[HCARC] Solar Activity - The Bottom Line

[Kerry Sandstrom]

Gary, N5BAA, told me at the May club meeting that I hadn't provided a 
bottom line to my April talk on solar activity.  I guess that's 
correct.  i probably didn't emphasize what a new ham should be looking 
for.  This is the bottom line in a few parts.  The first part 
re-emphasizes the need for detailed record keeping.  The second part 
expands a little bit on MUF, Maximum Usable Frequency.  The third part 
describes the normal daily cycle on the HF bands.

Detailed record keeping is a big help in your determination of what to 
expect with your station at your location.  It also lets you see how 
changes you make affects your station performance.  Without detailed 
records, you will never know if you have made improvements or not.  Not 
only should you keep track of who you work, but also who you hear and 
what the solar/geomagnetic conditions are at the time.

MUF is used in several different ways by different people.  f_0 F_2 is 
the vertical incidence critical frequency for the F_2 -layer , the 
highest frequency that will be returned to earth.  If you are interested 
in NVIS, Near Vertical Incidence Skywave, this is the parameter you are 
interested in.   As the distance increases, the MUF also increases until 
you reach the maximum single hop F-layer distance,  approximately 4000 
km/2500 miles.  This assumes a takeoff angle of 0º, hopeful but unlikely 
for any ham station.  Typical takeoff angles are 30º or more and the 
maximum single hop distance is considerably less than 4000 km.  
Distances beyond your maximum single hop distance require 2 or more 
hops.  The MUF for multiple hops is lower than for single hops and the 
signal losses are higher.  Stations with antennas high above the ground, 
1 or 2 or more wavelengths, will have lower takeoff angles and have 
better signals working DX.  Stations with vertical antennas and good 
ground systems will also have lower takeoff angles.  When you make 
changes in your antenna, records of who you have heard and their signal 
strengths are the only practical way you can compare the performance of 
your antenna before and after you make changes.

The normal daily cycle of an MF/HF band is as follows.  Early morning, 
well before sunrise, the MUF will be at its minimum.  You will not hear 
close in stations on skywave.  As sunrise approaches, you will notice 
signals from the east will start to come from closer and closer 
locations.  This happens very quickly between the half hour before 
sunrise and the half hour after sunrise.  The band will be fairly stable 
for the rest of the day.  On the lower frequencies, the signals, 
particularly from the farther distance, will become noticeably weaker 
during the day as the D-layer becomes denser and the absorption 
increases.  This is very noticeable on 80 and 40 m and not noticeable on 
10 m.  As sunset approaches, the reverse process occurs, the closer in 
stations from the east start to drop out and eventually the stations 
farther to the east drop out. Eventually the close in stations from the 
west drop out and eventually the west coast stations drop out.  The 
stations from the east coast sometimes last well beyond sunset and the 
west coast seems to hang in there until quite late.  On 40 meters you 
will hear stations talk about the band is "going long" in the evening.  
This is when the close in stations start to drop out because the MUF is 
dropping.  Once in a while you will hear them talking about the band is 
"going short" in the morning.  This is when the absorption is starting 
to increase and the more distant stations are fading out. Because the 
distant stations signals go through the ionosphere at a smaller angle, 
they are in the absorption area longer and experience more absorption.  
NVIS signals, on the other hand, go through the D-layer at a very steep, 
near vertical, angle and experience much less absorption.  This 
situation occurs on every one of our HF bands.

Here in South Texas, we are fortunate in that we are close to the 
equator.  The MUF is usually higher near the equator for a variety of 
reasons.  Because of our location, we can easily work South America, the 
South Pacific and South Africa.  Unfortunately we are double hop or more 
from Europe and Asia so we have a more difficult time working them than 
stations on the east and west coasts. Conditions seem to generally be 
best during the spring and fall equinoxes.  Noise levels seem generally 
down and propagation is good in both the northern and southern 
hemispheres.  If you want to work stations in the southern hemisphere, 
these are the best times of the year.  The MUF on cross equatorial paths 
is higher than any other path and we, in South Texas, can use 15, 10 and 
even 6 meters when most of the country thinks these bands are "dead".  
Not only can we work DX on these bands, we can also work other US 
stations via backscatter where both stations point there antennas at a 
common point in the Soth Atlantic or South Pacific and bounce signals 
off this common point.

So in summary, depending on the sunspot number, you should choose your 
band.  The higher the sunspot number, the higher the band.  In the 
morning, look towards the south east, east and north east.  Mid day is 
good any direction and the higher bands to avoid absorption on the lower 
frequencies.  In the evening look towards the north west, west and south 
west.  South is good any time of day and most of the night.  Late at 
night and early morning. 80 meters is the best choice unless you have a 
40 meter antenna with a low takeoff angle, ie. not a low dipole!

If you have any questions, just send an e-mail.

Kerry