[TAC] Propagation - or why does HF sound that way?

[Hank Kohl K8DD]

 From qrp-l and the propagation guru:


Gang,
There is a coronal hole on the sun right now that is producing above
normal solar wind velocity. Normal "background" solar wind is about
350 km/sec., and today we've been seeing over 700 km/sec.  This
increased solar wind pushes against the earth's magnetic field, squashing
it more than normal, which produces currents flowing along our magnetic 
field lines.  Tens of thousands of amps flying along these field lines
above our heads produces higher than normal noise on HF. The lower in
frequency, the more noise is heard.  This will dictate the noise on
40M tonite for the FOX hunt - not the solar flux.
The solar flux is a measure of the ionizing energy put out by the sun.
The higher the solar flux, the more ionization that takes place in the
D, E and F layers.  During the active sun, when solar fluxes are 150,
200, 250 or more, the E and F layers are highly ionized and hence,
highly reflective to HF signals (which is why the MUF is much higher
during than active sun).  Also, with such high ionization, the E/F
layers remain very reflective for many hours after the sun goes away
at sundown.
Right now, with solar flux around 100, ionization is weak, the E/F layers
are not very reflective, and the resulting MUF fairly low.  Furthermore,
when the sun goes away, the ionization of the E/F layers quickly halts,
the free electrons quickly recombine with their host atoms, and the
reflectivity quickly reverts to what the night time MUF will be.  This is
why during the active sun, 20M is open around the clock, 15M stays open
1-2 hours after sundown.  Because of the high reflectivity of the E/F
layers, even after the sun goes away.  But right now, all the frequencies
above 10MHz dies by shortly after sundown, because the ionization is so
weak during the quiet sun.
If you learn anything about propagation, make it this:
The solar flux has virtually NOTHING to do with the ham bands below the
MUF.  Generally, this is around 10 MHz after sunset right now.
40M is NOT affected by the solar flux, except during the mother of all
flares that penetrates and ionizes the D-layer, which usually produces an
all-out HF blackout.  That is rare, and ever more rare-er now that we're
in the quiet sun.  Also ... the solar flux, again, is a measure of the
ionizing radiation from the sun.  DURING THE NIGHT TIME HOURS, there
is NO ionizing radiation from the sun doing anything.  So, solar flux
has nothing to do with 40M propagation, generally, and certainly has
nothing to do with it at 8 p.m. when the sun is not even shining on the
E/F layers above your head.
So -- for 10 MHz and above, look at the solar flux to see what propagation
will be doing.  For 10 MHz and below, propagation is dictated mostly by
the noise being generated by our geomagnetic field.  That is, look at the
K-index for 40M.  If our geomagnetic field is active or in storm levels,
K4 or higher, there will be gobs of noise and QRP "propagation" difficult,
not because of poor reflection, but due to noise and signal absorption.
If the geomagnetic field is quiet, K=1,2 or 3, normal, seasonal noise
levels will exist and "propagation" will be good, due to the low noise
floor of the band and small signal absorption.
And finally, we can't blame all the 40M noise on the sun.  On a quiet
night, with K<4, there is still lots of noise.  This is caused by a huge
array of electro-magnetic (EM) devices within hundreds of miles of you,
everything from EM noise generated by the high voltage AC lines all over
the place, to that welding shop down the street, and never mind the
computer, monitor, and game-boy running in the next room.  You mix 43
jillion sources of low level noise like that together, and you end up
with many dBs of "white noise" all over the HF bands.  Most of these
propagate via ground wave, so you have to blame every single living
human consuming electricity within several hundred miles of you! :-)
Right now, K=3, for a slightly unsettled geomagnetic field, and the
noise levels on 40M tonight should be about normal.  However, with the
weather, more people using electricity, thick, moist clouds forming a
canopy above your head, noise may be higher.  But you can't blame the
sun, and certainly not the solar flux, on that part.  And you certainly
can't blame the sun if it ain't shining -hi.
Have a good FOX hunt.
72, Paul NA5N

-- 


'Never argue with an idiot. They drag you down to their 
 level then beat you  with experience.'           -anon