[evlatests] EVLA X-Band gain stability

[Peggy Perley]

Antenna 22 has a maintenance form on this - I believe that on Bob 
Hayward's suggestion, the feed was lowered on Wed. to clean off metal 
shavings.  So, this antenna/band should be rechecked as soon as 
practical to see if this solved the problem.

Peggy

Rick Perley wrote:
>     I am now beginning the data reduction of the ~400 GB of data taken 
> during the 30 hour 'flux densities' run.  Data quality is excellent 
> overall.  I've completed basic reduction of the X-band data, using a 
> single subband centered at 8457 MHz.    Clean switched power was 
> obtained for all antennas, so a full and proper calibration utilizing 
> the PDif values was executed, and a fit for elevation gain dependency 
> was made. 
>     Antenna gain stability is outstanding on all antennas except two -- 
> noted below.  The antenna gain dependency is at the expected level 
> typically 0.5% (in amplitude) from high to low elevations.  The 
> solutions for RCP and LCP are always similar, lending credence to an 
> interpretation that the dependency is truly due to the antenna, rather 
> than the electronics. 
> 
>     There are two antennas that show notable -- indeed, radical! -- 
> departures from this happy picture:
> 
>     1) Antenna 1:  Shows a 24-hour sinusoidal dependence in gain, with 
> +/- 2% amplitude variation.  The low gain is seen at 0h IAT (5PM in the 
> afternoon), the high gain 12 hours later.   The effect is identical in 
> RCP and LCP. 
> 
>     2) Antenna 22:  Shows large (+/- 10%) and apparently random changes 
> in gain throughout, which are not corrected by the switched power 
> monitoring.  The switched power looked normal, so these changes are not 
> due to the electronics.  Curiously, the RCP and LCP gains are 
> approximately anticorrelated -- when one is up, the other is down ...  
> No obvious explanation comes to mind...
> 
>     Antenna 19, RCP only, is the only other discrepant antenna -- and 
> the effect here is quite small, or order 1% gain variations.  This 
> antenna/polarization also showed major changes in bandpass shape 
> throughout the run, so there is likely a correlation. 
> 
>     All other antennas are at least as stable as the VLA antennas were 
> 'back in their prime'. 
> 
>    
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