(28) (29) where: : height of the top of the obstacle above the straight line joining the two ends of h the path. If the height is below this line, is negative; h and : distances of the two ends of the path from the top of the obstacle; d1 d2 : length of the path; d : angle of diffraction (rad); its sign is the same as that of The angle is h. assumed to be less than about 0.2 rad, or roughly 12º; 1 and 2 : angles in radians between the top of the obstacle and one end as seen from the other end. 1 and 2 are of the sign of in the above equations. h NOTE 1 In equations (26) to (29) inclusive and should be in self-consistent units. h, d, d1, d2 – FIGURE 8 Geometrical elements Figure 9 gives, as a function of , the loss (dB). J() is by: J() (30) where and are the real and imaginary parts respectively of the complex Fresnel integral C() S() F() defined in § 2.7. For greater than an approximate value can be obtained from the expression: –0.78 (31) FIGURE 9 Knife-edge diffraction loss 4.2 Single rounded obstacle The geometry of a rounded obstacle of radius is illustrated in Fig 8c). Note that the distances R d1 and and the height the baseline, are all measured to the vertex where the projected rays d2, h intersect above the obstacle. The diffraction loss for this geometry may be calculated as: (32)
(28) (29) where: h : height of the top of the obstacle above the straight line joining the two ends of the path. If the height is below this line, h is negative; d1 and d2 : distances of the two ends of the path from the top of the obstacle; d : length of the path; : angle of diffraction (rad); its sign is the same as that of h. The angle is assumed to be less than about 0.2 rad, or roughly 12º; 1 and 2 : angles in radians between the top of the obstacle and one end as seen from the other end. 1 and 2 are of the sign of h in the above equations. NOTE 1 – In equations (26) to (29) inclusive h, d, d1, d2 and should be in self-consistent units. FIGURE 8 Geometrical elements Figure 9 gives, as a function of , the loss J() (dB). J() is by: (30) where C() and S() are the real and imaginary parts respectively of the complex Fresnel integral F() defined in § 2.7. For greater than –0.78 an approximate value can be obtained from the expression: (31) FIGURE 9 Knife-edge diffraction loss 4.2 Single rounded obstacle The geometry of a rounded obstacle of radius R is illustrated in Fig 8c). Note that the distances d1 and d2, and the height h the baseline, are all measured to the vertex where the projected rays intersect above the obstacle. The diffraction loss for this geometry may be calculated as: (32)
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