LF: Fast QSB and SXV ripples

[MarkusVester@aol.com]

Dear Mike, Hartmut, Roeloff and LF Group,

congratulations again to Hartmut and Joe on the aural CW copy, this is excellent work.

In my opinion, the rapid amplitude fluctuations seen by Hartmut are not really QSB, but rather due of the marginal SNR, as the superposition of the randomly varying noise vector will often result in apparent signal cancellation or amplification. On strong signals like CFH, the periods of multipath QSB I have seen were always on the order of several minutes.

However just today I stumbled across a different effect which I cannot explain. Browsing through  last night's colour-DF grabber captures, I observed a very unusual ripple in the apparent direction of arrival of SXV between 22:55 and 01:10 UT. There is a complete image of the event at http://members.aol.com/df6nm2/sxv_ripples.gif (288 kB).

Slow colour changes (like to the one near 01:35) happen quite often and are usually related to deep multipath fades. However the fine yellow-red ripple structure first visible at 22:53 has a period of only about 5 seconds (!). In the next half hour it becomes more pronounced, and slows down to about 30 seconds near midnight. Perhaps even more stunning is the fact that the pattern has a frequency dependence, with the falling slope indicating a periodicity of ~100 Hz. Interpreted as multipath, this would be a 10 ms delay, with a Doppler shift changing from +0.2 Hz to +0.03 Hz. For the 1600 km path geometry from Greece, this would imply a reflecting layer at about 2200 km height, having a downward velocity of up to 200 m/s! After midnight, the pattern is superimposed by a second, rising component (receding Doppler shift -0.03 Hz), which finally dominates for about an hour after 0:10.

This is all very odd, and I have not seen it before. But I'm fairly sure it's not an artifact of the receiving system. I did look for ~0.2 Hz sidebands on fading signals in the narrowband plots (CT1DRP, 137689.6) but found nothing.

Any explanations?

73 de Markus,
DF6NM in JN59NK


In einer eMail vom 11.02.2006 17:32:19 Westeuropäische Normalzeit schreibt wolff.h@t-online.de:

attached some screenshots made from the audiofile which I recorded
during the reception. The "30sec dot" image shows the first dot of the
"V" after the CW 30sec long at about 05:00 UTC. At that time the
signal was not so strong as 15min earlier, but it shows exactly what I
meant. There are many gaps of rapid fading.