Winter 2003 Issue

A Look at Long-Range North-South Path F2-Skip on 50 MHz

During his many years on 6 meters WB2AMU has made some fascinating observations concerning transequatorial propagation. See if you agree with them.

By Ken Neubeck,* WB2AMU

Figure 1. This figure shows how the signals are carried in the
F-layer of the ionosphere over the geomagnetic equator to create TEP. The stations involved are located at equal distances from the geomagnetic equator, and the overall distance covered
is on the order of 6000 miles.

Over the past several years, there have been some misunderstandings concerning the interpretation of 50-MHz transequatorial propagation (TEP) and related F2 north-south paths. Because of limited observations over the years, little has been written on the subject. Indeed, very few VHF column writers will tackle this subject with any great confidence, frequently avoiding any discussion of the subject. Consequently, many 6-meter operators may not be adequately informed about when these events can occur, and thus they may not get the chance to observe such activity when it does occur.
With the 2001 second sunspot peak of Cycle 23, many more observations made by operators on 6 meters have reinforced the knowledge base of TEP and related F2 north-south paths, allowing the opportunity for the actual path mechanisms to be better defined for these propagation modes.

In 2000, Jim Kennedy, K6MIO/KH6, published “50 MHz F2 Propagation Mechanisms,”1 in which he describes the actual mechanics of TEP. One of the terms Jim uses to explain the TEP effect is chordal hop. He describes the action of 50-MHz radio waves at the edges of the TEP zone as follows:

This upward tilt is such that an upcoming wave hits the near corner at a shallower angle of attack to the tilted layer than it would to the usual spherical layer. This means that it will have a higher MUF for the same value of electron density. That is, the M factor is larger than 3.4, perhaps by quite a bit. The wave is not bent all the way back toward the ground. However, it is bent enough to cross the equator and hit the tilted layer on the far side, without ever coming back to Earth. (p. 94)

Figure 1 shows a typical chordal-hop TEP path on 6 meters between a station located in Miami, Florida and one in Buenos Aires, Argentina. Often this path is observed during the higher sunspot count years, and generally during the months of the fall and spring equinoxes.

As noted by Jim in his paper, the reason why the best time for north-south TEP skip is during the fall and spring equinoxes is because the winter anomaly effects for F2 propagation overlap a bit in both the Northern and Southern Hemispheres. Otherwise, when it is winter in one hemisphere and F2 is possible (during the higher solar-count year), it is summer in the other hemisphere (when no F2 is present), and thus there is no chance for north-south F2 paths between the hemispheres to occur.

During the late afternoon and evening, the F-layer near the magnetic equator has a build up of electron density known as the fountain effect, which allows for this TEP path. With regard to the fountain effect, Jim comments: “In order for this form of propagation to function, both the north-side and the south-side tilted regions need to be ionized enough to make the path work.” (p. 94) He adds:

For operators who have the good fortune to be in the TEP zone, the paths themselves do not have to be especially north-south. In the simplest case, the two stations are on opposite sides of the magnetic equator, although they can be at a considerable angle to the north-south line. All that is necessary is that the two ±20-degree corners be at usable chordal skip points. (p. 97)

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A Look at Long-Range North-South Path F2-Skip on 50 Mhz

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