Tropospheric DX Modes
by William R. Hepburn, WTFDA

"Tropo" Modes 

  Tropospheric propagation is radio propagation that occurs in the lowest layer of the Earth's atmosphere - the troposphere. It is 100% weather-related. A Tropo DX mode is any abnormal condition that scatters, reflects or refracts VHF, UHF and/or microwave signals in the troposphere causing changes to their normal coverage. Another name for this is anomalous propagation, or AP for short. Tall mountain ranges form a physical barrier to tropospheric propagation, thus tropo is more rare in or across mountainous regions. Deserts are usually too dry to support the long distance tropospheric modes. There are six main tropospheric DX modes. The refraction and ducting effects are similar to those that cause visual mirages. Therefore, the distant signals received via tropo can be considered "radio mirages". Signals that are normally below the radio horizon and out-of-range instead appear above the radio horizon and receivable. (Note that due to the difference in wavelengths, the radio horizon is farther away than the visual horizon). A description of the tropo modes follows:

1) LINE-OF-SIGHT (GW) - Also known as Groundwave. Normal continuous reception where the receiving and transmitting antennas can see each other, taking into account the 4/3 Earth curvature of radio waves. Figure 1 illustrates normal conditions where the receiver is out-of-range. The illustration is not-to-scale.

2) TROPOSPHERIC SCATTER (TrS)  - An almost ever-present condition that brings in distant fluttery signals beyond normal line-of-sight. Scattering of the signals occurs in contact with discontinuities in the troposphere. These discontinuities can be small temperature or humidity variations, such as can be found around cloud layers, individual clouds, updrafts, downdrafts, the tropopause (the boundary between the troposphere and the stratosphere), small particles and droplets such as drizzle, mist, haze, dust, smoke, volcanic ash, etc., or even flocks of birds and large swarms of insects.

3) TROPOSPHERIC SUPER-REFRACTION (TrE) - Also known as Tropospheric Enhancement or Tropospheric Bending. Super-refraction occurs when the lower troposphere becomes stratified into two stable layers. A warm dry layer over a cool moist layer (with warm and cool being relative terms). The boundary between these two layers is called an inversion. Normally in the lower troposphere, temperatures decrease with height and humidity increases with height - thus 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 effective bending is now less than 4/3 Earth radius, but still more than 1 Earth radius. The base of the inversion is considered to be the ground. The top of the inversion is the airmass boundary. The thickness of the inversion can roughly determine the lowest usable (affected) frequency (LUF). Figure 2 illustrates super-refraction.

4) TROPOSPHERIC DUCTING (TrD) - Ducting occurs when the super-refractive bending becomes 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 "trapping layer" or duct. The effective bending is now less than 1 Earth radius. The base of the duct is the ground. The top of the duct is the inversion. Signals do weaken when reflecting off the ground. Signals that reflect off of a water surface instead (ocean/lake) retain much more strength. One side-effect of ducting is reduced range for aircraft flying above the inversion. Most ducting occurs below 3000 m (10,000'). Above that level, the air starts to get too sparse in density for inversions to be strong enough. Figure 3 illustrates ducting.

5) ELEVATED TROPOSPHERIC DUCTING - In cases where the top of the inversion is very high above the ground, it may become possible for the low level moisture to rise and pool beneath the inversion top. In these cases, 3 different airmass layers can form - with the surface layer being somewhat warmer and drier than the cool moist air higher up. This results in a duct that is elevated above the ground. 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. Often this is the case with the transmitter as well, as tall masts or locations on high hills poking into the duct get direct access to it. Although the bulk of the trapped signals stay within the duct, they may occasionally escape allowing for random and spotty reception beneath the duct. Often, long ducts may consist of portions that are surface-based and portions that are elevated. Figure 4 illustrates elevated ducting.
Elevated Ducting

6) TROPOSPHERIC SUB-REFRACTION - Also known colloquially as "Anti-Tropo". Sub-refraction occurs when the lower troposphere becomes unstable with a greater-than-normal dropoff of temperature with height. Signals gradually bend upwards. The effective bending is greater than the normal 4/3 Earth radius. This results in reduced range for all signals. The degree of abnormal bending during an extreme sub-refraction event is much less than that observed during extreme super-refraction. Figure 5 illustrates sub-refraction.

Main Types of Tropo, according to Meteorology

1) RADIATION TROPO - 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.

2) HIGH-PRESSURE TROPO - 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.

3) FRONTAL TROPO - 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 fast motion of cold fronts, ducting events are often short-lived.

4) DOWNSLOPE TROPO - Also known as Chinook Tropo, Santa Ana Tropo, Fhn 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.

5) ADVECTION TROPO - Also known as Warm Air Advection Tropo. It occurs when warm dry air overrides cooler moist land (example: recently rain-soaked land) resulting in a shallow inversion.

6) VALLEY TROPO - 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.

7) MARINE TROPO - 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 miles 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.

Exotic Tropospheric DX Modes

  Here are some other tropospheric modes that are more rare and exotic :

1) RAIN SCATTER (RS) - Rain scatter mainly occurs in the higher UHF and microwave bands. A band of very heavy rain at a distance can scatter or even reflect signals. The effect is the one used for Meteorological Radars. (Note that snow is not an useful reflector). Signals may appear to arrive from the wrong direction - that of the rain band rather than the transmitter.

2) HAIL SCATTER (HS) - Similar to Rain Scatter but caused by Hail.

3) ICE PELLET SCATTER (SS) - Known as Sleet Scatter in the USA. Similar to Rain Scatter but caused by Ice Pellets.

4) LIGHTNING SCATTER (LS) - Lightning Scatter is a seldom-reported mode that affects UHF and microwave. It is very similar to Meteor Scatter (MS). Quick short bursts of reception are caused by reflection off of the ionized trails forged by lightning strikes.

5) AIRCRAFT SCATTER (AS) - Caused by reflections off of aircraft. Aircraft scatter is more common within 10 miles of an active airport.

  Reflections off of hills, mountains, buildings and Knife-Edge Diffraction by itself are not considered true DX modes since they are omni-present. However, when present in combination with tropo they can help to extend signal range even further.

   There are also conditions in the ionosphere that produce distant reception via a whole different set of modes. Ionospheric "Skip" and Scatter are not caused by the weather, but instead by the interaction between the Sun and the Earth's outer atmosphere, or by objects such as meteors (and possibly thunderstorm sprites). For more information on these modes, consult Wikipedia's article on VHF-UHF Propagation.

2018 William R. Hepburn