2) where d = distance between the transmitting antenna and the passive repeater d = distance between the receiving antenna and the passive repeater AR = projected area 3.5 Microwave Propagation The link budget example shown above is the first step towards a final microwave plan. In any given microwave link, there will be variations in the received signal level about its median value, even when the transmitted power is constant. These variations are sometimes SO high that the link may go down, which means that the received signal is below the threshold value (a condition also known as 'blackout' as no signal is received by the receiving antenna). This is called as fading. Microwave engineers should be able to predict this fading as close by as possible and hence design the link accordingly. Fading can be broadly divided into two types: slow fading and fast fading. 3.5.1 Slow Fading As the name suggests, the signal may fade in hours. The signal variations that result in fading are mainly due to changes in atmospheric conditions. Due to the changes in temperature, pressure and humidity, the radio refractivity changes, which varies the k-factor. This results in a variation of the path taken by the microwave signal and then variations in the angle of arrival, resulting in less (or no) power being received by the receiving antenna. This phenomenon is also called k-fading. Although the standard value of k is taken to be 4/3 or 1.33 for the path calculations, microwave planners should be aware of the atmospheric variation for the region when they design microwave links (shown in Figure 3.45). There are two types of refractive conditions: subrefractive and superrefractive. When the variability becomes greater than -40 N/km, the k-value decreases and hence the signal becomes a straight line. This would result in the microwave signal getting closer to the surface of earth, increasing the chances of surface or vegetation obstructing the signal and decreasing the received power at the receiver. If the variability increases fu