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ARRL Propagation Bulletin ARLP028 (2001)

ARLP028 Propagation de K7VVV

QST de W1AW  
Propagation Forecast Bulletin 28  ARLP028
From Tad Cook, K7VVV
Seattle, WA  July 6, 2001
To all radio amateurs 

ARLP028 Propagation de K7VVV

It has been a quiet week.  There were no active or disturbed days,
and solar activity declined.  Average daily solar flux, compared the
past week to the previous, declined over 51 points and average
sunspot numbers were down over 86 points.  Activity will probably
reach a short term minimum on Friday and Saturday when solar flux is
around 115, then it is expected to rise back to 130 by July 10, and
near 200 around July 13-20.  There is no forecast for increased
geomagnetic activity, but emergence of new sunspots could change

With summer here, MUF (Maximum Usable Frequency) is depressed, and
about the only openings on 10 meters are E-layer skip, which is
somewhat sporadic and doesn't offer the longer distances of F-layer
communications.  20 meters is open well into the night.

In last week's review of Field Day, it was claimed that 40 meters
was the best band all around, but this drew a response from K7JA,
who reported that around 0300-0400 UTC on Sunday morning (Saturday
night on the West Coast, where he was) 40 meters went bad.  He also
reported that 15 and 20 meters were never good early in the day on
Saturday.  The 40 meter report in this bulletin was probably due to
talking with a local Seattle area club that had an excellent 40
meter antenna, plus the numbers generated by W6ELprop looked good.

Judging from recent email, it is probably time to repeat the
occasional explanation of the indices reported in this bulletin.

Amateur Radio operators who use HF generally like increased sunspots
because they correlate with better worldwide radio propagation.
When there are more sunspots, the sun puts out radiation which
charges particles in the earth's ionosphere.  Radio waves bounce off
of (refract from) these charged particles, and the denser these
clouds of ions, the better the HF propagation.

When the ionosphere is denser, higher frequencies will refract off
the ionosphere rather than passing through to outer space.  This is
why every 11 years or so when this activity is higher, 10 meters
gets exciting.  10 meters is at a high enough frequency, right near
the top of the HF spectrum, that radio waves propagate very
efficiently when the sunspot count is high.  Because of the shorter
wavelength, smaller antennas are very efficient on this band, so
mobile stations running low power on 10 meters can communicate world
wide on a daily basis when the sunspot cycle is at its peak.  There
are also seasonal variations, and 10 meters tends to be best near
the spring or fall equinox.

The sunspot numbers used in this bulletin are calculated by counting
the spots on the visible solar surface and also measuring their

Solar flux is another value reported in this bulletin, and it is
measured at an observatory in Penticton, British Columbia using an
antenna pointed toward the sun hooked to a receiver tuned to 2.8
GHz, which is at a wavelength of 10.7 cm.  Energy detected seems to
correlate somewhat with sunspots and with the density of the

Other solar activity of concern to HF operators are solar flares and
coronal holes, which emit protons.  Since the charged ions in the
ionosphere are negative, a blast of protons from the sun can
neutralize the charge and make the ionosphere less refractive.
These waves of protons can be so intense that they may trigger an
event called a geomagnetic storm.  In addition, energy from a solar
flare may energize the D-layer of the ionosphere, which absorbs
radio waves.

The Planetary A index relates to geomagnetic stability.
Magnetometers around the world are used to generate a number called
the Planetary K index.

A one point change in the K index is quite significant.  K index
readings below 3 generally mean good stable conditions, and above 3
can mean high absorption of radio waves.  Each point change reflects
a big change in conditions.

Every 24 hours the K index is summarized in a number called the A
index.  A one point change in A value is not very significant.  A
full day with the K index at 3 will produce an A index of 15, K of 4
means A of 27, K of 5 means A of 48, and K of 6 means A of 80.  You
can find an explanation of these numbers on the web at

The geomagnetic number reported here is the Planetary A index, which
is a worldwide average based on the K index readings from a number
of magnetometers. The numbers reported on WWV are the Boulder K and
A index, measured in Colorado.  Generally the higher the latitude of
the measuring station, the higher the K and A indices reported.
This is because the effects of geomagnetic instability tend to
concentrate toward the polar regions of the globe.  You can hear the
Boulder K index updated every three hours on WWV, or by calling

Sunspot numbers for June 28 through July 4 were 143, 98, 115, 108,
130, 132 and 106 with a mean of 118.9. 10.7 cm flux was 140.2,
139.9, 136.6, 135.4, 134.3, 131.9 and 127, with a mean of 135, and
estimated planetary A indices were 5, 7, 10, 12, 8, 9 and 8 with a
mean of 8.4.


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