Ducting Propagation 2a13y

TROPOSPHERIC - DUCTING PROPAGATION

Before proceeding towards the tropospheric propagation, we must know about the troposphere. We basically have the four layers of atmosphere as given as under: 1. Troposphere 2. Stratosphere 3. Mesosphere 4. Thermosphere Restricting our area of concern towards troposphere only, The troposphere begins at the Earth's surface and extends up to 4-12 miles (620 km) high. This is where we live. As the gases in this layer decrease with height, the air become thinner. Therefore, the temperature in the troposphere also decreases with altitude. As the lowest region of the Earth's atmosphere, the troposphere extends from the Earth's surface to a height of slightly over 7 miles. Virtually all weather phenomena occur in this region. Generally, the troposphere is characterized by a steady decrease in both temperature and pressure as height is increased. However, the many changes in weather phenomena cause variations in humidity and an uneven heating of the Earth's surface.

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Radio waves can propagate over the horizon when the lower atmosphere of the earth bends, scatters, and/or reflects the electromagnetic fields. These effects are collectively known as tropospheric propagation, or tropo for short.

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wavelength decreases as frequency increases and vice versa. Radio waves of frequencies below 30 megahertz normally have wavelengths longer than the size of weather turbulences. These radio waves are, therefore, affected very little by the turbulences. On the other hand, as the frequency increases into the vhf range and above, the wavelengths decrease in size, to the point that they become subject to tropospheric scattering. The usable frequency range for tropospheric scattering is from about 100 megahertz to 10 gigahertz.

There are probably three modes that describes the tropospheric propagation at its best as given under: 1. Tropospheric: bending 2. Tropospheric: scattering 3. Tropospheric: ducting. 

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The most well-known form of tropospheric propagation is called bending. Air reduces radio-wave propagation speed compared with the speed in a vacuum. The greater the air density, the more the air slows the waves, and thus the greater is the index of refraction. The density and index of refraction are highest near the surface, and steadily decrease with altitude. This produces a tendency for radio waves at very-high frequencies (VHF, 30 to 300 MHz) and ultra-high frequencies (UHF, 300 MHz to 3 GHz) to be refracted toward the surface. A wave beamed horizontally can follow the curvature of the earth for hundreds of miles.

Yet 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.  In general, tropo scatter is most pronounced at UHF and microwave radio frequencies (300 MHz and above). A radio wave beamed slightly above the horizon can be scattered at altitudes up to several miles, making over-the-horizon communication possible. The greatest communications range can be realized over flat land or over water. Scattered 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 visualized as a relay station located above the horizon; it receives the transmitted energy and then reradiates it in a forward direction to some point beyond the lineof-sight distance. A high gain receiving antenna aimed toward this scattered energy can then capture it.  The magnitude of the received signal depends on 1. 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 the diffracted space wave drops to an unusable level.

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 the dielectric property of the air. This property depends on the pressure, temperature and humidity of the air.  In general as we move upwards through the atmosphere the pressure decreases and temperature falls. This means that the dielectric property changes with height and allows a slight 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 90 degrees, then the upper part of the wave travels faster than the lower part. Therefore even under normal conditions this can in effect bend, or refract, the wave back down to earth.  Tropospheric ducting of UHF television signals is relatively common during the summer and autumn months, and is the result of change in the refractive index of the atmosphere at the boundary between air masses of different temperatures and humidities. The denser air at ground level slows the wave front a little more than does the rare upper air, imparting a downward curve to the wave travel.

Temperature Inversion / Troposphere Ducting: Certain weather conditions produce a layer of air in the Troposphere that will be at a higher temperature than the layers of air above and below it. Such a layer will provide a "duct" creating a path through the warmer layer of air which has less signal loss than cooler layers above and below. These ducts occur over relatively long distances and at varying heights from almost ground level to several hundred meters above the earth's surface. This propagation takes place when hot days are followed by rapid cooling at night and affects propagation in the 50 MHz - 450 MHz range (6 meter, 2 meter, 1 1/4 meter and 70 centimeter bands). Signals can propagate hundreds of kilometers up to about 2,000 kilometers (1,300 mi).

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Have you ever seen a mirage on the road that looks like water, that actually the sky you are seeing. Light 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 VHF, UHF 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. Ducting 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 extend for 1,000 miles (1,600 km). It occurs most frequently along coastal areas bordering large bodies of water. This is the result of natural onshore movement of cool, humid air shortly after sunset when the ground air cools more quickly than the upper air layers. The same action may take place in the morning when the rising sun warms the upper layers. Even though tropospheric ducting has been occasionally observed down to 40 MHz, the signal levels are usually very weak. Higher frequencies above 90 MHz are generally more favorably propagated. High mountainous areas and undulating terrain between the transmitter and receiver can form an effective barrier to tropospheric signals. Ideally, a relatively flat land path between the transmitter and receiver is ideal for tropospheric ducting. Sea paths also tend to produce superior results.

Tropospheric ducting occurs when there is a sharp rate of change in the dielectric constant as we move 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

If 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 yellow shaded area.

Under these conditions we now have the radio wave bent back towards the earth. However, 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 horizon.

Typical conditions required for a good duct to occur are: 1. An increase in temperature by 3 degree celcius or more per 100ft. 2. A rapid decrease of RH (dew-point) with height. The depth of the duct required for varying wave-lengths is: 1. 50ft for wavelengths around 3cm (approx. 1000MHz) 2. 600ft for wavelengths around 1m (approx. 300MHz) Typical meteorological conditions which can be favorable for ducting are: 1. Warm dry air over a cooler surface, especially a cool sea 2. Surface cooling under clear skies overland 3. Anticyclone (high pressure) or developing high pressure ridges with a cold surface. 4. Sea breezes undercutting warm air overland 5. At fronts with a strong thermal contrast

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Tropospheric scatter propagation is used for point-to-point communications. A correctly designed tropospheric scatter circuit will provide highly reliable service for distances ranging from 50 miles to 500 miles. Tropospheric scatter systems may be particularly useful for communications to locations in rugged terrain that are difficult to reach with other methods of propagation. Due to ducting, radio waves travels in ducts several hundred feet above the ground. Tropospheric signals exhibit a slow cycle of fading and will occasionally produce signals sufficiently strong for noise-free stereo, reception of Radio Data System(RDS) data, and solid locks of HD Radio streams on FM or noise-free, color TV pictures. Considering the one of the real time applications, one way to find out if a duct is opening up is to monitor tv channels that are not used in your area for example if your local tv channels are 2 and 6 try to see if 4 or 9 are coming.

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