Satgen461 Space Propagation Pt1   by GM4IHJ  (BID SGEN461) 1998-01-24

Most satellite and EME operators know, there is a deal of difference
between, the simple descriptions in amateur radio handbooks of the
theory of what happens to a signal in space or travelling through the
ionosphere, and, what actually arrives at the ground station receiver.
Signals can be disturbed by Faraday effect, Sporadic E, Field aligned
effects, Aurora, Scintillation, the Weather, and the Atmosphere.

Taking Faraday effect first . Does a signal coming through the
ionosphere simply have its phase rotated such that, EG vertical
polarisation at the transmitter appears as a signal with something
other than vertical polarisation at the ground station antenna ? No it
does not , as a simple experiment listening to a signal from space
with a ground antenna which can be turned to present varying angles of
polarisation, soon reveals.  But you have to be careful. A Sat over
Latitude X with its antenna pointing verticallly at the ground beneath
does not have the same vertical as a ground station antenna aligned
verticaly at Latitude Y. Many satellite operators ( particularly geo
weather sats and TVRO sats ops, adjust the antenna at the satellite to
match say the vertical at the centre of the ground service area of a
spot beam . So under a spot beam the theoretical alignment ought not
to be far out. But with a wide hemisphere beam on say a geosat you can
be 60 degrees out at a station at latitude 60 degrees. So be careful
even as you start simple one way experiments.

On TV signals at 10GHz Faraday rotation is not a problem. But at UHF
frequencies it is becoming a problem and by VHF frequencies it is a
serious problem on satellite signals which come from linearly
polarised antennas and also from those that come from supposedly
circularly polarised sat antennas which produce varying polarisation
the further you are off the sat antenna central axis. 

Worse still when you take the trouble to set up an experiment targeted
on a geosat fixed in your sky sending linear polarisation , and you
receive it on an antenna which can be turned to align it to the
strongest signal ie correct for the transmitted polarisation plus
Faraday rotation, you invariably get a shock . When you discover that
instead of the 20 dB difference in signal expected between correct
polarisation match and, 90 degrees away at total mismatch, you only
get about 6 dBs of difference, because whatever it started out as , a
linear signal coming through the ionosphere is inevitably elliptically
polarised when you receive it at the ground. A phenomenon fully
reported nearly 70 years ago by Appleton and Hartree.

So if this simple test already reveals that a one way trip through the
ionosphere produces elliptical polarisation, the results of a two way
trip to a satellite or to the Moon will be very different from simple
theory. Hence the move in most professional systems to the much more
complex circular polarisation mode, where, if helical antennas are
used instead of XY or spaced dipoles to get the circularity , signals
will be almost completely unaffected by Faraday rotation at any
frequency above 136 MHz. Below 136 MHz antennas producing circular
polarisation are too bulky and rarely produce reasonable circularity.