Jerry Martes wrote:

wrote in message
ups.com...
On Mar 28, 3:25 pm, "Jerry Martes" wrote:
Hi

I want to find a place where I can learn about the effects the
atmosphere
has on the polarization of signals from satellites. Does anyone know of
any web sites where that might be discussed?
Thanks
Jerry


Referred to as Faraday Rotation.. ARRL handbook talks about it in the
EME section. Rotation is less as frequency increases (so VHF is
affected a lot more than UHF is affected more than microwaves)
Amount of rotation is determined by the total electron content in the
path, which, of course, varies with everything you can imagine, but
primarily the amount of sunlight.
It's related to the difference (dispersion) in propagation speed with
frequency (or, index of refraction, if you want to work it that
way)..

More than 360 degrees of rotation isn't unusual at VHF.

What's really interesting is that it rotates the same way no matter
which direction you go, so if you had a planar reflector above the
ionosphere, and you launched a wave that's say polarized North-South,
and there's 45 degrees of rotation, when it came back down it would be
rotated 90 and be polarized East West...

This is one reason why GPS radiates on more than one frequency (you
measure the phase difference between the two, and that allows you to
estimate the TEC, which in turn allows you to estimate the change in
propagation velocity, which improves the accuracy of your fix.)

Most high accuracy orbiting radars that operate in P or L band do
something similar, especially if they are making polarimetric
measurements (handy if you want to do things like distinguish between
trees and grass, for instance)

How much detail do you need? I can probably give you URLs to cover
everything from simple estimates of magnitude to mind bendingly
detailed theoretical discussions.


Hi Jim

I am considering the design of a horn to illuminate an 8 foot diameter
solid surface dish at 4 GHz, for reception of geosynchronous satellite
signals that are linearly polarized.

One rraa reader has informed me that the rotation is refered to as Faraday
rotation and gave some links to it. I see that the amount of rotation
may be small enough to be negligable for my application.

I would like to know how rapid the rotation changes with time.

That depends on geomagnetic activity, because Faraday rotation is also a
function of the earth's magnetic field.

With a signal that started out linearly polarized, there are three
sources of change:

1. Simple geometry - if a geostationary satellite orbiting above the
0deg meridian has an antenna that is horizontally polarized when viewed
from a ground location on the same meridian, that same antenna will
appear vertically polarized when viewed from a location 90deg west. This
effect is easily calculated and compensated.

2. Faraday rotation, as above.

3. Dispersion ("smearing") of the linear polarization, at times of high
geomagnetic activity. From experience with a 432MHz array that had fully
rotatable polarization, polarization could range from accurately linear
at geomagnetically quiet times (with a very deep null at 90deg offset),
all the way to dispersion of the signal around the whole 360 degrees. A
completely dispersed signal will have a 3dB loss compared with
accurately aligned linear polarization, so you have to include that
possibility in your path loss budget. (At lower frequencies, dispersion
is also accompanied by ionospheric absorption, but at 4GHz you may not
need to worry about that.)

Bottom line: if you have enough signal/noise ratio to handle a permanent
3dB loss, then it will be much more convenient to ignore the fact that
the signal started out linearly polarized. Instead, use a
circularly-polarized feedhorn which will be insensitive to the
polarization of the arriving signal.


--

73 from Ian GM3SEK