Hi Stefan,

You wrote:
> ...
> I found the Q of the
> loop is just 8! Is this normal? Which Q do others achieve, with a
> similar antenna for 137 kHz?
> ...

Confirming your findings. In the late 90th I built a 2m-loop from 1"
coax ("Flexwell") for nulling out Loran-C splatter. I remember a
bandwidth of ~12-15 kHz => Q ~= 10 with 8.2uH/165nF.
http://www.qsl.net/df3lp/projects/lfloop/index.html

> ...
> It is fascinating to null out s signal, which i have done the first
> time now. I can reduce the signal of DCF39 by 25 dB by turning the
> loop...

That's somewhat different to what could be found here. I observed very
sharp Nulls well >40dB below main lobes. May be due do the orthodox
"screened" design including the usual gap. But as you suggested that's only
true for pure groundwave reception. At VLF I'm observing the deepest null
of DHO38 (~200km; noon) at ~30deg. of elevation.

Good luck for tomorrow,
Peter, df3lp