[evlatests] S-band polarization

[Michael Rupen]

Have we done S- or L-band VLBI lately?  Should be a direct indicator of 
consistency at least.

        Michael

>    Some time ago, I wondered aloud about whether we had the S-band 
> polarizations reversed.  The evidence was that the effect of the XM and 
> Sirius satellites was much stronger on the LCP channel than the RCP.  This is 
> still the case.  According to their websites, those satellites broadcast in 
> RCP.    A well known antenna theorem tells us that an antenna which 
> broadcasts in one polarization also receives that polarization.
>
>    There is an easy way to see if the polarizations are correct -- or at 
> least if they are the same as at L-band.  This is to use the beam squint. 
> The L-band and S-band feeds are arranged nearly exactly below, and above 
> (respectively) the antenna center, along the vertical plane.  This should 
> mean that the beam squint (separation of RCP and LCP beams) should be 
> reversed on the sky.
>
>    Ken reviewed the results of recent pointing scans, and confirms that 
> indeed the squint parameters for S-band are opposite those of L-band (and 
> that they scale correctly, and are wholly in the horizontal direction, as 
> expected).
>
>    So it seems the S-band polarizations are correct.  (I discount the other 
> possibility -- that they are both wrong).
>
>    Which raises again the question of why the LCP channel is much more 
> strongly affected by supposedly circularly polarized RFI...  Is it possible 
> that the engineering for Sirius and XM were using the Physics definition of 
> circular polarization, rather than the IAU/IEEE definition?  Seems unlikely 
> ...
>
>
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