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ARRL Propagation Bulletin ARLP040 (2009)

ARLP040 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 40  ARLP040
From Tad Cook, K7RA
Seattle, WA  October 2, 2009
To all radio amateurs 

ARLP040 Propagation de K7RA

With the appearance of two sunspots, the past eleven days had some
nice HF propagation.  Combined with quiet geomagnetic conditions and
month after month of quiet sun, it seems quite a dramatic relief.
Sunspot 1026 has faded away, and sunspot 1027, which appeared later
than 1026 but actually led it in the sun's rotation, has just
rotated out of view, over the sun's western limb.  You could see
this early Friday morning by viewing the STEREO animation at

Remember that the 0 degree meridian is facing us, and the two 90
degree longitudinal marks are at the sun's eastern and western
horizons.  It seems odd, since we are accustomed to viewing an atlas
with west on the left and east to the right, but when we say a
sunspot is rotating over the western limb, it is moving to the
right, from our point of view.

Early Friday morning (evening on the West Coast in North America)
you could see that Northern Heliosphere bright spot just beyond the
+90 degree mark.  There is a bright spot in the Southern
Heliosphere where I think sunspot 1026 was, but it doesn't appear as
a sunspot to any of the observatories that track them.  This can be
puzzling, because if I look at a spot that bright via the STEREO
animation that appears to the left, beyond the eastern limb past
-90 degrees, I might assume that is a sunspot coming our way.

Perhaps someone is expecting a reappearance of either 1026 or 1027,
because if we look at the October 1 forecast from the U.S. Air Force
and NOAA at, it
shows an expectation of a rising solar flux above 70 beginning
October 17.  It looks like a spot isn't expected any time soon, as
they have the predicted solar flux for Friday (today) and Saturday,
October 2-3 at 70, then down to 69 for October 4-8, 68 on October
9-14, then rising back to 70, then 72 for October 17-28.  This
forecast is likely to change, and is updated daily.

The same forecast projects very quiet geomagnetic conditions, with a
planetary A index of five off into the indefinite future.  Sunspot
number on Thursday, October 1 was 11, same as Wednesday, and today's
is likely to be zero.

The three-month moving average of sunspot numbers, which now
includes data from September, is the same as last month's three
month average, which was calculated from sunspot numbers for June
through August.  It seems that both June and September had identical
average sunspot numbers, so for the latest reading, the June data
was dropped, the September data added, and the same 3-month average
of 4.0 is the result.

The monthly averages of daily sunspot numbers for January through
September were 2.81, 2.54, 0.77, 1.27, 3.97, 6.6, 5.07, 0.39 and

Our three-month moving average always is labeled with the middle
month, so with the September data, we now have the August average,
which is for July, August and September.

For October 2008 through August 2009, the moving average is 4.52,
4.39, 3.62, 2.19, 2.02, 1.49, 2.01, 4.23, 5.2, 4 and 4.

Dan Prebenda, K9DP of Richmond, Indiana is back on the air after 10
years QRT, and is frustrated trying to work fairly short skip on 30
and 40 meters CW around noon to 3:00 PM local time (1600-1900z when
local time is Eastern Daylight Time, which ends November 1 this
year).  He is experiencing a lot of fading, and the stations he is
trying to work are about 300 to 700 miles away, mostly to the east
and south of him.

Dan wonders if 30 meters will get better for this path with higher
sunspot counts?

I suggested he test some paths and seasonal variation using
W6ELprop, which is free at  Using
either zero or a smoothed sunspot number of 10, it looks like 40
meters is the best band, although 30 meters looks good 700 miles out
to the southwest.  I used 39.826 degrees north latitude, 84.896
degrees west longitude for Dan's location, based on the address in
his FCC licensing record.

Philadelphia is about 500 miles almost directly east of him.  At
that distance, 30 meters doesn't look good until the smoothed
sunspot number gets above 50, which is not where we are in the
cycle.  Same thing with Atlanta, at about 400 miles, almost due
south.  But Dallas at about 800 miles and 237 degrees looks a lot
better, with strong signals on 30 meters, 1400-2300 UTC (10am to 7pm
EDT) and a smoothed sunspot number of 2.

But 40 meters looks great for any of those paths, even with no
sunspots, and he can test some assumptions using W6ELprop.  He can
look at any path on different dates, too, and should see seasonal

Instead of solar flux, use sunspot number with W6ELprop.  If the
program isn't set for sunspot number in the options, just precede
the number by the letter S.  The program is designed to give a rough
forecast using the predicted smoothed sunspot number for the month.
It was 9 for September, 10 for October.  The most recent prediction
is in PRF1774 at on page 8.  There
may be a new prediction next week in PRF1779.  In the past couple of
weeks, when we have actually had sunspots, I would average the three
latest known sunspot numbers (a day's number is not available until
the evening) from

The smoothed number changes for any month because it is based on
actual sunspot numbers for the previous six months, and predicted
sunspot numbers for about 6 months into the future.  So the only
smoothed sunspot numbers that we know for sure are at least six
months old because they are all based on real sunspot data, with
nothing predicted.

To use current numbers, on September 26 during the day the three
latest known sunspot numbers were 31, 32 and 25, which averages to
about 29.3.  You can get the latest mid-latitude K index from WWV
off the air, or at or
by calling 303-497-3235.  It is updated every three hours.

Meyersdale, PA is about 300 miles due east of Dan, and for that
short skip, 80 meters is the only realistic option, except for a
dead period, 0230-1100 UTC.  40 meters is possible from 1600-1800
UTC, but the odds are against it on most days (this time of year).
It looks better in the spring, 1600-2200 UTC, and a LOT better when
we get some sunspots.

Von Weddige, W5COW of Hope, New Mexico (how did I guess his was a
vanity callsign?) reported a surprising six meter opening on
September 22.  He made 19 contacts on SSB in 31 minutes, from
0022-0053z.  Von is in gridsquare DM72, and he worked stations in
EM53, EM54, EM57, EM63, EM68, EM73, EM75, EM79, EM81, EM86, EM89,
EN50, EN70, and EN71.

In last week's bulletin, ARLP039, Ken Tata, K1KT talked about
interesting conditions on two meters from his location in Rhode
Island during a weeknight VHF sprint.  Leo Halverson, WA2AMW
commented in response, "These conditions aren't all that rare at
this time of year, although I usually encountered them later at
night.  They often go undetected because most of these openings
happen during work-nights, when most people are asleep.  I worked
the second shift for many years and encountered these conditions
during the homeward-bound commute".

"I found that they were most common, of greater duration, and with
longer-distance propagation during the first two weeks of October.
I also noticed that they were much more likely to occur on nights
when a (mostly low) ground-fog had risen and was very wide-spread
and thick, but very clear above.  It also seemed that the sky was
very clear and with good visibility of stars and distant
high-altitude aircraft.  The stars even seemed to have an extra
twinkle to them".

"One night we had a round-table going on one of the shore town
Repeaters, with stations from Nova Scotia to Kitty Hawk, NC.  One
guy was on a Hand-held in his garage in Virginia Beach.  I could
copy all the stations on the input frequency on my mobile dual-band.
The opening started fading out about 0330 hrs EST".

"I never did find out why the ground-fog was so indicative of these

"I'm glad to be working the day-shift, but I sure do miss those
Autumn midnight commutes".

Ken responded, "I don't have decades-long experience with fall tropo
conditions, but Leo described the conditions to a 'T.'  It was
virtually dead calm.  I also noticed the sky overhead was unusually
clear.  The stars did not twinkle and seemed a bit brighter than
usual, perhaps because convection currents were minimal.  We do get
occasional ground fog here at this time of year but I didn't see
any.  As it cooled on my hilltop that evening the formerly clear
conditions gave way to a soaking fog at about 9:30.  I was about 8
miles north of the beach and two miles west of Narragansett Bay".

"I think ground fog is not just an indicator.  It's a medium with
higher mass than the clear air above.  It slows the lower part of
the wavefront relative to the clear air above, tending to bend the
wave along the surface of the earth.  Leo's observation of a
'...(mostly low) ground-fog had risen...' seems right on".

"As far as being not all that rare, well, I'll pay closer attention
from now on.  It was still a fun contest.  And while I have
experienced more extensive tropo propagation than that, I don't
think I've ever heard so much widespread activity on two".

Walt commented, "I vaguely remember someone telling me (years ago)
that he thought the ground-fog could be (but not always) an
indication of a temperature inversion or some other atmospheric
anomaly that created some kind of ducting which quite often ran
parallel to the shore line.  Thinking back, I did notice during
those events that the propagation was no where near as long-distance
east-west as it was north-south.  I wonder if that extra twinkle I
thought I noticed in the stars was also there because of some
atmospheric thing going on?"

Tim Hickman, N3JON confirmed that his long-path contact with VK4MA,
mentioned in last week's bulletin, was in fact on 20 meters.

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

Instructions for starting or ending email distribution of this
bulletin are at

Sunspot numbers for September 24 through 30 were 32, 25, 14, 11, 11,
14, and 11 with a mean of 16.9.  10.7 cm flux was 74.6, 72.4, 71.8,
72.3, 73.2, 72.3, and 72 with a mean of 72.7.  Estimated planetary A
indices were 1, 2, 3, 8, 8, 2 and 4 with a mean of 4.  Estimated
mid-latitude A indices were 1, 1, 3, 8, 6, 0 and 5 with a mean of


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