1. 1 Diffraction Phenomena: Radio signal can propagate around the curved surface of the earth, beyond the horizon and behind obstructions. Huygen’s principle: All points on a wavefront can be considered as point sources for the production of secondary wavelets and these wavelets combine to produce a new wavefront in the direction of propagation. 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 obstacles.

2. 2 Fresnel Zone Geometry The wave propagating from the transmitter to the receiver via the top of the screen travels a longer distance than if a direct line-of-sight path exists.

3. 3 Fresnel Zone Geometry(Cont’d) Angle, Fresnel-Kirchoff diffraction parameter Normalizing,

4. 4 Fresnel Zone Geometry(Cont’d) The concentric circles on the plane are Fresnel Zones.

5. 5 The radius of the nth Fresnel zone circle The excess total path length traversed by a ray passing through each circle is

6. 6 Consider a receiver at point R, located in the shadowed region. The electric field strength Ed, where E0 is the free space field strength

7. 7 Graphical representation of The diffraction gain:

8. 8 Lee’s approximate solution:

9. 9 Multiple Knife-edge Diffraction