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ARRL Propagation Bulletin ARLP034 (2007)

ARLP034 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 34  ARLP034
From Tad Cook, K7RA
Seattle, WA  August 17, 2007
To all radio amateurs 

ARLP034 Propagation de K7RA

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 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  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,

For more information concerning radio propagation, see the ARRL
Technical Information Service at  For a detailed
explanation of the numbers used in this bulletin, see  An archive of past
propagation bulletins is at .
Monthly propagation charts between four USA regions and twelve
overseas locations are at

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


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