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

John Passaneau wrote in
:

....
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.

Yes John, I talked about that and I incorrectly used the term
"diversity", but it is frequency reuse as I noted using polarisation.

This can be done with circular (LH and RH) polarisation without a need to
rotate the feed unit, which for many installations will mean another
rotary waveguide joint which is to be avoided.

Launching circular polarisation in a feed horn is not difficult, and
developing autotrack error signals is relatively easy. An interesting
feed for amateur applications is the septum feed which gives access to
both circular polarisations from a fairly simple feed unit.

Owen

Owen Duffy wrote in
:

John Passaneau wrote in
:

...
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.

Yes John, I talked about that and I incorrectly used the term
"diversity", but it is frequency reuse as I noted using polarisation.

This can be done with circular (LH and RH) polarisation without a need
to rotate the feed unit, which for many installations will mean
another rotary waveguide joint which is to be avoided.

Launching circular polarisation in a feed horn is not difficult, and
developing autotrack error signals is relatively easy. An interesting
feed for amateur applications is the septum feed which gives access to
both circular polarisations from a fairly simple feed unit.

Owen

Hi Owen:

In this case the feed point has for lack of the right name a "pick up
loop" that is motorized and that is what turns for the differnt
polarisation. It is linear, not circular polarisation, at least on the
geostationary satellites they use. I would guess that linear is easier
for them to change. The way it works is they call a satatellite broker
and buy time, he gives them a satellite, a frequency and a polarisation
to use. Before the requested time the satellite transmits a test signal.
They tune to that frequency and tune their antenna for max signal, then
they transmit a test signal to the satellite, the satellite operator
verfies that their signal meets spec.'s and then the channel is theirs
for how every long they have paid for. It's more of a hands on operation
than I would have though. Their main busness is sports as they up load
all the sports events here on campus and for many of the non sports
events like news events and some classes that are held via satellite.

John Passaneau W3JXP