[evlatests] L-Band test results

[Rick Perley]

    In a word, simply fantastic (with one significant caveat, noted 
below). 

    Now for the details.

    The point of this experiment was to test both the usual parameters 
(stability,
sensitivity, etc.), and get an elementary polarization determination.  
To do this,
I observed in spectral line mode, with 12.5 MHz BW, alternatively in
mode PA with PB.
    For the former mode, I selected 1485 MHz, as at this frequency, Bob's
polarization measurements show rather poor behavior -- caused by the
existing OMT.  For the latter mode, I used 1340 MHz, as the lab measurements
show low cross-polarization.  The EVLA antennas have the new quad
hybrid to convert the (native) linear to circular polarization.   All 
but one
VLA antennas have the original polarizers.  The differing polarizers are
expected to result in significant polarization on VLA-EVLA baselines.
We anticipate very low polarization on EVLA-EVLA baselines (at
frequencies away from the problems induced by the old VLA OMT --
these will soon be replaced by Paul Lillie's new super-OMT). 

    Results:

    Amplitude and phase behavior on 13, 14 and 16, at both frequencies,
is superb.  There is nothing bad to report.  The 'bi-stable' state in 14A is
not visible in this dataset (taken two days earlier). 

    Antenna 14 gave no fringes at 1340 MHz on any source -- whereas
it always gave fringes at 1485 MHz.  Note, however, that this could be
an AC vs. BD IF issue -- we used 1485 MHz for the AC IFs, 1340 at
the  BD IFs. 

    Sensitivity is not superb.  The observations were made at very high
elevation -- 82 degrees, for which the EVLA advantage of lower spillover
will not be seen.  At 1485 MHz, antennas 13, 14 and 16 are all well below
the VLA antennas in sensitivity.  However, at 1340 MHz, although they
are not amongst the best, neither are they the worst.  (It might be useful
to now do a spot check of sensitivity as a function of frequency).   

    Polarization: 

    At 1485 MHz, and with a VLA antenna as reference, the indicated
antenna polarizations vary from 1 to 5% for VLA antennas,  and
5 to 10% for EVLA antennas.  With EVLA antennas as reference,
all others (VLA and EVLA) antennas show high (5 - 10%) polarization.
Using 13 as a reference certainly results in much higher apparent
polarizations -- a result probably explained from Bob's measurements
which show 13 with very poor circularity. 
   
    At 1340 MHz,  there is less variation, as anticipated from Bob's
measurements.  With VLA antenna 9 as a reference, all other antennas
show 5 to 8 % polarization.  With EVLA antenna 13 as a reference,
VLA baselines are uniformly at 10%, but antenna 16 is a sparkling
1% polarization!  Unfortunately, as noted above, antenna 14 was
not operating.