Amateur Radio Propagation Studies
Science, research, engineering, operating
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Amateur Radio Propagation Studies | | |||||||||
Science, research, engineering, operating | ||||||||||
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Polar lights, i.e. the "Aurora Borealis" in the northern and the "Aurora Australis" in the southern hemisphere may be considered one of the most fascinating phenomena in nature. In ham radio, "Aurora" denotes the backscatter of radio waves which strongly correlates to E region irregularities aligned to the Earth magnetic field. Aurora dx communication is therefore closely related to geomagnetic disturbances. A very similiar backscatter scenario may be found even in mid-latitudes independent from geomagnetic disturbances though, i.e. the FAI scatter mode. The term FAI ("field-aligned irregularities") is actually no good choice because field-aligned irregularities also apply to Auroral backscatter - ham radio terminology is indeed very often inconsistent and partially misleading, unfortunately. Aurora and FAI backscatter are both considered examples of coherent scatter of radio waves and radio amateurs can indeed provide many observation reports, some of them are discussed in this section (further below, we will also catch a glimpse on the incoherent scatter of radio waves which is however inaccessible to amateur radio stations). The | ||
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Unusual Aurora QSOs in 144 MHz The so-called unusual Aurora QSOs do not follow the 'geometrical rules' in field-aligned backscatter resulting in long-distance QSOs which shouldn't exist at all. Radio amateurs know this type of extraordinary Aurora communication for many years, it is a rare phenomenon though and its scatter geometry was a mystery. In 2002, I received Oliver's (DL1EJA) email in which he reports his Aurora QSOs with Peter (SM2CEW) in northern Sweden. Evidently, the dx distance and the corresponding antenna headings (see the upper figure on the right) did not correspond to the 'ordinary' backscatter geometry in Aurora dxing. Using the BeamFinder software, the mystery was finally solved: that QSOs may be explained quite accurately by introducing aspect angles to the Earth-magnetic fieldlines which considerably deviate from perpendicularity. In ionospheric research, similiar results are known for long time, in amateur radio it was reported the very first time. Thanks to Jan Erik, SM2EKM, I received even more data on unusual Aurora QSOs between northern Sweden and central Europe and the BeamFinder software was capable to explain the scatter geometry more or less perfectly in all cases. Eddi, DK3UZ, finally draw my attention to his (and Y22ME's) remarkable Aurora QSO to UA1ZCL in Murmansk (see the figure on the right). In this example of unusual Aurora QSOs, the aspect angle deviates from the ideal scatter geometry by 20 degree which corresponds to the largest ever observed deviation from 'ordinary Auroral backscatter' in amateur radio I am aware of. However, aspect angles violating the 90-degree-rule must not be considered an exclusive feature restricted to the most spectacular dx QSOs in Aurora communication. Even short range Aurora QSOs may result from 'unusual backscatter' as described in the latest paper (May 2004) which discusses the Aurora QSO between IK2YXK and HB0/HA5OJ.
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What is your dx radius in Aurora and FAI communication? Aurora and FAI propagation may be considered the backscattering version of sporadic E in which the direction of the scattering is correlated with the direction of the Earth magnetic fieldline passing through the scatter volume. As a result, radio and visual observation of Aurora follows different rules. You may view visual Aurora Borealis anywhere in the sky but you cannot receive radio signals from Auroral backscatter in all antenna directions. Thus, Aurora and FAI backscatter is restricted to particular directions in azimuth and elevation corresponding to scatter volumes located in particular geographical regions. Those regions may differ considerably depending on your actual geographical (read geomagnetical) position. This leads, by the way, in a curiosity because radio amateurs in the very far north of Europe may enjoy magnificent polar lights in the sky but cannot enjoy Aurora dx QSOs at all (there is one exception though: please refer to the section Unusual Aurora QSOs in 144 MHz available on this webpage). The BeamFinder software may display all positions of Auroral and FAI backscatter corresponding to your home location (see the green areas in the figures). The software may also display the corresponding geographical distribution of dx stations you may access (see the blue dx target areas in the figures). In the following papers, a variety of geographical positions in Europe and North America are analysed in detail.
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Northern lights over Athens on November 20, 2003 [December 2003]. Joachim, DL8HCZ, editor in chief of the Dubus magazine, send me this photograph showing an impressive display of Northern lights taken in Athens on November 20, 2003. Does this photograph indicate Auroral backscatter of radiowaves over Greece? No, it doesn't, i.e. this Northern light does not generate any dx QSO and it isn't even located above Greece. | |||||||||||||
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Auroral backscatter originates in a height from 100 to 110 kilometers. In this height, Northern lights vary between white and green color. The photograph, on the other hand, shows an intense red Aurora originating from atmospheric oxygen (630.0nm and 636.4nm) in a height between 300 and 700 kilometers. We may estimate that this Northern light corresponds to an elevation lower than 10 degree above Athen's horizon, which is in particular true for its lower parts. Assuming further, the height is 300 kilometers we may estimate the Northern light's geographical position, see the map showing a surprising result: this Northern light is located somewhere above Poland, Belarus and the Ukraine, i.e. far away from Greece. | |||||||||||||
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Non-reciprocal dx conditions in Auroral backscatter? In Aurora dx communication, the propagation path separates into two legs, i.e. the uplink from the transmitter to the scatterer and the downlink from the scatterer to the receiver. The length of the uplink and downlink may differ considerably in practice. The red lines in the figure show an example where the propagation path separates into a short and a long leg, the total length is however identical to the green lines When using the red path, does the station in the north (A) benefit from reduced path loss while the station in the south (B) finds Aurora dx communication more difficult to achieve? Many radio operators raise this question by speculating about non-reciprocal Auroral scattering. Analysing the situation mathematically, surprising results are found: Aurora QSOs along the red lines benefit from higher fieldstrengths compared to QSOs along the green lines. Thus, it is advantageous to have unequal up- and downlink path lengths in Aurora dx communication. However, it is also shown that there is no reason to assume non-reciprocal dx conditions. Meanwhile I became aware that the same type of analysis was already communicated by Geoff, G3NAQ, many years ago.
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Doppler effect in Auroral backscatter The Doppler effect in Auroral backscatter was recently discussed in N1BUG's Aurora email forum. I realized that the mathematical framework described in the short paper from 1989 cannot interpret all features of Doppler-shifted Aurora signals. So, I went back to the roots resulting in a paper (Febuary 2003) which provides a more elaborated discussion of the Aurora Doppler effect and its implications on VHF dxing. Note that radio amateurs focus on questions different from those discussed by scientists in ionospheric research: scientists operate radar systems on well-defined frequencies and in fixed position allowing precise measurements of the actual Doppler shift. Radio amateurs, on the other hand, cannot measure the Doppler shift because we typically do not know the dx station's actual transmitting frequency and we need to consider many dx stations in a wide geographical area, i.e. the backscatter geometry is highly dynamic and may change from one QSO to another considerably. Which dx station shows positive and which shows negative Doppler shift? There is no general answer on this question because the scenario is quite complicated. Even more surprising: in an Aurora QSO, we have two radio stations operating on three different frequencies. This may cause severe practical complications in Aurora QSOs on very high frequencies, for example in 432 MHz and 1296 MHz.
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Plotting the number of QSOs versus dx distance is a simple method of studying the characteristics of dx openings. Analysing Aurora QSO data, I often found unusual dx maxima around 1.500 km. Please refer to sporadic E section to view the details. | ||||||||
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Analysing dx openings and unusual radio propagation phenomena is one issue. What about predicting dx opportunties which still await its discovery? Here we go: We are familiar with Auroral backscatter which correlates to geomagnetic storms in polar latitudes. A very similiar ionospheric backscatter mode exists in mid-latitudes independent from geomagnetic disturbances. In the amateur radio community, this scatter mode is known at least since Tom's (K4GFG) famous article in QST, 1982. Since then, we refer to FAI synonymous to 'field-aligned irregularities'. Analysing this scatter mode more carefully, we may indeed find attractive dx opportunities in 144 MHz long-distance communication. The pictures on the right indicates a potential dx path between the UK and northern Spain and between Austria and the Balearic Islands, respectively: Find your sked buddy, target a common volume within that green line of potential scatterers in the E-region of the ionosphere and go for a test. You probably need to implement a long test run because FAI is a rare phenomenon. However, it is worth a try.
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There is no such thing like 'hot spots' of FAI backscatter In amateur radio literature (in particular from the 1980s and 1990s), we often read about this scatterer above Budapest, the scatterer above Geneva and other places which appear to have some importance in FAI dx communcation. There was even an attempt at discovering new scatterers, i.e. particular regions in the E-layer of the ionosphere from which FAI backscatter originates more often compared to other regions. This was indeed a speciality in the European sector because there was no equivalent discussion, for example, in the U.S. ham community. That approach was misleading because there is no such thing like "hot spots" of FAI backscatter, in fact. The VHF/UHF DX Book (editor: Ian White, G3SEK, 1995) apparently shares my scepticism about "FAI hop spots". The below papers, published long time ago in 1988, refer to the AURORA software which was available on Atari ST computers. AURORA was a realtime program dealing with real observation data, providing highly accurate results and was something new in ham radio in those days. The same analysis routines are now available in the BeamFinder software.
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Übersichtsartikel über Aurora und FAI Es läßt sich möglicherweise gar nicht mehr zweifelsfrei feststellen, wann Rückstreuungen von ultrakurzen Radiowellen an Polarlichtern erstmalig Erwähnung in der Amateurfunkliteratur fanden. Im Falle der FAI-Ausbreitung ("field-aligned irregularities - an den Erdmagnetfeldlinien ausgerichtete Irregularitäten") gilt der Aufsatz von Tom Kneisel, K4GFG, QST, 1982, als wegweisend - im Vergleich zu den ersten wissenschaftlichen Quellen (z.B. Heritage et al., 1959) sind wir Funkamateure jedoch erst spät auf diesen Rückstreuprozeß aufmerksam geworden. Während Radio-Polarlichter eindeutig mit geomagnetischen Störungen in Verbindung stehen, kann für die in mittleren Breiten beobachteten FAI-Rückstreuungen eine solche Abhängigkeit nicht festgestellt werden. Dennoch können beide Funkausbreitungsmoden durch eine identische Streugeometrie beschrieben werden, was die geophysikalischen Hintergründe zwar nicht deutet, der Amateurfunk-Praxis jedoch vielfältige Auswertungs- und Prognosemöglichkeiten ermöglicht.
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Principles and applications in Incoherent Scatter It is our expectation that UHF radiowaves escape into space very easily because the ionosphere does not play any role at this high frequencies contrary to the shortwave and the low VHF range. Not quite right. A small fraction of radio energy returns to Earth, actually a very small fraction. Geophysicists are highly interested in studying the spectral shape and the Doppler shift of those radio echoes because we may learn a wealth of information about the ionosphere and its structure by analysing incoherently scattered radiowaves. Incoherent scatter radars may be considered the most powerful radar facilities on Earth, at least in civil applications. Therefore, only very few incoherent scatter radars exist in the world - the European Incoherent Scatter Association (EISCAT) operates the most modern and most flexible systems in Norway, Sweden, Finland and on Svalbard. What is incoherent scatter and how does it work? Read the below articles which discuss the basics of this fascinating remote-sensing technique.
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D and E region ionization measured by the EISCAT Svalbard radar | ||
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 | [December 2003]. The European Incoherent Scatter Association (EISCAT) operates one of its powerful radar systems near Longyearbyen on Svalbard (78 degree northern latitude). By measuring radar echoes from the ionosphere, scientists study the polar cap region to understand the complex system of solar events and its impacts on the Earth's upper atmosphere (radio amateurs interested in ionospheric research are advised to visit the EISCAT webpages). During the October/November 2003 geomagnetic storm, EISCAT measured high electron densities even in low altitudes (see, e.g., the red spots at 65 kilometers on October 30). | |||||||||||
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Acknowledgements: You may have noticed the "not for publication" label on the figure - it doesn't apply here. I am grateful to J. Röttger and M. Rietveld for authorization to use that material on this web site - thanks Jürgen and thanks Michael for brilliant support. | ||||||||||||
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