Tropospheric Waves
Transcript of Tropospheric Waves
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TROPOSPHERIC
WAVESAND
ITS
APPLICATIONSGUNJAN VERMA (U11EC143)
KULDEEP SINGH (U11EC145)
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Tropospheric waves
The troposphere is the region of the atmosphere
adjacent to the earth and extending upto 10km.
The temperature in the troposphere decreases
with the height at the rate of 6.5C per km.
Wave propagation beyond the line of sight with
in the troposphere can result from severalmechanisms such as diffraction ,normalrefraction,abnormal reflection and refraction and
tropospheric scatter.
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Normal refractionRadio wave travelling horizontally in the earths
atmosphere follows a path which has a slight
downward curvature due to refraction of the wave
in the atmosphere .this curvature of the path helps
in overcoming the loss of signal due to curvature ofthe earth
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The normal refraction of radio wave in the
atmosphere occurs because of the refractive index
of the atmosphere ,decreases slowly with theheight above the earth .since . .due to very
small change in n we use scaled up value of n
called refractivity N ;N=(n-1)10^6
N can be calculated using relation
Where, p: total pressure (milli bars).
e: partial pressure of water vapour (milli bars)
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Relation b/w radius of curvature of path and
change of refractive index with height
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The radius of curvature of path being a function of the
rate of change of the dielectric constant with height,
varies from hour to hour ,day to day and season to
season.
we can compensate curvature of path by using the
larger value for the effective radius of the earth
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the effective radius of the earth =(4/3)*a
Where, a: actual radius of the earth. By using this
effective radius instead of actual radius we can
replace curved path by straight line path.
Abnormal Reflection
there are chances of reflection which are due to
the abrupt variation in the refractive index and itsgradient and such reflections can produce usable
signal beyond the range compared with only ground
wave propagation.
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Modified index curves and duct
propagation
Modified index (M)=(n-1+(h/a))*10^6a: radius of earth .
h: height above ground.
n: refractive index.
Importance of M in radio propagation is because ofthe variation of dM/dh of M with height.
Usually M increases 0.158M units per metre,
however near earth surface increases linearly withconstant rate less than 0.158M units per metre.
with the help of M curves we can predict thetransmission path of radio waves.
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Duct propagationIf M modified index decreases with height (M
curve slopes to left ) over a portion of range ofheight the waves will be curved downwards (over
the flat earth) and condition is known as trapping or
duct propagation.
this range of height over the surface of the earth
is called duct or wave guide this wave guide will be
leaky and is called surface duct if the lower side of
duct is at the surface of the earth.
When the lower side of the duct is elevated then
that duct is called elevated duct.
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Surface ducting elevated ducting
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Tropospheric scatterIt is method of transmitting and receiving radio
signals over considerable distances often upto300km.
In this method radio signals are transmitted in atight beam midway b/w transmitter and receiversites ,as the signals pass through the tropospherethey are scattered ,allowing receiver to pick up thesignals.
Because troposphere is turbulant and has a highproportion of moisture radio signals are refractedand consequently only a small proportion of radioenergy is collected by the receiving antenna .
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Scattering in the common volume of transmitting
and receiving antenna
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Applications of tropospheric propagation
Tropospheric scatter propagation is used for
point to point communication ranging from 80kmto 800km.
Tropospheric scatter propagation particularlyusefull for communication to location in ruggedterrain that are difficult to reach with othermethod of propagation .
Tropospheric scatter propagation is not affectedby ionospheric and auroral disturbance.
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End