Propagation Primer 101
Paul Harden - NA5N
Updated 01 July 1999
For those of you who seem hopelessly lost with all this solar and
propagation stuff, I will attempt a quick primer that will hopefully
We all know the sun goes through a solar cycle about every 11 years.
During the minimun, or QUIET SUN, there are few sunspots, the solar
flux is very low (<100), which means the sun's ionizing radiation is
quite low. As a result, our upper atmosphere, where the E and F layers
reside, are not well ionized. This means the E and F layers do not
reflect HF radio waves very well ... and most of your signals will pass
right on through to space to be picked up by Jodie Foster in the sequel
to "Contact." One measure of how well ionized our E and F layers are
is the MUF, or Maximum Usable Frequency. During the quiet sun, the MUF
is often below 15-18MHz. This is why 15M and 10M are "dead" during the
quiet sun, except for local (line-of-sight) communications.
However, during the solar maximum or ACTIVE SUN, there are many sunspots,
the solar flux is high, and this highly ionizes our ionosphere. This in
turn means our E and F layers become very reflective to HF signals.
Virtually all the power hitting the E and F layers will be reflected back
to Earth and Jodie Foster will hear nothing out in space. This high
reflectivity causes the MUF to rise, often to above 30MHz. And when
this occurs, 10M will be open all day long to support global
communications by using "skip propagation" ... in that your signals are
skipping (or being reflected) off the ionosphere back to earth.
OK ... now a couple of definitions:
SOLAR FLUX (SF) is a number that attempts to describe the total power
output of the sun at radio wavelengths, which in turn helps describe the
total ionizing power delivered to our ionosphere. The higher the SF,
the more ionization, and the more reflective our ionosphere is to HF.
An SF <100 is fairly poor propagation, the MUF <15MHz
An SF >150 is fairly good propagation, the MUF >25MHz
A general rule of the thumb is 10M is open when the solar flux >150.
IONIZATION. The solar radiation reaching the Earth contains IONIZING
radiation. This means the incoming solar radiation can rip electrons
away from the oxygen molecules high in our atmosphere. So now you have
all these "free electrons" roaming around that makes the upper atmosphere
(or ionosphere) more dense. Now the mass or weight doesn't change, it's
just denser. Think of a bunch of popcorn balls on a floor, and shooting
a marble through the open spaces without hitting a popcorn ball. Likely,
not hard to do. The marble represents your radio signal passing through
to space. Now go out there and stomp those popcorn balls so a bunch
of individual popcorn kernels are scattered all over the floor. The
mass of the popcorn has not changed, but it is distributed to make the
field more dense. Now try to shoot that marble across the floor without
touching a piece of popcorn. Gonna be very hard to do. The marble, or
your RF signal, does NOT pass on to space. In the real case, your RF
signal strikes all these free electrons, and that is what reflects them
back to earth ... DURING DAYLIGHT HOURS when ionization occurs.
Now the really amazing thing that happens in our ionosphere, is when the
ionizing radiation from the sun goes away (night time), all these free
electrons rejoin (or recombine) with their host molecules, making intact
oxygen molecules again. In our example, this would be like watching all
the popcorn kernels on the floor magically turning back into popcorn
balls again. (Woah ... what have you been drinking?) Of course, this
means RF signals will again pass through on to space and will not be
By the way ... when electrons are stripped away from oxygen, it turns the
oxygen molecules into helium. Another way of measuring the extent of
ionization is to measure the amount of helium in our upper atmosphere.
This is usually done through optical spectral line equipment or launching
high altitude instrument balloons. However, this is seldom done today
since other means and satellite surveillence is far more superior for
measuring the extent of ionization.
This is why the higher bands, such as 15M and 10M, are open (that is,
signals being reflected back to earth) during the DAYLIGHT HOURS, but
these same bands go dead (no reflective propagation) nearly as soon as
the sun sets - because the sun's ionizing radiation goes away.
This is also why these same bands tend to be completely dead during the
quiet sun, because there is insufficient ionizing radiation to cause
ionization for reflection. This is a phenomenon of the active sun, the
period we are well into right now. And, during a quiet sun, the
ionization can be so low, that the MUF drops below 14MHz at night, which
is why even 20M can go dead at night. During an active sun, the MUF
almost always remains above 15MHz even at night, which is why 20M often
becomes a 'round-the-clock band during the active sun.
So what about 40M? Truth is, the solar cycle has virtually no effect
on 40M or below. Propagation on 40M remains pretty much the same during
the active sun as it does the quiet sun, because the MUF seldom drops
below 10MHz. This is why 40M is the main nighttime band, year in and
year out. Even with low ionization, the very long wavelengths of the
lower frequencies will be reflected by the ionosphere. This would be
like rolling a basketball through the popcorn balls ... while the high
frequency RF (the marbles) pass through pretty easy, certainly the low
frequencies (basketball) would not. Quiet sun or active sun.
The active sun DOES effect 40M in that absorption to RF can be very good
to very bad, or very high noise levels from geomagnetic storms ...
both due to solar flare activity that occurs only during an active sun.
A large solar flare sends an extra dose of ionizing radiation to the
Earth. This can raise the MUF to very high frequencies (>100MHz), but
this radiation can also penetrate far into our atmosphere to ionize the
lower D-layer. RF signals must pass through the D-layer on their way
to the upper E and F layers, where the reflection occurs. The more
ionized the D-layer is, the more collisions that will take place with your
RF signal, absorbing or attenuating some of its power. Thus, high
aborption to HF signals can occur during and after a solar flare. This
would be like rolling that marble across the popcorn covered floor,
which encounters so many collisions with the popcorn that the marble comes
to a halt. Now that is total attenuation or absorption. Your poor little
QRP signals just vanish on their way to the E and F layers!
80M signals are almost always highly or fully attenuated by the D-layer,
and what "propagation" that occurs on 80M is actually by the signals
traveling across the Earth's surface, or "ground wave" propagation.
The wave front is confined between the Earth's surface and the D-layer,
which causes attenuation to the power as it travels along the ground,
skims the D-layer, and propagates through the dense atmosphere near
the surface. This is why QRP on 80M is challenging at best since the
absorption rates are fairly high - day and night, quiet sun or active.
The other major effect to HF propagation during the active sun is
geomagnetic storms. Very briefly, this is caused by a shock wave from
a solar flare hitting the Earth's magnetic field, causing it to compress
and wiggle for awhile. And while it's wiggling, it's generating huge
electrical currents, which in turn creates gobs of noise on HF. I'll
present geomagnetic storms in another lesson.
....BAND.... THE QUIET SUN.......... / ..........THE ACTIVE SUN
....80M.... Seldom has skip propagation.....Seldom has skip propagation
....40M.... Open around the clock.................Open around the clock
....30M.... Open daylight hours......................Open around the clock
....20M.... Open daylight hours......................Open around the clock (usually)
....15M.... Dead - no skip propagation..........Open - daylight hours only
....10M.... Dead - no skip propagation..........Open - daylight hours only
If you have questions about the above, I suggest you pose them on qrp-l
so I and others can answer them for the benefit of all those interested.
72, Paul NA5N
PS - I do not, however, fully understand the effects of the solar cycle
on dummy load operations, the effects on drilling steel, how it effects
the NC-20 AGC, or that Canadian Bouquet thing. :-)