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ARRL Propagation Bulletin ARLP020 (2007)

ARLP020 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 20  ARLP020
From Tad Cook, K7RA
Seattle, WA  May 11, 2007
To all radio amateurs 

ARLP020 Propagation de K7RA

Over the past week, sunspot activity has again declined, with the
average daily sunspot number down over 7 points to 18.  Geomagnetic
activity was much more stable this week, with the planetary and
mid-latitude A indexes less than half what they were the week
before.  The mid-latitude A index was actually down to 0 on both
weekend days, May 5-6.

The next period of higher geomagnetic activity is expected around
May 25-27.  Geophysical Institute Prague projects quiet conditions
for May 11-12, quiet to unsettled May 13, unsettled May 14-15, and
quiet again on May 16-17.  The US Air Force over those same days,
May 11-17, expects a planetary A index of 5, 5, 8, 8, 10, 10 and 5,
a slightly different scenario.

May 6 was the halfway point between the Vernal (Spring) equinox and
the Summer solstice.  This weekend, compared to the first day of
Spring, when observing propagation between the center of the United
States and Europe, we expect much shorter openings on the low
portion of the HF spectrum.  This is because the hours of darkness
are best for 80, 60 and 40 meters, and the first day of Spring has a
longer night than what we currently see.  For much of the 24-hour
day the MUF over this path would be higher than in March, except the
six hours between 1430-2030z, a period over most of which there is
daylight on both ends of the path.

April 30 through May 3 saw sunspot numbers ranging from 32-38, more
than 20 points higher than the usual daily readings of late.

Several readers reported better propagation on these dates.  Randy
Crews, W7TJ of Spokane, Washington said that 20 meters was still
good for the Pacific and for stateside stations until 0600z.  Vic
Alfonsi, WA6MCL of Corona, California noted similar propagation
until around 0400z.

Probably the reason for 20 appearing to close earlier in Southern
California than in Washington State was because during Spring and
Summer, the higher latitudes have more daylight.  On May 4, my
sunset in Seattle at 47.7 degrees north latitude was around 0318z
(approximately the same as Spokane), but Southern California at 34
degrees north was 45 minutes earlier at 0233z.  But Seattle's
longitude is about 4 degrees to the west of Los Angeles, which is
another factor for a later sunset.

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 May 3 through 9 were 33, 19, 18, 14, 12, 12 and
18 with a mean of 18. 10.7 cm flux was 83.1, 82.5, 81, 78.1, 75.7,
73.4, and 72.1, with a mean of 78. Estimated planetary A indices
were 4, 2, 3, 2, 18, 14 and 5 with a mean of 6.9. Estimated
mid-latitude A indices were 2, 1, 0, 0, 11, 11 and 3, with a mean of


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