Tropospheric Propagation.
~ a primer ~
~ a primer ~
VHF propagation beyond that of line of sight occurs by using the troposphere, the Ionosphere, or an obstacle such as the moon or a building.
The troposphere is the lowest part of the atmosphere, and what takes place within its boundaries largely determines our weather. Its upper boundary is the temperature-inversion layer known as the tropopause, which lies between 8km and 16km high depending on the latitude and prevailing conditions.
The key to tropospheric propagation is refraction.
Refraction is the change in direction of a wave passing from one medium to another. Keep in mind that radio waves and light behave in a very similar way.
The refractive index is a measure of how fast light travels through a specific medium and determines how much the path of light is bent, or refracted when entering a material of different density.
The most common VHF/UHF propagation occurs due to changes of refractive index within the troposphere. Amateur Radio operators broadly refer to this form of propagation as “tropo”
Starting from ground level, the layers include the troposphere, stratosphere, mesosphere and thermosphere and the exosphere.
There are three types of tropospheric propagation, or “tropo”, which take signals a significant distance beyond the horizon:
These modes occur almost exclusively in the lowest few kilometres of the troposphere where the air density and water-vapour content are greatest.
It’s also worth noting that the refractive index decreases with height.
1. Enhanced tropospheric refraction occurs when there is a significant increase above the normal value of the refractive index in the atmosphere.
2. Tropospheric ducting occurs when radio waves are trapped within channels bounded by sharp changes in refractive index and…
3. Tropospheric scattering, which arises from small-scale variations in the refractive index.
There is also evidence of tropospheric scatter in bands between 1296 MHz and 10 GHz.
Enhanced tropospheric refraction occurs when the lower troposphere divides into two stable layers, typically a warm dry layer over a cool moist layer.
The boundary between these two layers is called an inversion. Normally in the lower troposphere, temperatures decrease with height and humidity increases with height – which is why they are called inversions.
Signals bend as they cross the inversion.
When they start bending downwards, the signals can travel farther, reaching places that are normally beyond the radio horizon and out-of-range.
The base of the inversion is considered to be the ground. The top of the inversion is the airmass boundary.
Tropospheric ducting occurs when radio waves are trapped between two boundaries. Ducts fall into two categories – Surface ducts and Elevated ducts.
Ducting occurs when the refractive bending increases so much that the signal hits the ground at a distance far away from the transmitter and is then reflected back up to the inversion, to then be refracted back down again. In essence, the signals become trapped in a layer or surface duct.
Surface ducts occur when a steep negative gradient in refractive index forms immediately above the ground or sea, trapping the radio waves by refraction from above, and by reflection from the surface below.
In a normal atmosphere, the air temperature and dew point – and hence the refractive index –all decrease with height above ground. The top of the surface duct is where the inversion occurs.
Elevated ducts form when a double discontinuity in refractive index occurs.
In this case, the waves are bent upwards from the lower boundary of the duct, and downwards from the upper boundary. The waves can be guided over irregular terrain until the duct is broken by mountains or other changes that the duct cannot follow.
Elevated ducts form at heights typically between 450 and 2000m, and are the origin of the longest overland tropo openings. Even longer distances can be worked via elevated ducts over the sea.
Although the signals are being carried far from the transmitter, receivers at low elevation will not be able to receive them. Only tall masts or locations on high hills that “poke” into the duct will be able to receive the signals.
Tropospheric scatter is an almost ever-present condition that brings in distant fluttery signals beyond normal line-of-sight.
Scatter arises from small variations in the refractive index of the troposphere. Changes in refractive index occur wherever there is turbulence, for example in convection cells and in any strong wind.
Scatter arises from small variations in the refractive index of the troposphere. Changes in refractive index occur wherever there is turbulence, for example in convection cells and in any strong wind.
The atmosphere is never completely uniform and differences in temperature, pressure and water-vapour content are always present – if it were otherwise, we would have no weather. Changes in refractive index occur wherever there is turbulence, for example in convection cells and in any strong wind. These irregularities extend beyond the troposphere into the stratosphere.
In the lower troposphere, differences in water-vapour content dominate the changes in refractive index. At greater heights the humidity is much lower so that temperature variations become the dominant factor and the changes in refractive index are smaller. However, the lower density of the stratosphere leads to physically larger irregularities.
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.