[amsat-bb] Re: Antenna Polarization Technical Question
bruninga at usna.edu
Sat Aug 4 15:24:38 PDT 2012
> Not sure why anyone would want to maintain the
> orientation of the satellite in such a way that
> would cause the direction of circular
> polarization to change during the path.
Lets try this approach... As I said before, By the laws of physics, what comes out one side of a circular polarized low gain antenna as RHCP comes out the opposite side as LHCP.
Now given that, and the fact that someone in Maryland is in the center of the RHCP beam, then by the laws of physics, the guy in California must see mostly LHCP. No matter how much one of those persons demands that he deserves the RHCP beam, by definition, someone else somewhere will get the LHCP one, and the geometry changes at least every 10 minutes or so and every time the spacecraft rotates a bit.
So one might say, "point it down" then only the person in Kansas will see the main beam and those in CA or MD will be completely off the sides almost 70 degrees from the main beam. Mot people do not realize how LOW these satellites are. The only solution is to put satellites so high, that "down" is about the same to everyone (geostationary altitude). But then that takes 100 times more altitude, and that takes 10,000 times more power.
Better to just live with the laws of physics... I guess.
>On Sat, Aug 4, 2012 at 2:47 PM, Bob Bruninga <bruninga at usna.edu> wrote:
>>> I believe that is true but that does not explain
>>> why the optimum polarity setting on the receive
>>> end would change during a pass.
>> That's easy. The circularity on a pair of crossed dipoles (about all you can get on a spacecraft) May be designed for Right hand circularity when viewed from the prime direction. But by definition, that save waveform will be LHC when viewed from the opposite direction.
>> And since the geometry to any one observer is constantly changing by almost 180 degrees during an overhead pass, that is why it is very easy to see, complete change in circularity.
>> Bob, WB4APR
>Sent from my computer.
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