Owen Duffy wrote in
:

"Jerry Martes" wrote in
news:iyJOh.23960$FD1.9394@trnddc05:

.
A long time ago, I worked with the Intelsat series, and they were
circular polarisation. Earth stations had no means of adjusting the
orientation of feeds, they were RH or LH circular, the uplink was
opposite to the downlink IIRC.

More recently, I worked on the design of a bird that used polarisation
diversity. It used LH and RH circular, and reused the same frequency
band on both polarisations.

If your bird is truly linear, you could use a circular antenna with a
slight reduction in G/T, but with the flexibility of eliminating the
orientation variable and the mechanical aspects of an antenna with
adjustable orientation (remembering that the feed orientation will
vary with position of the earth station).

Notwithstanding that transmission might be circular, the received
signal might not be perfectly circular as a result of some of the
effects you have described.

Owen

Hi:
We just had a member that works in the satellite uplink/downlink business
at Penn State University give a talk on the subject. The satellites use
polarization as part of the frequency sharing system in geostationary
satellites. When they buy time on a satellite they are given a frequency
and a polarization to use. As there is a limited band of frequencies they
use polarization to help share frequencies with some working horizontal
and some vertical. The feeds on the dishes they use for uplinks and
downlinks have motorized polarization feeds and they adjust them to the
requested polarization.


John Passaneau W3JXP