Propagation expert I am not...but, I might have some answers...please
forgive me if someone has already responded since I'm reading through
the archives. I acknowledge a quick and insightful discussion with
Erhan Kudeki, who is an expert on the ionosphere.
In an anisotropic medium (such as a magnetized plasma like our
ionosphere), there may exist more types of propagating wave than an
ordinary TEM "plane" wave. Recall that in a TEM wave, the electric
and magnetic fields are transverse (normal, at right angles to) the
direction of propagation. This is the "ordinary" or "O" mode. In the
case of a uniaxial medium (the special case where the medium is
"uniform" along one direction, in this case the geomagnetic field),
there is a second mode called the "extraordinary" or "X" mode. The X
mode has components of E or H in the direction of propagation. If
you've ever seen the "double image" when looking through a quartz
crystal, it's an example of such anisotropy.
Magneto-ionic propagation (a fancy keyword for what was described
above when it happens in the ionosphere) has been modeled through the
famous Appleton-Hartree Equation, which is too dense to repeat here.
It relates refractive index (essentially a measure of the wave
velocity) to the propagation direction and, radio, gyro, and plasma
frequencies. Where there are discontinuities in the refractive index,
there is no propagation.
Ionosondes (aka "Digisondes") differentiate between their O and X mode
returns. It's instructive to look at them, particularly during the
time of interest. Here's one in the Czech Republic:
http://147.231.47.3/
This is a good example for the following discussion, if it's still
archived on the Digisonde when you read this:
http://147.231.47.3/cgi-bin/UniSearch.exe?Data=1&Column=3&Row=5&Width=240&ImageOrder=ascending&Page=3&SelectedPic=/IonoGIF/PQ052_2007186024500_IO.PNG
If you look in the post-midnight time period, you can see the red (O
mode) and green (X mode) traces quite clearly. The red trace always
extends lower in frequency than the green. The scraggly "double"
traces above 400 km or so are multiple reflections (ionosphere,
ground, ionosphere).
The example above illustrates the attenuation of the X mode at lower
frequencies. Although, this is not universally the case, as can be
seen in other ionograms during this night. This attenuation is
primarily due to reduced group velocity (notice the slight upward
curvature of the X mode trace at lower frequencies). In this sense,
the X mode had "more opportunity" for dissipation.
You can see that even over the course of one night, this condition
changes. So, I'm not entirely sure how useful or practical it is to
try to "match" the propagation mode. I don't have a good feel for how
much power is lost to the X mode, either. Of course, that depends on
the geometry and ionosphere.
If you're interested in this topic, allow me to suggest the following
references:
Terman, F. E. Radio Engineering. (I have the 3rd edition; it's a
great, readable, reference for the amateur.)
Davies, K. Ionospheric Radio Propagation. NBS Monograph 80, 1965.
(Updated and more detailed than Terman.)
73,
--Ethan, K8GU/9.
http://www.k8gu.com/
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