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ARRL Propagation Bulletin ARLP037 (2006)

ARLP037 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 37  ARLP037
From Tad Cook, K7RA
Seattle, WA  September 8, 2006
To all radio amateurs 

ARLP037 Propagation de K7RA

Average daily sunspot numbers were down only slightly this week,
from 27 to 25.3. There were two days this week when the sunspot
number was 0, and we should see more zero sunspot days than we're
currently experiencing as we move closer to the bottom of the
sunspot cycle.

The last solar minimum was centered near October 1996, according to
the NOAA SEC graphs on page 11 of their weekly report at,

If you look at the average sunspot numbers reported in our
propagation forecast bulletins from September 13, 1996 to October
25, 1996 (located on the web at, you'll see week after
week of no sunspots.

Currently we're observing average daily sunspot numbers near 25 for
this week, 27 for last, 22 the previous week, and 34 prior to that.
According to weekly NOAA SEC sunspot predictions, on the web at,, these averages are
above the high end for this month and last, and the minimum is about
six months away. But look back on that 1996 index mentioned above,
at the numbers reported in this bulletin six months prior to the
last minimum. Weekly averages reported here during April 1996 were
near 12, 2, 7, and 20. Does this suggest the sunspot minimum is
further away, rather than next spring?

Also note on page 11 in that first reference in the second
paragraph, the spacing of the sunspot cycle minimums. From the end
of cycles 17 to 19 they are a little over ten years apart, from 1944
to 1954 and 1964. Then it jumps less than 12 years (from October
1964 to June 1976), and it goes back to a little over ten years from
1976 to 1986, and then 1996. Next spring will be a bit less than 11
years since the last minimum.

Right now is a fairly good time for long-distance HF communications,
because the geomagnetic field is mostly stable, sunspots haven't
disappeared, and we are close to the Autumnal Equinox. Running
intercontinental paths with ACE-HF propagation software shows very
little in the way of openings on the upper bands above 20 meters,
with the best bets on 40, 30 and 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, .

Sunspot numbers for August 31 through September 6 were 39, 32, 27,
0, 0, 26 and 53 with a mean of 25.3. 10.7 cm flux was 83.2, 76.9,
75.6, 76.5, 79, 80.4, and 84, with a mean of 79.4. Estimated
planetary A indices were 8, 13, 7, 6, 23, 8 and 7 with a mean of
10.3. Estimated mid-latitude A indices were 6, 8, 6, 6, 15, 7 and 6,
with a mean of 7.7.


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