# [amsat-bb] Re: Antenna Polarization Technical Question

Thomas Doyle tomdoyle1948 at gmail.com
Sat Aug 4 13:20:20 PDT 2012

```Bob,

Thanks for the reply. A student of mine once told me that if someone
tells you something is easy - it is not. Even though this is not only
easy but "very easy" I still need a slight clarification. I think it
boils down to the orientation of the satellite relative to the center
of the earth.

If the satellite was a clock and the face of the clock was oriented
toward the center of the earth the I believe the clock would appear to
rotate CW on both ends of the pass. If the side of the clock rather
than the face of the clock was oriented toward the center of the earth
it would appear to rotate one way at the start of the pass and the
other way at the end of the pass because we would be looking at the
clock from the other side. I believe this is the basis of the "very
easy" explanation you offered.

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. Perhaps people selling antenna
circularity switches would like it but other than that I do not
understand why it would be done. I am most likely missing something
important.

tnx & 73 W9KE Tom Doyle

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.

tom ...

```