Amateur Radio Propagation Studies
Science, research, engineering, operating
"Sporadic E" may be considered the most important ionospheric propagation mode in VHF dxing supporting long distance communication even with low power radio stations. The term "sporadic E" is however a geophysical term denoting localized enhancements of the electron density in the E region of the ionosphere with a duration between seconds and several ten minutes. Those electron density enhancements may be observed quite often in the E region of ionosphere which is however not true for sporadic E forward scatter of VHF radio waves. In fact, the electron density is only one of the many controlling parameters in sporadic E dx openings. The geophysical phenomenon has been exhaustively studied experimentally and theoretically by scientists, open questions however remain and some of them are addressed in this section based on observations made by radio amateurs. The -icon indicates articles referring to the BeamFinder analysis software.
An international team of radio amateurs has studied potential thunderstorm effects on the generation of sporadic E by analysing scientific results and QSO data compiled during the 2004 sporadic E season. In a detailed paper, Sabine (DL1DBC), Allard (PE1NWL), Udo (DK5YA), Gabriel (EA6VQ), Joachim (DL8HCZ) and Volker (DF5AI) discuss a possible model of thunderstorm-triggered sporadic E propagation on very high frequencies. In the 2004 data, a remarkable geographical correlation was found between 144 MHz sporadic E QSOs and sferic events, i.e. short term radio pulses emitted by lightning strikes.
Part 1: Discussion of the May 20, 2003 dx opening
On May 20, 2003 radio amateurs from the Canary Islands reported many 144 MHz QSOs to central Europe and England corresponding to radio path lengths of 3.000 to more than 3.500 kilometers. At the same time, 144 MHz sporadic E QSOs as well as tropospheric dx propagation was reported. Does this dx event represent double hop sporadic E or did tropospheric dx propagation extend the radius of single hop sporadic E? Udo (DK5YA) and Volker (DF5AI) discuss a variety of possible propagation modes by analysing dx reports from radio amateurs, vertical profiles of meteorological data, ionograms and by using the BeamFinder software. Surprising results are found in this Sherlock Holmes type of investigation including new ideas in double hop sporadic E radio propagation on very high frequencies. Surprisingly, this paper has been discussed even by scientific experts, i.e. by commission G (Ionospheric physics and radio wave propagation) of U.R.S.I. (Union Radio-Scientifique Internationale) at the annual national conference in Miltenberg, Germany, October 2003.
An attempt was made at identifying the possibility of atmospheric gravity waves in the May 20, 2003 dx event. I am grateful to the Instituto Nacional de Técnica Aeroespacial, El Arenosillo (Spain), the Observatorio del Ebro, Roquetes (Spain), and to the Institut Royal Météorologique, Bruxelles (Belgium) for providing detailed Digisonde material for analysis purposes. Having analysed various ionospheric parameters and its variation, there is indeed evidence for wavelike oscillations in, for example, the critical frequency foEs and the total electron content over Spain on this particular day. However, I am reluctant to interpret those wavelike variations by gravity waves. The assumption of atmospheric tides appears more likely but final results are not yet available.
Part 2: Dx opportunities in the European sector
The analysis of the May 20, 2003 dx opening between the Canary Islands and central Europe motivates the assumption of double hop sporadic E propagation supported by surface reflection of radiowaves by inland lakes and major rivers. This assumption results from the geographical position of the corresponding path centers which appear to correlate to the location of inland water areas. Assuming this assumption is justified, the results may be extrapolated to other regions in Europe, e.g. the Lake Balaton in Hungary, the Lake Constance between Germany, Switzerland and Austria or, just another example, to the river Dnieper in the Ukraine which also represents large water expanses. Doing so, surprising results are found, i.e. scenarios which appear to correspond to 'historical' examples of very long distance QSOs in 144 MHz.
Results of the 2004 sporadic E season
[August 2004]. On July 11 this year, EA8BPX and EB8BTV both managed another series of long distance QSOs into Northern Italy, the sout-east of France and into Switzerland too. By refering to Fig. 6.24 in the initial paper (see page 30 in the above document by Grassmann and Langenohl), I realized that this QSOs indeed point in the same direction compared to the findings of last year. The figure displays the corresponding path centers of July 11. Note the circle labed "1" corresponds to seasurface reflection close to the Spanish sea coast, the circles labeled "2" denote path centers corresponding to the river Guadalquivir (see the four circles in the south), to a sidearm of the river Guadiana (see the two circles further north) and the river Guadiana itself (see the circle close to the center), respectively. Well, the two circles labeled "3" correspond to the highest position of the Sierra Morena. On August 06, IW2NOR worked EB8AYA and EA8BPX, the corresponding QSO midpoints are indicated by the green circles ("4") in the above image: another example of path centers located along the river Guadalquivir. Do you think all this is an accidental result?
Results of the 2005 sporadic E season
[September 2005]. Since 2003. we keep an eye on 144 MHz long distance QSOs between the Canary Islands and central Europe (path lengths around 3.500 km). Interpreting those QSOs by double hop sporadic E propagation, some sort of ground reflection must exist along the radiowaves' zigzag path between ground and the E region of the ionosphere. Calculating the geographical center of the QSOs, the reflection points are found on the Iberian peninsula. Referring to the 3/2005 issue of the Dubus magazine (see page 81), this type of QSOs are also reported by EB8BTV in 2005. The red circles display the geographical position of the QSO midpoints which appear to correlate to the position of large rivers, lakes and dams similar to the results we have already found in 2003 and 2004. In the analysis of the May 20, 2005 event, we have therefore speculated about seasurface reflection of the 2 meter radiowaves (we have used the term radioglint in inland water expanses). However, assuming radiowaves reflected at ground level, double hop sporadic E QSOs should be accompanied by single hop sporadic E QSOs originating in the central area of the double hop path (i.e. VHF radio stations located within the red circles from above can work into central Europe and also towards the Canary Islands at the same time). In practice, however, we generally find no such geographical correlation between double and single hop sporadic E QSOs which could have something to do with the geographical distribution of the VHF dx stations, of course. However, we have also presented an alternative interpretation, i.e. radiowaves not reflected at ground level but a few hundred meters above ground at the topside of local inversion layers associated with large rivers, lakes and dams. Anyhow, the results from the 2005 sporadic E season appear fully compatible to the previous results. What is the next step in the analyses of very long distance propagation in 144 MHz? I think there is a need to study the meteorological situations in more detail, e.g. (small-scale) inversion layers on the Iberian peninsula above inland water expanses in the months of May and June - with other words: can we identify any systematical features which could be used for prediction purposes (by using, for example, the vertical soundings distributed by the University of Wyoming)? Are there any meteorologists in the ham community willing to volunteer?
Part 3: Use of satellite and aerial images to analyse double hop sporadic E radio links
In 2006, the number of 144 MHz double hop sporadic E QSOs has increased considerably resulting in 128 dx QSO exceeding the distance of 3.000 kilometers (two QSO even represent distances beyond 4.000 kilometers). The corresponding path centers have been analysed with high geographical resolution by referring to the Google Earth internet service. 25 percent of all QSOs represent seasurface reflection of radiowaves in the ocean, all the remaining QSOs show path centers in close vicinity of major rivers (width of a few hundreds meters) and big lakes (size of 1x1 kilometers at least). The below paper therefore closes the case that was opened with the analysis of 144 MHz very long distance QSOs in 2003 (see above): the hypothsis of rivers and lakes enabling double hop sporadic E QSOs is strongly supported by the analysis, it is actually safe to say the hypothesis is verfied to a high degree of certainty.
From November 2001 to January 2002, many radio amateurs enjoyed a series of openings between Europe and North America in the 50 MHz band. On November 17, 2001, 1520-1630 UTC, dx stations lined up in multiples of approximately 2000 kilometers within a very small window of azimuth. This situation appears to represent a brilliant example of multiple hop propagation in 50 MHz but the number of skips along the zigzag path between the ionosphere and ground isn't easy to analyse.
Plotting the number of QSOs versus dx distance is a simple method of studying the characteristics of dx openings. Analysing sporadic E and Aurora QSO data, I often found unusual dx maxima around 1.500 km. For example: the upper panel on the right displays approx. 1.000 sporadic E stations spotted in the 50 MHz band from three independent locations in recent years. Unusual dx maxima are also found in Aurora dx communication, see, for example, the analysis of the geomagnetic storm from March 31, 2001 (see the lower diagram and the arrows therein). Joe, NA3T, found something similiar when analysing the Aurora QSO data from July 15, 2000. British radio amateurs analysing dx distances in Auroral backscatter also found a dx maximum between 1.400 and 1.500 km (info by Juho, OH8HFY). What's the meaning of this maxima? We don't know yet. In my very first internet publication (1999), I have speculated about an unknown mode of sporadic E which very soon became a matter of dispute, of course. The reader is therefore advised to visit "Six News Magazine" section on the UKSMG webpage which provides a critique on the below article.
VHF sporadic E propagation is associated with plasma instabilities which may cause strong scatter of radiowaves. This geophysical phenomena are not considered by the theory of electromagnetic wave propagation in an ionized media (i.e. in the basic versions we generally find in textbooks). However, we actually refer to this theory when analysing VHF sporadic E by extrapolating 'shortwave models' to very high frequencies. Very dense sporadic E layers may exceed critical frequencies of 16 MHz but this models result in critical frequencies around 25 MHz (in 144 MHz sporadic E) or even 35 to 39 MHz (in 222 MHz sporadic E) which evidently no longer reflects the true ionosphere. From this perspective, almost all MUF calculation programs used by VHF radio amateurs must be considered incorrect. However, we currently have no alternative model available which appears applicable in VHF dxing. Thus, it is very important to keep in mind that our current approach is nothing else than an imperfect approximation. In 1988, I have contributed to this approach and even the current version of the BeamFinder software provides its 'sporadic E analyser' module by refering to this paper. I am close to ban this module from future BeamFinder releases because this method is no longer acceptable in my view.