Amateur Radio Propagation Studies
Science, research, engineering, operating
The mystery of long delayed radio echoes (LDE) baffles radio amateurs and scientific experts for many decades. You may find qualitative interpretations in the literature but no quantitative model is yet available capable to explain the phenomenon with scientific conviction. The so-called 'multiple-tone effect' misguides VHF radio amateurs by giving the illusion of an Aurora band opening ... but there is none. Contrary to LDEs, the multi-tone finds a simple explanation because it is caused by radiowaves reflected by airplanes crossing the antenna beam which results in heavily distorted radio signals similar to the audio pitch which is well known in Aurora backscatter QSOs.
Long-delayed radio echoes (LDE) are defined as radio echoes received 1 to 40 seconds after transmission. LDE are mainly reported on HF but radio amateurs also report LDE on EME circuits in 432 as well as 1296 MHz. I was a teenaged SWL (many years ago, actually) when I heard about this mysterious radio echoes the very first time. In that book, Duncan Lunan, professor of the Scottish Association for Technology and Research was referenced with his idea of an alien radio repeater, i.e. he interpreted LDE by an extraterrestrial space probe travelling in our solar system for more than 30.000 years. I wasn't very much impressed about this idea but my interest was nevertheless attracted by this strange radio propagation phenomenon.
I therefore decided to contact Prof. Lunan but my letter returned several months later. The Scottish mail obviously tried very hard to identify this professor, without any success though and I was convinced the whole story is nothing else than a fake. One or two years later I was surprised by finding another article also featuring this long-delayed radio echoes - and this was a serious article, apparently, which finally referenced the JGR, i.e. the Journal of Geophysical Research. At that time I had no idea what the JGR is (it is in fact one of leading scientific journals in geophysical research) - and even the bookstore in my little hometown couldn't figure out where and how to get that document. Thus, I was facing another dead end and I finally gave up further investigation.
The situation changed at university because you cannot deal with ionospheric research without knowing this excellent scientific journal. And I indeed discovered that article in the library and I was speechless: Long-delayed radio echoes are discussed by serious scientists - in terms of a real geophysical phenomenon rather than in terms of alien space probes. In recent years, I compiled a hugh amount of scientific literature on this phenomenon including this review article from Muldrew which was published in JGR in 1979. He discussed all aspects of long-delayed radio echoes even far-fetched ideas and, believe it or not, he mentioned Duncan Lunan in that scientific paper. I was pretty much baffled to learn that Mr. Lunan is a real character being referenced in a scientific journal. However, in that article Mr. Lunan wasn't introduced as a professor in research and technology but was introduced as ... a science fiction author. From my perspective, long-delayed radio echoes is a curiosity - in all aspects.
Brummelnde und verzerrte Signale im 2m Band! Hoppla, offenbar befinden wir uns gerade in einer Aurora Bandöffnung - schnell die Antenne Richtung Norden drehen! Verblüfft stellen wir fest: keine Spur von Aurora. Ähnliches ist wahrscheinlich schon jeden UKW-Funkamateur widerfahren, d.h. der Empfang von teilweise erheblich verzerrten Funksignalen unter einer Antennenrichtung, welche deutlich von der geographischen Großkreisrichtung zum Sender abweicht. Bei einer detaillierten Untersuchung dieser seltsamen Signale anhand der 144 MHz Bake DL0PR konnte der Grund für den ungewöhnlichen Klangeindruck schnell ausfindig gemacht werden: zusätzliche Spektrallinien in wenigen Hertz Abstand von der eigentlichen Senderfrequenz bewirken die akustische Signalverzerrung. Jedoch blieb die Ursache für diese zusätzlichen Frequenzkomponenten unklar. Prof. Walter Dieminger, DL6DS, teilte mir bereits in den achtziger Jahren mit, dass dieser "Mehrton-Effekt" vermutlich durch Reflexionen an Flugzeugen gedeutet werden kann, welche dem Sendersignal zusätzliche Dopplerkomponenten aufprägen. Es scheint, Walter lag mit dieser Deutung sehr richtig, wie spätere Beobachtungen von DJ2LF eindrucksvoll belegen, siehe "2-m-Reflexionen an Flugzeugen", Staubach W., DJ2LF, cq-DL 11/2001, S. 812-813 (siehe auch G3CWIs Webseite Doppler radar und die Messungen des Autors im rechten Bild). Inzwischen bin ich etwas skeptisch geworden, denn es sind nicht die Dopplerspuren, welche die Signalverzerrung hervorrufen, sondern die Verbreiterung der Mittenfrequenz (siehe Abbildung).