Spring 2009 Issue

Crossing Oceans with VHF Ducts
Subsidence Temperature Inversions

In an effort to cross oceans with amateur radio signals, N7BHC has been working tirelessly to find propagation paths where no amateur radio signal seemingly has gone before. Here he discusses subsidence temperature inversions and how they may be used for making record-establishing transoceanic QSOs.

By Dave Pedersen, N7BHC


VHF and UHF operators have used temperature inversions and the ducting propagation they create for many decades. There are several types of ducts that occur over both land and sea. These include frontal, evaporative, and subsidence ducts. Surface ducts reach all the way to the Earth’s surface, while elevated ducts do not.

Ducts are caused by a change in the refractive qualities of the medium through which the radio signal is traveling—in this case, the lower atmosphere. The key mechanism is a temperature inversion. The sun’s energy heats the Earth, which in turn heats the air in contact with it. Thus, the warmest air is usually at the Earth’s surface. The air progressively cools as the altitude increases. This is shown in figure 1(A). The solid line is temperature, while the dashed line is humidity.

Under certain conditions, the temperature stops cooling and increases by as much as 5–15 degrees F over just a few hundred feet elevation. This is a temperature inversion and forms the top of the trapping layer, or duct. A temperature inversion is illustrated in figure 1(B). Note that in the temperature inversion, the humidity can decrease sharply. The humidity change actually has a much greater effect on the index of refractivity than does the temperature.

The keen observer will notice a small temperature inversion almost at the surface in figure 1(A). This is probably an evaporative duct, suitable for microwave paths in the 3–24 GHz range. Evaporative ducts are very reliable across the oceans and have the potential for very long-distance contacts. This propagation mode is worthy of a lot more amateur research for stations literally at the beach. Very little work has been done on this topic for point-to-point communications, and it may be covered in a future article.
This article is not an exhaustive explanation of temperature-inversion creation and physics. The reader is encouraged to conduct further research online. Several web pages are referenced in this article and in the references section at the end.

In this article we will focus on subsidence ducts. These subsidence ducts are a primary cause for extremely long-range temperature inversions across open ocean paths. We will explore these ducts, the mechanisms that cause them, and where they appear. We will also investigate why some paths have not been worked yet, and offer a few observations and suggestions on potential strategies to work these paths.

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