[TowerTalk] ionosphere and ground reflections of radio signals

jimlux jimlux at earthlink.net
Mon Apr 26 10:23:01 PDT 2010

```Al Williams wrote:
>> When it comes to "pure" reflection you are dealing with the normal
>>  >reciprocity.
>> When you are dealing with a wave traveling through a charged media
>> interacting with a magnetic field,
>> such as the ionosphere, now the reciprocity is no longer there.
>>
>> Hans- N2JFS
>
> I can't recall, in the dozen or so books on antennas etc. that I have, an
> explanation of the reflection mechanism.  Maybe it is in there in the math
> but not in more simple terms. I vaguely remember, but not where I read it,
> that the traveling wave into a ionosphere media gets bent because of the
> action of the wave on the existing electrons (or atoms?). But is it because
> of the electric or magnetic field? And what about the reflection from the
> ground --electric, magnetic field, horizontal or vertical polarization,
> etc.Why is the exit angle identical to the entrance angle.

the simple rules (e.g. angle of incidence = angle of reflection) are for
boundaries between isotropic media, which isn't a bad model for things
like air/soil.  As to "WHy this is so", that gets a bit trickier.

Isotropic means that the direction of the field doesn't change the
properties

For anisotropic media, this isn't the case.  The doubled image looking
through calcite crystals is an example.. the different polarizations of
light travel at different speeds.  A polarizer is another example (the
loss is greater for one polarization over another).  And those are
simple cases.  When you get to something like clumps of electrons in a
partly ionized gas (e.g. the ionosphere) it gets worse.

Most of the effects you see in radio propagation are from polarization
dependence (that is, the ray takes a different path if polarized one way
vs another)

A circular polarizer is a fascinating case.   A left hand CP wave
incident on a reflector comes back as right hand CP.  The new Real3D
movies use CP in the lenses of the glasses you wear, so if you look at
yourself in a mirror, the lenses look dark (because of the polarization
flip)

However, polarized sun glasses are linear pol, so you don't see the effect.

Anyway, for the ionosphere, the virtual reflecting height (it's actually
a bending refraction, but you can think of it as a reflection) is
different depending on the polarization.  So, if you have a ground
antenna looking at a reflected skywave, it actually sees more than one
incident signal coming from more than one elevation angle.  And, because
the propagation path length for the different paths are different, the
signals may combine destructively or constructively, depending on the
phase difference.  That would look like fading.

If you have an antenna that is primarily polarized one way (like most
ham antennas with any gain), then the "wrong pol" is suppressed, so its
effect is small (that is, say it's down 10dB.. and assume you have two
paths of equal strength... if the two paths are in phase, you see 1.1
times the single path, if they're out of phase, you see 0.9... either
way, it's a small change)

Now start looking at a multihop path, and it gets "real" complex,
because the polarization can rotate on the way through the ionosphere.

>
> Understanding radio waves and propagation is a rather challenging and slow
> process for me. It doesn't help to simply state that the action is identical
> to light rays!
>
> k7puc
>
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```