Winter 2003 Issue

Figure 1. A typical plot of the daytime electron density as a function of height above the ground, showing the C, D, E, F1, and F2 layers.

This seminal work on 50-MHz F2 propagation is reprinted by permission from the Proceedings of the 34th Conference of the Central States VHF Society, July 2000.

By Jim Kennedy,* KH6/K6MIO
Gemini Observatory,1 Hilo, Hawaii

The possibility of worldwide F-layer propagation is a particularly intriguing part of the challenge of 6-meter operation. Even the casual 6-meter operator will soon notice that there are some fairly mysterious things going on when it comes to ionospheric propagation. The more seasoned operator will notice that there are a number of prevalent patterns, but it is still very difficult to predict when the band will open, especially on a day-to-day basis. Unfortunately, there are no simple answers to this dilemma. Nevertheless, there are some pieces to the F2 puzzle that are known and understood, and some clues to those that remain mysterious. In order to understand (however imperfectly) when the band will open, it is essential to have some understanding of why the band will open.
A discussion of why signals propagate has to begin with some basic facts about how radio waves behave in the ionosphere. There are three basic elements that critically affect this propagation:

1. The amount of ionization present,
2. The angle of attack of the incoming signal to the ionosphere, and
3. The presence of large or small irregularities in the ionization.

These factors play key roles in the success or failure of a communications path via either E or F layers. Although there are many external things which influence the status of the three conditions, in the end, it is the combination of these three that make or break any path. The way in which external events affect these three parameters determines what kind of propagation will occur.
Six-meter F2 propagation is a very improbable event, from a statistical point of view. While this may seem obvious, it has a very important consequence. “Unlikely” events in complex physical systems are often the result of a combination of factors, some of which also may be fairly unlikely. This is certainly the case with most 6-meter F2 activity, where propagation is almost always at or very near the ultimate edge of what is possible.

The task of predicting band openings generally involves predicting not just one event, but the coincidence of several events, and not always the same ones or in the same combinations. In truth, we do not yet know what all the factors are, much less how they interact. On the other hand, there are a number of things that are known to be significant, and most of them involve the Sun in one way or another.

The Ionosphere and the Sun

The Earth’s atmosphere extends from the surface to heights well in excess of 1000 km (or 620 miles). The density of the static atmosphere is highest near the surface and decreases progressively as one goes upwards. Most of the atmospheric mass is located very near the Earth’s surface, with more than half the mass contained within just the first 6 km (or 4 miles).

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50-MHz F2 Propagation Mechanisms

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