I'm pretty sure now this is a propagation change.

Info from Bob via PM indicates that the antenna current is
essentially constant during the transmission with negligible
tuning adjustments.

I can think of no environmental factors around the tx that
would affect signal this much without affecting antenna current.

I'm doubtful that freezing ground over the NY land portion
of the path would have noticeable effect.  Skin depth is
substantial at these frequencies, tens of metres, maybe up
to 100m or more, in dry rocky ground and down to 10 or fewer
metres in soggy wet soil.   This is the reflecting volume and
a thin frozen layer at the surface isn't going to make much
difference to it.

That leaves propagation then.  Some little shift of phase
and amplitude of the modes involved, altering the interference
pattern.  We've seen on good nights, occasional dips of the
signal which indicate some competition between modes during
the night.  The outcome of that competition will vary from one
night to another.

We'll just have to be patient, it'll come back again.
There'll be times when WH2XBA/1 is coming in well and NAA
is low.   We could get a night that's really good.

Carrier tonight until 04:00.  Signal just now: 0.43 fT for 20 mins
which is strongest I've seen from Bob.   That'll make lines on
spectrograms if it keeps up!

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Paul Nicholson
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