Hidden Mode of Sporadic-E? Click here to go back to the home page

More Magic with the Magic Band

Dr. Volker Grassmann,  DF5AI
Theodor-Fliedner-Str. 19, 65510 Idstein, Germany

1. Introduction

In vhf dxing sporadic-E is one of the most popular propagation modes because it allows long-distance qsos, typically  between 1400-1600 km. Although other distances are also possible, very long-distance qsos, e.g. >2000 km, and short-range qsos, e.g. <700 km, are rare.

In this paper a statistical analysis of sporadic-E qsos in the 50 MHz frequency band is presented. The data base comprises 979 records of approx. 550 amateur radio stations worked or heard by the author between 1983 and 1999 from JO52CJ, JO31PG or JO40DF (figure 1).

Figure 1. Geographical distribution of stations worked or heard via 6m sporadic-E by the amateur radio station DF5AI between 1983 and 1999 from JO52CJ, JO31PG or JO40DF. The circular gap of about 500 km in radius results from the so-called dead zone which is a typical feature of ionospheric skip propagation.

2. Results

Figure 2 shows the number of stations worked or heard as a function of the great circle distance between the station and DF5AI's actual location. No stations at distances shorter than 500 km and very few stations at distances shorter than 700 km were heard or worked. The number of stations beyond 700 km first grows rapidly, reaching a maximum of 100 stations in the  900-1000 km range. The number of stations then decreases again to around 60 in the 1100-1200 km and 1200-1300 km ranges. Surprisingly, the number of stations increases then again, peaking in the 1500-1600 km range (125 stations), before it  drops sharply towards a second minimum in the 1700-1800 km range (30 stations). After peaking again in the 1800-1900 km range (50 stations), the distribution function fades out at distances over 2400 km, although a few stations also appear in the 2600-2900 km and 3000-3300 km ranges.
Figure 2. Range distribution of 979 stations worked or heard via 6m sporadic-E. The width of each of the range gates is 100 km e.g. range gate "800 km" refers to stations in the range 800-900 km from the amateur radio station DF5AI.

This distribution is somewhat unexpected: instead of having a single maximum around 1400-1600 km, a number of peaks are obvious,  i.e. at 900 km, 1500 km and 1800 km. Thus, it appears that skips of a particular distance are more likely than skips of any other distance, something which is not predicted by the theory of sporadic-E.

3. Discussion

Speculating that figure 2 would show the sum of two modes, a short-skip as well as a long-skip mode (Es(SS) and Es(LS), there is every reason to expect a double-peaked distribution function instead of a single-peak one. However, close inspection of figure 2 does not support the two-peak theory because a third peak is also evident.

Nevertheless, all three maxima may still be interpreted in terms of two propagation modes if multiple-hop propagation is taken into account: figure 3 interprets the sporadic-E analysis in terms of multiple-mode/multiple-hop propagation. The short-skip mode, Es(SS), generates the first peak at a skip distance of approx. 950 km. The same mode also appears as a 2nd hop at approx. 1800-1900 km. Finally the tripple-hop variant causes a third peak at approx. 2700-2800 km. Because  higher order hops occur less often than lower order ones, the third peak is smaller than the second and the second peak is smaller than the first. The long skip mode, Es(LS), accounts for the maximum which appears at approx. 1500-1600 km. Its double-hop variant also generates a small peak at 3000-3200 km, just above the three-hop type of Es(SS).

Figure 3. Interpretation of the data shown in figure 2 in terms of multiple-mode/multiple-hop propagation. The short-skip mode, Es(SS), generates peaks at multiples of approx. 930 km and the long-skip mode, Es(LS), provides peaks at multiples of approx. 1500 km.

In a further analysis, the short-skip and the long-skip modes were isolated from the database by selecting stations in the 800-1100km and 1300-1500km ranges, respectively. The two data subsets were analysed independently, as described in table 1.

Analysis Results
Season Both the short-skip and the long-skip modes typically appear in the months of May to September/October. Both commence very abruptly in May, although occasional Es(LS) was found earlier. No Es(SS) or Es(LS) were observed in November and December. 
Number of events The number of events differs significantly when comparing the Es(SS) and Es(LS) mode. The frequency of Es(SS) is a smooth function of time i.e. the season starts in May, peaks slightly in June and fades out gently after that. This is not the case for Es(LS): the long-skip mode also commences abruptly in May but the number doubles in June. In this month Es(LS) occurs twice as frequently as Es(SS). In July and August the numbers of Es(LS) are almost identical, but are lower than in May. Very few events were found in September and October - this is also so for the short-skip mode.
Diurnal occurrence Both modes, Es(SS) and Es(LS), have very similiar daily profiles. In particular, a daily peak exists in the late morning (9-11UT) and also in the early evening (17-19UT). However, there is very strong evidence that Es(LS) occurs one to two hours before the Es(SS) mode.
Azimuth Very often Es(SS) and Es(LS) propagation coexist with identical antenna headings. However, sometimes only one of the modes appears in a particular antenna direction.
Geography No distinct geographical distribution of Es(SS) and Es(LS) was found.
Table 1. Summary of the statistical analyses of Es(SS) and Es(LS).

The long-skip mode appears to be in agreement with the dxers´ sporadic-E expectations. However, the short-skip mode, Es(SS), is disturbing because the short distance (e.g. 700-900km) requires a relatively high penetration angle into the E-layer (elevation approx. 10-12 degrees) and this would be associated with a surprisingly high electron density. This is true when Es(SS) originates at the same height as Es(LS), i.e. in approx. 100-110 km. However one might speculate that short-skip propagation corresponds to a lower scattering height of around 80-90km. In fact, strong vhf radio echos from the mesosphere have been reported but forward scattering in the mesosphere at low antenna elevation would, however, be a new phenomenon.

Statistical data was insufficient for an evaluation of an equivalent range gate analysis for 144 MHz sporadic-E. However, preliminary results indicate a different behaviour for 144 MHz. In particular there is evidence that the major peak appears at longer distances and that the other peaks are less pronounced than in  50 MHz sporadic-E.

Possible sources of error

In order to rule out  contamination of the statistical data, possible sources of error were analysed (table 2).

Possible error Summary
Too few statistical data In the 1999 sporadic-E season, the number of records grew from approx. 650 to almost 1000 and, consequently, the distribution function stabilized. Even when dealing with much less data, results similiar to those in figure 2 were obtained. In the case of the 1998 sporadic-E season, for example, a multi-peak distribution function with peaks at similar distances was also found although only 165 stations were monitored (figure 4).  Hence there is strong evidence that figure 2 is not distorted by too few data.

Figure 4. Range gate analysis of 50 MHz sporadic-E in 1998.

Topographical features Local topographical features, such as hilly terrain, may cut off low antenna elevation and may, therefore, reduce the level of long distance propagation. However this is not the case here as 1) similiar distributions were found at two locations, JO31PG and JO40DF, and 2) the correlation of skip-distance and local antenna azimuth did not indicate any cutoff effects or preferred antenna directions (figure 5). Hence it is very unlikely that the distribution shown in figure 2 is a consequence of local terrain effects.

Figure 5. Sporadic-E skip distance versus local azimuth.

Continental shape The continental shape of Europe might also affect the range gate analysis. If, for example, sea water terrain would dominate a particular range gate, a corresponding minimum would occur in the distribution. In order to analyse this possible effect, each of the Maidenhead grid locators in Europe was characterized according to its dominant terrain type, i.e. land, sea or coastal terrain. In a second step, a range gate analysis was calculated which included all European grid squares. The resulting distribution indicated the land and sea areas as a function of distance from the oberserver's location (figure 6). No correlation between continental shape and number of amateur radio stations was found, so that  figure 2 may be assumed free from this possible error.

Figure 6. Sea, land and coastal terrain areas as a function of distance in reference to JO40DF.

Geographical density of radio amateurs The range gate analysis could be affected by an inhomogenous distribution of radio amateur stations. One could speculate that the first peak in figure 2 is a consequence of the large number of British 6m-operators. Exclusion of all stations located in this IO-square i.e. Great Britain and Ireland, resulted in a decrease in the magnitude of the 900 km peak, but did not remove the peak altogether (figure 7). The geographical density of 6m-operators may, nevertheless, influence the results shown in figure 2; only further observations from different locations can clarify this point. 

Figure 7. Range gate analysis similiar to figure 2 excluding stations located in the IO square.

Vertical antenna pattern Unwanted or unexpected side lobes in the H-plane antenna pattern may filter out particular elevation angles and so bias the communication range of a radio station. In order to investigate this possibility, the antenna elevation of each of the 979 records was calculated: for example, 10 degree elevation corresponds to qsos in the 920-930 km  range (height of scatter volume: 105 km). It was found that each of the stations was definitely captured by the antenna's main lobe (figure 8). Finally, different antennas were used by DF5AI (yagi as well as wire antenna) and all arrays provided results similiar to those of figure 2. Hence it is believed that the vertical antenna pattern only plays a minor role in figure 2.

Figure 8. Calculated elevation profile (979 records, scatter height 105 km).

Table 2. Discussion of possible effects which might cause data contamination. 

4. Conclusions

The statistical analysis of the sporadic-E skip distances revealed a number of unusal features for which no explanation have yet been found. There is some evidence for the existence of another propagation mode in parallel to the usual sporadic-E radio propagation. At least  the phenomena  discussed here are not conform with the common understanding of sporadic-E.

This work should encourage other radio amateurs to analyse their personal qso databases in a similiar fashion with the aim of investigating a possible unknown propagation mode:
  Do other radio amateurs confirm the existence of a double- or tripple-peak distribution function similiar to figure 2? If yes, what is the distance at which the maxima appear? What results are obtained when analysing 144MHz sporadic-E?

(C) Copyright, Volker Grassmann, 1999.

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