[evlatests] L-Band antenna polarization

[Rick Perley]

    Finally, I get to what I was originally seeking. 

    The idea of the test was to use polarization spectral line mode to 
see if the high and variable L-band antenna polarization seen last Sept. 
and Oct. have frequency structure. 

    I observed 3C48 (L-band polarization = 0.5%, X=band polarization = 
5.4%) for two hours, following transit.  Standard editing and 
calibration were applied.  Bandpass calibration was also done, as the 
data are taken in 'spectral line' mode. 
    3C48 transits very near the zenith.  Hence, over the HA range of 
+0.5 through +2.3, the elevation declined from 83 to 62 degrees, but the 
parallactic angle remained nearly fixed, at 80 degrees. 

    Results:

    A)  X-band:  All antenna polarizations are stable.   

    B) L-Band:  Only antenna 11 showed variable antenna 
cross-polarization.  The fractional value appeared roughly sinusoidal, 
of one hour period, with amplitude about 2%, about a mean of about 4% 
for RL, and 7.5% for LR.  The patterns were identical for all frequency 
channels.  (Channelwidth was 3 MHz, and we had 7 channels at a center 
frequency of 1385 MHz).  All other antennas were stable. 
    As usual, EVLA cross polarizations are considerably higher than VLA. 

    More testing will be required.   It is not reasonable to conclude 
that parallactic angle, alone, can be responsible for the observed 
variations.