SWL PROPAGATION
By Tomas Hood,
NW7US / KPC7USA
High-frequency
Propagation This Month
As
we move into spring in the Northern Hemisphere we experience great DX openings
from around the world on HF. This is
because the sun is mostly overhead over the equator, creating equal day and
night periods in both hemispheres. The
Vernal Equinox on March 20, 2014 marks the day when the hours of daylight and
darkness are about equal around the world. This creates an ionosphere of similar characteristics throughout more of
the world than is possible during other times when it is summer in one
hemisphere and winter in the other, and there are extreme differences in the
ionosphere. This equalization of the
ionospheric which takes place during the equinoctial periods (autumn and
spring) is responsible for optimum DX conditions, and starts late in February
and lasts through late April. The
improvement in propagation is most noticeable on long circuits between the
northern and southern hemispheres.
During this season conditions are also optimal for long-path as well as
short-path openings, and during gray line twilight periods associated with
sunrise and sunset.
Spring
is also the season of Aurora. Geomagnetic storms that ignite auroras occur more often during the
months around the equinoxes during early autumn and spring. This seasonal effect has been observed for
more than 100 years.
Look
for Aurora-mode propagation when the geomagnetic indices of the planetary K (Kp)
rises above 4, and look for visual Aurora after dark when the Kp rises above
5. The higher the Kp, the more likely
you may see the visual lights. But, you
don't have to see them to hear their influence on propagation. Listen for stations from over the poles that
sound raspy or fluttery. Look for VHF
DX. Sometimes it will enhance a path at
certain frequencies, other times it will degrade the signals. Sometimes signals will fade quickly, and then
come back with great strength. The
reason for this is that the radio signal is being refracted off of the more
highly ionized areas that are lit up.
These ionized areas ebb and flow, so the ability to refract changes,
sometimes quickly. I've observed the
effect of Aurora and associated geomagnetic storminess even on lower HF
frequencies. To monitor the live aurora
predictions and geomagnetic activity, visit http://aurora.sunspotwatch.com.
Because
of these various conditions occurring in the month of April, hot DX is possible
across the HF and low-VHF spectrum. The
seasonal change plays out on HF with activity moving up from 41 meters and down
from 11 meters. Propagation on the
higher HF frequencies (19 through 11 meters) begins to suffer late in April and
into the summer months due to lower MUFs (Maximum Usable Frequencies) in the
Northern Hemisphere. MUFs peak very late
in the day during summer. Summertime
MUFs are lower due to solar heating which cause the ionosphere to expand. An expanded ionosphere produces lower ion
density, which results in lower MUFs. Short-path propagation between countries in the Northern Hemisphere will
drop out entirely. Higher frequency
propagation peaks in the fall. April and
May are fall months in the Southern Hemisphere making long-path DX
possible. Short-path propagation from
South America, South Pacific, and other areas south of the equator will be
strong and reliable when open. However,
these do not happen every day on the higher frequencies.
From
April to June, excellent propagation occurs on both daytime and nighttime
paths. The strongest propagation occurs
on paths that span areas of both day and night, following the MUF. During April, peaking in May and still in
June, 16 meters may offer 24-hour DX to all parts of the world, with both
short- and long-path openings occurring, sometimes at the same time. If you hear a lot of echo on a signal, you
might be beamed in the wrong direction. Try the opposite azimuth.
Thirty-one through 19 meters are more stable as nighttime bands, with
propagation following gray line and nighttime paths.
Low-band
propagation is still hot on 41 meters, with Europe in the evening, and Asia in
the mornings. Occasional DX openings
will occur on 90 and 75 meters around sunrise.
VHF Ionospheric
Openings
The
possibilities for ionospheric openings on the VHF bands usually improve after
March and the spring months. Many of the
solar-ionospheric relationships that can produce ionospheric openings on the
VHF bands tend to peak during equinoctial periods. On VHF, many different types of propagation
modes can appear once or twice during this month. Combination propagation modes may be possible,
as well. Aurora is highly likely, as is an increase in Trans-equatorial (TE)
propagation. On days of high solar flux,
there might be F2-mode VHF openings.
Sporadic-E will play out just as we hit May, but until then, there is
opportunity for those caring to stay on low VHF spectrum, hunting for
signals. There are times when
sporadic-E, TE, and F2-layer propagation modes will link, providing strong DX
openings on VHF between North America and New Zealand, Australia, or other
areas.
If you use Twitter.com – you can follow
@hfradiospacewx for hourly updates that include the K index numbers (and,
follow this columnist – @nw7us). You can
also check the numbers at http://SunSpotWatch.com.
CURRENT
SOLAR CYCLE PROGRESS
The Royal
Observatory of Belgium, the world’s official keeper of sunspot records, reports
a monthly mean sunspot number of 82.0 for January 2014. The mean value for January
results in a 12-month running smoothed sunspot number of 65.5 centered on July
2013. Following the curve of the
13-month running smoothed values, a smoothed sunspot level of 82 is expected
for April 2014, plus or minus 14 points.
Canada's Dominion
Radio Astrophysical Observatory at Penticton, British Columbia reports a
10.7-cm observed monthly mean solar flux of 158.6 in January 2014, continuing
an upward trend (a second peak?). The 12-month smoothed 10.7-cm flux centered on July 2013 is 123.9, up from June’s
120.9. A smoothed 10.7-cm solar flux of
about 138 is predicted for April 2014.
The geomagnetic
activity as measured by the planetary-A index (Ap) for January 2014
is 6. The 12-month smoothed Ap index centered on July 2013 is a
steady 7.3. Geomagnetic activity should be much the same as we have had during March. Refer to the Last Minute Forecast for the
outlook on what days that this might occur (remember that you can get an
up-to-the-day Last Minute Forecast at http://SunSpotWatch.com on the
main page).
I'd like to hear
from you
I welcome your
thoughts, questions, and experiences regarding this fascinating science of
propagation. You may e-mail me, write me
a letter, or catch me on the HF Amateur bands.
On Twitter, please follow @NW7US (and if you wish to have an hourly automated
update on space weather conditions and other radio propagation-related updates,
follow @hfradiospacewx). I invite you to
visit my online propagation resource at http://sunspotwatch.com,
where you can get the latest space data, forecasts, and more, all in an
organized manner. If
you are on Facebook, check out http://www.facebook.com/spacewx.hfradio and http://www.facebook.com/NW7US.
Until next month,
73, Tomas, NW7US
P.O. Box 27654
Omaha, NE 68127
@NW7US
@hfradiospacewx
