Emil Pocock, W3EP
Issue 66, August 2000
Most six-metre intercontinental DX during the peak of the solar cycle is accomplished via conventional refraction from the F layer. For such contacts to take place, the maximum usable frequency (MUF) for each refractive hop must be at least as high as the operating frequency. This is an infrequent occurrence at 50 MHz, even at the peak of the cycle. It is much rarer still over high-latitude paths, such as Europe to Japan or to the western United States, and certainly much less common than contacts that cross only lower latitudes adjacent to the equator.
Extrapolated paths of reported winter 50 MHz scatter contacts are shown with solid lines. An analogous proposed path is shown with a dotted line. These paths sometimes open around noon local time when the solar flux is high. The responsible scattering region seems to correspond to a high-MUF area that forms in the F-layer just north of the geomagnetic equator and tracks westward with the sun.
Backscatter
There are other ways to make DX contacts. F-layer backscatter is one of those modes. The usual explanation of backscatter involves signals that take one conventional hop and are then scattered in many directions from the turbulent surface of a distant ocean. A small portion of the scattered signal returns toward the transmitter. Two stations 100 to 2,000 km apart, both pointing in the same general direction as the scattering area, may hear each other via these scattered signals, even though the direct path is too short for a direct, single-hop, contact.
Backscatter is commonly observed on 50 MHz when the MUF is just above the operating frequency. Backscatter signals are usually much weaker than signals coming in simultaneously from longer distances via conventional refraction. Backscatter signals often exhibit considerable multipath distortion, sometimes described as a slight echo or hollow sound. Antennas generally have to be pointed south of the direct path, toward the area of highest MUF, for the most effective use of backscatter in the northern hemisphere.
At other times, backscatter signals are present when the MUF within single-hop range is apparently lower than the operating frequency. Six-metre stations in Western Europe, for example, often use such backscatter as an indication that the MUF is close to 50 MHz and the band is about to open towards America on direct paths. In such cases, the strongest backscatter signals come from the west or more commonly south of west. It may be that an actual F-layer path is open to South America, but the lack of stations in the optimal places does not reveal this. It is also possible that such signals are scattered directly from the most dense portion of the F-layer, which forms just north of the geomagnetic equator by noon local time, rather than from the ocean or the ground. See the accompanying figure.
Whatever the precise mechanisms, backscatter is useful for filling in contacts at distances less than a full hop, normally at least 3,000 km at 50 MHz (for F2). Point your antennas somewhat south of expected direct paths to listen for backscatter signals. Once the band is clearly open, continue to look for the weak and oddly distorted backscatter signals also while pointed generally south.
Sidescatter
Six-metre operators have also observed a somewhat different form of scatter that makes even much longer distances possible. Stations separated by distances greater than normal single-hop F2 range at 50 MHz (3,000 to 5,000 km) can sometimes make contact via skewed scatter paths. Signals are generally much weaker than those via backscatter and also usually have a fluttery multipath-like sound. Stations running substantially more than 100W and at least a five-element yagi have the best chance of success via this mode.
A typical skewed sidescatter path is from Europe to the east coast of the US, via a scattering region that appears to be off the west coast of Africa. The first time I worked Scandinavia from Connecticut was via such a sidescatter path. On the morning of November 11th, 1989, I was beaming due east, expecting the first contacts of the day from Africa. Sure enough, I did start off the day with an easy direct-path contact with 6W1/F6CBC in Senegal at 1214 UTC for a new country. The very next station in the log was LA3EQ, followed quickly by SM7BAE, OH2TI (all for new countries as well), and half a dozen other northern Europeans. In contrast to the strong African signal, the fluttery European signals peaked 419 to 539 with a beam heading of 80 degrees, 30 degrees south of the direct heading. The Scandinavians reported they were beamed nearly due south. When we tried the direct path, signals disappeared. During the following three hours, by the way, the band opened directly to Western Europe with very loud signals—but not as far north as Norway, Sweden, or Finland.
The distance from the West African coast to New England is about 6,000 km, coincidentally, about the same distance to Scandinavia. This is somewhat longer than one conventional hop near the MUF and suggests that scattering took place from a common region off the coast of West Africa, perhaps in the ionosphere itself. The F-layer is most densely ionised over West Africa during late morning local time and consequently the MUF reaches its highest levels in that region.
This was not an isolated event. Those in the north-eastern US experienced at least half a dozen similar openings during Cycle 22. The point is that the MUF was well below 50 MHz to northern Europe via the shorter, direct path, but a scatter path via West Africa was available. Signals from Europe were weak and easily overlooked, but they were there. Similar sidescatter contacts have been reported from California to Japan and the Pacific in early afternoon local California time. In those cases, the US stations beamed west or south-west, while the Japanese beamed south and south-east.
The direct path between Europe and Japan traverses the auroral zone and rarely opens on 50 MHz, yet there is another way to make such contacts via an even more remarkable sidescatter path. Hatsuo Yoshida, JA1VOK, has reported several instances (most recently in November 1999) when well-equipped Japanese and European stations made weak scatter contacts when both were beamed toward a common region in the Indian Ocean. The Indian Ocean is about 7,500 km from both Western Europe and central Japan, suggesting that two hops were required for both Europeans and Japanese to reach another high MUF scattering region.
Other Possibilities
These examples of long-distance sidescatter are not isolated events and are probably more common than so far observed. They also suggest that analogous paths in other parts of the world might be possible. The mid-Atlantic Ocean at around 20 degrees north latitude, where the high MUF region forms after 1400 UTC, might also provide the necessary scattering region for contacts between Europe and the western US. The distance from this hypothetical scattering region to both Europe and California is about 7,500 km, or the same as the distances in the Europe to Japan case.
Other sidescatter paths might be found in the Northern Hemisphere. The best chances are for stations beaming south of the direct path around noon local time at path midpoint. Signals will be weak and fluttery, making CW the natural mode choice. Similar paths might exist in the Southern Hemisphere, in which case the beam headings will be northerly. This is a relatively unusual phenomenon at 50 MHz, even though ten-metre operators have reported similar conditions when the MUF is around 28 MHz for years. Please send me reports of any unusual scatter paths observed during this coming cycle, whether at 28 or 50 MHz. Give the usual particulars of date, time, stations and locations, signal strength and quality, and any other related observations that will be useful.
An earlier version of this article appeared in QST for November 1999.
