[evlatests] Antenna 16, Q-band, bad polarizer?

[Bob Hayward]

My guess is it's not the polarizer, but a metal shard from the feed-horn 
that has fallen onto the vacuum window. Corrugated feeds do seem to have 
that problem, alas.

We do not check the axial ratio in the lab of receivers that have 
commercial polarizers (like Q-Band). That was already done by the 
company we bought them from (Atlantic Microwave, in this case).

Debris on the vacuum window can be checked for on the antenna. It 
requires the receiver being dropped so the feed can be removed. If we're 
lucky, a piece of metal will be found. If not, the receiver would need 
to be returned to the lab and investigated further.

-Bob


Rick Perley wrote:
>     More interesting -- or at least curious -- information from the 
> short test today. 
> 
>     In plotting the visibility amplitudes at Q-band, I found large 
> 'closure errors', of the order of a few percent.  A quick check showed 
> that *all* of these come from antenna 16.  I suspected bad delay or 
> bandpass (since I created a 100 MHz-wide continuum by integrating across 
> the spectrum) -- but no, that wasn't the problem, since the closure 
> errors remained after proper delay and bandpass calibration.
>     The problem is a bad polarizer.  In plotting the cross-polarization 
> amplitudes, I find that this antenna shows a 10% cross-polarization!  I 
> guessing that this is cross-coupling the correlations, either through 
> the D*P  mechanism, or the D*D mechanism.  Neither is corrected with the 
> current way we handle polarization calibration ...
> 
> 
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