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circular polarization sense



Can someone help me understand this?
Ive been a bit confused about how much this affects signal strength if you
get it 'wrong'.
My first problem is I dont know which Sats actually transmit in circular
pol.

Any ? All? just the P3 series?

Where can I find out what this info? does anyone already have a compiled
summary of these kind of specs? (Im also interested in SAT Antenna, BeamW,
pol, power etc).

An article I read recently claimed that polarization will reverse sense
after passing overhead (IE SAT goes from approaching to receeding). Implying
that if an operator has a (say LH.) circular polarized RX ant, then he will
only have good sigs on half of the pass
To me this is counter intuitive if the sat is already transmitting in
circular polarization.

how much does the atmosphere affect polarization?

The way I understand it is that atmospheric effects cause a (slight?) random
rotation over space or time or both.
So a fixed pol antenna should have a good chance of getting a signal in its
vector.

But the other thing Id like to know is *if* a SAT is designed to transmit
only one polarization, even if we elminiate atmosphere, is it ALWAYS this
polarization? (IE can we be sure there is no small angular velocity on an
axis radial to earth which would alter this over time? )

thanks for any comments.

David Merrett,
RF Engineer, Jindalee project
BAE Systems, 
Ph   +618 8290 7001
Fax  +618 8290 8887

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