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.

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.
73, Bob

I can confirm that 137.71000 is clear in the UK, but the situation varies
around the world because of various Loran systems, and spurious emissions
and mixes from commercials. In practice, even a 120s dot window has a
viewable width of 1.5Hz so the real answer is for a DXpedition to be precise
about its frequency (which can be advertised in advance and objected to, if
necessary) and for callers to spread out. A pre-posted, or live, screen shot
will show callers which frequencies to avoid.

Mike, G3XDV
http://www.lf.thersgb.net
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