HF Propagation for October
By Tomas Hood, NW7US
A change in propagation conditions in the Northern Hemisphere
can be observed as we move away from the long sunlit days of summer into the
longer hours of winter’s darkness. With the shorter period of sunlight
each day, the ionosphere has more time during the dark hours to lose the energy
created during daylight hours. This affects the propagation of radio
signals by lowering the Maximum Usable Frequency (MUF) over many areas of the Earth.
However, the change in the length of daily darkness is not the only influence
on the propagation of radio waves through the atmosphere. The amount and
strength of radiation arriving and passing through our atmosphere varies from
season to season, as well as from the solar cycle minimum to the solar cycle
maximum.
During the Northern Hemisphere’s winter months, the Earth is
closer to the sun than during any other time of its orbit. This makes the
daytime ionization more intense than that of summer daytimes. In turn,
this higher-level energy during the day causes the average MUF to increase
slightly as compared to the same time of day during the summer season, over the
same radio signal path.
Then, with the longer winter hours of darkness, the ionosphere
has more time to lose its electrical charge. This causes the MUF to dip
lower at night than during the summer months.
These conditions cause a wide daily variation in the maximum
frequency that can be propagated by refraction of the radio waves by the
wintertime ionosphere. Many radio enthusiasts celebrate the arrival of
the winter shortwave season for these reasons.
Signals below 120 meters are improving, with nighttime paths
growing larger in the Northern Hemisphere. Seasonal static, which makes
it difficult to hear weak DX signals, is starting to decrease as we move into
winter. Expect a few DX openings during the hours of darkness and into
the sunrise period. These openings will often be weak due to the
relatively high signal absorption during the expected elevated geomagnetic
storminess through the rest of this year. Look for openings from Europe
and the south if you are listening in the eastern half of the United States,
and from the south, the Far East, Australasia, and the South Pacific if you are
in the western half of the country. The best propagation aid is a set of
sunrise and sunset curves, since DX signals tend to peak when it is local
sunrise at the easterly end of the path in question. A good Internet web
site featuring a grey line map display is found at < http://www.fourmilab.to/earthview/ >.
Follow the link, "map of the Earth" showing the day and night
regions.
Seventy-five through 120 meters are coming alive in late
October. Expect long-range DX on the low bands, starting close in right
after sunset, and extending farther as the night develops. Signals here
should peak from Europe and from a generally easterly direction
around midnight. DX paths will move farther west through the
night. By morning, openings from Asia should be common. For
openings in a generally western direction, expect a peak just after
sunrise. The band should remain open from the south throughout most of
the night. Propagation in this band is quite similar to that expected on
41 meters, except that signals will be somewhat weaker on the average, noise
levels will be a bit higher, and the period for band openings in a particular
direction will be a bit shorter.
Forty-one meters should be the hottest DX band during the dark
hours as the seasonal static levels are lower than they were during the
summer. The band should be open first for European DX in the eastern
United States during the late afternoon. Signals should increase in
intensity as darkness approaches. During the hours of darkness, expect
good DX openings from most areas of the world. Signals should peak from
an easterly direction about midnight, and from a westerly direction just
after sunrise. Excellent openings toward the south should be possible
throughout most of the nighttime period.
Paths on 31 through 19 meters are becoming ever more reliable
between North America and Europe in the morning and between North America and
Asia during the late afternoon hours. The strongest openings occur for a
few hours after sunrise and during the sunset hours.
Thirty-one and 25 meters will often remain open into many areas
late into the night and will open early in the morning, especially when part of
the propagation path moves through sunlit regions. However, these bands
are crowded and signals are usually very strong and steady. Twenty-five
meters is expected to be an excellent band for medium distance (500 to 1500
miles) reception during the daylight hours. Longer distance reception (up
to 2000 to 3000 miles) should be possible for an hour or two after local
sunrise, and again during the late afternoon and early evening.
Thirty-one meters will provide medium distance daytime reception ranging
between 400 and 1200 miles.
Twenty-two through 19 meters compete with 16 for the best
daytime DX band during October. They will open for DX just before sunrise
and should remain open from all directions throughout the day, with a peak in
the afternoon. Nighttime conditions will favor openings from the south
and tropical areas. Since the Southern Hemisphere has long daylight
hours, DX paths on these bands from stations in the south will be common.
Sixteen through 13 meters will occasionally open through October
when flux levels reach above 100. Paths from Europe and the South Pacific
as well as from Asia, at least during days of higher solar flux levels, are
common, especially on 16 meters. Look for best conditions from Europe and
the northeast before noon and from the rest of the world during the
afternoon hours. Reception from the South Pacific, Australia, New
Zealand, and the Far East should be possible well into the early evening.
VHF Conditions
Conditions during October should include moderate levels of
trans-equatorial propagation (TE) in which stations in the southern states and
parts of the Caribbean will be able to work into the northern areas of South
America during the late afternoon. During peak years of a solar cycle,
October is one of the best months for TE activity, especially later in the
month. Since we are in a rather moderate solar cycle (that’s a liberal
perspective), Sunspot Cycle 24, these openings may not occur often, but it is
possible an exciting opening might occur from time to time; look for them by
getting on the air and trying.
While sporadic-E activity is sparse during October in the
northern Temperate Zone (where much of the U.S. is located), there is some
possibility of extended tropospheric propagation conditions during October
because of the changing weather patterns. Higher VHF is the best
frequency range to watch for this.
