[evlatests] S-band polarization

[Vivek Dhawan]

I can verify that NASA's RCP from a variety of satellites is the same as our
VLA and VLBA pol sense. (mostly X and Ku band 12 GHz)

On 06/12/2015 12:30 PM, Bryan Butler wrote:
>
> we've done plenty of VLB; the problem is not with the VLA. i wonder if whatever rick is reading about XM is using the different convention for circular polarization (along or against the poynting vector).
>
>         -bryan
>
> Sent from my iPhone.
>
>
>> On Jun 12, 2015, at 12:19, Michael Rupen <mrupen at nrao.edu> wrote:
>>
>> 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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