of propagation modes in the troposphere 9/13 effect of reflection or scattering from the troposphere increases and the rate of attenuation with distance decreases. In an actual inhomo- The effects of irregular terrain are complex. There is some evi- geneous atmosphere the height-variation of n is the dominant factor dence that, for short, line-of-sight links, a small degree of surface in the scatter zone as illustrated in Figure 9.6. However, in practice roughness increases the field strength by reducing the destructive the situation is rarely as simple as that indicated by these simple interference between the direct and ground-reflected rays. Increas- models. ing the terrain irregularity then reduces the field strength, particu- At frequencies above about 40GHz, absorption in atmospheric larly at the higher frequencies, as a result of shadowing, scattering and absorption by hills, buildings and trees. However, in the particu- lar case of a single, obstructing ridge visible from both terminals it is sometimes possible to receive field strengths greater than those over level terrain at the same distance. This is the so-called obstacle Homogeneous gain. atmosphere Weak scattering In designing microwave radio-relay links for line-of-sight oper- Height ation it is customary to so locate the terminals that, even with unfavourable conditions in the vertical gradient of refractive index (with sub-refraction effects decreasing the effective radius of the Earth), the direct ray is well clear of any obstacle. However in addition to multipath fading caused by a ground-reflected ray, it is possible for line-of-sight microwave links to suffer fading caused Surface layer by multi-path propagation via strong scattering or abnormal refrac- tion in an elevated layer in the lower troposphere. This situation may Height Super-refraction lead to a significant reduction in usable bandwidth and to distortion, but the use of spaced antennae (space diversity) or different frequen- cies (frequency diversity) can reduce these effects. Eve