Thanks to Paul for reporting and running fbins, and to Stefan for setting up a new 424 uHz colour spectrogram. It looks like there was a faint yellow line in Heidelberg, but it is pretty marginal due to upcoming QRN and approaching the midday minimum from destructive skywave.

The most interesting aspect of this experiment was that Tom DK1IS also transmitted a carrier from the opposite side, a couple of mHz above mine. We had expected a pair of cyan and red traces, providing a beautiful demonstration of RDF without 180° ambiguity. However there was a surprise: While my trace appeared close to west (perhaps a little too greenish), Tom's trace up came out blue rather than red!

               azimuth
          actual   measured
DK1IS     74°       20°
DF6NM   268°    255° (+-5°)

How can this be?

The 360° RDF scheme is based on evaluating the phase between an electric antenna and a quadrature pair of loops:
http://df6nm.de/ColourDF/ColourDF.htm
The a priori unknown angle offset was approximately calibrated using MSK stations, but there is some uncertainty mainly from frequency responses (particularly of the earth loops with frequency-dependent skin effect), and also small sampling time offsets between the two independent soundcards. So I believe that the ~13° error for DF6NM is due to a calibration error. However this still leaves an offset of 74-20-13 = 41 ° for Tom, much too large to be accounted to measurement errors.

I believe the true explanation is rooted in Maxwell's equations themselves. Tom's antenna is 5.97 km from DL0AO, just outside the nearfield boundary lambda/2pi = 5.773 km (often denoted as kr=1). Looking at Hertzian dipole antenna fields, we find that electric and magnetic fields are 45° out of phase at the boundary:
E ~ kr^-1 - j kr^-2 - kr^-3 ~ -90°
H ~ kr^-1 - j kr^-2  ~ -45°

For Tom's distance, the phase difference would theoretically be -42°, perfectly explaining the remaining error. Note that the sign of the error depends on the sense of rotation which was arbitrarily selected when connecting the two loops to the IQ inputs of SpecLab's FFT Filter. If I had done otherwise, Tom would appeared bright red at 104°, and we might not even have noticed this interesting near-field effect. 

Best 73,
Markus


-----Ursprüngliche Mitteilung-----
Von: Markus Vester <[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: Sa, 14. Jul 2018 7:31
Betreff: VLF: 8270.0025 Hz Colour-DF Test