[evlatests] L-Band test results

[Jim Jackson]

Rick,

For the antenna 14 problem, we still need to know what frequencies the L301 
and L302 synthesizers were tuned to for this experiment.

Jim



At 04:22 PM 3/6/2006, you wrote:
>    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.
>
>
>
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