JPL's Wireless Communication Reference WebsiteChapter: Wireless Propagation Channels

Figure: Path profile model for (single) knife edge diffraction
If the direct lineofsight is obstructed by a single knifeedge type of obstacle, with height h_{m} we define the following diffraction parameter v:
where d_{t} and d_{R} are the terminal distances from the knife edge. The diffraction loss, additional to free space loss and expressed in dB, can be closely approximated by
A_{D}=0  if v < 0  
A_{D}=6 + 9 v + 1.27 v^{2}  if 0 < v < 2.4  
A_{D}=13 + 20 log v  if v >2.4 
Approximate techniques to compute the diffraction loss over multiple knife edges have been proposed by
The method by Bullington defines a new `effective' obstacle at the point where the lineofsight from the two antennas cross.
Epstein and Peterson suggested to draw linesofsight between relevant obstacles, and to add the diffraction losses at each obstacle.
Deygout suggested to search the `main' obstacle, i.e., the point with the highest value of v along the path. Diffraction losses over `secondary' obstacles are added to the diffraction loss over the main obstacle.
Many measurements of propagation losses for paths with combined diffraction and ground reflection losses indicate that knife edge type of obstacles significantly reduce ground wave losses. Blomquist suggested two methods to find the total loss:
and the empirical formula
where A_{fs} the free space loss, A_{R} the ground reflection loss and A_{D} the multiple knifeedge diffraction loss in dB values.