VLA Test/Observation Coordination Meeting
                           B.G. Clark
                        October 28, 2004


1.  eVLA 21cm

B. Clark states that antenna 13 is now up and running again, after a
long hiatus where its modules were removed and taken back to the lab.
However, the antenna is not time synchronized, so it is not yet possible
to check even lobe rotation, much less phase stability.  Two things
have been addressed.  We have set the delays so that 50 MHz bandwidth
works (more or less - there is a large phase excursion at the lower
end of the band which we have not yet cured which costs some sensitivity).
We have managed to synchronize with the Modcomps, so that VLA pointing
mode emulation now works.  A 20cm pointing run was taken, but K.
Sowinski reports fairly indecisive results.  It is not entirely clear
whether this is due to amplitude eVLA/Modcomp synchronization problems,
amplitude instability problems, or just source confusion.

Since we have continuum mode working now, we can comment on the 
polarization, which seems to be remarkably high.  The RL and LR
amplitudes on antenna 13 crosscorrelated with VLA antennas are 
about 10%.  Within VLA antennas we are used to seeing numbers of
2% or 3%.  The other two antennas using the cooled hybrid for 
circular polarization formation show crossed hands of about 5% with
the rest of the VLA, and still about 10% with antenna 13, so it
is not a simple systematic difference in feed ellipticity.

R. Perley reported on single dish measurements at 20cm.  A memo will
be soon forthcoming.  He used filters which happened to be available, 
at 1325, 1425, 1675, and 1975 MHz.  At the highest frequency, the 
receiver temperature is quite high, because it is a VLA receiver, not 
the new, wide bandwidth, EVLA receiver.  As expected, the best focus is 
a function of frequency.  The good news is that at the best compromise 
focus, there is a loss of only a percent or two all across the band.

The system temperature vs elevation curves are quite flat to 40d 
elevation, then curve upward, adding 20 K by 8d elevation.  (The
lowest frequency is slightly worse than the others.)  This is very
much less than the VLA feed shows.

Measured efficiencies are about 0.43-0.45, with possibly somewhat 
larger numbers at the highest frequency.  This is lower than originally
hoped, and, indeed, lower than the current feed.  This is compensated
partly by the fact that the zenith system temperature is lower
than the VLA system.  The zenith System Equivalent Flux Density is
about 350 Jy on antenna 13, 335 on antenna 28, a difference comparable
to the measurement errors.  Antenna 13 is substantially more sensitive
than VLA antennas at more than 30d zenith distance, however.

The beam width is essentially the same as VLA antennas.  The first
sidelobe is higher in the principal directions (up-down, cw-ccw)
than at 45d - an expected result, the influence of feed leg shadowing.

One interesting difference is that the beam squint is different from
that of the VLA antennas.  On antenna 13 the pointing difference 
between the R and L beams is almost entirely in elevation.