[HCARC] Radio Wave Propagation Question

[Kerry Sandstrom]

Gary,

There is not a simple answer to your question.

First, lets cover a few basic points.  What we call layers as in E-layer ,
F2-layer, etc are not really layers but just inflection points in the
electron density vs. altitude curve.  The D, E and F1-layers are well
behaved in that their distribution is smooth and peaks approximately at the
sub-solar point, that is the position on the earth directly under the sun.
This is seldom on the equator because of the earth's tilt relative to the
plane of its orbit around the earth.

 The F2-layer is far from well behaved.  It is strongly influenced by the
earth's geomagnetic field.  The earth's geomagnetic field is aligned with
the earth's rotational axis, ie. the geomagnetic poles aren't the same as
the physical poles.  While there are very simple equations that describe the
D-, E- and F1-layers, there is no simple equation to describe the F2 layer.
When one does ionospheric propagation predictions one uses a "map" of the F2
electron density.  In the old days these "maps" were published by the CRPL
(Cenral Radio Propagation Lab), now these "maps"  are digital files embedded
in the propagation programs such as VOACAP.

The ionosphere is ionized primarily by the sun.  The primary wavelengths 
involved appears to be EUV and soft x-rays.  The electrons are moved by the 
earth's magnetic fields.  After sunset, the electrons begin to recombine 
with the positive ions in the ionosphere.  This recombination occurs fairly 
quickly for the D- and E-layers but very slowly for the F-layers.  This is 
because the ion density at F-layer altitudes is so low.  During an 
ionospheric/geomagnetic storm, the earth's geomagnetic field is very 
disturbed and changing.  That is why HF propagation is so strange during a 
geomagnetic storm.

So, what happens after sunset?  First the D-layer quickly disappears after 
sunset.  The D-layer is the primary source of absorption so we see an 
increase in signal levels.  The E-layer is the source of a lot of the less 
distance signals.  It also quickly disappears.  The sun is no longer 
ionizing the F-layers but recombination is slow at F-layer altitudes so the 
F1- and F2-layers combine and the electron density slowly drops.  With this 
drop in electron density, the MUF (Maximum Usable Frequency) also starts 
dropping.  The first paths that close are the NVIS and 15/10 meter paths. 
Gradually as we get farther past sunset, the MUF continues to drop.  The 
higher bands go out first and the lower frequency bands like 40 m "go long". 
Remember the MUF is highest for the longest paths and lowest for the close 
paths.  What we find are the longest paths, DX and the east and west coasts 
are what we hear while the 5 region is only very local stations.  During 
winter nights at solar minimum, its not unusual for the MUF to be so low 
that you can't hear any CONUS stations.

The local "ground wave" stations on 40 m are the same strength all the time. 
The background noise, static/QRN, is lower at night, but the longer distance 
stations we couldn't hear during the daytime are now quite strong because a 
lot of the D-layer absorption is gone.  It is a challenge to work locals 
because the signals from longer distances are too plentiful and too strong.

Polar regions have their own unique characrteristics.  Yes, the MUF is down 
during local winter when the sun doesn't get above the horizon.  The charged 
particles from the sun carried by the solar wind to the earth are funneled 
to the two poles where they precipate down onto the earths atmosphere. 
These cause some increased absorption of signals going through the polar 
regions.  There is never a good time to work the poles.

The equatorial region is also different.  The ionosphere bulges outward over 
the geomagnetic equator (which is not the same as the physical equator) and 
seems to have a lot of variations in density which leads to what we call 
Trasn-equatorial Scatter (TE) which permits staions close to the equator to 
use 6 m and sometimes 2 m for long distance North-South propagation.  we are 
too far North for TE unless we have a sporadic-E path to get our signals to 
an area near the equator.

Our signals don't reflect off the ionosphere.  They are refracted/bent.  An 
effective reflection point is sometimes calculated, but this has nothing to 
do with reality.  Not only are our signals bent, but they are also split 
into an 'ordinary' ray and an 'extraodinary' ray.   These two different rays 
basically correspond to right and left circular polarization.  They follow 
different paths through the ionosphere and have different time delays. 
Sometimes thes delays are not very great and the effect is to rotate the 
polarization of the initial linear polarized wave after it recombines on the 
way back down.  It is possible to see the two different rays on ionograms so 
there is no question that it really occurs.

I know you want the bottom line so here it is.  After dark the propagation 
on our HF bands slowly change so the skip zone, the distance where we can't 
hear any ionospherically propagated signals, gradually increases until the 
signals no longer are returned to the earth.  This happens on the higher 
frequencies first, but it also happens on 40 m.  The absorption is lower 
which further benefits long distance signals.   Local ground-wave signals 
which were easy to copy during the daytime are now covered by QRM from the 
distant signals.  Propagation in the polar regions is difficult anytime.  If 
you can work a staion inside the arctic or antarctic circle, be happy. 
Propagation near the equator is better, however, the noise level 
(static/QRN) is higher because of normal weather conditions at low 
latitudes.

Kerry