Winter 2008 Issue
Are Tropo Extensions to
Sporadic-E Openings Possible?
Many of us weak-signal operators occasionally have experienced a DX
QSO that is beyond what we might expect for the single-hop sporadic-E
propagation range. WB2AMU offers a possible explanation of how it was
possible to complete such a QSO.
By Ken Neubeck, WB2AMU
Sometimes veteran 6-meter operators wonder whether there are other forces at play that seem to extend an apparent sporadic-E opening beyond the normal single-hop distance. This question often comes to mind with regard to long-range paths when two-hop sporadic-E is part of the equation but still seems to need an additional mode to carry the signal.
In previous articles presented in CQ VHF and CQ magazines, discussions were presented about “mix and match” propagation, where different modes of propagation combine to carry a 6-meter signal over specific distances. For example, during the height of the latest sunspot peak, it was observed that F2 paths combined with sporadic-E openings to extend distances, and sometimes past sunset. One mix-and-match mode that would seem possible, but hard to prove based strictly on radio observations, is the combination of tropospheric-ducting paths with sporadic-E skip.
In this article we will explore the
possibility of this combination mode as well as examine the type of
geometry that would be involved if a tropospheric-duct path was
connected to the end of a sporadic-E path. The emphasis primary will be
on 50-MHz events. However, a special case of a 144-MHz event will be
discussed as well.
One path that comes to mind is the path between the northeast US and the UK during the summer months. Conventional thinking states that this path is at least a two-hop sporadic-E opening, and sometimes a three-hop sporadic-E opening. However, another model that could fit is a two-hop sporadic-E opening with tropo extensions on one or both sides of the path of the contact. Figure 1 shows the geometry of such a potential combination.
With long-range contacts such as these, it is almost impossible to tell by listening to the signal quality whether there may be some tropo enhancement on one end of the path. If there was such an extension to the sporadic-E link, it would be just as hard to determine which side of the link the tropo extension was on. Sometimes tropo has a unique fading quality of the signal that differs from the rapid fading experienced with sporadic-E. However, if the two modes are combined, it basically would be impossible to tell which of the modes was causing the signal fading.
Indeed, it generally is impossible by just listening to the signals to tell for sure if there is such a combination. However, a map-plot approach may actually provide clues. For example, during the heat of the summertime sporadic-E season, stations in much of Florida can work stations in South America and a good portion of the Caribbean on 6 meters. However, there are some Caribbean stations that are difficult for Florida stations to work, because the distance falls beyond a single-hop sporadic-E opening and somewhat short for a double-hop sporadic-E opening. It is kind of like a one-and-a-half sporadic-E path, and would almost be like very short sporadic-E skips lined up back-to-back, which is pretty rare. If two very short skips on 6x meters actually happened, there probably would be a possibility of 2-meter sporadic-E at the same time.
The evidence, however, appears to point to a single-hop sporadic-E coupled with tropo enhancement on one or both ends of the path. A single-hop sporadic-E opening could cover about 1000 miles. Coupled with a tropo enhancement that covers an additional 200 to 400 miles, you actually could get the effect of a one-and-a-half sporadic-E path!
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