At 06:16 1/13/2009, you wrote:
>Unfortunately I don't have the knowledge to debate the physics theory
>behind TB propagation, but my experience in ZL, which is now approaching
>one solar cycle, is starting to indicate the opposite. Certainly the
>long path between ZL and G has already shown a couple of good openings
>this season, and I made more Eu contacts in Mar/Apr 2008 than ever
>before in a similar period.
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
*********************
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Nick Hall-Patch
Victoria, BC
Canada
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