[Elecraft] Propagation Modes

[Douglas Westover]

I must disagree, in part,  with AC7ACs assessment of propagation
modes. The most common modes are, indeed, earth / ionosphere
"reflections" (actually refractions). Part of the problem here is
the simplistic "copper sheet" model of the ionosphere which
makes the assumption of specular reflection of the wave.
In fact the ray is refracted in the ionosphere with the refractive
index varying as a function of electron density which, in turn,
is a function of height. Therefore, the ray is "bent" within the
medium and in fact does travel further than the specular
reflection model would suggest but is returned to earth at
the same angle that it entered the ionosphere. Multiple hops
are easily verified by oblique back scatter sounding records
which are made routinely as part of the propagation
prediction process.

However, If you do the path geometry for a particular "hop" distance
you can easily compute an equivalent, or "virtual"  height based
on the specular reflection model. Then by integrating the
refractive index over this "virtual path" you can compute the
group phase path length (Breit and Tuve's Theorem) and from
there obtain the "true" height of the ionosphere.

That was the easy part, there ARE modes which don't behave
in such a well defined manner. If in addition to the electron density
varying with height  it also varies with range along the path then
you can, indeed, get longer than expected hops (often called
ionospheric "tilts"  which are most pronounced when there is
sunset or sunrise along the path). You can also get "cordal modes"
if the refractive index profile is "just right" which is what I think
Ron was probably referring to. Another interesting mode is one
where the ray is trapped in the ionosphere, again due to the
"just right" electron density distribution and you can get
multiple transits of the earth before it finally exits. Think of
round-the-world propagation and, possibly, so called long
delay echoes. 

Oh, by the way, if you're dealing with Es (sporadic E) the
"copper sheet" specular reflection model works just fine
because the electron density is so high and for all practical
purposes you can assume a reflection height of 100km.
And, of course, it is true that a path with the fewest earth /
ionosphere transitions will produce the strongest signals.

Doug
W6JD