While performing my beacon check on December 16th, 2013 at 8:30 UTC, I stated that all beacons seemed to be at a normal level. Except, when looking for DB0FGB (JO50WB) with the dish heading 85°. There was a constant carrier of about 20 dB 15 kHz up, keying F1ZBK (JN38BP). Expecting a big signal, I turned the dish to France. But the beacon was only 15 dB with QSB, when beaming directly to it at 228°. Then I turned the dish back to 85° and noticed, that HB9EME (JN37KB) joined the party.
DB0FGB, F1ZBK and HB9EME with dish heading 85°
The rain radar didn´t show anything unusual and the F5LEN Refractive index forecast for Europe promised only marginal tropo conditions for this area. Only DL7QYs Condition Index for south Germany has been on a level of 7 (1 = poor; 10 = extreme).
F5LEN Refractive Index
DB0FGB was a bit above normal at about 15 db with little QSB up and down. F1ZBK was constantly at 20 dB, when beaming 85° and around 15 dB with QSB heading 228° (direct). HB9EME was up to 20 dB in 85° and peaking 15 dB in fast QSB when beaming 198° (direct).
Green: Paths from Mainz to the beacons
Orange: Paths of the beacons to the area of the supposed reflection point
I checked HB9BBD, F5ZBS, F1ZBI and others, but couldn´t see any similar effects. The angle was very small. When turning the 3 m dish 5° left or right, the signals disappeared. These conditions lasted for about one hour.
Topographic profile between DJ5AR and DB0FGB
The horizon in 85° is about 80 to 90 km off. A plausible explanation could be a strong troposheric duct between the beacons and a reflecting topographic formation.
In issue 2/14 the german magazine “Funkamateur” published an article by Matthias Hornsteiner, DG4MHM “Prognosekarten für troposphärische Überreichweiten” (Prediction maps for troposheric ducting). Matthias explains the basics of ducting and analyses the models, the well known refractive index maps of F5LEN and W. Hepburn are basing on.
So I had the intention, to ask him about a possibility for a reconstruction of the wheater constellation of that day. I have to thank him very much for his comprehensive answer.
As Matthias explained, the situation, shown on the map, was quite common for this, as he describes it, “anti-winter”. But it holds the possibility, that warm air might glide onto cold air at ground level. And indeed, when he reviewed the data of weather balloons launched at Stuttgart on that day, he found indications for a strong inversion in an estimated level of 400 to 600 m over south Germany. That matches!
In a distance of 200 km from here, the the topographic profile shows a formation, high enough to reach the level of this duct, as well as the mountains of the Fichtelgebirge do, where DB0FGB is located. The distances from Mainz to the area of the supposed reflection point is between 200 and 250 km. From F1ZBK and HB9EME it varies between 400 and 500 km. So the total path should have been between 600 and 750 km.