I originally thought that 137.7000 kHz was clear of QRM in this corner of
the world, however lots of capture time on trying to find a UA9OC signal
(with success on 13 March) found a weak QRM typically 0.35 Hz higher than
where I think is spot on 137.7000 kHz. I'm still not sure where the QRM
comes from. Luckily the UA9OC signal was offset slightly, but it was luck.
I did a full night checking on 137.7100 kHz, with Argo on 120 second dot
length, and that was genuinely "clean".
I can't help thinking that the QRM is harmonic content of mains frequency,
and there are lots of peak rectifiers in domestic use, and the likes of TV
sets, VCRs and microwave ovens run with mains live, to keep the clocks
going. When I listen in AM mode, I hear "mains hash" over all the LF band.
If you can hear mains hash (in AM mode), then Mr Fourier has good advice
about where the (highest density) spectral lines are.
So my suggestion for selecting frequencies for long haul DX between amateur
stations is to avoid multiples of 50 or 60 Hz (50 Hz mains being used in
some regions, 60 Hz in others). 137.7100 kHz is an example of being clear
of 50 Hz harmonics, whereas 137.7000 kHz is dicing with the 2754th harmonic
of 50 Hz, plus or minus some tolerance for the national grid.