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Re: radio propagation



this is in addition to McInnes' discussion:

as BM mentions, the density of the plasma increases with height; this is
similar to grin lenses and wave guides. this results in refraction
which is the mode of "reflection."

A discussion of this is in Harnwell --Princs. of elec and Electromag.
2nd ed. (49) p 587 ff.

bc

P.s. a non-intuitive result discussed is that the ionosphere is
birefringent! (because of the earth's B field.)

Ben Crowell wrote:

I was looking through some of my father's ham radio books
recently, and came across something I didn't understand.
They describe how radio signals (we're talking
about 20-40 meters typically) propagate over long
distances by multiple reflections from the ground
and the ionosphere. (There are actually lots of
ionospheric layers, but let's keep this simple.)
They assume there's 100% reflection from the ground,
which makes sense, since it's a conductor. When the
radio waves hit the ionosphere, however, they describe
some behavior that I don't understand. Waves that strike
the ionosphere close to normal can be 100% transmitted,
while beyond some critical angle they're partially
transmitted and partially reflected. For this reason,
there's a certain radius within which it's not possible
to communicate. (Apparently the critical angle depends
on conditions, and under some conditions there's even
reflection for theta=0.)

This is obviously different from ordinary optics,
where there's always some reflection -- the reflection
decreases with theta, but never reaches zero. I
assume this unfamiliar behavior has something to
do with the fact that the ionosphere is a plasma.
Is this something that should be thought about in
terms of a complex index of refraction or
something? Is it analogous to reflection from
a solid metal conductor? I'm sure a lot of us
(me included, probably) could break out their
copies of Jackson and try to figure it out, but
part of the question in my mind is what are the
real physical parameters of the ionosphere. Also,
I'd be grateful for an answer that would be simple
enough to be intelligible not just to me but to
my students as well.