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

....
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.

Jerry, I am a little surprised that a geostationary satellite would use
linear polarisation on a 4GHz feed, but that might just express a lack of
experience.

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