VLA Test/Observation Coordination Meeting
                           B.G. Clark
                        November 19, 2004


1.  Antenna 13 20cm efficiency

P. Napier reports from Shrikanth that the behavior of the 20 cm feed
is now understood.  Previous predictions were made assuming the position
of the horn phase center was located at the true focus.  The effective
phase center at a frequency depended location well down the horn.  The
effect of this out-of-focus position is to cut the main reflector 
illumination off more sharply than for something at the focus.  This
results in both a lower efficiency measured a lower than expected 
zenith spillover.  The bottom line is that the system has about the
same sensitivity as the current VLA system at the zenith, and much higher
sensitivity at lower elevations, about a factor of two more sensitive 
at 20d elevation.

2.  Antenna 13 6cm efficiency

R. Perley reports on efficiency measurements on the C band horn.
Tipping curves show that the system temperature is rather flat with
elevation until about 20 or 25d.  At that point it increases both
due to spillover striking the ground and increasing atmospheric 
emission.  At 8  degrees elevation these amount to about 12 K each.
The focus is frequency dependent, but again, it is possible to find
a focus position that works well for all the band, losing no more
than 5% at the extreme band edges.  Preliminary results for efficiency
are 58% at 3.9 GHz, 68% at 6GHz, 51% (with especially large error) at
8 GHz.

3.  Antenna 13 20cm fringes

B. Clark reports that the antenna is now synchronized, with true time,
as far as is known.  There was a longish (2.5h) simple track at L 
band.  Amplitudes are reasonably stable.  It shows a bit more variation
than other antennas, but it remains possible to attribute that to 
the fact that the switching noise source is not yet turned on to 
remove small system temperature variations.  (Weather was pretty good,
but still a few scattered clouds that could affect system temperature.)
There are occasional downward glitches accompanied by a phase reversal.
This appears to be due to the direct digital synthesizer (DDS) being 
used as a loberotator is having its new phase strobed in at the wrong
time.

R. Perley notes that the phase scatter is somewhat larger for antenna
13.  The cause of this is unknown.  He also analyzed this track for
polarization.  Relative to the rest of the array, Antenna 13 has D
terms of 0.10 and 0.13.  The scatter among VLA antennas is about
0.02 to 0.03, and the two other antennas which have a hybrid 
polarizer only run about 0.05.  When PCAL is applied, however, the
scatter in antenna 13 is down around 0.002, so the D terms are 
reasonably stable, at least on this two hour timescale.