JPL's Wireless Communication Reference Website

Chapter: Wireless Propagation Channels
Section: Path Loss

Diffraction loss

Figure: Path profile model for (single) knife edge diffraction

If the direct line-of-sight is obstructed by a single knife-edge type of obstacle, with height hm we define the following diffraction parameter v:

where dt and dR 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
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
The attenuation over rounded obstacles is usually higher than AD in the above formula.

Single Knife Edge Calculator

Input parameters:

Distance between transmitter and obstacle meter
Distance between receiver and obstacle meter
height of the obstacle meter
Frequency MHz

Run calculation:


Free space loss dB
SKE loss dB
Attenuation dB

Approximate techniques to compute the diffraction loss over multiple knife edges have been proposed by

Total Path loss

The previously presented methods for ground reflection loss and diffraction losses suggest a "Mondriaan" interpretation of the path profile: Obstacles occur as straight vertical lines while horizontal planes cause reflections. That is the propagation path is seen as a collection of horizontal and vertical elements. Accurate computation of the path loss over non-line-of-sight paths with ground reflections is a complicated task and does not allow such simplifications.

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 Afs the free space loss, AR the ground reflection loss and AD the multiple knife-edge diffraction loss in dB values.

JPL's Wireless Communication Reference Website 1993, 1995.