NNING IN TRANSMISSION NETWORK Figure 3.11 Multipath propagation Generally, most surfaces reflect the signal, but the losses incurred during the reflections can make the reflected signal SO weak that it is either not able to reach the receiving antenna or is not able to make any impact on the received signal level. The term multipath fading is used for the fading of the signal caused by (Earth) surface reflections, while fading caused by reflections/refractions from the atmosphere is called selective fading. The layers in the atmosphere that reflect the microwave signal will cause fades in the received signal that can range from tens of decibels to 100 dB/s, but the duration of these fades is only a fraction of a second. This is also known as the layering phenomenon. The layering of the atmosphere can be caused by things like radiation nights, advection, subsidence, frontal systems, etc. Radiation nights occur over land when a cold night follows a warm day, resulting in the surface of the Earth cooling and the air at the surface cooling down faster than the air higher up in the atmosphere, causing a temperature inversion. Advection is caused by a high-pressure system moving land-based warm dry air over the cooler moist air above the sea, causing a temperature inversion. Subsidence is the descent of dry air in a high-pressure system that becomes heated by compression and spreads out over the cooler moist air, causing a temperature inversion. Localised inversion can also be caused by a frontal system driving cold air beneath warm air. Thus, the layering phenomenon is usually caused in hot climatic regions and on land near large water bodies. Multipath fading has a substantial impact on signal strength at frequencies up to 10 GHz. With increasing frequency the impact of this kind of fading diminishes and signal fading is more to do with rain effects. The effects of multipath fading can be countered by using diversity. Usually space diversity is used, i.e. two receiving antennas separated from each other in space. Whe