Friends in Radio Land-
I do believe that geometers have shown centuries ago that the
shortest distance between two points on a sphere is the great-
circle distance between those points obtained by the intersection
on the spherical surface of a plane passing through the two points
and the center of the sphere. Given the total circumference of
great circle made in that manner, the longest distance between the
two point is that along the other part of the great-circle. For
our purposes, the former is what we call "short-path" and the
latter is what we should call "long-path", in the direction
opposite to short-path. Physically, those paths would be followed
in the ionosphere if there were no variations in electron density
across, i.e. perpendicular to, the path direction.
Any other RF paths between those two points are deviated from the
great-circle paths. Ionospherically, deviations of radio paths
between two points occur because of good and sufficient physical
reasons. Since propagation take place because of either wave
scattering, reflection or refraction, the problem is to identify
the cause and location of any deviations from great-circle paths.
Well-known causes of deviations are found during auroral displays
and magnetic storms when energetic electrons precipitate on the
atmosphere. Depending on their geometry and extent, those
electron events may give rise to scattering or wave reflection.
In the former, there are concentrations of electron density which
are small in extent compared to the wavelength under consideration
while during reflections, the electron density in the region shows
a large variation over a distance comparable to a wavelength.
Another example of reflections is of atmospheric origin, the case
of sporadic E reflections where an ionospheric region appears to
be more metallic than a lightly ionized dielectric.
Wave refraction takes place when the electron density in the
ionosphere changes only by a small amount over regions with
dimensions comparable to a wavelength. Surfaces of relatively
constant electron density, as used in propagation prediction
programs, are assumed to be parallel to the surface of the earth.
That condition, as well as the one involving small variations, is
not always fulfilled, particularly over the large regions required
for waves on the 160 meter band, because of disturbances in the
neutral atmosphere. As a result, irregularities may be present
which either tilt the regions of constant electron density, thus
changing the direction of wave refraction, or breaking up the
region, bringing the wave refraction closer to the one involving
scattering, less efficient but still with the majority of the
radiation going in the forward direction.
Some causes of wave deviation are quite obvious while others are
very subtle, even defying the best efforts and searches for an
explanation. But path deviations do have physical causes; that
should never be forgotten in our discussions.
73,
Bob, NM7M
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