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18 August, 2007
We're on the road this week, and post this bulletin from
Hillsborough County, New Hampshire.
Solar activity continues low. The sun is currently spotless, but sunspots
may return around August 20. This week's average daily sunspot numbers
were down about a point from last week's, from 12.4 to 11.3.
Expect quiet geomagnetic conditions over the next week, according to
a forecast from the US Air Force, which predicts planetary A index for
August 17-23 of 8, 5, 10, 5, 5, 10 and 8. But Geophysical Institute Prague
has quite a different prediction for August 18.
They predict quiet conditions for August 17, unsettled to active on
August 18, unsettled August 19, quiet August 20-21, unsettled August 22,
and quiet to unsettled August 23.
Jerry Reimer, KK5CA of Spring, Texas, sent in some interesting comments
about NVIS (Near Vertical Incidence Skywave) propagation and antennas,
and ionospheric data available on the internet. An automated ionospheric
sounder, or isosonde, beams energy straight up while sweeping the signal
up in frequency, thereby determining the MUF or Maximum Usable Frequency
of that area by measuring the
reflected signal.
Jerry says that NVIS communication (which is used to communicate with
stations out to about 200 miles maximum) is best at a frequency 50 to
80 percent below the MUF from the isosonde. So if the MUF of the patch
of ionosphere overhead is 10 MHz, then NVIS is best between 2-5 MHz. With
NVIS, users are trying to get high angle radiation instead of low angle,
which is usually the goal with
other modes of HF communication. A page explaining Vertical
Incidence Soundings is linked from http://www.ngdc.noaa.gov/stp/IONO/
by clicking on the Vertical Soundings link on the left of the page.
He also pointed out some interesting real-time maps showing continent-wide
communication between various points at http://www.ips.gov.au/HF_Systems/4/1.
For instance, if you select Hourly HAP Charts, then select Kansas City,
what you will see is the best frequencies for communications with Kansas
City from across the continent at that time. So you can look at the color
region over any point on the map, and this is keyed to the best frequency
for communicating with Kansas City from that point.
More about NVIS and ionospheric soundings in next week's bulletin.
If you would like to make a comment or have a tip for our readers, email
the author at, k7ra@arrl.net.
For more information concerning radio propagation, see the ARRL Technical
Information Service at
http://www.arrl.org/tis/info/propagation.html.
For a detailed explanation of the numbers used in this bulletin, see
http://www.arrl.org/tis/info/k9la-prop.html.
An archive of past propagation bulletins is at http://www.arrl.org/w1aw/prop/.
Monthly propagation charts between four USA regions and twelve overseas
locations are at http://www.arrl.org/qst/propcharts/.
Sunspot numbers for August 9 through 15 were 14, 14, 13, 11, 13, 14 and
0 with a mean of 11.3. 10.7 cm flux was 67.4, 67.5, 67.6, 68.1, 67.7,
68.5, and 67.6, with a mean of 67.8. Estimated planetary A indices were
4, 13, 12, 6, 3, 6 and 8 with a mean of 7.4. Estimated mid-latitude A
indices were 2, 10, 7, 5, 2, 3 and 6, with a mean of 5.
Source: The
American Radio Relay League
•
DX Spots popup
• All propagation
reports can be found at:
http://www.southgatearc.org/propagation
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