f 0. The angles between the wedge surfaces and these made with a cylindrical surface stylisation of 4 radii joining each horizon lines are Q and . The diffraction loss is given by Equation other with common tangents, as shown in Figure 6.21. Point 'a' is 6.78 to 6.83 where F is given in Equations 6.24. the transmitter horizon, and 'b' is the receiver horizon. Arc ac just clears the terrain between point 'a' and the transmitter site, and has AD-20ogF(v1)+p,F(v2 a tangent to the horizon line Ta at point 'a'. This condition defines (6.78) the radius n. Radius In is defined similarly at the receiver end. The radii [1 and r2 are defined such that arcs ae and be form tangents with V1=0u (6.79) horizon lines Ta and Rb at points a and b, respectively, and have a common tangent at e. The curved surface attenuation can then be approximated by Equations 6.73 to 6.77. (6.80) (6.81) (6.73) (6.82) (6.74) (6.83) G(Z)0058-10lo2 (6.75) < 2000 : R(Z)0.058 -33 (6.76) Simulated reflection coefficients P, and P, depend on how the actual profile deviates from the wedge surface and the values should Figure 6.21 Geometry for curved surface diffraction calculation The space wave 6/17 Figure 6.22 Geometry for wedge diffraction calculation range from about -0.9 for a very smooth wedge like profile, to about direct by o and a reflection coefficient of magnitude p and phase -0.1 for a rough 'peaky' edge. change Q. From a summation of vectors (applying the cosine rule), the received field is reduced from the free space value by Equation 6.4.10 Attenuation by buildings and trees 6.84. Attenuation is also caused by buildings and trees. The effects are A = - 10 log 1+p2-2pcas(p8-v+x) + ] (6.84) complicated in these conditions, but allowance can be made by specifying categories. For horizontal polarisation and angles of incidence below 10 For reception at 10m above ground level, a clutter loss ACL is degrees at frequencies above 400MHz it is reasonable to assume p given approximately by Table 6.1. = 1 and Q=r. 6.4.11 Ground reflections 6.4.12 Tro