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 favourably propagated.
Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anticyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent.
Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 1,300 km although some people have received “tropo” beyond 1,600 km. While with tropospheric-bending, stable signals with good signal strength from 800+ km away are not uncommon when the refractive index of the atmosphere is fairly high.
Tropospheric ducting of radio and 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. Using an analogy, it can be said that 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.
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 1,500 km or more along a stationary weather front.
This diagram illustrates some of the features and phenomena found in the various layers of Earth’s atmosphere.
Starting from ground level, the layers include the troposphere, stratosphere, mesosphere and thermosphere. The exosphere, which is above the thermosphere, is not shown in the diagram. Source: ACAR
Tropospheric scatter is the most common form of tropospheric enhancement. Tropo-scatter is always present to some degree just about everywhere.
Tropospheric scatter at FM and TV frequencies is caused when the paths of radio signals are altered by slight changes in the refractive index in the lower atmosphere caused by air turbulence, and small changes in temperature, humidity and barometric pressure. The signal is scattered in a random fashion. The tiny portion of the transmitted signal that is scattered forward and downward from what is called the “common scattering volume” is responsible for signal paths longer than the normal line-of-sight horizon.
The height of the scattering volume that is common to both the transmitting and receiving stations determines the maximum tropo-scatter path distance. Above about 10km refraction in the troposphere becomes insufficient to return any signal to Earth.
Tropo-scatter enables the reception of signals from out to about 800km, depending primarily on the power of the transmitting station and the quality of the receiving equipment being used. Maximum tropo-scatter path distances of 200 to 300 are more typical on a day-to-day basis. Tropo scattered signals are characteristically weak, “fluttery” signals that often suffer from random fading.
This is where things start getting interesting for the DXer. Strong temperature inversions with very well defined boundaries sometimes form from as high as several thousand feet above the surface of the Earth.
If the inversion is strong enough, a signal crossing the boundary into the inversion will be bent sufficiently to return it to Earth. The inversion boundary layer and the surface of the Earth form the upper and lower walls of a “duct” that acts much like an open-ended wave guide. Signals “trapped” in the duct follow the curvature of the Earth, sometimes for hundreds or even thousands of kilometers. In the tropics and over large bodies of water, strong inversions that cover large geographic areas are quite common, and stable ducts can remain in tact for days on end.
An interesting characteristic of this form of ducting is that both the transmitting and receiving antennas must be inside of the duct to gain the maximum signal enhancement. A receiving antenna located outside of the duct will hear little or no signal from a transmitting antenna located inside the duct.
For this type of duct to be useful to us, the signal must get in and exit the duct somewhere along the signal path. This can occur if the ends of a duct are open at each end, or through “holes” that form along the bottom layer of the duct.
The illustration above shows high altitude troposheric ducting.
An interesting characteristic of this form of ducting is that both the transmitting and receiving antennas must be inside of the duct to gain the maximum signal enhancement. A receiving antenna located outside of the duct will hear little or no signal from a transmitting antenna located inside the duct. For this type of duct to be useful to us, the signal must get in and exit the duct somewhere along the signal path. This can occur if the ends of a duct are open at each end (see below), or through “holes” that form along the bottom layer of the duct.
Tropospheric ducting most often occurs because of a dramatic increase in temperature at higher altitudes. If the temperature inversion layer has lower humidity than the air below or above it, the refractive index of the layer will be enhanced further. There are several common weather conditions that often bring about strong temperature inversions.
While not usually the cause of strong ducting, radiation inversions can bring about pronounced signal enhancement, extending the DX range up to a few hundred kilometres. This is probably the most common and widespread form of inversion a DXer is likely to encounter on a regular basis.
A radiation inversion forms over land after sunset. The Earth cools by radiating heat into space. This is a progressive process where the radiation of surface heat upwards causes further cooling at the Earth’s surface as cooler air moves in to replace the upward moving warm air. At higher altitudes the air tends to cool more slowly, thus setting up the inversion. This process often continues all the way through the night until dawn, sometimes producing inversion layers at 1,000 to 2,000 feet above the ground.
Radiation inversions are most common during the summer months on clear, calm nights. The effect is diminished by blowing winds, cloud cover and wet ground. Radiation inversions are often more pronounced in dry climates, in valleys and over large expanses of flat, open ground.
Another meteorological process called “subsidence” often produces strong ducting conditions and excellent DX. Subsidence is the process of sinking air that becomes compressed and heated as it descends over a rather large area. This process often causes strong temperature inversions to form at altitudes ranging from 1,000 feet to as high as 10,000 feet.
These almost stationary high-pressure zones often form over Australia during the summer and early autumn months.
Temperature inversions occur 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. Smoke and pollution can also be trapped by temperature inversions.
Rising smoke in Lochcarron (Scotland) forms a ceiling over the valley due to a temperature inversion.
There are several types of tropospheric propagation according to meteorologists:
Radiation Tropo is also known as Radiative Cooling Tropo or Nocturnal Tropo. A common nocturnal event that often occurs during clear, calm nights on land. Radiative cooling results in cooler more humid conditions near the surface which forms a shallow inversion. This inversion usually “burns off” shortly after sunrise. Due to its shallow nature, Radiation Tropo often follows the topography of the land.
Also known as Subsidence Tropo. Sinking air (subsidence) in a high-pressure system warms and dries as it descends. Often cool moist air can become trapped underneath forming an inversion. High-Pressure tropo can last all day. Often, Radiation Tropo occurs simultaneously at night, blocking more distant signals from High-Pressure Tropo. As a result, conditions can often be better during the day.
Frontal inversions can be found in the area ahead of an approaching warm front, behind a departing cold front, or north of a quasi-stationary front. Inclement weather often accompanies fronts and may hinder duct formation. Due to the normally fast motion of cold fronts, cold frontal tropo events are often short-lived.
Advection Tropo comes in two forms. Warm Air Advection Tropo occurs when warm dry air overrides cooler moist land (example: recently rain-soaked land) resulting in a shallow inversion. Cold Air Advection Tropo occurs when cool moist air undercuts warmer drier air aloft. This can often occur along the northern and western flanks of tropical cyclones as they advance into the temperate zones.
Also known as Chinook Tropo, Santa Ana Tropo, Fœhn Tropo, Bora Tropo, Zonda Tropo, etc. Downslope Tropo is caused by air descending down a mountainside that warms and dries as it descends. If the pre-existing airmass is cool enough, it may become trapped under an inversion.
Warm dry air can override cooler moist air trapped in a valley under the resulting inversion. This is different from topography-conforming Radiation Tropo in that the inversion can persist all day long, long after any radiative effects have dissipated.
Also known as Maritime Tropo, Oceanic Tropo or Lake-Effect Tropo. Marine Tropo occurs when warm dry air overrides a cooler body of water. Marine inversions often extend the entire breadth of lakes and can extend for thousands of kilometres over the ocean. It also spreads into coastal areas by way of sea or lake breezes. Marine tropo can become enhanced or combined with other types such as High-Pressure Tropo. It normally peaks during the afternoon when the inversion is the strongest. Outside of the equatorial zone, spring and early summer is the best season.
A basic principle of radio is that the wavelength of a signal gets shorter as the frequency of the signal is increased. Because of this, the size of the tropospheric duct determines the lowest signal frequency that it can successfully propagate. This is known as the Lowest Usable Frequency or LUF of the duct.
A physically small duct, a duct with its upper and lower boundaries close together, will propagate only higher frequency signals with very short wavelengths. As the distance between the boundaries of the duct increases, the signal frequency the duct will propagate decreases. In other words, a larger duct will accommodate a lower frequency signal having a physically longer wavelength. It’s possible for a duct to form that only supports signal propagation at UHF, while not effectively passing anything in the VHF bands.
Ducted signals from 1400 – 1600 km are fairly common, but it’s more common for ducted signals to travel 800 – 1300km. Ducted signals are typically quite strong, sometimes so strong that they can cause interference to local signals on the same
frequency.
Weather Suitable for a Duct – Tropospheric ducting most often occurs because of a dramatic increase in temperature at higher altitudes. If the temperature inversion layer has lower humidity than the air below or above it, the refractive index of the layer will be enhanced further. There are several common weather conditions that often bring about strong temperature inversions.
While not usually the cause of strong ducting, radiation inversions can bring about pronounced signal enhancement, extending the DX range up to a few hundred kilometres. This is probably the most common and widespread form of inversion a DXer is likely to encounter on a regular basis.
A radiation inversion forms over land after sunset. The Earth cools by radiating heat into space. This is a progressive process where the radiation of surface heat upwards causes further cooling at the Earth’s surface as cooler air moves in to replace the upward moving warm air. At higher altitudes the air tends to cool more slowly, thus setting up the inversion. This process often continues all the way through the night until dawn, sometimes producing inversion layers at 1,000 to 2,000 feet above the ground. Radiation inversions are most common during the summer months on clear, calm nights.
The effect is diminished by blowing winds, cloud cover and wet ground. Radiation inversions are often more pronounced in dry climates, in valleys and over large expanses of flat, open ground.
Sources: DXInfo, BOM, Wiki, ACAR.
Check out other forms of troposheric propagation such as Sporadic E.
