Multiple-skip propagation in the 6 meter band
Dr. Volker Grassmann, DF5AI
Febuary 06, 2002
From November 2001 to January 2002, many radio amateurs enjoyed a series of openings between Europe and North America in the 50 MHz band. The dx situation of November 17, 2001, 1520-1630 UTC, is distinguished because radio observations were restricted to a small window in azimuth. In fact, the 18 dx stations which were received at the author's location near Frankurt, JO40DF, were all found between 291 and 297 degrees azimuth. On the other hand, the corresponding distances ranged from 4200 km to 8200 km (see the azimuth/distance distribution in figure 1).
Furthermore, the stations seemed to line up in multiples of approximately 2000 km, i.e. at a distance of 4000 km, 6000 km and 8000 km. Taking the fact that the azimuth is almost identical in all cases into consideration, we may speculate that the radio waves travelled along the same path, more or less, across the Atlantic Ocean.
These findings motivated the assumption of a multiple-skip propagation between Europe and North America. To verify this hypothesis the BeamFinder analysis software was used to identify the number of skips along the zigzag ray path between the surface of the Earth and the ionosphere.
2-, 3- and 4-skip radio propagation
The path Newfoundland - New York - New Orleans could be interpreted as two consecutive skips of a length of approximately 2000 km each (see figure 1). Unfortunately, no information is available concerning the propagation path from Germany to Newfoundland. However, we speculated that the first skip terminates in the Atlantic Ocean where dx stations are not available for observation. Thus, it is the second skip which hit Newfoundland and, therefore, it is skips #3 and #4 which enabled long-distance communication to New York and New Orleans, respectively.
The above scenario may be analysed using the BeamFinder script displayed in the box. Please note the script commands "ShowSkips s2", "ShowSkips s3" and "ShowSkips s4" which initialize the BeamFinder software to calculate and display the geographical footprints corresponding to double, triple and 4-hop radio propagation, respectively. Figure 3 shows the function of the "ShowSkips" command in more detail.
Figure 2 shows the results of the BeamFinder analysis. The first skip hits the Atlantic Ocean in the eastern part of the HO grid-square (see the circular markers labeled "1"). The second skip terminates in Newfoundland (see the markers labeled "2" and also the blue grid locators in the GN square). The touch-down areas of the third and fourth skips are found in the FN grid-square and in southern USA, respectively.
3-, 4- and 5-skip radio propagation
It is important to note that the BeamFinder software cannot identify the type of multiple-skip propagation, i.e. the findings in figure 2 reflect the initial assumption of a 2-, 3- and 4-skip scenario.
Figure 4 displays an alternative solution in which the path from Germany to Newfoundland is no longer interpreted in terms of a double-hop, but a triple-hop propagation. Comparing figures 2 and 4 there are two scenarios, surprisingly, and each is able to interpret the dx situation of November 17, 2001.
The BeamFinder user may test alternative s-parameters in the "ShowSkips" commands e.g. by using "ShowSkips s4", "... s5" and "... s6" in the above BeamFinder script. Using this combination, or any other combination, the BeamFinder user will hardly obtain a pattern similiar to that shown in figures 2 and 4. A picture with footprints scattered randomly along the propagation path from Europe to North America is more probable. Therefore, there is no reason to believe that any skip combination of the type n, n+1, n+2 will create results similiar to those in above figures.
However, the existence of two alternative interprations is surprising and requires further analysis. The average skip distances in figure 2 and 4 are 2010 km and 1520 km. This reflects a difference of about 500 km. In the New York area, no discrepancy exists between figure 2 and 4 because 3 x 2010 km = 6030 km is almost identical to 4 x 1520 km = 6080 km, i.e. we cannot judge which multiple-skip scenario reflects the dx situation of Novemver 17, 2001. In the New Orleans and Newfoundland areas the difference between the average skip distance is 440 km and 540 km, respectively. This is comparable to the geographical spread associated to the dx stations (note, for example, the dx-station in Nova Scotia assigns to none of the skip areas accurately, see the blue grid-locator in the eastern part of the FN square). Geographical spreading, however, affects the accuracy of the average skip distance because it is calculated from observations. In fact, we cannot justify discussing precise quantities such as 2010 km and 1520 km because the analysis lacks the necessary geographical resolution. We must conclude that the geographical resolution is indeed not better than 500 km, otherwise we could separate the scenarios of figure 2 and 4. It is believed that the analysis would benefit from observations made in the Atlantic Ocean, but this sort of data is not available unfortunately.
There is evidence that the dx situation of November 17, 2001, may be interpreted in terms of a multiple-skip propagation across the Atlantic Ocean. Similiar dx conditions are well known to many 6m operators. However, it is a remarkable fact that the 6m band may support long-distance propagation modes equivalent to shortwave propagation because 50 MHz is considered a very high frequency in ionospheric plasma physics.
The BeamFinder software proved to be a valuable tool in radio propagation analyses. The creation of displays similiar to figure 2 and 4 is actually a matter of a few seconds using the BeamFinder Command Language. Simply by changing three parameters in the corresponding script file, we identifed alternative multiple-skip scenarios even in a complex geographical situation.
Finally a word of caution regarding many amateur radio publications declaring actual dx situations, e.g. as double-hop, triple-E, F2, Auroral-E or any other type of radio propagation mode. One should hesitate to wholly accept these declarations because even in cases we may consider obvious (such as the November 17, 2001), we may fail to identify the number of skips and (neglected in this paper) the ionospheric layer which enabled the opening. Note: radio waves don't tell us the way they have travelled.