Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges).
- Diffraction allows radio signals to propagate around the curved surface of the earth, beyond the horizon, and to propagate behind obstructions.
- The received field strength decreases rapidly as a receiver moves deeper into the obstructed (shadowed) region, the diffraction field still exist and often have sufficient strength to produce a useful signal.
- The phenmenon of diffraction can be explained by Huygen’s principle.
Huygen’s principle: It states that all points on a wave front can be considered as point sources for the production of secondary wavelets, and that these 'wavelets combine to produce a new wave front in the direction of propagation. As shown in the figure 10.6
- The field strength of a diffracted wave in the shadowed region is the vector sum of the electric field components of all the secondary wavelets in the space around the obstacle.
At high frequencies, diffraction, like reflection, depends on the geometry of the object, as well as the amplitude, phase, and polarization of the incident wave at the point of diffraction.
Fresnel Zone Geometry:
Consider a transmitter and receiver separated in free space as shown in Figure.Let an obstructing screen of effective height h with infinite width (going into and out of the paper) be placed between them at a distance d, from the transmitter and d: from the receiver. Assuming h λ, then the difference between the direct path and the diffracted path, called the excess path length (Δ) , can be obtained from the geometry
The corresponding phase difference is given by
and when tanx =x, then α =β γ