Dr. S. Mary Praveena discusses path loss models related to electromagnetic wave propagation, focusing on reflection, diffraction, and scattering as the three basic mechanisms. The ground reflection model is effective for tall towers in urban settings, emphasizing the phase differences between direct and reflected paths. The document explains the impact of these factors on signal strength and wave behavior at varying distances.

1. PATH LOSS MODELS
Presented by
Dr.S.Mary Praveena
Associate Professor
ECE Department
Sri Ramakrishna Institute of Technology
Coimbatore
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2. The Three Basic Propagation Mechanisms
• Basic propagation mechanisms
– reflection
– diffraction
– scattering
• Reflection occurs when a propagating electromagnetic
wave impinges upon an object which has very large
dimensions when compared to the wavelength, e.g.,
buildings, walls.
• Diffraction occurs when the radio path between the
transmitter and receiver is obstructed by a surface that
has sharp edges.
• Scattering occurs when the medium through which the
wave travels consists of objects with dimensions that are
small compared to the wavelength.
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Ground Reflection (2-Ray)
Model
– Good for systems that use tall towers (over 50 m
tall)
– Good for line-of-sight microcell systems in urban
environments
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• ETOT is the electric field that results from a
combination of a direct line-of-sight path and a
ground reflected path
is the amplitude of the electric field at distance d
ωc = 2πfc where fc is the carrier frequency of the signal
Notice at different distances d the wave is at a different
phase because of the form similar to
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• For the direct path let d = d’ ; for the reflected path
d = d” then
for large T−R separation : θi goes to 0 (angle of
incidence to the ground of the reflected wave) and
Γ = −1
• Phase difference can occur depending on the phase
difference between direct and reflected E fields
• The phase difference is θ∆ due to Path difference ,
∆ = d”− d’, between
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Using method of images (fig below) , the path difference can be
expressed as:-
• From two triangles with sides d and (ht + hr) or (ht – hr)
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• Using taylor series the expression can be simplified as:-
• Now as P.D is known , Phase difference and time delay
can be evaluated as:-
• If d is large than path difference become negligible and
amplitude ELOS & Eg are virtually identical and differ only in
phase,.i.e
&

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If than ETOT can be expressed as:-

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Referring to the Phasor diagram below:-

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or
As E-field is function of “sin” it decays in oscillatory fashion with
local maxima being 6dB greater than free space and local minima
reaching to-∞ dB.

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note that the magnitude is with respect to a
reference of E0=1 at d0=100 meters, so near 100
meters the signal can be stronger than E0=1
– the second ray adds in energy that would have
been lost otherwise
for large distances it can be shown that
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