[evlatests] More on EVLA and VLA L-Band Polarization

[Rick Perley]

    The report I sent around a few days back concerning EVLA 
polarization was highly influenced by the behavior of antenna 11, which 
is perhaps the most bizarre of them all.  I have now reviewed the 
behavior of all antennas (EVLA and VLA), and have recognized other 
characteristics which may prove useful in uncovering the origin of the 
variable polarization. 

    To recap -- these observations were made in Sept and Oct., of two 
essentially unpolarized (< 1%) sources, at the standard bands.   Careful 
editing calibration has been done.  Conclusions are based on examination 
of the normalized (i.e. RL/RR, LR/RR) X-hand amplitudes and phases. 

    1)  Overall polarization.  One can easily find the *average* 
cross-polarization by matrix listing the VLA x VLA, then EVLA x EVLA 
antennas.   I find:

               IF 1      IF 2
   VLA    2.4%   3.2%
 EVLA   7.3%    6.2%

          The EVLA has, on average, 2  to 3 times the cross polarization.
    I did not compute the VLA x EVLA products, but visual inspection 
makes it clear that the values will be similar to EVLA x EVLA baselines. 

    2) Variability.  In order to get a sensible picture of the 
variability, one has to find an antenna whose polarization is believed 
to be non-changing.  By inspection of the cross-products, it appeared 
that VLA antenna 2 has both low and steady polarization.  Hence, 
judgments on antenna polarization variability are based on plots versus 
antenna 2. 

    It is clear that both VLA and EVLA antennas have slow (timescale of 
an hour or more) variability in their antenna polarization.  The 
*fractional* variations for VLA antennas are at least as high as, and 
sometimes higher than,  EVLA antennas.  What distinguishes EVLA antennas 
is that the amplitudes are 2 to 3 times higher.  Not all antennas are 
equally variable -- there are VLA and EVLA antennas which are relatively 
stable. 

    3)  Repeatability.  For many antennas -- both EVLA and VLA -- the 
characteristic polarization accurately repeats on all four 
observations.  For three of these -- all of the same source, 3C287 -- 
the repeatability is very high.  For the fourth observation (0555+398), 
the agreement is less impressive.  (This source transits to the north, 
while the other transits to the south). 

    4) Differences between the two IF pairs.  A mixed result here.  For 
most antennas, the two IFs give very similar variability patterns.  It 
appears this is more true, the higher the cross-polarization.  For some 
antennas, notably those with lower polarization, there are marked 
differences. 

    5) Symmetries.  One might hope that patterns would be symmetric 
w.r.t. hour angle (or, equivalently for these data, parallactic angle or 
elevation).  Such evidence would point strongly to an antenna-based 
geometric origin (as opposed to electronics).  But the evidence is very 
mixed.  It's almost true for antennas 11 and 13.  But antennas 17 and 24 
(for example) are anti-symmetric.  And (of course!) others show no 
particular symmetry at all. 

    Tomorrow, I plan to review the data at the other bands.