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The Grey Line is the band around
the Earth that separates day from night. There is always a short period,
around one hour, of semi darkness, hence "Grey". On one side of this area
the Earth is coming into daylight, and on the other the Earth is darkening
to night. Propagation along the Grey Line is usually very good, albeit of
short duration. This line generally runs in a north-south direction
depending on the time of year and it can have a tilt of as much as 23°.
On the sunset side
of the Grey Line, the D Layer disappears quite quickly and on the sunrise
side the D Layer takes time to form. During this period propagation is
enhanced along the line. The effect is most noticed on the low bands, 1.8MHz
to 10MHz. Of course hf yagi antennas are the best to use to take advantage
of this propagation, but wire antennas also work well if aligned to take
advantage of the Grey Line, especially since below 7MHz yagis are somewhat
large!
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