re 6.20. The diffraction attenuation assumed for the path is as in Equation 6.4.6 Rounded hill tops 6.72. One method of stylising the profile is to assume a rounded top on each of the obstructions, rather than assuming a knife edge. This Ak = A1 + A2' + (6.72) technique has a problem in its application because it requires the radius of a hill top to be known and, with most maps in common use, Diffraction loss (dB) Figure 6.20 Fresnel diffraction loss chart 6/16 Electromagnetic waves it is not possible to determine this radius with sufficient accuracy. Z > 2000 : R(Z) = G(Z) (6.77) However, a useful analysis of this method is given in an IEE paper by Hacking (1970), where extra terms are derived to add to the knife The calculation does not allow for the roughness of the terrain edge loss. and, as a result, the loss of signal on the path is usually over estimated. A better result is obtained from an interpolation between 6.4.7 Diffraction over a sea path this and the knife edge calculation. The form of interpolation can be determined by obtaining a best fit with a set of measured data. In the If it is only sea which interrupts the line of sight path, a spherical UK it was found that an averaging of losses, expressed in decibels, diffraction calculation can be made from a method which has its gave good results. origins in work by Van der Pol and Bremmer (1937, 1949). These methods have a complicated formulation and will not be detailed 6.4.9 Approximating a single hill to a wedge here. However, for many applications a simplification, based on a shape paper by Vogler (1964) is of value for frequencies above 400MHz. On a land path with one diffraction edge, a prediction can be made 6.4.8 Approximating a land path to a cylindrical by assuming a wedge shaped profile. This wedge has its peak at the surface diffraction edge, and surfaces which clear the terrain on both sides of the profile, as shown in Figure 6.22, with an angle between the On a land path with many diffraction edges, a useful prediction is horizon lines o