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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.
2
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.
4

The radius of the nth Fresnel zone circle
The
excess total path length traversed by a ray
passing through each circle is
5

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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The diffraction gain:

Graphical representation of
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
Lee’s approximate solution:
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Multiple Knife-edge Diffraction
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