The diffraction parameter v is defined as

where

h_m is the height of the obstacle, and

d_t is distance transmitter - obstacle

d_r is distance receiver - obstacle

The diffraction loss L_d, expressed in dB, is approximated by

	AD=0	if v < 0
	AD=6 + 9 v + 1.27 v2	if 0 < v < 2.4
	AD=13 + 20 log v	if v >2.4

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