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TROPOSPHERIC - DUCTING PROPAGATION

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Before proceeding towards the tropospheric propagation, we must knowabout the troposphere. We basically have the four layers of atmosphere asgiven as under:1. Troposphere2. tratosphere

!. "esosphere#. Thermosphere

  $estricting our area of concern towards troposphere only, Thetroposphere begins at the %arth&s surface and e'tends up to #(12 miles )*(2+ km high. This is where we live. -s the gases in this layer decrease withheight, the air become thinner. Therefore, the temperature in the

troposphere also decreases with altitude.-s the lowest region of the %arth&s atmosphere, the troposphere e'tendsfrom the %arth&s surface to a height of slightly over miles. /irtually allweather phenomena occur in this region. 0enerally, the troposphere ischaracteried by a steady decrease in both temperature and pressure asheight is increased. owever, the many changes in weather phenomenacause variations in humidity and an uneven heating of the %arth&s surface.

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$adio waves can propagate over the horion when the lower atmosphere of theearth bends, scatters, and3or reflects the electromagnetic fields. These effects arecollectively known as tropospheric propagation, or tropo for short.

wavelength decreases as fre4uency increases and vice versa. $adio waves of

fre4uencies below !+ megahert normally have wavelengths longer than thesie of weather turbulences. These radio waves are, therefore, affected verylittle by the turbulences. 5n the other hand, as the fre4uency increases into thevhf range and above, the wavelengths decrease in sie, to the point that they

 become sub6ect to tropospheric scattering. The usable fre4uency range fortropospheric scattering is from about 1++ megahert to 1+ gigahert.

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There are probably three modes that describes the tropospheric propagation at its bestas given under:

1. Tropospheric: bending

2. Tropospheric: scattering

!. Tropospheric: ducting.

The most well(known form of tropospheric propagation is called bending. -irreduces radio(wave propagation speed compared with the speed in a vacuum. Thegreater the air density, the more the air slows the waves, and thus the greater is theinde' of refraction. The density and inde' of refraction are highest near the surface,and steadily decrease with altitude.

This produces a tendency for radio waves at very(high fre4uencies )/7, !+ to !++" and ultra(high fre4uencies )87, !++ " to ! 0 to be refracted towardthe surface. - wave beamed horiontally can follow the curvature of the earth forhundreds of miles.

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9et another aspect of tropospheric propagation is troposphere scattering. The lower atmosphere scatters electromagnetic radiation over a vast range, including radio

wavelengths. This effect is known as tropospheric scatter, or tropo scatter. n general, tropo scatter is most pronounced at 87 and microwave radio fre4uencies )!++

" and above. - radio wave beamed slightly above the horion can be scattered at altitudesup to several miles, making over(the(horion communication possible. The greatestcommunications range can be realied over flat land or over water. cattered waves are weak,

so high(power transmitters and sensitive receivers are necessary. This is because of the height at which scattering takes place. The turbulence that causes the

scattering can be visualied as a relay station located above the horion; it receives thetransmitted energy and then reradiates it in a forward direction to some point beyond the line(of(sight distance. - high gain receiving antenna aimed toward this scattered energy can thencapture it.

The magnitude of the received signal depends on1. the number of turbulences causing scatter in the desired direction and

2. the gain of the receiving antenna. This scattering mode of propagation enables vhf and uhf signals to be transmitted far beyond

the normal line(of(sight. provides a usable signal at distances beyond the point where thediffracted space wave drops to an unusable level.

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The most important feature of tropospheric propagation is the tropospheric ducting. Tropospheric ducting is a type of radio propagation that tends to happen during periods of

stable, anticyclonic weather. The speed of a radio wave in the atmosphere is determined by thedielectric property of the air. This property depends on the pressure, temperature and humidityof the air.

n general as we move upwards through the atmosphere the pressure decreases andtemperature falls. This means that the dielectric property changes with height and allows aslight increase in the speed of a radio wave as we move upwards through the atmosphere.

This in turn means that if a radio wave moves away from the earth at an angle less than

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Temperature Inversion / Troposphere Ducting: Certainweather conditions produce a layer of air in the Troposphere thatwill be at a higher temperature than the layers of air above and

below it. Such a layer will provide a "duct" creating a path throughthe warmer layer of air which has less signal loss than cooler layersabove and below. These ducts occur over relatively long distancesand at varying heights from almost ground level to several hundredmeters above the earth's surface. This propagation takes place whenhot days are followed by rapid cooling at night and aects

propagation in the 50 M! " #50 M! range $% meter& meter& ( ()#meter and *0 centimeter bands+. Signals can propagate hundreds ofkilometers up to about &000 kilometers $(&,00 mi+.

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ave you ever seen a mirage on the road that looks like water, that actually the sky you areseeing. =ight waves that normally travel in a straight line bounce of the super heated windless pavement and are reflected back to your eyes. The same concept during a duct( but backwards. Typically straight line /7, 87 signals begin to travel up and away, but are bent back by a sharp boundary layer of warm, moist air overlying cool dryer air below and above.

>ucting can occur on a very large scale when a large mass of cold air is overrun by warm air.This is termed a temperature inversion, and the boundary between the two air masses may

e'tend for 1,+++ miles )1,*++ km. t occurs most fre4uently along coastal areas borderinglarge bodies of water. This is the result of natural onshore movement of cool, humid air shortlyafter sunset when the ground air cools more 4uickly than the upper air layers. The same actionmay take place in the morning when the rising sun warms the upper layers.

%ven though tropospheric ducting has been occasionally observed down to #+ ", thesignal levels are usually very weak. igher fre4uencies above

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Tropospheric ducting occurs when there is a sharp rate of change in the dielectric constant as wemove upwards through the atmosphere. That probably occurs as a result of

1. rapid increase of temperature and

2. a rapid decrease in humidity )dew(point with height

f we look at a vertical profile of the

atmosphere showing ducting potential, we can

see that there is a sharp increase in temperature

)an inversion, coupled with a sharp fall in

dew(point )indicating a fall in humidity. The

duct occurs below this inversion in the yellowshaded area.

8nder these conditions we now have the radio wave bent back towards the earth. owever, the

radio wave can then reflect back of the earth and become refracted again to return earthwards

once more. This can sometimes occur a number of times with little attenuation but some fading.

The result can be long distance reception of radio waves that would normally have been far

 beyond the radio horion.

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Typical conditions re4uired for a good duct to occur are:

1. -n increase in temperature by ! degree celcius or more per 1++ft.

2. - rapid decrease of $ )dew(point with height.

The depth of the duct re4uired for varying wave(lengths is:

1. ?+ft for wavelengths around !cm )appro'. 1+++"

2. *++ft for wavelengths around 1m )appro'. !++"

Typical meteorological conditions which can be favorable for ducting are:

1. Warm dry air over a cooler surface, especially a cool sea

2. urface cooling under clear skies overland

!. -nticyclone )high pressure or developing high pressure ridges with a coldsurface.

#. ea breees undercutting warm air overland

?. -t fronts with a strong thermal contrast

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Tropospheric scatter propagation is used for point(to(point communications. -correctly designed tropospheric scatter circuit will provide highly reliable servicefor distances ranging from ?+ miles to ?++ miles. Tropospheric scatter systems may be particularly useful for communications to locations in rugged terrain that aredifficult to reach with other methods of propagation.

>ue to ducting, radio waves travels in ducts several hundred feet above the ground.

Tropospheric signals e'hibit a slow cycle of fading and will occasionally producesignals sufficiently strong for noise(free stereo, reception of $adio >ataystem)$> data, and solid locks of > $adio streams on 7" or noise(free, colorT/ pictures.

@onsidering the one of the real time applications, one way to find out if a duct isopening up is to monitor tv channels that are not used in your area for e'ample if

your local tv channels are 2 and * try to see if # or < are coming.

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 T/S