra attenuation losses (e.g. wet radome losses) need to be taken into account. Frequency-selective Fading With ducting, the entire signal is trapped and (usually) lost. In contrast, with frequency- selective fading only a part of the signal may be lost. The atmosphere acts in a way that the beam is not trapped completely, but only deflected. The signal may still reach the receiving antenna, but rarely in phase with the direct-path signal. The total fading (PTF) in a system can be considered to be a sum of flat and selective fading: (3.14) where PTF is the probability of fading (combined effect of flat and selective fading), PFF is the probability of fade due to flat fading, and PSF is the probability of fade due to selective fading. Other Types of Fading Apart from flat fading and frequency-selective fading, there are some other types of fading that transmission/microwave planning engineers should be aware of. k-fading As explained and shown above (see Figure 3.10), there is fading of the microwave signal to variations in atmospheric conditions - pressure, temperature and humidity - which Variation in k- value Site A; 60 m Site B:30 m Diversity Diversity at Site A: 20 m Frequency of Operation: 7.5 GHz Figure 3.15 Received fi