ARRL

ARRL Propagation Bulletin ARLP017 (2010)

SB PROP @ ARL $ARLP017
ARLP017 Propagation de K7RA

ZCZC AP17
QST de W1AW  
Propagation Forecast Bulletin 17  ARLP017
From Tad Cook, K7RA
Seattle, WA  April 30, 2010
To all radio amateurs 

SB PROP ARL ARLP017
ARLP017 Propagation de K7RA

Our Sun has again become very quiet.  We saw 13 days with no
sunspots, April 15-27, then new sunspot group 1063 appeared April
28, and on April 29 it was gone again. Although the sunspot number
for April 29 is zero, early on April 30 I can still see group 1063
in a magnetogram, so perhaps it rises again.

We watch the STEREO mission at http://stereo.gsfc.nasa.gov/, looking
for bright hints of activity, but lately they just seem to be
magnetically complex areas that don't turn into sunspots.

With no sunspot expected for today, the last day of April, this
should be the second monthly decline in a row for monthly sunspot
averages.  Monthly averages of daily sunspot numbers for February,
March and April are 30.5, 25.2 and 10.8.

USAF/NOAA predicts solar flux of 76 for April 30 through May 6,
rising to 80 on May 7-8, 78 May 9, and 75 after that.  They also
predict higher geomagnetic activity for May 4, with planetary A
index from April 30 to May 7 at 5, 5, 5, 8, 18, 10, 5, and 5.
Geophysical Institute Prague says April 30 through May 2 should be
quiet, quiet to unsettled May 3, unsettled May 4-5, and quiet May 6.

This week many comments came in about the volcanic ash cloud in
Europe and possibly effects to VHF propagation, perhaps due to lack
of aircraft in the sky.

Brett Graham, VR2BG wrote, "No aircraft in the air removes a lot of
reflective surface, though 'aircraft bounce' isn't generally thought
of at lower frequencies.  This is a bit like an equation with
several variables, somewhat difficult to solve.

"Trails of meteors somehow change due to presence of ash, in a way
that seemingly affects their reflectivity at higher frequencies but
not so noticeable at lower frequencies.  Did the ash cloud reach
that sort of altitude?  If the trails are in the E-layer region,
then it is about 90-150 km up.  Maybe just a little ash gets that
high up and could have an effect.

"Though aircraft do provide more reflective surface at higher
frequencies, they will move into and out of random points in the
sky, potentially including area where reflectivity is needed for a
particular path.

"The big aircraft usually move quite a bit faster than the wind,
which is what moves meteor trails around and causes the Doppler
shift seen when working meteor scatter.  Of course, Doppler is a
function of frequency, but perhaps could help reduce the rather big
variable of a lot less metal up in the sky.  Ah, the black magic of
RF - will we ever figure it all out?"

Brett sent along this instructive link:
http://www.imo.net/radio/reflection.

Bill Echols, NI5F wrote, "Will the strongly reduced amount of air
traffic over Europe affect the ability to complete MS QSO in as
rapid a time as usual? In other words, does it have to be ash
attenuation making the difference?"

Budd Hippisley, W2RU wrote, "How about the total absence of aluminum
aircraft in the European air space?" in response to the assertion
that volcanic ash was attenuating signals.

Likewise, Don Kerns, AE6RF wrote, "Degraded propagation in the 4m
band during the time the ash was active MIGHT have been due to the
stark DECREASE in air traffic. 4m should 'bounce' off of airliners
pretty well. If there were fewer of them in the sky during the same
time period."

Dan Zimmerman, N3OX wrote, "I don't know if the procedures in meteor
scatter operation might rule out airplane scatter, but it seems like
it might be the simplest explanation.

"I get a lot of what I assume is airplane scatter around here ...
pretty useful in contests, because I have low power and low
antennas.

"I think that all the Doppler shifted traces in this spectrogram of
WA1ZMS/B on 2m are airplanes,
http://n3ox.net/files/WA1ZMS0042Z082309.jpg."

Dan sent along some information on airplane scatter here.  You can
read it on the web at,
http://www.qsl.net/oz1rh/troposcatter99/troposcatter99.htm#Toc458130270.

J.D. Erskine, VA7OTC wrote, "While attenuation of the signal from
the one mentioned satellite might provide an independent space
signal, signal from outside the atmosphere, for comparison with
terrestrial VHF signals, two events occurred during this period. As
well as the presence of volcanic ash, there was an absence of
aircraft, a significant one at that. Might that have as much to do
with the attenuation, real or apparent, of longer distance reception
of lower frequency VHF signals?"

Gene Zimmerman, W3ZZ writes the monthly "World Above 50 MHz" column
for QST, and cc'd us on an email to LA4LN, mentioned in last week's
bulletin.

"Some have speculated that the volcanic ash might affect VHF
propagation both in terms of MS (meteor scatter) and in terms of Es.
While random MS conditions are reasonable in April, Es is usually
quite poor. Yours is the first report I have seen that indicates
that MS was significantly poorer in regions covered by the ash
cloud.

"You have indicated why a mechanism to explain any effect of
volcanic ash would be difficult to find. This volcano has driven ash
to only ~6 km heights. MS reflections occur at ~60 km or 10x higher.
As you accurately point out, 4 meter signals are much longer
wavelengths than the diameter of the ash particles. I see no
mechanism by which the ash cloud could affect gas ionized by meteor
burns in the E layer where all the refraction is taking place. It is
also difficult but less so to postulate that the fairly dense ash
particle cloud could scatter the signals refracted from meteor
trails such that FSK441 would detect them but not be able to decode
them. However you indicate that many fewer signals were even
detected and NOT that these were more difficult to decode."

In relation to reflections from aircraft, John Sahr, WB7NWP built a
passive radar system that correlates FM broadcast signals with their
reflections from aircraft and other objects in the sky.  Read about
it on the web at, http://rrsl.ee.washington.edu.

Chip Margelli, K7JA, writes from Garden Grove, California (DM03xs):
"I worked Fred, KH7Y in Hawaii around 2010 UTC on April 24th on
50.110 MHz. His signal came up to over S9, and this is rather early
for us to work Hawaii on 6 meters. I hope it is a good omen."

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 web page at,
http://arrl.org/propagation-of-rf-signals. For an explanation of the
numbers used in this bulletin see,
http://arrl.org/the-sun-the-earth-the-ionosphere. An archive of past
propagation bulletins is at,
http://arrl.org/w1aw-bulletins-archive-propagation.

Monthly propagation charts between four USA regions and twelve
overseas locations are at, http://arrl.org/propagation.

Instructions for starting or ending email distribution of ARRL
bulletins are at, http://arrl.org/bulletins.

Sunspot numbers for April 22 through 28 were 0, 0, 0, 0, 0, 0, and
12 with a mean of 1.7. 10.7 cm flux was 76.1, 75, 74.2, 75.4, 76,
74.8 and 76.1 with a mean of 75.4. Estimated planetary A indices
were 6, 13, 8, 3, 2, 4 and 4 with a mean of 5.7. Estimated
mid-latitude A indices were 4, 7, 8, 2, 0, 3 and 2 with a mean of
3.7.
NNNN
/EX