Tropospheric Waves

8/13/2019 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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Tropospheric Waves