Satellite Antennas Pt3 by GM4IHJ
Faraday rotation of the polarisation of a radio signal from space means
that what starts from the satellite as horizontally polarised gets down to
your ground antenna perhaps as vertical one minute then horizontal then
vertical again a minute later. Thus making it difficult for you to match
your antenna polarisation for maximum signal to your receiver. The reason
for this is basic electrics. Electric motors turn because the conducting
cable is in a magnetic field, so we get rotation. The situation in space
is similar accept it is the radio signal which turns thereby shifting its
polarisation perhaps 4 or 5 times as it transits the ionosphere.
We rarely bother about this at HF frequencies because there are so many
rotations, any fading they cause due to antenna mismatch is only of short
duration. But at VHF we get say 4 rotations when the satellite is just
above the horizon with a long path through the ionosphere, to perhaps only
one turn when the satellite is overhead taking a short path directly down
through the ionosphere.
The end result at VHF is that we see the signal appear to rotate 4 - 1
turns of polarisation as it comes from horizon to overhead, then another 4
- 1 turns as it goes from overhead to the far horizon. A total of 6 turns
in an orbit pass , which if it takes 12 minutes, means a long deep
reception fade every two minutes unless we can compensate for this change
in polarisation.
The simplest way to do this is to use a crossed dipole as reported in Part
2, but this suffers because its antennas are never in exactly the right
polarisation agreement. So a better way is to use two yagi beams. You can
mount then on the same antenna shaft and put their elements at 90 degrees
to one another, or you can use two quite separate yagis , one mounted with
its elements at 90 degrees to one another and both yagis kept pointed at
the satellite. Given this double array you can route the coax cable from
each yagi to a change over switch and when your signal begins to fade you
switch to the other yagi. For use on say Mir VHF signals this simple
system works well.
Please do not confuse the above use of separate yagis on the same shaft or
set apart, with circular polarisation. This is not circular polarisation.
Circular polarisation only works well when the signal transmitted from the
satellite is sent on a circularly polarised antenna, and this downlink is
received at your station on a similarly circularly polarised antenna. More
about circular polarisation in Antennas Pt4.