This image is from a 122 hours (~5 days) recording the frequency in Denmark [0] The red bar is avg. over 40000 sec (11 hours).

[1] is the original source translated from danish.

[0] http://webx.dk/oz2cpu/radios/50hz-kurve122h.gif [1] http://translate.google.com/translate?sl=da&tl=en&js...