Tropospheric Propagation1

7/31/2019 Tropospheric Propagation1

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Tropospheric Propagation

(above 300 MHz)


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CONTENTS

What is Atmosphere??!

Structure of Atmosphere

Definition of Troposphere Tropospheric Propagation

(above 300 MHz)


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What is Atmosphere??!

The atmosphere of Earth is a layer ofgases surrounding the planet Earth that isretained by Earth's gravity.

The atmosphere protects life on Earth byabsorbing ultra violet solar radiation,warming the surface through heat

retention (green house effect ), andreducing temperature extremes betweenday and night.


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Structure Of Atmosphere

The atmosphere, which broken intothree major regions:

Atmospheric regions

Troposphere Stratosphere Ionosphere


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Diagram

EARTH

UV / visibleSun-light

Ionosphere

Stratosphere

TroposphereOzonelayer

Infra-redradiation


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Troposphere It is the region of the atmosphere within 16

km from the earths surface.

This is the layer of the atmosphere in which welive.

It is that portion of the earth atmosphere fromearths surface up to :

1) a height of 8 to 10 km at polar latitude,

2) 10 to 12 km at moderate latitude and3) up to 16 to 18 km at the equator.


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Properties1. With increase of height, the percentage of the

gas components remains almost constant.

2. The water vapour components sharplydecreases with height.

3. The temp. decreases with increases of heightat the rate of 6.5 per km and reaches atabout -50C at the upper boundary.

4. The region next higher to troposphere is thestratosphere, where the temp. almost remainsconstant to -50C.


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5. The dielectric constant of the atmosphere isslightly greater than unity at the earths surface

& decreases to unity at grater than heights whereair density approaches zero.

6. Thus, the troposphere is an inhomogeneousdielectric medium whose refractive index varies

with height.n = c / v = r

r = dielectric constant of the median = refractive index

c = velocity of lightv = velocity of the wave in the media

7. Due to change in refractive index in it the wavevelocity changes.


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Tropospheric propagation(above 300 MHz)

The tropospheric is a region of the atmosphereadjacent to the earth, extending up to about 16kilometers.

The wave propagation beyond the line-of-sight is

possible using this mode of propagation. The several mechanisms responsible are classified

as:

1. Diffraction

2. Normal refraction3. Abnormal reflection and refraction

4. Tropospheric scatter


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1) Diffraction : When the waves aretraveling over a plane ground surfacemeet some obstacle then the waves donot stop at the obstacle, they bend overit and proceed.

ex: the closely spaced tracks on a CD or DVD

act as a diffraction grating to form thefamiliar rainbow pattern we see whenlooking at a disk

Diffracted

RaysIncidentwaves


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2) Refraction : When the waves passfrom one propagating media to a

medium having different density, thedirection of the wave changes.


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3) Abnormal reflection and refraction :

It is a phenomenon relating to

the wavelength of reflected light,whether in x-rays,interactions with metals, or fluids.

Incidentwaves

Normal

Reflectedwaves

Reflectingsurface


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4) Tropospheric scatter :

It is possible to communicate over long distances by

means of tropospheric scatter.

This phenomenon is due to irregularities in theatmosphere.

Tropospheric scatter is a method of transmittingand receiving microwave radio signals overconsiderable distances often up to 300 km.

Now consider a radio wave is transmitted into thetroposphere.


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The magnitude of the received signal depends on

the number of turbulences causing scatter in thedesired direction and the gain of the receivingantenna.

ex : point to point communicationi.e. in broadcasting system.


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Tropospheric Propagation1