> Some years ago, with a great deal of help from NM7M, I wrote an article
on propagation monitoring in QEX, "Medium Frequency Sunrise Enhancements",
based on automated monitoring of signal strengths of overseas AM broadcast
band signals near 1600kHz. Local sunrise enhancements seem to usually
provide the best signal strengths during a darkness path for these
frequencies (which are somewhat relevant for 160m), and it was speculated in
the article that ducted signals left the duct at that time of day due to
ionospheric tilting.
>
> In that article there's a chart of frequency of occurrence of sunrise
enhancements by month between September 1997 and February 2000 for two
Trans-Pacific paths, one over the north Pacific, the other across the
equator. It shows the most frequent enhancements for the northerly path
occurred in the local spring and fall, with a decided dip in January.
>
> Although the monitoring is still ongoing (now covering one solar cycle), I
haven't been able to find the time to do a serious analysis of what has
happened since, but my impression is that throughout the solar cycle, that
annual pattern has repeated itself. This corresponds with the experience
of medium-wave DXers, who observe poorer quality reception of overseas
signals (on average) around midwinter, and this has little correspondence
with geomagnetic activity. The mechanism for this remains a mystery,
though at one time stratospheric warming was considered. K9LA's article in
the October 1997 QST found little effect from stratospheric warming however.
>
> So, it may be wisest to (as always) keep an ear on the band in spite of
generally poor conditions in midwinter (exceptions to the rule do occur),
and expect conditions to improve as spring approaches.
>
> best wishes,
>
> Nick
> VE7DXR
> *********************
Nick. . .could you specify which QEX issue contains the article? Sounds
like it would make interesting reading. I would be curious to know what
percentage of the time you saw worthwhile local sunrise enhancements
(worthwhile being perhaps at least 2 db). Having spent a lot of time on
160m at local SR during the past decade, I see SR enhancement fairly
infrequently. . . maybe 10% or 15% of the time overall, if that. Most of
my observations are taken on JA, VK, and ZL paths to the US Midwest.
73. . . Dave
W0FLS
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