Dear LF,

 

the Australian VLF transmitter North West Cape (NWC, 19.8 kHz) can be received regularly in Europe. This morning I had a detailed look on the power spectrum of the MSK signal, and found interesting 21 Hz periodic ripples. I believe this is an interference effect, due to the same signal being simultaneously received on both short path and long path. The geometrical path difference (12.8 vs 27.2 Mm) does indeed match the observed ripple period.

 

I have switched my VLF grabber http://www.mydarc.de/df6nm/vlf/vlfgrabber.htm to zoom in on the signal, and have slowed it down to be able to see diurnal changes. During the last hours between 7 and 13 UT, the fading pattern has not disappeared completely. The frequency of the minima has shifted upwards by one-and-a-half ripple periods, most likely due to a variable differential phase shift between the two routes.

 

I'd be interested to see if others observe this effect as well. If the explanation is correct, the null positions should depend on east-west receiver location on a half-wavelength scale.

 

Fascinating stuff!

 

Kind regards
Markus (DF6NM)

PS - This is not an April joke.