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The lowest HF band is similar to 160 m in many
respects. Daytime absorption is significant, but not quite as extreme as at
1.8 MHz. High-angle signals may penetrate to the E and F layers. Daytime
communication range is typically limited to 400 km, primarily via ground-wave
propagation. At night, signals are often propagated halfway around the world.
As at 1.8 MHz, atmospheric noise is a nuisance, making winter the most
attractive season for the 80 m Dxer.
7000-7300 KHz (7.0-7.3 MHz) or 40
m band
The popular 40 m band has clearly defined
skip zone during the day. D-layer absorption is not as severe as on the lower
bands, so short-distance skip via the E and F layers is possible. During the
day, a typical station can cover a radius of approximately 800 km. Ground-wave
propagation is not important. At night, reliable worldwide communication via
F2 is common on the 40 m band. Atmospheric noise is less troublesome than on
160 and 80 m, and 40 m DX signals are often of sufficient strength to override
even high-level summer static. For these reasons, 40 m is the lowest-frequency
amateur band considered reliable for DX communication in all seasons. Even
during the lowest point in the solar cycle, 40 m may be open for worldwide DX
throughout the night.
Source: ARRL
Handbook, 1993
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