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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.

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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.

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Fresnel Zone Geometry(Cont’d)

Angle ,

Fresnel-Kirchoff diffraction parameter

Normalizing ,

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Fresnel Zone Geometry(Cont’d)

The concentric circles on the plane are Fresnel Zones.

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The radius of the nth Fresnel zone circle

The excess total path length traversed by a ray passing through each circle is

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Consider a receiver at point R, located in the shadowed region.

The electric field strength Ed,

where E0 is the free space field strength

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Graphical representation of

The diffraction gain:

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Lee’s approximate solution:

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Multiple Knife-edge Diffraction