[evlatests] S-band polarization

[Rick Perley]

     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 ...