Satgen232   Satellite Propagation Worldwide Part 5  by GM4IHJ   4 Sept 93
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The Atmosphere.   Anyone who has watched the Sun change shape as it
approaches the horizon, has seen a mirage. Most of the Sun has set below
the horizon but it all remains visible because the light coming from the
sub horizon Sun is bent around the curvature of the earth by the thick
lower atmosphere. Light is just that small part of the radio frequency
spectrum which is detectable by the two receivers in our heads. All radio
signals transitting through a variable density atmosphere will be bent. So
it is no surprise that when geometry says RS12 has gone below your
horizon, you can still hear it, perhaps for 2 or 3 minutes provided you
have an unrestricted zero degree elevation clear horizon.This bending of
signal, is sometimes due to atmospheric anomalies, it may be local or it
may form a long duct hugging the ground, thin ducts propagate only high
frequency signals. High ducts propagate much lower frequencies ie 29 MHz.
The above generalisation results in duct propagation / bending being most
common when the earth's surface is smooth and unobstructed. So tropical
stations have an advantage over stations in the latitude belts 40 to 70
degs N or S, because seas are calmer and winds are lighter. DX paths
across tropical seas can extend for thousands of kilometres particularly
on calm humid evenings. This is particularly true for terrestrial
microwave signals. The trick is to get your antenna right down on the
water surface, (or on the height adjustable periscope of a nuclear sub).
Then a man wading in the North Sea off Norfolk England with a dish inches
above the surface can regularly put a microwave signal into Holland across
the sea, to a man similarly standing in the sea. But if either of them
begin to move up the beach and go inland they rapidly lose contact. Trying
the same trick from stormy Scotland rarely brings success. All of which
helps the terrestrial DXer but is hopeless for the satellite user who
wishes to get through the roof of any duct and into space. For the sat
user poor ducting conditions are good news. This is particularly noticable
when trying for a geosat at low elevation from a position on or near the
sea at say latitude 65N or S. The book says geosats can be "seen " up to
75 degrees or so but in practise ducting can make this impossible. Over
the Equator by contrast the geosat is at very high elevation so the signal
has no problem penetrating vertically through the roof of a duct.
SCINTILLATION . This is a phenomenon equivalent to signal multipathing -
twinkle twinkle little star is the "visible version " of scintillation of
a radio signal. This effect occurs in the ionosphere and in deep space ,
indeed anywhere there is ionised gas plasma. The solar wind from the Sun
is the worst offender as it spreads through space. Some of the plasma gets
into the polar ionosphere down open magnetic field lines and it breaks up
satellite signals at high latitudes. Middle latitudes suffer least, but
another form of scintillation is present which affects Equatorial
satellite signals. Scintillation almost totally ruins some HF terrestrial
and some VHF/UHF/microwave satellite packet radio, particularly at slow
keying speeds. There is some evidence that higher keying speeds are less
affected, but more research is necessary to definitely prove this.
General Comment. When asked to write these world wide satellite
propagation notes. GM4IHJ quickly discovered that there is almost no
published information on Equatorial or Southern Hemisphere satellite
propagation. GM4IHJ would very much like to hear from anyone from those
regions who has information/comment on this subject. 73 GM4IHJ @ GB7SAN