 in Figure 6.18, being for transmitter and receiver. which is stylised into the construction shown in Figure 6.19. From Again some workers prefer to base an equation on the isotropic this, three Fresnel limit parameters may be derived, of the type antenna. In which case the gains G, and G are each increased by introduced in Equation 6.24, as in Equations 6.67 to 6.69. 2.1dB to be relative to isotropic antennas instead of half wavelength dipoles, which is compensated by the constant 17.7 becoming 21.9. (6.67) There may also be a preference to convert d in metres to D in kilometres, and a (metres) wavelength to frequency in Megahertz, which must be matched by an appropriate change of constant. (6.68) 6.4.3 Variation from the free space values (6.69) Free space values rarely occur in practice because factors such as the Earth's surface, buildings, trees, and the atmosphere cause variations. It is, therefore, necessary to be able to predict an excess Virtual edge giving loss A1 Edge giving loss A/2 Edge giving loss A 3 Transmitter Receiver Figure 6.18 Example of a path profile with multiple obstructions The space wave 6/15 Figure 6.19 Construction for the Deygout method Let us assume that V1 is the largest of these SO that we then get As stated, this method is generally the most accurate of the Equations 6.70 and 6.71. approximations. However, it is liable to overestimate the loss by up to 6dB if either A2 or A is is nearly equal to A 1 (6.70) 6.4.5 Surface diffraction The rounded nature of hill tops, and reflections from elsewhere on (6.71) the Earth's surface, is liable to make losses greater than that derived from the simple screens assumed for the Deygout approximations. To overcome this, the profile can be assumed to consist of geometric To avoid the complication of evaluating the Fresnel integral, it is shapes, for which calculations are possible, such as a system of possible to go directly from the above parameters to the required spheres, cylinders or wedges. losses in dB via a graph of the type shown in Figu